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MAX8724
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Maxim Integrated MAX1908, MAX8724, MAX8765, MAX8765A Schematic

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General Description
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
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
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.
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
________________________________________________________________
Maxim Integrated Products
1
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
19-2764; Rev 5; 11/09
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.
EVALUATION KIT
AVAILABLE
+
Denotes a lead(Pb)-free/RoHS-compliant package.
*
EP = Exposed pad.
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*
TOP VIEW
VCTL
BATT
CELLS
CSIN
CSIP
PGND
DLO
15161718192021
22
DLOV
BST
DHI
CSSN
CSSP
IINP
23
LX
24
25
26
27
28
+
MAX1908 MAX8724 MAX8765
MAX8765A
7654321
CCI
REF
CLS
DCIN
LDO
THIN QFN
CCS
CCV
14
GND
13
ICTL
REFIN
12
11
ACOK
ACIN
10
9
ICHG
SHDN
8
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
DCIN, 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 (V
BST
+ 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 (V
LDO
+ 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 (V
DLOV
+ 0.3V)
LDO Short-Circuit Current...................................................50mA
Continuous Power Dissipation (T
A
= +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
V
Battery-Regulation Voltage
Accuracy (MAX1908/MAX8724/MAX8765 (2, 3, or 4 Cells) and MAX8765A (4 Cells Only))
Battery-Regulation Voltage
Accuracy (MAX8765A, 2 or 3 Cells Only)
VCTL Default Threshold V
= V
V
V
V V
V
VCTL
VCTL
VCTL
VCTL
VCTL
VCTL
VCTL
REFIN
= V
REFIN
= V
LDO
= V
REFIN
= V
REFIN
= V
LDO
rising 4.0 4.1 4.2 V
REFIN Range (Note 1) 2.5 3.6 V REFIN Undervoltage Lockout V
falling 1.20 1.92 V
REFIN
CHARGE-CURRENT REGULATION CSIP-to-CSIN Full-Scale Current-
Sense Voltage
Charging-Current Accuracy
V
V V V
ICTL
ICTL
ICTL
ICTL
= V
= V
= V
= V
REFIN
REFIN
REFIN
LDO
MAX8765/MAX8765A only; V
0.036
MAX8724 only; V
Charge-Current Gain Error
(MAX8765/MAX8765A Only)
Charge-Current Offset
(MAX8765/MAX8765A Only)
ICTL Default Threshold V BATT/CSIP/CSIN Input Voltage
Range
-2 +2 %
-2 +2 mV
rising 4.0 4.1 4.2 V
ICTL
0 19 V
-0.5 +0.5
/20 -0.5 +0.5
-0.5 +0.5
-0.4 +0.4
/20 -0.4
-0.4
+0.4
+0.4
71.25 75 78.75 mV
-5 +5
x 0.6 -5 +5
-6 +6
ICTL
= V
= V
ICTL
x 0.058 -33 +33
REFIN
REFIN
x
-45 +45
%
%
%
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
CSIP/CSIN Input Current
Cycle-by-Cycle Maximum Current
Limit
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
V
Charging 400
DCIN
= 0V or V
ICTL
RS2 = 0.015mΩ 6.0 6.8 7.5
ICTL Power-Down Mode
Threshold Voltage (MAX1908/MAX8724 Only)
V
ICTL, VCTL Input Bias Current
REFIN Input Bias Current
ICHG Transconductance
(MAX1908/MAX8724 Only)
ICHG Transconductance
(MAX8765/MAX8765A Only)
ICHG Transconductance Error
(MAX8765/MAX8765A Only)
ICHG Transconductance Offset
(MAX8765/MAX8765A Only)
G
ICHG VCSIP
G
ICHG VCSIP
-5 +5 %
-5 +5 µA
ICHG Output Current V ICHG Output Voltage V
rising
VCTL
V
VCTL
V
DCIN
V
DCIN
V
REFIN
V
CSIP
V
CSIP
V
CSIP
CSIP
CSIP
= V
= 0V, V
= 5V, V
- V
- V
- V
- V
- V
- V
- V
= 0 or 3V -1 +1
ICTL
= V
VCTL
REFIN
= 5V -1 +1
= 45mV 2.7 3 3.3 µA/mV
CSIN
= 45mV 2.85 3 3.15 µA/mV
CSIN
= 75mV -6 +6
CSIN
= 45mV -5 +5 ICHG Accuracy
CSIN
= 5mV -40 +40
CSIN
= 150mV, V
CSIN
= 150mV, ICHG = open 3.5 V
CSIN
INPUT-CURRENT REGULATION CSSP-to-CSSN Full-Scale
Current-Sense Voltage
Input Current-Limit Accuracy
Input Current-Limit Gain Error
(MAX8765/MAX8765A Only)
Input Current-Limit Offset
(MAX8765/MAX8765A Only)
CSSP, CSSN Input Voltage
Range
CSSP, CSSN Input Current
(MAX1908/MAX8724 Only)
CSSP Input Current
(MAX8765/MAX8765A Only)
72 75 78 mV
V
= V
CLS
REF
V
= V
/2 -7.5 +7.5
REF
= 1.1V (MAX8765/MAX8765A only) -10 +10
V
CLS
CLS
-2 +2 %
-2 +2 mV
8 28 V
V
= 0V 0.1 1
V
DCIN
V
CSSP
CSSP
= V
= V
CSSN
CSSN
= V
= 28V
= 0V or V
SHDN
= 0V
1
650
REFIN/
= V
ICTL
= 3V -1 +1
ICHG
= 5V -1 +1
REFIN
= 0V 350 µA
100
REFIN/55 REFIN/
33
-4 +4
> 8V 350 600
DCIN
V
= 0V 0.1 1
DCIN
V
DCIN
= 28V
400 650
µA
A
V
µA
µA
%
%
µA
µA
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
CSSN Input Current
(MAX8765/MAX8765A Only)
CLS Input Range
(MAX1908/MAX8724 Only)
CLS Input Range
(MAX8765/MAX8765A Only)
CLS Input Bias Current V
IINP Transconductance
(MAX1908/MAX8724 Only)
IINP Accuracy
IINP Transconductance
(MAX8765/MAX8765A Only)
IINP Transconductance Error
(MAX8765/MAX8765A Only)
IINP Transconductance Offset
(MAX8765/MAX8765A Only)
IINP Output Current V IINP Output Voltage V SUPPLY AND LDO REGULATOR DCIN Input Voltage Range V
DCIN Undervoltage-Lockout Trip
Point
DCIN Quiescent Current I
BATT Input Current I
LDO Output Voltage 8V < V LDO Load Regulation 0 < I
LDO Undervoltage-Lockout Trip
Point
REFERENCE REF Output Voltage 0 < I
REF Undervoltage-Lockout Trip
Point
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
V
CSSP
= V
= 28V
CSSN
V
= 0V 0.1 1
DCIN
V
= 28V 0.1 1
DCIN
1.6 REF V
1.1 REF V
= 2V -1 +1 µA
CLS
G
IINP VCSSP
G
IINP VCSSP
V
CSSP
V
CSSP
- V
- V
- V
- V
= 75mV 2.7 3 3.3 µA/mV
CSSN
= 75mV -5 +5
CSSN
= 37.5mV -7.5 +7.5
CSSN
= 75mV 2.82 3 3.18 µA/mV
CCSN
-6 +6 %
-10 +10 µA
- V
DCIN
DCIN
BATT
V
V
- V
CSSN
CSSN
= 150mV, V
= 150mV, V
CSSP
CSSP
8 28 V V
falling 7 7.4
DCIN
V
rising 7.5 7.85
DCIN
8.0V < V V
BATT
V
BATT
LDO
DCIN
REF
falling 3.1 3.9 V
REF
< 28V 3.2 6 mA
DCIN
= 19V, V
= 2V to 19V, V
< 28V, no load 5.25 5.4 5.55 V
DCIN
< 10mA 34 100 mV
= 8V 3.20 4 5.15 V
< 500µA 4.072 4.096 4.120 V
= 0V 1
DCIN
DCIN
= 0V 350 µA
IINP
= open 3.5 V
IINP
= 19.3V 200 500
µA
%
V
µA
1bios.ru
MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
_______________________________________________________________________________________ 5
ELECTRICAL CHARACTERISTICS (continued)
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
TRIP POINTS
V
falling, referred to V
DCIN
(MAX1908/MAX8724 only)
CSIN
50 100 150
BATT Power-Fail Threshold
V
falling, referred to V
BATT Power-Fail Threshold
Hysteresis
ACIN Threshold
CSSP
(MAX8765/MAX8765A only)
200 mV
ACIN rising (MAX8765/MAX8765A only) 2.028 2.048 2.068
ACIN rising (MAX1908/MAX8724 only) 2.007 2.048 2.089
CSIN
50 100 150
ACIN Threshold Hysteresis 0.5% of REF 20 mV ACIN Input Bias Current V
= 2.048V -1 +1 µA
ACIN
SWITCHING REGULATOR
DHI Off-Time
DHI Minimum Off-Time
V
BATT
V
CELLS
V
BATT
V
CELLS
= 16V, V
= V
REFIN
= 16V, V
= V
REFIN
DCIN
DCIN
= 19V,
= 17V,
0.36 0.4 0.44 µs
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
LX Input Bias Current V LX Input Quiescent Current V
DCIN
V
BATT
DCIN
DCIN
= 0V, V
= V
= 28V, V
= 0V, V
LX
BST
= 20V
BATT
BATT
= 24.5V,
= V
LX
= V
LX
0.3 1 µA
= 20V
= 20V 0.3 1 µA
150 500 µA
V
DHI Maximum Duty Cycle 99 99.9 % Minimum Discontinuous-Mode
Ripple Current
Battery Undervoltage Charge
Current
Battery Undervoltage Current
Threshold
DHI On-Resistance High V DHI On-Resistance Low V DLO On-Resistance High V DLO On-Resistance Low V
0.5 A
V
= 3V per cell (RS2 = 15mΩ),
BATT
MAX1908 only, V
BATT
rising
CELLS = GND, MAX1908 only, V CELLS = open, MAX1908 only, V
CELLS = V
- VLX = 4.5V, I
BST
- V
BST
LX
= 4.5V, I
DLOV
= 4.5V, I
DLOV
, MAX1908 only, V
REFIN
DHI
= 4.5V, I
DHI
= +100mA 4 7 Ω
DLO
= -100mA 1 3.5 Ω
DLO
= +100mA 4 7 Ω
= -100mA 1 3.5 Ω
150 300 450 mA
rising 6.1 6.2 6.3
BATT
rising
BATT
rising 12.2 12.4 12.6
BATT
9.15 9.3 9.45
mV
V
V
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
6 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
ERROR AMPLIFIERS
GMV Amplifier Transconductance GMV
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
V
= V
V C T L
C E LLS = V
, V
LD O
RE F IN
BAT T
= 16.8V ,
0.0625 0.125 0.2500 µA/mV
GMI Amplifier Transconductance GMI V
GMS Amplifier Transconductance GMS V
CCI, CCS, CCV Clamp Voltage 0.25V < V
ICTL
CLS
= V
RE F IN
= V
REF
CCV,CCS,CCI
, V
, V
CSIP
CSSP
- V
- V
= 75mV 0.5 1 2.0 µA/mV
CSIN
= 75mV 0.5 1 2.0 µA/mV
CSSN
< 2V 150 300 600 mV
LOGIC LEVELS CELLS Input Low Voltage 0.4 V
CELLS Input Open Voltage CELLS = open
CELLS Input High Voltage
CELLS Input Bias Current CELLS = 0V or V
RE F IN
(V
REFIN
/2) -
0.2V
V
- 0.4V
V
REFIN
-2 +2 µA
( V
R E F IN
/
R E F IN
/2) +
2
0.2V
V
ACOK AND SHDN ACOK Input Voltage Range 0 28 V ACOK Sink Current V ACOK Leakage Current V
A COK
A COK
= 0.4V, V
= 28V, V
= 3V 1 mA
ACIN
= 0V 1 µA
ACIN
SHDN Input Voltage Range 0 LDO V
V
SHDN
= 0V OR V
LDO
-1 +1
SHDN Input Bias Current
V
SHDN
= 0V OR V
= 5V -1 +1
SHDN
V
µA
SHDN Threshold V
SHDN Threshold Hysteresis 1
falling 22 23.5 25
S HDN
% of
V
REFIN
% of
V
REFIN
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MAX1908/MAX8724/MAX8765/MAX8765A
ELECTRICAL CHARACTERISTICS
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= -40°C to +85°C, unless otherwise noted.) (Note 2)
Low-Cost Multichemistry Battery Chargers
_______________________________________________________________________________________ 7
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
CHARGE-VOLTAGE REGULATION
Battery Regulation Voltage
Accuracy (2, 3, or 4 Cells)
= V
VCTL
V
V
VCTL
VCTL
= V
= V
REFIN
/20 -0.6 +0.6
REFIN
LDO
-0.6 +0.6
-0.6 +0.6
V
REFIN Range (Note 1) 2.5 3.6 V REFIN Undervoltage Lockout V
falling 1.92 V
REFIN
CHARGE CURRENT REGULATION
CSIP-to-CSIN Full-Scale Current-
Sense Voltage
Charging-Current Accuracy
Charge-Current Gain Error
(MAX8765/MAX8765A Only)
Charge-Current Offset
(MAX8765/MAX8765A Only)
BATT/CSIP/CSIN Input Voltage
Range
CSIP/CSIN Input Current
Cycle-by-Cycle Maximum Current
Limit
ICTL Power-Down Mode
Threshold Voltage (MAX1908/MAX8724 Only)
V
= V
V V V
ICTL
ICTL
ICTL
ICTL
= V
= V
= V
REFIN
REFIN
x 0.6 -7.5 +7.5
REFIN
LDO
MAX8765/MAX8765A only; V
0.036
MAX8724 only; V
ICTL
= V
REFIN
x 0.058
ICTL
= V
REFIN
70.5 79.5 mV
-6 +6
-7.5 +7.5
x
-50 +50
-33 +33
-2 +2 %
-2 +2 mV
0 19 V
V
I
MAX
V
= 0V or V
DCIN
Charging 650
RS2 = 0.015Ω 6.0 7.5 A
rising
ICTL
ICTL
= 0V or V
= 0V 1
SHDN
REFIN/
100
REFIN/
33
%
%
µA
V
ICHG Transconductance
(MAX1908/MAX8724 Only)
ICHG Transconductance
(MAX8765/MAX8765A Only)
ICHG Transconductance Error
(MAX8765/MAX8765A Only)
ICHG Transconductance Offset
(MAX8765/MAX8765A Only)
G
ICHG VCSIP
G
ICHG VCSIP
- V
= 45mV 2.7 3.3 µA/mV
CSIN
- V
= 45mV 2.785 3.225 µA/mV
CSIN
-7.5 +7.5 %
-6.5 +6.5 µA
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
8 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= -40°C to +85°C, unless otherwise noted.) (Note 2)
ICHG Accuracy
INPUT-CURRENT REGULATION CSSP-to-CSSN Full-Scale
Current-Sense Voltage
Input Current-Limit Accuracy
Input Current-Limit Gain Error
(MAX8765/MAX8765A Only)
Input Current-Limit Offset
(MAX8765/MAX8765A Only)
CSSP, CSSN Input Voltage
Range
CSSP, CSSN Input Current
(MAX1908/MAX8724 Only)
CSSP Input Current
(MAX8765/MAX8765A Only)
CSSN Input Current
(MAX8765/MAX8765A Only)
CLS Input Range
(MAX1908/MAX8724 Only)
CLS Input Range
(MAX8765/MAX8765A Only)
IINP Transconductance
(MAX1908/MAX8724 Only)
IINP Transconductance
(MAX8765/MAX8765A Only)
IINP Transconductance Error
(MAX8765/MAX8765A Only)
IINP Transconductance Offset
(MAX8765/MAX8765A Only)
IINP Accuracy
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
V V
V
CSIP
CSIP
CSIP
- V
- V
- V
= 75mV -7.5 +7.5
CSIN
= 45mV -7.5 +7.5
CSIN
= 5mV -40 +40
CSIN
71.25 78.75 mV
V
= V
CLS
REF
V
= V
/2 -7.5 +7.5
REF
= 1.1V (MAX8765/MAX8765A only) -10 +10
V
CLS
CLS
-5 +5
-2 +2 %
-2 +2 mV
8 28 V
V
= 0V 1
V
V
DCIN
V
CSSP
CSSP
CSSP
= V
= V
= V
CSSN
CSSN
CSSN
= V
DCIN
= 28V
= 28V
> 8V 600
V
= 0V 1
DCIN
V
= 28V 650
DCIN
V
= 0V 1
DCIN
V
= 28V 1
DCIN
1.6 REF V
1.1 REF V
G
IINP VCSSP
G
IINP VCSSP
- V
- V
= 75mV 2.7 3.3 µA/mV
CSSN
= 75mV 2.785 3.225 µA/mV
CCSN
-7.5 +7.5 %
-12 +12 µA
V
- V
CSSP
V
CSSP
= 75mV -7.5 +7.5
CSSN
- V
= 37.5mV -7.5 +7.5
CSSN
%
%
µA
µA
µA
%
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Low-Cost Multichemistry Battery Chargers
_______________________________________________________________________________________ 9
ELECTRICAL CHARACTERISTICS (continued)
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= -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 V DCIN Quiescent Current I
BATT Input Current I
DCIN
DCIN
BATT
LDO Output Voltage 8V < V LDO Load Regulation 0 < I
8 28 V
8.0V < V V
BATT
V
BATT
LDO
< 28V 6 mA
DCIN
= 19V, V
= 2V to 19V, V
< 28V, no load 5.25 5.55 V
DCIN
< 10mA 100 mV
= 0V 1
DCIN
= 19.3V 500
DCIN
REFERENCE REF Output Voltage 0 < I
< 500µA 4.065 4.120 V
REF
TRIP POINTS
V
falling, referred to V
BATT Power-Fail Threshold
ACIN Threshold
DCIN
(MAX1908/MAX8724 only)
V
falling, referred to V
CSSP
(MAX8765/MAX8765A only)
ACIN rising (
ACIN rising (MAX1908/MAX8724 only) 2.007 2.089
MAX8765/MAX8765A
CSIN
CSIN
50 150
50 150
only) 2.028 2.068
SWITCHING REGULATOR
DHI Off-Time
DHI Minimum Off-Time
BATT
V
CELLS
V
BATT
V
CELLS
= 16V, V
= V
REFIN
= 16V, V
= V
REFIN
DCIN
DCIN
= 19V,
= 17V,
0.35 0.45 µs
0.24 0.33 µs
V
DHI Maximum On-Time 2.5 7.5 ms DHI Maximum Duty Cycle 99 %
Battery Undervoltage Charge
Current
Battery Undervoltage Current
Threshold
DHI On-Resistance High V DHI On-Resistance Low V DLO On-Resistance High V DLO On-Resistance Low V
V
= 3V per cell (RS2 = 15mΩ),
BATT
MAX1908 only, V
BATT
CELLS = GND, MAX1908 only, V CELLS = open, MAX1908 only, V
CELLS = V
- VLX = 4.5V, I
BST
- VLX = 4.5V, I
BST
= 4.5V, I
DLOV
= 4.5V, I
DLOV
, MAX1908 only, V
REFIN
DLO
DLO
rising
rising 6.09 6.30
BATT
rising 9.12 9.45
BATT
rising 12.18 12.60
BATT
= +100mA 7 Ω
DHI
= -100mA 3.5 Ω
DHI
= +100mA 7 Ω = -100mA 3.5 Ω
150 450 mA
µA
mV
V
V
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Low-Cost Multichemistry Battery Chargers
10 ______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(V
DCIN
= V
CSSP
= V
CSSN
= 18V, V
BATT
= V
CSIP
= V
CSIN
= 12V, V
REFIN
= 3V, V
VCTL
= V
ICTL
= 0.75 x V
REFIN
, CELLS = open, CLS =
REF, V
BST
- VLX= 4.5V, ACIN = GND = PGND = 0, C
LDO
= 1µF, LDO = DLOV, C
REF
= 1µF; CCI, CCS, and CCV are compensated
per Figure 1a; T
A
= -40°C to +85°C, unless otherwise noted.) (Note 2)
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 T
A
= -40°C are guaranteed by design and not production tested.
LOAD-TRANSIENT RESPONSE
(BATTERY INSERTION AND REMOVAL)
MAX1908 toc01
1ms/div
I
BATT
2A/div
V
BATT
5V/div
V
CCI
500mV/div
V
CCV
500mV/div
ICTL = LDO VCTL = LDO
CCV
CCI
LOAD-TRANSIENT RESPONSE
(STEP IN-LOAD CURRENT)
MAX1908 toc02
1ms/div
V
BATT
2V/div
V
CCI
500mV/div
V
CCS
500mV/div
16.8V
0
0
LOAD
CURRENT
5A/div
ADAPTER
CURRENT
5A/div
ICTL = LDO CHARGING CURRENT = 3A V
BATT
= 16.8V LOAD STEP = 0 TO 4A I
SOURCE
LIMIT = 5A
CCS
CCS
CCI
CCI
V
BATT
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 V
BATT
= 16.8V LOAD STEP = 0 TO 4A I
SOURCE
LIMIT = 5A
Typical Operating Characteristics
(Circuit of Figure 1, V
DCIN
= 20V, TA= +25°C, unless otherwise noted.)
ERROR AMPLIFIERS
GMV Amplifier Transconductance GMV
GMI Amplifier Transconductance GMI V GMS Amplifier Transconductance GMS V CCI, CCS, CCV Clamp Voltage 0.25V < V LOGIC LEVELS CELLS Input Low Voltage 0.4 V
CELLS Input Open Voltage CELLS = open
CELLS Input High Voltage
ACOK AND SHDN ACOK Input Voltage Range 0 28 V ACOK Sink Current V SHDN Input Voltage Range 0 LDO V
SHDN Threshold V
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
V
= V
, V
, V
= 16.8V ,
BAT T
, V
CSSP
CSIP
- V
- V
= 75mV 0.5 2.0 µA/mV
CSIN
= 75mV 0.5 2.0 µA/mV
CSSN
0.0625 0.250 µA/mV
< 2V 150 600 mV
(V
REFIN
/2) -
0.2V
V
REFIN
- 0.4V
= 3V 1 mA
ACIN
V
( V
/2) +
0.2V
R E F IN
V C T L
C E LLS = V
ICTL
CLS
ACOK
SHDN
LD O
RE F IN
= V
RE F IN
= V
REF
CCV,CCS,CCI
= 0.4V, V
falling 22 25
% of
V
REFIN
V
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Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________
11
Typical Operating Characteristics (continued)
(Circuit of Figure 1, V
DCIN
= 20V, TA= +25°C, unless otherwise noted.)
LINE-TRANSIENT RESPONSE
V
DCIN
10V/div
V
BATT
500mV/div
INDUCTOR
CURRENT
500mA/div
ICTL = LDO VCTL = LDO
= 3A
I
CHARGE
LINE STEP 18.5V TO 27.5V
REF VOLTAGE LOAD REGULATION
0
-0.01
-0.02
-0.03
-0.04
-0.05
ERROR (%)
-0.06
REF
V
-0.07
-0.08
-0.09
-0.10 0 200100 300 400 500
MAX1908 toc04
10ms/div
REF CURRENT (µA)
0
-0.1
-0.2
-0.3
-0.4
-0.5
ERROR (%)
-0.6
LDO
V
-0.7
-0.8 V
LDO
-0.9
-1.0
0 2341 567 9810
REF VOLTAGE ERROR vs. TEMPERATURE
0.10
0.08
MAX1908 toc07
0.06
0.04
0.02
0
ERROR (%)
-0.02
REF
V
-0.04
-0.06
-0.08
-0.10
-40 10-15 35 60 85
LDO LOAD REGULATION
= 5.4V
LDO CURRENT (mA)
TEMPERATURE (°C)
MAX1908 toc05
MAX1908 toc08
0.05
I
0.04
0.03
0.02
0.01
0
ERROR (%)
-0.01
LDO
V
-0.02
-0.03
-0.04
-0.05
= 0
LDO
= 5.4V
V
LDO
8 12141610 18 20 22 2624 28
VIN (V)
EFFICIENCY vs. CHARGE CURRENT
100
90
80
V
= 16V
LDO LINE REGULATION
70
60
50
40
EFFICIENCY (%)
30
20
10
0
0.01 1010.1 CHARGE CURRENT (A)
BATT
V
= 12V
BATT
V
= 8V
BATT
MAX1908 toc06
MAX1908 toc09
500
450
400
350
300
250
200
FREQUENCY (kHz)
150
100
50
0
FREQUENCY vs. VIN - V
4 CELLS
0462 8 10 12 14 16 18 20 22
(VIN - V
BATT
BATT
3 CELLS
I
= 3A
CHARGE
VCTL = ICTL = LDO
) (V)
OUTPUT V/I CHARACTERISTICS
0.5
0.4
MAX1908 toc10
0.3
0.2
0.1
0
-0.1
-0.2
BATT VOLTAGE ERROR (%)
-0.3
-0.4
-0.5 01234
3 CELLS
4 CELLS
BATT CURRENT (A)
2 CELLS
0.08
0.07
MAX1908 toc11
0.06
0.05
0.04
0.03
BATT VOLTAGE ERROR (%)
0.02
0.01
0
0 0.2 0.3 0.4 0.50.1 0.6 0.7 0.8 0.9
BATT VOLTAGE ERROR vs. VCTL
4 CELLS REFIN = 3.3V NO LOAD
VCTL/REFIN (%)
MAX1908 toc12
1.0
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Low-Cost Multichemistry Battery Chargers
12 ______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1, V
DCIN
= 20V, TA= +25°C, unless otherwise noted.)
CURRENT-SETTING ERROR vs. ICTL
5
4
3
2
1
CURRENT-SETTING ERROR (%)
0
-1
01.00.5 1.5 2.0
V
REFIN
= 3.3V
V
ICTL
(V)
IINP ERROR vs. SYSTEM LOAD CURRENT
40
30
20
10
0
IINP ERROR (%)
-10
-20
-30
-40 01234
I
= 0
BATT
SYSTEM LOAD CURRENT (A)
MAX1908 toc13
MAX1908 toc15
ICHG ERROR vs. CHARGE CURRENT
5.0
4.5
4.0
3.5
3.0
2.5
ICHG (%)
2.0
1.5
1.0
0.5
0
01.00.5 1.5 2.0 2.5 3.0
V
= 3.3V
REFIN
V
= 16V
BATT
V
= 12V
BATT
V
= 8V
BATT
I
(A)
BATT
IINP ERROR vs. INPUT CURRENT
80
60
40
20
0
IINP ERROR (%)
-20
-40
-60
-80
ERROR DUE TO SWITCHING NOISE
SYSTEM LOAD = 0
0 0.5 1.0 1.5 2.0
INPUT CURRENT (A)
MAX1908 toc14
MAX1908 toc16
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 13
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
9 ICHG
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 V 12 REFIN Reference Input. Allows the ICTL and VCTL inputs to have ratiometric ranges for increased accuracy.
13 ICTL
14 GND Analog Ground
15 VCTL
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
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.
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.
Output Current-Limit Set Input. ICTL input voltage range is V
down if ICTL is forced below V LDO, the set point for CSIP - CSIN is 45mV.
/100 while the MAX8765/MAX8765A does not. When ICTL is equal to
REFIN
REFIN
Output Voltage-Limit Set Input. VCTL input voltage range is 0 to V
point is (4.2 x CELLS)V.
is less than V
ACIN
/32 to V
REFIN
. The MAX1908/MAX8724 shut
REFIN
. When VCTL is equal to LDO, the set
REF
/2.
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.
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
14 ______________________________________________________________________________________
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 V
RE-
FIN
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
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 V
REFIN
, provid-
ing a 10% adjustment range on the V
BATT
regulation
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 V
BATT
= 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 reg­ulation and current regulation loops allows for optimal compensation (see the
Compensation
section).
Table 2. Cell-Count Programming
Table 1. Versions Comparison
V CELLS V
=×+×
BATT
⎛
⎛
404.
⎜
⎜
⎝
⎝
DESCRIPTION MAX1908 MAX8724
Conditioning Charge Feature
ICTL Shutdown Mode
ACOK Enable Condition
Yes No No
Yes Yes No
REFIN must
be ready
REFIN must
be ready
CELLS CELL COUNT
GND 2
Open 3
V
REFIN
⎞
⎞
V
VCTL
⎟
V
REFIN
⎟
⎠
⎠
MAX8765/
MAX8765A
Independent
of REFIN
4
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Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 15
MAX1908/MAX8724/MAX8765/MAX8765A
Figure 1. µC-Controlled Typical Application Circuit
Typical Application Circuits
AC ADAPTER INPUT
8.5V TO 28V
DAC OUTPUT
V
CC
OUTPUT
ADC INPUT
ADC INPUT
HOST
AVDD/REF
C14
0.1µF
R6
59kΩ
1%
R8 1MΩ
R9 20kΩ
R19, R20, R21 10kΩ
D2
R7
19.6kΩ
1%
12.6V OUTPUT VOLTAGE
C20
0.1µF
0.01µF
C5 1µF
R10 10kΩ
C9
D1
R5 1kΩ
C11
0.1µF
C10
0.01µF
0.01Ω
0.1µF
CSSP CSSN
DCIN
LDO
VCTL
ICTL
REFIN
ACIN
ACOK
SHDN
ICHG
MAX1908 MAX8724
C12 1µF
MAX8765
MAX8765A
IINP
CCV
CCI
CCS
RS1
0.1µF
CELLS
CLSREF
7.5A INPUT CURRENT LIMIT
LDO
BST
DLOV
DHI
DLO
PGND
CSIP
CSIN
BATT
GND
LX
OPEN (3 CELLS SELECT)
C13 1µF
D3
C15
0.1µF
N1b
R13 33Ω
C16 1µF
N1a
C4 22µF
L1 10µH
RS2
0.015Ω
BATT
TO EXTERNAL
C1 2 × 10µF
+
SMART
BATTERY
LOAD
SCL
SDA
ADC INPUT
GND
SCL
SDA
TEMP
BATT-
PGND GND
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Low-Cost Multichemistry Battery Chargers
16 ______________________________________________________________________________________
Figure 2. Typical Application Circuit with Fixed Charging Parameters
Typical Application Circuits (continued)
AC ADAPTER
INPUT
8.5V TO 28V
R6
59kΩ
LDO
R14
10.5kΩ 1%
R15
8.25kΩ 1%
R16
8.25kΩ 1%
FROM HOST µP
(SHUTDOWN)
1%
19.6kΩ
N
1%
R7
R19 10kΩ 1%
0.01µF
P1
R11
15kΩ
R12
12kΩ
D2
C5 1µF
LDO
16.8V OUTPUT VOLTAGE
2.5A CHARGE LIMIT
C12
1.5nF
R20
10kΩ
1%
R5 1kΩ
C11
0.1µF
C9
C10
0.01µF
CSSP CSSN ACOK DCIN
VCTL
REFIN
ICTL
ACIN
SHDN
ICHG
IINP
CCV
CCI
CCS
C12 1µF
RS1
0.01Ω
0.01µF
0.01µF
MAX1908 MAX8724 MAX8765
MAX8765A
R17
19.1kΩ 1%
REFIN (4 CELLS SELECT)
CELLS
LDO
C13 1µF
BST
DLOV
C15
0.1µF
DHI
LX
DLO
PGND
CSIP
CSIN
BATT
GND
CLSREF
N1b
R13 33Ω
D3
C16 1µF
N1a
L1 10µH
RS2
0.015Ω
C4 22µF
GNDPGND
TO EXTERNAL
C1 2 × 10µF
+
BATT
BATTERY
THM
BATT-
LOAD
R18 22kΩ 1%
4A INPUT CURRENT LIMIT
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 17
MAX1908/MAX8724/MAX8765/MAX8765A
Figure 3. Functional Diagram
Functional Diagram
SHDN
GND
CCS
CLS
CSSP
CSSN
CSIP
CSIN
ICTL
CCI
BATT
23.5% REFIN
REFIN
GND
DCIN
75mV
x
LEVEL
SHIFTER
LEVEL
SHIFTER
75mV
x
REFIN
REF
R1
3.1V/CELL
MAX1908 MAX8724 MAX8765
MAX8765A
GMS
GMI
MAX1908 ONLY
BAT_UV
SRDY
LOGIC
BLOCK
CSI
LVC
LVC
RDY
5.4V
LINEAR
REGULATOR
REF/2
CONVERTER
MAX1908/MAX8724 ONLY
GM
GM
LEVEL
SHIFTER
DC-DC
4.096V
REFERENCE
1/55
ICTL
DRIVER
DCIN
LDO
REF
REFIN
ACIN
ACOK
N
IINP
ICHG
BST
DHI
LX
CELLS
CCV
VCTL
CELL
SELECT
LOGIC
x
400mV
REFIN
4V
GMV
DRIVER
DLOV
DLO
PGND
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
18 ______________________________________________________________________________________
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:
The input voltage range for ICTL is V
REFIN
/32 to V
REFIN
. The MAX1908/MAX8724 shut down if ICTL is forced below V
REFIN
/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:
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
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:
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 V
CLS
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 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 V
REF
/2 with 20mV hysteresis. ACOK is an open-drain
output and is high impedance when ACIN is less than
V
I
CHG
ICTL
=×
VRS
REFIN
I
=
CHG
0 0452.
RS
0 0752.
V
II
=+
INPUT LOAD
I
INPUT
V
=×
VRS
⎛
IV
CHG BATT
⎜
V
⎝
IN
0 0751.
CLS
REF
×
⎞ ⎟
×
η
⎠
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 19
V
REF
/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:
V
ICHG
= ICHG x RS2 x G
ICHG
x R9
where I
CHG
is the battery-charging current, G
ICHG
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:
V
IINP
= I
INPUT
x RS1 x G
IINP
x R10
where I
INPUT
is the DC current being supplied by the AC
adapter power, G
IINP
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%
V
REFIN
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) < V
BATT
< (0.88 x V
DCIN
)
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.
MAX1908/MAX8724/MAX8765/MAX8765A
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
20 ______________________________________________________________________________________
Figure 4. DC-DC Functional Diagram
DC-DC Functional Diagram
5ms
RESET
1.8V
0.15V
0.1V
IMAX
CCMP
IMIN
ZCMP
S
BST
RQ
LVC
MAX1908 MAX8724 MAX8765
MAX8765A
RQ
CHG
S
Q
t
OFF
GENERATOR
GMS
GMI
GMV
DHI
DLO
CSS X20
CSI X20
CSSP
CSSN
BST
DHI
LX
DLO
CSIP
CSIN
BATT
AC ADAPTER
RS1
N1a
N1b
C
OUT
BATTERY
D3
L1
RS2
LDO
C
BST
SETV
CONTROL
CCVCCICCS
SETI CLS
CELLS
CELL
SELECT
LOGIC
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 21
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 V
DCIN
and V
BATT
. 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 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 (V
DCIN
-
V
BATT
) differential becomes too small. If V
BATT
≥ 0.88 ×
V
DCIN
, then the threshold for minimum off-time is
reached and the t
OFF
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:
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:
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 VLXand V
BST
. 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 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.
MAX1908/MAX8724/MAX8765/MAX8765A
ts
OFF
t
I
RIPPLE
f
=
=×
25. µ
ON
f
VV
()
CSSN BATT
VV
LI
×
=
VV
CSSN BATT
Vt
BATT OFF
=
=
tt
ON OFF
LI
×
RIPPLE
−
−
DCIN BATT
V
DCIN
RIPPLE
−
×
L
1
+
+103. µ
s
V
IMIN
015
RSA=×
20 2
=
05..
1
f
==
C
RC F
2
ππ µΩ
1
××
2331
=
48
.
kHz
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
22 ______________________________________________________________________________________
CCV Loop Definitions
Compensation of the CCV loop depends on the para­meters and components shown in Figure 5. CCVand RCVare the CCV loop compensation capacitor and series resistor. R
ESR
is the equivalent series resistance
(ESR) of the charger output capacitor (C
OUT
). RLis the
equivalent charger output load, where RL= V
BATT
/
I
CHG
. The equivalent output impedance of the GMV
amplifier, R
OGMV
≥ 10MΩ . The voltage amplifier transconductance, GMV = 0.125µA/mV. The DC-DC converter transconductance, GM
OUT
= 3.33A/V:
where A
CSI
= 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 R
L
varies with load according to RL= V
BATT/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 fre­quency, the crossover frequency is given by:
To calculate R
CV
and CCVvalues of the circuit of Figure 2: Cells = 4 C
OUT
= 22µF
V
BATT
= 16.8V
I
CHG
= 2.5A GMV = 0.125µA/mV GM
OUT
= 3.33A/V
R
OGMV
= 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:
f
CO_CV
= 3kHz
Choose the output-capacitor ESR so the output-capacitor zero is 10 times the crossover frequency:
Figure 5. CCV Loop Diagram
CCV
REF
BATT
R
C
ESR
OUT
R
L
GM
OUT
GMV
R
C
R
CV
CV
OGMV
f
Z ESR
LTF GM R GMV R
=××××
11
()
11
()
=
_
OUT L OGMV
+×
sC R sC R
+×
2π
sC R sC R
OUT ESR CV CV
CV OGMV OUT L
1
RC
×
ESR OUT
()
()
GMV R GM
××
f
CO CV
GM
OUT
=
ARS
×12
CSI
f
PCV
_
=
2π
1
RC
×
OGMV CV
f
CO CV
_
=
_
GMV R GM
=
××
CV OUT
C
2π
OUT
CV OUT
C
π
OUT
f
ZCV
_
=
1
RC
2π
×
CV CV
R
ESR
=
×× ×
210
π
1
fC
CO CV OUT
_
+×
+×
kHz
=280
=
. Ω
024
f
P OUT
_
=
2π
1
RC
×
L OUT
f
Z ESR
_
1
RC
2
×
π
ESR OUT
=
2 412
MHz
.=
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 23
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 (f
Z_CV
) so it is equivalent to
the output pole (f
P_OUT
= 1.08kHz), effectively produc­ing a pole-zero cancellation and maintaining a single­pole system response:
Choose CCV= 100nF, which sets the compensation zero (f
Z_CV
) at 1.6kHz. This sets the compensation pole:
CCI Loop Definitions
Compensation of the CCI loop depends on the parame­ters and components shown in Figure 7. CCIis the CCI loop compensation capacitor. A
CSI
is the internal gain of the current-sense amplifier. RS2 is the charge cur­rent-sense resistor, RS2 = 15mΩ. R
OGMI
is the equiva­lent output impedance of the GMI amplifier ≥ 10MΩ. GMI is the charge-current amplifier transconductance = 1µA/mV. GM
OUT
is the DC-DC converter transcon­ductance = 3.3A/V. The CCI loop is a single-pole sys­tem with a dominant pole compensation set by f
P_CI
:
The loop transfer function is given by:
Since:
The loop transfer function simplifies to:
MAX1908/MAX8724/MAX8765/MAX8765A
Figure 6. CCV Loop Gain/Phase vs. Frequency
f
P OUT
_
∆
R
==
L
∆
f
ZCV
_
RC
2
π
L OUT
V
BATT
I
CHG
1
×
Battery ESR
=
.=
108
kHz
=
1
RC
2π
×
CV CV
f
=
PCI
_
RC
2π
OGMI CI
LTF GM A RS GMI
=×××
OUT CSI
2
1
×
R
OGMI
+×
1
sR C
OGMI CI
C
=
CV
2108
1
R kHz
×
π .
CV
=
147
nF
f
PCV
_
80
60
40
20
GAIN (dB)
0
-20
-40
2
1
RC
π
OGMV CV
CCV LOOP GAIN vs. FREQUENCY
=
×
.=
016
Hz
GM
OUT
=
ARS
×12
CSI
R
OGMI
+×1
sR C
OGMI CI
-45
-60
-75
-90
PHASE (DEGREES)
-105
-120
LTF GMI
=×
CCV LOOP PHASE
vs. FREQUENCY
-60 11M
FREQUENCY (Hz)
100k10k1k10010
-135 11M
FREQUENCY (Hz)
100k10k1k10010
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
24 ______________________________________________________________________________________
The crossover frequency is given by:
The CCI loop dominant compensation pole:
where the GMI amplifier output impedance, R
OGMI
=
10MΩ.
To calculate the CCI loop compensation pole, C
CI
:
GMI = 1µA/mV
GM
OUT
= 3.33A/V
R
OGMI
= 10MΩ
f = 400kHz
Choose crossover frequency f
CO_
CI
to be 1/5th the MAX1908/MAX8724/MAX8765/MAX8765A switching frequency:
Solving for C
CI
, CCI= 2nF.
To be conservative, set CCI= 10nF, which sets the crossover frequency at:
The compensation pole, f
P_CI
is set at:
CCS Loop Definitions
Compensation of the CCS loop depends on the parame­ters and components shown in Figure 9. CCSis the CCS loop compensation capacitor. A
CSS
is the internal gain of the current-sense amplifier. RS1 is the input current­sense resistor, RS1 = 10mΩ. R
OGMS
is the equivalent
output impedance of the GMS amplifier ≥ 10MΩ. GMS is
Figure 7. CCI Loop Diagram
GM
CCI
R
C
CI
OGMI
OUT
GMI
CSIP CSIN
RS2
CSI
ICTL
f
CO CICI_
GMI
=
2π
C
f
PCI
_
=
2π
1
RC
×
OGMI CI
GMI
==
280π
C
kHz
f
CO CICI_
f
CO CI_
f
PCI
_
GMI
==
210
π
GMI
π
RC
OGMI CI
×
nF
16
=20 0016
kHz
.=
Hz
CCI LOOP GAIN
vs. FREQUENCY
100
80
60
40
20
GAIN (dB)
0
-20
-40
-60
0.1 1M FREQUENCY (Hz)
CCI LOOP PHASE
vs. FREQUENCY
0
-15
-30
-45
-60
PHASE (DEGREES)
-75
-90
100k10k1 10 100 1k
-105
0.1 1M FREQUENCY (Hz)
100k10k1k100101
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 25
the charge-current amplifier transconductance = 1µA/mV. GMINis the DC-DC converter transconductance =
3.3A/V. The CCS loop is a single-pole system with a dom­inant pole compensation set by f
P_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, R
OGMS
=
10MΩ.
To calculate the CCI loop compensation pole, CCS:
GMS = 1µA/mV
GMIN= 3.33A/V
R
OGMS
= 10MΩ
f = 400kHz
MAX1908/MAX8724/MAX8765/MAX8765A
Figure 9. CCS Loop Diagram
Figure 10. CCS Loop Gain/Phase vs. Frequency
f
=
PCS
_
RC
2π
OGMS CS
LTF GM A RS GMS
=××××
IN CSS
1
GM
=
IN
ARS
CSS
1
×
R
OGMS
+×
1
sR C
OGMS CS
×11
CCS
GM
IN
GMS
R
C
CS
OGMS
CSSP CSSN
RS1
CSS
CLS
R
LTF GMS
=×
OGMS
+×1
sR C
OGMS CS
f
CO CSCS_
100
80
60
40
20
GAIN (dB)
0
-20
-40
-60
0.1 1M
GMS
=
2π
C
CCS LOOP GAIN vs. FREQUENCY
FREQUENCY (Hz)
f
=
PCS
_
2π
1
RC
×
OGMS CS
CCS LOOP PHASE
vs. FREQUENCY
0
-15
-30
-45
-60
PHASE (DEGREES)
-75
-90
100k10k1 10 100 1k
-105
0.1 1M FREQUENCY (Hz)
100k10k1k100101
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
26 ______________________________________________________________________________________
Choose crossover frequency f
CO_CS
to be 1/5th the MAX1908/MAX8724/MAX8765/MAX8765A switching frequency:
Solving for CCS, CCS= 2nF.
To be conservative, set C
CS
= 10nF, which sets the
crossover frequency at:
The compensation pole, f
P_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 (I
CHG
), plus 1/2 the current rip-
ple I
RIPPLE
:
I
SAT
= I
CHG
+ (1/2) I
RIPPLE
Ripple current varies according to the equation:
I
RIPPLE
= (V
BATT
) × t
OFF
/L
where:
t
OFF
= 2.5µs × (V
DCIN
– V
BATT
)/V
DCIN
V
BATT
< 0.88 × V
DCIN
or:
t
OFF
= 0.3µs
V
BATT
> 0.88 × V
DCIN
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, I
RIPPLE
= 0.3 × I
CHG
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:
I
C1
= input capacitor ripple current.
D = DC-DC converter duty ratio.
I
CHG
= 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 × I
CHG
. 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:
Figure 11. Ripple Current vs. Battery Voltage
f
CO CSCS_
GMS
==
280π
C
f
CO CS_
GMS
==
21016π
nF
kHz
kHz
f
PCS
_
RC
2
π
1
×
OGMS CS
=
0 0016
Hz
.=
II DD
=−
C CHG1
1.5
1.0
0.5
RIPPLE CURRENT (A)
0
RIPPLE CURRENT vs.
BATTERY VOLTAGE
3 CELLS
V
= 19V
DCIN
VCTL = ICTL = LDO
8 15161718
V
BATT
14131211109
(V)
4 CELLS
05<.
ESR
CC1
V
I
1
I
C1
2
dV
C
1=
dt
2
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 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):
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 VGSof 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 (R
DS(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 I
GATE
= Q
GATE
× 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.
C
I
s
V
C
1
22505
1
>×
..µ
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
28 ______________________________________________________________________________________
Table 3. Component List for Circuit of Figure 2
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.
DESIGNATION QTY DESCRIPTION
10µF, 50V 2220-size ceramic
C1 2
C4 1
C5 1
C9, C10 2
C11, C14,
C15, C20
C12, C13, C16 3
D1 (optional) 1
D2 1
capacitors TDK C5750X7R1H106M
22µF, 25V 2220-size ceramic capacitor TDK C5750X7R1E226M
1µF, 25V X7R ceramic capacitor (1206) Murata GRM31MR71E105K Taiyo Yuden TMK316BJ105KL TDK C3216X7R1E105K
0.01µF, 16V cer am i c cap aci tor s ( 0402) Murata GRP155R71E103K Taiyo Yuden EMK105BJ103KV TDK C1005X7R1E103K
0.1µF, 25V X7R ceramic capacitors (0603)
4
Murata GRM188R71E104K TDK C1608X7R1E104K
1µF, 6.3V X5R ceramic capacitors (0603) Murata GRM188R60J105K Taiyo Yuden JMK107BJ105KA TDK C1608X5R1A105K
10A Schottky diode (D-PAK) Diodes, Inc. MBRD1035CTL ON Semiconductor MBRD1035CTL
Schottky diode Central Semiconductor CMPSH1–4
DESIGNATION QTY DESCRIPTION
D3 1
L1 1
N1 1
P1 1
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
RS2 1
U1 1
Schottky diode Central Semiconductor CMPSH1-4
10µH, 4.4A inductor Sumida CDRH104R-100NC TOKO 919AS-100M
Dual, n-channel, 8-pin SO MOSFET Fairchild FDS6990A or FDS6990S
Single, p-channel, 8-pin SO MOSFET Fairchild FDS6675
0.01Ω ±1%, 0.5W 2010 sense resistor Vishay Dale WSL2010 0.010 1.0% IRC LRC-LR2010-01-R010-F
0.015Ω ±1%, 0.5W 2010 sense resistor Vishay Dale WSL2010 0.015 1.0% IRC LRC-LR2010-01-R015-F
MAX1908ETI+, MAX8724ETI+, or MAX8765ETI+, MAX8765AETI+
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 29
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 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
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MAX1908/MAX8724/MAX8765/MAX8765A
Low-Cost Multichemistry Battery Chargers
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
30
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
0 2/03 Initial release —
5 11/09 Added the MAX8765A to the data sheet. 1–30
REVISION
DATE
DESCRIPTION
PAGES
CHANGED
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