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MAX1908
Technical data
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MAXIM MAX1908, MAX8724, MAX8765 Technical data

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General Description
The MAX1908/MAX8724/MAX8765 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 achieve high efficiency using a buck topology with synchronous rectification.
The MAX1908 includes a conditioning charge feature while the MAX8724/MAX8765 do not.
The MAX1908/MAX8724/MAX8765 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
♦ ±0.5% Output Voltage Accuracy Using Internal
Reference (0°C to +85°C)
♦ ±4% Accurate Input Current Limiting
♦ ±5% Accurate Charge Current
♦ Analog Inputs Control Charge Current and
Charge Voltage
♦ Outputs for Monitoring
Current Drawn from AC Adapter Charging Current AC Adapter Presence
♦ Up to 17.6V Battery-Voltage Set Point
♦ Maximum 28V Input Voltage
♦ > 95% Efficiency
♦ Shutdown Control Input
♦ Charge Any Battery Chemistry
Li+, NiCd, NiMH, Lead Acid, etc.
MAX1908/MAX8724/MAX8765
Low-Cost Multichemistry Battery Chargers
________________________________________________________________ Maxim Integrated Products 1
28
27
26
25
24
23
22
IINP
CSSP
CSSN
DHI
BST
LX
DLOV
8
9
10
11
12
13
14
SHDN
ICHG
ACIN
ACOK
REFIN
ICTL
GND
15161718192021
VCTL
BATT
CELLS
CSIN
CSIP
PGND
DLO
7654321
CCV
CCI
CCS
REF
CLS
LDO
DCIN
MAX1908 MAX8724 MAX8765
THIN QFN
TOP VIEW
Pin Configuration
Ordering Information
MAX1908 MAX8724 MAX8765
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 4; 7/05
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
PART
TEMP RANGE
PIN-
PKG CODE
MAX1908ETI
T2855-6
MAX8724ETI
T2855-6
MAX8765ETI
T2855-6
PACKAGE
-40°C to +85°C 28 Thin QFN
-40°C to +85°C 28 Thin QFN
-40°C to +85°C 28 Thin QFN
MAX1908/MAX8724/MAX8765
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 = float, 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
CONDITIONS
UNITS
CHARGE-VOLTAGE REGULATION
V
VCTL
= V
REFIN
(2, 3, or 4 cells)
V
VCTL
= V
REFIN
/ 20 (2, 3, or 4 cells)
Battery-Regulation Voltage
Accuracy
V
VCTL
= V
LDO
(2, 3, or 4 cells)
%
VCTL Default Threshold V
VCTL
rising 4.0 4.1 4.2 V
REFIN Range (Note 1) 2.5 3.6 V REFIN Undervoltage Lockout V
REFIN
falling
V
CHARGE-CURRENT REGULATION
CSIP-to-CSIN Full-Scale Current-
Sense Voltage
V
ICTL
= V
REFIN
75
mV
V
ICTL
= V
REFIN
-5 +5
V
ICTL
= V
REFIN
x 0.6 -5 +5
V
ICTL
= V
LDO
-6 +6
MAX8765 only; V
ICTL
= V
REFIN
x 0.036 -45
Charging-Current Accuracy
MAX8724 only; V
ICTL
= V
REFIN
x 0.058 -33
%
Charge-Current Gain Error
(MAX8765 Only)
-2 +2 %
Charge-Current Offset
(MAX8765 Only)
-2 +2 mV
ICTL Default Threshold V
ICTL
rising 4.0 4.1 4.2 V
BATT/CSIP/CSIN Input Voltage
Range
0 19 V
V
DCIN
= 0 or V
ICTL
= 0 or SHDN = 0 1
CSIP/CSIN Input Current
Charging
650
µA
SYMBOL
MIN TYP MAX
-0.5
-0.5
-0.5
71.25
1.20 1.92
400
+0.5 +0.5 +0.5
78.75
+45 +33
Low-Cost Multichemistry Battery Chargers
ELECTRICAL CHARACTERISTICS (continued)
(V
= V
DCIN
REF, V
BST
per Figure 1a; T
Cycle-by-Cycle Maximum Current
Limit
= V
CSSP
- VLX= 4.5V, ACIN = GND = PGND = 0, C
A
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
= 18V, V
CSSN
= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
BATT
= V
I
MAX
CSIP
= V
= 12V, V
CSIN
LDO
REFIN
= 1µF, LDO = DLOV, C
RS2 = 0.015Ω 6.0 6.8 7.5
ICTL Power-Down Mode
Threshold Voltage (MAX1908/MAX8724 Only)
ICTL, VCTL Input Bias Current
REFIN Input Bias Current
ICHG Transconductance
(MAX1908/MAX8724 Only)
ICHG Transconductance
(MAX8765 Only)
ICHG Transconductance Error
(MAX8765 Only)
ICHG Transconductance Offset
(MAX8765 Only)
ICHG Accuracy
ICHG Output Current V ICHG Output Voltage V
V
G
ICHG VCSIP
G
ICHG VCSIP
rising
ICTL
V
VCTL
V
DCIN
V
DCIN
V
REFIN
= V
= 0, V
= 5V, 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
-5 +5 %
-5 +5 µA
V
- V
= 75mV -6 +6
CSIN
- V
= 45mV -5 +5
CSIN
- V
= 5mV -40 +40
CSIN
- V
= 150mV, V
CSIN
- V
= 150mV, ICHG = float 3.5 V
CSIN
V
V
CSIP
CSIP
CSIP
CSIP
CSIP
INPUT-CURRENT REGULATION CSSP-to-CSSN Full-Scale
Current-Sense Voltage
Input Current-Limit Gain Error
(MAX8765 Only)
Input Current-Limit Offset
(MAX8765 Only)
CSSP, CSSN Input Voltage
Range
CSSP, CSSN Input Current
(MAX1908/MAX8724 Only)
CSSP Input Current
(MAX8765 Only)
72 75 78 mV
V
= V
CLS
REF
V
= V
/ 2 -7.5 +7.5 Input Current-Limit Accuracy
REF
= 1.1V (MAX8765 only) -10 +10
V
CLS
CLS
-2 +2 %
-2 +2 mV
8 28 V
V
= 0 0.1 1
V
DCIN
V
CSSP
CSSP
= V
= V
CSSN
CSSN
= V
= 28V
= 3V, V
= V
ICTL
= 3V -1 +1
ICHG
= V
VCTL
REF
= 5V -1 +1
REFIN
= 0 350 µA
= 0.75 x V
ICTL
= 1µF; CCI, CCS, and CCV are compensated
REFIN /
100
, CELLS = float, CLS =
REFIN
REFIN /55 REFIN
/ 33
-4 +4
> 8V 350 600
DCIN
V
= 0V 0.1 1
DCIN
= 28V
V
DCIN
400 650
MAX1908/MAX8724/MAX8765
A
V
µA
µA
%
%
µA
µA
_______________________________________________________________________________________ 3
MAX1908/MAX8724/MAX8765
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 = float, 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
V
DCIN
= 0 0.1 1
CSSN Input Current
(MAX8765 Only)
V
CSSP
= V
CSSN
= 28V
V
DCIN
= 28V 0.1 1
µA
CLS Input Range
(MAX1908/MAX8724 Only)
1.6
V
CLS Input Range
(MAX8765 Only)
1.1
V
CLS Input Bias Current V
CLS
= 2V -1 +1 µA
IINP Transconductance
(MAX1908/MAX8724 Only)
G
IINP
V
CSSP
- V
CSSN
= 75mV 2.7 3 3.3
µA/mV
V
CSSP
- V
CSSN
= 75mV -5 +5
IINP Accuracy
V
CSSP
- V
CSSN
= 37.5mV
%
IINP Transconductance
(MAX8765 Only)
G
IINP
V
CSSP
- V
CCSN
= 75mV
3
µA/mV
IINP Transconductance Error
(MAX8765 Only)
-6 +6 %
IINP Transconductance Offset
(MAX8765 Only)
-10
µA
IINP Output Current V
CSSP
- V
CSSN
= 150mV, V
IINP
= 0
µA
IINP Output Voltage V
CSSP
- V
CSSN
= 150mV, V
IINP
= float 3.5 V
SUPPLY AND LDO REGULATOR DCIN Input Voltage Range V
DCIN
8 28 V V
DCIN
falling 7 7.4
DCIN Undervoltage-Lockout Trip
Point
V
DCIN
rising 7.5
V
DCIN Quiescent Current I
DCIN
8.0V < V
DCIN
< 28V 3.2 6 mA
V
BATT
= 19V, V
DCIN
= 0 1
BATT Input Current I
BATT
V
BATT
= 2V to 19V, V
DCIN
= 19.3V
500
µA
LDO Output Voltage 8V < V
DCIN
< 28V, no load
5.4
V
LDO Load Regulation 0 < I
LDO
< 10mA
34 100 mV
LDO Undervoltage-Lockout Trip
Point
V
DCIN
= 8V
4
V
REFERENCE REF Output Voltage 0 < I
REF
< 500µA
V
REF Undervoltage-Lockout Trip
Point
V
REF
falling 3.1 3.9 V
REF
REF
-7.5
2.82
350
5.25
3.20
4.072 4.096 4.120
200
+7.5
3.18
+10
7.85
5.55
5.15
MAX1908/MAX8724/MAX8765
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 = float, 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
DCIN
falling, referred to V
CSIN
(MAX1908/MAX8724 only)
50
150
BATT Power-Fail Threshold
V
CSSP
falling, referred to V
CSIN
(MAX8765 only)
50
150
mV
BATT Power-Fail Threshold
Hysteresis
mV
ACIN rising (MAX8765 only)
V
ACIN Threshold
ACIN rising (MAX1908/MAX8724 only)
V ACIN Threshold Hysteresis 0.5% of REF 20 mV ACIN Input Bias Current V
ACIN
= 2.048V -1 +1 µA
SWITCHING REGULATOR
DHI Off-Time
V
BATT
= 16V, V
DCIN
= 19V,
V
CELLS
= V
REFIN
0.4
µs
DHI Minimum Off-Time
V
BATT
= 16V, V
DCIN
= 17V,
V
CELLS
= V
REFIN
µs
DHI Maximum On-Time 2.5 5 7.5 ms DLOV Supply Current I
DLOV
DLO low 5 10 µA
BST Supply Current I
BST
DHI high 6 15 µA
BST Input Quiescent Current
V
DCIN
= 0, V
BST
= 24.5V,
V
BATT
= VLX = 20V
0.3 1 µA
LX Input Bias Current V
DCIN
= 28V, V
BATT
= VLX = 20V
500 µA
LX Input Quiescent Current V
DCIN
= 0, V
BATT
= VLX = 20V 0.3 1 µA
DHI Maximum Duty Cycle 99
%
Minimum Discontinuous-Mode
Ripple Current
0.5 A
Battery Undervoltage Charge
Current
V
BATT
= 3V per cell (RS2 = 15mΩ),
MAX1908 only, V
BATT
rising
450 mA
CELLS = GND, MAX1908 only, V
BATT
rising 6.1 6.2 6.3
CELLS = float, MAX1908 only, V
BATT
rising
9.3
Battery Undervoltage Current
Threshold
V
DHI On-Resistance High V
BST
- VLX = 4.5V, I
DHI
= +100mA 4 7 Ω
DHI On-Resistance Low V
BST
- V
LX
= 4.5V, I
DHI
= -100mA 1 3.5 Ω
DLO On-Resistance High V
DLOV
= 4.5V, I
DLO
= +100mA
4 7 Ω
DLO On-Resistance Low V
DLOV
= 4.5V, I
DLO
= -100mA
1 3.5 Ω
100
100
200
2.028 2.048 2.068
2.007 2.048 2.089
0.36
0.24 0.28 0.33
150
99.9
CELLS = V
, MAX1908 only, V
REFIN
BATT
rising 12.2 12.4 12.6
150 300
9.15
0.44
9.45
MAX1908/MAX8724/MAX8765
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 = float, 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
ERROR AMPLIFIERS
GMV Amplifier Transconductance
GMV
V
V C T L
= V
LD O
, V
BAT T
= 16.8V ,
C E LLS = V
RE F IN
µA/mV
GMI Amplifier Transconductance
GMI V
ICTL
= V
RE F IN
, V
CSIP
- V
CSIN
= 75mV 0.5 1 2.0
µA/mV
GMS Amplifier Transconductance
GMS V
CLS
= V
REF
, V
CSSP
- V
CSSN
= 75mV 0.5 1 2.0
µA/mV
CCI, CCS, CCV Clamp Voltage 0.25V < V
CCV,CCS,CCI
< 2V
600 mV LOGIC LEVELS CELLS Input Low Voltage 0.4 V
CELLS Input Float Voltage CELLS = float
(V
REFIN
/ 2) -
0.2V
V
REFIN
/ 2
( V
R E F IN
/ 2) +
V
CELLS Input High Voltage
V
REFIN
V
CELLS Input Bias Current CELLS = 0 or V
REFIN
-2 +2 µA
ACOK AND SHDN ACOK Input Voltage Range 0 28 V ACOK Sink Current V
ACOK
= 0.4V, V
ACIN
= 3V 1 mA
ACOK Leakage Current V
ACOK
= 28V, V
ACIN
= 0 1 µA
SHDN Input Voltage Range 0
V
V
SHDN
= 0 or V
LDO
-1 +1
SHDN Input Bias Current
V
DCIN
= 0, V
SHDN
= 5V -1 +1
µA
SHDN Threshold V
SHDN
falling 22
25
% of
V
REFIN
SHDN Threshold Hysteresis 1
% of
V
REFIN
0.0625 0.125 0.2500
150 300
- 0.4V
0.2V
23.5
LDO
MAX1908/MAX8724/MAX8765
Low-Cost Multichemistry Battery Chargers
_______________________________________________________________________________________ 7
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 = FLOAT, 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
CHARGE-VOLTAGE REGULATION
V
VCTL
= V
REFIN
(2, 3, or 4 cells)
V
VCTL
= V
REFIN
/ 20 (2, 3, or 4 cells)
Battery Regulation Voltage
Accuracy
V
VCTL
= V
LDO
(2, 3, or 4 cells)
%
REFIN Range (Note 1) 2.5 3.6 V REFIN Undervoltage Lockout V
REFIN
falling
V
CHARGE CURRENT REGULATION
CSIP-to-CSIN Full-Scale Current-
Sense Voltage
V
ICTL
= V
REFIN
mV
V
ICTL
= V
REFIN
-6 +6
V
ICTL
= V
REFIN
x 0.6
V
ICTL
= V
LDO
MAX8765 only; V
ICTL
= V
REFIN
x 0.036 -50
Charging-Current Accuracy
MAX8724 only; V
ICTL
= V
REFIN
x 0.058
-33
%
Charge-Current Gain Error
(MAX8765 Only)
-2 +2 %
Charge-Current Offset
(MAX8765 Only)
-2 +2 mV
BATT/CSIP/CSIN Input Voltage
Range
0 19 V
V
DCIN
= 0 or V
ICTL
= 0 or SHDN = 0 1
CSIP/CSIN Input Current
Charging 650
µA
Cycle-by-Cycle Maximum Current
Limit
I
MAX
RS2 = 0.015Ω 6.0 7.5 A
ICTL Power-Down Mode
Threshold Voltage (MAX1908/MAX8724 Only)
V
ICTL
rising
REFIN /
100
REFIN /
33
V
ICHG Transconductance
(MAX1908/MAX8724 Only)
V
CSIP
- V
CSIN
= 45mV 2.7 3.3
µA/mV
ICHG Transconductance
(MAX8765 Only)
V
CSIP
- V
CSIN
= 45mV
µA/mV
ICHG Transconductance Error
(MAX8765 Only)
%
ICHG Transconductance Offset
(MAX8765 Only)
µA
-0.6
-0.6
-0.6
70.5
-7.5
-7.5
G
ICHG
G
ICHG
2.785
-7.5
-6.5
+0.6 +0.6 +0.6
1.92
79.5
+7.5 +7.5
+50
+33
3.225
+7.5
+6.5
MAX1908/MAX8724/MAX8765
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 = FLOAT, 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
V
CSIP
- V
CSIN
= 75mV
V
CSIP
- V
CSIN
= 45mV
ICHG Accuracy
V
CSIP
- V
CSIN
= 5mV -40
%
INPUT-CURRENT REGULATION CSSP-to-CSSN Full-Scale
Current-Sense Voltage
mV
V
CLS
= V
REF
-5 +5
V
CLS
= V
REF
/ 2
Input Current-Limit Accuracy
V
CLS
= 1.1V (MAX8765 only) -10
%
Input Current-Limit Gain Error
(MAX8765 Only)
-2 +2 %
Input Current-Limit Offset
(MAX8765 Only)
-2 +2 mV
CSSP, CSSN Input Voltage
Range
8 28 V
V
DCIN
= 0 1
CSSP, CSSN Input Current
(MAX1908/MAX8724 Only)
V
CSSP
= V
CSSN
= V
DCIN
> 8V 600
µA
V
DCIN
= 0V 1
CSSP Input Current
(MAX8765 Only)
V
CSSP
= V
CSSN
= 28V
V
DCIN
= 28V 650
µA
V
DCIN
= 0V 1
CSSN Input Current
(MAX8765 Only)
V
CSSP
= V
CSSN
= 28V
V
DCIN
= 28V 1
µA
CLS Input Range
(MAX1908/MAX8724 Only)
1.6
V
CLS Input Range
(MAX8765 Only)
1.1
V
IINP Transconductance
(MAX1908/MAX8724 Only)
G
IINP
V
CSSP
- V
CSSN
= 75mV 2.7 3.3
µA/mV
IINP Transconductance
(MAX8765 Only)
G
IINP
V
CSSP
- V
CCSN
= 75mV
µA/mV
IINP Transconductance Error
(MAX8765 Only)
%
IINP Transconductance Offset
(MAX8765 Only)
-12
µA
V
CSSP
- V
CSSN
= 75mV
IINP Accuracy
V
CSSP
- V
CSSN
= 37.5mV
%
-7.5
-7.5
71.25
-7.5
2.785
-7.5
-7.5
-7.5
+7.5 +7.5
+40
78.75
+7.5
+10
REF
REF
3.225
+7.5
+12
+7.5 +7.5
MAX1908/MAX8724/MAX8765
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 = FLOAT, 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
CONDITIONS
UNITS
SUPPLY AND LDO REGULATOR DCIN Input Voltage Range
8 28 V
DCIN Quiescent Current I
DCIN
8V < V
DCIN
< 28V 6 mA
V
BATT
= 19V, V
DCIN
= 0 1
BATT Input Current I
BATT
V
BATT
= 2V to 19V, V
DCIN
= 19.3V 500
µA
LDO Output Voltage 8V < V
DCIN
< 28V, no load
V
LDO Load Regulation 0 < I
LDO
< 10mA 100 mV
REFERENCE REF Output Voltage 0 < I
REF
< 500µA
V
TRIP POINTS
V
DCIN
falling, referred to V
CSIN
(MAX1908/MAX8724 only)
50 150
BATT Power-Fail Threshold
V
CSSP
falling, referred to V
CSIN
(MAX8765 only)
50 150
mV
ACIN rising (MAX8765 only)
ACIN Threshold
ACIN rising (MAX1908/MAX8724 only)
V
SWITCHING REGULATOR
DHI Off-Time
V
BATT
= 16V, V
DCIN
= 19V,
V
CELLS
= V
REFIN
µs
DHI Minimum Off-Time
V
BATT
= 16V, V
DCIN
= 17V,
V
CELLS
= V
REFIN
µs
DHI Maximum On-Time 2.5 7.5 ms DHI Maximum Duty Cycle 99 %
Battery Undervoltage Charge
Current
V
BATT
= 3V per cell (RS2 = 15mΩ),
MAX1908 only, V
BATT
rising
450 mA
CELLS = GND, MAX1908 only, V
BATT
rising
CELLS = float, MAX1908 only, V
BATT
rising
Battery Undervoltage Current
Threshold
V
DHI On-Resistance High V
BST
- VLX = 4.5V, I
DHI
= +100mA 7 Ω
DHI On-Resistance Low V
BST
- VLX = 4.5V, I
DHI
= -100mA 3.5 Ω
DLO On-Resistance High V
DLOV
= 4.5V, I
DLO
= +100mA 7 Ω
DLO On-Resistance Low V
DLOV
= 4.5V, I
DLO
= -100mA 3.5 Ω
SYMBOL
V
DCIN
MIN TYP MAX
5.25
4.065
2.028
2.007
0.35
0.24
CELLS = V
, MAX1908 only, V
REFIN
150
6.09
9.12
rising 12.18
BATT
5.55
4.120
2.068
2.089
0.45
0.33
6.30
9.45
12.60
MAX1908/MAX8724/MAX8765
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 = FLOAT, 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
ERROR AMPLIFIERS
GMV Amplifier Transconductance
GMV
V
V C T L
= V
LD O
, V
BAT T
= 16.8V ,
C E LLS = V
RE F IN
µA/mV
GMI Amplifier Transconductance
GMI V
ICTL
= V
RE F IN
, V
CSIP
- V
CSIN
= 75mV 0.5 2.0
µA/mV
GMS Amplifier Transconductance
GMS V
CLS
= V
REF
, V
CSSP
- V
CSSN
= 75mV 0.5 2.0
µA/mV
CCI, CCS, CCV Clamp Voltage 0.25V < V
CCV,CCS,CCI
< 2V
600 mV LOGIC LEVELS CELLS Input Low Voltage 0.4 V
CELLS Input Float Voltage CELLS = float
(V
REFIN
/ 2) -
0.2V
( V
R E F IN
/ 2) +
V
CELLS Input High Voltage
V
REFIN
V
ACOK AND SHDN ACOK Input Voltage Range 0 28 V ACOK Sink Current V
A COK
= 0.4V, V
ACIN
= 3V 1 mA
SHDN Input Voltage Range 0
V
SHDN Threshold V
S HDN
falling 22 25
% of
V
REFIN
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 -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.)
0.0625
150
- 0.4V
0.250
0.2V
LDO
MAX1908/MAX8724/MAX8765
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 11
INDUCTOR
CURRENT
500mA/div
V
DCIN
10V/div
V
BATT
500mV/div
LINE-TRANSIENT RESPONSE
MAX1908 toc04
10ms/div
ICTL = LDO VCTL = LDO I
CHARGE
= 3A
LINE STEP 18.5V TO 27.5V
-1.0
-0.8
-0.9
-0.6
-0.7
-0.4
-0.5
-0.3
-0.1
-0.2
0
0 2341 567 9810
LDO LOAD REGULATION
MAX1908 toc05
LDO CURRENT (mA)
V
LDO
ERROR (%)
V
LDO
= 5.4V
-0.05
-0.03
-0.04
-0.01
-0.02
0.01
0
0.02
0.04
0.03
0.05
8 12141610 18 20 22 2624 28
LDO LINE REGULATION
MAX1908 toc06
VIN (V)
V
LDO
ERROR (%)
I
LDO
= 0
V
LDO
= 5.4V
-0.10
-0.07
-0.08
-0.09
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
0 200100 300 400 500
REF VOLTAGE LOAD REGULATION
MAX1908 toc07
REF CURRENT (μA)
V
REF
ERROR (%)
-0.10
-0.04
-0.06
-0.08
-0.02
0
0.02
0.04
0.06
0.08
0.10
-40 10-15 35 60 85
REF VOLTAGE ERROR vs. TEMPERATURE
MAX1908 toc08
TEMPERATURE (°C)
V
REF
ERROR (%)
90
0
0.01 1010.1
EFFICIENCY vs. CHARGE CURRENT
30
10
70
50
100
40
20
80
60
MAX1908 toc09
CHARGE CURRENT (A)
EFFICIENCY (%)
V
BATT
= 16V
V
BATT
= 8V
V
BATT
= 12V
0
100
50
250
200
150
300
350
450
400
500
0462 8 10 12 14 16 18 20 22
FREQUENCY vs. VIN - V
BATT
MAX1908 toc10
(VIN - V
BATT
) (V)
FREQUENCY (kHz)
I
CHARGE
= 3A
VCTL = ICTL = LDO
3 CELLS
4 CELLS
-0.4
-0.1
-0.3
-0.5
0
0.2
0.3
0.4
0.5
01234
OUTPUT V/I CHARACTERISTICS
MAX1908 toc11
BATT CURRENT (A)
BATT VOLTAGE ERROR (%)
0.1
-0.2
2 CELLS
3 CELLS
4 CELLS
0
0.02
0.01
0.03
0.06
0.07
0.05
0.04
0.08
0 0.2 0.3 0.4 0.50.1 0.6 0.7 0.8 0.9
1.0
BATT VOLTAGE ERROR vs. VCTL
MAX1908 toc12
VCTL/REFIN (%)
BATT VOLTAGE ERROR (%)
4 CELLS REFIN = 3.3V NO LOAD
Typical Operating Characteristics (continued)
(Circuit of Figure 1, V
DCIN
= 20V, TA= +25°C, unless otherwise noted.)
MAX1908/MAX8724/MAX8765
Low-Cost Multichemistry Battery Chargers
12 ______________________________________________________________________________________
-1
1
0
3
2
4
5
01.00.5 1.5 2.0
CURRENT-SETTING ERROR vs. ICTL
MAX1908 toc13
V
ICTL
(V)
CURRENT-SETTING ERROR (%)
V
REFIN
= 3.3V
0
1.5
1.0
0.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
01.00.5 1.5 2.0 2.5 3.0
ICHG ERROR vs. CHARGE CURRENT
MAX1908 toc14
I
BATT
(A)
ICHG (%)
V
BATT
= 16V
V
BATT
= 12V
V
BATT
= 8V
V
REFIN
= 3.3V
-40
-30
-20
-10
0
10
20
30
40
01234
IINP ERROR vs. SYSTEM LOAD CURRENT
MAX1908 toc15
SYSTEM LOAD CURRENT (A)
IINP ERROR (%)
I
BATT
= 0
-80
-60
-40
-20
0
20
40
60
80
0 0.5 1.0 1.5 2.0
IINP ERROR vs. INPUT CURRENT
MAX1908 toc16
INPUT CURRENT (A)
IINP ERROR (%)
SYSTEM LOAD = 0
ERROR DUE TO SWITCHING NOISE
Typical Operating Characteristics (continued)
(Circuit of Figure 1, V
DCIN
= 20V, TA= +25°C, unless otherwise noted.)
MAX1908/MAX8724/MAX8765
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 13
Pin Description
PIN
NAME
FUNCTION
1
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 a ss 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
Shutdown Control Input. Drive SHDN logic low to shut down the MAX1908/MAX8724/MAX8765. Use with a
thermistor to detect a hot battery and suspend charging.
9
Charge-Current Monitor Output. ICHG is a scaled-down replica of the charger output current. Use ICHG to
monitor the charging current and detect when the chip changes from constant-current mode to constant-
voltage mode. The transconductance of (CSIP - CSIN) to ICHG is 3µA/mV.
10 ACIN AC Detect Input. Input to an uncommitted comparator. ACIN can be used to detect AC-adapter presence. 11
AC Detect Output. High-voltage open-drain output is high impedance when V
ACIN
is less than V
REF
/ 2.
12
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 V
REFIN
/ 32 to V
REFIN
. The MAX1908/MAX8724
shut down if ICTL is forced below V
REFIN
/ 100 while the MAX8765 does not. When ICTL is equal to LDO, the
set point for CSIP - CSIN is 45mV.
14 GND Analog Ground
15
Output Voltage-Limit Set Input. VCTL input voltage range is 0 to V
REFIN
. When VCTL is equal to LDO, the set
point is (4.2 x CELLS)V.
16
Battery Voltage Input
17
Cell Count Input. Tri-level input for setting number of cells. GND = 2 cells, float = 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
Power Ground
21 DLO Low-Side Power MOSFET Driver Output. Connect to low-side nMOS gate. 22
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
Input Current-Sense Negative Input
27
Input Current-Sense Positive Input. Connect a current-sense resistor from CSSP to CSSN.
28 IINP
Input-Current Monitor Output. IINP is a scaled-down replica of the input current. IINP monitors the total
system current. The transconductance of (CSSP - CSSN) to IINP is 3µA/mV.
DCIN
SHDN
ICHG
ACOK REFIN
VCTL
BATT
CELLS
PGND
DLOV
CSSN
CSSP
Low-Cost Multichemistry Battery Chargers
14 ______________________________________________________________________________________
MAX1908/MAX8724/MAX8765
Detailed Description
The MAX1908/MAX8724/MAX8765 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
REFIN
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 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 battery­charging 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 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
, providing a 10% adjust-
ment 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 per-cell 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).
V CELLS V
V
V
BATT
VCTL
REFIN
=×+×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
404.
Table 2. Cell-Count Programming
DESCRIPTION
MAX1908
MAX8724
MAX8765
Conditioning Charge Feature
Yes No No
ICTL Shutdown Mode
Yes Yes No
ACOK Enable Condition
REFIN must
be ready
REFIN must
be ready
Independent
of REFIN
Table 1. Versions Comparison
CELLS CELL COUNT
GND 2
Float 3
V
REFIN
4
MAX1908/MAX8724
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 15
MAX1908/MAX8724/MAX8765
DCIN
LDO
MAX1908 MAX8724 MAX8765
CLSREF
GND
CELLS
DLOV
AC ADAPTER INPUT
8.5V TO 28V
12.6V OUTPUT VOLTAGE
7.5A INPUT CURRENT LIMIT
DHI
D3
BST
SMART
BATTERY
HOST
ACIN
D2
R6
59kΩ
1%
R7
19.6kΩ
1%
C5 1μF
VCTL
ICTL
REFIN
ACOK
ICHG
IINP
R8 1MΩ
R9 20kΩ
R10 10kΩ
C14
0.1μF
C20
0.1μF
CCV
C11
0.1μF
R5 1kΩ
CCI
CCS
C10
0.01μF
C9
0.01μF
C12 1μF
C1 2 × 10μF
C13 1μF
C15
0.1μF
LX
C16 1μF
LDO
R13 33Ω
CSSP CSSN
FLOAT (3 CELLS SELECT)
D1
RS1
0.01Ω
L1 10μH
RS2
0.015Ω
CSIP
CSIN
PGND
DLO
N1b
N1a
BATT
C4 22μF
BATT
+
R19, R20, R21 10kΩ
AVDD/REF
SCL
SDA
TEMP
BATT-
ADC INPUT
ADC INPUT
OUTPUT
DAC OUTPUT
V
CC
SCL
SDA
ADC INPUT
GND
PGND GND
TO EXTERNAL
LOAD
SHDN
0.1μF
0.1μF
Figure 1. µC-Controlled Typical Application Circuit
Typical Application Circuits
Low-Cost Multichemistry Battery Chargers
16 ______________________________________________________________________________________
MAX1908/MAX8724/MAX8765
TO EXTERNAL
LOAD
MAX1908 MAX8724 MAX8765
CLSREF
GND
CELLS
REFIN (4 CELLS SELECT)
DLOV
AC ADAPTER
INPUT
8.5V TO 28V
DHI
D3
BST
BATTERY
ACIN
D2
LDO
LDO
16.8V OUTPUT VOLTAGE
2.5A CHARGE LIMIT
4A INPUT CURRENT LIMIT
R6
59kΩ
1%
R7
19.6kΩ
1%
R11
15kΩ
R12
12kΩ
C5 1μF
C12
1.5nF
SHDN
ICHG
IINP
R19 10kΩ 1%
R20
10kΩ
1%
CCV
C11
0.1μF
R5 1kΩ
CCI
CCS
C10
0.01μF
C9
0.01μF
C12 1μF
C1 2 × 10μF
C13 1μF
C15
0.1μF
LX
C16 1μF
LDO
R13 33Ω
CSSP CSSN
RS1
0.01Ω
L1 10μH
RS2
0.015Ω
CSIP
CSIN
PGND
DLO
N1b
N1a
FROM HOST μP
(SHUTDOWN)
N
BATT
GNDPGND
C4 22μF
BATT
+
REFIN
VCTL
DCIN
BATT-
THM
ICTL
R14
10.5kΩ 1%
R15
8.25kΩ 1%
R16
8.25kΩ 1%
P1
R17
19.1kΩ 1%
R18 22kΩ 1%
ACOK
0.01μF
0.01μF
Figure 2. Typical Application Circuit with Fixed Charging Parameters
Typical Application Circuits (continued)
MAX1908/MAX8724
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 17
MAX1908/MAX8724/MAX8765
MAX1908 MAX8724 MAX8765
LOGIC
BLOCK
GMS
SHDN
GND
CLS
CCS
CSSP
CSSN
CSIP
CSIN
ICTL
CCI
BATT
CELLS
CCV
VCTL
23.5% REFIN
GND
DCIN
SRDY
5.4V
LINEAR
REGULATOR
1/55
ICTL
MAX1908/MAX8724 ONLY
REF/2
RDY
4V
CELL
SELECT
LOGIC
4.096V
REFERENCE
LVC
REFIN
CSI
BAT_UV
3.1V/CELL
R1
LVC
DCIN
LDO
REF
REFIN
ACIN
ACOK
IINP
ICHG
BST
DHI
LX
DLOV
DLO
PGND
MAX1908 ONLY
x
75mV
REF
LEVEL
SHIFTER
x
75mV REFIN
x
400mV
REFIN
DC-DC
CONVERTER
GMI
GMV
GM
LEVEL
SHIFTER
N
GM
LEVEL
SHIFTER
DRIVER
DRIVER
Figure 3. Functional Diagram
Functional Diagram
Low-Cost Multichemistry Battery Chargers
18 ______________________________________________________________________________________
MAX1908/MAX8724/MAX8765
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
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 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 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 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
REF
/ 2. Since ACOK can withstand 30V (max), ACOK
I
V
VRS
INPUT
CLS
REF
=×
0 0751.
II
IV
V
INPUT LOAD
CHG BATT
IN
=+
×
×
⎛
⎝
⎜
⎞
⎠
⎟
η
I
V
RS
CHG
=
0 0452.
I
V
VRS
CHG
ICTL
REFIN
=×
0 0752.
MAX1908/MAX8724
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 19
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, 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 feature a low-power shutdown mode. Driving SHDN low shuts down the MAX1908/MAX8724/MAX8765. In shutdown, the DC­DC 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 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 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 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 G
M
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 feature a variable off­time, cycle-by-cycle current-mode control scheme. Depending upon the conditions, the MAX1908/MAX8724/ MAX8765 work in continuous or discontinuous-conduc­tion mode.
Continuous-Conduction Mode
With sufficient charger loading, the MAX1908/MAX8724/ MAX8765 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
Low-Cost Multichemistry Battery Chargers
20 ______________________________________________________________________________________
MAX1908/MAX8724/MAX8765
IMAX
RESET
1.8V
0.15V
0.1V
5ms
LVC
CONTROL
CELLS
SETV
SETI
CCVCCICCS
GMS
GMI
GMV
CLS
DLO
DHI
CSI
X20
t
OFF
GENERATOR
BST
S
RQ
CCMP
ZCMP
IMIN
CHG
RQ
S
CSS X20
CSSP
AC ADAPTER
CSSN
BST
DHI
LX
RS1
LDO
D3
N1a
N1b
C
BST
L1
RS2
DLO
CSIP
CSIN
C
OUT
BATT
BATTERY
MAX1908 MAX8724 MAX8765
Q
CELL
SELECT
LOGIC
Figure 4. DC-DC Functional Diagram
DC-DC Functional Diagram
MAX1908/MAX8724
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 enter discontinuous­conduction 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 have 99% duty-cycle capability with a 5ms (max) on-time and 0.3µs (min) off­time. 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.
f
RC F
kHz
C
==
××
=
1
2
1
2331
48
ππ μΩ
.
IMIN
V
RSA=×
=
015
20 2
05..
f
LI
VV
s
RIPPLE
CSSN BATT
=
×
−
()
+103. μ
f
tt
ON OFF
=
+
1
I
Vt
L
RIPPLE
BATT OFF
=
×
t
LI
VV
ON
RIPPLE
CSSN BATT
=
×
−
ts
VV
V
OFF
DCIN BATT
DCIN
=×
−
25. μ
MAX1908/MAX8724/MAX8765
Low-Cost Multichemistry Battery Chargers
22 ______________________________________________________________________________________
MAX1908/MAX8724/MAX8765
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 RLvaries with load according to RL= V
BATT
/ I
CHG.
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 RCVand 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 R
CV
= 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:
f
RC
MHz
Z ESR
ESR OUT
_
.=
×
=
1
2
2 412
π
R
fC
ESR
CO CV OUT
=
×× ×
=
1
210
024
π
_
. Ω
f
GMV R GM
C
kHz
CO CV
CV OUT
OUT
_
=
××
=280
π
f
GMV R GM
C
CO CV
CV OUT
OUT
_
=
××
2π
LTF GM R GMV R
sC R sC R
sC R sC R
OUT L OGMV
OUT ESR CV CV
CV OGMV OUT L
=××××
+×
()
+×
()
+×
()
+×
()
11
11
f
RC
Z ESR
ESR OUT
_
=
×
1
2π
f
RC
P OUT
L OUT
_
=
×
1
2πfRC
ZCV
CV CV
_
=
×
1
2π
f
RC
PCV
OGMV CV
_
=
×
1
2πGMARS
OUT
CSI
=
×12
GM
OUT
BATT
CCV
GMV
REF
R
CV
C
CV
R
OGMV
R
ESR
R
L
C
OUT
Figure 5. CCV Loop Diagram
MAX1908/MAX8724
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 equiva-
lent 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:
LTF GMI
R
sR C
OGMI
OGMI CI
=×
+×1
GM
ARS
OUT
CSI
=
×12
LTF GM A RS GMI
R
sR C
OUT CSI
OGMI
OGMI CI
=×××
+×
2
1
f
RC
PCI
OGMI CI
_
=
×
1
2π
f
RC
Hz
PCV
OGMV CV
_
.=
×
=
1
2
016
π
C
R kHz
nF
CV
CV
=
×
=
1
2108
147
π .
f
RC
ZCV
CV CV
_
=
×
1
2π
R
V
I
Battery ESR
L
BATT
CHG
==
Δ
Δ
f
RC
kHz
P OUT
L OUT
_
.=
×
=
1
2
108
π
MAX1908/MAX8724/MAX8765
CCV LOOP GAIN vs. FREQUENCY
FREQUENCY (Hz)
GAIN (dB)
100k10k1k10010
-40
-20
0
20
40
60
80
-60 11M
CCV LOOP PHASE
vs. FREQUENCY
FREQUENCY (Hz)
PHASE (DEGREES)
100k10k1k10010
-120
-105
-90
-75
-60
-45
-135 11M
Figure 6. CCV Loop Gain/Phase vs. Frequency
Low-Cost Multichemistry Battery Chargers
24 ______________________________________________________________________________________
MAX1908/MAX8724/MAX8765
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 switching frequency:
Solving for CCI, 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
f
GMI
RC
Hz
PCI
OGMI CI
_
.=
×
=20 0016
π
f
GMI
nF
kHz
CO CI_
==
210
16
π
f
GMI
C
kHz
CO CICI_
==
280π
f
RC
PCI
OGMI CI
_
=
×
1
2π
f
GMI
C
CO CICI_
=
2π
GM
OUT
CCI
GMI
ICTL
C
CI
R
OGMI
CSIP CSIN
CSI
RS2
Figure 7. CCI Loop Diagram
CCI LOOP GAIN
vs. FREQUENCY
FREQUENCY (Hz)
GAIN (dB)
100k10k1 10 100 1k
-40
-20
0
20
40
60
80
100
-60
0.1 1M
CCI LOOP PHASE
vs. FREQUENCY
FREQUENCY (Hz)
PHASE (DEGREES)
100k10k1k100101
-90
-75
-60
-45
-30
-15
0
-105
0.1 1M
Figure 8. CCI Loop Gain/Phase vs. Frequency
MAX1908/MAX8724
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
f
RC
PCS
OGMS CS
_
=
×
1
2π
f
GMS
C
CO CSCS_
=
2π
LTF GMS
R
sR C
OGMS
OGMS CS
=×
+×1
GM
ARS
IN
CSS
=
×11
LTF GM A RS GMS
R
sR C
IN CSS
OGMS
OGMS CS
=××××
+×
1
1
f
RC
PCS
OGMS CS
_
=
×
1
2π
MAX1908/MAX8724/MAX8765
GM
IN
CCS
GMS
CLS
C
CS
R
OGMS
CSSP CSSN
CSS
RS1
Figure 9. CCS Loop Diagram
CCS LOOP GAIN
vs. FREQUENCY
FREQUENCY (Hz)
GAIN (dB)
100k10k1 10 100 1k
-40
-20
0
20
40
60
80
100
-60
0.1 1M
CCS LOOP PHASE
vs. FREQUENCY
FREQUENCY (Hz)
PHASE (DEGREES)
100k10k1k100101
-90
-75
-60
-45
-30
-15
0
-105
0.1 1M
Figure 10. CCS Loop Gain/Phase vs. Frequency
Low-Cost Multichemistry Battery Chargers
26 ______________________________________________________________________________________
MAX1908/MAX8724/MAX8765
Choose crossover frequency f
CO_CS
to be 1/5th the
MAX1908/MAX8724/MAX8765 switching frequency:
Solving for CCS, CCS= 2nF.
To be conservative, set CCS= 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:
I
C
dV
dt
C1
2
1=
ESR
V
I
CC1
1
05<.
II DD
C CHG1
2
=−
f
RC
Hz
PCS
OGMS CS
_
.=
×
=
1
2
0 0016
π
f
GMS
nF
kHz
CO CS_
==
210
16
π
f
GMS
C
kHz
CO CSCS_
==
280π
RIPPLE CURRENT vs.
BATTERY VOLTAGE
V
BATT
(V)
RIPPLE CURRENT (A)
14131211109
0.5
1.0
1.5
0
8 15161718
V
DCIN
= 19V
VCTL = ICTL = LDO
4 CELLS
3 CELLS
Figure 11. Ripple Current vs. Battery Voltage
MAX1908/MAX8724
Low-Cost Multichemistry Battery Chargers
______________________________________________________________________________________ 27
MAX1908/MAX8724/MAX8765
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 V
GS
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 (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 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/MAX8765s’ 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
Low-Cost Multichemistry Battery Chargers
28 ______________________________________________________________________________________
MAX1908/MAX8724/MAX8765
Table 3. Component List for Circuit of Figure 2
DESIGNATION
QTY
DESCRIPTION
C1 2
10µF, 50V 2220-size ceramic capacitors TDK C5750X7R1H106M
C4 1
22µF, 25V 2220-size ceramic capacitor TDK C5750X7R1E226M
C5 1
1µF, 25V X7R ceramic capacitor (1206) Murata GRM31MR71E105K Taiyo Yuden TMK316BJ105KL TDK C3216X7R1E105K
C9, C10 2
0.01µF, 16V cer am i c cap aci tor s ( 0402) Murata GRP155R71E103K Taiyo Yuden EMK105BJ103KV TDK C1005X7R1E103K
C11, C14,
C15, C20
4
0.1µF, 25V X7R ceramic capacitors (0603) Murata GRM188R71E104K TDK C1608X7R1E104K
C12, C13, C16
3
1µF, 6.3V X5R ceramic capacitors (0603) Murata GRM188R60J105K Taiyo Yuden JMK107BJ105KA TDK C1608X5R1A105K
D1 (optional)
1
10A Schottky diode (D-PAK) Diodes, Inc. MBRD1035CTL ON Semiconductor MBRD1035CTL
D2 1
Schottky diode Central Semiconductor CMPSH1–4
Chip Information
TRANSISTOR COUNT: 3772
PROCESS: BiCMOS
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. Connect the power ground and normal ground to this node.
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
MAX1908/MAX8724
Low-Cost Multichemistry Battery Chargers
MAX1908/MAX8724/MAX8765
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 29
© 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages
.)
QFN THIN.EPS
D2
(ND-1) X e
e
D
C
PIN # 1 I.D.
(NE-1) X e
E/2
E
0.08 C
0.10 C
A
A1
A3
DETAIL A
E2/2
E2
0.10 M C A B
PIN # 1 I.D.
b
0.35x45°
D/2
D2/2
L
C
L
C
e e
L
CC
L
k
L
L
DETAIL B
L
L1
e
AAAAA
MARKING
I
1
2
21-0140
PACKAGE OUTLINE, 16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm
-DRAWING NOT TO SCALE-
L
e/2
COMMON DIMENSIONS
MAX.
EXPOSED PAD VARIATIONS
D2
NOM.MIN.
MIN.
E2
NOM. MAX.
NE
ND
PKG.
CODES
1. DIMENSIONING & TOLERANCING CONFORM TO ASME Y14.5M-1994.
2. ALL DIMENSIONS ARE IN MILLIMETERS. ANGLES ARE IN DEGREES.
3. N IS THE TOTAL NUMBER OF TERMINALS.
4. THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION SHALL CONFORM TO JESD 95-1 SPP-012. DETAILS OF TERMINAL #1 IDENTIFIER ARE OPTIONAL, BUT MUST BE LOCATED WITHIN THE ZONE INDICATED. THE TERMINAL #1 IDENTIFIER MAY BE EITHER A MOLD OR MARKED FEATURE.
5. DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN
0.25 mm AND 0.30 mm FROM TERMINAL TIP.
6. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY.
7. DEPOPULATION IS POSSIBLE IN A SYMMETRICAL FASHION.
8. COPLANARITY APPLIES TO THE EXPOSED HEAT SINK SLUG AS WELL AS THE TERMINALS.
9. DRAWING CONFORMS TO JEDEC MO220, EXCEPT EXPOSED PAD DIMENSION FOR T2855-3 AND T2855-6.
NOTES:
SYMBOL
PKG.
N
L1
e
E
D
b
A3
A
A1
k
10. WARPAGE SHALL NOT EXCEED 0.10 mm.
JEDEC
0.70 0.800.75
4.90
4.90
0.25
0.250--
4
WHHB
4
16
0.350.30
5.10
5.105.00
0.80 BSC.
5.00
0.05
0.20 REF.
0.02
MIN. MAX.NOM.
16L 5x5
L
0.30 0.500.40
---
---
WHHC
20
5
5
5.00
5.00
0.30
0.55
0.65 BSC.
0.45
0.25
4.90
4.90
0.25
0.65
--
5.10
5.10
0.35
20L 5x5
0.20 REF.
0.75
0.02
NOM.
0
0.70
MIN.
0.05
0.80
MAX.
---
WHHD-1
28
7
7
5.00
5.00
0.25
0.55
0.50 BSC.
0.45
0.25
4.90
4.90
0.20
0.65
--
5.10
5.10
0.30
28L 5x5
0.20 REF.
0.75
0.02
NOM.
0
0.70
MIN.
0.05
0.80
MAX.
---
WHHD-2
32
8
8
5.00
5.00
0.40
0.50 BSC.
0.30
0.25
4.90
4.90
0.50
--
5.10
5.10
32L 5x5
0.20 REF.
0.75
0.02
NOM.
0
0.70
MIN.
0.05
0.80
MAX.
0.20 0.25 0.30
DOWN BONDS
ALLOWED
YES3.103.00 3.203.103.00 3.20T2055-3
3.103.00 3.203.103.00 3.20
T2055-4
T2855-3 3.15 3.25 3.35 3.15 3.25 3.35
T2855-6
3.15 3.25 3.35 3.15 3.25 3.35
T2855-4 2.60 2.70 2.80 2.60 2.70 2.80 T2855-5 2.60 2.70 2.80 2.60 2.70 2.80
T2855-7 2.60 2.70
2.80
2.60 2.70 2.80
3.20
3.00 3.10T3255-3 3 .203.00 3.10
3.203.00 3.10T3255-4 3 .203.00 3.10
NO
NO NO
NO
YES YES
YES
YES
3.203.00T1655-3 3.10 3.00 3.10 3.20 NO NO3.203.103.003.10T1655N-1 3.00 3.20
3.353.15T2055-5 3.25 3.15 3.25 3.35
YES
3.35
3.15
T2855N-1
3.25 3.15 3.25 3.35
NO
3.353.15T2855-8 3.25 3.15 3.25 3.35
YES
3.203.10T3255N-1 3.00
NO
3.203.103.00
L
0.40
0.40
** ** **
**
** ** ** ** **
** ** **
**
**
SEE COMMON DIMENSIONS TABLE
±0.15
11. MARKING IS FOR PACKAGE ORIENTATION REFERENCE ONLY.
I
2
2
21-0140
PACKAGE OUTLINE, 16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm
-DRAWING NOT TO SCALE-
12. NUMBER OF LEADS SHOWN ARE FOR REFERENCE ONLY.
3.30T4055-1 3.20 3.40 3.20 3.30 3.40
**
YES
0.050 0.02
0.600.40 0.50
10
-----
0.30
40 10
0.40 0.50
5.10
4.90 5.00
0.25 0.35 0.45
0.40 BSC.
0.15
4.90
0.250.20
5.00 5.10
0.20 REF.
0.70
MIN.
0.75 0.80
NOM.
40L 5x5
MAX.
13. LEAD CENTERLINES TO BE AT TRUE POSITION AS DEFINED BY BASIC DIMENSION "e", ±0.05.
T1655-2
**
YES3.203.103.003.103.00 3.20
T3255-5 YES3.003.103.00
3.20
3.203.10
**
exceptions
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