MAXIM MAX1909, MAX8725 User Manual

General Description

The MAX1909/MAX8725 highly integrated control ICs simplify construction of accurate and efficient multichemistry battery chargers. The MAX1909/MAX8725 use analog inputs to control charge current and voltage, and can be programmed by a host microcontroller (µC) or hardwired. High efficiency is achieved through use of buck topology with synchronous rectification.

The maximum current drawn from the AC adapter is programmable to avoid overloading the AC adapter when supplying the load and the battery charger simultaneously. The MAX1909/MAX8725 provide a digital output that indicates the presence of an AC adapter, and an analog output that monitors the current drawn from the AC adapter. Based on the presence or absence of the AC adapter, the MAX1909/MAX8725 automatically select the appropriate source for supplying power to the system by controlling two external p-channel MOSFETs. Under system control, the MAX1909/MAX8725 allow the battery to undergo a relearning or conditioning cycle in which the battery is completely discharged through the system load and then recharged.

The MAX1909 includes a conditioning charge feature while the MAX8725 does not. The MAX1909/MAX8725 are available in space-saving 28-pin, 5mm  5mm thin QFN packages and operate over the extended -40°C to +85°C temperature range. The MAX1909/MAX8725 are now available in lead-free packages.

Applications

Notebook and Subnotebook Computers Hand-Held Data Terminals

Features

o ±0.5% Accurate Charge Voltage (0°C to +85°C) o ±3% Accurate Input Current Limiting o ±5% Accurate Charge Current o Programmable Charge Current >4A o Automatic System Power-Source Selection o Analog Inputs Control Charge Current and

Charge Voltage

o Monitor Outputs for

Current Drawn from AC Input Source AC Adapter Presence

o Up to 17.65V (max) Battery Voltage o Maximum 28V Input Voltage o Greater than 95% Efficiency o Charge Any Battery Chemistry: Li+, NiCd, NiMH,

Lead Acid, etc.

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

________________________________________________________________

Maxim Integrated Products

Ordering Information

19-2805; Rev 2; 9/04

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.

EVALUATION KIT AVAILABLE

PART TEMP RANGE PIN-PACKAGE

MAX1909ETI -40°C to +85°C 28 Thin QFN

MAX1909ETI+ -40°C to +85°C 28 Thin QFN

MAX8725ETI -40°C to +85°C 28 Thin QFN

MAX8725ETI+ -40°C to +85°C 28 Thin QFN

28 27 26 25 24 23 22

8 9 10 11 12 13 14

MAX1909 MAX8725

THIN QFN

TOP VIEW

LDO

DCIN

ACIN

REF

GND/PKPRES

ACOK

MODE

PDL

PDS

CSSP

CSSN

SRC

DHI

DHIV

DLOV

DLO

PGND

CSIP

CSIN

BATT

GND

CCS

CCV

CCI

VCTL

ICTL

CLS

IINP

Pin Configuration

CSSP CSSN

LDO

DHI

DLOV

DLO

PGND

CSIP

CSIN

BATT GND

DCIN

VCTL

ICTL

MODE

ACIN

ACOK

CLS

CCV CCI

CCS

REF

LDO

AC ADAPTER: INPUT

0.01Ω

10μH

0.015Ω

EXTERNAL LOAD

LDO

PDS

PDL

SRC

LDO

REF

IINP

DHIV

SRC

MAX1909 MAX8725

PKPRES

MAX8725 ONLY

Minimum Operating Circuit

Denotes lead-free package.

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

= 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, CSSP, CSSN, SRC, ACOK to GND..............-0.3V to +30V

DHIV ........................................................…SRC + 0.3, SRC - 6V

DHI, PDL, PDS to GND ...............................-0.3V to (V

SRC

+ 0.3)

BATT, CSIP, CSIN to GND .....................................-0.3V to +20V

CSIP to CSIN or CSSP to CSSN or PGND to GND ...-0.3V to +0.3V CCI, CCS, CCV, DLO, IINP, REF,

ACIN to GND ........................................-0.3V to (V

LDO

+ 0.3V)

DLOV, VCTL, ICTL, MODE, CLS, LDO,

PKPRES to GND ...................................................-0.3V to +6V

DLOV to LDO.........................................................-0.3V to +0.3V

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

LDO Short-Circuit Current...................................................50mA

Continuous Power Dissipation (T

= +70°C)

28-Pin TQFN (derate 20.8mW/°C above +70°C) .......1666mW

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

CHARGE VOLTAGE REGULATION

VCTL Range 0 3.6 V

VCTL

= 3.6V (3 or 4 cells);

not including VCTL resistor tolerances

VCTL

= 3.6V/20 (3 or 4 cells); not including

VCTL resistor tolerances

VCTL

= 3.6V (3 or 4 cells); including VCTL

resistor tolerances of 1%

Battery Regulation Voltage Accuracy

VCTL

= V

LDO

(3 or 4 cells, default

threshold of 4.2V/cell)

VCTL

Default Threshold V

VCTL

rising 4.1 4.3 V

VCTL

= 3V 0 2.5

VCTL Input Bias Current

DCIN

= 0, V

VCTL

= 5V 0 12

µA

CHARGE-CURRENT REGULATION

MAX1909 0 3.6

ICTL Range

MAX8725 0 3.2

CSIP-to-CSIN Full-Scale Current-

Sense Voltage

MAX1909: V

ICTL

= 3.6V (not including ICTL

resistor tolerances)

MAX8725: V

ICTL

= 3.2V (not including ICTL

resistor tolerances)

-5 +5

MAX1909: V

ICTL

= 3.6V x 0.5, MAX8725:

ICTL

= 3.2V x 0.5 (not including ICTL

resistor tolerances)

-5 +5

MAX1909: V

ICTL

= 0.9V (not including ICTL

resistor tolerances)

Charge-Current Accuracy

MAX8725: V

ICTL

= 0.18V (not including

ICTL resistor tolerances)

-30

-0.8

-1.0

-0.5

69.37 75.00 80.63

-7.5

+0.8 +0.8 +1.0 +0.5

+7.5

+30

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

MAX1909: V

ICTL

= 3.6V x 0.5, MAX8725:

ICTL

= 3.2V x 0.5 (including ICTL resistor

tolerances of 1%)

Charge-Current Accuracy

ICTL

= V

LDO (default threshold of 45mV)

-5 +5

ICTL

Default Threshold V

ICTL

rising 4.1 4.2 4.3 V

BATT/CSIP/CSIN Input Voltage

Range

0 19 V

Charging enabled

650

CSIP/CSIN Input Current

0.1 1

µA

MAX1909

ICTL Power-Down Mode

Threshold Voltage

MAX8725

MAX1909

ICTL Power-Up Mode Threshold

Voltage

MAX8725

ICTL

= 3V -1 +1

ICTL Input Bias Current

DCIN

= 0V, V

ICTL

= 5V

-1 +1

µA

INPUT CURRENT REGULATION CSSP-to-CSSN Full-Scale

Current-Sense Voltage

CLS

= REF -3 +3

CLS

= REF x 0.75 -3 +3

Input Current-Limit Accuracy

CLS

= REF x 0.5 -4 +4

8.0 28 V V

CSSP

= V

CSSN

= V

DCIN

> 8.0V

730

CSSP/CSSN Input Current

DCIN

= 0 0.1 1

µA

CLS Input Range 1.6

CLS Input Bias Current V

CLS

= 2.0V -1 +1 µA

IINP Transconductance V

CSSP

- V

CSSN

= 56mV 2.7 3.0 3.3

mA/V

CSSP

- V

CSSN

= 75mV, terminated with

10kΩ

CSSP

- V

CSSN

= 56mV, terminated with

10kΩ

-5 +5IINP Accuracy

CSSP

- V

CSSN

= 20mV, terminated with

10kΩ

-10

IINP Output Current V

CSSP

- V

CSSN

= 150mV, V

IINP

= 0V

µA

IINP Output Voltage V

CSSP

- V

CSSN

= 150mV, V

IINP

= float

3.5 V

C S S P /C S S N Inp ut V ol tag e Rang e

Charging disabled; V

DCIN

= 0 or V

ICTL

= 0

-7.0

+7.0

350

0.75

0.06

0.85

0.11

72.75 75.00 77.25

450

REF

-7.5

+7.5

+10

350

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

SUPPLY AND LINEAR REGULATOR

DCIN Input Voltage Range V

DCIN

8.0 28 V DCIN falling 7 7.4

DCIN Undervoltage-Lockout Trip Point

DCIN rising 7.5

DCIN Quiescent Current I

DCIN

8.0V < V

DCIN

< 28V 2.7 6 mA

BATT

= 19V, V

DCIN

= 0V, or ICTL = 0V 0.1 1

BATT

= 16.8V, V

DCIN

= 19V, ICTL = 0V 0.1 1

BATT Input Current I

BATT

= 2V to 19V, V

DCIN

> V

BATT

+ 0.3V

500

µA

LDO Output Voltage 8.0V < V

DCIN

< 28V, no load

5.4

LDO Load Regulation 0 < I

LDO

< 10mA 80 115 mV

LDO Undervoltage-Lockout Trip

Point

DCIN

= 8.0V

REFERENCE

REF Output Voltage Ref 0 < I

REF

< 500µA

REF Undervoltage-Lockout Trip

Point

REF falling 3.1 3.9 V

TRIP POINTS

BATT POWER_FAIL Threshold V

DCIN

- V

BATT

, V

DCIN

falling 50

150 mV

BATT POWER_FAIL Threshold

Hysteresis

300 mV

ACIN Threshold ACIN rising

V ACIN Threshold Hysteresis 10 20 30 mV ACIN Input Bias Current V

ACIN

= 2.048V -1 +1 µA

SWITCHING REGULATOR

DHI Off-Time V

B AT T

= 16.0V , V

D C I N

= 19V , V

M OD E

= 3.6V

440 ns

DHI Minimum Off-Time V

B AT T

= 16.0V , V

D C I N

= 17V , V

M OD E

= 3.6V

350 ns

DLOV Supply Current I

DLOV

DLO low 5 10 µA

Sense Voltage for Minimum

Discontinuous Mode Ripple Current

7.5 mV

Cycle-by-Cycle Current-Limit

Sense Voltage

97 mV

Sense Voltage for Battery

Undervoltage Charge Current

MAX1909 only, BATT = 3.0V per cell 3 4.5 6 mV

MAX1909 only, MODE = float (3 cell),

BATT

rising

Battery Undervoltage Threshold

MAX1909 only, MODE = LDO (4 cell),

BATT

rising

DHIV Output Voltage With respect to SRC

7.85

200

5.25

3.20

4.2023 4.2235 4.2447

100

100 200

2.007 2.048 2.089

360 400 260 300

9.18

12.235

-4.5 -5.0 -5.5

5.55

5.15

9.42

12.565

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

_______________________________________________________________________________________ 5

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

DHIV Sink Current 10 mA DHI On-Resistance Low DHI = V

DHIV

, I

DHI

= -10mA 2 5 Ω

DHI On-Resistance High DHI = V

CSSN

, I

DHI

= 10mA 2 4 Ω

DLO On-Resistance High V

DLOV

= 4.5V, I

DLO

= +100mA 3 7 Ω

DLO On-Resistance Low V

DLOV

= 4.5V, I

DLO

= -100mA 1 3 Ω

ERROR AMPLIFIERS

V C TL = 3.6, V

BATT

= 16.8V , M OD E = LD O

GMV Loop Transconductance

V C TL = 3.6, V

BATT

= 12.6V , M OD E = FLOAT

mA/V

GMI Loop Transconductance

MAX1909: ICTL = 3.6V, MAX8725: V

ICTL

3.2V, V

CSSP

- V

CSIN

= 75mV

0.5 1 2

mA/V

GMS Loop Transconductance V

CLS

= 2.048V, V

CSSP

- V

CSSN

= 75mV 0.5 1 2

mA/V

CCI/CCS/CCV Clamp Voltage

0.25V < V

CCS

< 2.0V

600 mV

LOGIC LEVELS

MODE Input Low Voltage 0.8 V MODE Input Middle Voltage 1.6 1.8 2.0 V MODE Input High Voltage 2.8 V MODE Input Bias Current MODE = 0V or 3.6V -2 +2 µA

ACOK AND PKPRES

ACOK Input Voltage Range 0 28 V ACOK Sink Current V

ACOK

= 0.4V, ACIN = 1.5V 1 mA

ACOK Leakage Current V

ACOK

= 28V, ACIN = 2.5V 1 µA

PKPRES Input Voltage

Range

PKPRES Input Bias Current -1 +1 µA PKPRES Battery Removal Detect

Threshold

MAX8725, PKPRES rising 55

% of

LDO

PKPRES Hysteresis MAX8725 1 %

PDS, PDL SWITCH CONTROL

PDS Switch Turn-Off Threshold V

DCIN

- V

BATT

, V

DCIN

falling 50

150 mV

DCIN

- V

BATT

300 mV

PDS Output Low Voltage, PDS

Below SRC

PDS

= 0A 8 10 12 V

PDS Turn-On Current PDS = SRC 6 12 mA PDS Turn-Off Current V

PDS

= V

SRC

- 2V, V

DCIN

= 16V 10 50 mA

PDL Switch Turn-On Threshold V

DCIN

- V

BATT

, V

DCIN

falling 50

150 mV

DCIN

- V

BATT

300 mV

0.0625 0.125 0.2500

0.0833 0.167 0.3330

0.25V < V

< 2.0V, 0.25V < V

CCV

< 2.0V,

CCI

150 300

PDS Switch Threshold Hysteresis

100 200

100

PDL Switch Threshold Hysteresis

100 200

100

LDO

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

6 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

CHARGE VOLTAGE REGULATION

VCTL Range 0 3.6 V

VCTL

= 3.6V (3 or 4 cells); not including

VCTL resistor tolerances

VCTL

= 3.6V/20 (3 or 4 cells); not including

VCTL resistor tolerances

VCTL

= 3.6V (3 or 4 cells); including VCTL

resistor tolerances of 1%

Battery Regulation Voltage Accuracy

VCTL

= V

LDO

(3 or 4 cells, default

threshold of 4.2V/cell)

VCTL

Default Threshold V

VCTL

rising 4.1 4.3 V

VCTL

= 3V 0 2.5

VCTL Input Bias Current

DCIN

= 0V, V

VCTL

= 5V

0 12

µA

CHARGE-CURRENT REGULATION

MAX1909 0 3.6

ICTL Range

MAX8725 0 3.2

CSIP-to-CSIN Full-Scale Current-

Sense Voltage

MAX1909: V

ICTL

= 3.6V (not including ICTL

resistor tolerances)

MAX8725: V

ICTL

= 3.2V (not including ICTL

resistor tolerances)

-5 +5

MAX1909: V

ICTL

= 3.6V x 0.5, MAX8725:

ICTL

= 3.2V x 0.5 (not including ICTL

resistor tolerances)

-5 +5

MAX1909: V

ICTL

= 0.9V (not including ICTL

resistor tolerances)

Charge-Current Accuracy

MAX8725: V

ICTL

= 0.18V (not including

ICTL resistor tolerances)

-30

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

PDL Turn-On Resistance PDL = GND 50

150 kΩ

PDL Turn-Off Current V

SRC

- V

PDL

= 1.5V 6 12 mA

SRC = 19V, DCIN = 0V 1

SRC Input Bias Current

SRC = 19, V

BATT

= 16V

µA

Delay Time Between PDL and

PDS Transitions

2.5 5 7.5 µs

100

450 1000

-0.8

-1.0

-0.8

69.37

-7.5

+0.8 +0.8 +1.0 +0.8

80.63 +7.5

+7.5

+30

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

_______________________________________________________________________________________ 7

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

MAX1909: V

ICTL

= 3.6V x 0.5, MAX8725:

ICTL

= 3.2V x 0.5 (including ICTL resistor

tolerances of 1%)

Charge-Current Accuracy

ICTL

= V

LDO (default threshold of 45mV)

-5 +5

ICTL

Default Threshold V

ICTL

rising 4.3 V

BATT/CSIP/CSIN Input Voltage

Range

0 19 V

CSIP/CSIN Input Current Charging enabled 650 µA

MAX1909

ICTL Power-Down Mode

Threshold Voltage

MAX8725

MAX1909

ICTL Power-Up Mode Threshold

Voltage

MAX8725

INPUT CURRENT REGULATION CSSP-to-CSSN Full-Scale

Current-Sense Voltage

CLS

= REF -3 +3

CLS

= REF x 0.75 -3 +3Input Current-Limit Accuracy

CLS

= REF x 0.5 -4 +4

8.0 28 V

CSSP/CSSN Input Current V

CSSP

= V

CSSN

= V

DCIN

> 8.0V 730 µA

CLS Input Range 1.6

IINP Transconductance V

CSSP

- V

CSSN

= 56mV 2.7 3.3

mA/V

CSSP

- V

CSSN

= 75mV, terminated with

10kΩ

CSSP

- V

CSSN

= 56mV, terminated with

10kΩ

-5 +5IINP Accuracy

CSSP

- V

CSSN

= 20mV, terminated with

10kΩ

-10

IINP Output Current V

CSSP

- V

CSSN

= 150mV, V

IINP

= 0V

µA

IINP Output Voltage V

CSSP

- V

CSSN

= 150mV, V

IINP

= float

3.5 V

SUPPLY AND LINEAR REGULATOR

DCIN Input Voltage Range V

DCIN

8.0 28 V DCIN falling 7

D C IN U nd er vol tag e- Lockout Tr i p P oi nt

DCIN rising

DCIN Quiescent Current I

DCIN

8.0V < V

DCIN

< 28V 6 mA

BATT Input Current I

BATT

= 2V to 19V, V

DCIN

> V

BATT

+ 0.3V 500 µA

LDO Output Voltage 8.0V < V

DCIN

< 28V, no load

LDO Load Regulation 0 < I

LDO

< 10mA 115 mV

-7.0

CSSP/CSSN Input Voltage Range

0.85

0.11

72.75

-7.5

350

5.25

+7.0

0.75

0.06

77.25

REF

+7.5

+10

7.85

5.55

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

8 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

LDO Undervoltage-Lockout Trip

Point

DCIN

= 8.0V

REFERENCE

REF Output Voltage Ref 0 < I

REF

< 500µA

REF Undervoltage-Lockout Trip

Point

REF falling 3.9 V

TRIP POINTS

BATT POWER_FAIL Threshold V

DCIN

- V

BATT

, V

DCIN

falling 50 150 mV

BATT POWER_FAIL Threshold

Hysteresis

300 mV

ACIN Threshold ACIN rising

ACIN Threshold Hysteresis 10 30 mV

SWITCHING REGULATOR

DHI Off-Time

440 ns

DHI Minimum Off-Time

350 ns

DLOV Supply Current I

DLOV

DLO low 10 µA

Sense Voltage for Battery

Undervoltage Charge Current

MAX1909 only, BATT = 3.0V per cell 3 6 mV

MAX1909 only, MODE = float (3 cell),

BATT

rising

Battery Undervoltage Threshold

MAX1909 only, MODE = LDO (4 cell),

BATT

rising

DHIV Output Voltage With respect to SRC

V DHIV Sink Current 10 mA DHI On-Resistance Low DHI = V

DHIV

, I

DHI

= -10mA 5 Ω

DHI On-Resistance High DHI = V

CSSN

, I

DHI

= 10mA 4 Ω

DLO On-Resistance High V

DLOV

= 4.5V, I

DLO

= +100mA 7 Ω

DLO On-Resistance Low V

DLOV

= 4.5V, I

DLO

= -100mA 3 Ω

ERROR AMPLIFIERS

V C TL = 3.6, V

BATT

= 16.8V , M OD E = LD O

GMV Loop Transconductance

V C TL = 3.6, V

BATT

= 12.6V , M OD E = FLOAT

mA/V

GMI Loop Transconductance

MAX1909: ICTL = 3.6V, MAX8725: V

ICTL

3.2V, V

CSSP

- V

CSIN

= 75mV

0.5 2.0

mA/V

GMS Loop Transconductance V

CLS

= 2.048V, V

CSSP

- V

CSSN

= 75mV 0.5 2.0

mA/V

CCI/CCS/CCV Clamp Voltage

0.25V < V

CCV

< 2.0V, 0.25V < V

CCI

< 2.0V,

0.25V < V

CCS

< 2.0V

600 mV

LOGIC LEVELS

MODE Input Low Voltage 0.8 V MODE Input Middle Voltage 1.6 2.0 V

3.20

4.1960

V V

100

2.007

= 16.0V, V

BATT

= 16.0V, V

BATT

DCIN

= 19V, V

= 17V, V

= 3.6V 360

MODE

= 3.6V 260

MODE

9.18

12.235

-4.5

0.0625

0.0833

150

5.15

4.2520

2.089

9.42

12.565

-5.5

0.2500

0.3330

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

_______________________________________________________________________________________ 9

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

BATT

= V

CSIP

= V

CSIN

= 12V, V

VCTL

= V

ICTL

= 1.8V, MODE = float, ACIN = 0, CLS =

REF, GND = PGND = 0, PKPRES = GND, LDO = DLOV, T

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

PARAMETER

CONDITIONS

UNITS

MODE Input High Voltage 2.8 V

ACOK AND PKPRES

ACOK Input Voltage Range 0 28 V ACOK Sink Current V

ACOK

= 0.4V, ACIN = 1.5V 1 mA

PKPRES Input Voltage Range 0

PKPRES Battery Removal Detect

Threshold

MAX8725, PKPRES rising 55

% of

LDO

PDS, PDL SWITCH CONTROL

PDS Switch Turn-Off Threshold V

DCIN

- V

BATT

, V

DCIN

falling 50 150 mV

P D S S w i tch Thr eshol d H yste r esi s V

DCIN

- V

BATT

300 mV

PDS Output Low Voltage, PDS

Below SRC

PDS

= 0A 8 12 V

PDS Turn-On Current PDS = SRC 6 mA PDS Turn-Off Current V

PDS

= V

SRC

- 2V, V

DCIN

= 16V 10 mA

PDL Switch Turn-On Threshold V

DCIN

- V

BATT

, V

DCIN

falling 50 150 mV

P D L S w i tch Thr eshol d H yster esi s V

DCIN

- V

BATT

300 mV PDL Turn-On Resistance PDL = GND 50 150 kΩ PDL Turn-Off Current V

SRC

- V

PDL

= 1.5V 6 mA

SRC Input Bias Current SRC = 19, V

BATT

= 16V

µA

Note 1: Guaranteed by design. Not production tested.

BATTERY INSERTION

AND REMOVAL RESPONSE

MAX1909/MAX8725 toc01

500μs/div

16V

17V

BATT

CCI

, V

CCV

5A/div

CCI

CCV

CCI

SYSTEM LOAD-TRANSIENT RESPONSE

MAX1909/MAX8725 toc02

100μs/div

BATT

SYSTEMLOAD

CCS

CCI

CCS

CCI

Typical Operating Characteristics

(Circuit of Figure 2, V

DCIN

= 20V, charge current = 3A, 4 Li+ series cells, TA= +25°C, unless otherwise noted.)

SYMBOL

MIN TYP MAX

100

LDO

1000

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

10 ______________________________________________________________________________________

LDO LOAD REGULATION

MAX1909/MAX8725 toc04

LDO CURRENT (mA)

LDO OUTPUT ERROR (%)

987654321

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

-1.4 010

LINE-TRANSIENT RESPONSE

MAX1909/MAX8725 toc03

500μs/div

1.8V

BATT

AC-COUPLED

200mV/div

INDUCTOR CURRENT 200mA/div

1.6V

20V

30V

DCIN

CCV

REF vs. TEMPERATURE

MAX1909/MAX8725 toc07

TEMPERATURE (°C)

REF OUTPUT ERROR (%)

603510-15

-0.15

-0.10

-0.05

0.05

0.10

-0.20

-40 85

EFFICIENCY vs. CHARGE CURRENT

MAX1909/MAX8725 toc08

CHARGE CURRENT (A)

EFFICIENCY (%)

2.52.01.51.00.5

100

03.0

4 CELLS

3 CELLS

LDO LINE REGULATION

MAX1909/MAX8725 toc05

INPUT VOLTAGE (V)

LDO OUTPUT ERROR (%)

2010

-0.05

0.05

0.10

-0.10 030

REF LOAD REGULATION

MAX1909/MAX8725 toc06

REF CURRENT (μA)

REF OUTPUT ERROR (%)

800600400200

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

-0.14 0 1000

Typical Operating Characteristics (continued)

(Circuit of Figure 2, V

DCIN

= 20V, charge current = 3A, 4 Li+ series cells, TA= +25°C, unless otherwise noted.)

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________

SWITCHING FREQUENCY vs. VIN - V

BATT

MAX1909/MAX8725 toc09

VIN - V

BATT

(V)

SWITCHING FREQUENCY (kHz)

8642

100

150

200

250

300

350

400

450

500

010

IINP ERROR vs. INPUT CURRENT

MAX1909/MAX8725 toc10

INPUT CURRENT (A)

IINP (%)

3.02.50.5 1.0 1.5 2.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

03.5

CHARGER DISABLED

-3

-1

-2

1.5 2.52.0 3.0 3.5

INPUT CURRENT-LIMIT ACCURACY vs. V

CLS

MAX1909/MAX8725 toc14

CLS

(V)

INPUT CURRENT-LIMIT ACCURACY (%)

-8

-4

-6

-2

1.5 3.0 3.52.0 2.5 4.0 4.5 5.0 5.5 6.0

IINP ACCURACY vs. INPUT CURRENT

MAX1909/MAX8725 toc11

INPUT CURRENT (A)

IINP ACCURACY (%)

-2

-1

0.5 1.51.0 2.0 2.5 3.0

INPUT CURRENT-LIMIT ACCURACY

vs. SYSTEM LOAD

MAX1909/MAX8725 toc12

SYSTEM LOAD (A)

INPUT CURRENT-LIMIT ACCURACY (%)

BATT

= 10V

BATT

= 13V

BATT

= 12V

BATT

= 16V

CHARGE

= 3A

MAX1909 ONLY

Typical Operating Characteristics (continued)

(Circuit of Figure 2, V

DCIN

= 20V, charge current = 3A, 4 Li+ series cells, TA= +25°C, unless otherwise noted.)

INPUT CURRENT-LIMIT ACCURACY

vs. SYSTEM LOAD

MAX1909/MAX8725 toc13

SYSTEM LOAD (A)

INPUT CURRENT-LIMIT ACCURACY (%)

3.02.50.5 1.0 1.5 2.0

-1

-2

03.5

BATT

= 16V V

BATT

= 12V

BATT

= 13V

BATT

= 10V

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

12 ______________________________________________________________________________________

Typical Operating Characteristics (continued)

(Circuit of Figure 2, V

DCIN

= 20V, charge current = 3A, 4 Li+ series cells, TA= +25°C, unless otherwise noted.)

PDL-PDS SWITCHING,

AC ADAPTER INSERTION

MAX1909/MAX8725 toc15

100μs/div

10V

20V

10V

20V

10V

20V

WALLADAPTER

SYSTEMLOAD

, V

PDS

PDL

, V

BATT

PDL

SYSTEM LOAD

PDL

PDS

PDS-PDL SWITCHOVER,

WALL ADAPTER REMOVAL

MAX1909/MAX8725 toc16

500μs/div

10V

20V

10V

20V

10V

20V

WALLADAPTER

SYSTEMLOAD

PDS

PDL

BATT

PDL

PDS-PDL SWITCHOVER,

BATTERY INSERTION

MAX1909/MAX8725 toc17

50μs/div

10V

15V

10V

15V

20V

SYSTEM

PDS

PDL

BATT

PKDET

CONDITIONING MODE WALL ADAPTER = 18V

PKPRES

PDL-PDS SWITCHING,

BATTERY REMOVAL

MAX1909/MAX8725 toc18

10μs/div

10V

15V

10V

15V

20V

SYSTEM

PDS

PDL

BATT

CONDITIONING MODE WALL ADAPTER = 18V

PKPRES

MAX8725 ONLY

MAX1909/MAX8725

Pin Description

PIN NAME FUNCTION

1 DCIN DC Supply Voltage Input. Bypa ss DCIN with a 1μF capacitor to power ground.

2 LDO Device Power Supply. Output of the 5.4V linear regulator supplied from DCIN. Bypass with a 1μF capacitor.

3 ACIN

AC Detect Input. This uncommitted comparator input can be used to detect the presence of the charger’s power source. The comparator’s open-drain output i s the ACOK signal.

4 REF 4.2235V Voltage Reference. Bypass with a 1μF capacitor to GND.

GND MAX1909: Ground thi s pin.

PKPRES MAX8725: Pull PKPRES high to disable charging. Used for detecting presence of battery pac k.

6 ACOK

AC Detect Output. High-voltage open-drain output is high impedance when ACIN is greater than 2.048V. The ACOK output remains a high impedance when the MAX1909/MAX8725 are powered down.

7 MODE

Trilevel Input for Setting Number of Cells and Asserting the Conditioning Mode: MODE = GND; asserts conditioning mode. MODE = float; charge with 3 times the cell vo ltage programmed at VCTL. MODE = LDO; charge with 4 times the cell voltage programmed at VCTL.

8 IINP

Input Current Mon itor Output. The current delivered at the IINP output is a scaled-down replica of the sy stem load current plus the input-referred charge current sensed across CSSP and CSSN inputs. The transconductance of (CSSP - CSSN) to IINP is 3mA/V.

9 CLS Source Current-Limit Input. Voltage input for setting the current limit of the input source.

10 ICTL Input for Setting Maximum Output Current

11 VCTL Input for Setting Ma ximum Output Voltage

12 CCI Output Current-Regulation Loop-Compensat ion Point. Connect 0.01μF to GND.

13 CCV Voltage-Regulation Loop-Compensation Point. Connect 10k in series with 0.1μF to GND.

14 CCS Input Current-Regulat ion Loop-Compensation Point. Use 0.01μF to GND.

15 GND Analog Ground

16 BATT Battery Voltage Feedback Input

17 CSIN Output Current-Sense Negative Input

18 CSIP Output Current-Sense Positive Input. Connect a current-sense resistor from CSIP to CSIN.

19 PGND Power Ground

20 DLO

Low-Side Power-MOSFET Driver Output. Connect to low-side NMOS gate. When the MAX1909/MAX8725 are shut down, the DLO output is low.

21 DLOV Low-Side Driver Supply. Bypas s w ith a 1μF capacitor to ground.

22 DHIV High-Side Dri ver Supply. Bypass with a 0.1μF capacitor to SRC.

23 DHI

High-Side Power-MOSFET Driver Output. Connect to high-side PMOS gate. When the MAX1909/MAX8725 are shut down, the DHI output i s

high.

24 SRC Source Connection for Driver for PDS/PDL Switches. Bypass SRC to power ground with a 1μF capacitor.

25 CSSN Input Current Sense for Charger (Negative Input)

26 CSSP Input Current Sense for Charger (Positive Input). Connect a current-sen se resistor from CSSP to CSSN.

27 PDS

Power-Source PMOS Switch Dri ver Output. When the MAX1909/MAX8725 are powered down, the PDS output is pulled to SRC through an internal 1M resistor.

28 PDL

System-Load PMOS Switch Driver Output. When the MAX1909/MAX8725 are powered down, the PDL output is p ul led to ground through an internal 100k resistor.

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 13

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

14 ______________________________________________________________________________________

CSSP CSSN

LDO

DHI

DLOV

DLO

PGND

CSIP

CSIN BATT

GND

DCIN

VCTL

ICTL

MODE

ACIN

ACOK

CLS

CCV CCI

CCS

REF

GND

HOST

SYSTEM

BATT +

TEMP BATT -

BATTERY

AC ADAPTER

590kΩ

196kΩ

C5 1μF

RS1

0.01Ω

R5 10kΩ

C11

0.1μF

C10

0.01μF

C12 1μF

C4 22μF

C16 1μF

C13 1μF

R13 33Ω

C1 22μF

GND

PGND

RS2

0.015Ω

SYSTEM LOAD

R8 1M

LDO

OUTPUT

(INPUT I LIMIT: 7.5A)

OUTPUT VOLTAGE: 12.6V

CHARGE I LIMIT: 3.0A

PDS

PDL

SRC

LDO

REF

DHIV

C17

0.1μF

100kΩ

SRC

C22 1μF

R9 10kΩ

MAX1909 MAX8725

PKPRES (MAX8725 ONLY)

L1 10μH

LDO

0.1μF

Figure 1. Typical Operating Circuit Demonstrating Hardwired Control

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 15

CSSP CSSN

LDO

DHI

DLOV

DLO

PGND

CSIP

CSIN BATT

GND

DCIN

VCTL

ICTL

ACIN

MODE

ACOK

IINP

CCV

CCI

CCS

REF

/REF

SCL

SDA

GND

HOST

BATT +

TEMP

SDA

SCL

BATT -

SMART

BATTERY

AC ADAPTER

590kΩ

196kΩ

C5 1μF

P3 P4

RS1

0.01Ω

10kΩ

C11

0.1μF

C10

0.01μF

C12 1μF

C4 22μF

C16 1μF

C13 1μF

R13 33Ω

C1 22μF

GND

PGND

RS2

0.015Ω

SYSTEM LOAD

R8 1MΩ

LDO

OUTPUTS

OUTPUT

INPUT

A/D INPUT

OPEN-DRAIN

OUTPUT VOLTAGE: 16.8V

PDS

PDL

SRC

LDO

CLSREF

DHIV

C17

0.1μF

SRC

C15 1μF

R21 10kΩ

MAX1909 MAX8725

C14

0.1μF

R9 10kΩ

R19, R20

10kΩ

(INPUT I LIMIT: 7.5A)

L1 10μH

D/A OUTPUT

0.1μF

LDO

PKPRES (MAX8725 ONLY)

Figure 2. Smart-Battery Charger Circuit Demonstrating Operation with a Host Microcontroller

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

16 ______________________________________________________________________________________

CHG

LOGIC

5.4V

LINEAR

REGULATOR

4.2235V

REFERENCE

LDO

DCIN

REF

ACOK

2.048V

IINP

DC-DC

CONVERTER

PDS

DHI

PDL

DRIVER

DLOV

DLO

PGND

LVC

BATT

MODE

VCTL

CSIP

CSIN

LEVEL

SHIFTER

LEVEL

SHIFTER

CSSP

CSSN

ICTL

CLS

SRDY

GND

GMV

GMI

GMS

CCS

CCI

CCV

CELL SELECT

LOGIC AND

BATTERY VOLTAGE-

DIVIDER

ACIN

0.9 * LDO

RDY

SRC

SWITCH LOGIC

REF

MODE

DHIV

DCIN

0.8V

BATT

PKPRES

3.0V/CELL

BATT_UV

ICTLOK

PACK_ON

CHG

SRC

MAX1909 MAX8725

SRC-10V

100k

MAX1909 ONLY

MAX8725 ONLY

Figure 3. Functional Diagram

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 17

Detailed Description

The MAX1909/MAX8725 include all of the functions necessary to charge Li+, NiMH, and NiCd batteries. A high-efficiency, synchronous-rectified step-down DCDC converter is used to implement a precision constant-current, constant-voltage charger with input current limiting. The DC-DC converter uses external p-channel/n-channel MOSFETs as the buck switch and synchronous rectifier to convert the input voltage to the required charge current and voltage. The charge current and input current-limit sense amplifiers have lowinput-referred offset errors and can use small-value sense resistors. The MAX1909/MAX8725 feature a voltage-regulation loop (CCV) and two current-regulation loops (CCI and CCS). The CCV voltage-regulation loop monitors BATT to ensure that its voltage never exceeds the voltage set by VCTL. The CCI battery current-regulation loop monitors current delivered to BATT to ensure that it never exceeds the current limit set by ICTL. A third loop (CCS) takes control and reduces the charge current when the sum of the system load and the inputreferred charge current exceeds the power source current limit set by CLS. Tying CLS to the reference voltage provides a 7.5A input current limit with a 10mΩ sense resistor.

The ICTL, VCTL, and CLS analog inputs set the charge current, charge voltage, and input current limit, respectively. For standard applications, internal set points for ICTL and VCTL provide a 3A charge current using a 15mΩ sense resistor and a 4.2V per-cell charge voltage. The variable for controlling the number of cells is set with the MODE input. The MAX8725 includes a PKPRES input used for battery-pack detection.

Based on the presence or absence of the AC adapter, the MAX1909/MAX8725 automatically provide an opendrain logic output signal ACOK and select the appropriate source for supplying power to the system. A p-channel load switch controlled from the PDL output and a similar p-channel source switch controlled from the PDS output are used to implement this function. Using the MODE control input, the MAX1909/MAX8725 can be programmed to perform a relearning, or conditioning, cycle in which the battery is isolated from the charger and completely discharged through the system load. When the battery reaches 100% depth of discharge, it is recharged to full capacity.

The circuit shown in Figure 1 demonstrates a simple hardwired application, while Figure 2 shows a typical application for smart-battery systems with variable charge current and source switch configuration that supports battery conditioning. Smart-battery systems typically use a host µC to achieve this added functionality.

Setting the Charge Voltage

The MAX1909/MAX8725 use a high-accuracy voltage regulator for charge voltage. The VCTL input adjusts the battery output voltage. In default mode (VCTL = LDO), the overall accuracy of the charge voltage is ±0.5%. VCTL is allowed to vary from 0 to 3.6V, which provides a 10% adjustment range of the battery voltage. Limiting the adjustment range reduces the sensitivity of the charge voltage to external resistor tolerances from ±1% to ±0.05%. The overall accuracy of the charge voltage is better than ±1% when using ±1% resistors to divide down the reference to establish VCTL. The per-cell battery termination voltage is a function of the battery chemistry and construction. Consult the battery manufacturer to determine this voltage. The battery voltage is calculated by the equation:

where V

REF

= 4.2235V, and CELL is the number of cells selected with the MAX1909/MAX8725s’ trilevel MODE control input. When MODE is tied to the LDO output, CELL = 4. When MODE is left floating, CELL = 3. When MODE is tied to ground, the charger enters conditioning mode, which is used to isolate the battery from the charger and discharge it through the system load. See the

Conditioning Mode

section. The internal error amplifier (GMV) maintains voltage regulation (see Figure 3 for the

Functional Diagram

). The voltage-error amplifier is compensated at CCV. The component values shown in Figures 1 and 2 provide suitable performance for most applications. Individual compensation of the voltage regulation and current-regulation loops allow for optimal compensation. See the

Compensation

section.

Setting the Charge Current

The voltage on the ICTL input sets the maximum voltage across current-sense resistor RS2, which in turn determines the charge current. The full-scale differential voltage between CSIP and CSIN is 75mV; thus, for a

0.015Ω sense resistor, the maximum charge current is 5A. In default mode (ICTL = LDO), the sense voltage is 45mV with an overall accuracy of ±5%. The charge current is programmed with ICTL using the equation:

CHG

ICTL

=×

0 075

236

V CELL V

BATT REF

VCTL

⎛ ⎝

⎜

⎞ ⎠

⎟

⎛ ⎝

⎜

⎞ ⎠

⎟

−18

952..

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

18 ______________________________________________________________________________________

The input range for ICTL is 0 to 3.6V on the MAX1909, and 0 to 3.2V on the MAX8725. The charger shuts down if ICTL is forced below 0.75V for the MAX1909 and 0.06V for the MAX8725. When choosing current-sense resistor RS2, note that it must have a sufficient power rating to handle the full-load current. The sense resistor’s I

R power loss reduces charger efficiency. Adjusting ICTL to drop the voltage across the current-sense resistor improves efficiency, but may degrade accuracy due to the current-sense amplifier’s input offset error. The charge-current error amplifier (GMI) is compensated at the CCI pin. See the

Compensation

section.

Conditioning Charge

The MAX1909 includes a battery voltage comparator that allows a conditioning charge of overdischarged Li+ battery packs. If the battery-pack voltage is less than 3.1V x the number of cells programmed by CELLS, the MAX1909 charges the battery with 300mA current when using sense resistor RS2 = 0.015Ω. After the battery voltage exceeds the conditioning charge threshold, the MAX1909 resumes full-charge mode, charging to the programmed voltage and current limits. The MAX8725 does not provide automatic support for providing a conditioning charge. To configure the MAX8725 to provide a conditioning charge current, ICTL should be directly driven.

Setting the Input Current Limit

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. The MAX1909/MAX8725 reduce the source current by decreasing the charge current when the input current exceeds the set input current limit. This technique does not truly limit the input current. As the system supply current rises, the available charge current drops proportionally to zero. Thereafter, the total input current can increase without limit.

An internal amplifier compares the differential voltage between CSSP and CSSN to a scaled voltage set with the CLS input. V

CLS

can be driven directly or set with a resistive voltage-divider between REF and GND. Connect CLS to REF to set the input current-limit sense voltage to the maximum value of 75mV. Calculate the input current as follows:

CLS

determines the reference voltage of the GMS error amplifier. Sense resistor RS1 sets the maximum allowable source current. Once the input current limit is reached, the charge current is decreased linearly until the input current is below the desired threshold.

Duty cycle affects the accuracy of the input current limit. AC load current also affects accuracy (see the

Typical Operating Characteristics

). Refer to the MAX1909/MAX8725 EV kit data sheet for more details on reducing the effects of switching noise.

When choosing the current-sense resistor RS1, carefully calculate its power rating. Take into account variations in the system’s load current and the overall accuracy of the sense amplifier. Note that the voltage drop across RS1 contributes additional power loss, which reduces efficiency.

System currents normally fluctuate as portions of the system are powered up or put to sleep. Without input current regulation, the input source must be able to deliver the maximum system current and the maximum charger input current. By using the input current-limit circuit, the output current capability of the AC wall adapter can be lowered, reducing system cost.

Current Measurement

The MAX1909/MAX8725 include an input current monitor IINP. The current delivered at the IINP output is a scaleddown replica of the system load current plus the inputreferred charge current that is sensed across CSSP and CSSN inputs. The output voltage range is 0 to 3V. The voltage of IINP is proportional to the input current according to the following equation:

IINP

= I

SOURCE

 RS1 G

IINP

 R

where I

SOURCE

is the DC current supplied by the AC

adapter power, G

IINP

is the transconductance of IINP

(3mA/V typ), and R9 is the resistor connected between IINP and ground.

Leave the IINP pin unconnected if not used.

LDO Regulator

LDO provides a 5.4V supply derived from DCIN and can deliver up to 10mA of extra load current. The lowside MOSFET driver is powered by DLOV, which must be connected to LDO as shown in Figure 1. LDO also supplies the 4.2235V reference (REF) and most of the control circuitry. Bypass LDO with a 1µF capacitor.

Shutdown and Charge Inhibit (

PKPRES

)

When the AC adapter is removed, the MAX1909/ MAX8725 shut down to a low-power state that does not significantly load the battery. Under these conditions, a maximum of 6µA is drawn from the battery through the combined load of the SRC, CSSP, CSSN, CSIP, CSIN, and BATT inputs. The charger enters this low-power state when DCIN falls below the undervoltage-lockout (UVLO) threshold of 7V. The PDS switch turns off, the PDL switch turns on, and the system runs from the battery.

RSVV

CLS

REF

=×

0 0751.

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 19

The body diode of the PDL switch prevents the voltage on the power source output from collapsing.

Charging can also be inhibited by driving ICTL below

0.035V, which suspends switching and pulls CCI, CCS, and CCV to ground. The PDS and PDL drivers, LDO, input current monitor, and control logic (ACOK) all remain active in this state. Approximately 3mA of supply current is drawn from the AC adapter and 3µA (max) is drawn from the battery to support these functions.

In smart-battery systems, PKPRES is usually driven from a voltage-divider formed with a low-value resistor or PTC thermistor inside the battery pack and a local resistive pullup. This arrangement automatically detects the presence of a battery. The MAX8725 threshold voltage is 55% of V

LDO

, with hysteresis of 1% V

LDO

to prevent erratic

transitions.

AC Adapter Detection and

Power-Source Selection

The MAX1909/MAX8725 include a hysteretic comparator that detects the presence of an AC power adapter and automatically delivers power to the system load from the appropriate available power source. When the adapter is present, the open-drain ACOK output becomes high impedance. The switch threshold at ACIN is 2.048V. Use a resistive voltage-divider from the adapter’s output to the ACIN pin to set the appropriate detection threshold. When charging, the battery is isolated from the system load with the p-channel PDL switch, which is biased off. When the adapter is absent, the drives to the switches change state in a fast breakbefore-make sequence. PDL begins to turn on 7.5µs after PDS begins to turn off.

The threshold for selecting between the PDL and PDS switches is set based on the voltage difference between the DCIN and the BATT pins. If this voltage difference drops below 100mV, the PDS is switched off and PDL is switched on. Under these conditions, the MAX1909/MAX8725 are completely powered down. The PDL switch is kept on with a 100kΩ pulldown resistor when the charger is powered down through ICTL or PKPRES, or when the AC adapter is removed.

The drivers for PDL and PDS are fully integrated. The positive bias inputs for the drivers connect to the SRC pin and the negative bias inputs connect to a negative regulator referenced to SRC. With this arrangement, the drivers can swing from SRC to approximately 10V below SRC.

Conditioning Mode

The MAX1909/MAX8725 can be programmed to perform a conditioning cycle to calibrate the battery’s fuel gauge. This cycle consists of isolating the battery from the charger and discharging it through the system load. When the battery reaches 100% depth of discharge, it is then recharged. Driving the MODE pin low places the MAX1909/MAX8725 in conditioning mode, which stops the charger from switching, turns the PDS switch off, and turns the PDL switch on.

To utilize the conditioning mode function, the configuration of the PDS switch must be changed to two sourceconnected FETs to prevent the AC adapter from supplying current to the system through the MOSFET’s body diode. See Figure 2. The SRC pin must be connected to the common source node of the back-to-back FETs to properly drive the MOSFETs.

It is essential to alert the user that the system is performing a conditioning cycle. If the user terminates the cycle prematurely, the battery can be discharged even though the system was running off the AC adapter for a substantial period of time. If the AC adapter is in fact removed during conditioning, the MAX1909/MAX8725 keep the PDL switch on and the charger remains off as it would in normal operation.

In the MAX8725, if the battery is removed during conditioning mode, the PKPRES control overrides conditioning mode. When MODE is grounded and PKPRES goes high, the PDS switch starts turning on within 7.5µs and the system is powered from the AC adapter.

In the MAX1909, disable conditioning mode before the battery is overdischarged or removed.

DC-DC Converter

The MAX1909/MAX8725 employ a buck regulator with a PMOS high-side switch and a low-side NMOS synchronous rectifier. The MAX1909/MAX8725 feature a pseudo-fixed-frequency, cycle-by-cycle current-mode control scheme. The off-time is dependent upon V

DCIN

BATT

, and a time constant, with a minimum t

OFF

of 300ns. The MAX1909/MAX8725 can also operate in discontinuous conduction for improved light-load efficiency. The operation of the DC-DC controller is determined by the following four comparators as shown in Figure 4:

• CCMP: Compares the control point (lowest voltage

clamp (LVC)) against the charge current (CSI). The high-side MOSFET on-time is terminated if the CCMP output is high.

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

20 ______________________________________________________________________________________

COMP

IMAX

IMIN

ZCMP

CSS 20X

DHI

DLO

GMS

GMILVC

CSI

20X

GMV

CLS

ICTL

VCTL

1.94V

0.15V

0.1V

LVC

AC ADAPTER

CSSP CSSN

DHI

DLO

CSIP

CSIN

BATT

CCSCCICCV

TOFF

CCV

CCV CCI CCS

OUT

MAX1909 MAX8725

Figure 4. DC-DC Converter Functional Diagram

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 21

• IMIN: Compares the control point (LVC) against

0.15V (typ). If IMIN output is low, then a new cycle cannot begin. This comparator determines whether the regulator operates in discontinuous mode.

• IMAX: Compares the charge current (CSI) to the internally fixed cycle-by-cycle current limit. The current-sense voltage limit is 97mV. With RS2 =

0.015Ω, this corresponds to 6A. The high-side MOSFET on-time is terminated if the IMAX output is high and a new cycle cannot begin until IMAX goes low. IMAX protects against sudden overcurrent faults.

• ZCMP: Compares the charge current (CSI) to 333mA (RS2 = 0.015Ω). The current-sense voltage threshold is 5mV. If ZCMP output is high, then both MOSFETs are turned off. The ZCMP comparator terminates the switch on-time in discontinuous mode.

CCV, CCI, CCS, and LVC Control Blocks

The MAX1909/MAX8725 control charge voltage (CCV control loop), charge current (CCI control loop), or input current (CCS control loop), depending on the operating conditions. The three control loops, CCV, CCI, and CCS, are brought together internally at the LVC amplifier. The output of the LVC amplifier is the feedback control signal for the DC-DC controller. The minimum voltage at CCV, CCI, or CCS appears at the output of the LVC amplifier and clamps the other two control loops to within 0.3V above the control point. 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 battery current loading, the MAX1909/ MAX8725s’ inductor current never reaches zero, which is defined as continuous conduction mode. If the BATT voltage is within the following range:

3.1V  (number of cells) < V

BATT

< (0.88  V

DCIN

)

the regulator is not in dropout and switches at f

NOM

= 400kHz. The controller starts a new cycle by turning on the high-side p-channel MOSFET and turning off the low-side n-channel MOSFET. When the charge current is greater than the control point (LVC), CCMP goes high and the off-time is started. The off-time turns off the high-side p-channel MOSFET and turns on the low-side n-channel MOSFET. The operating frequency is governed by the off-time and is dependent upon V

DCIN

and V

BATT

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

where f

NOM

= 400kHz:

These equations describe the controller’s pseudo-fixedfrequency performance over the most common operating conditions.

At the end of the fixed off-time, the controller can initiate a new cycle if the control point (LVC) is greater than

0.15V (IMIN = high) and the peak charge current is less than the cycle-by-cycle limit (IMAX = low). If the charge current exceeds I

MAX

, the on-time is terminated by the

IMAX comparator.

If during the off-time the inductor current goes to zero, ZCMP = high, both the high- and low-side MOSFETs are turned off until another cycle is ready to begin. This condition is discontinuous conduction. See the

Discontinuous Conduction

section.

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

DCIN

BATT

) differential becomes too small. If V

BATT

≥ 0.88 x

DCIN

, then the threshold for minimum off-time is

reached and the t

OFF

is fixed at 0.3µs. The switching

frequency in this mode varies according to the equation:

Discontinuous Conduction

The MAX1909/MAX8725 enter discontinuous-conduction mode when the output of the LVC control point falls below 0.15V. For RS2 = 0.015Ω, this corresponds to

0.5A:

MIN

015

20 2

05..

OFF

BATT

CSSN BATT

−

⎛ ⎝

⎜

⎞ ⎠

⎟

ON OFF

where I

RIPPLE

BATT OFF

RIPPLE

CSSN BATT

−

OFF

NOM

CSSN BATT

CSSN

−1

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

22 ______________________________________________________________________________________

In discontinuous mode, a new cycle is not started until the LVC voltage rises above 0.15V. 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.

Compensation

The charge voltage, charge current, and input currentlimit regulation loops are compensated separately and independently at the CCV, CCI, and CCS pins.

CCV Loop Compensation

The simplified schematic in Figure 5 is sufficient to describe the operation of the MAX1909/MAX8725 when the voltage loop (CCV) is in control. The required compensation network is a pole-zero pair formed with C

and RCV. The pole is necessary to roll off the voltage loop’s response at low frequency. 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= ΔV

BATT

ΔI

CHG

. The equivalent output impedance of the GMV

amplifier, R

OGMV

, is greater than 10MΩ. The voltage

loop transconductance (GMV = I

CCV

/ V

BATT

) depends on the MODE input, which determines the number of cells. GMV = 0.125mA/mV for 4 cells and GMV =

0.167mA/mV for 3 cells. The DC-DC converter transconductance is dependent upon the charge current-sense resistor RS2:

where A

CSI

= 20, and RS2 = 0.015Ω in the

Typical

Operating Circuits

(Figures 1 and 2), so GM

OUT

3.33A/V.

The loop transfer function is:

LTF GM

RsCR

sC R

GsCR

OUT

OGMV CV CV

CV OGMV

OUT L

MV OUT ESR

=×

×+ ×

()

+×

()

+×

()

+×

()

ARS

OUT

CSI

×12

OUT

ESR

OGMV

CCV

BATT

GMV

REF

OUT

Figure 5. CCV Loop Diagram

NAME CALCULATION DESCRIPTION

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

OGMV

is very large and not well controlled, the exact value for the pole frequency is also not well controlled (R

OGMV

> 10MΩ).

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

to place this zero at least one

decade below crossover to ensure adequate phase margin.

Output pole formed with the effective load resistance RL 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 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 1. Poles and Zeros of the Voltage-Loop Transfer Function

PCV

OGMV CV

2π

ZCV

CV CV

2π

P OUT

L OUT

2π

Z OUT

ESR OUT

2π

NO.

CCV pole

CCV zero

Output pole

Output zero

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

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______________________________________________________________________________________ 23

The poles and zeros of the voltage-loop transfer function are listed from lowest frequency to highest frequency in Table 1.

Near crossover, C

has a much lower impedance

than R

OGMV

. Since CCVis in parallel with R

OGMV, CCV

dominates the parallel impedance near crossover. Additionally, RCVhas a much higher impedance than CCVand dominates the series combination of RCVand C

, so:

OUT

also has a 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 the LTF = 1 to solve for the unity-gain frequency yields:

For stability, choose a crossover frequency lower than 1/10th of the switching frequency. Choosing a crossover frequency of 30kHz and solving for R

using the component values listed in Figure 1 yields:

MODE = VCC(4 cells)

GMV = 0.125µA/mV

OUT

= 22µF

BATT

= 16.8V

RL= 0.2Ω

OUT

= 3.33A/V

CO_CV

= 30kHz

OSC

= 400kHz

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

Z_CV

≤ f

P_OUT

CCV≥ (RL/RCV) C

OUT

where CCV≥ 4nF (assuming 4 cells and 4A maximum charge current).

Figure 6 shows the Bode plot of the voltage-loop frequency response using the values calculated above.

CCI Loop Compensation

The simplified schematic in Figure 7 is sufficient to describe the operation of the MAX1909/MAX8725 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 single pole is required to compensate this loop. A

CSI

is the internal gain of the current-sense amplifier. RS2 is the charge current-sense resistor, RS2 = 15mΩ. 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 = 3.3A/V.

The loop transfer function is given by:

LTF GM A RS GMI

sR C

OUT CSI

OGMI

OGMI CI

=×××

+×

GMV GM

OUT CO CV

OUT

××

=210

CO CV GM GMV

OUT

⎛ ⎝

⎜

⎞ ⎠

⎟

2π

LTF GM

GMV

OUT

=×

sC R sC

OUT L OUT

11+×

()

≅

RsCR

sC R

OGMV CV CV

CV OGMV

×+ ×

()

+×

()

≅

FREQUENCY (Hz)

MAGNITUDE (dB)

PHASE (DEGREES)

100k10k1k100101

-20

-40

-90

-45

-135

0.1 1M

MAG PHASE

Figure 6. CCV Loop Response

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

24 ______________________________________________________________________________________

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 1/10th of the switching frequency:

= GMI / (2π f

O_CI

)

Choosing a crossover frequency of 30kHz and using the component values listed in Figure 1 yields CCI> 5.4nF. Values for CCIgreater than 10 times the minimum value may slow down the current-loop response excessively. Figure 8 shows the Bode plot of the current-loop frequency response using the values calculated above.

CCS Loop Compensation

The simplified schematic in Figure 9 is sufficient to describe the operation of the MAX1909/MAX8725 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 amplifier. RS1 is the input current-sense resistor; RS1 = 10mΩ in the typical operating circuits. R

OGMS

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

OUT

= (1/D)

3.3A/V.

GMI

CO CICI_

2π

LTF GMI

sR C

OGMI

OGMI CI

+×1

ARS

OUT

CSI

×12

FREQUENCY (Hz)

MAGNITUDE (dB)

100k1k10

-20

100

-40

-45

-90

0.1

MAG PHASE

Figure 8. CCI Loop Response

OGMS

GMS

CSS

CLS

CCS

CSSP

RS1

CSSN

SYSTEM

LOAD

ADAPTER

INPUT

Figure 9. CCS Loop Diagram

OGMI

CCI

GMI

CSI

ICTL

OUT

CSIP

RS2

CSIN

Figure 7. CCI Loop Diagram

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 25

The loop transfer function is given by:

Since:

the loop transfer function simplifies to:

The crossover frequency is given by:

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

CCS= GMS / (2π f

CO_CS

)

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

5.4nF. Values for CCSgreater than 10 times the minimum value may slow down the input current-loop response excessively. Figure 10 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 moderately-sized power MOSFETs. The MOSFET drive capability is the same for both the low-side and highside 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. An adaptive dead-time circuit monitors the DLO output and prevents the high-side FET from turning on until DLO is fully off. There must be a low-resistance, low-inductance path from the DLO driver to the MOSFET gate for the adaptive dead-time circuit to work properly. Otherwise, the sense circuitry in the MAX1909/MAX8725 interpret the MOSFET gate as “off” while there is still charge left on the gate. Use very short, wide traces measuring 10 squares 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 fixed-delay 50ns time to prevent the low-side FET from turning on until DHI is fully off. The same layout considerations should be used for routing the DHI signal to the high-side FET.

Since the transition time for a p-channel switch can be much longer than an n-channel switch, the dead time prior to the high-side PMOS turning on is more pronounced than in other synchronous step-down regulators, which use high-side n-channel switches. On the high-to-low transition, the voltage on the inductor’s “switched” terminal flies below ground until the low-side switch turns on. A similar dead-time spike occurs on the opposite low-to-high transition. Depending upon the magnitude of the load current, these spikes usually have a minor impact on efficiency.

The high-side driver (DHI) swings from SRC to 5V below SRC and typically sources 0.9A and sinks 0.5A from the gate of the p-channel FET. The internal pullhigh transistors that drive DHI high are robust, with a

2.0Ω (typ) on-resistance.

The low-side driver (DLO) swings from DLOV to ground and typically sources 0.5A and sinks 0.9A from the gate of the n-channel FET. The internal pulldown transistors that drive DLO low are robust, with a 1.0Ω (typ) onresistance. 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 FETs that can be used. Using a low-side FET with smaller gate-to-drain capacitance can prevent these problems.

GMS

CO CSCS_

2π

LTF GMS

sR C

OGMS

OGMS CS

+×1

ARS

CSS

×11

LTF GM A RS GMS

sR C

IN CSS

OGMS

OGMS CS

=×××

+×

FREQUENCY (Hz)

MAGNITUDE (dB)

100k 10M1k10

-20

100

-40

-45

-90

0.1

MAG PHASE

PHASE (DEGREES)

Figure 10. CCS Loop Response

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

26 ______________________________________________________________________________________

Design Procedure

Table 2 lists the recommended components and refers to the circuit of Figure 2. The following sections describe how to select these components.

MOSFET Selection

MOSFETs P2 and P3 (Figure 1) provide power to the system load when the AC adapter is inserted. These devices may have modest switching speeds, but must be able to deliver the maximum input current as set by RS1. As always, care should be taken not to exceed the device’s maximum voltage ratings or the maximum operating temperature.

The p-channel/n-channel MOSFETs (P1, N1) are the switching devices for the buck controller. The guidelines

for these devices focus on the challenge of obtaining high load-current capability when using high-voltage (>20V) AC adapters. Low-current applications usually require less attention. The high-side MOSFET (P1) must be able to dissipate the resistive losses plus the switching losses at both V

DCIN(MIN)

and V

DCIN(MAX)

Ideally, the losses at V

DCIN(MIN)

should be roughly equal

to losses at V

DCIN(MAX)

, with lower losses in between. If

the losses at V

DCIN(MIN)

are significantly higher than the

losses at V

DCIN(MAX)

, consider increasing the size of P1.

Conversely, if the losses at V

DCIN(MAX)

are significantly

higher than the losses at V

DCIN(MIN),

consider reducing the size of P1. If DCIN does not vary over a wide range, the minimum power dissipation occurs where the resistive losses equal the switching losses.

DESCRIPTION

C1, C4

22µF ±20%, 35V E-size low-ESR tantalum capacitors AVX TPSE226M035R0300 Kemet T495X226M035AS

C5, C15

1µF ±10%, 25V, X7R ceramic capacitors (1206) Murata GRM31MR71E105K Taiyo Yuden TMK316BJ105KL TDK C3216X7R1E105K

C9, C10

0.01µF ±10%, 25V, X7R ceramic capacitors (0402) Murata GRP155R71E103K TDK C1005X7R1E103K

C11, C14,

C17

0.1µF ±10%, 25V, X7R ceramic capacitors (0603) Murata GRM188R71E104K TDK C1608X7R1E104K

C12, C13,

C16

1µF ±10%, 6.3V, X5R ceramic capacitors (0603) Murata GRM188R60J105K Taiyo Yuden JMK107BJ105KA TDK C1608X5R1A105K

Schottky diode, 0.5A, 30V SOD-123 Diodes Inc. B0530W General Semiconductor MBR0530 ON Semiconductor MBR0530

25V ±1% zener diode CMDZ5253B

10µH, 4.4A inductor Sumida CDRH104R-100NC TOKO 919AS-100M

Table 2. Recommended Components

REFERENCE

QTY

DESCRIPTION

N1/P1

Dual n- and p-channel MOSFETs, 7A, 30V and -5A, -30V, 8-pin SO, MOSFET Fairchild FDS8958A or Single n-channel MOSFETs, +13.5A, +30V FDS6670S and Single p-channel MOSFETs, -13.5A,

-30V FDS66709Z

Single, p-channel, -11A, -30V, 8-pin SO MOSFETs Fairchild FDS6675

100kΩ, ±5% resistor (0603) 10kΩ ±1% resistors (0603)

590kΩ ±1% resistor (0603)

196kΩ ±1% resistor (0603)

1MΩ ±5% resistor (0603)

R11

1kΩ ±5% resistor (0603)

R16

33Ω ±5% resistor (0603)

R19, R20

10kΩ ±5% resistors (0603)

RS1

0.01Ω ±1%, 0.5W sense resistor (2010) Vishay Dale WSL2010 0.010 1.0% IRC LRC-LR2010-01-R010-F

RS2

0.015Ω ±1%, 0.5W sense resistor (2010)

Vishay Dale WSL2010 0.015 1.0% IRC LRC-LR2010-01-R015-F

MAX1909ETI/MAX8725ETI (28-pin thin QFN-EP)

REFERENCE QTY

P2, P3, P4 3

R5, R9, R21 2

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

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______________________________________________________________________________________ 27

Choose a low-side MOSFET that has the lowest possible on-resistance (R

DS(ON)

), comes in a moderatesized package, and is reasonably priced. Make sure that the DLO gate driver can supply sufficient current to support the gate charge and the current injected into the parasitic gate-to-drain capacitor caused by the high-side MOSFET turning on; otherwise, cross-conduction problems can occur.

The MAX1909/MAX8725 have an adaptive dead-time circuit that prevents the high-side and low-side MOSFETs from conducting at the same time (see the

MOSFET

Drivers

section). Even with this protection, it is still possible for delays internal to the MOSFET to prevent one MOSFET from turning off when the other is turned on.

Select devices that have low turn-off times. To be conservative, make sure that P1(t

DOFF(MAX)

) -

N1(t

DON(MIN)

) < 40ns. Failure to do so may result in efficiency-killing shoot-through currents. If delay mismatch causes shoot-through currents, consider adding extra capacitance from gate to source on N1 to slow down its turn-on time.

MOSFET Power Dissipation

Worst-case conduction losses occur at the duty factor extremes. For the high-side MOSFET, the worst-case power dissipation (PD) due to resistance occurs at the minimum supply voltage:

Generally, a small high-side MOSFET is desired to reduce switching losses at high input voltages. 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 (I2R

DS(ON)

) losses. High-side switching losses do not usually become an issue until the input is greater than approximately 15V. Switching losses in the high-side MOSFET can become an insidious heat problem when maximum AC adapter voltages are applied, due to the squared term in the CV2f switching-loss equation. If the highside MOSFET that was chosen for adequate R

DS(ON)

at low supply voltages becomes extraordinarily hot when subjected to V

DCIN(MAX),

then choose a MOSFET with lower losses. Calculating the power dissipation in P1 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 only a very rough estimate and is no substitute for breadboard evaluation, preferably including a verification using a thermocouple mounted on P1:

where C

RSS

is the reverse transfer capacitance of P1,

and I

GATE

is the peak gate-drive source/sink current.

For the low-side MOSFET (N1), the worst-case power dissipation always occurs at maximum input voltage:

Choose a Schottky diode (D1, Figure 2) with a forward voltage low enough to prevent the N1 MOSFET body diode from turning on during the dead time. As a general rule, a diode with a DC current rating equal to 1/3rd the load current is sufficient. This diode is optional and can be removed if efficiency is not critical.

Inductor Selection

The charge current, ripple, and operating frequency (off-time) determine the inductor characteristics. Inductor L1 must have a saturation current rating of at least the maximum charge current plus 1/2 of the ripple current (ΔIL):

SAT

= I

CHG

+ (1/2) ΔIL

PD N

V V

BATT

DCIN

LOAD

DS ON

()

⎛ ⎝

⎜

⎞ ⎠

⎟

⎡

⎣

⎢ ⎢

⎤

⎦

⎥ ⎥

⎛ ⎝

⎜

⎞ ⎠

⎟

−

PD P Switching

VCfI

DCIN MAX RSS SW LOAD

GATE

(_ )

()

×××

PD P

V V

BATT

DCIN

LOAD

DS ON

()

⎛ ⎝

⎜

⎞ ⎠

⎟

⎛ ⎝

⎜

⎞ ⎠

⎟

1.0

0.5

1.5

8 101112139 1415161718

BATT

(V)

RIPPLE CURRENT (A)

DCIN

= 19V

VCTL = ICTL = LDO

3 CELLS

4 CELLS

Figure 11. Ripple Current vs. Battery Voltage (MAX1909)

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

28 ______________________________________________________________________________________

The ripple current is determined by:

ΔIL = V

BATTtOFF

/ L

where:

OFF

= 2.5µs (V

DCIN

- V

BATT

) / V

DCIN

for

BATT

< 0.88 V

DCIN

or:

OFF

= 0.3µs for V

BATT

> 0.88 V

DCIN

Figure 11 illustrates the variation of the ripple current vs. battery voltage when the circuit is charging at 3A with a fixed input voltage of 19V.

Higher inductor values decrease the ripple current. Smaller inductor values require high-saturation current capabilities and degrade efficiency. Designs that set LIR = ΔIL / I

CHG

= 0.3 usually result in a good balance

between inductor size and efficiency.

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  V

BATT

, which equates to 0.5  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 the 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

Startup Conditioning Charge for

Overdischarged Cells

It is desirable to charge deeply discharged Li+ batteries at a low rate to improve cycle life. The MAX1909/MAX8725 automatically reduces the charge current when the voltage per cell is below 3.1V. The charge current-sense voltage is set to 4.5mV (I

CHG

= 300mA with RS2 = 15mΩ) until the battery voltage rises above the threshold. There is approximately 300mV for 3 cell, 400mV for 4 cell of hysteresis to prevent the charge-current magnitude from chattering between the two values.

For the MAX8725, control the ICTL voltage to set a conditioning charge rate.

Layout and Bypassing

Bypass DCIN with a 1µF capacitor to ground (Figure 1). D4 protects the MAX1909/MAX8725 when the DC power source input is reversed. A signal diode for D4 is adequate because DCIN only powers the LDO and the internal reference. Bypass LDO, DHIV, DLOV, and other pins as shown in Figure 1.

Good PC board layout is required to achieve specified noise, 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 MAX1909/MAX8725 evaluation kit for examples. A ground plane is essential for optimum performance. In most applications, the circuit is 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 an uninterrupted ground plane.

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-current

paths.

c) Minimize other trace lengths in the high-current

paths.

d) Use >5mm wide traces.

VV V

RMS CHG

BATT DCIN BATT

DCIN

⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟

−()

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 29

e) Connect C1 and C2 to the high-side MOSFET

(10mm max length). Return these capacitors to the power ground plane.

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

ode, inductor (15mm max length)).

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 ground plane is connected to the normal inner-layer ground plane at the output ground terminals, which ensures that the IC’s analog ground is sensing at the supply’s output terminals without interference from IR drops and ground noise. Other high-current paths should also be minimized, but focusing primarily on short ground and current-sense connections eliminates about 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 should be less than

10mm from the current-sense resistors.

Quiet connections to REF, VCTL, ICTL, CCV, CCI, CCS, IINP, ACIN, and DCIN should be returned to a separate ground (GND) island. The appropriate traces are marked on the schematic with the ground symbol ( ). 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.

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. See Figure 12.

Chip Information

TRANSISTOR COUNT: 2720

PROCESS: BiCMOS

INDUCTOR

OUT

INPUT

OUTPUT

KELVIN-SENSE VIAS

UNDER THE SENSE

RESISTOR

(REFER TO EVALUATION KIT)

GND

PGND

POWER PATH

QUIET GROUND

ISLAND

Figure 12. PC Board Layout Examples

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic System Power Selector

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.

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

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

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

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

QFN THIN.EPS

MAXIM MAX1909, MAX8725 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual