Datasheet MAX8731ETI Datasheet (Maxim) [ru]

General Description

The MAX8731 is an SMBus™ programmable multichem­istry battery charger. The MAX8731 uses a minimal
command set to easily program the charge voltage,
charge current, and adapter current limit.

The MAX8731 charges one to four Li+ series cells and
delivers up to 8A charge current. The MAX8731 drives
n-channel MOSFETs for improved efficiency and
reduced cost. Low-offset current-sense amplifiers pro­vide high accuracy with 10mΩ sense resistors.

The MAX8731 current-sense amplifiers provide high
accuracy (3% at 3.5A) and also provide fast cycle-by­cycle current-mode control to protect against battery
short circuit and system load transients.

The charger employs dual remote-sense, which reduces
charge time by measuring the feedback voltage directly
at the battery, improving accuracy of initial transition into
constant-voltage mode. The MAX8731 provides 0.5%
battery voltage accuracy directly at the battery terminal.

The MAX8731 provides a digital output that indicates the
presence of the AC adapter, as well as an analog output
that indicates the adapter current within 4% accuracy.

The MAX8731 is available in a small 5mm x 5mm,
28-pin, thin (0.8mm) QFN package. An evaluation kit is
available to reduce design time. The MAX8731 is avail­able in lead-free packages.

Applications

Notebook Computers
Tablet PCs
Medical Devices
Portable Equipment with Rechargeable Batteries

Features

♦ 0.5% Battery Voltage Accuracy
♦ 3% Input Current-Limit Accuracy
♦ 3% Charge-Current Accuracy
♦ SMBus 2-Wire Serial Interface
♦ Cycle-by-Cycle Current Limit

Battery Short-Circuit Protection
Fast Response for Pulse Charging
Fast System-Load-Transient Response

♦ Dual-Remote-Sense Inputs
♦ Monitor Outputs for

Adapter Current (4% Accuracy)
AC Adapter Detection

♦ 11-Bit Battery Voltage Setting
♦ 6-Bit Charge-Current/Input-Current Setting
♦ 8A (max) Battery Charger Current
♦ 11A (max) Input Current
♦ +8V to +26V Input Voltage Range
♦ Charges Li+, NiMH, and NiCd Battery Chemistries

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

Typical Operating Circuit

19-3923; Rev 0; 1/06

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

SMBus is a trademark of Intel Corp.

+Indicates lead-free packaging.

EVALUATION KIT

AVAILABLE

Pin Configuration

TOP VIEW

DCIN

DHI

BST

V

CSSN

CSSP

22

LX

23

24

25

26

CC

27

28

12

PGND

DLO

CSIP

LDO

2021 19 17 16 15

*EXPOSED PADDLE

GND

ACIN

THIN QFN

5mm x 5mm

CSIN

18

MAX8731

4567

3

CCI

REF

CCS

FBSB

CCV

FBSA

DAC

14

BATSEL

ACOK

13

12

GND

V

11

DD

10

SCL

SDA

9

8

IINP

PART TEMP RANGE PIN-PACKAGE

MAX8731ETI+ -40°C to +85°C 28 Thin QFN (5mm x 5mm)

OPTIONAL

HOST

SCL
SDA
VDD

CSSP CSSN

ACIN
DCIN
ACOK

REF

GND

BATSEL
SCL

SDA

V

DD

GND

IINP CCV

DAC

MAX8731

CCS

DHI

DLO

PGND

BST
CSIP

CSIN

FBSA

FBSB

LDO

V

CCI

LX

CC

N

N

EXTERNAL

LOAD

SELECTOR

BATSEL

BATTERY
A

BATTERY
B

SMBus Level 2 Battery Charger with
Remote Sense

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(V

DCIN

= VLX= V

CSSP

= V

CSSN

= 19V, V

BST

- VLX= 4.5V, V

FBSA

= V

FBSB

= V

CSIP

= V

CSIN

= 16.8V, BATSEL = GND = PGND = 0,

C

LDO

= 1µF, VCC= LDO, C

REF

= 1µF, C

DAC

= 0.1µF, VDD= 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per

Figure 1; T

A

= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

DCIN, CSSN, CSIN, FBSA, FBSB to GND..............-0.3V to +28V

CSSP to CSSN, CSIP to CSIN, PGND to GND ......-0.3V to +0.3V

BST to GND ............................................................-0.3V to +32V

BST to LX..................................................................-0.3V to +6V

DHI to LX.................................................-0.3V to +(V

BST

+ 0.3)V

DLO to PGND..........................................-0.3V to +(LDO + 0.3)V

LX to GND .................................................................-6V to +28V

CCI, CCS, CCV, DAC, REF,

IINP to GND...........................................-0.3V to (V

VCC

+ 0.3)V

V

DD

, SCL, SDA, BATSEL, ACIN, ACOK, VCCto GND,

LDO to PGND ......................................................-0.3V to +6V

Continuous Power Dissipation (T

A

= +70°C)

28-Pin Thin QFN

(derate 20.8mW/°C above +70°C)........................1666.7 mW

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-60°C to +150°C

Lead Temperature (soldering, 10s) .................................+300°C

MAX8731

PARAMETER CONDITIONS MIN TYP MAX UNITS

CHARGE-VOLTAGE REGULATION

ChargingVoltage() = 0x41A0

ChargingVoltage() = 0x3130

Battery Full-Charge Voltage and

Accuracy

ChargingVoltage() = 0x20D0

ChargingVoltage() = 0x1060

Battery Undervoltage-Lockout

Trip Point for Trickle Charge

2.5 V

CHARGE-CURRENT REGULATION

CSIP to CSIN Full-Scale Current-

Sense Voltage

78.22 80.64 83.06 mV

RS2 (Figure 1) = 10mΩ;

ChargingCurrent() = 0x1f80

Charge Current and Accuracy

RS2 (Figure 1) = 10mΩ;

ChargingCurrent() = 0x0f80

RS2 (Figure 1) = 10mΩ;

ChargingCurrent() = 0x0080 (128mA)

Charge-Current Gain Error Based on ChargeCurrent() = 128mA and 8.064A -2 +2 %

FBSA/FBSB/CSIP/CSIN

Input Voltage Range

0 19 V

16.716 16.8 16.884 V

-0.5 +0.5 %

12.491 12.592 12.693 V

-0.8 +0.8 %

8.333 8.4 8.467 V

-0.8 +0.8 %

4.15 4.192 4.234 V

-1.0 +1.0 %

7.822 8.064 8.306 A

-3 +3 %

3.809 3.968 4.126 A

-4 +4 %

64 400 mA

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(V

DCIN

= VLX= V

CSSP

= V

CSSN

= 19V, V

BST

- VLX= 4.5V, V

FBSA

= V

FBSB

= V

CSIP

= V

CSIN

= 16.8V, BATSEL = GND = PGND = 0,

C

LDO

= 1µF, VCC= LDO, C

REF

= 1µF, C

DAC

= 0.1µF, VDD= 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per

Figure 1; T

A

= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

PARAMETER CONDITIONS MIN TYP MAX UNITS

Battery Quiescent Current

Adapter Quiescent Current

INPUT-CURRENT REGULATION
CSSP to CSSN Full-Scale

Current-Sense Voltage

Input Current Accuracy

POR Input Current RS1 (Figure 1) = 10mΩ 256 mA
Input Current-Limit Gain Error -2 +2 %
Input Current-Limit Offset
CSSP/CSSN Input Voltage Range 8 26 V
IINP Transconductance V
IINP Offset Based on V

IINP Accuracy

IINP Output Voltage Range 0 3.5 V
SUPPLY AND LINEAR REGULATOR
DCIN, Input Voltage Range 8.0 26.0 V

DCIN Undervoltage-Lockout

Trip Point

Power-Fail Threshold

Adapter present, not charging, I

V

Ad ap ter ab sent, I

+ I

I

DCIN

I

CSSP

I

CSSN

V

RS1 (Figure 1) = 10mΩ, InputCurrent() = 11004mA or

3584mA

= VLX = V

FBS_

, V

C S S N

FBS _

+

+

= 19V 106.7 110 113.3 mV

FBS_

CSIN

C S I P

= V

LX

V

Adapter

charging

V

Adapter

V

Battery

V

Adapter

V

Battery

= V

+ I
= V

= 26V, V

= 19V,

= 16.8V

= 8V,

= 4V

CSIP

C S I N

C S I N

= 19V

+ I

= V

Battery

LX

CSIP

+ I

C S I P

+ I

CSIN

FBS A

= 19V , V

= 16.8V, not

+ I

FBS B

+ ILX + I

+ I

= 0V

D C I N

FBS,

C S S P

2 5

+1

200 500 µA

Charging 0.4 1 mA
Not charging 200 500 µA
Charging 0.4 1 mA
Not charging 200 500 µA

-3 +3 %

RS1 (Figure 1) = 10mΩ, InputCurrent() = 2048mA -5 +5 %

Based on InputCurrent() = 1024mA and 11004mA

CSSP - CSSN

V

CSSP - CSSN

V

CSSP - CSSN

V

CSSP - CSSN

= 110mV 2.85 3.0 3.15 mA/V

CSSP - CSSN

= 110mV -5 +5
= 55mV or 35mV -4 +4
= 20mV -10 +10

= 110mV and 20mV -1.5 +1.5 mV

-1 +1 mV

DCIN falling 7 7.4
DCIN rising 7.5 7.85
V

- V

CSSP

V

CSSP

falling 9 15 21

CSIN

- V

rising 160 210 271

CSIN

µA

%

V

mV

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(V

DCIN

= VLX= V

CSSP

= V

CSSN

= 19V, V

BST

- VLX= 4.5V, V

FBSA

= V

FBSB

= V

CSIP

= V

CSIN

= 16.8V, BATSEL = GND = PGND = 0,

C

LDO

= 1µF, VCC= LDO, C

REF

= 1µF, C

DAC

= 0.1µF, VDD= 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per

Figure 1; T

A

= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

PARAMETER CONDITIONS MIN TYP MAX UNITS

LDO Output Voltage 8.0V < V
LDO Load Regulation 0 < I
LDO Undervoltage-Lockout Threshold V
V

Range 2.7 5.5 V

DD

V

UVLO Rising 2.5 2.7 V

DD

V

UVLO Hysteresis 100 mV

DD

V

Quiescent Current DCIN < 6V, VDD = 5.5V, SCL = SDA = 5.5V 16 27 µA

DD

REFERENCE
REF Output Voltage 0 < I
REF Undervoltage-Lockout Trip Point REF falling 3.1 3.9 V
ACOK
ACOK Sink Current V
ACOK Leakage Current V
ACIN
ACIN Threshold 2.007 2.048 2.089 V
ACIN Threshold Hysteresis 10 20 30 mV
ACIN Input Bias Current -1 +1 µA
REMOTE-SENSE INPUTS
FBS_ Range V
FBS_ Gain ∆V
CSIN-FBS_ Clamp Voltage 225 250 275 mV
FBS_ Bias Current Charger switching, FBS_ selected 14 µA
FBS_ Bias Current Charger not switching or FBS_ not selected -2 +2 µA
SWITCHING REGULATOR

Off-Time

BST Supply Current DHI high 500 800 µA
LX Input Bias Current V

Maximum Discontinuous-Mode Peak

Current (I

MIN

)

DHI On-Resistance Low I
DHI On-Resistance High I
DLO On-Resistance High I
DLO On-Resistance Low I

< 28V, no load 5.25 5.4 5.55 V

DCIN

< 30mA 34 100 mV

LDO

= 8.0V, V

DCIN

REF

= 0.4V, ACIN = 1.5V 1 mA

ACOK

= 5.5V, ACIN = 2.5V 1 µA

ACOK

- V

CSIN

/ ∆(V

CSIN

V

= 16.0V, V

CSIN

V

= 16.0V, V

CSIN

= 28V, V

DCIN

LDO

< 500µA 4.071 4.096 4.120 V

FBS

CSIN

CSIN

falling 3.20 4 5.15 V

0 200 mV

- V

) 0.95 1.00 1.05 V/V

FBS_

= 19V 360 400 440

CSSP

= 17V 260 300 360

CSSP

= VLX

= 20V, DHI low

2 µA

0.5 A

= -10mA 1 3 Ω

DHI

= 10mA 3 5 Ω

DHI

= 10mA 3 5 Ω

DLO

= -10mA 1 3 Ω

DLO

ns

ELECTRICAL CHARACTERISTICS (continued)

(V

DCIN

= VLX= V

CSSP

= V

CSSN

= 19V, V

BST

- VLX= 4.5V, V

FBSA

= V

FBSB

= V

CSIP

= V

CSIN

= 16.8V, BATSEL = GND = PGND = 0,

C

LDO

= 1µF, VCC= LDO, C

REF

= 1µF, C

DAC

= 0.1µF, VDD= 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per

Figure 1; T

A

= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

_______________________________________________________________________________________ 5

ERROR AMPLIFIERS

GMV Amplifier Transconductance C har g i ng V ol tag e( ) = 16.8V , V

GMI Amplifier Transconductance ChargingCurrent() = 3968mA, V

GMS Amplifier Transconductance InputCurrent() = 3968mA, V

CCI/CCS/CCV Clamp Voltage

LOGIC LEVELS

SDA/SCL Input Low Voltage VDD = 2.7V to 5.5V 0.8

SDA/SCL Input High Voltage VDD = 2.7V to 5.5V 2.1

SDA/SCL Input Bias Current VDD = 2.7V to 5.5V -1 +1

BATSEL Input Low Voltage 0.8

BATSEL Input High Voltage 2.1

BATSEL Input Bias Current -1 +1

SDA, Output Sink Current V

SMBus TIMING SPECIFICATIONS (VDD = 2.7V to 5.5V) (see Figures 4 and 5)

SMBus Frequency f

Bus Free Time t

Start Condition Hold Time from
SCL

Start Condition Setup Time from
SCL

Stop Condition Setup Time from
SCL

SDA Hold Time from SCL t

SDA Setup Time from SCL t

SCL Low Timeout t

SCL Low Period T

SCL High Period T

Maximum Charging Period
Without a ChargeVoltage() or
ChargeCurrent() Command

PARAMETER CONDITIONS

PARAMETERS SYMBOL CONDITIONS MIN TYP MAX UNITS

0.25V < V

(SDA)

t

t

t

TIMEOUT

< 2.0V

CCI/S/V

= 0.4V 6

SMB

BUF

HD:STA

SU:STA

SU:STO

HD:DAT

SU:DAT

(Note 1) 25 35

LOW

HIGH

FBS_

CSSP

= 16.8V

- V

CSIP

- V

CSSN

= 39.68mV

CSIN

= 79.36mV

MIN TYP MAX UNITS

0.0625 0.125 0.2500 mA/V

0.5

0.5

120

10 100

4.7

4

4.7

4

300

250

4.7

4

140 175 210

1

1

250

2.0

2.0

600

mA/V

mA/V

mV

V

V

µA

V

V

µA

mA

kHz

µs

µs

µs

µs

ns

ns

ms

µs

µs

s

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

6 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS

(V

DCIN

= VLX= V

CSSP

= V

CSSN

= 19V, V

BST

- V

LX

= 4.5V, V

FBSA

= V

FBSB

= V

CSIP

= V

CSIN

= 16.8V, BATSEL = GND = PGND = 0,

C

LDO

= 1µF, VCC= LDO, C

REF

= 1µF, C

DAC

= 0.1µF , VDD= 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per

Figure 1; T

A

= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER CONDITIONS

MIN

TYP

MAX

UNITS

CHARGE-VOLTAGE REGULATION

V

ChargingVoltage() = 0x41A0

-1 +1 %
V

ChargingVoltage() = 0x3130

-1 +1 %
V

ChargingVoltage() = 0x20D0

-1 +1 %
V

Battery Full-Charge Voltage and

Accuracy

ChargingVoltage() = 0x1060

%

CHARGE-CURRENT REGULATION
CSIP to CSIN Full-Scale Current-

Sense Voltage

mV

A

RS2 (Figure 1) = 10mΩ;

ChargingCurrent()= 0x1f80

-3 +3 %
A

RS2 (Figure 1) = 10mΩ;

ChargingCurrent() = 0x0f80

-4 +4 %

Charge Current and Accuracy

RS2 (Figure 1) =10mΩ;

ChargingCurrent() = 0x0080

30

mA

Charge-Current Gain Error Based on ChargeCurrent() = 128mA and 8.064A -2 +2 %
FBSA/FBSB/CSIP/CSIN Input

Voltage Range

0 19 V

Adapter present, not charging, I

CSIP

+ I

CSIN

+ ILX + I

FBS

,

V

FBS_

= VLX = V

CSIN

= V

CSIP

= 19V

5

Battery Quiescent Current

Adapter absent, I

CSIP

+ I

CSIN

+ ILX + I

FBSA

+ I

FBSB

+

I

CSSP

+ I

CSSN

, V

FBS_

= VLX = V

CSIN

= V

CSIP

= 19V,

V

DCIN

= 0V

1

µA

µA

Charging 1 mA

V

Adapter

= 19V,

V

Battery

= 16.8V

Not charging

µA

Charging 1 mA

Adapter Quiescent Current

I

DCIN

+

I

CSSP

+

I

CSSN

V

Adapter

= 8V,

V

Battery

= 4V

Not charging

µA

16.632

12.466

8.316

4.129

-1.5

78.22

7.822

3.809

V

A d a p t er

= 26V , V

= 16.8V , not char g i ng

B at te r y

16.968

12.717

8.484

4.255
+1.5

83.05

8.305

4.126

400

500

500

500

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

_______________________________________________________________________________________ 7

ELECTRICAL CHARACTERISTICS (continued)

(V

DCIN

= VLX= V

CSSP

= V

CSSN

= 19V, V

BST

- V

LX

= 4.5V, V

FBSA

= V

FBSB

= V

CSIP

= V

CSIN

= 16.8V, BATSEL = GND = PGND = 0,

C

LDO

= 1µF, VCC= LDO, C

REF

= 1µF, C

DAC

= 0.1µF , VDD= 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per

Figure 1; T

A

= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER CONDITIONS

UNITS

INPUT-CURRENT REGULATION

CSSP to CSSN Full-Scale

Current-Sense Voltage

V

FBS_

= 19V

mV

RS1 (Figure 1) = 10mΩ;

InputCurrent() = 11004mA or 3584mA

-6 +6

Input Current Accuracy

RS1 (Figure 1) = 10mΩ;

InputCurrent() = 2048mA

-5 +5

%

Input Current-Limit Gain Error Based on InputCurrent() = 1024mA and 11004mA -5 +5 %
Input Current-Limit Offset Based on InputCurrent() = 1024mA and 11004mA -1 +1 mV

8 26 V

IINP Transconductance V

CSSP - CSSN

= 110mV 2.7 3.3

mA/V

IINP Offset Based on V

CSSP - CSSN

= 110mV and 20mV

mV

V

CSSP - CSSN

= 110mV -5 +5

V

CSSP - CSSN

= 55mV or 35mV -4 +4 IINP Accuracy

V

CSSP - CSSN

= 20mV -10

%

IINP Output Voltage Range 0 3.5 V
SUPPLY AND LINEAR REGULATOR
DCIN, Input Voltage Range 8.0

V

DCIN falling 7

DCIN Undervoltage-Lockout

Trip Point

DCIN rising

V

V

CSSP

- V

CSIN

falling 9 21

POWER_FAIL Threshold

V

CSSP

- V

CSIN

rising

mV

LDO Output Voltage 8.0V < V

DCIN

< 28V, no load

V

LDO Load Regulation 0 < I

LDO

< 30mA

mV

LDO Undervoltage-Lockout

Threshold

V

DCIN

= 8.0V, V

LDO

falling

V

V

DD

Range 2.7 5.5 V

V

DD

UVLO Rising 2.7 V

V

DD

Quiescent Current DCIN < 6V, VDD = 5.5V, SCL = SDA = 5.5V 27 µA

REFERENCE
REF Output Voltage 0 < I

REF

< 500µA

V

REF Undervoltage-Lockout

Trip Point

REF falling 3.9 V

ACOK
ACOK Sink Current V

ACOK

= 0.4V, ACIN = 1.5V 1 mA

MIN TYP MAX

CSSP/CSSN Input Voltage Range

103.3

-1.5

116.6

+1.5

+10

26.0

7.85

271

5.55
100

5.15

160

5.25

3.20

4.053

4.139

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

8 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(V

DCIN

= VLX= V

CSSP

= V

CSSN

= 19V, V

BST

- V

LX

= 4.5V, V

FBSA

= V

FBSB

= V

CSIP

= V

CSIN

= 16.8V, BATSEL = GND = PGND = 0,

C

LDO

= 1µF, VCC= LDO, C

REF

= 1µF, C

DAC

= 0.1µF , VDD= 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per

Figure 1; T

A

= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER CONDITIONS

UNITS

ACIN
ACIN Threshold

V
ACIN Threshold Hysteresis 10 30 mV
REMOTE-SENSE INPUTS
FBS_ Range V

CSIN

- V

FBS

0

mV

FBS_ Gain ∆V

CSIN

/ ∆(V

CSIN

- V

FBS_

) 0.9 1.1 V/V

CSIN-FBS_ Clamp Voltage

mV
FBS_ Bias Current Charger switching, FBS_ selected 14 µA
SWITCHING REGULATOR

V

CSIN

= 16.0V, V

CSSP

= 19V

Off-Time

V

CSIN

= 16.0V, V

CSSP

= 17V

ns

BST Supply Current DHI high

µA

DHI On-Resistance Low I

DHI

= -10mA 3 Ω

DHI On-Resistance High I

DHI

= 10mA 5 Ω

DLO On-Resistance High I

DLO

= 10mA 5 Ω

DLO On-Resistance Low I

DLO

= -10mA 3 Ω

ERROR AMPLIFIERS

C har g i ng V ol tag e( ) = 16.8V , V

FBS_

= 16.8V

mA/V

ChargingCurrent() = 3968mA, V

CSIP

- V

CSIN

= 39.68mV 0.5 2.0

mA/V

InputCurrent() = 3968mA, V

CSSP

- V

CSSN

= 79.36mV 0.5 2.0

mA/V

CCI/CCS/CCV Clamp Voltage 0.25V < V

CCI/S/V

< 2.0V

mV
LOGIC LEVELS

SDA/SCL Input Low Voltage VDD = 2.7V to 5.5V 0.8 V

SDA/SCL Input High Voltage VDD = 2.7V to 5.5V 2.3 V
BATSEL Input Low Voltage 0.8 V
BATSEL Input High Voltage 2.3 V

SDA, Output Sink Current V

(SDA)

= 0.4V 6 mA

MIN TYP MAX

2.007

220

360
260

GMV Amplifier Transconductance
GMI Amplifier Transconductance
GMS Amplifier Transconductance

0.0625

150

2.089

200

280

440
350
800

0.2500

600

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

_______________________________________________________________________________________ 9

ELECTRICAL CHARACTERISTICS (continued)

(V

DCIN

= V

LX

= V

CSSP

= V

CSSN

= 19V, V

BST

- VLX= 4.5V, V

FBSA

= V

FBSB

= V

CSIP

= V

CSIN

= 16.8V, BATSEL = GND = PGND = 0,

C

LDO

= 1µF, VCC= LDO, C

REF

= 1µF, C

DAC

= 0.1µF , VDD= 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated

per Figure 1; T

A

= -40°C to +85°C, unless otherwise noted.) (Note 2)

SMB TIMING SPECIFICATION (VDD = 2.7V to 5.5V) (see Figures 4 and 5)

PARAMETERS

CONDITIONS

UNITS

SMBus Frequency f

SMB

10

kHz

Bus Free Time t

BUF

4.7 µs

Start Condition Hold Time from
SCL

4 µs

Start Condition Setup Time from
SCL

t

SU:STA

4.7 µs

Stop Condition Setup Time from
SCL

4 µs

SDA Hold Time from SCL

ns

SDA Setup Time from SCL

ns

SCL Low Timeout

(Note 1) 25 35 ms

SCL Low Period T

LOW

4.7 µs

SCL High Period T

HIGH

4 µs

Maximum Charging Period
Without a ChargeVoltage() or
ChargeCurrent() Command

s

Note 1: Devices participating in a transfer will timeout when any clock low exceeds the 25ms minimum timeout period. Devices that

have detected a timeout condition must reset the communication no later than the 35ms maximum timeout period. Both a
master and a slave must adhere to the maximum value specified as it incorporates the cumulative stretch limit for both a
master (10ms) and a slave (25ms).

Note 2: Specifications to -40°C are guaranteed by design, not production tested.

SYMBOL

t

HD:STA

t

SU:STO

t

HD:DAT

t

SU:DAT

t

TIMEOUT

MIN TYP MAX

100

300

250

140 210

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

10 ______________________________________________________________________________________

Typical Operating Characteristics

(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25°C,
unless otherwise noted.)

INPUT CURRENT-LIMIT ERROR

vs. INPUT CURRENT-LIMIT SETTING

INPUT CURRENT-LIMIT SETTING (A)

INPUT CURRENT-LIMIT ERROR (%)

MAX8731 toc01

0246810

-6

-2

-4

0

2

4

6

MAXIMUM

MINIMUM

TYPICAL

INPUT CURRENT-LIMIT ERROR

vs. SYSTEM CURRENT

SYSTEM CURRENT (A)

INPUT CURRENT-LIMIT ERROR (%)

MAX8731 toc02

01234

-0.4

-0.2

0

0.2

0.4

INPUT CURRENT LIMIT = 2.048A

INPUT CURRENT LIMIT = 3.584A

INPUT CURRENT
LIMIT = 4.096A

INPUT CURRENT-LIMIT ERROR

vs. SYSTEM CURRENT

SYSTEM CURRENT (A)

INPUT CURRENT-LIMIT ERROR (%)

MAX8731 toc03

01234

-1.0

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

V

BATT

= 8.4V

V

BATT

= 16.8V

V

BATT

= 12.6V

INPUT CURRENT LIMIT = 3.584A

IINP ERROR vs. SYSTEM CURRENT

SYSTEM CURRENT (A)

IINP ERROR (%)

MAX8731 toc04

01.00.5 2.01.5 2.5 3.0 3.5 4.0

-1.0

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

INPUT CURRENT LIMIT = 2.048A

INPUT CURRENT LIMIT = 3.584A

INPUT CURRENT LIMIT = 4.096A

OPERATING AT INPUT CURRENT LIMIT

IINP ERROR vs. SYSTEM CURRENT

SYSTEM CURRENT (A)

IINP ERROR (%)

MAX8731 toc05

01234

0

0.5

1.0

1.5

2.0

2.5

V

BATT

= 8.4V

V

BATT

= 12.6V

V

BATT

= 16.8V

INPUT CURRENT LIMIT = 3.584A

IINP ERROR vs. INPUT CURRENT

INPUT CURRENT (A)

IINP ERROR (%)

MAX8731 toc06

0123456

-10

-6

-8

-2

2

6

-4

0

4

8

10

MAXIMUM

MINIMUM

TYPICAL

NOT SWITCHING

CHARGE-CURRENT ERROR vs.

CHARGE CURRENT-LIMIT SETTING

CHARGE-CURRENT SETTING (A)

CHARGE-CURRENT LIMIT ERROR (%)

MAX8731 toc07

024 86

-10

-6

-8

-2

2

6

-4

0

4

8

10

MAXIMUM

MINIMUM

TYPICAL

CHARGE-CURRENT ERROR

vs. BATTERY VOLTAGE

BATTERY VOLTAGE (V)

CHARGE-CURRENT ERROR (%)

MAX8731 toc08

3 6 9 12 15 18

-4

-2

0

2

4

3.072A

3.968A

8.064A

TRICKLE-CHARGE CURRENT ERROR

vs. BATTERY VOLTAGE

BATTERY VOLTAGE (V)

TRICKLE-CHARGE CURRENT ERROR (%)

MAX8731 toc09

0 3 6 9 12 15 18

-30

-25

-20

-15

-10

-5

0

ChargeCurrent( ) = 128mA

CHARGE-VOLTAGE ERROR

vs. CHARGE-VOLTAGE SETTING

CHARGE-VOLTAGE SETTING (V)

CHARGE-VOLTAGE ERROR (%)

MAX8731 toc10

4 8 12 16 20

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

BATTERY-VOLTAGE ERROR

vs. CHARGE CURRENT

CHARGE CURRENT (A)

BATTERY-VOLTAGE ERROR (%)

MAX8731 toc11

0123456

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

3 CELLS

2 CELLS

4 CELLS

BATTERY REMOVAL

20µs/div

MAX8731 toc12

13.5V

13.0V

12.5V

V

OUT

OUTPUT CAPACITOR = 22µF

ChargeVoltage( ) = 12.6V

V

OUT

OUTPUT CAPACITOR = 10µF

SYSTEM LOAD TRANSIENT

200µs/div

MAX8731toc13

LOAD

CURRENT

ADAPTER

CURRENT

INDUCTOR

CURRENT

CCS VOLTAGE

500 mV/div

CCI VOLTAGE

500 mV/div

5A

0A

0A

5A

5A

0A

500mV/div

500mV/div

CCI

CCS

CCI

CCS

EFFICIENCY vs. CHARGE CURRENT

CHARGE CURRENT (A)

EFFICIENCY (%)

MAX8731 toc14

02468

60

65

70

75

80

85

90

95

100

2 CELLS

3 CELLS

4 CELLS

LDO LOAD REGULATION

I

LDO

(mA)

LDO ERROR (mV)

MAX8731 toc15

0 20406080100

-40

-35

-30

-25

-20

-15

-10

-5

0

CHARGER OFF

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 11

Typical Operating Characteristics (continued)

(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25°C,
unless otherwise noted.)

Typical Operating Characteristics (continued)

(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25°C,
unless otherwise noted.)

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

12 ______________________________________________________________________________________

0

LDO LINE REGULATION

-1

-2

-3

LDO ERROR (mV)

-4

-5

-6
8 131823

V

DCIN

450

400

350

300

FREQUENCY (kHz)

250

200

NOT SWITCHING

(V)

SWITCHING FREQUENCY

MAX8731 toc16

0.20

0.15

0.10

0.05

0

-0.05

REF ERROR (%)

-0.10

-0.15

-0.20
0 0.2 0.4 0.6 0.8 1.0

MAX8731 toc19

I

(mA)

REF

NOT SWITCHING

CHARGE CURRENT (A)

REF LOAD REGULATION

5

BATTERY-CHARGE CURVE

2.8Ah x 3S3P BATTERY

4

3

2

1

MAX8731 toc17

REF ERROR vs. TEMPERATURE

0.3

0.2

0.1

0.0

REF ERROR (%)

-0.1

-0.2

-0.3

-40-200 20406080

TEMPERATURE (°C)

MAX8731 toc20

BATTERY VOLTAGE

CHARGE CURRENT

13.0

12.5

12.0

11.5

11.0
BATTERY VOLTAGE (V)

10.5

MAX8731 toc18

150

0 5 10 15 20

V

- V

ADAPTER

BATTERY

ADAPTER CURRENT

vs. ADAPTER VOLTAGE

3.0
SWITCHING, NO LOAD

2.5

2.0

1.5

1.0

ADAPTER CURRENT (mA)

0.5

0

0 5 10 15 20 25 30

ChargeVoltage( ) = 4.192V

NOT SWITCHING

ADAPTER VOLTAGE (V)

(V)

MAX8731 toc21

0

0123456

TIME (h)

BATTERY-LEAKAGE CURRENT

vs. BATTERY VOLTAGE

2.5
ADAPTER PRESENT OR ABSENT

2.0

1.5

1.0

BATTERY CURRENT (µA)

0.5

0

0 5 10 15 20

BATTERY VOLTAGE (V)

10.0

MAX8731 toc22

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 13

Pin Description

PIN NAME FUNCTION

1, 12 GND Analog Ground. Connect directly to the paddle.

2 ACIN AC Adapter Detect Input. ACIN is the input to an uncommitted comparator.
3 REF 4.096V Voltage Reference. Bypass REF with a 1µF capacitor to GND.
4 CCS Input Current Regulation Loop-Compensation Point. Connect 0.01µF from CCS to GND.
5 CCI Output Current Regulation Loop-Compensation Point. Connect 0.01µF from CCI to GND.
6 CCV Voltage Regulation Loop-Compensation Point. Connect 10kΩ in series with 0.01µF to GND.
7 DAC DAC Voltage Output. Bypass with 0.1µF from DAC to GND.

8 IINP

9 SDA S M Bus D ata I/O. Op en- d r ai n outp ut. C onnect an exter nal p ul l up r esi stor accor d i ng to S M Bus sp eci fi cati ons.
10 SCL SMBus Clock Input. Connect an external pullup resistor according to SMBus specifications.
11 V

DD

13 ACOK

14 BATSEL

15 FBSA

16 FBSB

17 CSIN Charge Current-Sense Negative Input
18 CSIP C har g e C ur r ent- S ense P osi ti ve I np ut. C onnect a 10m Ω cur r ent- sense r esi stor b etw een C S IP and C S IN .
19 PGND Power Ground

20 DLO

Input Current Monitor Output. IINP sources the current proportional to the current sensed across

CSSP and CSSN. The transconductance from (CSSP - CSSN) to IINP is 3mA/V.

Logic Circuitry Supply-Voltage Input. Bypass with a 0.1µF capacitor to GND.

AC D etect Outp ut. Thi s op en- d r ai n outp ut i s hi g h i m p ed ance w hen AC IN i s g r eater than RE F/2. The

AC O K outp ut r em ai ns l ow w hen the M AX 8731 i s p ow er ed d ow n. C onnect a 10kΩ p ul l up r esi stor fr om
V

to AC O K.

C C

Batter y V ol tag e S el ect Inp ut. D r i ve BATS E L hi g h to sel ect b atter y B, or d r i ve BATS E L l ow to sel ect b atter y A.

Any chang e of BATS E L i m m ed i atel y stop s char g i ng . C har g i ng b eg i ns ag ai n i n ap p r oxi m atel y 10m s.

Remote Sense Input for the Output Voltage of Battery A. Connect a 100Ω resistor from FBSA to the

battery connector, and a 10nF capacitor from FBSA to PGND.

Remote Sense Input for the Output Voltage of Battery B. Connect a 100Ω resistor from FBSB to the

battery connector, and a 10nF capacitor from FBSB to PGND.

Low-Side Power MOSFET Driver Output. Connect to low-side n-channel MOSFET. DLO drives

between LDO and PGND.

Linear-Regulator Output. LDO is the output of the 5.4V linear regulator supplied from DCIN. LDO also

21 LDO

directly supplies the DLO driver and the BST charge pump. Bypass with a 1µF ceramic capacitor
from LDO to PGND.

22 DCIN Charger Bias Supply Input. Bypass DCIN with a 0.1µF capacitor to PGND.

23 LX

H i g h- S i d e P ow er M OS FE T D r i ver S our ce C onnecti on. C onnect to the sour ce of the hi g h- si d e n- channel

M OS FE T.

24 DHI High-Side Power MOSFET Driver Output. Connect to the high-side n-channel MOSFET gate.
25 BST H i g h- S i d e P ow er M OS FE T D r i ver P ow er - S up p l y C onnecti on. C onnect a 0.1µF cap aci tor fr om BS T to LX .
26 V

CC

D evi ce P ow er - S up p l y I np ut. C onnect to LD O thr oug h an RC fi l ter as show n i n Fi g ur e 1.

27 CSSN Input Current-Sense Negative Input
28 CSSP Inp ut C ur r ent- S ense P osi ti ve Inp ut. C onnect a 10m Ω cur r ent- sense r esi stor b etw een C S S P and C S S N .
29 BP Backside Paddle. Connect the backside paddle to analog ground.

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

14 ______________________________________________________________________________________

Figure 1. Typical Dual-Battery Application Circuit

Detailed Description

The typical operating circuit is shown in Figure 1. The
MAX8731 includes all the functions necessary to
charge Li+, NiMH, and NiCd smart batteries. A high­efficiency, synchronous-rectified, step-down DC-DC
converter is used to implement a precision constant­current, constant-voltage charger. The DC-DC convert­er drives a high-side n-channel MOSFET and provides
synchronous rectification with a low-side n-channel
MOSFET. The charge current and input current-sense

amplifiers have low input-offset error (±64µV typ),
allowing the use of small-valued sense resistors.

The MAX8731 features a voltage-regulation loop (CCV)
and two current-regulation loops (CCI and CCS). The
loops operate independently of each other. The CCV
voltage-regulation loop monitors either FBSA or FBSB
to ensure that its voltage never exceeds the voltage set
by the ChargeVoltage() command. The CCI battery cur­rent-regulation loop monitors current delivered to the
selected battery to ensure that it never exceeds the
current limit set by the ChargeCurrent() command. The

ADAPTER
INPUT

R2

0.1µF

0.1µF

0.01µF

C6

0.01µF

100kΩ

MAX8731

C8

0.1µF

BATSEL

BP

CSSP

CSSN

V

LDO

BST

DHI

DLO

PGND

CSIP

CSIN

FBSB

FBSA

GND

RS1
10mΩ

CC

C12
1µF

R12
33Ω

LDO

C11

D2

1µF

N1, N2,: SI4800BDY
N3: SI4810BDY

C10

0.1µF

N1

R11

LDO

R10

100Ω

R9

100Ω

C9
220pF

D3

R13
1kΩ

1Ω

N3

SELECTOR

BATTERY

A

LX

C

IN1

10µF

DHI

L1

4.3µH

L1: SUMIDA
CEP125-4R3MC-U

RS2
10mΩ

V

OUT

C

OUT1

10µF

BATTERY

B

SYSTEM

LOAD

C

IN2

10µF

N2

DCIN

ACIN

ACOK

V

DD

SCL

C2

SDA

IINP

C3

CCV

C7
1µF

CCI

CCS

REF

DAC

C5

KBC

D1

C1

1µF

INPUT

VDD

SCL

SDA

150kΩ

49.9kΩ

LDO

10kΩ

10kΩ

C4

0.01µF

R1

R3

R5

R8

R4

10kΩ

10kΩ

R6

R7
10kΩ

N

C

OUT2

10µF

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 15

Figure 2. Functional Diagram

charge current-regulation loop is in control as long as
the selected battery voltage is below the charge volt­age set point. When the selected battery voltage reach­es its set point, the voltage-regulation loop takes control
and maintains the battery voltage at the set point. A

third loop (CCS) takes control and reduces the charge
current when the adapter current exceeds the input
current limit set by the InputCurrent() command.

A functional diagram is shown in Figure 2.

ACIN

ACOK

GND

CCV

CCI

CCS

MAX8731

CSSP

CSIN

VCC

CSA: CURRENT-SENSE

AMPLIFIER

GM

REF/2

LOWEST VOLTAGE CLAMP

GMS

CSS

CSA

POWER-FAIL

LVC

CSI

CSA

A = 20V/V

(750mA FOR RS2 = 10mΩ)

100mV

GMI

150mV

A = 1V/V

ZCMD

ENABLE

IMIN

CCMP

GMV

DC-DC

CONVERTER

CHARGE VOLTAGE( )

11-BIT DAC

IMAX

OVP

+100mV

6-BIT DAC

6-BIT DAC

2V
(10A FOR RS2 = 10mΩ)

HIGH­SIDE
DRIVER

LEVEL
SHIFT

LOW­SIDE
DRIVER

5.4V

LINEAR

REGULATOR

4.096V

REFERENCE

SMBus LOGIC

CHARGE VOLTAGE ( )

CHARGE CURRENT ( )

INPUT CURRENT ( )

BST

DHI

LX

LDO

DLO

PGND

DCIN

V

CC

REF

SCL

SDA

V

DD

IINP

CSSN

A = 20V/V

CSSP

CSIP

CSIN

BATSEL

FBSB

FBSA

DAC

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

16 ______________________________________________________________________________________

Table 1. ChargeVoltage () (0x15)

Setting Charge Voltage

To set the output voltage, use the SMBus to write a 16­bit ChargeVoltage() command using the data format
listed in Table 1. The ChargeVoltage() command uses
the Write-Word protocol (see Figure 3). The command
code for ChargeVoltage() is 0x15 (0b00010101). The
MAX8731 provides a 1.024V to 19.200V charge voltage

range, with 16mV resolution. Set ChargeVoltage()
below 1.024V to terminate charging. Upon reset, the
ChargeVoltage() and ChargeCurrent() values are
cleared and the charger remains off until both the
ChargeVoltage() and the ChargeCurrent() command
are sent. Both DHI and DLO remain low until the charg­er is restarted.

BIT BIT NAME DESCRIPTION

0 — Not used. Normally a 1mV weight.

1 — Not used. Normally a 2mV weight.

2 — Not used. Normally a 4mV weight.

3 — Not used. Normally a 8mV weight.

4 Charge voltage, DACV 0

5 Charge voltage, DACV 1

6 Charge voltage, DACV 2

7 Charge voltage, DACV 3

8 Charge voltage, DACV 4

9 Charge voltage, DACV 5

10 Charge voltage, DACV 6

11 Charge voltage, DACV 7

12 Charge voltage, DACV 8

13 Charge voltage, DACV 9

14 Charge voltage, DACV 10

15 — Not used. Normally a 32,768mV weight.

0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 16mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 32mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 64mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 128mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 256mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 512mV of charger voltage compliance.

0 = Adds 0mA of charger voltage compliance.
1 = Adds 1024mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance.
1 = Adds 2048mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance.
1 = Adds 4096mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance.
1 = Adds 8192mV of charger voltage compliance.

0 = Adds 0mV of charger voltage compliance.
1 = Adds 16,384mV of charger voltage compliance, 19,200mV max.

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 17

Table 2. ChargeCurrent() (0x14) (10mΩ Sense Resistor, RS2)

Setting Charge Current

To set the charge current, use the SMBus to write a 16­bit ChargeCurrent() command using the data format
listed in Table 2. The ChargeCurrent() command uses
the Write-Word protocol (see Figure 3). The command
code for ChargeCurrent() is 0x14 (0b00010100). When
RS2 =10mΩ, the MAX8731 provides a charge current
range of 128mA to 8.064A, with 128mA resolution. Set
ChargeCurrent() to 0 to terminate charging. Upon reset,
the ChargeVoltage() and ChargeCurrent() values are
cleared and the charger remains off until both the
ChargeVoltage() and the ChargeCurrent() commands
are sent. Both DHI and DLO remain low until the charger
is restarted.

The MAX8731 includes a foldback current limit when
the battery voltage is low. If the battery voltage is less
than 2.5V, the charge current is temporarily set to
128mA. The ChargeCurrent() register is preserved and
becomes active again when the battery voltage is high­er than 2.5V. This function effectively provides a fold­back current limit, which protects the charger during
short circuit and overload.

Setting Input Current Limit

System current normally fluctuates as portions of the
system are powered up or put to sleep. By using the
input-current-limit circuit, the output-current require­ment of the AC wall adapter can be lowered, reducing
system cost.

The total input current, from a wall cube or other DC
source, is the sum of the system supply current and the
current required by the charger. When the input current
exceeds the set input current limit, the MAX8731
decreases the charge current to provide priority to sys­tem load current. As the system supply rises, the avail­able charge current drops linearly to zero. Thereafter,
the total input current can increase without limit.

The internal amplifier compares the differential voltage
between CSSP and CSSN to a scaled voltage set by
the InputCurrent() command (see Table 3). The total
input current is the sum of the device supply current,
the charger input current, and the system load current.
The total input current can be estimated as follows:

BIT BIT NAME DESCRIPTION

0 — Not used. Normally a 1mA weight.

1 — Not used. Normally a 2mA weight.

2 — Not used. Normally a 4mA weight.

3 — Not used. Normally an 8mA weight.

4 — Not used. Normally a 16mA weight.

5 — Not used. Normally a 32mA weight.

6 — Not used. Normally a 64mA weight.

7 Charge Current, DACI 0

8 Charge Current, DACI 1

9 Charge Current, DACI 2

10 Charge Current, DACI 3

11 Charge Current, DACI 4

12 Charge Current, DACI 5

13 — Not used. Normally a 8192mA weight.

14 — Not used. Normally a 16,386mA weight.

15 — Not used. Normally a 32,772mA weight.

0 = Adds 0mA of charger current compliance.
1 = Adds 128mA of charger current compliance.

0 = Adds 0mA of charger current compliance.
1 = Adds 256mA of charger current compliance.

0 = Adds 0mA of charger current compliance.
1 = Adds 512mA of charger current compliance.

0 = Adds 0mA of charger current compliance.
1 = Adds 1024mA of charger current compliance.

0 = Adds 0mA of charger current compliance.
1 = Adds 2048mA of charger current compliance.

0 = Adds 0mA of charger current compliance.
1 = Adds 4096mA of charger current compliance, 8064mA max.

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

18 ______________________________________________________________________________________

Table 3. InputCurrent() (0x3F) (10mΩ Sense Resistor, RS1)

where η is the efficiency of the DC-DC converter (typi­cally 85% to 95%).

To set the input current limit, use the SMBus to write a
16-bit InputCurrent() command using the data format
listed in Table 3. The InputCurrent() command uses the
Write-Word protocol (see Figure 3). The command
code for InputCurrent() is 0x3F (0b00111111). When
RS1 = 10mΩ, the MAX8731 provides an input-current­limit range of 256mA to 11.004A, with 256mA resolu­tion. InputCurrent() settings from 1mA to 256mA result
in a current limit of 256mA. Upon reset the input current
limit is 256mA.

Charger Timeout

The MAX8731 includes a timer to terminate charging if
the charger does not receive a ChargeVoltage() or
ChargeCurrent() command within 175s. If a timeout
occurs, both ChargeVoltage() and ChargeCurrent()
commands must be resent to reenable charging.

Remote Sense

The MAX8731 features dual remote sense, which allows
the rejection of board resistance and selector resistance
when used in either single- or dual-battery systems. To
fully utilize remote sensing, connect FBS_ directly to the
battery interface through an unshared battery sense
trace in series with a 100Ω resistor, and 10nF capacitor
(see Figure 1). In single-battery systems, connect
BATSEL directly to GND and use only FBSA.

Remote sensing cancels the effect of impedance in
series with the battery. This impedance normally caus­es the battery charger to prematurely enter constant­voltage mode with reducing charge current. The result
is that the last 20% of charging takes longer than nec­essary. When in constant-voltage mode, the remaining
charge time is proportional to the total resistance in
series with the battery. Remote sensing reduces
charge time according to the following equation:

⎣

⎦

⎡

IV

()

II

=+

INPUT LOAD

CHARGE BATTERY

⎢
⎢

×

η

×

V

()

IN

⎤

+

I

⎥

BIAS

⎥

BIT BIT NAME DESCRIPTION

0 — Not used. Normally a 2mA weight.

1 — Not used. Normally a 4mA weight.

2 — Not used. Normally an 8mA weight.

3 — Not used. Normally a 16mA weight.

4 — Not used. Normally a 32mA weight.

5 — Not used. Normally a 64mA weight.

6 — Not used. Normally a 128mA weight.

7 Input Current, DACS 0

8 Input Current, DACS 1

9 Input Current, DACS 2

10 Input Current, DACS 3

11 Input Current, DACS 4

12 Input Current, DACS 5

13 — Not used. Normally a 16,384mA weight.

14 — Not used. Normally a 32,768mA weight.

15 — Not used. Normally a 65,536mA weight.

0 = Adds 0mA of input current compliance.
1 = Adds 256mA of input current compliance.

0 = Adds 0mA of input current compliance.
1 = Adds 512mA of input current compliance.

0 = Adds 0mA of input current compliance.
1 = Adds 1024mA of input current compliance.

0 = Adds 0mA of input current compliance.
1 = Adds 2048mA of input current compliance.

0 = Adds 0mA of input current compliance.
1 = Adds 4096mA of input current compliance.

0 = Adds 0mA of input current compliance.
1 = Adds 8192mA of input current compliance, 11,004mA max.

tt

=×

CVRS CV

0

R

Pack

+

RR

Pack Board

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 19

where R

Pack

is the total resistance in the battery pack,

R

Board

is the board resistance in series with the battery

charge path, t

CV0

is the constant-voltage charge time

without remote sense, and t

CVRS

is the constant-volt-

age charge time with remote sense.

The MAX8731 includes a safety feature, which limits the
charge voltage when FBS_ or the selector is discon­nected. The MAX8731 guarantees that CSIN does not
regulate more than 200mV above the selected charg­ing voltage. This also limits the extent to which remote
sense can cancel charge-path impedance.

Input Current Measurement

Use IINP to monitor the system-input current sensed
across CSSP and CSSN. The voltage at IINP is propor­tional to the input current by the equation:

V

IINP

= I

INPUT

x RS1 x G

IINP

x R8

where I

INPUT

is the DC current supplied by the AC

adapter, G

IINP

is the transconductance of IINP (3mA/V
typ), and R8 is the resistor connected between IINP
and ground. Typically, IINP has a 0 to 3.5V output volt­age range. Leave IINP open if not used.

LDO Regulator

An integrated low-dropout (LDO) linear regulator pro­vides a 5.4V supply derived from DCIN, and delivers over
30mA of load current. The LDO powers the gate drivers
of the n-channel MOSFETs. See the MOSFET Drivers
section. LDO has a minimum current limit of 35mA. This
allows the MAX8731 to work with 87nC of total gate
charge (both high-side and low-side MOSFETs). Bypass
LDO to PGND with a 1µF or greater ceramic capacitor.

AC Adapter Detection

The MAX8731 includes a hysteretic comparator that
detects the presence of an AC power adapter. When
ACIN is greater than 2.048V, the open-drain ACOK out­put becomes high impedance. Connect 10kΩ pullup
resistance between LDO and ACOK. Use a resistive
voltage-divider from the adapter’s output to the ACIN
pin to set the appropriate detection threshold. Select
the resistive voltage-divider not to exceed the 6V
absolute maximum rating of ACIN.

VDDSupply

The VDDinput provides power to the SMBus interface.
Connect VDDto LDO, or apply an external supply to
VDDto keep the SMBus interface active while the sup­ply to DCIN is removed. When V

DD

is biased the inter­nal registers are maintained. Bypass VDDto GND with
a 0.1µF or greater ceramic capacitor.

Operating Conditions

The MAX8731 has the following operating states:

• Adapter Present: When DCIN is greater than 7.5V,

the adapter is considered to be present. In this con­dition, both the LDO and REF function properly and
battery charging is allowed:

a) Charging: The total MAX8731 quiescent current
when charging is 1mA (max) plus the current required
to drive the MOSFETs.

b) Not Charging: To disable charging, set either
ChargeCurrent() or ChargeVoltage() to zero. When the
adapter is present and charging is disabled, the total
adapter quiescent current is less than 1mA and the
total battery quiescent current is less than 5µA.

• Adapter Absent (Power Fail): When V

CSSP

is less

than V

CSIN

+ 10mV, the MAX8731 is in the power-fail
state, since the DC-DC converter is in dropout. The
charger does not attempt to charge in the power-fail
state. Typically, this occurs when the adapter is
absent. When the adapter is absent, the total MAX8731
quiescent battery current is less than 1µA (max).

•V

DD

Undervoltage (POR): When VDDis less than

2.5V, the V

DD

supply is in an undervoltage state and
the internal registers are in their POR state. The
SMBus interface does not respond to commands.
When VDDrises above 2.5V, the MAX8731 is in a
power-on reset state. Charging does not occur until
the ChargeVoltage() and ChargeCurrent() com­mands are sent. When V

DD

is greater than 2.5V,

SMBus registers are preserved.

The MAX8731 allows charging under the following conditions:

1) DCIN > 7.5V, LDO > 4V, REF > 3.1V

2) V

CSSP

> V

CSIN

+ 210mV (15mV falling threshold)

3) VDD> 2.5V

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

20 ______________________________________________________________________________________

Figure 3. SMBus Write-Word and Read-Word Protocols

SMBus Interface

The MAX8731 receives control inputs from the SMBus
interface. The MAX8731 uses a simplified subset of the
commands documented in System Management Bus
Specification V1.1, which can be downloaded from
www.smbus.org. The MAX8731 uses the SMBus Read­Word and Write-Word protocols (Figure 3) to communi­cate with the smart battery. The MAX8731 performs
only as an SMBus slave device with address
0b0001001_ (0x12) and does not initiate communica­tion on the bus. In addition, the MAX8731 has two iden­tification (ID) registers (0xFE): a 16-bit device ID
register and a 16-bit manufacturer ID register (0xFF).

The data (SDA) and clock (SCL) pins have Schmitt-trig­ger inputs that can accommodate slow edges. Choose
pullup resistors (10kΩ) for SDA and SCL to achieve rise
times according to the SMBus specifications.

Communication starts when the master signals a
START condition, which is a high-to-low transition on
SDA, while SCL is high. When the master has finished
communicating, the master issues a STOP condition,
which is a low-to-high transition on SDA, while SCL is
high. The bus is then free for another transmission.
Figures 4 and 5 show the timing diagram for signals on
the SMBus interface. The address byte, command
byte, and data bytes are transmitted between the
START and STOP conditions. The SDA state changes
only while SCL is low, except for the START and STOP
conditions. Data is transmitted in 8-bit bytes and is
sampled on the rising edge of SCL. Nine clock cycles
are required to transfer each byte in or out of the
MAX8731 because either the master or the slave
acknowledges the receipt of the correct byte during the
ninth clock cycle. The MAX8731 supports the charger
commands as described in Table 4.

a) Write-Word Format

SLAVE

S

ADDRESS

7 BITS 8 BITS1b

MSB LSB MSB LSB

PRESET TO

0b0001001

b) Read-Word Format

SLAVE

S

ADDRESS

7 BITS 8 BITS1b

MSB LSB

Preset to

0b0001001

W ACK ACK ACK P

W ACK ACK NACK P

COMMAND

BYTE

ChargerMode() = 0x12
ChargeCurrent() = 0x14
ChargeVoltage() = 0x15
AlarmWarning() = 0x16
InputCurrent() = 0x3F

COMMAND

BYTE

ChargerSpecInfo() = 0x11
ChargerStatus() = 0x13

LOW DATA

ACK

BYTE

8 BITS

1b

MSB LSB

0

D7 D0 D15 D8

SLAVE

SACK

ADDRESS

1b

0

7 BITS

MSB LSBMSB LSB

PRESET TO
0b0001001

1b

0

R ACK

1b11b

HIGH DATA

BYTE

8 BITS

MSB LSB01b0

0

1b

0

LOW DATA

BYTE

8 BITS

MSB LSB

D7 D0 D15 D8

1b

0

HIGH DATA

BYTE

8 BITS

MSB LSB01b0

1b

1

LEGEND:
S = START CONDITION OR REPEATED START CONDITION
ACK = ACKNOWLEDGE (LOGIC-LOW)
W = WRITE BIT (LOGIC-LOW)

MASTER TO SLAVE
SLAVE TO MASTER

P = STOP CONDITION
NACK = NOT ACKNOWLEDGE (LOGIC-HIGH)
R = READ BIT (LOGIC-HIGH)

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 21

Figure 4. SMBus Write Timing

Figure 5. SMBus Read Timing

AB CDEFG HIJ

t

LOWtHIGH

SMBCLK

SMBDATA

t

t

HD:STA

SU:STA

A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW

AB CDEFG H

t

LOW

t

HIGH

t

t

SU:DAT

HD:DAT

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = SLAVE PULLS SMBDATA LINE LOW

t

HD:DAT

K

J = ACKNOWLEDGE CLOCKED INTO MASTER
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION, DATA EXECUTED BY SLAVE
M = NEW START CONDITION

I

t

SU:STO

L

t

BUF

J

K

M

SMBCLK

SMBDATA

t

SU:STAtHD:STA

A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE

t

SU:DAT

t

HD:DAT

E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER

t

SU:DAT

I = ACKNOWLEDGE CLOCK PULSE
J = STOP CONDITION
K = NEW START CONDITION

t

SU:STO

t

BUF

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

22 ______________________________________________________________________________________

Battery-Charger Commands

The MAX8731 supports four battery-charger com­mands that use either Write-Word or Read-Word proto­cols, as summarized in Table 4. ManufacturerID() and
DeviceID() can be used to identify the MAX8731. On
the MAX8731, the ManufacturerID() command always
returns 0x004D and the DeviceID() command always
returns 0x0008.

DC-DC Converter

The MAX8731 employs a synchronous step-down DC­DC converter with an n-channel high-side MOSFET
switch and an n-channel low-side synchronous rectifier.
The MAX8731 features a pseudo-fixed-frequency, cur­rent-mode control scheme with cycle-by-cycle current
limit. The controller’s constant off-time (t

OFF

) is calculat-

ed based on V

CSSP

, V

CSIN

, and a time constant with a
minimum value of 300ns. The MAX8731 can also oper­ate in discontinuous-conduction mode for improved
light-load efficiency. The operation of the DC-DC con­troller is determined by the following four comparators
as shown in the functional diagrams in Figures 2 and 6:

The IMIN comparator triggers a pulse in discontinuous
mode when the accumulated error is too high. IMIN
compares the control signal (LVC) against 100mV (typ).
When LVC is less than 100mV, DHI and DLO are both
forced low. Indirectly, IMIN sets the peak inductor cur­rent in discontinuous mode.

The CCMP comparator is used for current-mode regu-
lation in continuous-conduction mode. CCMP com­pares LVC against the inductor current. The high-side
MOSFET on-time is terminated when the CSI voltage is
higher than LVC.

The IMAX comparator provides a secondary cycle-by­cycle current limit. IMAX compares CSI to 2V (corre­sponding to 10A when RS2 = 10mΩ). The high-side
MOSFET on-time is terminated when the current-sense
signal exceeds 10A. A new cycle cannot start until the
IMAX comparator’s output goes low.

The ZCMP comparator provides zero-crossing detec­tion during discontinuous conduction. ZCMP compares
the current-sense feedback signal to 750mA (RS2 =
10mΩ). When the inductor current is lower than the
750mA threshold, the comparator output is high and
DLO is turned off.

The OVP comparator is used to prevent overvoltage at
the output due to battery removal. OVP compares FBS_
against the set voltage (ChargeVoltage()). When FBS_
is 100mV above the set value, the OVP comparator out­put goes high and the high-side MOSFET on-time is ter­minated. DHI and DLO remain off until the OVP
condition is removed.

CCV, CCI, CCS, and LVC Control Blocks

The MAX8731 controls input current (CCS control loop),
charge current (CCI control loop), or charge voltage
(CCV control loop), depending on the operating condi­tion. The three control loops—CCV, CCI, and CCS—are
brought together internally at the lowest voltage-clamp
(LVC) amplifier. The output of the LVC amplifier is the
feedback control signal for the DC-DC controller. The
minimum voltage at the CCV, CCI, or CCS appears at
the output of the LVC amplifier and clamps the other
control loops to within 0.3V above the control point.

COMMAND

COMMAND NAME READ/WRITE DESCRIPTION POR STATE

0x14 ChargeCurrent() Write Only 6-Bit Charge-Current Setting 0x0000

0x15 ChargeVoltage() Write Only 11-Bit Charge-Voltage Setting 0x0000

0x3F InputCurrent() Write Only 6-Bit Charge-Current Setting 0x0080

0xFE ManufacturerID() Read Only Manufacturer ID 0x004D

0xFF DeviceID() Read Only Device ID 0x0008

Table 4. Battery-Charger Command Summary

IMAX

CCMP

IMIN

ZCMP

OVP

CSI

2V

100mV

150mV

ChargeVoltage ( )

+100mV

FBS_

DCIN

CSIN

LVC

RSQ

Q

OFF-TIME

ONE-SHOT

OFF-TIME

COMPUTE

DH
DRIVER

DL
DRIVER

Figure 6. DC-DC Converter Functional Diagram

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 23

Clamping the other two control loops close to the low­est control loop ensures fast transition with minimal
overshoot when switching between different control
loops (see the Compensation section).

Continuous-Conduction Mode

With sufficient charge current, the MAX8731’s inductor
current never crosses zero, which is defined as contin­uous-conduction mode. The regulator switches at
400kHz (nominal) if V

CSIN

< 0.88 x V

CSSP

. The con­troller starts a new cycle by turning on the high-side
MOSFET and turning off the low-side MOSFET. When
the charge-current feedback signal (CSI) is greater
than the control point (LVC), the CCMP comparator out­put goes high and the controller initiates the off-time by
turning off the high-side MOSFET and turning on the
low-side MOSFET. The operating frequency is gov­erned by the off-time and is dependent upon V

CSIN

and

V

CSSP

. The off-time is set by the following equation:

The on-time can be determined using the following
equation:

where:

The switching frequency can then be calculated:

These equations describe the controller’s pseudo­fixed-frequency performance over the most common
operating conditions.

At the end of the fixed off-time, the controller initiates a
new cycle if the control point (LVC) is greater than
100mV and the peak charge current is less than the
cycle-by-cycle current limit. Restated another way,

IMIN must be high, IMAX must be low, and OVP must
be low for the controller to initiate a new cycle. If the
peak inductor current exceeds the IMAX comparator
threshold or the output voltage exceeds the OVP
threshold, then the on-time is terminated. The cycle-by­cycle current limit effectively protects against overcur­rent and short-circuit faults.

If during the off-time the inductor current goes to zero,
the ZCMP comparator output pulls high, turning off the
low-side MOSFET. Both the high- and low-side
MOSFETs are turned off until another cycle is ready to
begin. ZCOMP causes the MAX8731 to enter into dis­continuous-conduction mode (see the Discontinuous
Conduction section).

There is a 0.3µs minimum off-time when the (V

CSSP

-

V

CSIN

) differential becomes too small. If V

CSIN

≥ 0.88 x

V

CSSP

, then the threshold for the 0.3µs minimum off­time is reached. The switching frequency in this mode
varies according to the equation:

Discontinuous Conduction

The MAX8731 can also operate in discontinuous-con­duction mode to ensure that the inductor current is
always positive. The MAX8731 enters discontinuous­conduction mode when the output of the LVC control
point falls below 100mV. This corresponds to peak
inductor current = 500mA:

charge current for RS2 = 10mΩ.

In discontinuous mode, a new cycle is not started until
the LVC voltage rises above 100mV. Discontinuous­mode operation can occur during conditioning charge
of overdischarged battery packs, when the charge cur­rent has been reduced sufficiently by the CCS control
loop, or when the charger is in constant-voltage mode
with a nearly full battery pack.

ts

=×

25. µ

OFF

t

ON

I

RIPPLE

f

SW

VV

CSSP CSIN

V

CSSP

LI

×

=

RIPPLE

VV

−

CSSN BATT

Vt

×

BATT OFF

=

=

L

1

tt

+

ON OFF

−

f

=

LI

VV

CSSN BATT

×

RIPPLE

1

03. µ

s

−

+

100

I

CHG

1

=×

2

mV

×

20 2

RS

=

250

mA

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

24 ______________________________________________________________________________________

Compensation

The charge-voltage and charge-current regulation
loops are independent and compensated separately at
the CCV, CCI, and CCS.

CCV Loop Compensation

The simplified schematic in Figure 7 is sufficient to
describe the operation of the MAX8731 when the volt­age loop (CCV) is in control. The required compensa­tion network is a pole-zero pair formed with C

CV

and
RCV. The zero is necessary to compensate the pole
formed by the output capacitor and the load. R

ESR

is
the equivalent series resistance (ESR) of the charger
output capacitor (C

OUT

). RLis the equivalent charger

output load, where RL= ∆V

BATT

/ ∆I

CHG

. The equiva-

lent output impedance of the GMV amplifier, R

OGMV

, is

greater than 10MΩ. The voltage amplifier transconduc-

tance, GMV = 0.125µA/mV. The DC-DC converter
transconductance is dependent upon the charge-cur­rent sense resistor RS2:

GM

OUT

=

where A

CSI

= 20V/V, and RS2 = 10mΩ in the typical

application circuits, so GM

OUT

= 5A/V. The loop-trans-

fer function is given by:

The poles and zeros of the voltage loop-transfer func­tion are listed from lowest frequency to highest frequen­cy in Table 5.

Near crossover C

CV

is much lower impedance than

R

OGMV

. Since CCVis in parallel with R

OGMV

, CCVdom­inates the parallel impedance near crossover.
Additionally, RCVis much higher impedance than C

CV

and dominates the series combination of RCVand CCV,
so near crossover:

Figure 7. CCV Loop Diagram

NAME EQUATION DESCRIPTION

Lowest frequency pole created by CCV and GMV’s finite output resistance.

Voltage-loop compensation zero. If this zero is at the same frequency or
lower than the output pole f

P_OUT

, then the loop-transfer function
approximates a single-pole response near the crossover frequency. Choose
C

CV

to place this zero at least 1 decade below crossover to ensure

adequate phase margin.

Output

Pole

Output pole formed with the effective load resistance R

L

and the output

capacitance C

OUT

. RL influences the DC gain but does not affect the

stability of the system or the crossover frequency.

Output

Zero

Output ESR Zero. This zero can keep the loop from crossing unity gain if
f

Z_OUT

is less than the desired crossover frequency; therefore, choose a

capacitor with an ESR zero greater than the crossover frequency.

Table 5. CCV Loop Poles and Zeros

f

RC

PCV

OGMV CV

_

=

×

1

2πfRC

ZCV

CV CV

_

=

×

1

2π

f

RC

P OUT

L OUT

_

=

×

1

2π

f

RC

P OUT

L OUT

_

=

×

1

2π

FBS_

ChargeVoltage( )

ESR

C

OUT

CCV

R

C

GM

OUT

GMV

CV

CV

R

OGMV

R

LR

1

×

CSI

2ARS

LTF GM R GMV R

=×××

OUT L OGMV

sC R sC R

+×+×

()()

11

×

OUT ESR CV CV

sC R sC R

+× + ×

()()

11

CV OGMV OUT L

RsCR

OGMV CV CV

()

1

+×

×

()

1

sC R

+×

CV OGMV

R

≅

CV

CCV Pole

CCV Zero

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 25

C

OUT

is also much lower impedance than RLnear
crossover so the parallel impedance is mostly capaci­tive and:

If R

ESR

is small enough, its associated output zero has
a negligible effect near crossover and the loop-transfer
function can be simplified as follows:

Setting LTF = 1 to solve for the unity-gain frequency
yields:

For stability, choose a crossover frequency lower than
1/10 the switching frequency. For example, choose a
crossover frequency of 50kHz and solve for RVCusing
the component values listed in Figure 1 to yield RCV=
10kΩ:

GMV = 0.125µA/mV

GM

OUT

= 5A/V

C

OUT

= 2 x 10µF

F

OSC

= 400kHz

RL= 0.2Ω

F

CO_CV

= 50kHz

To ensure that the compensation zero adequately can­cels the output pole, select f

Z_CV

≤ f

P_OUT

:

CCV≥ (RL/ RCV) C

OUT

CCV≥ 400pF (assuming 2 cells and 2A maximum
charge current.)

Figure 8 shows the Bode plot of the voltage-loop fre­quency response using the values calculated above.

CCI Loop Compensation

The simplified schematic in Figure 9 is sufficient to
describe the operation of the MAX8731 when the bat­tery current loop (CCI) is in control. Since the output
capacitor’s impedance has little effect on the response
of the current loop, only a simple single pole is required
to compensate this loop. A

CSI

is the internal gain of the

current-sense amplifier. RS2 is the charge current­sense resistor (10mΩ). R

OGMI

is the equivalent output

impedance of the GMI amplifier, which is greater than
10MΩ. GMI is the charge-current amplifier transcon­ductance = 1µA/mV. GM

OUT

is the DC-DC converter

transconductance = 5A/V.

Figure 8. CCV Loop Response Figure 9. CCI Loop Diagram

sC

≅

R

CV

OUT

OUT

1

G

MV

R

CV

×2π

C

OUT

_

10

Ω

k

≅

R

⋅

L

+×

()1

sC R sC

OUT L OUT

LTF GM

=×

OUT

fGMG

CO CV OUT MV

=××

_

π

=

GMV GM

×

R

CV

×2

Cf

OUT CO CV

×

80

60

40

20

MAGNITUDE (dB)

0

-20

-40

0.1 1M

MAG
PHASE

FREQUENCY (Hz)

0

GM

-45

PHASE (DEGREES)

-90

CCI

C

-135

100k10k1k100101

CI

R

OGMI

OUT

GMI

CSIP

ChargeCurrent( )

CSIN

RS2

CSI

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

26 ______________________________________________________________________________________

Figure 10. CCI Loop Response

Figure 11. CCS Loop Diagram

The loop-transfer function is given by:

This describes a single-pole system. Since:

the loop-transfer function simplifies to:

The crossover frequency is given by:

For stability, choose a crossover frequency lower than
1/10 the switching frequency:

CCI> 10 × GMI / (2π f

OSC

) = 4nF, for a 400kHz switch-

ing frequency.

Values for CCIgreater than 10 times the minimum value
can slow down the current-loop response. Choosing C

CI

= 10nF yields a crossover frequency of 15.9kHz. Figure
10 shows the Bode plot of the current-loop frequency
response using the values calculated above.

CCS Loop Compensation

The simplified schematic in Figure 11 is sufficient to
describe the operation of the MAX8731 when the input
current-limit loop (CCS) is in control. Since the output
capacitor’s impedance has little effect on the response
of the input current-limit loop, only a single pole is
required to compensate this loop. A

CSS

is the internal

gain of the current-sense resistor; RS1 = 10mΩ in the
typical application circuits. R

OGMS

is the equivalent

output impedance of the GMS amplifier, which is
greater than 10MΩ. GMS is the charge-current amplifier
transconductance = 1µA/mV. GMINis the DC-DC con­verter’s input-referred transconductance = (1/D) x
GM

OUT

= (1 / D) x 5A/V.

The loop-transfer function is given by:

Since:

the loop-transfer function simplifies to:

R

LTF GM A RS GMI

=×××

OUT CSI

2

OGMI

+

1

sR C

OGMI CI

×

GM

LTF GMI

=

OUT

+

=

sR C

ARS

×12

CSI

R

OGMI

×1

OGMI CI

f

CO CICI_

GMI

=

2π

C

LTF GM A RSI GMS

=×××

IN CSS

GM

=

IN

ARS

CSS

SR C

+×1

×12

R

OGMS

SR C

+×1

OGMS CS

LTF GMS

=

R

OGMS

OGMS CS

100

80

60

40

20

MAGNITUDE (dB)

0

-20

-40

0.1
FREQUENCY (Hz)

MAG
PHASE

0

InputCurrent( )

-45

-90

100k1k10

CCS

GMS

C

CS

R

OGMS

GM

CSS

IN

ADAPTER

INPUT

CSSP

RS1

CSSI

SYSTEM

LOAD

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 27

The crossover frequency is given by:

For stability, choose a crossover frequency lower than
1/10 the switching frequency:

Choosing a crossover frequency of 30kHz and using
the component values listed in Figure 1 yields CCS>

5.4nF. Values for CCS greater than 10 times the mini­mum value may slow down the current-loop response
excessively. Figure 12 shows the Bode plot of the input
current-limit-loop frequency response using the values
calculated above.

MOSFET Drivers

The DHI and DLO outputs are optimized for driving
moderate-sized power MOSFETs. The MOSFET drive
capability is the same for both the low-side and high­sides switches. This is consistent with the variable duty
factor that occurs in the notebook computer environ­ment where the battery voltage changes over a wide
range. There must be a low-resistance, low-inductance
path from the DLO driver to the MOSFET gate to pre­vent shoot-through. Otherwise, the sense circuitry in the
MAX8731 interprets the MOSFET gate as “off” while
there is still charge left on the gate. Use very short,
wide traces measuring 10 to 20 squares or less
(1.25mm to 2.5mm wide if the MOSFET is 25mm from

the device). Unlike the DLO output, the DHI output uses
a 50ns (typ) delay time to prevent the low-side MOSFET
from turning on until DHI is fully off. The same consider­ations should be used for routing the DHI signal to the
high-side MOSFET.

The high-side driver (DHI) swings from LX to 5V above
LX (BST) and has a typical impedance of 3Ω sourcing
and 1Ω sinking. The low-side driver (DLO) swings from
DLOV to ground and has a typical impedance of 1Ω
sinking and 3Ω sourcing. This helps prevent DLO from
being pulled up when the high-side switch turns on, due
to capacitive coupling from the drain to the gate of the
low-side MOSFET. This places some restrictions on the
MOSFETs that can be used. Using a low-side MOSFET
with smaller gate-to-drain capacitance can prevent
these problems.

Design Procedure

MOSFET Selection

Choose the n-channel MOSFETs according to the maxi­mum required charge current. The MOSFETs must be
able to dissipate the resistive losses plus the switching
losses at both V

DCIN(MIN)

and V

DCIN(MAX)

.

For the high-side MOSFET, the worst-case resistive
power losses occur at the maximum battery voltage
and minimum supply voltage:

Generally a low-gate charge high-side MOSFET is pre­ferred to minimize switching losses. However, the
R

DS(ON)

required to stay within package power-dissi­pation limits often limits how small the MOSFET can be.
The optimum occurs when the switching (AC) losses
equal the conduction (R

DS(ON)

) losses. Calculating the
power dissipation in N1 due to switching losses is diffi­cult since it must allow for difficult quantifying factors
that influence the turn-on and turn-off times. These fac­tors include the internal gate resistance, gate charge,
threshold voltage, source inductance, and PC board
layout characteristics. The following switching-loss cal­culation provides a rough estimate and is no substitute
for breadboard evaluation, preferably including a verifi­cation using a thermocouple mounted on N1:

Figure 12. CCS Loop Response

f

CO CSCS_

C GMS f

=×52/( )π

CS OSC

100

80

60

40

20

MAGNITUDE (dB)

0

-20

-40

0.1

GMS

=

2π

C

FREQUENCY (Hz)

0

MAG
PHASE

-45

PHASE (DEGREES)

-90

100k 10M1k10

PD HighSide

CONDUCTION

()

PD High Side t V I f

SWITCHING Trans DCIN CHG SW

()=× × × ×

V

FBS

=××

V

CSSP

1
2

_

IR

CHG

2

DS ON

()

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

28 ______________________________________________________________________________________

where t

TRANS

is the driver’s transition time and can be

calculated as follows:

I

GATE

is the peak gate-drive current.

The following is the power dissipated due to the high­side n-channel MOSFET’s output capacitance (C

RSS

):

The total high-side MOSFET power dissipation is:

Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum AC adapter
voltages are applied. If the high-side MOSFET chosen
for adequate R

DS(ON)

at low-battery voltages becomes

hot when biased from V

IN(MAX)

, consider choosing
another MOSFET with lower parasitic capacitance. For
the low-side MOSFET (N2), the worst-case power dissi­pation always occurs at maximum input voltage:

The following additional loss occurs in the low-side
MOSFET due to the reverse-recovery charge of the
MOSFET’s body diode and the body diode conduction
losses:

The total power low-side MOSFET dissipation is:

These calculations provide an estimate and are not a sub­stitute for breadboard evaluation, preferably including a
verification using a thermocouple mounted on the MOSFET.

Inductor Selection

The charge current, ripple, and operating frequency
(off-time) determine the inductor characteristics. For
optimum efficiency, choose the inductance according
to the following equation:

This sets the ripple current to 1/3 the charge current
and results in a good balance between inductor size
and efficiency. Higher inductor values decrease the rip­ple current. Smaller inductor values save cost but
require higher saturation current capabilities and
degrade efficiency.

Inductor L1 must have a saturation current rating of at
least the maximum charge current plus 1/2 the ripple
current (∆IL):

I

SAT

= I

CHG

+ (1/2) ∆IL

The ripple current is determined by:

∆IL = V

BATT

× t

OFF

/ L

where:

t

OFF

= 2.5µs (V

DCIN

- V

BATT

) / V

DCIN

for V

BATT

< 0.88

V

DCIN

or during dropout:

t

OFF

= 0.3µs for V

BATT

> 0.88 V

DCIN

⎛

t

TRANS

112

=+

⎜

IIQI

⎝

Gsrc GsnkGGATE

⎞

×≈

⎟
⎠

and f kHz

SW

400,

PD HighSide

COSS

()≈

2

VCf

××

DCIN

RSS SW

2

PD HighSide PD HighSide

() ()

++

() ()

TOTAL CONDUCTION

PD HighSide PD HighSide

SWITCHING COSS

≈

PD Low Side

CONDUCTION

()

⎛

1

=−

⎜
⎝

IR

××

CHG

FBS

V

CSSP
2

⎞

_

⎟
⎠

DS ON

()

V

Vt

BATT OFF

L

=

×03.

×

I

CHG

PD Low Side Q V f I V

() (. .)=× ×+××

QRR RR DCIN SW PEAK

2

PD Low Side PD Low Side

()

+

() ()

TOTAL CONDUCTION

PD HighSide

QRR

≈

005 04

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 29

Input Capacitor Selection

The input capacitor must meet the ripple current
requirement (I

RMS

) imposed by the switching currents.
Nontantalum chemistries (ceramic, aluminum, or OS­CON) are preferred due to their resilience to power-up
surge currents:

The input capacitors should be sized so that the temper­ature rise due to ripple current in continuous conduction
does not exceed approximately 10°C. The maximum rip­ple current occurs at 50% duty factor or V

DCIN

= 2 x

V

BATT

, which equates to 0.5 x I

CHG

. If the application of
interest does not achieve the maximum value, size the
input capacitors according to the worst-case conditions.

Output Capacitor Selection

The output capacitor absorbs the inductor ripple current
and must tolerate the surge current delivered from the
battery when it is initially plugged into the charger. As
such, both capacitance and ESR are important parame­ters in specifying the output capacitor as a filter and to
ensure stability of the DC-DC converter (see the
Compensation section). Beyond the stability require­ments, it is often sufficient to make sure that the output
capacitor’s ESR is much lower than the battery’s ESR.
Either tantalum or ceramic capacitors can be used on the
output. Ceramic devices are preferable because of their
good voltage ratings and resilience to surge currents.

Applications Information

Smart-Battery System Background

Information

Smart-battery systems have evolved since the concep­tion of the smart-battery system (SBS) specifications.
Originally, such systems consisted of a smart battery
and smart-battery charger that were compatible with the
SBS specifications and communicated directly with one
another using SMBus protocols. Modern systems still
employ the original commands and protocols, but often
use a keyboard controller or similar digital intelligence to
mediate the communication between the battery and the
charger (Figure 13). This arrangement permits consider­able freedom in the implementation of charging algo­rithms at the expense of standardization. Algorithms can
vary from the simple detection of the battery with a fixed
set of instructions for charging the battery to highly com­plex programs that can accommodate multiple battery

configurations and chemistries. Microcontroller pro­grams can perform frequent tests on the battery’s state
of charge and dynamically change the voltage and cur­rent applied to enhance safety. Multiple batteries can
also be utilized with a selector that is programmable over
the SMBus.

Setting Input Current Limit

The input current limit should be set based on the cur­rent capability of the AC adapter and the tolerance of
the input current limit. The upper limit of the input cur­rent threshold should never exceed the adapter’s mini­mum available output current. For example, if the
adapter’s output current rating is 5A ±10%, the input
current limit should be selected so that its upper limit is
less than 5A × 0.9 = 4.5A. Since the input current-limit
accuracy of the MAX8731 is ±3%, the typical value of
the input current limit should be set at 4.5A / 1.03 ≈

4.36A. The lower limit for input current must also be
considered. For chargers at the low end of the spec,
the input current limit for this example could be 4.36A ×

0.95, or approximately 4.14A.

Layout and Bypassing

Bypass DCIN with a 1µF ceramic to ground (Figure 1).
D1 protects the MAX8731 when the DC power source
input is reversed. Bypass VDD, DCIN, LDO, VCC, DAC,
and REF as shown in Figure 1.

Figure 13. Typical Smart-Battery System

AC-TO-DC

CONVERTER

(ADAPTER)

SYSTEM

POWER

SUPPLIES

II

RMS CHG

=

⎛

VV V

()

BATT DCIN BATT

⎜
⎜

⎝

V

DCIN

−

⎞
⎟

⎟
⎠

MAX8731

SMART-BATTERY

CHARGER/

POWER-SOURCE

SELECTOR

SMBus

CONTROL

SIGNALS

FOR

BATTERY

BATT+

BATT-

SYSTEM HOST

(KEYBOARD CONTROLLER)

SMART

BATTERY

SMBus

CONTROL

SIGNALS

FOR

BATTERY

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

30 ______________________________________________________________________________________

Good PC board layout is required to achieve specified
noise immunity, efficiency, and stable performance. The
PC board layout artist must be given explicit instruc­tions—preferably, a sketch showing the placement of
the power-switching components and high-current rout­ing. Refer to the PC board layout in the MAX8731 evalu­ation kit for examples. A ground plane is essential for
optimum performance. In most applications, the circuit
will be located on a multilayer board, and full use of the
four or more copper layers is recommended. Use the
top layer for high-current connections, the bottom layer
for quiet connections, and the inner layers for uninter­rupted ground planes.

Use the following step-by-step guide:

1) Place the high-power connections first, with their

grounds adjacent:

a) Minimize the current-sense resistor trace

lengths, and ensure accurate current sensing
with Kelvin connections.

b) Minimize ground trace lengths in the high-cur-

rent paths.

c) Minimize other trace lengths in the high-current

paths.

Use > 5mm wide traces in the high-current
paths.

d) Connect C1 and C2 to high-side MOSFET

(10mm max length). Place the input capacitor
between the input current-sense resistor and
drain of the high-side MOSFET.

e) Minimize the LX node (MOSFETs, rectifier cath-

ode, inductor (15mm max length)). Keep LX on
one side of the PC board to reduce EMI radiation.

f) Since the return path of DHI is LX, route DHI near

LX. Optimally, LX and DHI should overlap. The
same principle is applied to DLO and PGND.

g) Ideally, surface-mount power components are

flush against one another with their ground termi­nals almost touching. These high-current
grounds are then connected to each other with a
wide, filled zone of top-layer copper, so they do

not go through vias. The resulting top-layer sub­ground plane is connected to the normal inner­layer ground plane at the paddle. Other
high-current paths should also be minimized, but
focusing primarily on short ground and current­sense connections eliminates approximately 90%
of all PC board layout problems.

2) Place the IC and signal components. Keep the
main switching node (LX node) away from sensitive
analog components (current-sense traces and REF
capacitor).

Important: The IC must be no further than 10mm
from the current-sense resistors. Quiet connections
to REF, CCS, DAC, CCV, CCI, ACIN, and VCC
should be returned to a separate ground (GND)
island. The analog ground is separately worked
from power ground in Figure 1. There is very little
current flowing in these traces, so the ground island
need not be very large. When placed on an inner
layer, a sizable ground island can help simplify the
layout because the low-current connections can be
made through vias. The ground pad on the back­side of the package should also be connected to
this quiet ground island.

3) Keep the gate-drive traces (DHI and DLO) as short
as possible (L < 20mm), and route them away from
the current-sense lines and REF. These traces
should also be relatively wide (W > 1.25mm).

4) Place ceramic bypass capacitors close to the IC.
The bulk capacitors can be placed further away.
Place the current-sense input filter capacitors under
the part, connected directly to the GND pin.

5) Use a single-point star ground placed directly
below the part at the PGND pin. Connect the power
ground (ground plane) and the quiet ground island
at this location.

Chip Information

TRANSISTOR COUNT: 10,234

PROCESS: BiCMOS

MAX8731

SMBus Level 2 Battery Charger with

Remote Sense

______________________________________________________________________________________ 31

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages

.)

MARKING

PIN # 1

I.D.

D

D/2

AAAAA

C

E/2

E

e

L

L1

0.10 C

A

0.08 C

A3

A1

(NE-1) X e

DETAIL A

L

D2

C

L

k

D2/2

e/2

e

(ND-1) X e

L

e e

b

L

DETAIL B

0.10 M C A B

E2/2

C

E2

L

PIN # 1 I.D.

0.35x45°

CC

L

QFN THIN.EPS

L

-DRAWING NOT TO SCALE-

PACKAGE OUTLINE,
16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm

21-0140

1

I

2

MAX8731

SMBus Level 2 Battery Charger with
Remote Sense

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

32 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2006 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. Inc.

MAX8731

Package Information (continued)

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages

.)

PKG.

SYMBOL

JEDEC

NOTES:

1. DIMENSIONING & TOLERANCING CONFORM TO ASME Y14.5M-1994.

2. ALL DIMENSIONS ARE IN MILLIMETERS. ANGLES ARE IN DEGREES.

3. N IS THE TOTAL NUMBER OF TERMINALS.

4. THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION SHALL
CONFORM TO JESD 95-1 SPP-012. DETAILS OF TERMINAL #1 IDENTIFIER ARE
OPTIONAL, BUT MUST BE LOCATED WITHIN THE ZONE INDICATED. THE TERMINAL #1
IDENTIFIER MAY BE EITHER A MOLD OR MARKED FEATURE.

5. DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN

0.25 mm AND 0.30 mm FROM TERMINAL TIP.

6. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY.

7. DEPOPULATION IS POSSIBLE IN A SYMMETRICAL FASHION.

8. COPLANARITY APPLIES TO THE EXPOSED HEAT SINK SLUG AS WELL AS THE TERMINALS.

9. DRAWING CONFORMS TO JEDEC MO220, EXCEPT EXPOSED PAD DIMENSION FOR
T2855-3 AND T2855-6.

10. WARPAGE SHALL NOT EXCEED 0.10 mm.

11. MARKING IS FOR PACKAGE ORIENTATION REFERENCE ONLY.

12. NUMBER OF LEADS SHOWN ARE FOR REFERENCE ONLY.

13. LEAD CENTERLINES TO BE AT TRUE POSITION AS DEFINED BY BASIC DIMENSION "e", ±0.05.

-DRAWING NOT TO SCALE-

MIN. MAX.NOM.

A

0.70 0.800.75
A1
A3

b

0.25

D

4.90

E

4.90

e

0.250--

k
L

0.30 0.500.40

L1

N
ND
NE

16L 5x5

0.02

0.20 REF.

5.00

0.80 BSC.

--­16

4
4

WHHB

0.05

0.350.30

5.10

5.105.00

COMMON DIMENSIONS

20L 5x5

NOM.

MIN.

MAX.

MIN.

0.70

0.25

0.25

0

0.20 REF.

4.90

4.90

0.65 BSC.

0.45

---

0.75

0.02

0.30

5.00

5.00

0.55

20

5
5

WHHC

0.80

0.05

0.35

5.10

5.10

0.65

--

0.70
0

0.20 REF.

0.20

4.90

4.90

0.25

0.45

---

28L 5x5

NOM.

0.75

0.02

0.25

5.00

5.00

0.50 BSC.

0.55

28

7
7

WHHD-1

MAX.

MIN.

0.80

0.70

0.05

0

0.30

0.20 0.25 0.30

5.10

4.90

5.10

4.90

--

0.25

0.65

0.30

---

32L 5x5

NOM.

0.75

0.02

0.20 REF.

5.00

5.00

0.50 BSC.

0.40

32

8
8

WHHD-2

MAX.

MIN.

0.70

0.80

0.05

0.15

5.10

4.90

5.10

4.90 5.00

--

0.25 0.35 0.45

0.50

0.30

40L 5x5

NOM.

0.75 0.80

0.20 REF.

5.00 5.10

0.40 BSC.

0.40 0.50
40
10

10

-----

MAX.

0.050 0.02

0.250.20

5.10

0.600.40 0.50

EXPOSED PAD VARIATIONS

PKG.
CODES

D2

MAX.

NOM.MIN.

MIN.

E2

NOM. MAX.

T1655-2

3.203.00T1655-3 3.10 3.00 3.10 3.20 NO

T2055-4

3.103.00 3.203.103.00 3.20

3.353.15T2055-5 3.25 3.15 3.25 3.35
T2855-3 3.15 3.25 3.35 3.15 3.25 3.35
T2855-4 2.60 2.70 2.80 2.60 2.70 2.80
T2855-5 2.60 2.70 2.80 2.60 2.70 2.80
T2855-6
T2855-7 2.60 2.70

3.15 3.25 3.35 3.15 3.25 3.35

2.80

2.60 2.70 2.80

3.353.15T2855-8 3.25 3.15 3.25 3.35

3.15

3.25 3.15 3.25 3.35

T2855N-1

3.00 3.10T3255-3 3 3.203.00 3.10

T3255-5 YES3.003.103.00

3.35

3.20

3.203.00 3.10T3255-4 3 3.203.00 3.10

3.20

3.203.10T3255N-1 3.00

3.203.10

3.203.103.00

3.30T4055-1 3.20 3.40 3.20 3.30 3.40

SEE COMMON DIMENSIONS TABLE

**

PACKAGE OUTLINE,
16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm

21-0140

exceptions

±0.15

**

**
**
**

**

0.40
**

**
**

**
**

0.40
**

**
**

**

**
**

DOWN

L

BONDS

ALLOWED

YES3.203.103.003.103.00 3.20

NO3.203.103.003.10T1655N-1 3.00 3.20

YES3.103.00 3.203.103.00 3.20T2055-3

NO
YES
YES
YES

NO

NO
YES
YES

NO
YES

NO

NO
YES

2

I

2