General Description
The MAX1757 is a switch-mode lithium-ion (Li+) battery
charger that charges one to three cells. It provides a
regulated charging current accurate to ±10% and a
regulated voltage with only a ±0.8% total voltage error
at the battery terminals. The internal high-side switch
delivers a programmable current of up to 1.5A to
charge the battery. The built-in safety timer automatical-
ly terminates charging once the adjustable time limit
has been reached.
The MAX1757 regulates the voltage set point and
charging current using two loops that work together to
transition smoothly between voltage and current regula-
tion. An additional control loop monitors the total cur-
rent drawn from the input source (charging + system)
and by automatically reducing battery-charging current
prevents overload of the input supply, allowing the use
of a low-cost wall adapter.
The per-cell battery regulation voltage is set between
4.0V and 4.4V using standard 1% resistors. The num-
ber of cells is set from 1 to 3 by pin strapping. Battery
temperature is monitored by an external thermistor to
prevent charging outside the acceptable temperature
range.
The MAX1757 is available in a space-saving 28-pin
SSOP package. Use the MAX1757EVKIT to help reduce
design time. For a stand-alone charger with a 28V
switch, refer to the MAX1758 data sheet. For a charger
controller capable of up to 4A charging current, refer to
the MAX1737 data sheet.
________________________Applications
Features
♦Stand-Alone Charger for Up to 3 Li+ Batteries
♦±0.8% Battery Regulation Voltage Accuracy
♦Low-Dropout 98% Duty Cycle
♦Safely Precharges Near-Dead Cells
♦Continuous Voltage and Temperature Monitoring
♦0.1µA Shutdown Battery Current
♦Input Voltage Up to 14V
♦Up to 1.5A Programmable Charge Current
♦Input Current Limiting
♦Space-Saving 28-Pin SSOP
♦300kHz PWM Oscillator Reduces Noise
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
________________________________________________________________ Maxim Integrated Products 1
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
DCIN
CSSP
CSSN
CCV
CCI
CCS
FASTCHG
BST
CS
LX
LX
PGND
SHDN
FULLCHG
FAULT
TIMER2
TIMER1
CELL
HSD
HSD
BATT
VADJ
GND
REF
THM
ISETOUT
ISETTIN
VL
SSOP
TOP VIEW
MAX1757
Pin Configuration
19-1754; Rev 0; 6/00
For free samples and the latest literature, visit www.maxim-ic.com or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
EVALUATION KIT
AVAILABLE
Ordering Information
28 SSOP
PIN-PACKAGETEMP. RANGE
-40°C to +85°CMAX1757EAI
PART
GND
DCIN
VIN
6V to 14V
CSSP
CSSN
LX
BST
VL
PGND
CS
BATT
THM
THERM
FASTCHG
FULLCHG
FAULT
HSD
SYSTEM
LOAD
Li+ BATTERY
1 TO 3 CELLS
MAX1757
ISETOUT
ISETIN
VADJ
CCS
CCI
CCV
REF
TIMER1
TIMER2
SHDN
ON
OFF
CELL
Li+ Battery Packs
Notebook Computers
Hand-Held Instruments
PDAs
Desktop Cradle Chargers
Typical Operating Circuit
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VDCIN = VHSD = VCSSP = VCSSN = 12V, VSHDN = VVL, VCELL = GND, VBATT = VCS = 4.2V, VVADJ = VREF/2,
VISETIN = VISETOUT = VREF, RTHM = 10kΩ, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
BATT, CS, DCIN, CSSP, CSSN, HSD to GND ........-0.3V to +15V
CSSP to CSSN.......................................................-0.6V to +0.6V
BST to GND ............................................................-0.3V to +21V
BST to LX..................................................................-0.3V to +6V
LX to PGND ..............................................-0.6V to (VHSD + 0.3V)
VL, SHDN, ISETIN, ISETOUT, REF, VADJ, CELL, TIMER1,
TIMER2, CCI, CCS, CCV, THM to GND ................-0.3V to +6V
FASTCHG, FULLCHG, FAULT to GND ..................-0.3V to +30V
CS to BATT Current ............................................................±3.5A
PGND to GND .......................................................-0.3V to +0.3V
VL Source Current...............................................................50mA
Continuous Power Dissipation (TA= +70°C)
28-Pin SSOP (derate 9.5mW/°C above +70°C) ...........762mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
VBATT = 14V, done state
Falling edge
CELL = REF, VBATT = 12V, any charging state
VSHDN = GND, VBATT =14V
6V < VDCIN < 14V
Internal resistor between CS and BATT,
1.5A RMS operating
See PWM Controller section
VBST = VLX + 4.5V
VLX = VHSD = VDCIN =14V, VSHDN = GND
IREF = 0 to 1mA
VLX = PGND, VHSD = VDCIN = 14V,
VSHDN = GND
VCSSN = VCSSP = VDCIN = 14V, VSHDN = GND
6V < VDCIN < 14V
Rising edge
6V < VDCIN < 14V
IVL = 0 to 15mA
In dropout, fOSC / 4
6V < VDCIN < 14V
Nondropout fOSC
CONDITIONS
µA150 270
BATT, CS Input Current µA280 540
µA0.1 5
mΩ110 170RCS
CS to BATT Current-Sensing
Resistance
Ω12LX to PGND On-Resistance
mΩ150 250HSD to LX On-Resistance
µA0.1 10LX Off-State Leakage
µA0.1 10HSD Off-State Leakage
µA210CSSN/CSSP Off-State Leakage
%97 98LX Maximum Duty Cycle
kHz270 300 330fOSC
PWM Oscillator Frequency
V0.075 0.125 0.175
DCIN to BATT Dropout
Threshold, DCIN Falling
mA57DCIN Quiescent Supply Current
mV614REF Load Regulation
mV26REF Line Regulation
V
V0.20 0.30 0.40
DCIN to BATT Dropout
Threshold, DCIN Rising
V5.10 5.40 5.70VL Output Voltage
mV44 65
VREF 4.179 4.20 4.221REF Output Voltage
UNITSMIN TYP MAXSYMBOLPARAMETER
V614DCIN Input Voltage Range
SUPPLY AND REFERENCE
SWITCHING REGULATOR
VL Output Load Regulation
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VDCIN = VHSD = VCSSP = VCSSN = 12V, VSHDN = VVL, VCELL = GND, VBATT = VCS = 4.2V, VVADJ = VREF/2,
VISETIN = VISETOUT = VREF, RTHM = 10kΩ, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
VADJ = GND
Instantaneous peak current limit
With 1% VADJ resistors
Not including VADJ resistor tolerances
CELL = float, GND, or REF
CONDITIONS
3.948 3.979 4.010
BATT Regulation Voltage
Adjustment Range
-1 1
Absolute Voltage Accuracy %
-0.8 0.8
V/cell4.167 4.2 4.233VBATTR
Battery Regulation Voltage
V014BATT, CS Input Voltage Range
A2.4 2.7 3.0CS to BATT Hard Current Limit
UNITSMIN TYP MAXSYMBOLPARAMETER
VADJ = REF V/cell
4.386 4.421 4.453
VCCV = 2V mS ×
cells
0.4 0.7 1.0
CCV Amplifier
Transconductance
VCCV = 2V µA±50
CCV Amplifier Maximum Output
Current
A1.35 1.5 1.65BATT Full-Scale Charge Current
VISETOUT = VREF/10 mA100 150 200
BATT 1/10-Scale Charge
Current (Note 1)
VBATT < 2.4V per cell mA100 150 200
BATT Charge Current in
Prequalification State
VCCI = 2V µA/A60 130 240CCI Battery Current Sense Gain
VCCI = 2V µA±100
CCI Amplifier Maximum Output
Current
mV90 100 115
CSSP to CSSN Full-Scale
Current-Sense Voltage
VISETIN = VREF/10 mV51015
CSSP to CSSN 1/10-Scale
Current-Sense Voltage
VCCS = 2V mS1.0 2.0 3.0CCS Amplifier Transconductance
VCCS = 2V µA±100
CCS Amplifier Maximum Output
Current
THM low-temp or high-temp current V1.386 1.40 1.414VTRT
THM Trip Threshold Voltage
VTHM = 1.4V µA46.2 49 51.5ITLTC
THM Low-Temp Current
VTHM = 1.4V µA344 353 362ITHTC
THM High-Temp Current
Combines THM low-temp current and THM
threshold, VTRT / ITLTC kΩ26.92 28.70 30.59
THM COLD Threshold
Resistance (Note 2)
Combines THM high-temp current and THM
threshold, VTRT / ITHTC kΩ3.819 3.964 4.115
THM HOT Threshold Resistance
(Note 2)
mV25 200
CCI, CCS Clamp Voltage with
Respect to CCV
mV25 200
CCV Clamp Voltage with
Respect to CCI, CCS
SWITCHING REGULATOR
ERROR AMPLIFIERS
STATE MACHINE
VOLTAGE LIMIT ACCURACY
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VDCIN = VHSD = VCSSP = VCSSN = 12V, VSHDN = VVL, VCELL = GND, VBATT = VCS = 4.2V, VVADJ = VREF/2,
VISETIN = VISETOUT = VREF, RTHM = 10kΩ, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
CONDITIONS
% of
VBATTR x
cells
94 95 96
BATT Recharge Voltage
Threshold (Note 6)
mA250 330 400
FULLCHG BATT Current
Termination Threshold (Note 5)
V/cell4.55 4.67 4.8
BATT Overvoltage Threshold
(Note 4)
V/cell2.4 2.5 2.6
BATT Undervoltage Threshold
(Note 3)
UNITSMIN TYP MAXSYMBOLPARAMETER
TIMER1 and TIMER2
Oscillation Frequency 2.1 2.33 2.6 kHz
Prequalification Timer 6.25 7.5 8.75
min
Fast-Charge Timer 81 90 100
Full-Charge Timer 81 90 100
Top-Off Timer 40.5 45 49.8
Temperature Measurement
Frequency 0.98 1.12 1.32 Hz
SHDN Input Voltage High VIH 1.4 V
SHDN Input Voltage Low VIL 0.6 V
VADJ, ISETIN, ISETOUT Input
Voltage Range 0V
REF V
VVADJ, VISETIN, VISETOUT = 0 or 4.2V -50 50 nA
SHDN Input Bias Current VSHDN = 0 or VVL -1 1 µA
ISETOUT Shutdown Threshold
Voltage (Note 3) 150 220 300 mV
CELL Input Bias Current VCELL = 0 or VVL -5 5 µA
VADJ, ISETIN, ISETOUT Input
Bias Current
CELL Input Voltage
For 1 cell 00.5
V
For 3 cells VREF - 0.3 VREF + 0.3
For 2 cells (floating) 1.5 2.5
STATE MACHINE
CONTROL INPUTS/OUTPUTS
min
min
min
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
_______________________________________________________________________________________ 5
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VDCIN = VHSD = VCSSP = VCSSN = 12V, V SHDN = VVL, VCELL = GND, VBATT = VCS = 4.2V, VVADJ = VREF/2,
VISETIN = VISETOUT = VREF, RTHM = 10kΩ, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VDCIN = VHSD = VCSSP = VCSSN = 12V, V SHDN = VVL, VCELL = GND, VBATT = VCS = 4.2V, VVADJ = VREF/2,
VISETIN = VISETOUT = VREF, RTHM = 10kΩ, TA= -40°C to +85°C, unless otherwise noted.) (Note 7)
CONDITIONS
DCIN Input Voltage Range 614V
UNITSMIN TYP MAXSYMBOLPARAMETER
VL Output Voltage 5.1 5.7 V
REF Output Voltage 6V < VDCIN < 14V 4.166 4.242 V
REF Line Regulation 6V < VDCIN < 14V 6mV
PWM Oscillator Frequency fOSC Nondropout fOSC 260 340 kHz
HSD to LX On-Resistance VBST = VLX + 4.5V 250 mΩ
LX to PGND On-Resistance 2Ω
CS to BATT Hard Current Limit Instantaneous peak current limit 2.2 3.2 A
BATT, CS Input Voltage Range 014V
Absolute Voltage Accuracy Not including VADJ resistor tolerances -0.8 0.8 %
With 1% VADJ resistors -1 1
BATT Regulation Voltage CELL = float, GND, or REF 4.158 4.242 V/cell
BATT Full-Scale Charge Current 1.3 1.7 A
BATT 1/10-Scale Charge
Current (Note 1) VISETOUT = VREF/ 10 100 200 mA
BATT Charge Current in
Prequalification State VBATT < 2.4V per cell 100 200 mA
CSSP to CSSN Full-Scale
Current-Sense Voltage 85 115 mV
CSSP to CSSN 1/10-Scale
Current-Sense Voltage VISETIN = VREF/ 10 515mV
VFASTCHG, VFULLCHG, VFAULT = 28V,
VSHDN = GND
ISINK = 5mA
CONDITIONS
µA1
FASTCHG, FULLCHG, FAULT
Output High Leakage
V0.5VOL
FASTCHG, FULLCHG, FAULT
Output Low Voltage
UNITSMIN TYP MAXSYMBOLPARAMETER
CONTROL INPUTS/OUTPUTS
THM Trip Threshold Voltage VTRT THM low-temp or high-temp current 1.386 1.414 V
THM Low-Temp Current ITLTC VTHM = 1.4V 46.2 51.5 µA
BATT Undervoltage Threshold
(Note 3) 2.4 2.6 V/cell
BATT Overvoltage Threshold
(Note 4) 4.55 4.8 V/cell
SUPPLY AND REFERENCE
SWITCHING REGULATOR
STATE MACHINE
ACCURACY AND ERROR AMPLIFIERS
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
6 _______________________________________________________________________________________
Note 1: When VISETOUT = 0, battery charger turns off.
Note 2: See Thermistor section.
Note 3: Below this threshold, charger reverts to a prequalification mode with IBATT reduced to 10% of full scale.
Note 4: Above this threshold, charger is disabled.
Note 5: After full-charge state is complete and BATT current falls below this threshold, FULLCHG output switches high. Battery
charging continues until top-off timeout occurs. See Table 1.
Note 6: After charging is complete, when BATT voltage falls below this threshold, a new charging cycle is initiated.
Note 7: Specifications to -40°C are guaranteed by design, not production tested.
CONDITIONS
Hz0.93 1.37
Temperature Measurement
Frequency
mA250 400
FULLCHG BATT Current
Termination Threshold (Note 5)
UNITSMIN TYP MAXSYMBOLPARAMETER
V1.4VIH
SHDN Input Voltage High
V0.6VIL
SHDN Input Voltage Low
CONTROL INPUTS/OUTPUTS
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VDCIN = VHSD = VCSSP = VCSSN = 12V, V SHDN = VVL, VCELL = GND, VBATT = VCS = 4.2V, VVADJ = VREF/2,
VISETIN = VISETOUT = VREF, RTHM = 10kΩ, TA= -40°C to +85°C, unless otherwise noted.) (Note 7)
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
_______________________________________________________________________________________ 7
4.194
4.196
4.195
4.197
4.200
4.201
4.199
4.198
4.202
0 200 300 400 500100 600 700 800 900 1000
REFERENCE LOAD REGULATION
MAX1757 TOC07
REFERENCE LOAD (µA)
REFERENCE VOLTAGE (V)
1000
0.1
0.1 1 10
TIMEOUT vs. TIMER1 CAPACITANCE
1
MAX1757 TOC08
CAPACITANCE (nF)
TIMEOUT (MINUTES)
10
100
PREQUALIFICATION MODE
TOP-OFF MODE
VOLTAGE MODE
1000
1
0.1 1 10
FAST-CHARGE TIMEOUT
vs. TIMER2 CAPACITANCE
10
MAX1757 TOC09
CAPACITANCE (nF)
TIMEOUT (MINUTES)
100
50
60
70
80
90
100
6 8 10 12 14
EFFICIENCY
vs. INPUT VOLTAGE
MAX1757 TOC10
INPUT VOLTAGE (V)
EFFICIENCY (%)
ICHG = 1.0A
2 CELLS
1 CELL
3.95
4.05
4.00
4.15
4.10
4.25
4.20
4.30
4.40
4.35
4.45
0 1.0 1.5 2.00.5 2.5 3.0 3.5 4.0 4.5
VOLTAGE LIMIT
vs. VADJ VOLTAGE
MAX1757 TOC04
VADJ VOLTAGE (V)
VOLTAGE LIMIT (V)
4.185
4.195
4.190
4.205
4.200
4.210
4.215
-40 20 40-20 0 60 80 100
REFERENCE VOLTAGE
vs. TEMPERATURE
MAX1757 TOC05
TEMPERATURE (°C)
REFERENCE VOLTAGE (V)
Typical Operating Characteristics
(Circuit of Figure 1, VDCIN = 12V, VSHDN = VVL, VCELL = GND, VVADJ = VREF/2, VISETIN = VISETOUT = VREF, see Figure 1, TA= +25°C,
unless otherwise noted.)
0
1.0
0.5
2.5
2.0
1.5
4.0
3.5
3.0
4.5
0 0.6 0.80.2 0.4 1.0 1.2 1.4 1.6
BATTERY VOLTAGE
vs. CHARGING CURRENT
MAX1757 TOC01
CHARGING CURRENT (A)
BATTERY VOLTAGE (V)
0
0.4
0.2
1.0
0.8
0.6
1.4
1.2
1.6
0 1.5 2.00.5 1.0 2.5 3.0 3.5 4.0 4.5
CHARGING CURRENT
vs. ISETOUT VOLTAGE
MAX1757 TOC02
ISETOUT VOLTAGE (V)
CHARGING CURRENT (A)
0
20
40
60
80
100
120
01.00.5 1.5 2.0 2.5 3.0 3.5 4.0 4.5
INPUT CURRENT-SENSE REGULATION VOLTAGE
vs. ISETIN VOLTAGE
MAX1757 TOC03
ISETIN VOLTAGE (V)
INPUT CURRENT-SENSE VOLTAGE (mV)
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
8 _______________________________________________________________________________________
NAME FUNCTION
1VL Chip Power Supply. Output of the 5.4V linear regulator from DCIN. Bypass VL to GND with 2.2µF or larger
ceramic capacitor.
2ISETIN Input Current Limit Adjust. Use a voltage divider to set the voltage between 0 and VREF. See Input Current
Regulator section.
PIN
3ISETOUT Battery Charging Current Adjust. Use a voltage divider to set the voltage between 0 and VREF. See
Charging Current Regulator section.
4THM Thermistor Input. Connect a thermistor from THM to GND to set qualification temperature range. If unused,
connect a 10kΩresistor from THM to GND. See Thermistor section.
8BATT Battery Voltage-Sense Input and Current-Sense Negative Input
7VADJ Voltage Adjustment. Use a voltage divider to set the voltage between 0 and VREF to adjust the battery reg-
ulation voltage by ±5%. See Battery Regulation Voltage section.
6GND Analog Ground
5REF 4.2V Reference Voltage Output. Bypass REF to GND with 1µF or larger ceramic capacitor.
13 TIMER2 Timer2 Adjustment. Connect a capacitor from TIMER2 to GND to set the fast-charge time. See Timers section.
12 TIMER1 Timer1 Adjustment. Connect a capacitor from TIMER1 to GND to set the prequalification, full-charge, and
top-off times. See Timers section.
11 CELL Cell-Count Programming Input. Connect CELL to GND or REF to set 1 or 3 cells, or leave unconnected to
set 2 cells.
9, 10 HSD High-Side Drain. This is the drain of the internal high-side FET. See Figure 3.
Pin Description
14 FAULT Charge Fault Indicator. Open-drain output pulls low when charging terminates abnormally. See Table 1.
15 FASTCHG Fast-Charge Indicator. Open-drain output pulls low when charging with constant current.
16 FULLCHG Full-Charge Indicator. Open drain output pulls low when charging with constant voltage in full-charge state.
17 SHDN Shutdown Input. Drive SHDN low to disable charging. Connect SHDN to VL for normal operation.
18 PGND Power Ground. Current from the low-side power MOSFET switch source flows through PGND.
19, 20 LX Power Inductor Switching Node and High-Side Power MOSFET Source
21 CS Battery Current-Sense Positive Input. Connects to internal 0.1Ωresistor between BATT and CS.
22 BST High-Side MOSFET Gate Drive Bias. Connect a 0.1µF capacitor from BST to LX.
23 CCS Charger Source Current Regulation Loop Compensation Point. See Compensation section.
24 CCI Battery Charge Current Regulation Loop Compensation Point. See Compensation section.
25 CCV Voltage Regulation Loop Compensation Point. See Compensation section.
26 CSSN Source Current-Sense Negative Input. See Input Current Regulator section.
27 CSSP Source Current-Sense Positive Input. See Input Current Regulator section.
28 DCIN Power-Supply Input. DCIN is the input supply for the VL regulator. Bypass DCIN to GND with a 0.1µF or
greater capacitor. See Detailed Description.
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
_______________________________________________________________________________________ 9
General Description
The MAX1757 includes all of the functions necessary to
charge 1, 2, or 3 Li+ battery cells in series. It includes a
step-down DC-DC converter that controls charging
voltage and current. It also includes input source cur-
rent limiting, battery temperature monitoring, battery
undervoltage precharging, battery fault indication, and
a state machine with timers for charge termination.
The DC-DC converter uses an internal power MOSFET
switch to convert the input voltage to the charging cur-
rent or voltage. Figure 1 shows the typical application
circuit. Figure 2 shows a typical charging sequence
and Figure 3 shows the functional diagram. The charg-
ing current is set by the voltage at ISETOUT. The bat-
tery regulation voltage is measured at the BATT pin.
The battery voltage limit is set to 4.2V per cell and can
be adjusted ±5% by changing the voltage at the VADJ
pin. By limiting the adjust range, the voltage limit accu-
racy is better than 1% while using 1% setting resistors.
27
26
1
22
10
9
20
19
21
18
D4
MBR5340
8
4
28 CSSP
DCIN
CSSN
VL
BST
HSD
HSD
LX
LX
PGND
CS
BATT
THM
REF
ISETIN
TO VL SHDN
ISETOUT
VADJ
CELL
GND
CCV
CCI
CCS
TIMER1
TIMER2
R1
0.05Ω
L1
22µH
FAULT
FULLCHG
FULL CHARGE
FAST CHARGE
FAULT
FASTCHG
C2
0.1µF
C5
1nF
C6
1nF
C1
0.1µF
C3
1µF
C12
0.22µF
C10
22µF
TO
SYSTEM
LOAD
INPUT
SUPPLY
C7
0.1µFC8
0.1µF
R4
D2
D3
D1
R5
C4
0.1µF
R6
10k
15
24
25
6
11
3
7
2
5
17
23
12
13
16
14
C15
68µF
C9
0.1µF
C13
4.7µF
C14
0.1µF
C16
0.1µF
MAX1757
+C11
22µF
++
C17
1nF
MBR5340
C18
0.1µF
THERM
Li+ BATTERY
1 TO 3 CELLS
Figure 1. Typical Application Circuit
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
10 ______________________________________________________________________________________
STATE ENTRY CONDITIONS STATE CONDITIONS
Reset
From initial power on
or
From done state if battery voltage < recharge voltage
threshold
or
VDCIN - VBATT < dropout threshold
or
VBATT > battery overvoltage threshold
Timers reset, charging current = 0,
FASTCHG = high, FULLCHG = high,
FAULT = high
Prequalification From reset state if input power, reference, and internal
bias are within limits
Battery voltage ≤undervoltage threshold,
charging current = (fast-charge current / 10),
timeout = 7.5min typ (CTIMER1 = 1nF),
FASTCHG = low, FULLCHG = high,
FAULT = high
Fast Charge
(Constant Current)
From prequalification state if battery voltage >
undervoltage threshold
Undervoltage threshold ≤battery voltage
≤battery regulation voltage,
charging current = charge current limit,
timeout = 90min typ (CTIMER2 = 1nF),
FASTCHG = low, FULLCHG = high,
FAULT = high
Full Charge
(Constant Voltage)
From fast-charge state if battery voltage = battery
regulation voltage
Battery voltage = battery regulation
voltage,
charging current ≤330mA,
timeout = 90min typ (CTIMER1 = 1nF),
FASTCHG = high, FULLCHG = low,
FAULT = high
Top-Off
(Constant Voltage)
From full-charge state if full-charge timer expires
or
If charging current ≤330mA
Battery voltage = battery regulation voltage,
charging current ≤330mA
timeout = 45min typ
(CTIMER1 = 1nF), FASTCHG = high,
FULLCHG = high, FAULT = high
Done From top-off state if top-off timer expires
Recharge voltage threshold ≤battery,
voltage ≤voltage limit, charging current = 0,
FASTCHG = high, FULLCHG = high,
FAULT = high
Over/Undertemperature From fast-charge state or full-charge state if battery
temperature is outside limits
Charge current = 0, timers suspended,
FASTCHG = no change,
FULLCHG = no change,
FAULT = no change
Fault
From reset state if battery temperature ≥maximum
battery temperature
or
From prequalification state if prequalification timer expires
or
From fast-charge state if fast-charge timer expires
Charging current = 0, FASTCHG = high,
FULLCHG = high, FAULT = low
Table 1. Charging State Table
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
______________________________________________________________________________________ 11
The MAX1757 includes a state machine that controls
the charging algorithm. Figure 4 shows the state dia-
gram. Table 1 is the charging state table. When power
is applied, or SHDN input is driven high, the part goes
into the reset state where the timers are reset to zero to
prepare for charging. From the reset state, it enters the
prequalification state. In this state, 1/10 of the fast-
charge current charges the battery, and the battery
temperature and voltage are measured. If the voltage is
above the undervoltage threshold and the temperature
is within the limits, then it will enter the fast-charge
state. If the battery voltage does not rise above the
undervoltage threshold before the prequalification timer
expires, the charging terminates and the FAULT output
goes low. The prequalification time is set by the
TIMER1 capacitor (CTIMER1). If the battery is outside
the temperature limits, charging and the timer are sus-
pended. Once the temperature is back within limits,
charging and the timer resume.
In the fast-charge state, the FASTCHG output goes low
and the batteries charge with a constant current (see
Charging Current Regulator section). If the battery volt-
age reaches the voltage limit before the fast timer
expires, the part enters the full-charge state. If the fast-
charge timer expires before the voltage limit is
reached, charging terminates and the FAULT output
goes low. The fast-charge time limit is set by the
TIMER2 capacitor (CTIMER2). If the battery temperature
is outside the limits, charging pauses and the timers
are suspended until the temperature returns to within
the limits.
In the full-charge state, the FULLCHG output goes low
and the batteries charge at a constant voltage (see
Voltage section). When the charging current drops
below 150mA (330mA peak inductor current), or if the
full-charge timer expires, the state machine enters the
top-off state. In the top-off state, the batteries continues
to charge at a constant voltage until the top-off timer
expires, at which time it enters the done state. In the
done state, charging stops until the battery voltage
drops below the recharge-voltage threshold, at which
time it enters the reset state to start the charging
process again. In the full-charge or the top-off state, if
the battery temperature is outside the limits, charging
pauses and the timers are suspended until the battery
temperature returns to within limits.
Voltage Regulator
Li+ batteries require a high-accuracy voltage limit while
charging. The MAX1757 uses a high-accuracy voltage
regulator (±0.8%) to limit the charging voltage. The bat-
tery regulation voltage is nominally set to 4.2V per cell
and can be adjusted ±5% by changing the voltage at
the VADJ pin between reference voltage and ground.
By limiting the adjust range of the regulation voltage, an
overall voltage accuracy of better than 1% is main-
tained while using 1% resistors. CELL sets the cell
count from 1 to 3 series cells (see Setting the Battery
Regulation Voltage section).
An internal error amplifier (GMV) maintains voltage reg-
ulation (Figure 3). The GMV amplifier is compensated
at CCV. The component values shown in Figure 1 pro-
vide suitable performance for most applications.
Individual compensation of the voltage regulation and
current regulation loops allows for optimum stability.
Charging Current Regulator
The charging current-limit regulator limits the charging
current. Current is sensed by measuring the voltage
across the internal current-sense resistor RCS between
BATT and CS. The voltage at ISETOUT adjusts the
charging current. Full-scale charging current is
achieved when ISETOUT is connected to REF.
The charging current error amplifier (GMI) is compen-
sated at CCI. A 0.1µF capacitor at CCI provides suit-
able performance for most applications.
FAST-
CHARGE
STATE
OPEN-
DRAIN
LOW
OPEN-
DRAIN
LOW
BATTERY
CURRENT
BATTERY
VOLTAGE
FASTCHG
OUTPUT
FULLCHG
OUTPUT
FULL-
CHARGE
STATE TOP-OFF
STATE DONE
CHARGE I = 1C
BATTERY
INSERTION
OR SHDN HIGH
TRANSITION TO
VOLTAGE MODE
(APPROX 85% CHARGE)
FULL-CHARGE TIMER
TIMES OUT OR
BATTERY CURRENT
DROPS TO C/10
(APPROX 95% CHARGE)
TOP-OFF TIMER
TIMES OUT, END OF ALL
CHARGE FUNCTIONS
Figure 2. Charge State and Indicator Output Timing for a
Typical Charging Sequence
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
12 ______________________________________________________________________________________
CSS
LEVEL SHIFT
AND
GAIN OF 10
ON
CCV
RCS
Rx
R2
BDIV
TO
BATT
R
3Rx
+1
GMS
TO
BATT
ENABLE
GMI
MIN AND CLAMP
REF
VL
CCI
CCS
BST
PGND
HSD
LX
CSSN
CSSP
GMV
CS
BATT
ISETIN
ISETOUT
CELL
VADJ
CNTRL
LOGIC
CSI
LEVEL SHIFT
AND
GAIN OF 7
ON
ON
SUMMING
COMPARATOR
BLOCK
LEVEL
SHIFT
REF/2 =
ZERO
CURRENT
V/I MODE
TO REF
OSCILLATOR, SM, TIMERS
THERM CONTROL
TEST CIRCUITRY
R2 = R(2N - 1)
WHERE
N = CELL NUMBER
Rx
REF/2
3Rx
9R R
R
DCIN
THM
5.4V
REGULATOR
INTERNAL
REFERENCE
DRIVER
+1
FASTCHG
FULLCHG
FAULT
TIMER 1
TIMER 2
GND
+1
MAX1757
Figure 3. Functional Diagram
Input Current Regulator
The total input current (from a wall cube or other DC
source) is the sum of system load current plus the bat-
tery-charging current. The input current regulator limits
the source current by reducing charging current when
input current exceeds the set input current limit. System
current will normally fluctuate as portions of the system
are powered up or put to sleep. Without input current
regulation, the input source must be able to supply the
maximum system load current plus the maximum
charger input current. By using the input current limiter,
the current capability of the AC wall adapter may be
lowered, reducing system cost.
Input current is measured through an external sense
resistor at CSSP and CSSN. The voltage at ISETIN also
adjusts the input current limit. Full-scale input current is
achieved when ISETIN is connected to REF, setting the
full-scale current-sense regulation voltage to 100mV.
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
______________________________________________________________________________________ 13
When choosing the current-sense resistor, note that the
voltage drop across this resistor adds to the power
loss, reducing efficiency. Reducing the voltage across
the current-sense resistor may degrade input current
limit accuracy due to the input offset of the input cur-
rent-sense amplifier.
The input current error amplifier (GMS) is compensated
at CCS. A 0.1µF capacitor at CCS provides suitable
performance for most applications.
PWM Controller
The PWM controller drives the internal high-side MOS-
FET to control charging current or voltage. The input to
the PWM controller is the lowest of CCI, CCV, or CCS.
An internal clamp limits the noncontrolling signals to
within 200mV of the controlling signal to prevent delay
when switching between regulation loops.
The current mode PWM controller measures the induc-
tor current to regulate the output voltage or current,
simplifying stabilization of the regulation loops.
Separate compensation of the regulation circuits allows
each to be optimally stabilized. Internal slope compen-
sation is included, ensuring stable operation over a
wide range of duty cycles.
The controller drives an internal N-channel MOSFET
switch to step the input voltage down to the battery
voltage. The high-side MOSFET gate is driven to a volt-
age higher than the input source voltage by a bootstrap
capacitor. This capacitor (between BST and LX) is
charged through a diode from VL when LX is low. An
internal N-channel MOSFET turns on momentarily after
the high-side switch turns off, pulling LX to PGND to
ensure that the bootstrap capacitor charges. The high-
side MOSFET gate is driven from BST, supplying suffi-
SHUTDOWN
FASTCHG = HIGH
FULLCHG = HIGH
FAULT = HIGH
RESET
FASTCHG = HIGH
FULLCHG = HIGH
FAULT = HIGH
PREQUAL
FASTCHG = LOW
FULLCHG = HIGH
FAULT = HIGH
FAULT
FASTCHG = HIGH
FULLCHG = HIGH
FAULT = LOW
FAST CHARGE
FASTCHG = LOW
FULLCHG = HIGH
FAULT = HIGH
FULL CHARGE
FASTCHG = HIGH
FULLCHG = LOW
FAULT = HIGH
DONE
FASTCHG = HIGH
FULLCHG = HIGH
FAULT = HIGH
TOP-OFF
FASTCHG = HIGH
FULLCHG = HIGH
FAULT = HIGH
TEMP
OK
TEMP
OK
TEMP
OK
TEMP
OK
TEMP
NOT OK
TOP-OFF
TIMEOUT
ICHARGE < IMIN OR
FULL-CHARGE
TIMEOUT
ONCE PER
SECOND
ONCE PER
SECOND
TEMP
QUAL
VBATT > 2.5V
VBATT < 0.95 × VBATTR
VBATT < 0.95 × VBATTR
VDCIN < BATT
VBATT < UNDERVOLTAGE
THRESHOLD
VBATT = BATTERY
REGULATION VOLTAGE (VBATTR)
FAST-CHARGE
TIMEOUT
PREQUAL
TIMEOUT
TEMP
NOT OK
TEMP
NOT OK
SHUTDOWN IS
ENTERED FROM ALL STATES
WHEN SHDN IS LOW.
SHDN HIGH
VDCIN > VBATT
Figure 4. State Diagram
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
14 ______________________________________________________________________________________
cient voltage to fully drive the MOSFET gate even when
its source is near the input voltage.
Timers
The MAX1757 includes safety timers to terminate
charging and to ensure that faulty batteries are not
charged indefinitely. TIMER1 and TIMER2 set the time-
out periods.
TIMER1 controls the maximum prequalification time,
maximum full-charge time, and the top-off time. TIMER2
controls the maximum fast-charge time. The timers are
set by external capacitors. The typical times of 7.5 min-
utes for prequalification, 90 minutes for full charge, 45
minutes for top-off, and 90 minutes for fast charge are
set by using a 1nF capacitor on TIMER1 and TIMER2
(Figure 1).
Charge Monitoring Outputs
FASTCHG, FULLCHG, and FAULT are open-drain out-
puts that can be used as LED drivers. FASTCHG indi-
cates the battery is being fast charged. FULLCHG
indicates the charger has completed the fast-charge
cycle (approximately 85% charge) and is operating in
voltage mode. The FASTCHG and FULLCHG outputs
can be tied together to indicate charging or done
(Figure 2). FAULT indicates the charger has detected a
charging fault and that charging has terminated. The
charger can be brought out of the FAULT condition
only by removing and reapplying the input power, or by
pulling SHDN low.
Thermistor
The intent of THM is to inhibit charging when the bat-
tery is too cold or too hot (+2.5°C ≤TOK ≤+47.5°C),
using an external thermistor. THM time multiplexes two
sense currents to test for both hot and cold qualifica-
tion. The thermistor should be 10kΩat +25°C and have
a negative temperature coefficient (NTC); the THM pin
expects 3.97kΩat +47.5°C and 28.7kΩat +2.5°C.
Connect the thermistor between THM and GND. If no
temperature qualification is desired, replace the ther-
mistor with a 10kΩresistor. Thermistors by Philips/
BCcomponents (2322-640-63103), Cornerstone
Sensors (T101D103-CA), and Fenwall Electronics (140-
103LAG-RB1) work well. The battery temperature is
measured at a 1.12Hz rate (CTIMER1 = CTIMER2 = 1nF).
Charging is briefly halted to allow accurate measure-
ment.
If the temperature goes out of limits while charging is in
progress, charging will be suspended until the temper-
ature returns to within the limits. While charging is sus-
pended, the timers will also be suspended but will
continue counting from where they left off when charg-
ing resumes.
Shutdown
When SHDN is pulled low, the MAX1757 enters the
shutdown mode and charging is stopped. In shutdown,
the internal resistive voltage divider is removed from
BATT to reduce the current drain on the battery to less
than 5µA. The high-side power MOSFET switch is off.
However, the internal linear regulator (VLO) and the ref-
erence (REF) remain on. Status outputs FASTCHG,
FULLCHG, and FAULT are high impedance. When exit-
ing the shutdown mode, the MAX1757 goes to the
power-on reset state, which resets the timers and
begins a new charge cycle.
Source Undervoltage Shutdown (Dropout)
If the voltage on DCIN drops within 100mV of the volt-
age on BATT, the charger turns off. This prevents bat-
tery discharge by the charger during low input voltage
conditions.
Design Procedure
Setting the Battery Regulation Voltage
VADJ sets the per-cell voltage limit. To set the VADJ
voltage, use a voltage-divider from REF to VADJ. A
GND-to-VREF change at VADJ results in a ±5% change
in the battery limit voltage. Since the full VADJ range
results in only a 10% change on the battery regulation
voltage, the resistor-divider’s accuracy need not be as
high as the output-voltage accuracy. Using 1% resis-
tors for the voltage dividers results in no more than
0.1% degradation in output-voltage accuracy. VADJ is
internally buffered so that high-value resistors can be
used. Set VVADJ by choosing a value less than 100kΩ
for R5 (Figure 1) from VADJ to GND. The per-cell bat-
tery termination voltage is a function of the battery
chemistry and construction; thus, consult the battery
manufacturer to determine this voltage. Once the per-
cell voltage limit battery regulation voltage is deter-
mined, the VADJ voltage is calculated by the equation:
VVADJ = (9.5 VBATTR / N) - (9.0 ✕VREF)
CELL CELL COUNT (N)
GND 1
Float 2
REF 3
Table 2. Cell-Count Programming Table
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
______________________________________________________________________________________ 15
CELL is the programming input for selecting cell count
N. Table 2 shows how CELL is connected to charge 1,
2, or 3 cells.
Setting the Charging Current Limit
A resistor-divider from REF to GND sets the voltage at
ISETOUT (VISETOUT). This determines the charging cur-
rent during the current-regulation (fast-charge) mode.
The full-scale charging current is 1.5A.
The charging current (ICHG) is, therefore:
Connect ISETOUT to REF to get the full-scale current
limit.
Setting the Input Current Limit
A resistor-divider from REF to GND sets the voltage at
ISEVTIN (VISETIN). This sets the maximum source cur-
rent allowed at any time during charging. The source
current IFSS is set by the current-sense resistor
RSOURCE between CSSP and CSSN. The full-scale
source current is IFSS = 0.1V / R1 (Figure 1).
The input current limit (IIN) is therefore:
Connect ISETIN to REF to get the full-scale input cur-
rent limit. Short CSSP and CSSN if the input source cur-
rent limit is not used.
In choosing the current-sense resistor, it should be noted
that the drop across this resistor adds to the power loss
and thus reduces efficiency. However, too low a resistor
value may degrade input current-limit accuracy.
Inductor Selection
The inductor value may be changed for more or less
ripple current. The higher the inductance, the lower the
ripple current will be; however, as the physical size is
kept the same, typically, higher inductance will result in
higher series resistance and lower saturation current. A
good tradeoff is to choose the inductor so that the rip-
ple current is approximately 30% to 50% of the DC
average charging current. The ratio of ripple current to
DC charging current (LIR) can be used to calculate the
optimal inductor value:
where fOSC is the switching frequency (300kHz).
The peak inductor current is given by:
Capacitor Selection
The input capacitor shunts the switching current from
the charger input and prevents that current from circu-
lating through the source, typically an AC wall cube.
Thus, the input capacitor must be able to handle the
input RMS current. Typically, at high charging currents,
the converter will operate in continuous conduction (the
inductor current does not go to 0). In this case, the
RMS current of the input capacitor may be approximat-
ed by the equation:
where:
ICIN is the input capacitor RMS current.
D is the PWM converter duty ratio
(typically VBATT / VDCIN).
ICHG is the battery charging current.
The maximum RMS input current occurs at 50% duty
cycle; thus, the worst-case input ripple current is 0.5 ✕
ICHG. If the input-to-output voltage ratio is such that the
PWM controller will never work at 50% duty cycle, then
the worst-case capacitor current will occur where the
duty cycle is nearest 50%.
The input capacitor impedance is critical to preventing
AC currents from flowing back into the wall cube. This
requirement varies depending on the wall cube imped-
ance and the requirements of any conducted or radiat-
ed EMI specifications that must be met. Aluminum
electrolytic capacitors are generally the cheapest, but
usually are a poor choice for portable devices due to
their large size and poor equivalent series resistance
(ESR). Tantalum capacitors are better in most cases, as
are high-value ceramic capacitors. For equivalent size
and voltage rating, tantalum capacitors will have higher
capacitance, but also higher ESR than ceramic capaci-
tors. This makes it more critical to consider RMS cur-
rent and power dissipation ratings when using tantalum
capacitors.
The output filter capacitor is used to absorb the induc-
tor ripple current. The output capacitor impedance
must be significantly less than that of the battery to
ensure that it will absorb the ripple current. Both the
IIDD
CIN CHG
≅−
2
II LIR
PEAK ISETOUT
=+
12
L
VV V
V x f x x LIR
BATT DCIN MAX BATT
DCIN MAX OSC CHG
=−
()
()
()
I
II
V
V
IN FSS ISETIN
REF
=
IA
V
V
CHG ISETOUT
REF
.=
15
MAX1757
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
16 ______________________________________________________________________________________
capacitance and ESR rating of the capacitor are impor-
tant for its effectiveness as a filter and to ensure stabili-
ty of the PWM circuit. The minimum output capacitance
for stability is:
where:
COUT is the total output capacitance.
VREF is the reference voltage (4.2V).
VBATT is the maximum battery regulation voltage
(typically 4.2V per cell).
VDCIN(MIN) is the minimum source input voltage.
The maximum output capacitor ESR required for stabili-
ty is:
where:
RESR is the output capacitor ESR.
RCS is the current-sense resistor from CS to BATT
(100mΩtyp).
Setting the Timers
The MAX1757 contains four timers: a prequalification
timer, fast-charge timer, full-charge timer, and top-off
timer. Connecting a capacitor from TIMER1 to GND
and TIMER2 to GND sets the timer periods. The
TIMER1 input controls the prequalification, full-charge,
and top-off times while TIMER2 controls the fast-charge
timeout. The typical timeouts for a 1C charge rate are
set to 7.5 minutes for the prequalification timer, 90 min-
utes for the fast-charge timer, 90 minutes for the full-
charge timer, and 45 minutes for the top-off timer by
connecting 1nF capacitors to TIMER1 and TIMER2.
Each timer period is directly proportional to the capaci-
tance at the corresponding pin (see Typical Operating
Characteristics).
Compensation
Each of the three regulation loops—the input current
limit, the charging current limit, and the charging volt-
age limit—can be compensated separately at the CCS,
CCI, and CCV pins, respectively.
The charge-current loop error amp output is brought
out at CCI. Likewise, the source-current error amplifier
output is brought out at CCS; 0.1µF capacitors to
ground at CCI and CCS compensate the current loops
in most charger designs. Raising the value of these
capacitors reduces the bandwidth of these loops.
The voltage-regulating loop error amp output is brought
out at CCV. Compensate this loop by connecting a
capacitor in parallel with a series resistor-capacitor (RC)
from CCV to GND. Recommended values are shown in
Figure 1.
Applications Information
Diode Selection
A Schottky rectifier with a rating of at least 1.5A must
be connected from LX to PGND.
VL and REF Bypassing
The MAX1757 uses an internal linear regulator to drop
the input voltage down to 5.4V, which powers the inter-
nal circuitry. The output of the linear regulator is the VL
pin. The internal linear regulator may also be used to
power external circuitry as long as the maximum cur-
rent of the linear regulator is not exceeded.
A 4.7µF bypass capacitor is required at VL to ensure that
the regulator is stable. A 1µF bypass capacitor is also
required between REF and GND to ensure that the inter-
nal 4.2V reference is stable. In both cases, use a low-ESR
ceramic capacitor.
Chip Information
TRANSISTOR COUNT: 5996
RRxV
V
ESR CS BATT
REF
<
C
VV
V
VxfxR
OUT
REF BATT
DCIN MIN
BATT OSC CS
>
+
()
1
Package Information
SSOP.EPS
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 17
© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Stand-Alone, Switch-Mode
Li+ Battery Charger with Internal 14V Switch
