Maxim Integrated MAX1909, MAX8725 Schematic

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.

Applications

Notebook and Subnotebook Computers
Hand-Held Data Terminals

Features

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

Charge Voltage

♦ Monitor Outputs for

Current Drawn from AC Input Source
AC Adapter Presence

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

Lead Acid, etc.

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

19-2805; Rev 1; 4/04

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

28 27 26 25 24 23 22

891011 12 13 14

15

16

17

18

19

20

21

7

6

5

4

3

2

1

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

P3

0.01Ω

10µH

N1

P1

0.015Ω

TO

EXTERNAL LOAD

LDO

PDS

PDL

SRC

LDO

REF

IINP

IINP

DHIV

SRC

P2

MAX1909
MAX8725

PKPRES

MAX8725 ONLY

Minimum Operating Circuit

Functional Diagrams appear at end of data sheet.

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, TA= 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

A

= +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

V

VCTL

= 3.6V (3 or 4 cells);

not including VCTL resistor tolerances
V

VCTL

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

VCTL resistor tolerances
V

VCTL

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

resistor tolerances of 1%

Battery Regulation Voltage
Accuracy

V

VCTL

= V

LDO

(3 or 4 cells, default

threshold of 4.2V/cell)

%

V

VCTL

Default Threshold V

VCTL

rising 4.1 4.3 V

V

VCTL

= 3V 0 2.5

VCTL Input Bias Current

V

DCIN

= 0, V

VCTL

= 5V 0 12

µA

CHARGE-CURRENT REGULATION

MAX1909 0 3.6

ICTL Range

MAX8725 0 3.2

V

CSIP-to-CSIN Full-Scale Current-

Sense Voltage

mV

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:

V

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

-0.8

-1.0

-0.5

69.37 75.00 80.63

-7.5

-7.5

+0.8

+0.8

+1.0

+0.5

+7.5

+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

A

= 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:

V

ICTL

= 3.2V x 0.5 (including ICTL resistor

tolerances of 1%)

Charge-Current Accuracy

V

ICTL

= V

LDO (default threshold of 45mV)

-5 +5

%

V

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

V

MAX1909

ICTL Power-Up Mode Threshold

Voltage

MAX8725

V

V

ICTL

= 3V -1 +1

ICTL Input Bias Current

V

DCIN

= 0V, V

ICTL

= 5V

-1 +1

µA

INPUT CURRENT REGULATION
CSSP-to-CSSN Full-Scale

Current-Sense Voltage

mV

V

CLS

= REF -3 +3

V

CLS

= REF x 0.75 -3 +3

Input Current-Limit
Accuracy

V

CLS

= REF x 0.5 -4 +4

%

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

8.0 28 V
V

CSSP

= V

CSSN

= V

DCIN

> 8.0V

730

CSSP/CSSN Input Current

V

DCIN

= 0 0.1 1

µA

CLS Input Range 1.6

V

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

V

CSSP

- V

CSSN

= 75mV, terminated with

10kΩ

V

CSSP

- V

CSSN

= 56mV, terminated with

10kΩ

-5 +5IINP Accuracy

V

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

Charging disabled; V

DCIN

= 0 or V

= 0

ICTL

-7.0

350

0.85

0.11

72.75 75.00 77.25

450

-7.5

350

+7.0

0.75

0.06

REF

+7.5

+10

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

A

= 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

V

DCIN Quiescent Current I

DCIN

8.0V < V

DCIN

< 28V 2.7 6 mA

V

BATT

= 19V, V

DCIN

= 0V, or ICTL = 0V 0.1 1

V

BATT

= 16.8V, V

DCIN

= 19V, ICTL = 0V 0.1 1

BATT Input Current I

BATT

V

BATT

= 2V to 19V, V

DCIN

> V

BATT

+ 0.3V

500

µA

LDO Output Voltage 8.0V < V

DCIN

< 28V, no load

5.4

V

LDO Load Regulation 0 < I

LDO

< 10mA 80 115 mV

LDO Undervoltage-Lockout Trip

Point

V

DCIN

= 8.0V

4

V

REFERENCE

REF Output Voltage Ref 0 < I

REF

< 500µA

V

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),

V

BATT

rising

Battery Undervoltage Threshold

MAX1909 only, MODE = LDO (4 cell),

V

BATT

rising

V

DHIV Output Voltage With respect to SRC

V

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

A

= 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

0

V

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

V

DCIN

- V

BATT

300 mV

PDS Output Low Voltage, PDS

Below SRC

I

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

V

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

A

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

CHARGE VOLTAGE REGULATION

VCTL Range 0 3.6 V

V

VCTL

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

VCTL resistor tolerances
V

VCTL

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

VCTL resistor tolerances
V

VCTL

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

resistor tolerances of 1%

Battery Regulation Voltage
Accuracy

V

VCTL

= V

LDO

(3 or 4 cells, default

threshold of 4.2V/cell)

%

V

VCTL

Default Threshold V

VCTL

rising 4.1 4.3 V

V

VCTL

= 3V 0 2.5

VCTL Input Bias Current

V

DCIN

= 0V, V

VCTL

= 5V

0 12

µA

CHARGE-CURRENT REGULATION

MAX1909 0 3.6

ICTL Range

MAX8725 0 3.2

V

CSIP-to-CSIN Full-Scale Current-

Sense Voltage

mV

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:

V

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

A

= 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

-0.8

-1.0

-0.8

69.37

-7.5

-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

A

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

MAX1909: V

ICTL

= 3.6V x 0.5, MAX8725:

V

ICTL

= 3.2V x 0.5 (including ICTL resistor

tolerances of 1%)

Charge-Current Accuracy

V

ICTL

= V

LDO (default threshold of 45mV)

-5 +5

%

V

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

V

MAX1909

ICTL Power-Up Mode Threshold

Voltage

MAX8725

V

INPUT CURRENT REGULATION
CSSP-to-CSSN Full-Scale

Current-Sense Voltage

mV

V

CLS

= REF -3 +3

V

CLS

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

V

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

V

IINP Transconductance V

CSSP

- V

CSSN

= 56mV 2.7 3.3

mA/V

V

CSSP

- V

CSSN

= 75mV, terminated with

10kΩ

V

CSSP

- V

CSSN

= 56mV, terminated with

10kΩ

-5 +5IINP Accuracy

V

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

V

DCIN Quiescent Current I

DCIN

8.0V < V

DCIN

< 28V 6 mA

BATT Input Current I

BATT

V

BATT

= 2V to 19V, V

DCIN

> V

BATT

+ 0.3V 500 µA

LDO Output Voltage 8.0V < V

DCIN

< 28V, no load

V

LDO Load Regulation 0 < I

LDO

< 10mA 115 mV

-7.0

0.85

0.11

72.75

CSSP/CSSN Input Voltage Range

-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

A

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

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

LDO Undervoltage-Lockout Trip

Point

V

DCIN

= 8.0V

V

REFERENCE

REF Output Voltage Ref 0 < I

REF

< 500µA

V

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

V

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),

V

BATT

rising

Battery Undervoltage Threshold

MAX1909 only, MODE = LDO (4 cell),

V

BATT

rising

V

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
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

A

= -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

V
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 yster esi s V

DCIN

- V

BATT

300 mV

PDS Output Low Voltage, PDS

Below SRC

I

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

1V

0V

2V

3V

16V

17V

0A

0A

I

IN

I

BATT

V

BATT

V

CCI

, V

CCV

5A/div

5A/div

V

CCI

V

CCV

V

CCV

V

CCV

V

CCI

V

CCI

SYSTEM LOAD-TRANSIENT RESPONSE

MAX1909/MAX8725 toc02

100µs/div

1V

0V

2V

3V

5A

5A

0A

5A

0A

0A

I

BATT

I

IN

I

SYSTEMLOAD

V

CCS

V

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

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

0

-1.4
010

LINE-TRANSIENT RESPONSE

MAX1909/MAX8725 toc03

500µs/div

1.8V

V

BATT

AC-COUPLED

200mV/div

INDUCTOR CURRENT
200mA/div

1.6V

3A

20V

30V

V

DCIN

V

CCV

REF vs. TEMPERATURE

MAX1909/MAX8725 toc07

TEMPERATURE (°C)

REF OUTPUT ERROR (%)

603510-15

-0.15

-0.10

-0.05

0

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

82

84

86

88

90

92

94

96

98

100

80

0 3.0

4 CELLS

3 CELLS

LDO LINE REGULATION

MAX1909/MAX8725 toc05

INPUT VOLTAGE (V)

LDO OUTPUT ERROR (%)

2010

-0.05

0

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

-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

______________________________________________________________________________________ 11

SWITCHING FREQUENCY vs. VIN - V

BATT

MAX1909/MAX8725 toc09

VIN - V

BATT

(V)

SWITCHING FREQUENCY (kHz)

8642

50

100

150

200

250

300

350

400

450

500

0

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

0

0 3.5

CHARGER

DISABLED

-3

-1

-2

1

0

2

3

1.5 2.52.0 3.0 3.5

INPUT CURRENT-LIMIT ACCURACY vs. V

CLS

MAX1909/MAX8725 toc14

V

CLS

(V)

INPUT CURRENT-LIMIT ACCURACY (%)

-8

-4

-6

2

0

-2

6

4

8

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

0

-1

2

1

3

4

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 (%)

V

BATT

= 10V

V

BATT

= 13V

V

BATT

= 12V

V

BATT

= 16V

I

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

0

1

2

3

4

-2
0 3.5

V

BATT

= 16V V

BATT

= 12V

V

BATT

= 13V

V

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

0V

V

WALLADAPTER

V

SYSTEMLOAD

, V

PDS

V

PDS

V

PDL

, V

BATT

V

PDL

SYSTEM LOAD

V

PDL

V

PDS

PDS-PDL SWITCHOVER,

WALL ADAPTER REMOVAL

MAX1909/MAX8725 toc16

500µs/div

10V

20V

0V

10V

20V

10V

20V

0V

V

WALLADAPTER

V

SYSTEMLOAD

V

SYSTEMLOAD

V

PDS

V

PDL

V

BATT

V

PDL

PDS-PDL SWITCHOVER,

BATTERY INSERTION

MAX1909/MAX8725 toc17

50µs/div

10V

15V

0V

5V

10V

15V

20V

5V

0V

V

SYSTEM

V

PDS

V

PDL

V

BATT

V

PKDET

CONDITIONING MODE
WALL ADAPTER = 18V

V

PKPRES

PDL-PDS SWITCHING,

BATTERY REMOVAL

MAX1909/MAX8725 toc18

10µs/div

10V

15V

0V

5V

10V

15V

20V

5V

0V

V

SYSTEM

V

PDS

V

PDL

V

BATT

CONDITIONING MODE
WALL ADAPTER = 18V

V

PKPRES

MAX8725 ONLY

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 13

Pin Description

PIN

FUNCTION

1 DCIN DC Supply Voltage Input. Bypass DCIN with a 1µF capacitor to power ground.
2 LDO D evi ce P ow er S up p l y. Outp ut of the 5.4V l i near r eg ul ator sup p l i ed fr om D C IN . Byp ass w i th a 1µF cap aci tor .

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 is the ACOK signal.

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

GND MAX1909: Ground this pin

5

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

6

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

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 voltage programmed at VCTL.
MODE = LDO; charge with 4 times the cell voltage programmed at VCTL.

8 IINP

Input Current Monitor Output. The current delivered at the IINP output is a scaled-down replica of the system

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 Maximum Output Voltage
12 CCI Output Current-Regulation Loop-Compensation Point. Connect 0.01µF to GND.
13 CCV Voltage-Regulation Loop-Compensation Point. Connect 10kΩ in series with 0.1µF to GND.
14 CCS Input Current-Regulation 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

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. Bypass with a 1µF capacitor to ground.
22 DHIV High-Side Driver 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 is 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-sense resistor from CSSP to CSSN.

27 PDS

Power-Source PMOS Switch Driver Output. When the MAX1909/MAX8725 are powered down, the PDS output

is pulled to SRC through an internal 1MΩ resistor.

28 PDL

System-Load PMOS Switch Driver Output. When the MAX1909/MAX8725 are powered down, the PDL output

is pulled to ground through an internal 100kΩ resistor.

NAME

PKPRES

ACOK

MODE

PGND

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

TO

HOST

SYSTEM

BATT +

TEMP
BATT -

BATTERY

AC ADAPTER

R6

590kΩ

1%

R7

196kΩ

1%

C5
1µF

D4

P3

RS1

0.01Ω

R5
10kΩ

C11

0.1µF

C10

0.01µF

C9

0.01µF

C12
1µF

C4
22µF

N1

P1

C16
1µF

C13
1µF

R13
33Ω

C1
22µF

GND

PGND

RS2

0.015Ω

TO

SYSTEM LOAD

R8
1M

Ω

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

R4

100kΩ

SRC

C22
1µF

R9
10kΩ

P2

MAX1909
MAX8725

PKPRES (MAX8725 ONLY)

L1
10µH

LDO

LDO

0.1µF

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

AV

DD

/REF

SCL

SDA

GND

HOST

BATT +

TEMP

SDA

SCL

BATT -

SMART

BATTERY

AC ADAPTER

R6

590kΩ

1%

R7

196kΩ

1%

C5
1µF

D4

P3 P4

RS1

0.01Ω

R5

10kΩ

C11

0.1µF

C10

0.01µF

C9

0.01µF

C12
1µF

C4
22µF

N1

P1

C16
1µF

C13
1µF

R13
33Ω

C1
22µF

GND

PGND

RS2

0.015Ω

TO

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

P2

MAX1909
MAX8725

C14

0.1µF

R9
10kΩ

R19, R20

10kΩ

(INPUT I LIMIT: 7.5A)

L1
10µH

D/A OUTPUT

0.1µF

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

DRIVER

DRIVER

DRIVER

DLOV

DLO

PGND

LVC

BATT

MODE

VCTL

CSIP

CSIN

LEVEL

SHIFTER

LEVEL

SHIFTER

CSSP

CSSN

ICTL

CLS

SRDY

GND

GND

GMV

GMI

GMS

CCS

CCI

CCV

CELL SELECT

LOGIC AND

BATTERY VOLTAGE-

DIVIDER

ACIN

0.9 * LDO

RDY

SRC

SWITCH LOGIC

R

R

9R

REF

MODE

DHIV

DCIN

0.8V

Gm

BATT

PKPRES

3.0V/CELL

BATT_UV

ICTLOK

PACK_ON

CHG

SRC

MAX1909
MAX8725

SRC-10V

100kΩ

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:

I

RS

V

V

CHG

ICTL

=×

0 075

236

.

.

V CELL V

VV

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 I2R
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:

V

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:

V

IINP

= I

SOURCE

✕ RS1✕ G

IINP

✕ R

9

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.

I

RSVV

IN

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 condition-
ing 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

,

V

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

R

S

Q

Q

DHI

DLO

CSIP

CSIN

BATT

CCSCCICCV

TOFF

R

CCV

CCV CCI CCS

C

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

-

V

BATT

) differential becomes too small. If V

BATT

≥ 0.88 x

V

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:

where RS2 = 0.015Ω.

I

V

RS

A

MIN

=

×

=

015

20 2

05..

f

t

V

VV

OFF

BATT

CSSN BATT

=

−

+













1

1

f

tt

ON OFF

=

+

1

where I

Vt

L

RIPPLE

BATT OFF

=

×

t

LI

VV

ON

RIPPLE

CSSN BATT

=

×

−

t

f

VV

V

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

CV

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= ∆V

BATT

/

∆I

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

R

sC R

GsCR

OUT

OGMV CV CV

CV OGMV

L

OUT L

MV OUT ESR

=×

×+ ×

()

+×

()

×

+×

()

+×

()

1

1

1

1

GM

ARS

OUT

CSI

=

×12

C

CV

C

OUT

R

CV

R

LR

ESR

R

OGMV

CCV

BATT

GMV

REF

GM

OUT

Figure 5. CCV Loop Diagram

NAME CALCULATION DESCRIPTION

1

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).

2

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 one

decade below crossover to ensure adequate phase margin.

3

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.

4

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

f

RC

PCV

OGMV CV

_

=

×

1

2π

f

RC

ZCV

CV CV

_

=

×

1

2π

f

RC

P OUT

L OUT

_

=

×

1

2π

f

RC

Z OUT

ESR OUT

_

=

×

1

2π

NO.

CCV pole

CCV zero

Output pole

Output zero

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 23

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

Near crossover, CCVhas 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
CCV, so:

C

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

CV

using the component values listed in Figure 1 yields:
MODE = VCC(4 cells)
GMV = 0.125µA/mV
C

OUT

= 22µF

V

BATT

= 16.8V

R

L

= 0.2Ω

GM

OUT

= 3.33A/V

f

CO_CV

= 30kHz

f

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

R

sR C

OUT CSI

OGMI

OGMI CI

=×××

+×

2

1

R

Cf

GMV GM

k

CV

OUT CO CV

OUT

=

××

×

=210

π

_

Ω

f

CO CV GM GMV

R

C

OUT

CV

OUT

_

=

×

×













2π

LTF GM

R

sC

GMV

OUT

CV
OUT

=×

R

sC R sC

L

OUT L OUT

11+×

()

≅

RsCR

sC R

R

OGMV CV CV

CV OGMV

CV

×+ ×

()

+×

()

≅

1

1

FREQUENCY (Hz)

MAGNITUDE (dB)

PHASE (DEGREES)

100k10k1k100101

-20

0

20

40

60

80

-40

-90

-45

0

-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:

CCI= 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 charge-current amplifier transcon­ductance = 1µA/mV. GMINis the DC-DC converter’s
input-referred transconductance = (1/D) GM

OUT

=

(1/D) 3.3A/V.

f

GMI

C

CO CICI_

=

2π

LTF GMI

R

sR C

OGMI

OGMI CI

=

+×1

GM

ARS

OUT

CSI

=

×12

FREQUENCY (Hz)

MAGNITUDE (dB)

100k1k10

-20

0

20

40

60

100

80

-40

-45

0

-90

0.1

MAG
PHASE

Figure 8. CCI Loop Response

C

CS

R

OGMS

GMS

CSS

CLS

CCS

CSSP

RS1

CSSN

GM

IN

SYSTEM

LOAD

ADAPTER

INPUT

Figure 9. CCS Loop Diagram

C

CI

R

OGMI

CCI

GMI

CSI

ICTL

GM

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 CCIgreater than 10 times the mini­mum value may slow down the 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 pull­down 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.

f

GMS

C

CO CSCS_

=

2π

LTF GMS

R

sR C

OGMS

OGMS CS

=

+×1

GM

ARS

IN

CSS

=

×11

LTF GM A RS GMS

R

sR C

IN CSS

OGMS

OGMS CS

=×××

+×

1

1

FREQUENCY (Hz)

MAGNITUDE (dB)

100k 10M1k10

-20

0

20

40

60

100

80

-40

-45

0

-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.

REFERENCE

QTY

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

D4

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

D5

25V ±1% zener diode
CMDZ5253B

L1

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

R4

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

R6

590kΩ ±1% resistor (0603)

R7

196kΩ ±1% resistor (0603)

R8

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

U1

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

2

1

2

P2, P3, P4 3

2

3

3

1

1

1

1

R5, R9, R21 2

1
1
1
1
1
2

1

1

1

MAX1909/MAX8725

Multichemistry Battery Chargers with Automatic

System Power Selector

______________________________________________________________________________________ 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 CRSS 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 (∆IL):

I

SAT

= I

CHG

+ (1/2) ∆IL

PD N

V
V

I

R

BATT

DCIN

LOAD

DS ON

()

()

11

2

2

=





































×

−

PD P Switching

VCfI

I

DCIN MAX RSS SW LOAD

GATE

(_ )

()

1

2

2

=

×××

PD P

V
V

I

R

BATT
DCIN

LOAD

DS ON

()

()

1

2

2

=























×

0

1.0

0.5

1.5

81011121391415161718

V

BATT

(V)

RIPPLE CURRENT (A)

V

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:

∆IL = V

BATTtOFF

/ L

where:

t

OFF

= 2.5µs (V

DCIN

- V

BATT

) / V

DCIN

for

V

BATT

< 0.88 V

DCIN

or:

t

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 = ∆IL / 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.

II

VV V

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

C

IN

C

OUT

C

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.

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

© 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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

D2

(ND-1) X e

e

D

C

PIN # 1
I.D.

(NE-1) X e

E/2

E

0.08 C

0.10 C

A

A1 A3

DETAIL A

0.15

C B

0.15 C A

E2/2

E2

0.10 M C A B

PIN # 1 I.D.

b

0.35x45∞

L

D/2

D2/2

L

C

L

C

e e

L

CC

L

k

k

LL

E

1

2

21-0140

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

DETAIL B

L

L1

e

COMMON DIMENSIONS

3.353.15

T2855-1 3.25 3.353.15 3.25

MAX.

3.20

EXPOSED PAD VARIATIONS

3.00T2055-2 3.10

D2

NOM.MIN.

3.203.00 3.10

MIN.E2NOM. MAX.

NE

ND

PKG.

CODES

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 T2855-1,
T2855-3 AND T2855-6.

NOTES:

SYMBOL

PKG.

N

L1

e

E

D

b

A3

A

A1

k

10. WARPAGE SHALL NOT EXCEED 0.10 mm.

JEDEC

T1655-1

3.203.00 3.10 3.00 3.10 3.20

0.70 0.800.75

4.90

4.90

0.25

0.250--

4

WHHB

4

16

0.350.30

5.10

5.105.00

0.80 BSC.

5.00

0.05

0.20 REF.

0.02

MIN. MAX.NOM.

16L 5x5

3.10

T3255-2

3.00

3.20

3.00 3.10 3.20

2.70

T2855-2 2.60 2.602.80 2.70 2.80

E

2

2

21-0140

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

L

0.30 0.500.40

---

---

WHHC

20

5

5

5.00

5.00

0.30

0.55

0.65 BSC.

0.45

0.25

4.90

4.90

0.25

0.65

--

5.10

5.10

0.35

20L 5x5

0.20 REF.

0.75

0.02

NOM.

0

0.70

MIN.

0.05

0.80

MAX.

---

WHHD-1

28

7

7

5.00

5.00

0.25

0.55

0.50 BSC.

0.45

0.25

4.90

4.90

0.20

0.65

--

5.10

5.10

0.30

28L 5x5

0.20 REF.

0.75

0.02

NOM.

0

0.70

MIN.

0.05

0.80

MAX.

---

WHHD-2

32

8

8

5.00

5.00

0.40

0.50 BSC.

0.30

0.25

4.90

4.90

0.50

--

5.10

5.10

32L 5x5

0.20 REF.

0.75

0.02

NOM.

0

0.70

MIN.

0.05

0.80

MAX.

-

40

10

10

5.00

5.00

0.20

0.50

0.40 BSC.

0.40

0.25

4.90

4.90

0.15

0.60

5.10

5.10

0.25

40L 5x5

0.20 REF.

0.75

NOM.

0

0.70

MIN.

0.05

0.80

MAX.

0.20 0.25 0.30

-

0.35 0.45

0.30 0.40 0.50

DOWN
BONDS
ALLOWED

NO

YES3.103.00 3.203.103.00 3.20T2055-3

3.103.00 3.203.103.00 3.20T2055-4

T2855-3 3.15 3.25 3.35 3.15 3.25 3.35

T2855-6 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

T2855-7 2.60 2.70

2.80

2.60 2.70 2.80

3.20

3.00 3.10T3255-3 3.203.00 3.10

3.203.00 3.10T3255-4 3.203.00 3.10

3.403.20 3.30T4055-1 3.20 3.30 3.40

NO

NO
NO

NO

NO

NO

NO

NO

YES
YES

YES

YES

YES

3.203.00T1655-2 3.10 3.00 3.10 3.20 YES