Maxim MAX8724 Low-cost multichemistry battery charger Datasheet

19-2764; Rev 5; 11/09
KIT
ATION
EVALU
E
L
B
A
AVAIL
Low-Cost Multichemistry Battery Chargers
The MAX1908/MAX8724/MAX8765/MAX8765A highly
integrated, multichemistry battery-charger control ICs
simplify the construction of accurate and efficient chargers. 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 outputs to monitor current drawn from the AC adapter (DC
input source), battery-charging current, and the presence 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
Minimum Operating Circuit
o ±4% Accurate Input Current Limiting
o ±5% Accurate Charge Current
o Analog Inputs Control Charge Current and
Charge Voltage
o Outputs for Monitoring
Current Drawn from AC Adapter
Charging Current
AC Adapter Presence
o Up to 17.6V Battery-Voltage Set Point
o Maximum 28V Input Voltage
o > 95% Efficiency
o Shutdown Control Input
o Charge Any Battery Chemistry
Li+, NiCd, NiMH, Lead Acid, etc.
Ordering Information
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.
Pin Configuration
TO EXTERNAL
LOAD
0.01Ω
DLOV
ACIN
FROM HOST µP
SHDN
MAX1908
MAX8724
MAX8765
MAX8765A
DHI
CELLS
BATT
VCTL
17
16
15
DLOV
22
14
GND
LX
23
13
ICTL
BST
24
12
REFIN
LX
DHI
25
CSSN
26
CSSP
27
IINP
28
MAX1908
MAX8724
MAX8765
MAX8765A
DLO
ICHG
10µH
IINP
PGND
CCV
CSIP
0.015Ω
CCI
CSIN
BATT
CCS
REF
18
CLS
BATT+
+
1
2
LDO
ACOK
20 19
3
4
5
6
7
CCV
ICTL
21
CCI
BST
CCS
VCTL
DCIN
LDO
CSIN
LDO
REFIN
REF
DCIN
CSIP
CELLS
PGND
TOP VIEW
CSSN
CLS
CSSP
o ±0.5% Output Voltage Accuracy Using Internal
Reference (±0.4% for MAX8765A, 2-/3-Cell Only)
DLO
AC ADAPTER
INPUT
Features
11
ACOK
10
ACIN
9
ICHG
8
SHDN
GND
THIN QFN
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
1
MAX1908/MAX8724/MAX8765/MAX8765A
General Description
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
ABSOLUTE MAXIMUM RATINGS
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
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.
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.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
CHARGE-VOLTAGE REGULATION
Battery-Regulation Voltage
Accuracy
(MAX1908/MAX8724/MAX8765
(2, 3, or 4 Cells) and MAX8765A
(4 Cells Only))
VVCTL = VREFIN
-0.5
+0.5
VVCTL = VREFIN/20
-0.5
+0.5
VVCTL = VLDO
-0.5
+0.5
Battery-Regulation Voltage
Accuracy (MAX8765A, 2 or 3
Cells Only)
VVCTL = VREFIN
-0.4
VVCTL = VREFIN/20
-0.4
+0.4
+0.4
VVCTL = VLDO
-0.4
+0.4
VCTL Default Threshold
VVCTL rising
4.0
REFIN Range
(Note 1)
2.5
REFIN Undervoltage Lockout
VREFIN falling
4.1
%
%
4.2
V
3.6
V
1.20
1.92
V
75
78.75
mV
CHARGE-CURRENT REGULATION
CSIP-to-CSIN Full-Scale CurrentSense Voltage
Charging-Current Accuracy
VICTL = VREFIN
71.25
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
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
4.2
V
19
V
ICTL Default Threshold
BATT/CSIP/CSIN Input Voltage
Range
2
VICTL rising
4.0
4.1
0
_______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
(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
400
650
6.0
6.8
7.5
A
REFIN/
100
REFIN/
55
REFIN/
33
V
VDCIN = 0V or VICTL = 0V or V SHDN = 0V
CSIP/CSIN Input Current
1
Charging
Cycle-by-Cycle Maximum Current
Limit
RS2 = 0.015mΩ
ICTL Power-Down Mode
Threshold Voltage
(MAX1908/MAX8724 Only)
VVCTL rising
ICTL, VCTL Input Bias Current
REFIN Input Bias Current
VVCTL = VICTL = 0 or 3V
-1
+1
VDCIN = 0V, VVCTL = VICTL = VREFIN = 5V
-1
+1
VDCIN = 5V, VREFIN = 3V
-1
+1
VREFIN = 5V
-1
+1
UNITS
µA
µA
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
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
ICHG Accuracy
%
INPUT-CURRENT REGULATION
CSSP-to-CSSN Full-Scale
Current-Sense Voltage
72
VCLS = VREF
75
78
mV
-4
+4
VCLS = VREF/2
-7.5
+7.5
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
Input Current-Limit Accuracy
CSSP, CSSN Input Current
(MAX1908/MAX8724 Only)
CSSP Input Current
(MAX8765/MAX8765A Only)
VDCIN = 0V
0.1
1
VCSSP = VCSSN = VDCIN > 8V
350
600
VCSSP = VCSSN = 28V
VDCIN = 0V
0.1
1
VDCIN = 28V
400
650
%
µA
µA
_______________________________________________________________________________________
3
MAX1908/MAX8724/MAX8765/MAX8765A
ELECTRICAL CHARACTERISTICS (continued)
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
CSSN Input Current
(MAX8765/MAX8765A Only)
CONDITIONS
VCSSP = VCSSN = 28V
TYP
MAX
VDCIN = 0V
MIN
0.1
1
VDCIN = 28V
0.1
1
UNITS
µA
CLS Input Range
(MAX1908/MAX8724 Only)
1.6
REF
V
CLS Input Range
(MAX8765/MAX8765A Only)
1.1
REF
V
VCLS = 2V
-1
+1
µA
VCSSP - VCSSN = 75mV
2.7
3.3
µA/mV
CLS Input Bias Current
IINP Transconductance
(MAX1908/MAX8724 Only)
GIINP
VCSSP - VCSSN = 75mV
IINP Accuracy
IINP Transconductance
(MAX8765/MAX8765A Only)
GIINP
3
-5
+5
VCSSP - VCSSN = 37.5mV
-7.5
+7.5
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
VDCIN falling
DCIN Undervoltage-Lockout Trip
Point
DCIN Quiescent Current
IDCIN
BATT Input Current
IBATT
7
28
7.4
V
V
VDCIN rising
7.5
7.85
8.0V < VDCIN < 28V
3.2
6
200
500
5.4
5.55
V
34
100
mV
VBATT = 19V, VDCIN = 0V
1
VBATT = 2V to 19V, VDCIN = 19.3V
5.25
mA
µA
LDO Output Voltage
8V < VDCIN < 28V, no load
LDO Load Regulation
0 < ILDO < 10mA
LDO Undervoltage-Lockout Trip
Point
VDCIN = 8V
3.20
4
5.15
V
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
REFERENCE
4
_______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
(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
50
100
150
UNITS
TRIP POINTS
VDCIN falling, referred to VCSIN
(MAX1908/MAX8724 only)
BATT Power-Fail Threshold
mV
VCSSP falling, referred to VCSIN
(MAX8765/MAX8765A only)
50
BATT Power-Fail Threshold
Hysteresis
ACIN Threshold
100
150
200
mV
ACIN rising (MAX8765/MAX8765A only)
2.028
2.048
2.068
ACIN rising (MAX1908/MAX8724 only)
2.007
2.048
2.089
ACIN Threshold Hysteresis
0.5% of REF
ACIN Input Bias Current
VACIN = 2.048V
20
-1
V
mV
+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
2.5
DHI Maximum On-Time
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
Minimum Discontinuous-Mode
Ripple Current
Battery Undervoltage Charge
Current
Battery Undervoltage Current
Threshold
VBATT = 3V per cell (RS2 = 15mΩ),
MAX1908 only, VBATT rising
99.9
%
0.5
A
150
300
450
CELLS = GND, MAX1908 only, VBATT rising
6.1
6.2
6.3
CELLS = open, MAX1908 only, VBATT rising
9.15
9.3
9.45
CELLS = VREFIN, MAX1908 only, VBATT rising
12.2
12.4
12.6
mA
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
_______________________________________________________________________________________
5
MAX1908/MAX8724/MAX8765/MAX8765A
ELECTRICAL CHARACTERISTICS (continued)
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
0.0625
0.125
0.2500
µA/mV
ERROR AMPLIFIERS
GMV Amplifier Transconductance
GMV
VVCTL = VLDO, VBATT = 16.8V,
CELLS = VREFIN
GMI Amplifier Transconductance
GMI
VICTL = VREFIN, 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
0.25V < VCCV,CCS,CCI < 2V
150
300
600
mV
0.4
V
CCI, CCS, CCV Clamp Voltage
LOGIC LEVELS
CELLS Input Low Voltage
CELLS Input Open Voltage
CELLS = open
CELLS Input High Voltage
CELLS Input Bias Current
CELLS = 0V or VREFIN
(VREFIN
(VREFIN
VREFIN/
/2) /2) +
2
0.2V
0.2V
V
VREFIN
- 0.4V
V
-2
+2
µA
0
28
V
ACOK AND SHDN
ACOK Input Voltage Range
ACOK Sink Current
V A COK = 0.4V, VACIN = 3V
ACOK Leakage Current
V A COK = 28V, VACIN = 0V
1
mA
1
µA
0
LDO
V
VSHDN = 0V OR VLDO
-1
+1
VSHDN = 0V OR VSHDN = 5V
-1
+1
V SHDN falling
22
SHDN Input Voltage Range
SHDN Input Bias Current
SHDN Threshold
SHDN Threshold Hysteresis
6
µA
23.5
1
_______________________________________________________________________________________
25
% of
VREFIN
% of
VREFIN
Low-Cost Multichemistry Battery Chargers
(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
VVCTL = VLDO
-0.6
+0.6
REFIN Range
(Note 1)
2.5
3.6
V
REFIN Undervoltage Lockout
VREFIN falling
1.92
V
mV
Battery Regulation Voltage
Accuracy (2, 3, or 4 Cells)
%
CHARGE CURRENT REGULATION
CSIP-to-CSIN Full-Scale CurrentSense Voltage
VICTL = VREFIN
70.5
79.5
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
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
Charging-Current Accuracy
VDCIN = 0V or VICTL = 0V or VSHDN = 0V
CSIP/CSIN Input Current
Cycle-by-Cycle Maximum Current
Limit
%
1
Charging
IMAX
ICTL Power-Down Mode
Threshold Voltage
(MAX1908/MAX8724 Only)
RS2 = 0.015Ω
VICTL rising
650
µA
6.0
7.5
A
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
_______________________________________________________________________________________
7
MAX1908/MAX8724/MAX8765/MAX8765A
ELECTRICAL CHARACTERISTICS
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
ICHG Accuracy
CONDITIONS
MIN
TYP
MAX
VCSIP - VCSIN = 75mV
-7.5
+7.5
VCSIP - VCSIN = 45mV
-7.5
+7.5
VCSIP - VCSIN = 5mV
-40
+40
71.25
78.75
UNITS
%
INPUT-CURRENT REGULATION
CSSP-to-CSSN Full-Scale
Current-Sense Voltage
VCLS = VREF
Input Current-Limit Accuracy
-5
+5
VCLS = VREF/2
-7.5
+7.5
VCLS = 1.1V (MAX8765/MAX8765A only)
mV
%
-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
CSSP, CSSN Input Current
(MAX1908/MAX8724 Only)
1
VCSSP = VCSSN = VDCIN > 8V
CSSP Input Current
(MAX8765/MAX8765A Only)
VCSSP = VCSSN = 28V
CSSN Input Current
(MAX8765/MAX8765A Only)
VCSSP = VCSSN = 28V
600
VDCIN = 0V
1
VDCIN = 28V
650
VDCIN = 0V
1
VDCIN = 28V
1
µA
µA
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
VCSSP - VCSSN = 37.5mV
-7.5
+7.5
IINP Accuracy
8
_______________________________________________________________________________________
%
Low-Cost Multichemistry Battery Chargers
(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
28
V
8.0V < VDCIN < 28V
6
mA
VBATT = 19V, VDCIN = 0V
1
SUPPLY AND LDO REGULATOR
DCIN Input Voltage Range
VDCIN
DCIN Quiescent Current
IDCIN
BATT Input Current
IBATT
8
VBATT = 2V to 19V, VDCIN = 19.3V
LDO Output Voltage
8V < VDCIN < 28V, no load
LDO Load Regulation
0 < ILDO < 10mA
500
5.25
µA
5.55
V
100
mV
4.065
4.120
V
50
150
REFERENCE
REF Output Voltage
0 < IREF < 500µA
TRIP POINTS
VDCIN falling, referred to VCSIN
(MAX1908/MAX8724 only)
BATT Power-Fail Threshold
mV
VCSSP falling, referred to VCSIN
(MAX8765/MAX8765A only)
50
150
ACIN rising (MAX8765/MAX8765A only)
2.028
2.068
ACIN rising (MAX1908/MAX8724 only)
2.007
2.089
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
2.5
7.5
ms
ACIN Threshold
V
SWITCHING REGULATOR
DHI Maximum On-Time
DHI Maximum Duty Cycle
Battery Undervoltage Charge
Current
Battery Undervoltage Current
Threshold
99
%
VBATT = 3V per cell (RS2 = 15mΩ),
MAX1908 only, VBATT rising
150
450
CELLS = GND, MAX1908 only, VBATT rising
6.09
6.30
CELLS = open, MAX1908 only, VBATT rising
9.12
9.45
CELLS = VREFIN, MAX1908 only, VBATT rising
12.18
12.60
mA
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
_______________________________________________________________________________________
9
MAX1908/MAX8724/MAX8765/MAX8765A
ELECTRICAL CHARACTERISTICS (continued)
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
0.0625
0.250
µA/mV
ERROR AMPLIFIERS
GMV
VVCTL = VLDO, VBATT = 16.8V,
CELLS = VREFIN
GMI Amplifier Transconductance
GMI
VICTL = VREFIN, VCSIP - VCSIN = 75mV
0.5
2.0
µA/mV
GMS Amplifier Transconductance
GMS
VCLS = VREF, VCSSP - VCSSN = 75mV
0.5
2.0
µA/mV
0.25V < VCCV,CCS,CCI < 2V
150
600
mV
0.4
V
(VREFIN
/2) +
0.2V
V
GMV Amplifier Transconductance
CCI, CCS, CCV Clamp Voltage
LOGIC LEVELS
CELLS Input Low Voltage
CELLS Input Open Voltage
(VREFIN
/2) 0.2V
CELLS = open
VREFIN
- 0.4V
CELLS Input High Voltage
V
ACOK AND SHDN
ACOK Input Voltage Range
0
ACOK Sink Current
V ACOK = 0.4V, VACIN = 3V
SHDN Input Voltage Range
SHDN Threshold
28
1
V SHDN falling
V
mA
0
LDO
V
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.
Typical Operating Characteristics
(Circuit of Figure 1, VDCIN = 20V, TA = +25°C, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE
(BATTERY INSERTION AND REMOVAL)
MAX1908 toc03
MAX1908 toc02
ADAPTER
CURRENT
5A/div
IBATT
2A/div
LOAD
CURRENT
5A/div
VBATT
5V/div
CCV
CCI
VCCI
500mV/div
VCCV
500mV/div
ICTL = LDO
VCTL = LDO
10
LOAD-TRANSIENT RESPONSE
(STEP IN-LOAD CURRENT)
LOAD-TRANSIENT RESPONSE
(STEP IN-LOAD CURRENT)
MAX1908 toc01
1ms/div
0
0
VBATT
2V/div
VCCI
500mV/div
VCCS
500mV/div
16.8V
CCS
CCI
CCS
1ms/div
ICTL = LDO
CHARGING CURRENT = 3A
VBATT = 16.8V
LOAD STEP = 0 TO 4A
ISOURCE LIMIT = 5A
LOAD
CURRENT
5A/div
0
ADAPTER
CURRENT
5A/div
0
CHARGE
CURRENT
2A/div
CCI
0
VBATT
2V/div
1ms/div
ICTL = LDO
CHARGING CURRENT = 3A
VBATT = 16.8V
LOAD STEP = 0 TO 4A
ISOURCE LIMIT = 5A
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
LDO LINE REGULATION
LDO LOAD REGULATION
MAX1908 toc04
VDCIN
10V/div
-0.1
-0.2
INDUCTOR
CURRENT
500mA/div
-0.3
-0.4
-0.5
0.03
0.01
0
-0.02
-0.7
-0.8
-0.03
VLDO = 5.4V
-0.9
-0.04
-0.05
-1.0
0
1
2
3
4
5
6
7
8
9
8
10
VIN (V)
0.06
VREF ERROR (%)
VREF ERROR (%)
-0.03
0.08
-0.04
-0.05
-0.06
0.04
90
80
EFFICIENCY (%)
0.02
0
-0.02
-0.04
30
-0.06
20
-0.09
-0.08
10
-0.10
-0.10
500
0
-40
-15
10
REF CURRENT (µA)
350
4 CELLS
300
250
200
150
ICHARGE = 3A
VCTL = ICTL = LDO
100
50
3 CELLS
0.3
0.2
0.1
0
4 CELLS
-0.1
-0.2
-0.3
-0.5
0
2
4
6
8 10 12 14 16 18 20 22
(VIN - VBATT) (V)
0.08
0.07
10
0.06
0.05
0.04
0.03
0.02
4 CELLS
REFIN = 3.3V
NO LOAD
0.01
-0.4
0
1
BATT VOLTAGE ERROR vs. VCTL
2 CELLS
0.4
0.1
CHARGE CURRENT (A)
OUTPUT V/I CHARACTERISTICS
MAX1908 toc10
3 CELLS
400
0.01
85
0.5
BATT VOLTAGE ERROR (%)
450
60
TEMPERATURE (°C)
FREQUENCY vs. VIN - VBATT
500
35
BATT VOLTAGE ERROR (%)
400
MAX1908 toc11
300
VBATT = 8V
40
-0.08
200
VBATT = 12V
50
-0.07
100
VBATT = 16V
70
60
MAX1908 toc12
-0.02
100
MAX1908 toc08
MAX1908 toc07
-0.01
EFFICIENCY vs. CHARGE CURRENT
REF VOLTAGE ERROR vs. TEMPERATURE
0.10
MAX1908 toc09
REF VOLTAGE LOAD REGULATION
0
10 12 14 16 18 20 22 24 26 28
LDO CURRENT (mA)
0
FREQUENCY (kHz)
0.02
-0.01
-0.6
10ms/div
ICTL = LDO
VCTL = LDO
ICHARGE = 3A
LINE STEP 18.5V TO 27.5V
ILDO = 0
VLDO = 5.4V
0.04
VLDO ERROR (%)
VLDO ERROR (%)
VBATT
500mV/div
0.05
MAX1908 toc05
0
MAX1908 toc06
LINE-TRANSIENT RESPONSE
0
0
1
2
BATT CURRENT (A)
3
4
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
VCTL/REFIN (%)
______________________________________________________________________________________
11
MAX1908/MAX8724/MAX8765/MAX8765A
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VDCIN = 20V, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VDCIN = 20V, TA = +25°C, unless otherwise noted.)
ICHG ERROR vs. CHARGE CURRENT
CURRENT-SETTING ERROR vs. ICTL
4
MAX1908 toc14
5.0
MAX1908 toc13
4.5
4.0
VREFIN = 3.3V
VREFIN = 3.3V
3.5
3
ICHG (%)
CURRENT-SETTING ERROR (%)
5
2
1
3.0
VBATT = 16V
VBATT = 12V
VBATT = 8V
2.5
2.0
1.5
1.0
0
0.5
0
-1
0
0.5
1.0
1.5
0
2.0
0.5
1.0
IINP ERROR vs. SYSTEM LOAD CURRENT
2.0
2.5
3.0
IINP ERROR vs. INPUT CURRENT
30
MAX1908 toc16
80
MAX1908 toc15
40
60
20
40
IINP ERROR (%)
IBATT = 0
10
0
-10
ERROR DUE TO SWITCHING NOISE
20
0
-20
-20
-40
-30
-60
-40
SYSTEM LOAD = 0
-80
0
1
2
3
SYSTEM LOAD CURRENT (A)
12
1.5
IBATT (A)
VICTL (V)
IINP ERROR (%)
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
4
0
0.5
1.0
1.5
2.0
INPUT CURRENT (A)
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
PIN
NAME
FUNCTION
1
DCIN
Charging Voltage Input. Bypass DCIN with a 1µF capacitor to PGND.
2
LDO
Device Power Supply. Output of the 5.4V linear regulator supplied from DCIN. Bypass with a 1µF capacitor to GND.
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 constantvoltage 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
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
Cell Count Input. Tri-level input for setting number of cells. GND = 2 cells, open = 3 cells, REFIN = 4 cells.
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
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
High-Side Power MOSFET Driver Power-Return Connection. Connect to the source of the high-side nMOS.
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.
______________________________________________________________________________________
13
MAX1908/MAX8724/MAX8765/MAX8765A
Pin Description
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 VREFIN to the reference of the host. For standard applications, internal set points for ICTL and VCTL provide 3A
charge current (with 0.015Ω sense 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
0.015Ω sense 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 current, 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 regulation 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 current 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
voltage. By limiting the adjust range to 10% of the regulation 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 percell battery termination voltage is a function of the battery 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:
⎛
⎛
⎞⎞
V
VBATT = CELLS × ⎜ 4 V + ⎜ 0.4 × VCTL ⎟ ⎟
VREFIN ⎠ ⎠
⎝
⎝
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 regulation and current regulation loops allows for optimal
compensation (see the Compensation section).
Table 1. Versions Comparison
MAX1908
MAX8724
MAX8765/
MAX8765A
Conditioning
Charge Feature
Yes
No
No
ICTL Shutdown
Mode
Yes
Yes
No
REFIN must
be ready
REFIN must
be ready
Independent
of REFIN
DESCRIPTION
ACOK Enable
Condition
Table 2. Cell-Count Programming
CELLS
CELL COUNT
GND
2
Open
3
VREFIN
4
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, providing a 10% adjustment range on the VBATT regulation
14
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
AC ADAPTER INPUT
8.5V TO 28V
RS1
0.01Ω
D1
0.1µF
D2
R6
59kΩ
1%
R7
19.6kΩ
1%
TO EXTERNAL
LOAD
CSSP
CSSN
CELLS
DCIN
C5
1µF
C1
2 × 10µF
0.1µF
OPEN (3 CELLS SELECT)
LDO
LDO
C13
1µF
VCTL
R13
33Ω
D3
BST
DAC OUTPUT
ICTL
DLOV
12.6V OUTPUT VOLTAGE
VCC
C15
0.1µF
REFIN
R8
1MΩ
ACIN
LX
SHDN
OUTPUT
N1a
DHI
ACOK
C16
1µF
N1b
DLO
ICHG
ADC INPUT
IINP
ADC INPUT
MAX1908
MAX8724
MAX8765
MAX8765A
CCV
C14
0.1µF
R9
20kΩ
C20
0.1µF
R10
10kΩ
HOST
L1
10µH
PGND
CSIP
R5
1kΩ
RS2
0.015Ω
C11
0.1µF
CSIN
CCI
C10
0.01µF
GND
REF
AVDD/REF
R19, R20, R21
10kΩ
BATT+
BATT
CCS
C9
0.01µF
C12
1µF
C4
22µF
CLS
7.5A INPUT
CURRENT LIMIT
SMART
BATTERY
SCL
SCL
SDA
SDA
ADC INPUT
TEMP
GND
BATT-
PGND
GND
Figure 1. µC-Controlled Typical Application Circuit
______________________________________________________________________________________
15
MAX1908/MAX8724/MAX8765/MAX8765A
Typical Application Circuits
Low-Cost Multichemistry Battery Chargers
MAX1908/MAX8724/MAX8765/MAX8765A
Typical Application Circuits (continued)
AC ADAPTER
INPUT
8.5V TO 28V
RS1
0.01Ω
P1
TO EXTERNAL
LOAD
R11
15kΩ
C1
2 × 10µF
0.01µF 0.01µF
R12
12kΩ
LDO
R6
59kΩ
1%
CSSP
ACOK
DCIN
D2
R7
19.6kΩ
1%
CSSN
CELLS
C5
1µF
R14
10.5kΩ
1%
LDO
VCTL
LDO
C13
1µF
R13
33Ω
D3
REFIN
BST
R15
8.25kΩ
1%
DLOV
16.8V OUTPUT VOLTAGE
2.5A CHARGE LIMIT
R16
8.25kΩ
1%
FROM HOST µP
(SHUTDOWN)
REFIN (4 CELLS SELECT)
N
C15
0.1µF
ICTL
C16
1µF
N1a
DHI
ACIN
LX
R19
10kΩ
1%
C12
1.5nF
DLO
MAX1908
MAX8724
MAX8765
MAX8765A
SHDN
R20
10kΩ
1%
ICHG
IINP
N1b
L1
10µH
PGND
CSIP
CCV
R5
1kΩ
RS2
0.015Ω
C11
0.1µF
CSIN
CCI
CCS
C9
0.01µF
C10
0.01µF
BATT+
BATT
GND
REF
CLS
C4
22µF
BATTERY
THM
BATT-
C12
1µF
R17
19.1kΩ
1%
PGND GND
R18
22kΩ
1%
4A INPUT CURRENT LIMIT
Figure 2. Typical Application Circuit with Fixed Charging Parameters
16
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
DCIN
SHDN
23.5%
REFIN
LDO
RDY
GND
MAX1908
MAX8724
MAX8765
MAX8765A
GND
LOGIC
BLOCK
5.4V
LINEAR
REGULATOR
4.096V
REFERENCE
REF
MAX1908/MAX8724 ONLY
1/55
REFIN
ICTL
SRDY
ACIN
ACOK
DCIN
N
REF/2
CCS
x
CLS
CSSP
75mV
REF
CSIP
GM
ICHG
CSI
LEVEL
SHIFTER
CSIN
IINP
GMS
LEVEL
SHIFTER
CSSN
GM
BST
x
ICTL
GMI
75mV
REFIN
LEVEL
SHIFTER
DRIVER
DHI
CCI
MAX1908 ONLY
3.1V/CELL
BATT
BAT_UV
LX
LVC
LVC
REFIN
CELLS
R1
DC-DC
CONVERTER
GMV
CELL
SELECT
LOGIC
DLOV
DRIVER
DLO
CCV
PGND
VCTL
400mV
x
REFIN
4V
Figure 3. Functional Diagram
______________________________________________________________________________________
17
MAX1908/MAX8724/MAX8765/MAX8765A
Functional Diagram
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
0.015Ω sense resistor, the maximum charging current
is 5A. Battery-charging current is programmed with
ICTL using the equation:
ICHG =
VICTL
0.075
×
VREFIN
RS2
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 fullscale, charge-current sense voltage of 45mV. The
charge current when ICTL = LDO is:
ICHG =
0.045V
RS2
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 chargingcurrent-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 fluctuates 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
18
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. V CLS can be set by a
resistive divider between REF and GND. Connect CLS
to REF for the full-scale input current limit. The CLS voltage 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:
⎛I
× VBATT ⎞
IINPUT = ILOAD + ⎜ CHG
⎟
VIN × η
⎝
⎠
where η is the efficiency of the DC-DC converter.
V CLS 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:
V
0.075
IINPUT = CLS ×
VREF
RS1
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 threshold, 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
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
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 circuitry. 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 V REFIN 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
GM amplifier 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 ontime 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.
______________________________________________________________________________________
19
MAX1908/MAX8724/MAX8765/MAX8765A
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.
Low-Cost Multichemistry Battery Chargers
MAX1908/MAX8724/MAX8765/MAX8765A
DC-DC Functional Diagram
5ms
S
RESET
CSSP
MAX1908
MAX8724
MAX8765
MAX8765A
BST
IMAX
R
1.8V
Q
R
CSS
X20
AC ADAPTER
RS1
D3
BST
Q
N1a
DHI
DHI
CCMP
LDO
CSSN
CBST
LX
CHG
S
IMIN
Q
0.15V
DLO
N1b
DLO
L1
tOFF
GENERATOR
CSIP
ZCMP
0.1V
CSI
X20
LVC
RS2
CSIN
GMS
BATT
COUT
BATTERY
GMI
GMV
SETV
CONTROL SETI
CLS
CELLS
CCS
CCI
CELL
SELECT
LOGIC
CCV
Figure 4. DC-DC Functional Diagram
20
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
V
− VBATT
t OFF = 2.5µs × DCIN
VDCIN
t ON =
L × IRIPPLE
VCSSN − VBATT
where:
V
×t
IRIPPLE = BATT OFF
L
f=
1
t ON + t OFF
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 comparator. 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 ×
V DCIN , then the threshold for minimum off-time is
reached and the tOFF is fixed at 0.3µs. A maximum ontime 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:
1
f=
L × IRIPPLE
+ 0.3µs
(VCSSN − VBATT )
Discontinuous Conduction
The MAX1908/MAX8724/MAX8765/MAX8765A 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:
IMIN =
0.15V
= 0.5A
20 × RS2
for RS2 = 0.015Ω.
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 battery pack is near full charge (constant-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 bootstrapped 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):
fC =
1
1
=
= 4.8kHz
2πRC 2π × 33Ω × 1µF
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 voltage 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.
______________________________________________________________________________________
21
MAX1908/MAX8724/MAX8765/MAX8765A
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 operational frequency is governed by the off-time and is
dependent upon VDCIN and VBATT. The off-time is set
by the following equations:
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
where RL varies with load according to RL = VBATT/ICHG.
Output zero due to output capacitor ESR:
BATT
fZ _ ESR =
GMOUT
RESR
RL
The loop transfer function is given by:
LTF = GMOUT × RL × GMV × ROGMV ×
CCV
GMV
RCV
COUT
ROGMV
(1+ sCOUT × RESR )(1+ sCCV × RCV )
(1+ sCCV × ROGMV )(1+ sCOUT × RL )
REF
Assuming the compensation pole is a very low
frequency, and the output zero is a much higher frequency, the crossover frequency is given by:
CCV
fCO _ CV =
Figure 5. CCV Loop Diagram
CCV Loop Definitions
Compensation of the CCV loop depends on the parameters 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). RL is the
equivalent charger output load, where R L = V BATT/
ICHG. The equivalent output impedance of the GMV
amplifier, R OGMV ≥ 10MΩ. The voltage amplifier
transconductance, GMV = 0.125µA/mV. The DC-DC
converter transconductance, GMOUT = 3.33A/V:
GMOUT =
1
ACSI × RS2
where A CSI = 20, and RS2 is the charging currentsense resistor in the Typical Application Circuits.
The compensation pole is given by:
fP _ CV =
1
2πROGMV × CCV
The compensation zero is given by:
fZ _ CV =
1
2πRCV × CCV
fP _ OUT =
1
2πRL × COUT
GMV × RCV × GMOUT
2πCOUT
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:
fCO _ CV =
GMV × RCV × GMOUT
= 80kHz
2πCOUT
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:
RESR =
1
= 0.24Ω
2π × 10 × fCO _ CV × COUT
The output pole is given by:
22
1
2πRESR × COUT
fZ _ ESR =
1
= 2.412MHz
2πRESR × COUT
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
The output pole is set at:
fP _ OUT =
1
= 1.08kHz
2πRL × COUT
where:
RL =
∆VBATT
= Battery ESR
∆ICHG
Set the compensation zero (fZ_CV) so it is equivalent to
the output pole (fP_OUT = 1.08kHz), effectively producing a pole-zero cancellation and maintaining a singlepole system response:
fZ _ CV =
1
2πRCV × CCV
CCI Loop Definitions
Compensation of the CCI loop depends on the parameters 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 current-sense resistor, RS2 = 15mΩ. ROGMI is the equivalent output impedance of the GMI amplifier ≥ 10MΩ.
GMI is the charge-current amplifier transconductance
= 1µA/mV. GMOUT is the DC-DC converter transconductance = 3.3A/V. The CCI loop is a single-pole system with a dominant pole compensation set by fP_CI:
fP _ CI =
1
2πROGMI × CCI
The loop transfer function is given by:
LTF = GMOUT × A CSI × RS2 × GMI
ROGMI
1+ sROGMI × CCI
Since:
1
CCV =
= 147nF
2πRCV × 1.08kHz
GMOUT =
Choose CCV = 100nF, which sets the compensation
zero (fZ_CV) at 1.6kHz. This sets the compensation pole:
fP _ CV =
The loop transfer function simplifies to:
1
= 0.16Hz
2πROGMV × CCV
LTF = GMI ×
ROGMI
1+ sROGMI × CCI
CCV LOOP PHASE
vs. FREQUENCY
CCV LOOP GAIN
vs. FREQUENCY
80
-45
60
-60
PHASE (DEGREES)
40
GAIN (dB)
1
ACSI × RS2
20
0
-20
-75
-90
-105
-120
-40
-60
-135
1
10
100
1k
10k
100k
FREQUENCY (Hz)
1M
1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 6. CCV Loop Gain/Phase vs. Frequency
______________________________________________________________________________________
23
MAX1908/MAX8724/MAX8765/MAX8765A
The 22µF ceramic capacitor has a typical ESR of
0.003Ω, which sets the output zero at 2.412MHz.
CSIP
To calculate the CCI loop compensation pole, CCI:
GMI = 1µA/mV
GMOUT = 3.33A/V
CSIN
GMOUT
ROGMI = 10MΩ
f = 400kHz
Choose crossover frequency fCO_CI to be 1/5th the
MAX1908/MAX8724/MAX8765/MAX8765A switching
frequency:
RS2
CSI
CCI
ROGMI
CCI
Solving for CCI, CCI = 2nF.
To be conservative, set C CI = 10nF, which sets the
crossover frequency at:
ICTL
Figure 7. CCI Loop Diagram
fCO _ CI =
The crossover frequency is given by:
fCO _ CI =
fP _ CI =
1
2πROGMI × CCI
GMI
= 0.0016Hz
2πROGMI × CCI
CCS Loop Definitions
where the GMI amplifier output impedance, ROGMI =
10MΩ.
Compensation of the CCS loop depends on the parameters 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 currentsense resistor, RS1 = 10mΩ. ROGMS is the equivalent
output impedance of the GMS amplifier ≥ 10MΩ. GMS is
CCI LOOP GAIN
vs. FREQUENCY
CCI LOOP PHASE
vs. FREQUENCY
100
0
80
-15
PHASE (DEGREES)
60
40
20
0
-30
-45
-60
-75
-20
-90
-40
-60
-105
0.1
1
10
100
1k
FREQUENCY (Hz)
24
GMI
= 16kHz
2π10nF
The compensation pole, fP_CI is set at:
GMI
2πCCI
The CCI loop dominant compensation pole:
fP _ CI =
GMI
= 80kHz
2πCCI
fCO _ CI =
GMI
GAIN (dB)
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
10k
100k
1M
0.1
1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
fP _ CS =
CSSP
CSSN
GMIN
RS1
1
2πROGMS × CCS
CSS
The loop transfer function is given by:
LTF = GMIN × A CSS × RS1× GMS ×
CCS
ROGMS
1+ sROGMS × CCS
GMS
Since:
ROGMS
CCS
CLS
1
GMIN =
ACSS × RS1
Then, the loop transfer function simplifies to:
LTF = GMS ×
Figure 9. CCS Loop Diagram
ROGMS
1+ sROGMS × CCS
The CCS loop dominant compensation pole:
fP _ CS =
The crossover frequency is given by:
fCO _ CS =
1
2πROGMS × CCS
where the GMS amplifier output impedance, ROGMS =
10MΩ.
To calculate the CCI loop compensation pole, CCS:
GMS
2πCCS
GMS = 1µA/mV
GMIN = 3.33A/V
ROGMS = 10MΩ
f = 400kHz
CCS LOOP GAIN
vs. FREQUENCY
CCS LOOP PHASE
vs. FREQUENCY
100
0
80
-15
PHASE (DEGREES)
GAIN (dB)
60
40
20
0
-30
-45
-60
-75
-20
-90
-40
-60
-105
0.1
1
10
100
1k
10k
FREQUENCY (Hz)
100k
1M
0.1
1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 10. CCS Loop Gain/Phase vs. Frequency
______________________________________________________________________________________
25
MAX1908/MAX8724/MAX8765/MAX8765A
the charge-current amplifier transconductance = 1µA/mV.
GM IN is the DC-DC converter transconductance =
3.3A/V. The CCS loop is a single-pole system with a dominant pole compensation set by fP_CS:
Choose crossover frequency fCO_CS to be 1/5th the
MAX1908/MAX8724/MAX8765/MAX8765A switching
frequency:
fCO _ CS =
GMS
= 80kHz
2πCCS
Solving for CCS, CCS = 2nF.
To be conservative, set CCS = 10nF, which sets the
crossover frequency at:
fCO _ CS =
GMS
= 16kHz
2π10nF
The compensation pole, fP_CS is set at:
fP _ CS =
1
2πROGMS × CCS
= 0.0016Hz
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 ripple IRIPPLE:
ISAT = ICHG + (1/2) IRIPPLE
Ripple current varies according to the equation:
IRIPPLE = (VBATT) × tOFF/L
RIPPLE CURRENT vs.
BATTERY VOLTAGE
1.5
3 CELLS
RIPPLE CURRENT (A)
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
4 CELLS
1.0
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 current 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:
IC1 = ICHG D − D2
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 maximum ripple current that occurs during continuous conduction. The maximum input ripple current occurs at
50% duty cycle; thus, the worst-case input ripple current 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:
ESRC1 <
0.5
VDCIN = 19V
VCTL = ICTL = LDO
0
8
9
0.5V
IC1
The input capacitor size should allow minimal output
voltage sag at the highest switching frequency:
10 11 12 13 14 15 16 17 18
VBATT (V)
Figure 11. Ripple Current vs. Battery Voltage
26
where:
IC1
dV
= C1
2
dt
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
I
2.5µs
C1 > C1 ×
2
0.5V
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 capacitors. A single ceramic capacitor often can replace two
tantalum capacitors in parallel.
Output Capacitor
The output capacitor absorbs the inductor ripple current. The output capacitor impedance must be significantly 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 effectiveness as a filter and to ensure stability of the DC-DC
converter (see the Compensation section). Either tantalum 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 performance, 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 (Q GATE ) 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 routing. Refer to the PC board layout in the MAX1908 evaluation 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’ backside 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.
______________________________________________________________________________________
27
MAX1908/MAX8724/MAX8765/MAX8765A
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 operating frequency (400kHz):
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
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 capacitors 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.
Table 3. Component List for Circuit of Figure 2
DESIGNATION QTY
C1
C4
C5
C9, C10
C11, C14,
C15, C20
C12, C13, C16
D1 (optional)
D2
28
DESCRIPTION
DESIGNATION QTY
DESCRIPTION
2
10µF, 50V 2220-size ceramic
capacitors
TDK C5750X7R1H106M
D3
1
Schottky diode
Central Semiconductor CMPSH1-4
L1
1
1
22µF, 25V 2220-size ceramic
capacitor
TDK C5750X7R1E226M
10µH, 4.4A inductor
Sumida CDRH104R-100NC
TOKO 919AS-100M
N1
1
Dual, n-channel, 8-pin SO MOSFET
Fairchild FDS6990A or FDS6990S
1
1µF, 25V X7R ceramic capacitor
(1206)
Murata GRM31MR71E105K
Taiyo Yuden TMK316BJ105KL
TDK C3216X7R1E105K
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)
2
4
3
0.01µF, 16V ceramic capacitors (0402)
Murata GRP155R71E103K
Taiyo Yuden EMK105BJ103KV
TDK C1005X7R1E103K
0.1µF, 25V X7R ceramic capacitors
(0603)
Murata GRM188R71E104K
TDK C1608X7R1E104K
1µF, 6.3V X5R ceramic capacitors
(0603)
Murata GRM188R60J105K
Taiyo Yuden JMK107BJ105KA
TDK C1608X5R1A105K
1
10A Schottky diode (D-PAK)
Diodes, Inc. MBRD1035CTL
ON Semiconductor MBRD1035CTL
1
Schottky diode
Central Semiconductor
CMPSH1–4
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+
______________________________________________________________________________________
Low-Cost Multichemistry Battery Chargers
TRANSISTOR COUNT: 3772
PROCESS: BiCMOS
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 drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE
PACKAGE CODE
DOCUMENT NO.
28 TQFN-EP
T2855-6
21-0140
______________________________________________________________________________________
29
MAX1908/MAX8724/MAX8765/MAX8765A
Package Information
Chip Information
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
Revision History
REVISION
NUMBER
REVISION
DATE
DESCRIPTION
0
2/03
Initial release
5
11/09
Added the MAX8765A to the data sheet.
PAGES
CHANGED
—
1–30
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.
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