AVAILABLE
EVALUATION KIT AVAILABLE
Functional Diagrams
Pin Configurations appear at end of data sheet.
Functional Diagrams continued at end of data sheet.
UCSP is a trademark of Maxim Integrated Products, Inc.
For pricing, delivery, and ordering information, please contact Maxim Direct
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
General Description
The MAX1908/MAX8724/MAX8765/MAX8765A highly
integrated, multichemistry battery-charger control ICs
simplify the construction of accurate and efficient charg-
ers. These devices use analog inputs to control charge
current and voltage, and can be programmed by the host
or hardwired. The MAX1908/MAX8724/MAX8765/
MAX8765A achieve high efficiency using a buck topology
with synchronous rectification.
The MAX1908/MAX8724/MAX8765/MAX8765A feature
input current limiting. This feature reduces battery
charge current when the input current limit is reached
to avoid overloading the AC adapter when supplying
the load and the battery charger simultaneously. The
MAX1908/MAX8724/MAX8765/MAX8765A provide out-
puts to monitor current drawn from the AC adapter (DC
input source), battery-charging current, and the pres-
ence of an AC adapter. The MAX1908’s conditioning
charge feature provides 300mA to safely charge deeply
discharged lithium-ion (Li+) battery packs.
The MAX1908 includes a conditioning charge feature
while the MAX8724/MAX8765/MAX8765A do not.
The MAX1908/MAX8724/MAX8765/MAX8765A charge two
to four series Li+ cells, providing more than 5A, and are
available in a space-saving, 28-pin, thin QFN package (5mm
×5mm). An evaluation kit is available to speed designs.
Applications
Notebook and Subnotebook Computers
Personal Digital Assistants
Handheld Terminals
Features
o±0.5% Output Voltage Accuracy Using Internal
Reference (±0.4% for MAX8765A, 2-/3-Cell Only)
o±4% Accurate Input Current Limiting
o±5% Accurate Charge Current
oAnalog Inputs Control Charge Current and
Charge Voltage
oOutputs for Monitoring
Current Drawn from AC Adapter
Charging Current
AC Adapter Presence
oUp to 17.6V Battery-Voltage Set Point
oMaximum 28V Input Voltage
o> 95% Efficiency
oShutdown Control Input
oCharge Any Battery Chemistry
Li+, NiCd, NiMH, Lead Acid, etc.
Low-Cost Multichemistry Battery Chargers
28
+
27
26
25
24
23
22
IINP
CSSP
CSSN
DHI
BST
LX
DLOV
8
9
10
11
12
13
14
SHDN
ICHG
ACIN
ACOK
REFIN
ICTL
GND
15161718192021
VCTL
BATT
CELLS
CSIN
CSIP
PGND
DLO
7654321
CCV
CCI
CCS
REF
CLS
LDO
DCIN
MAX1908
MAX8724
MAX8765
MAX8765A
THIN QFN
TOP VIEW
Pin Configuration
Ordering Information
MAX1908
MAX8724
MAX8765
MAX8765A
AC ADAPTER
INPUT
TO EXTERNAL
LOAD
LDO
FROM HOST µP
10µH
0.015Ω
BATT+
DCIN
REFIN
VCTL
ICTL
ACIN
ACOK
SHDN
ICHG
IINP
CCV
CCI
CCS
CELLS
LDO
BST
DLOV
DHI
LX
DLO
PGND
CSIP
CSIN
BATT
REF CLS GND
CSSP CSSN
0.01Ω
Minimum Operating Circuit
PART TEMP RANGE PIN-PACKAGE
MAX1908ETI+ -40°C to +85°C 28 Thin QFN-EP*
MAX8724ETI+ -40°C to +85°C 28 Thin QFN-EP*
MAX8765ETI+ -40°C to +85°C 28 Thin QFN-EP*
MAX8765AETI+-40°C to +85°C 28 Thin QFN-EP*
+
Denotes a lead(Pb)-free/RoHS-compliant package.
*
EP = Exposed pad.
19-2764; Rev 5; 11/09
MAX1908/MAX8724/
MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; 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, ACOK to GND.......................-0.3V to +30V
BST to GND ............................................................-0.3V to +36V
BST to LX..................................................................-0.3V to +6V
DHI to LX...................................................-0.3V to (VBST + 0.3V)
LX to GND .................................................................-6V to +30V
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, ICHG,
IINP, ACIN, REF to GND.......................-0.3V to (VLDO + 0.3V)
DLOV, VCTL, ICTL, REFIN, CELLS, CLS,
LDO, SHDN to GND .............................................-0.3V to +6V
DLOV to LDO.........................................................-0.3V to +0.3V
DLO to PGND .........................................-0.3V to (VDLOV + 0.3V)
LDO Short-Circuit Current...................................................50mA
Continuous Power Dissipation (TA= +70°C)
28-Pin Thin QFN (5mm ×5mm)
(derate 20.8mW/°C above +70°C) .........................1666.7mW
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
VVCTL = VREFIN -0.5 +0.5
VVCTL = VREFIN/20 -0.5 +0.5
Battery-Regulation Voltage
Accuracy
(MAX1908/MAX8724/MAX8765
(2, 3, or 4 Cells) and MAX8765A
(4 Cells Only))
VVCTL = VLDO -0.5 +0.5
%
VVCTL = VREFIN -0.4 +0.4
VVCTL = VREFIN/20 -0.4 +0.4
Battery-Regulation Voltage
Accuracy (MAX8765A, 2 or 3
Cells Only)
VVCTL = VLDO -0.4 +0.4
%
VCTL Default Threshold VVCTL rising 4.0 4.1 4.2 V
REFIN Range (Note 1) 2.5 3.6 V
REFIN Undervoltage Lockout VREFIN falling 1.20 1.92 V
CHARGE-CURRENT REGULATION
CSIP-to-CSIN Full-Scale Current-
Sense Voltage VICTL = VREFIN 71.25 75 78.75 mV
VICTL = VREFIN -5 +5
VICTL = VREFIN x 0.6 -5 +5
VICTL = VLDO -6 +6
MAX8765/MAX8765A only; VICTL = VREFIN x
0.036 -45 +45
Charging-Current Accuracy
MAX8724 only; VICTL = VREFIN x 0.058 -33 +33
%
Charge-Current Gain Error
(MAX8765/MAX8765A Only) -2 +2 %
Charge-Current Offset
(MAX8765/MAX8765A Only) -2 +2 mV
ICTL Default Threshold VICTL rising 4.0 4.1 4.2 V
BATT/CSIP/CSIN Input Voltage
Range 0 19 V
MAX1908/MAX8724/MAX8765/MAX8765A
2
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
VDCIN = 0V or VICTL = 0V or V SHDN = 0V 1
CSIP/CSIN Input Current
Charging 400 650 µA
Cycle-by-Cycle Maximum Current
Limit RS2 = 0.015mΩ 6.0 6.8 7.5 A
ICTL Power-Down Mode
Threshold Voltage
(MAX1908/MAX8724 Only)
VVCTL rising REFIN/
100
REFIN/
55
REFIN/
33 V
VVCTL = VICTL = 0 or 3V -1 +1
ICTL, VCTL Input Bias Current
VDCIN = 0V, VVCTL = VICTL = VREFIN = 5V -1 +1
µA
VDCIN = 5V, VREFIN = 3V -1 +1
REFIN Input Bias Current
VREFIN = 5V -1 +1
µA
ICHG Transconductance
(MAX1908/MAX8724 Only) GICHG VCSIP - VCSIN = 45mV 2.7 3 3.3 µA/mV
ICHG Transconductance
(MAX8765/MAX8765A Only) GICHG VCSIP - VCSIN = 45mV 2.85 3 3.15 µA/mV
ICHG Transconductance Error
(MAX8765/MAX8765A Only) -5 +5 %
ICHG Transconductance Offset
(MAX8765/MAX8765A Only) -5 +5 µA
VCSIP - VCSIN = 75mV -6 +6
VCSIP - VCSIN = 45mV -5 +5 ICHG Accuracy
VCSIP - VCSIN = 5mV -40 +40
%
ICHG Output Current VCSIP - VCSIN = 150mV, VICHG = 0V 350 µA
ICHG Output Voltage VCSIP - VCSIN = 150mV, ICHG = open 3.5 V
INPUT-CURRENT REGULATION
CSSP-to-CSSN Full-Scale
Current-Sense Voltage 72 75 78 mV
VCLS = VREF -4 +4
VCLS = VREF/2 -7.5 +7.5
Input Current-Limit Accuracy
VCLS = 1.1V (MAX8765/MAX8765A only) -10 +10
%
Input Current-Limit Gain Error
(MAX8765/MAX8765A Only) -2 +2 %
Input Current-Limit Offset
(MAX8765/MAX8765A Only) -2 +2 mV
CSSP, CSSN Input Voltage
Range 8 28 V
VDCIN = 0V 0.1 1
CSSP, CSSN Input Current
(MAX1908/MAX8724 Only)
VCSSP = VCSSN = VDCIN > 8V 350 600
µA
VDCIN = 0V 0.1 1
CSSP Input Current
(MAX8765/MAX8765A Only) VCSSP = VCSSN = 28V VDCIN = 28V 400 650
µA
Maxim Integrated
3
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
VDCIN = 0V 0.1 1
CSSN Input Current
(MAX8765/MAX8765A Only) VCSSP = VCSSN = 28V VDCIN = 28V 0.1 1
µA
CLS Input Range
(MAX1908/MAX8724 Only) 1.6 REF V
CLS Input Range
(MAX8765/MAX8765A Only) 1.1 REF V
CLS Input Bias Current VCLS = 2V -1 +1 µA
IINP Transconductance
(MAX1908/MAX8724 Only) GIINP VCSSP - VCSSN = 75mV 2.7 3 3.3 µA/mV
VCSSP - VCSSN = 75mV -5 +5
IINP Accuracy
VCSSP - VCSSN = 37.5mV -7.5 +7.5
%
IINP Transconductance
(MAX8765/MAX8765A Only) GIINP VCSSP - VCCSN = 75mV 2.82 3 3.18 µA/mV
IINP Transconductance Error
(MAX8765/MAX8765A Only) -6 +6 %
IINP Transconductance Offset
(MAX8765/MAX8765A Only) -10 +10 µA
IINP Output Current VCSSP - VCSSN = 150mV, VIINP = 0V 350 µA
IINP Output Voltage VCSSP - VCSSN = 150mV, VIINP = open 3.5 V
SUPPLY AND LDO REGULATOR
DCIN Input Voltage Range VDCIN 8 28 V
VDCIN falling 7 7.4
DCIN Undervoltage-Lockout Trip
Point
VDCIN rising 7.5 7.85
V
DCIN Quiescent Current IDCIN 8.0V < VDCIN < 28V 3.2 6 mA
VBATT = 19V, VDCIN = 0V 1
BATT Input Current IBATT VBATT = 2V to 19V, VDCIN = 19.3V 200 500
µA
LDO Output Voltage 8V < VDCIN < 28V, no load 5.25 5.4 5.55 V
LDO Load Regulation 0 < ILDO < 10mA 34 100 mV
LDO Undervoltage-Lockout Trip
Point VDCIN = 8V 3.20 4 5.15 V
REFERENCE
REF Output Voltage 0 < IREF < 500µA 4.072 4.096 4.120 V
REF Undervoltage-Lockout Trip
Point VREF falling 3.1 3.9 V
MAX1908/MAX8724/MAX8765/MAX8765A
4
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
TRIP POINTS
VDCIN falling, referred to VCSIN
(MAX1908/MAX8724 only) 50 100 150
BATT Power-Fail Threshold
VCSSP falling, referred to VCSIN
(MAX8765/MAX8765A only) 50 100 150
mV
BATT Power-Fail Threshold
Hysteresis 200 mV
ACIN rising (MAX8765/MAX8765A only) 2.028 2.048 2.068
ACIN Threshold
ACIN rising (MAX1908/MAX8724 only) 2.007 2.048 2.089
V
ACIN Threshold Hysteresis 0.5% of REF 20 mV
ACIN Input Bias Current VACIN = 2.048V -1 +1 µA
SWITCHING REGULATOR
DHI Off-Time
VBATT = 16V, VDCIN = 19V,
VCELLS = VREFIN
0.36 0.4 0.44 µs
DHI Minimum Off-Time
VBATT = 16V, VDCIN = 17V,
VCELLS = VREFIN
0.24 0.28 0.33 µs
DHI Maximum On-Time 2.5 5 7.5 ms
DLOV Supply Current DLO low 5 10 µA
BST Supply Current DHI high 6 15 µA
BST Input Quiescent Current
VDCIN = 0V, VBST = 24.5V,
VBATT = VLX = 20V 0.3 1 µA
LX Input Bias Current VDCIN = 28V, VBATT = VLX = 20V 150 500 µA
LX Input Quiescent Current VDCIN = 0V, VBATT = VLX = 20V 0.3 1 µA
DHI Maximum Duty Cycle 99 99.9 %
Minimum Discontinuous-Mode
Ripple Current 0.5 A
Battery Undervoltage Charge
Current
VBATT = 3V per cell (RS2 = 15mΩ),
MAX1908 only, VBATT rising 150 300 450 mA
CELLS = GND, MAX1908 only, VBATT rising 6.1 6.2 6.3
CELLS = open, MAX1908 only, VBATT rising 9.15 9.3 9.45
Battery Undervoltage Current
Threshold
CELLS = VREFIN, MAX1908 only, VBATT rising 12.2 12.4 12.6
V
DHI On-Resistance High VBST - VLX = 4.5V, IDHI = +100mA 4 7 Ω
DHI On-Resistance Low VBST - VLX = 4.5V, IDHI = -100mA 1 3.5 Ω
DLO On-Resistance High VDLOV = 4.5V, IDLO = +100mA 4 7 Ω
DLO On-Resistance Low VDLOV = 4.5V, IDLO = -100mA 1 3.5 Ω
Maxim Integrated
5
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
ERROR AMPLIFIERS
GMV Amplifier Transconductance GMV V
V C T L = V
LD O, V
BAT T = 16.8V ,
C E LLS = V
RE F IN
0.0625 0.125 0.2500 µA/mV
GMI Amplifier Transconductance GMI VICTL = V
RE F IN
, VCSIP - VCSIN = 75mV 0.5 1 2.0 µA/mV
GMS Amplifier Transconductance GMS VCLS = VREF, VCSSP - VCSSN = 75mV 0.5 1 2.0 µA/mV
CCI, CCS, CCV Clamp Voltage 0.25V < VCCV,CCS,CCI < 2V 150 300 600 mV
LOGIC LEVELS
CELLS Input Low Voltage 0.4 V
CELLS Input Open Voltage CELLS = open
(VREFIN
/2) -
0.2V
V
R E F IN
/
2
( V
R E F IN
/2) +
0.2V
V
CELLS Input High Voltage
VREFIN
- 0.4V V
CELLS Input Bias Current CELLS = 0V or V
RE F IN -2 +2 µA
ACOK AND SHDN
ACOK Input Voltage Range 0 28 V
ACOK Sink Current V
A COK = 0.4V, VACIN = 3V 1 mA
ACOK Leakage Current V
A COK = 28V, VACIN = 0V 1 µA
SHDN Input Voltage Range 0 LDO V
VSHDN = 0V OR VLDO -1 +1
SHDN Input Bias Current
VSHDN = 0V OR VSHDN = 5V -1 +1
µA
SHDN Threshold V
S HDN falling 22 23.5 25 % of
VREFIN
SHDN Threshold Hysteresis 1
% of
VREFIN
MAX1908/MAX8724/MAX8765/MAX8765A
6
Maxim Integrated
ELECTRICAL CHARACTERISTICS
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
CHARGE-VOLTAGE REGULATION
VVCTL = VREFIN -0.6 +0.6
VVCTL = VREFIN/20 -0.6 +0.6
Battery Regulation Voltage
Accuracy (2, 3, or 4 Cells)
VVCTL = VLDO -0.6 +0.6
%
REFIN Range (Note 1) 2.5 3.6 V
REFIN Undervoltage Lockout VREFIN falling 1.92 V
CHARGE CURRENT REGULATION
CSIP-to-CSIN Full-Scale Current-
Sense Voltage VICTL = VREFIN 70.5 79.5 mV
VICTL = VREFIN -6 +6
VICTL = VREFIN x 0.6 -7.5 +7.5
VICTL = VLDO -7.5 +7.5
MAX8765/MAX8765A only; VICTL = VREFIN x
0.036 -50 +50
Charging-Current Accuracy
MAX8724 only;
VICTL = VREFIN x 0.058 -33 +33
%
Charge-Current Gain Error
(MAX8765/MAX8765A Only) -2 +2 %
Charge-Current Offset
(MAX8765/MAX8765A Only) -2 +2 mV
BATT/CSIP/CSIN Input Voltage
Range 0 19 V
VDCIN = 0V or VICTL = 0V or VSHDN = 0V 1
CSIP/CSIN Input Current
Charging 650
µA
Cycle-by-Cycle Maximum Current
Limit IMAX RS2 = 0.015Ω 6.0 7.5 A
ICTL Power-Down Mode
Threshold Voltage
(MAX1908/MAX8724 Only)
VICTL rising REFIN/
100
REFIN/
33 V
ICHG Transconductance
(MAX1908/MAX8724 Only) GICHG VCSIP - VCSIN = 45mV 2.7 3.3 µA/mV
ICHG Transconductance
(MAX8765/MAX8765A Only) GICHG VCSIP - VCSIN = 45mV 2.785 3.225 µA/mV
ICHG Transconductance Error
(MAX8765/MAX8765A Only) -7.5 +7.5 %
ICHG Transconductance Offset
(MAX8765/MAX8765A Only) -6.5 +6.5 µA
Low-Cost Multichemistry Battery Chargers
Maxim Integrated
7
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
VCSIP - VCSIN = 75mV -7.5 +7.5
VCSIP - VCSIN = 45mV -7.5 +7.5
ICHG Accuracy
VCSIP - VCSIN = 5mV -40 +40
%
INPUT-CURRENT REGULATION
CSSP-to-CSSN Full-Scale
Current-Sense Voltage 71.25 78.75 mV
VCLS = VREF -5 +5
VCLS = VREF/2 -7.5 +7.5
Input Current-Limit Accuracy
VCLS = 1.1V (MAX8765/MAX8765A only) -10 +10
%
Input Current-Limit Gain Error
(MAX8765/MAX8765A Only) -2 +2 %
Input Current-Limit Offset
(MAX8765/MAX8765A Only) -2 +2 mV
CSSP, CSSN Input Voltage
Range 8 28 V
VDCIN = 0V 1
CSSP, CSSN Input Current
(MAX1908/MAX8724 Only)
VCSSP = VCSSN = VDCIN > 8V 600
µA
VDCIN = 0V 1
CSSP Input Current
(MAX8765/MAX8765A Only) VCSSP = VCSSN = 28V VDCIN = 28V 650
µA
VDCIN = 0V 1
CSSN Input Current
(MAX8765/MAX8765A Only) VCSSP = VCSSN = 28V VDCIN = 28V 1
µA
CLS Input Range
(MAX1908/MAX8724 Only) 1.6 REF V
CLS Input Range
(MAX8765/MAX8765A Only) 1.1 REF V
IINP Transconductance
(MAX1908/MAX8724 Only) GIINP VCSSP - VCSSN = 75mV 2.7 3.3 µA/mV
IINP Transconductance
(MAX8765/MAX8765A Only) GIINP VCSSP - VCCSN = 75mV 2.785 3.225 µA/mV
IINP Transconductance Error
(MAX8765/MAX8765A Only) -7.5 +7.5 %
IINP Transconductance Offset
(MAX8765/MAX8765A Only) -12 +12 µA
VCSSP - VCSSN = 75mV -7.5 +7.5
IINP Accuracy
VCSSP - VCSSN = 37.5mV -7.5 +7.5
%
MAX1908/MAX8724/MAX8765/MAX8765A
8
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
SUPPLY AND LDO REGULATOR
DCIN Input Voltage Range VDCIN 8 28 V
DCIN Quiescent Current IDCIN 8.0V < VDCIN < 28V 6 mA
VBATT = 19V, VDCIN = 0V 1
BATT Input Current IBATT VBATT = 2V to 19V, VDCIN = 19.3V 500
µA
LDO Output Voltage 8V < VDCIN < 28V, no load 5.25 5.55 V
LDO Load Regulation 0 < ILDO < 10mA 100 mV
REFERENCE
REF Output Voltage 0 < IREF < 500µA 4.065 4.120 V
TRIP POINTS
VDCIN falling, referred to VCSIN
(MAX1908/MAX8724 only) 50 150
BATT Power-Fail Threshold
VCSSP falling, referred to VCSIN
(MAX8765/MAX8765A only)
50 150
mV
ACIN rising (MAX8765/MAX8765A only) 2.028 2.068
ACIN Threshold
ACIN rising (MAX1908/MAX8724 only) 2.007 2.089
V
SWITCHING REGULATOR
DHI Off-Time
VBATT = 16V, VDCIN = 19V,
VCELLS = VREFIN
0.35 0.45 µs
DHI Minimum Off-Time
VBATT = 16V, VDCIN = 17V,
VCELLS = VREFIN
0.24 0.33 µs
DHI Maximum On-Time 2.5 7.5 ms
DHI Maximum Duty Cycle 99 %
Battery Undervoltage Charge
Current
VBATT = 3V per cell (RS2 = 15mΩ),
MAX1908 only, VBATT rising 150 450 mA
CELLS = GND, MAX1908 only, VBATT rising 6.09 6.30
CELLS = open, MAX1908 only, VBATT rising 9.12 9.45
Battery Undervoltage Current
Threshold
CELLS = VREFIN, MAX1908 only, VBATT rising 12.18 12.60
V
DHI On-Resistance High VBST - VLX = 4.5V, IDHI = +100mA 7 Ω
DHI On-Resistance Low VBST - VLX = 4.5V, IDHI = -100mA 3.5 Ω
DLO On-Resistance High VDLOV = 4.5V, IDLO = +100mA 7 Ω
DLO On-Resistance Low VDLOV = 4.5V, IDLO = -100mA 3.5 Ω
Maxim Integrated
9
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VCSSP = VCSSN = 18V, VBATT = VCSIP = VCSIN = 12V, VREFIN = 3V, VVCTL = VICTL = 0.75 x VREFIN, CELLS = open, CLS =
REF, VBST - VLX = 4.5V, ACIN = GND = PGND = 0, CLDO = 1µF, LDO = DLOV, CREF = 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
ERROR AMPLIFIERS
GMV Amplifier Transconductance GMV V
V C T L = V
LD O, V
BAT T = 16.8V ,
C E LLS = V
RE F IN
0.0625 0.250 µA/mV
GMI Amplifier Transconductance GMI VICTL = V
RE F IN
, VCSIP - VCSIN = 75mV 0.5 2.0 µA/mV
GMS Amplifier Transconductance GMS VCLS = VREF, VCSSP - VCSSN = 75mV 0.5 2.0 µA/mV
CCI, CCS, CCV Clamp Voltage 0.25V < VCCV,CCS,CCI < 2V 150 600 mV
LOGIC LEVELS
CELLS Input Low Voltage 0.4 V
CELLS Input Open Voltage CELLS = open
(VREFIN
/2) -
0.2V
( V
R E F IN
/2) +
0.2V
V
CELLS Input High Voltage
VREFIN
- 0.4V V
ACOK AND SHDN
ACOK Input Voltage Range 0 28 V
ACOK Sink Current V ACOK = 0.4V, VACIN = 3V 1 mA
SHDN Input Voltage Range 0 LDO V
SHDN Threshold V SHDN falling 22 25 % of
VREFIN
Note 1: If both ICTL and VCTL use default mode (connected to LDO), REFIN is not used and can be connected to LDO.
Note 2: Specifications to TA= -40°C are guaranteed by design and not production tested.
LOAD-TRANSIENT RESPONSE
(BATTERY INSERTION AND REMOVAL)
MAX1908 toc01
1ms/div
IBATT
2A/div
VBATT
5V/div
VCCI
500mV/div
VCCV
500mV/div
ICTL = LDO
VCTL = LDO
CCV
CCI
LOAD-TRANSIENT RESPONSE
(STEP IN-LOAD CURRENT)
MAX1908 toc02
1ms/div
VBATT
2V/div
VCCI
500mV/div
VCCS
500mV/div
16.8V
0
0
LOAD
CURRENT
5A/div
ADAPTER
CURRENT
5A/div
ICTL = LDO
CHARGING CURRENT = 3A
VBATT = 16.8V
LOAD STEP = 0 TO 4A
ISOURCE LIMIT = 5A
CCS
CCS
CCI
CCI
VBATT
2V/div
0
0
0
CHARGE
CURRENT
2A/div
LOAD
CURRENT
5A/div
ADAPTER
CURRENT
5A/div
LOAD-TRANSIENT RESPONSE
(STEP IN-LOAD CURRENT)
MAX1908 toc03
1ms/div
ICTL = LDO
CHARGING CURRENT = 3A
VBATT = 16.8V
LOAD STEP = 0 TO 4A
ISOURCE LIMIT = 5A
Typical Operating Characteristics
(Circuit of Figure 1, VDCIN = 20V, TA= +25°C, unless otherwise noted.)
MAX1908/MAX8724/MAX8765/MAX8765A
10
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
INDUCTOR
CURRENT
500mA/div
VDCIN
10V/div
VBATT
500mV/div
LINE-TRANSIENT RESPONSE
MAX1908 toc04
10ms/div
ICTL = LDO
VCTL = LDO
ICHARGE = 3A
LINE STEP 18.5V TO 27.5V
-1.0
-0.8
-0.9
-0.6
-0.7
-0.4
-0.5
-0.3
-0.1
-0.2
0
0 2341 567 9810
LDO LOAD REGULATION
MAX1908 toc05
LDO CURRENT (mA)
VLDO ERROR (%)
VLDO = 5.4V
-0.05
-0.03
-0.04
-0.01
-0.02
0.01
0
0.02
0.04
0.03
0.05
8 12141610 18 20 22 2624 28
LDO LINE REGULATION
MAX1908 toc06
VIN (V)
VLDO ERROR (%)
ILDO = 0
VLDO = 5.4V
-0.10
-0.07
-0.08
-0.09
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
0 200100 300 400 500
REF VOLTAGE LOAD REGULATION
MAX1908 toc07
REF CURRENT (µA)
VREF ERROR (%)
-0.10
-0.04
-0.06
-0.08
-0.02
0
0.02
0.04
0.06
0.08
0.10
-40 10-15 35 60 85
REF VOLTAGE ERROR vs. TEMPERATURE
MAX1908 toc08
TEMPERATURE (°C)
VREF ERROR (%)
90
0
0.01 1010.1
EFFICIENCY vs. CHARGE CURRENT
30
10
70
50
100
40
20
80
60
MAX1908 toc09
CHARGE CURRENT (A)
EFFICIENCY (%)
VBATT = 16V
VBATT = 8V
VBATT = 12V
0
100
50
250
200
150
300
350
450
400
500
0462 8 10 12 14 16 18 20 22
FREQUENCY vs. VIN - VBATT
MAX1908 toc10
(VIN - VBATT) (V)
FREQUENCY (kHz)
ICHARGE = 3A
VCTL = ICTL = LDO
3 CELLS
4 CELLS
-0.4
-0.1
-0.3
-0.5
0
0.2
0.3
0.4
0.5
01234
OUTPUT V/I CHARACTERISTICS
MAX1908 toc11
BATT CURRENT (A)
BATT VOLTAGE ERROR (%)
0.1
-0.2
2 CELLS
3 CELLS
4 CELLS
0
0.02
0.01
0.03
0.06
0.07
0.05
0.04
0.08
0 0.2 0.3 0.4 0.50.1 0.6 0.7 0.8 0.9 1.0
BATT VOLTAGE ERROR vs. VCTL
MAX1908 toc12
VCTL/REFIN (%)
BATT VOLTAGE ERROR (%)
4 CELLS
REFIN = 3.3V
NO LOAD
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VDCIN = 20V, TA= +25°C, unless otherwise noted.)
Maxim Integrated
11
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
-1
1
0
3
2
4
5
01.00.5 1.5 2.0
CURRENT-SETTING ERROR vs. ICTL
MAX1908 toc13
VICTL (V)
CURRENT-SETTING ERROR (%)
VREFIN = 3.3V
0
1.5
1.0
0.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
01.00.5 1.5 2.0 2.5 3.0
ICHG ERROR vs. CHARGE CURRENT
MAX1908 toc14
IBATT (A)
ICHG (%)
VBATT = 16V
VBATT = 12V
VBATT = 8V
VREFIN = 3.3V
-40
-30
-20
-10
0
10
20
30
40
01234
IINP ERROR vs. SYSTEM LOAD CURRENT
MAX1908 toc15
SYSTEM LOAD CURRENT (A)
IINP ERROR (%)
IBATT = 0
-80
-60
-40
-20
0
20
40
60
80
0 0.5 1.0 1.5 2.0
IINP ERROR vs. INPUT CURRENT
MAX1908 toc16
INPUT CURRENT (A)
IINP ERROR (%)
SYSTEM LOAD = 0
ERROR DUE TO SWITCHING NOISE
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VDCIN = 20V, TA= +25°C, unless otherwise noted.)
MAX1908/MAX8724/MAX8765/MAX8765A
12
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
Pin Description
PIN NAME FUNCTION
1 DCIN Charging Voltage Input. Bypass DCIN with a 1µF capacitor to PGND.
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 to GN D .
3 CLS Source Current-Limit Input. Voltage input for setting the current limit of the input source.
4 REF 4.096V Voltage Reference. Bypass REF with a 1µF capacitor to GND.
5 CCS Input-Current Regulation Loop-Compensation Point. Connect a 0.01µF capacitor to GND.
6 CCI Output-Current Regulation Loop-Compensation Point. Connect a 0.01µF capacitor to GND.
7 CCV Voltage Regulation Loop-Compensation Point. Connect 1kΩ in series with a 0.1µF capacitor to GND.
8 SHDN Shutdown Control Input. Drive SHDN logic low to shut down the MAX1908/MAX8724/MAX8765 MAX8765A.
Use with a thermistor to detect a hot battery and suspend charging.
9 ICHG
Charge-Current Monitor Output. ICHG is a scaled-down replica of the charger output current. Use ICHG to
monitor the charging current and detect when the chip changes from constant-current mode to constant-
voltage mode. The transconductance of (CSIP - CSIN) to ICHG is 3µA/mV.
10 ACIN AC Detect Input. Input to an uncommitted comparator. ACIN can be used to detect AC-adapter presence.
11 ACOK AC Detect Output. High-voltage open-drain output is high impedance when VACIN is less than VREF/2.
12 REFIN Reference Input. Allows the ICTL and VCTL inputs to have ratiometric ranges for increased accuracy.
13 ICTL
Output Current-Limit Set Input. ICTL input voltage range is VREFIN/32 to VREFIN. The MAX1908/MAX8724 shut
down if ICTL is forced below VREFIN/100 while the MAX8765/MAX8765A does not. When ICTL is equal to
LDO, the set point for CSIP - CSIN is 45mV.
14 GND Analog Ground
15 VCTL Output Voltage-Limit Set Input. VCTL input voltage range is 0 to VREFIN. When VCTL is equal to LDO, the set
point is (4.2 x CELLS)V.
16 BATT Battery Voltage Input
17 CELLS Cell Count Input. Tri-level input for setting number of cells. GND = 2 cells, open = 3 cells, REFIN = 4 cells.
18 CSIN Output Current-Sense Negative Input
19 CSIP Output Current-Sense Positive Input. Connect a current-sense resistor from CSIP to CSIN.
20 PGND Power Ground
21 DLO Low-Side Power MOSFET Driver Output. Connect to low-side nMOS gate.
22 DLOV Low-Side Driver Supply. Bypass DLOV with a 1µF capacitor to GND.
23 LX High-Side Power MOSFET Driver Power-Return Connection. Connect to the source of the high-side nMOS.
24 BST High-Side Power MOSFET Driver Power-Supply Connection. Connect a 0.1µF capacitor from LX to BST.
25 DHI High-Side Power MOSFET Driver Output. Connect to high-side nMOS gate.
26 CSSN Input Current-Sense Negative Input
27 CSSP Input Current-Sense Positive Input. Connect a current-sense resistor from CSSP to CSSN.
28 IINP Input-Current Monitor Output. IINP is a scaled-down replica of the input current. IINP monitors the total
system current. The transconductance of (CSSP - CSSN) to IINP is 3µA/mV.
Maxim Integrated
13
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
Detailed Description
The MAX1908/MAX8724/MAX8765/MAX8765A include
all the functions necessary to charge Li+ batteries. A
high-efficiency synchronous-rectified step-down DC-DC
converter controls charging voltage and current. The
device also includes input-source current limiting and
analog inputs for setting the charge current and charge
voltage. Control charge current and voltage using the
ICTL and VCTL inputs, respectively. Both ICTL and
VCTL are ratiometric with respect to REFIN, allowing
compatibility with DACs or microcontrollers (µCs).
Ratiometric ICTL and VCTL improve the accuracy of the
charge current and voltage set point by matching VRE-
FIN to the reference of the host. For standard applica-
tions, internal set points for ICTL and VCTL provide 3A
charge current (with 0.015Ωsense resistor), and 4.2V
(per cell) charge voltage. Connect ICTL and VCTL to
LDO to select the internal set points. The MAX1908
safely conditions overdischarged cells with 300mA (with
0.015Ωsense resistor) until the battery-pack voltage
exceeds 3.1V ×number of series-connected cells. The
SHDN input allows shutdown from a microcontroller or
thermistor.
The DC-DC converter uses external n-channel
MOSFETs as the buck switch and synchronous rectifier
to convert the input voltage to the required charging
current and voltage. The
Typical Application Circuit
shown in Figure 1 uses a µC to control charging cur-
rent, while Figure 2 shows a typical application with
charging voltage and current fixed to specific values
for the application. The voltage at ICTL and the value of
RS2 set the charging current. The DC-DC converter
generates the control signals for the external MOSFETs
to regulate the voltage and the current set by the VCTL,
ICTL, and CELLS inputs.
The MAX1908/MAX8724/MAX8765/MAX8765A feature
a voltage regulation loop (CCV) and two current regula-
tion loops (CCI and CCS). The CCV voltage regulation
loop monitors BATT to ensure that its voltage does not
exceed the voltage set by VCTL. The CCI battery cur-
rent regulation loop monitors current delivered to BATT
to ensure that it does not exceed the current limit set by
ICTL. A third loop (CCS) takes control and reduces the
battery-charging current when the sum of the system
load and the battery-charging input current exceeds
the input current limit set by CLS.
Setting the Battery-Regulation Voltage
The MAX1908/MAX8724/MAX8765/MAX8765A use a
high-accuracy voltage regulator for charging voltage.
The VCTL input adjusts the charger output voltage.
VCTL control voltage can vary from 0 to VREFIN, provid-
ing a 10% adjustment range on the VBATT regulation
voltage. By limiting the adjust range to 10% of the regu-
lation voltage, the external resistor mismatch error is
reduced from 1% to 0.05% of the regulation voltage.
Therefore, an overall voltage accuracy of better than
0.7% is maintained while using 1% resistors. The per-
cell battery termination voltage is a function of the bat-
tery chemistry. Consult the battery manufacturer to
determine this voltage. Connect VCTL to LDO to select
the internal default setting VBATT = 4.2V ×number of
cells, or program the battery voltage with the following
equation:
CELLS is the programming input for selecting cell count.
Connect CELLS as shown in Table 2 to charge 2, 3, or 4
Li+ cells. When charging other cell chemistries, use
CELLS to select an output voltage range for the charger.
The internal error amplifier (GMV) maintains voltage
regulation (Figure 3). 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 reg-
ulation and current regulation loops allows for optimal
compensation (see the
Compensation
section).
V CELLS V V
V
BATT VCTL
REFIN
=×+×
⎛
⎝
⎜⎞
⎠
⎟
⎛
⎝
⎜⎞
⎠
⎟
404.
Table 2. Cell-Count Programming
DESCRIPTION MAX1908 MAX8724 MAX8765/
MAX8765A
Conditioning
Charge Feature Yes No No
ICTL Shutdown
Mode Yes Yes No
ACOK Enable
Condition
REFIN must
be ready
REFIN must
be ready
Independent
of REFIN
Table 1. Versions Comparison
CELLS CELL COUNT
GND 2
Open 3
VREFIN 4
MAX1908/MAX8724/MAX8765/MAX8765A
14
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
DCIN
LDO
MAX1908
MAX8724
MAX8765
MAX8765A
CLSREF
GND
CELLS
DLOV
AC ADAPTER INPUT
8.5V TO 28V
12.6V OUTPUT VOLTAGE
7.5A INPUT
CURRENT LIMIT
DHI
D3
BST
SMART
BATTERY
HOST
ACIN
D2
R6
59kΩ
1%
R7
19.6kΩ
1%
C5
1µF
VCTL
ICTL
REFIN
ACOK
ICHG
IINP
R8
1MΩ
R9
20kΩ
R10
10kΩ
C14
0.1µF
C20
0.1µF
CCV
C11
0.1µF
R5
1kΩ
CCI
CCS
C10
0.01µF
C9
0.01µF
C12
1µF
C1
2 × 10µF
C13
1µF
C15
0.1µF
LX
C16
1µF
LDO
R13
33Ω
CSSP CSSN
OPEN (3 CELLS SELECT)
D1
RS1
0.01Ω
L1
10µH
RS2
0.015Ω
CSIP
CSIN
PGND
DLO
N1b
N1a
BATT
C4
22µF
BATT+
R19, R20, R21
10kΩ
AVDD/REF
SCL
SDA
TEMP
BATT-
ADC INPUT
ADC INPUT
OUTPUT
DAC OUTPUT
VCC
SCL
SDA
ADC INPUT
GND
PGND GND
TO EXTERNAL
LOAD
SHDN
0.1µF0.1µF
Figure 1. µC-Controlled Typical Application Circuit
Typical Application Circuits
Maxim Integrated
15
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
TO EXTERNAL
LOAD
MAX1908
MAX8724
MAX8765
MAX8765A
CLSREF
GND
CELLS
REFIN (4 CELLS SELECT)
DLOV
AC ADAPTER
INPUT
8.5V TO 28V
DHI
D3
BST
BATTERY
ACIN
D2
LDO
LDO
16.8V OUTPUT VOLTAGE
2.5A CHARGE LIMIT
4A INPUT CURRENT LIMIT
R6
59kΩ
1% R7
19.6kΩ
1%
R11
15kΩ
R12
12kΩ
C5
1µF
C12
1.5nF
SHDN
ICHG
IINP
R19
10kΩ
1%
R20
10kΩ
1%
CCV
C11
0.1µF
R5
1kΩ
CCI
CCS
C10
0.01µF
C9
0.01µF
C12
1µF
C1
2 × 10µF
C13
1µF
C15
0.1µF
LX
C16
1µF
LDO
R13
33Ω
CSSP CSSN
RS1
0.01Ω
L1
10µH
RS2
0.015Ω
CSIP
CSIN
PGND
DLO N1b
N1a
FROM HOST µP
(SHUTDOWN) N
BATT
GNDPGND
C4
22µF
BATT+
REFIN
VCTL
DCIN
BATT-
THM
ICTL
R14
10.5kΩ
1%
R15
8.25kΩ
1%
R16
8.25kΩ
1%
P1
R17
19.1kΩ
1%
R18
22kΩ
1%
ACOK
0.01µF0.01µF
Figure 2. Typical Application Circuit with Fixed Charging Parameters
Typical Application Circuits (continued)
MAX1908/MAX8724/MAX8765/MAX8765A
16
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
MAX1908
MAX8724
MAX8765
MAX8765A
LOGIC
BLOCK
GMS
SHDN
GND
CLS
CCS
CSSP
CSSN
CSIP
CSIN
ICTL
CCI
BATT
CELLS
CCV
VCTL
23.5%
REFIN
GND
DCIN
SRDY
5.4V
LINEAR
REGULATOR
1/55
ICTL
MAX1908/MAX8724 ONLY
REF/2
RDY
4V
CELL
SELECT
LOGIC
4.096V
REFERENCE
LVC
REFIN
CSI
BAT_UV
3.1V/CELL
R1
LVC
DCIN
LDO
REF
REFIN
ACIN
ACOK
IINP
ICHG
BST
DHI
LX
DLOV
DLO
PGND
MAX1908 ONLY
x75mV
REF
LEVEL
SHIFTER
x75mV
REFIN
x400mV
REFIN
DC-DC
CONVERTER
GMI
GMV
GM
LEVEL
SHIFTER
N
GM
LEVEL
SHIFTER
DRIVER
DRIVER
Figure 3. Functional Diagram
Functional Diagram
Maxim Integrated
17
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
Setting the Charging-Current Limit
The ICTL input sets the maximum charging current. The
current is set by current-sense resistor RS2, connected
between CSIP and CSIN. The full-scale differential
voltage between CSIP and CSIN is 75mV; thus, for a
0.015Ωsense resistor, the maximum charging current
is 5A. Battery-charging current is programmed with
ICTL using the equation:
The input voltage range for ICTL is VREFIN/32 to VREFIN.
The MAX1908/MAX8724 shut down if ICTL is forced
below VREFIN/100 (min), while the MAX8765/MAX8765A
does not.
Connect ICTL to LDO to select the internal default full-
scale, charge-current sense voltage of 45mV. The
charge current when ICTL = LDO is:
where RS2 is 0.015Ω, providing a charge-current set
point of 3A.
The current at the ICHG output is a scaled-down replica
of the battery output current being sensed across CSIP
and CSIN (see the
Current Measurement
section).
When choosing the current-sense resistor, note that the
voltage drop across this resistor causes further power
loss, reducing efficiency. However, adjusting ICTL to
reduce the voltage across the current-sense resistor
can degrade accuracy due to the smaller signal to the
input of the current-sense amplifier. The charging-
current-error amplifier (GMI) is compensated at CCI
(see the
Compensation
section).
Setting the Input Current Limit
The total input current (from an AC adapter or other DC
source) is a function of the system supply current and
the battery-charging current. The input current regulator
limits the input current by reducing the charging
current when the input current exceeds the input
current-limit set point. System current normally fluc-
tuates as portions of the system are powered up or
down. Without input current regulation, the source must
be able to supply the maximum system current and the
maximum charger input current simultaneously. By using
the input current limiter, the current capability of the AC
adapter can be lowered, reducing system cost.
The MAX1908/MAX8724/MAX8765/MAX8765A limit the
battery charge current when the input current-limit
threshold is exceeded, ensuring the battery charger
does not load down the AC adapter voltage. An internal
amplifier compares the voltage between CSSP and
CSSN to the voltage at CLS. VCLS can be set by a
resistive divider between REF and GND. Connect CLS
to REF for the full-scale input current limit. The CLS volt-
age range for the MAX1908/MAX8724 is from 1.6V to
REF, while the MAX8765/MAX8765A CLS voltage is
from 1.1V to REF.
The input current is the sum of the device current, the
charger input current, and the load current. The device
current is minimal (3.8mA) in comparison to the charge
and load currents. Determine the actual input current
required as follows:
where ηis the efficiency of the DC-DC converter.
VCLS determines the reference voltage of the GMS
error amplifier. Sense resistor RS1 and VCLS determine
the maximum allowable input current. Calculate the
input current limit as follows:
Once the input current limit is reached, the charging
current is reduced until the input current is at the
desired threshold.
When choosing the current-sense resistor, note that the
voltage drop across this resistor causes further power
loss, reducing efficiency. Choose the smallest value for
RS1 that achieves the accuracy requirement for the
input current-limit set point.
Conditioning Charge
The MAX1908 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 ×number of cells programmed by CELLS, the
MAX1908 charges the battery with 300mA current when
using sense resistor RS2 = 0.015Ω. After the
battery voltage exceeds the conditioning charge thresh-
old, the MAX1908 resumes full-charge mode, charging
to the programmed voltage and current limits. The
MAX8724/MAX8765/MAX8765A do not offer this feature.
AC Adapter Detection
Connect the AC adapter voltage through a resistive
divider to ACIN to detect when AC power is available,
as shown in Figure 1. ACIN voltage rising trip point is
VREF/2 with 20mV hysteresis. ACOK is an open-drain
output and is high impedance when ACIN is less than
IV
VRS
INPUT CLS
REF
=×
0 075
1
.
II IV
V
INPUT LOAD CHG BATT
IN
=+ ×
×
⎛
⎝
⎜⎞
⎠
⎟
η
IV
RS
CHG =0 045
2
.
IV
VRS
CHG ICTL
REFIN
=×
0 075
2
.
MAX1908/MAX8724/MAX8765/MAX8765A
18
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
VREF/2. Since ACOK can withstand 30V (max), ACOK
can drive a p-channel MOSFET directly at the charger
input, providing a lower dropout voltage than a
Schottky diode (Figure 2). In the MAX1908/MAX8724
the ACOK comparator is enabled after REFIN is ready.
In the MAX8765/MAX8765A, the ACOK comparator is
independent of REFIN.
Current Measurement
Use ICHG to monitor the battery-charging current being
sensed across CSIP and CSIN. The ICHG voltage is
proportional to the output current by the equation:
VICHG = ICHG x RS2 x GICHG x R9
where ICHG is the battery-charging current, GICHG is
the transconductance of ICHG (3µA/mV typ), and R9 is
the resistor connected between ICHG and ground.
Leave ICHG unconnected if not used.
Use IINP to monitor the system input current being
sensed across CSSP and CSSN. The voltage of IINP is
proportional to the input current by the equation:
VIINP = IINPUT x RS1 x GIINP x R10
where IINPUT is the DC current being supplied by the AC
adapter power, GIINP is the transconductance of IINP
(3µA/mV typ), and R10 is the resistor connected between
IINP and ground. ICHG and IINP have a 0 to 3.5V output
voltage range. Leave IINP unconnected if not used.
LDO Regulator
LDO provides a 5.4V supply derived from DCIN and
can deliver up to 10mA of load current. The MOSFET
drivers are powered by DLOV and BST, which must be
connected to LDO as shown in Figure 1. LDO supplies
the 4.096V reference (REF) and most of the control cir-
cuitry. Bypass LDO with a 1µF capacitor to GND.
Shutdown
The MAX1908/MAX8724/MAX8765/MAX8765A feature
a low-power shutdown mode. Driving SHDN low shuts
down the MAX1908/MAX8724/MAX8765/MAX8765A. In
shutdown, the DC-DC converter is disabled and CCI,
CCS, and CCV are pulled to ground. The IINP and
ACOK outputs continue to function.
SHDN can be driven by a thermistor to allow automatic
shutdown of the MAX1908/MAX8724/MAX8765/
MAX8765A when the battery pack is hot. The shutdown
falling threshold is 23.5% (typ) of VREFIN with 1%
VREFIN hysteresis to provide smooth shutdown when
driven by a thermistor.
DC-DC Converter
The MAX1908/MAX8724/MAX8765/MAX8765A employ
a buck regulator with a bootstrapped nMOS high-side
switch and a low-side nMOS synchronous rectifier.
CCV, CCI, CCS, and LVC Control Blocks
The MAX1908/MAX8724/MAX8765/MAX8765A control
input current (CCS control loop), charge current (CCI
control loop), or charge voltage (CCV control loop),
depending on the operating condition.
The three control loops, CCV, CCI, and CCS are brought
together internally at the LVC amplifier (lowest voltage
clamp). The output of the LVC amplifier is the feedback
control signal for the DC-DC controller. The output of the
GMamplifier that is the lowest sets the output of the LVC
amplifier and also 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.
DC-DC Controller
The MAX1908/MAX8724/MAX8765/MAX8765A feature a
variable off-time, cycle-by-cycle current-mode control
scheme. Depending upon the conditions, the MAX1908/
MAX8724/MAX8765/MAX8765A work in continuous or
discontinuous-conduction mode.
Continuous-Conduction Mode
With sufficient charger loading, the MAX1908/MAX8724/
MAX8765/MAX8765A operate in continuous-conduction
mode (inductor current never reaches zero) switching at
400kHz if the BATT voltage is within the following range:
3.1V x (number of cells) < VBATT < (0.88 x VDCIN )
The operation of the DC-DC controller is controlled by
the following four comparators as shown in Figure 4:
• IMIN—Compares the control point (LVC) against
0.15V (typ). If IMIN output is low, then a new cycle
cannot begin.
• CCMP—Compares the control point (LVC) against the
charging current (CSI). The high-side MOSFET on-
time is terminated if the CCMP output is high.
• IMAX—Compares the charging current (CSI) to 6A
(RS2 = 0.015Ω). The high-side MOSFET on-time is
terminated if the IMAX output is high and a new cycle
cannot begin until IMAX goes low.
• ZCMP—Compares the charging current (CSI) to
333mA (RS2 = 0.015Ω). If ZCMP output is high, then
both MOSFETs are turned off.
Maxim Integrated
19
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
IMAX
RESET
1.8V
0.15V
0.1V
5ms
LVC
CONTROL
CELLS
SETV
SETI
CCVCCICCS
GMS
GMI
GMV
CLS
DLO
DHI
CSI
X20
tOFF
GENERATOR
BST
S
RQ
CCMP
ZCMP
IMIN
CHG
RQ
S
CSS
X20
CSSP AC ADAPTER
CSSN
BST
DHI
LX
RS1 LDO
D3
N1a
N1b
CBST
L1
RS2
DLO
CSIP
CSIN
COUT
BATT
BATTERY
MAX1908
MAX8724
MAX8765
MAX8765A
Q
CELL
SELECT
LOGIC
Figure 4. DC-DC Functional Diagram
DC-DC Functional Diagram
MAX1908/MAX8724/MAX8765/MAX8765A
20
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
In normal operation, the controller starts a new cycle by
turning on the high-side n-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 n-channel MOSFET and
turns on the low-side n-channel MOSFET. The opera-
tional frequency is governed by the off-time and is
dependent upon VDCIN and VBATT. The off-time is set
by the following equations:
where:
These equations result in fixed-frequency operation
over the most common operating conditions.
At the end of the fixed off-time, another cycle begins if
the control point (LVC) is greater than 0.15V, IMIN =
high, and the peak charge current is less than 6A (RS2
= 0.015Ω), IMAX = high. If the charge current exceeds
IMAX, the on-time is terminated by the IMAX compara-
tor. IMAX governs the maximum cycle-by-cycle current
limit and is internally set to 6A (RS2 = 0.015Ω). IMAX
protects against sudden overcurrent faults.
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.
There is a minimum 0.3µs off-time when the (VDCIN -
VBATT) differential becomes too small. If VBATT ≥0.88 ×
VDCIN, then the threshold for minimum off-time is
reached and the tOFF is fixed at 0.3µs. A maximum on-
time of 5ms allows the controller to achieve > 99% duty
cycle in continuous-conduction mode. The switching
frequency in this mode varies according to the equation:
Discontinuous Conduction
The MAX1908/MAX8724/MAX8765/MAX8765A enter dis-
continuous-conduction mode when the output of the LVC
control point falls below 0.15V. For RS2 = 0.015Ω, this
corresponds to 0.5A:
for RS2 = 0.015Ω.
In discontinuous mode, a new cycle is not started until
the LVC voltage rises above 0.15V. Discontinuous-
mode operation can occur during conditioning charge
of overdischarged battery packs, when the charge cur-
rent has been reduced sufficiently by the CCS control
loop, or when the battery pack is near full charge (con-
stant-voltage-charging mode).
MOSFET Drivers
The low-side driver output DLO switches between
PGND and DLOV. DLOV is usually connected through
a filter to LDO. The high-side driver output DHI is boot-
strapped off LX and switches between VLX and VBST.
When the low-side driver turns on, BST rises to one
diode voltage below DLOV.
Filter DLOV with a lowpass filter whose cutoff frequency
is approximately 5kHz (Figure 1):
Dropout Operation
The MAX1908/MAX8724/MAX8765/MAX8765A have 99%
duty-cycle capability with a 5ms (max) on-time and 0.3µs
(min) off-time. This allows the charger to achieve dropout
performance limited only by resistive losses in the DC-DC
converter components (D1, N1, RS1, and RS2, Figure 1).
Replacing diode D1 with a p-channel MOSFET driven by
ACOK improves dropout performance (Figure 2). The
dropout voltage is set by the difference between DCIN
and CSIN. When the dropout voltage falls below 100mV,
the charger is disabled; 200mV hysteresis ensures that
the charger does not turn back on until the dropout volt-
age rises to 300mV.
Compensation
Each of the three regulation loops—input current limit,
charging current limit, and charging voltage limit—are
compensated separately using CCS, CCI, and CCV,
respectively.
fRC F kHz
C==
××
=
1
2
1
2331 48
ππ µΩ.
IMIN V
RS A=×=
015
20 2 05
..
fLI
VV s
RIPPLE
CSSN BATT
=×
−
()
+
1
03.µ
ftt
ON OFF
=+
1
IVt
L
RIPPLE BATT OFF
=×
tLI
VV
ON RIPPLE
CSSN BATT
=×
−
ts
VV
V
OFF DCIN BATT
DCIN
=× −
25.µ
Maxim Integrated
21
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
CCV Loop Definitions
Compensation of the CCV loop depends on the para-
meters and components shown in Figure 5. CCV and
RCV are the CCV loop compensation capacitor and
series resistor. RESR is the equivalent series resistance
(ESR) of the charger output capacitor (COUT). RLis the
equivalent charger output load, where RL= VBATT/
ICHG. The equivalent output impedance of the GMV
amplifier, ROGMV ≥10MΩ. The voltage amplifier
transconductance, GMV = 0.125µA/mV. The DC-DC
converter transconductance, GMOUT = 3.33A/V:
where ACSI = 20, and RS2 is the charging current-
sense resistor in the
Typical Application Circuits
.
The compensation pole is given by:
The compensation zero is given by:
The output pole is given by:
where RLvaries with load according to RL= VBATT/ICHG.
Output zero due to output capacitor ESR:
The loop transfer function is given by:
Assuming the compensation pole is a very low
frequency, and the output zero is a much higher fre-
quency, the crossover frequency is given by:
To calculate RCV and CCV values of the circuit of Figure 2:
Cells = 4
COUT = 22µF
VBATT = 16.8V
ICHG = 2.5A
GMV = 0.125µA/mV
GMOUT = 3.33A/V
ROGMV = 10MΩ
f = 400kHz
Choose crossover frequency to be 1/5th the
MAX1908’s 400kHz switching frequency:
Solving yields RCV = 26kΩ.
Conservatively set RCV = 1kΩ, which sets the crossover
frequency at:
fCO_CV = 3kHz
Choose the output-capacitor ESR so the output-capacitor
zero is 10 times the crossover frequency:
fRC MHz
Z ESR ESR OUT
_.=×=
1
22 412
π
RfC
ESR CO CV OUT
=×× × =
1
210 024
π_
.Ω
fGMV R GM
CkHz
CO CV CV OUT
OUT
_=×× =
280
π
fGMV R GM
C
CO CV CV OUT
OUT
_=××
2π
LTF GM R GMV R
sC R sC R
sC R sC R
OUT L OGMV
OUT ESR CV CV
CV OGMV OUT L
=××××
+×
()
+×
()
+×
()
+×
()
11
11
fRC
Z ESR ESR OUT
_=×
1
2π
fRC
P OUT L OUT
_=×
1
2π
fRC
ZCV CV CV
_=×
1
2π
fRC
PCV OGMV CV
_=×
1
2π
GM ARS
OUT CSI
=×
1
2
GMOUT
BATT
CCV
GMV
REF
RCV
CCV
ROGMV
RESR RL
COUT
Figure 5. CCV Loop Diagram
MAX1908/MAX8724/MAX8765/MAX8765A
22
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
The 22µF ceramic capacitor has a typical ESR of
0.003Ω, which sets the output zero at 2.412MHz.
The output pole is set at:
where:
Set the compensation zero (fZ_CV) so it is equivalent to
the output pole (fP_OUT = 1.08kHz), effectively produc-
ing a pole-zero cancellation and maintaining a single-
pole system response:
Choose CCV = 100nF, which sets the compensation
zero (fZ_CV) at 1.6kHz. This sets the compensation pole:
CCI Loop Definitions
Compensation of the CCI loop depends on the parame-
ters and components shown in Figure 7. CCI is the CCI
loop compensation capacitor. ACSI is the internal gain
of the current-sense amplifier. RS2 is the charge cur-
rent-sense resistor, RS2 = 15mΩ. ROGMI is the equiva-
lent output impedance of the GMI amplifier ≥10MΩ.
GMI is the charge-current amplifier transconductance
= 1µA/mV. GMOUT is the DC-DC converter transcon-
ductance = 3.3A/V. The CCI loop is a single-pole sys-
tem with a dominant pole compensation set by fP_CI:
The loop transfer function is given by:
Since:
The loop transfer function simplifies to:
LTF GMI R
sR C
OGMI
OGMI CI
=×
+×1
GM ARS
OUT CSI
=×
1
2
LTF GM A RS GMI R
sR C
OUT CSI OGMI
OGMI CI
=×××
+×
21
fRC
PCI OGMI CI
_=×
1
2π
fRC Hz
PCV OGMV CV
_.=×=
1
2016
π
CR kHz nF
CV CV
=×=
1
2108
147
π.
fRC
ZCV CV CV
_=×
1
2π
RV
IBattery ESR
LBATT
CHG
==
∆
∆
fRC kHz
P OUT L OUT
_.=×=
1
2108
π
CCV LOOP GAIN
vs. FREQUENCY
FREQUENCY (Hz)
GAIN (dB)
100k10k1k10010
-40
-20
0
20
40
60
80
-60
11M
CCV LOOP PHASE
vs. FREQUENCY
FREQUENCY (Hz)
PHASE (DEGREES)
100k10k1k10010
-120
-105
-90
-75
-60
-45
-135
11M
Figure 6. CCV Loop Gain/Phase vs. Frequency
Maxim Integrated
23
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
The crossover frequency is given by:
The CCI loop dominant compensation pole:
where the GMI amplifier output impedance, ROGMI =
10MΩ.
To calculate the CCI loop compensation pole, CCI:
GMI = 1µA/mV
GMOUT = 3.33A/V
ROGMI = 10MΩ
f = 400kHz
Choose crossover frequency fCO_CI to be 1/5th the
MAX1908/MAX8724/MAX8765/MAX8765A switching
frequency:
Solving for CCI, CCI = 2nF.
To be conservative, set CCI = 10nF, which sets the
crossover frequency at:
The compensation pole, fP_CI is set at:
CCS Loop Definitions
Compensation of the CCS loop depends on the parame-
ters and components shown in Figure 9. CCS is the CCS
loop compensation capacitor. ACSS is the internal gain of
the current-sense amplifier. RS1 is the input current-
sense resistor, RS1 = 10mΩ. ROGMS is the equivalent
output impedance of the GMS amplifier ≥10MΩ. GMS is
fGMI
RC Hz
PCI OGMI CI
_.=×=
20 0016
π
fGMI
nF kHz
CO CI_==
210 16
π
fGMI
CkHz
CO CI CI
_==
280
π
fRC
PCI OGMI CI
_=×
1
2π
fGMI
C
CO CI CI
_=2π
GMOUT
CCI
GMI
ICTL
CCI ROGMI
CSIP CSIN
CSI
RS2
Figure 7. CCI Loop Diagram
CCI LOOP GAIN
vs. FREQUENCY
FREQUENCY (Hz)
GAIN (dB)
100k10k1 10 100 1k
-40
-20
0
20
40
60
80
100
-60
0.1 1M
CCI LOOP PHASE
vs. FREQUENCY
FREQUENCY (Hz)
PHASE (DEGREES)
100k10k1k100101
-90
-75
-60
-45
-30
-15
0
-105
0.1 1M
MAX1908/MAX8724/MAX8765/MAX8765A
24
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
the charge-current amplifier transconductance = 1µA/mV.
GMIN is the DC-DC converter transconductance =
3.3A/V. The CCS loop is a single-pole system with a dom-
inant pole compensation set by fP_CS:
The loop transfer function is given by:
Since:
Then, the loop transfer function simplifies to:
The crossover frequency is given by:
The CCS loop dominant compensation pole:
where the GMS amplifier output impedance, ROGMS =
10MΩ.
To calculate the CCI loop compensation pole, CCS:
GMS = 1µA/mV
GMIN = 3.33A/V
ROGMS = 10MΩ
f = 400kHz
fRC
PCS OGMS CS
_=×
1
2π
fGMS
C
CO CS CS
_=2π
LTF GMS R
sR C
OGMS
OGMS CS
=×
+×1
GM ARS
IN CSS
=×
1
1
LTF GM A RS GMS R
sR C
IN CSS OGMS
OGMS CS
=××××
+×
11
fRC
PCS OGMS CS
_=×
1
2π
GMIN
CCS
GMS
CLS
CCS ROGMS
CSSP CSSN
CSS
RS1
Figure 9. CCS Loop Diagram
CCS LOOP GAIN
vs. FREQUENCY
FREQUENCY (Hz)
GAIN (dB)
100k10k1 10 100 1k
-40
-20
0
20
40
60
80
100
-60
0.1 1M
CCS LOOP PHASE
vs. FREQUENCY
FREQUENCY (Hz)
PHASE (DEGREES)
100k10k1k100101
-90
-75
-60
-45
-30
-15
0
-105
0.1 1M
Figure 10. CCS Loop Gain/Phase vs. Frequency
Maxim Integrated
25
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
Choose crossover frequency fCO_CS to be 1/5th the
MAX1908/MAX8724/MAX8765/MAX8765A switching
frequency:
Solving for CCS, CCS = 2nF.
To be conservative, set CCS = 10nF, which sets the
crossover frequency at:
The compensation pole, fP_CS is set at:
Component Selection
Table 3 lists the recommended components and refers
to the circuit of Figure 2. The following sections
describe how to select these components.
Inductor Selection
Inductor L1 provides power to the battery while it is
being charged. It must have a saturation current of at
least the charge current (ICHG), plus 1/2 the current rip-
ple IRIPPLE:
ISAT = ICHG + (1/2) IRIPPLE
Ripple current varies according to the equation:
IRIPPLE = (VBATT) ×tOFF/L
where:
tOFF = 2.5µs ×(VDCIN – VBATT)/VDCIN
VBATT < 0.88 ×VDCIN
or:
tOFF = 0.3µs
VBATT > 0.88 ×VDCIN
Figure 11 illustrates the variation of ripple current vs.
battery voltage when charging at 3A with a fixed 19V
input voltage.
Higher inductor values decrease the ripple current.
Smaller inductor values require higher saturation cur-
rent capabilities and degrade efficiency. Designs for
ripple current, IRIPPLE = 0.3 ×ICHG usually result in a
good balance between inductor size and efficiency.
Input Capacitor
Input capacitor C1 must be able to handle the input
ripple current. At high charging currents, the DC-DC
converter operates in continuous conduction. In this
case, the ripple current of the input capacitor can be
approximated by the following equation:
where:
IC1 = input capacitor ripple current.
D = DC-DC converter duty ratio.
ICHG = battery-charging current.
Input capacitor C1 must be sized to handle the maxi-
mum ripple current that occurs during continuous con-
duction. The maximum input ripple current occurs at
50% duty cycle; thus, the worst-case input ripple cur-
rent is 0.5 ×ICHG. If the input-to-output voltage ratio is
such that the DC-DC converter does not operate at a
50% duty cycle, then the worst-case capacitor current
occurs where the duty cycle is nearest 50%.
The input capacitor ESR times the input ripple current
sets the ripple voltage at the input, and should not
exceed 0.5V ripple. Choose the ESR of C1 according to:
The input capacitor size should allow minimal output
voltage sag at the highest switching frequency:
ICdV
dt
C1
21=
ESR V
I
CC
11
05
<.
II DD
C CHG12
=−
fRC Hz
PCS OGMS CS
_.=×=
1
20 0016
π
fGMS
nF kHz
CO CS_==
210 16
π
fGMS
CkHz
CO CS CS
_==
280
π
RIPPLE CURRENT vs.
BATTERY VOLTAGE
VBATT (V)
RIPPLE CURRENT (A)
14131211109
0.5
1.0
1.5
0
8 15161718
VDCIN = 19V
VCTL = ICTL = LDO
4 CELLS
3 CELLS
Figure 11. Ripple Current vs. Battery Voltage
MAX1908/MAX8724/MAX8765/MAX8765A
26
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
where dV is the maximum voltage sag of 0.5V while
delivering energy to the inductor during the high-side
MOSFET on-time, and dt is the period at highest oper-
ating frequency (400kHz):
Both tantalum and ceramic capacitors are suitable in
most applications. For equivalent size and voltage
rating, tantalum capacitors have higher capacitance,
but also higher ESR than ceramic capacitors. This
makes it more critical to consider ripple current and
power-dissipation ratings when using tantalum capaci-
tors. A single ceramic capacitor often can replace two
tantalum capacitors in parallel.
Output Capacitor
The output capacitor absorbs the inductor ripple cur-
rent. The output capacitor impedance must be signifi-
cantly less than that of the battery to ensure that it
absorbs the ripple current. Both the capacitance and
ESR rating of the capacitor are important for its effec-
tiveness as a filter and to ensure stability of the DC-DC
converter (see the
Compensation
section). Either tanta-
lum or ceramic capacitors can be used for the output
filter capacitor.
MOSFETs and Diodes
Schottky diode D1 provides power to the load when the
AC adapter is inserted. This diode must be able to
deliver the maximum current as set by RS1. For
reduced power dissipation and improved dropout per-
formance, replace D1 with a p-channel MOSFET (P1)
as shown in Figure 2. Take caution not to exceed the
maximum VGS of P1. Choose resistors R11 and R12 to
limit the VGS.
The n-channel MOSFETs (N1a, N1b) are the switching
devices for the buck controller. High-side switch N1a
should have a current rating of at least the maximum
charge current plus one-half the ripple current and
have an on-resistance (RDS(ON)) that meets the power
dissipation requirements of the MOSFET. The driver for
N1a is powered by BST. The gate-drive requirement for
N1a should be less than 10mA. Select a MOSFET with a
low total gate charge (QGATE) and determine the
required drive current by IGATE = QGATE ×f (where f is
the DC-DC converter’s maximum switching frequency).
The low-side switch (N1b) has the same current rating
and power dissipation requirements as N1a, and
should have a total gate charge less than 10nC. N2 is
used to provide the starting charge to the BST capacitor
(C15). During the dead time (50ns, typ) between N1a
and N1b, the current is carried by the body diode of
the MOSFET. Choose N1b with either an internal
Schottky diode or body diode capable of carrying the
maximum charging current during the dead time. The
Schottky diode D3 provides the supply current to the
high-side MOSFET driver.
Layout and Bypassing
Bypass DCIN with a 1µF capacitor to power ground
(Figure 1). D2 protects the MAX1908/MAX8724/
MAX8765/MAX8765A when the DC power source input
is reversed. A signal diode for D2 is adequate because
DCIN only powers the internal circuitry. Bypass LDO,
REF, CCV, CCI, CCS, ICHG, and IINP to analog
ground. Bypass DLOV to power ground.
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 pencil sketch showing the placement of
the power-switching components and high-current rout-
ing. Refer to the PC board layout in the MAX1908 eval-
uation kit for examples. Separate analog and power
grounds are essential for optimum performance.
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.
e) Connect C1 to high-side MOSFET (10mm max
length).
f) LX node (MOSFETs, 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 power ground plane is
connected to the normal ground plane at the
MAX1908/MAX8724/MAX8765/MAX8765As’ back-
side exposed pad. Other high-current paths should
also be minimized, but focusing primarily on short
ground and current-sense connections eliminates
most PC board layout problems.
CIs
V
C
12
25
05
1
>×
.
.
µ
Maxim Integrated
27
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
Table 3. Component List for Circuit of Figure 2
DESIGNATION QTY DESCRIPTION
C1 2
10µF, 50V 2220-size ceramic
capacitors
TDK C5750X7R1H106M
C4 1
22µF, 25V 2220-size ceramic
capacitor
TDK C5750X7R1E226M
C5 1
1µF, 25V X7R ceramic capacitor
(1206)
Murata GRM31MR71E105K
Taiyo Yuden TMK316BJ105KL
TDK C3216X7R1E105K
C9, C10 2
0.01µF, 16V cer am i c cap aci tor s ( 0402)
Murata GRP155R71E103K
Taiyo Yuden EMK105BJ103KV
TDK C1005X7R1E103K
C11, C14,
C15, C20 4
0.1µF, 25V X7R ceramic capacitors
(0603)
Murata GRM188R71E104K
TDK C1608X7R1E104K
C12, C13, C16 3
1µF, 6.3V X5R ceramic capacitors
(0603)
Murata GRM188R60J105K
Taiyo Yuden JMK107BJ105KA
TDK C1608X5R1A105K
D1 (optional) 1
10A Schottky diode (D-PAK)
Diodes, Inc. MBRD1035CTL
ON Semiconductor MBRD1035CTL
D2 1
Schottky diode
Central Semiconductor
CMPSH1–4
DESIGNATION QTY DESCRIPTION
D3 1 Schottky diode
Central Semiconductor CMPSH1-4
L1 1
10µH, 4.4A inductor
Sumida CDRH104R-100NC
TOKO 919AS-100M
N1 1 Dual, n-channel, 8-pin SO MOSFET
Fairchild FDS6990A or FDS6990S
P1 1 Single, p-channel, 8-pin SO MOSFET
Fairchild FDS6675
R5 1 1kΩ ±5% resistor (0603)
R6 1 59kΩ ±1% resistor (0603)
R7 1 19.6kΩ ±1% resistor (0603)
R11 1 12kΩ ±5% resistor (0603)
R12 1 15kΩ ±5% resistor (0603)
R13 1 33Ω ±5% resistor (0603)
R14 1 10.5kΩ ±1% resistor (0603)
R15, R16 2 8.25kΩ ±1% resistors (0603)
R17 1 19.1kΩ ±1% resistor (0603)
R18 1 22kΩ ±1% resistor (0603)
R19, R20 2 10kΩ ±1% resistors (0603)
RS1 1
0.01Ω ±1%, 0.5W 2010 sense resistor
Vishay Dale WSL2010 0.010 1.0%
IRC LRC-LR2010-01-R010-F
RS2 1
0.015Ω ±1%, 0.5W 2010 sense
resistor
Vishay Dale WSL2010 0.015 1.0%
IRC LRC-LR2010-01-R015-F
U1 1 MAX1908ETI+, MAX8724ETI+, or
MAX8765ETI+, MAX8765AETI+
2) Place the IC and signal components. Keep the
main switching node (LX node) away from sensitive
analog components (current-sense traces and REF
capacitor). Important: The IC must be no further
than 10mm from the current-sense resistors.
Keep the gate-drive traces (DHI, DLO, and BST)
shorter than 20mm, and route them away from the
current-sense lines and REF. Place ceramic
bypass capacitors close to the IC. The bulk capac-
itors can be placed further away.
3) Use a single-point star ground placed directly
below the part at the backside exposed pad of the
MAX1908/MAX8724/MAX8765/MAX8765A.
Connect the power ground and normal ground to
this node.
MAX1908/MAX8724/MAX8765/MAX8765A
28
Maxim Integrated
Low-Cost Multichemistry Battery Chargers
Chip Information
TRANSISTOR COUNT: 3772
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages. Note that a "+", "#", or "-" in
the package code indicates RoHS status only. Package draw-
ings may show a different suffix character, but the drawing per-
tains to the package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
28 TQFN-EP T2855-6 21-0140
Maxim Integrated
29
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 2/03 Initial release —
5 11/09 Added the MAX8765A to the data sheet. 1–30
MAX1908/MAX8724/MAX8765/MAX8765A
30 Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000
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. The parametric values (min and max limits) shown in the Electrical
Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
© 2009 Maxim Integrated The Maxim logo and Maxim Integrated are trademarks of Maxim Integrated Products, Inc.
