●Notebook Computers
●Hand-Held Instruments
●Li+ Battery Packs
●Desktop Cradle Chargers
General Description
The MAX1737 is a switch-mode lithium-ion (Li+) battery
charger that charges one to four cells. It provides a
regulated charging current and a regulated voltage with
only a ±0.8% total voltage error at the battery terminals.
The external N-channel switch and synchronous rectifier
provide high efficiency over a wide input voltage range.
A built-in safety timer automatically terminates charging
once the adjustable time limit has been reached.
The MAX1737 regulates the voltage set point and charging
current using two loops that work together to transition
smoothly between voltage and current regulation. An
additional control loop monitors the total current drawn
from the input source to prevent overload of the input supply,
allowing the use of a low-cost wall adapter.
The per-cell battery voltage regulation limit is set between
+4.0V and +4.4V and can be set from one to four by pin
strapping. Battery temperature is monitored by an external
thermistor to prevent charging if the battery temperature
is outside the acceptable range.
The MAX1737 is available in a space-saving 28-pin
QSOP package. Use the evaluation kit (MAX1737EVKIT)
to help reduce design time.
Applications
Features
●Stand-Alone Charger for Up to Four Li+ Cells
●±0.8% Accurate Battery Regulation Voltage
●Low Dropout: 98% Duty Cycle
●Safely Precharges Near-Dead Cells
●Continuous Voltage and Temperature Monitoring
●<1µA Shutdown Battery Current
●Input Voltage Up to +28V
●Safety Timer Prevents Overcharging
●Input Current Limiting
●Space-Saving 28-Pin QSOP
●300kHz PWM Oscillator Reduces Noise
●90% Conversion Efficiency
PART TEMP. RANGE PIN-PACKAGE
MAX1737EEI -40°C to +85°C 28 QSOP
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
DHI
LX
BST
FAULT
VLO
DLO
PGND
CS
SHDN
FULLCHG
FASTCHG
TIMER2
TIMER1
CELL
CCI
CCS
CCV
VADJ
BATT
GND
REF
THM
ISETOUT
ISETIN
VL
QSOP
TOP VIEW
MAX1737
GND
DCIN CSSP
CSSN
LX
BST
VLO
DLO
PGND
CS
THM
BATT
FASTCHG
FULLCHG
FAULT
SHDN
ON
OFF
TIMER2
TIMER1
Li+
BATTERY
1 TO 4
CELLS
RS
CCI
CCV
CCS
VADJ
ISETIN
VL
REF
ISETOUT
INPUT SUPPLY
CELL
DHI
SYSTEM
LOAD
MAX1737
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
19-1626; Rev 5; 7/17
Ordering Information
Pin Conguration
Typical Operating Circuit
EVALUATION KIT AVAILABLE
CSSP, CSSN, DCIN to GND .................................-0.3V to +30V
BST, DHI to GND ..................................................-0.3V to +36V
BST to LX ................................................................-0.3V to +6V
DHI to LX ........................................ -0.3V to ((BST - LX) + 0.3V)
LX to GND ............................................. -0.3V to (CSSN + 0.3V)
FULLCHG, FASTCHG, FAULT to GND ................-0.3V to +30V
VL, VLO, SHDN, CELL, TIMER1, TIMER2, CCI,
CCS, CCV, REF, ISETIN, ISETOUT, VADJ,
THM to GND ........................................................ -0.3V to +6V
DLO to GND ............................................. -0.3V to (VLO + 0.3V)
BATT, CS to GND .................................................. -0.3V to +20V
PGND to GND, CSSP to CSSN ...........................-0.3V to +0.3V
VL to VLO ............................................................. -0.3V to +0.3V
VL Source Current. .............................................................50mA
Continuous Power Dissipation (TA = +70°C)
28-Pin QSOP (derate 10.8mW/°C above +70°C) .......860mW
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
(Circuit of Figure 1, VDCIN = VCSSN = VCSSP = +18V, SHDN = VL, CELL = GND, VBATT = VCS = +4.2V, VVADJ = VREF / 2, ISETIN =
ISETOUT = REF, RTHM = 10kΩ, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
SUPPLY AND REFERENCE
DCIN Input Voltage Range
6
28
V
DCIN Quiescent Supply Current 6.0V < VDCIN < 28V
5
7
mA
DCIN to BATT Undervoltage Threshold,
DCIN Falling 0.05 0.155 V
DCIN to BATT Undervoltage Threshold,
DCIN Rising
0.19 0.40
V
VL Output Voltage 6.0V < V
DCIN
< 28V
5.10 5.40 5.70
V
VL Output Load Regulation I
VL
= 0 to 15mA 44
65
mV
REF Output Voltage
4.179 4.20 4.221
V
REF Line Regulation 6V < V
DCIN
< 28V 2
6
mV
REF Load Regulation IREF = 0 to 1mA
6
14 mV
SWITCHING REGULATOR
PWM Oscillator Frequency VBATT = 15V, CELL = VL 270 300 330 kHz
LX Maximum Duty Cycle In dropout fOSC / 4, VCCV = 2.4V,
VBATT = 15V, CELL = VL
97 98
%
CSSN + CSSP Off-State Leakage VCSSN = VCSSP = VDCIN = 28V, SHDN = GND
2
10 µA
DHI, DLO On-Resistance 7Ω
LX Leakage
LX = VDCIN = 28V,
SHDN
= GND 0.1 10
µA
BATT, CS Input Current
SHDN = GND, V
BAT
T = 19V
0.1 5
µA
CELL = SHDN = VL, V
BATT
= 17V
225 500
BATT, CS Input Voltage Range
0
19 V
Battery Regulation Voltage (VBATTR)CELL = float, GND, VL, or REF (Note 1)
4.167 4.2 4.233
V/cell
Absolute Voltage Accuracy Not including VADJ resistor tolerances -0.8
+0.8
%
With 1% VADJ resistors -1
+1
Battery Regulation Voltage Adjustment
Range VCCV = 2V
V
VADJ
= GND
3.948 3.979 4.010 V/cell
VVADJ = REF
4.386 4.421
4.453
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
2
Electrical Characteristics
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.
Absolute Maximum Ratings
(Circuit of Figure 1, VDCIN = VCSSN = VCSSP = +18V, SHDN = VL, CELL = GND, VBATT = VCS = +4.2V, VVADJ = VREF / 2, ISETIN =
ISETOUT = REF, RTHM = 10kΩ, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS
MIN
TYP MAX
UNITS
ERROR
AMPLIFIERS
CCV Amplifier Transconductance (Note 2) 4.15V < VBATT < 4.25V, VCCV = 2V 0.39 0.584 0.80 mS
CCV Amplifier Maximum Output Current 3.5V < V
BATT
< 5V, VCCV = 2V
±50
µA
CS to BATT Current-Sense Voltage V
ISETOUT
= V
REF
/ 5
30 40 50
mV
CS to BATT Full-Scale Current-Sense
Voltage V
BATT
= 3V to 17V, CELL = GND or VL
185 200 215
mV
CS to BATT Current-Sense Voltage When in
Prequalification State
VBATT < 2.4V per cell 510 15 mV
CS to BATT Hard Current-Limit Voltage
355 385 415
mV
CSSP to CSSN Current-Sense Voltage 6V < V
CSSP
< 28V, V
ISETIN
= V
REF
/ 5,
V
CCS
= 2V
10 20 30
mV
CSSP to CSSN Full-Scale
Current-Sense Voltage
6V < VCSSP < 28V, VCCS = 2V 90 105 115 mV
CCI Amplifier Transconductance VCCI = 2V 0.6 11.4 mS
CCI Amplifier Output Current V
CS
- V
BATT
= 0, 400mV
±100
µA
CCS Amplifier Transconductance I
SET
= REF, V
CCS
= 2V
1.2 22.6
mS
CCS Amplifier Output Current VCSSP - VCSSN = 0, 200mV ±100 µA
CCI, CCS Clamp Voltage with Respect
to CCV
25 200 mV
CCV Clamp Voltage with Respect
to CCI, CCS
25 200
mV
STATE
MACHINE
THM Trip-Threshold Voltage THM low-temperature or high-temperature
current
1.386 1.4 1.414
V
THM Low-Temperature Current VTHM = 1.4V 46.2 49 51.5 µA
THM High-Temperature Current VTHM = 1.4V 344 353 362 µA
THM COLD Threshold Resistance (Note 3)
Combines THM low-temperature current and
THM rising threshold, VTRT/ITLTC 26.92 28.70 30.59
kΩ
THM HOT Threshold Resistance (Note 3)
Combines THM high-temperature current and
THM rising threshold, VTRT/ITHTC
3.819 3.964 4.115
kΩ
BATT Undervoltage Threshold (Note 4) 2.4 2.5 2.6 V/cell
BATT Overvoltage Threshold (Note 5) 4.55 4.67 4.8 V/cell
BATT Charge Current Full-Charge
Termination Threshold CS-BATT (Note 6)
35 44 55 mV
BATT Recharge Voltage Threshold (Note 7) 94 95 96
% of
VBATTR
TIMER1, 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
min
Full-Charge Timer 81
90 100 Min
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
3
Electrical Characteristics (continued)
(Circuit of Figure 1, VDCIN = VCSSN = VCSSP = +18V, SHDN = VL, CELL = GND, VBATT = VCS = +4.2V, VVADJ = VREF / 2, ISETIN =
ISETOUT = REF, RTHM = 10kΩ, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX
UNITS
Top-Off Timer
40.5 45 49.8
Min
Temperature Measurement Frequency 1nF on TIMER1 and TIMER2 0.98
1.12 1.32 Hz
CONTROL INPUTS/OUTPUTS
SHDN Input Voltage High
1.4 V
SHDN Input Voltage Low (Note 8)
0.6
V
VADJ, ISETIN, ISETOUT Input Voltage
Range
0
VREF V
VADJ, ISETIN, ISETOUT
Input Bias Current VVADJ, VISETIN, VISETOUT = 0 or 4.2V
-50
50 nA
SHDN Input Bias Current SHDN = GND or VL
-1
1µA
CELL Input Bias Current
-5
5µA
ISETIN Adjustment Range
VREF/5 VREF
V
ISETOUT Adjustment Range
VREF/5 VREF
V
ISETOUT Voltage for ICHG = 0
150 220 300
mV
CELL Input Voltage
For 1 cell
0 0.5
V
For 2 cells
1.5
2.5
For 3 cells VREF -
0.3
VREF +
0.3
For 4 cells
VVL -
0.4
V
VL
FASTCHG, FULLCHG, FAULT
Output Low Voltage ISINK = 5mA 0.5
V
FASTCHG, FULLCHG, FAULT Output High
Leakage
FASTCHG, FULLCHG, FAULT = 28V;
SHDN = GND
1
µA
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
4
Electrical Characteristics (continued)
(Circuit of Figure 1, VDCIN = VCSSN = VCSSP = +18V, SHDN = VL, CELL = GND, VBATT = VCS = +4.2V, VVADJ = VREF / 2, ISETIN =
ISETOUT = REF, RTHM = 10kΩ, TA = -40°C to +85°C, unless otherwise noted.) (Note 9)
PARAMETER CONDITIONS MIN TYP MAX UNITS
SUPPLY AND REFERENCE
DCIN Input Voltage Range
6
28
V
VL Output Voltage 6.0V < V
DCIN
< 28V 5.1
5.7
V
REF Output Voltage 4.166
4.242
V
REF Line Regulation 6V < V
DCIN
< 28V
6
mV
SWITCHING REGULATOR
PWM Oscillator Frequency VBATT = 15V, CELL = VL
260
340 kHz
DHI, DLO On-Resistance 7Ω
BATT, CS Input Voltage Range
0
19 V
Battery Regulation Voltage (VBATTR) CELL = float, GND, VL, or REF 4.158
4.242
V/cell
Absolute Voltage Accuracy Not including V
ADJ
resistor tolerances -1
+1
%
ERROR
AMPLIFIERS
CS to BATT Current-Sense Voltage V
ISETOUT
= V
REF
/5 25 55 mV
CS to BATT Full-Scale Current-Sense
Voltage V
BATT
= 3V to 17V, CELL = GND or VL 180 220 mV
CS to BATT Current-Sense Voltage When in
Prequalification State
VBATT < 2.4V per cell
3 17
mV
CS to BATT Hard Current-Limit Voltage 350 420 mV
CSSP to CSSN Current-Sense Voltage 6V < V
CSSP
< 28V, V
ISETIN
= V
REF
/ 5,
V
CCS
= 2V 5 35 mV
CSSP to CSSN Full-Scale
Current-Sense
Voltage
6V < VCSSP < 28V, VCCS = 2V
85 115
mV
STATE MACHINE
THM Trip-Threshold Voltage THM low-temperature or high-temperature
current 1.386 1.414 V
THM Low-Temperature Current VTHM = 1.4V 46.2 51.5 µA
THM COLD Threshold Resistance (Note 3)
Combines THM low-temperature current and
THM rising threshold, VTRT/ITLTC
26.92 30.59 kΩ
BATT Undervoltage Threshold (Note 4) 2.4 2.6 V/cell
BATT Overvoltage Threshold (Note 5) 4.55 4.8 V/cell
BATT Charge Current Full-Charge
Termination Threshold CS-BATT (Note 6)
35 55 mV
Temperature Measurement Frequency
1nF on TIMER1 and TIMER2 0.93 1.37
Hz
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
5
Electrical Characteristics
Note 1: Battery Regulation Voltage = Number of Cells × (3.979V + 0.10526 × VVADJ).
Note 2: This transconductance is for one cell. Divide by number of cells to determine actual transconductance.
Note 3: See Thermistor section.
Note 4: Below this threshold, the charger reverts to prequalification mode and ICHG is reduced to about 5% of full scale.
Note 5: Above this threshold, the charger returns to reset.
Note 6: After full-charge state is complete and peak inductor current falls below this threshold, FULLCHG output switches high
Battery charging continues until top-off timeout occurs.
Note 7: After charging is complete, when BATT voltage falls below this threshold, a new charging cycle is initiated.
Note 8: In shutdown, charging ceases and battery drain current drops to 5µA (max), but internal IC bias current remains on.
Note 9: Specifications to -40°C are guaranteed by design and not production tested.
(Circuit of Figure 1, VDCIN = VCSSN = VCSSP = +18V, SHDN = VL, CELL = GND, VBATT = VCS = +4.2V, VVADJ = VREF / 2, ISETIN =
ISETOUT = REF, RTHM = 10kΩ, TA = -40°C to +85°C, unless otherwise noted.) (Note 9)
PARAMETER CONDITIONS MIN TYP MAX UNITS
CONTROL INPUTS/OUTPUTS
SHDN Input Voltage High
1.4 V
SHDN Input Voltage Low (Note 8) 0.6 V
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
6
Electrical Characteristics (continued)
(Circuit of Figure 1, VDCIN = +18V, ISETIN = ISETOUT = REF, VVADJ = VREF / 2, TA = +25°C, unless otherwise noted.)
1000
1
0.1 1 10
FAST-CHARGE TIMEOUT
vs. TIMER2 CAPACITANCE
10
MAX1737 toc09
CAPACITANCE (nF)
TIMEOUT (MINUTES)
100
1000
0.1
0.1 1 10
TIMEOUT vs. TIMER1 CAPACITANCE
1
MAX1737 toc08
CAPACITANCE (nF)
TIMEOUT (MINUTES)
10
100
PREQUALIFICATION MODE
TOP-OFF MODE
FULL-CHARGE
MODE
4.190
4.194
4.192
4.198
4.196
4.202
4.200
4.204
4.208
4.206
4.210
0 200 300 400100 500 600 700 900800 1000
REFERENCE LOAD REGULATION
MAX1737 toc07
REFERENCE CURRENT (µA)
REFERENCE VOLTAGE (V)
50
60
70
80
90
100
8 12 16 20 24 28
EFFICIENCY vs. INPUT VOLTAGE
MAX1737 toc06
INPUT VOLTAGE (V)
EFFICIENCY (%)
CELL = FLOAT (2 CELLS)
VBATT = 7V
R18 = 0.1Ω (IBATT = 2A)
CELL = FLOAT (2 CELLS)
VBATT = 7V
R18 = 0.1Ω (IBATT = 2A)
CELL = FLOAT (2 CELLS)
VBATT = 7V
R18 = 0.1Ω (IBATT = 2A)
4.175
4.185
4.180
4.195
4.190
4.200
4.205
-40 20 40-20 0 60 80 100
REFERENCE VOLTAGE
vs. TEMPERATURE
MAX1737 toc05
TEMPERATURE (°C)
REFERENCE VOLTAGE (V)
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
MAX1737 toc04
VADJ VOLTAGE (V)
VOLTAGE LIMIT (V)
0
20
40
60
80
100
120
0 1.00.5 1.5 2.0 2.5 3.0 3.5 4.0 4.5
INPUT CURRENT-SENSE VOLTAGE
vs. ISETIN VOLTAGE
MAX1737 toc03
ISETIN VOLTAGE (V)
INPUT CURRENT-SENSE VOLTAGE (mV)
0
50
25
125
100
75
175
200
150
225
0 1.5 2.00.5 1.0 2.5 3.0 3.5 4.0 4.5
CHARGING CURRENT-SENSE VOLTAGE
vs. ISETOUT VOLTAGE
MAX1737 toc02
ISETOUT VOLTAGE (V)
CHARGING CURRENT-SENSE VOLTAGE (mV)
0
1.0
0.5
2.5
2.0
1.5
4.0
3.5
3.0
4.5
0 1.00.5 1.5 2.0 2.5
BATTERY VOLTAGE
vs. CHARGING CURRENT
MAX1737 toc01
CHARGING CURRENT (A)
BATTERY VOLTAGE (V)
R18 = 0.1Ω
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
Maxim Integrated
│
7
www.maximintegrated.com
Typical Operating Characteristics
PIN NAME FUNCTION
1VL Chip Power Supply. Output of the 5.4V linear regulator from DCIN. Bypass VL to GND with a
2.2µF or larger ceramic capacitor.
2 ISETIN
Input Current Limit Adjust. Use a voltage-divider to set the voltage between 0 and VREF.
See Input Current Regulator section.
3 ISETOUT 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 a qualification temperature
range. If unused, connect a 10kΩresistor from THM to ground. See Thermistor section.
5 REF 4.2V Reference Voltage Output. Bypass REF to GND with a 1µF or larger ceramic capacitor.
6GND Analog Ground
7 BATT Battery Voltage-Sense Input and Current-Sense Negative Input
8 VADJ
Voltage Adjust. Use a voltage-divider to set the VADJ voltage between 0 and VREF to adjust the
battery regulation voltage by ±5%. See Setting the Voltage Limit section.
9CCV Voltage Regulation Loop Compensation Point
10 CCS Input Source Current Regulation Compensation Point
11 CCI Battery-Current Regulation Loop Compensation Point
12 CELL Cell-Count Programming Input. See Table 2
13 TIMER1 Timer 1 Adjustment. Connect a capacitor from TIMER1 to GND to set the prequalification,
full-charge, and top-off times. See Timers section.
14 TIMER2
Timer 2 Adjustment. Connect a capacitor from TIMER2 to GND to set the fast-charge time. See
Timers section.
15
FAULT Charge Fault Indicator. Open-drain output pulls low when charging terminates abnormally
(Table 1).
16
FASTCHG
Fast-Charge Indicator. Open-drain output pulls low when charging with constant current.
17
FULLCHG Full-Charge Indicator. Open-drain output pulls low when charging with constant voltage in
full-charge state.
18 SHDN Shutdown Input. Drive SHDN low to disable charging. Connect SHDN to VL for normal
operation.
19 CS Battery Current-Sense Positive Input. See Charging Current Regulator section.
20 PGND Power Ground
21 DLO Synchronous-Rectifier MOSFET Gate-Drive Output
22 VLO Synchronous-Rectifier MOSFET Gate-Drive Bias. Bypass VLO to PGND with a 0.1µF capacitor.
23 BST High-Side MOSFET Gate Drive Bias. Connect a 0.1µF or greater capacitor from BST and LX.
24 LX Power Inductor Switching Node. Connect LX to the high-side MOSFET source.
25 DHI High-Side MOSFET Gate-Drive Output
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 capacitor. Also used for the source undervoltage sensing.
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
8
Pin Description
Detailed Description
The MAX1737 includes all of the functions necessary to
charge between one and four series Li+ battery cells.
It includes a high-efficiency synchronous-rectified step-
down DC-DC converter that controls charging voltage
and current. It also includes input source-current 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 external dual N-channel
MOSFET as a switch and a synchronous rectifier to
convert the input voltage to the charging current or voltage.
The typical application circuit is shown in Figure 1.
Figure 2 shows a typical charging sequence and
Figure 3 shows the block diagram. Charging current is
set by the voltage at ISETOUT and the voltage across
R18. The battery voltage is measured at the BATT
pin. The battery regulation voltage 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
Figure 1. Typical Application Circuit
27
26
22
23
25
24
21
20
19
7
4
28 CSSP
DCIN
CSSN
VLO
BST
DHI
LX
DLO
PGND
CS
BATT
THM
REF
ISETIN
VL
SHDN
ISETOUT
VADJ
CELL
GND
CCV
CCI
CCS
TIMER1
TIMER2
R12
Li+
BATTERY
(1 TO 4 CELLS)
L1
R18
22µH
FAULT
FULLCHG
FULL CHARGE
FAST CHARGE
FAULT
FASTCHG
C6
47nF
C13
1nF
C14
1nF
1nF
C5
47nF
C4
C3
1µF
C1
4.7µFC18
22µF
C19
22µF
SYSTEM
LOAD
INPUT
SUPPLY
C2
0.1µF
C7
0.1µF
C8
0.1µF
R1
10k
R8
D1
D3
D2
R9
0.1µF
16
11
9
6
12
3
8
2
5
18
1
10
13
14
17
15
0.1µF
0.1µF
C15
68µF
C9
0.1µF
C11
0.1µF
C10
0.1µF
MAX1737
++
THERMISTOR
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
9
tor (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
the Voltage Regulator section). When the charging current
drops below 10% of the charging current limit, or if the
full-charge timer expires, the state machine enters the
top-off state. In the top-off state, the batteries continue
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. It then 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 MAX1737 uses a high-accuracy voltage
regulator (±0.8%) to limit the charging voltage. The battery
regulation voltage is nominally set to 4.2V per cell and
can be adjusted ±5% by setting 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 maintained while using 1%
resistors. CELL sets the cell count from one to four series
cells (see Setting the Battery Regulation Voltage section).
An internal error amplifier (GMV) maintains voltage regulation
(Figure 3). The GMV amplifier is compensated at CCV.
The component values shown in Figure 1 provide suitable
performance for most applications. Individual compensation
of the voltage regulation and current regulation loops
allows for optimal compensation of each.
Charging Current Regulator
The charging current-limit regulator limits the charging
current. The current is sensed by measuring the voltage
across the current-sense resistor (R18, Figure 1) placed
between the BATT and CS pins. The voltage on the
ISETOUT pin also controls the charging current. Full-scale
charging current is achieved by connecting ISETOUT to
REF. In this case, the full-scale current-sense voltage is
200mV from CS to BATT.
When choosing the charging current-sense resistor,
note that the voltage drop across this resistor causes
further power loss, reducing efficiency. However,
adjusting ISETOUT to reduce the voltage across the
accuracy is better than 1% while using 1% setting
resistors.
The MAX1737 includes a state machine that controls
the charging algorithm. Figure 4 shows the state diagram.
Table 1 lists the charging state conditions. When power
is applied or SHDN 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, 1/20 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 suspended. 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
the Charging Current Regulator section). If the battery
voltage 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 with a fault indication.
The fast-charge time limit is set by the TIMER2 capaci-
Figure 2. Charge State and Indicator Output Timing for a
Typical Charging Sequence
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
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
10
current-sense resistor may degrade accuracy due to
the input offset of the current-sense amplifier.
The charging-current error amplifier (GMI) is compensated
at CCI. A 47nF capacitor at CCI provides suitable
performance for most applications.
Input Current Regulator
The total input current (from a wall cube or other DC
source) is the sum of system supply current plus the
battery-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 normally fluctuates as portions of the
system are powered up or put to sleep. Without input
Figure 3. PWM Controller Block Diagram
PWMCOMP
ON
BST
CCI
DHI
LX
DLO
VLO
PGND
CCV
CCS
LO
PWMCMP
ILIMIT
LOWILIM
OSC
160ns
160ns
PWMOSC
REF/42
REF/2
REF/2.6
CCI GND
CCS
LVC
GMS
GND
GND
R
GND
R
R/9
3R
DHI
DLO
GATE
CONTROL
CCV
SW+
SW-
CS+
CS-
EA+
EA-
GMI
10x
CSS
GMV
GND
GND
R
R
R/2
R/2R/2R
R
9R
CELL
CELL
REF
VADJ
3R
ISETOUT
ISETIN
REF/42
STOP
SLOPE
COMP
BATT
SAW
PREQ
BATT
SHDN
CS
CSSN
CSSP
ONE
TWO
THREE
FOUR
5x
CSI
MAX1737
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
11
current regulation, the input source must be able to supply
the maximum system 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 voltage to 100mV.
When choosing the input 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 current-sense
amplifier.
The input current error amplifier (GMS) is compensated at
CCS. A 47nF capacitor at CCS provides suitable performance
for most applications.
PWM Controller
The PWM controller drives the external MOSFETs to control
the 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.
Figure 4. State Diagram
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
DHI AND DLO HELD
LOW FOR 800µs
VBATT > 2.5V
VBATT < 0.95 × VBATTR
VBATT < 0.95 × VBATTR
VDCIN < BATT
VBATT < UNDERVOLTAGE
THRESHOLD
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
VBATT = BATTERY
REGULATION VOLTAGE (VBATTR)
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
12
Table 1. Charging State Conditions
STATE ENTRY CONDITIONS STATE CONDITIONS
Reset
From initial power on
or
From done state if battery voltage <
recharge voltage threshold
or
VDCIN - VBATT < 100mV or VBATT > bat-
tery 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 = C/20, 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 regu-
lation voltage, charging current = 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 ≤ current limit,
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 charging current ≤ 10% of
current limit
Battery voltage = battery regulation voltage, charging
current ≤ 10% of current limit, 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 ≤ battery
regulation voltage, charging current = 0, FASTCHG =
high,
FULLCHG = high, FAULT = high
Over/Under Temperature
From fast-charge state or full-charge
state if battery temperature is outside of
limits
Charge current = 0, timers suspended,
FASTCHG = no
change, FULLCHG = no change,
FAULT = no change
Fault
From prequalification state if prequalifi-
cation timer expires
or
From fast-charge state if fast-charge
timer expires
Charging current = 0,
FASTCHG
= high,
FULLCHG
= high,
FAULT
= low
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
13
The current-mode PWM controller uses the inductor
current to regulate the output voltage or current, simpli-
fying stabilization of the regulation loops. Separate
compensation of the regulation circuits allows each to
be optimally stabilized. Internal slope compensation is
included, ensuring stable operation over a wide range
of duty cycles.
The controller drives an external N-channel MOSFET
switch and a synchronous rectifier to step the input
voltage down to the battery voltage. A bootstrap capaci-
tor drives the high-side MOSFET gate to a voltage
higher than the input source voltage. This capacitor
(between BST and LX) is charged through a diode
from VLO when the synchronous rectifier is on. The
high-side MOSFET gate is driven from BST, supplying
sufficient voltage to fully drive the MOSFET gate even
when its source is near the input voltage. The synchro-
nous rectifier is driven from DLO to behave like a diode,
but with a smaller voltage drop for improved efficiency.
A built-in dead time (50ns typ) between switch and
synchronous rectifier turn-on and turn-off prevents crowbar
currents (currents that flow from the input voltage to
ground due to both the MOSFET switch and synchro-
nous rectifier being on simultaneously). This dead time
may allow the body diode of the synchronous rectifier
to conduct. If this happens, the resulting forward
voltage and diode recovery time will cause a small loss
of efficiency and increased power dissipation in the
synchronous rectifier. To prevent the body diode from
conducting, place an optional Schottky rectifier in parallel
with the drain and source of the synchronous rectifier. The
internal current-sense circuit turns off the synchronous
rectifier when the inductor current drops to zero.
Timers
The MAX1737 includes safety timers to terminate charg-
ing and to ensure that faulty batteries are not charged
indefinitely. TIMER1 and TIMER2 set the timeout 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
minutes 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). The timers cannot be disabled.
Charge Monitoring Outputs
FASTCHG, FULLCHG, and FAULT are open-drain
outputs that can be used as LED drivers. FASTCHG
indicates 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 (see 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 by removing and
reapplying the input power, or by pulling SHDN low.
Thermistor
The intent of THM is to inhibit fast-charging the cell
when it 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 qualification.
The thermistor should be 10kΩ at +25°C and have a
negative temperature coefficient (NTC); the THM pin
expects 3.97kΩ at +47.5°C and 28.7kΩ at +2.5°C.
Connect the thermistor between THM and GND. If
no temperature qualification is desired, replace
the thermistor with a 10kΩ resistor. Thermistors by
Philips/BCcomponents (2322-640-63103), Cornerstone
Sensors (T101D103-CA), and Fenwal Electronics
(140-103LAG-RB1) work well. Charging pauses briefly
(DHI and DLO are held Low for 800µs) to allow accurate
temperature measurement . The battery voltage will have
an undershoot when DHI and DLO stop switching during
the temperature measurement. The battery voltage will
have an overshoot when DHI and DLO resume switching
after the temperature measurement.
Shutdown
When SHDN is pulled low, the MAX1737 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 1µA. DHI and DLO are low. However, the
internal linear regulator (VLO) and the reference (REF)
remain on. The status outputs FASTCHG, FULLCHG, and
FAULT are high impedance. When exiting shutdown mode,
the MAX1737 goes back 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 voltage
on BATT, the charger resets.
Table 2. Cell-Count Programming
CELL CELL COUNT (N)
GND 1
Float 2
REF 3
VL 4
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
14
Design Procedure
Setting the Battery Regulation Voltage
VADJ sets the per-cell voltage limit. To set the VADJ
voltage, use a resistor-divider from REF to GND. 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% resistors
for the voltage-dividers results in no more than 0.1%
degradation in output voltage accuracy. VADJ is inter-
nally buffered so that high-value resistors can be used.
Set VVADJ by choosing a value less than 100kΩ for
R8 and R9 (Figure 1) from VADJ to GND. The per-cell
battery 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:
BATTR
ADJ REF
9.5 V
V (9.0 V )
N
×
= −×
where VBATTR is N x the cell voltage. CELL is the
programming input for selecting cell count N. Table 2
shows how CELL is connected to charge one to four cells.
Setting the Charging Current Limit
A resistor-divider from REF to GND sets the voltage
at ISETOUT (VISETOUT). This voltage determines the
charging current during the current-regulation fast-charge
mode. The full-scale charging current (IFSI) is set by the
current-sense resistor (R18, Figure 1) between CS and
BATT. The full-scale current is IFSI = 0.2V / R18.
The charging current ICHG is therefore:
ISETOUT
CHG FSI
REF
V
I I V
=
In choosing the current-sense resistor, note that the drop
across this resistor causes further power loss, reducing
efficiency. However, too low a value may degrade the
accuracy of the charging current.
Setting the Input Current Limit
A resistor-divider from REF to GND can set the voltage
at ISETIN (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 (R12,
Figure 1) between CSSP and CSSN. The full-scale
source current is IFSS = 0.1V / R12.
The input current limit (IIN) is therefore:
ISETIN
IN FSS
REF
V
I I V
=
Set ISETIN to REF to get the full-scale current limit. Short
CSSP and CSSN to DCIN if the input source current limit
is not used.
In choosing the current-sense resistor, note that the drop
across this resistor causes further power loss, reducing
efficiency. However, too low a resistor value may degrade
input current limit accuracy.
Inductor Selection
The inductor value may be changed to achieve 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, higher inductance typically will result in
higher series resistance and lower saturation current. A
good trade-off is to choose the inductor so that the ripple
current is approximately 30% to 50% of the DC average
charging current. The ratio of ripple current to DC charg-
ing current (LIR) can be used to calculate the optimal
inductor value:
BATT DCIN(MAX) BATT
DCIN(MAX) CHG
V (V V )
LV f I LIR
−
=×× ×
where f is the switching frequency (300kHz).
The peak inductor current is given by:
PEAK CHG
LIR
I I 1
2
= +
Capacitor Selection
The input capacitor absorbs the switching current from
the charger input and prevents that current from circulat-
ing 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 cur-
rent does not go to 0). In this case, the RMS current of
the input capacitor may be approximated by the equation:
2
CIN CHG
I I DD
≈−
where ICIN = the input capacitor RMS current, D =
PWM converter duty ratio (typically VBATT/VDCIN), and
ICHG = battery charging current.
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
15
The maximum RMS input current occurs at 50%
duty cycle, so 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 impedance of the input capacitor is critical to pre-
venting AC currents from flowing back into the wall cube.
This requirement varies depending on the wall cube’s
impedance and the requirements of any conducted or
radiated EMI specifications that must be met. Aluminum
electrolytic capacitors are generally the least costly, but
are usually a poor choice for portable devices due to their
large size and low 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
and ESR than ceramic capacitors. This makes it more
critical to consider RMS current and power dissipation
when using tantalum capacitors.
The output filter capacitor is used to absorb the inductor
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 capacitance and
ESR rating of the capacitor are important for its effective-
ness as a filter and to ensure stability of the PWM circuit.
The minimum output capacitance for stability is:
BATT
REF DCIN(MIN)
OUT BATT CS
V
V1
V
CV f R
+
>××
where COUT is the total output capacitance, VREF is the
reference voltage (4.2V), VBATT is the maximum battery
voltage (typically 4.2V per cell), and VDCIN(MIN) is the
minimum source input voltage.
The maximum output capacitor ESR allowed for stability
is:
CS BATT
ESR
REF
RV
RV
×
<
where RESR is the output capacitor ESR and RCS is the
current-sense resistor from CS to BATT.
Setting the Timers
The MAX1737 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 fast-charge
timeout. The typical timeouts for a 1C charge rate are set
to 7.5 minutes for the prequalification timer, 90 minutes
for the fast-charge timer, 90 minutes for the full-charge
timer, and 45 minutes for the top-off timer by connecting a
1nF capacitor to TIMER1 and TIMER2. Each timer period
is directly proportional to the capacitance at the corre-
sponding pin. See the 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 using 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; 47nF 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
MOSFET Selection
The MAX1737 uses a dual N-channel external power
MOSFET switch to convert the input voltage to the charg-
ing current or voltage. The MOSFET must be selected to
meet the efficiency and power-dissipation requirements
of the charging circuit, as well as the temperature rise of
the MOSFETs. The MOSFET characteristics that affect
the power dissipation are the drain-source on-resistance
(RDS(ON)) and the gate charge. In general, these are
inversely proportional.
To determine the MOSFET power dissipation, the oper-
ating duty cycle must first be calculated. When the
charger is operating at higher currents, the inductor
current will be continuous (the inductor current will not
drop to 0A) and, in this case, the high-side MOSFET
duty cycle (D) can be approximated by the equation:
BATT
DCIN
V
DV
≈
and the synchronous-rectifier MOSFET duty cycle (D′) will
be 1 - D or:
DCIN BATT
DCIN
VV
DV
−
′≈
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
16
For the high-side switch, the worst-case power dissipa-
tion due to on-resistance occurs at the minimum source
voltage VDCIN(MIN) and the maximum battery voltage
VBATT(MAX), and can be approximated by the equation:
BATT(MAX)
R DS(ON) CHG
DCIN(MIN)
V
P R I 2
V
≈ ××
The transition loss can be approximated by the equation:
DCIN CHG TR
T
V I f t
P
3
× ××
≈
where tTR is the MOSFET transition time. So the total
power dissipation of the high-side switch is PTOT = PR
+ PT.
The worst-case synchronous-rectifier power occurs at the
minimum battery voltage VBATT(MIN) and the maximum
source voltage VDC(MAX), and can be approximated by:
DCIN(MAX) BATT(MIN)
DL DS(ON) CHG
DCIN(MAX)
VV
P R I2
V
−
≈ ××
There is a brief dead time where both the high-side
switch and synchronous rectifier are off. This prevents
crowbar currents that flow directly from the source volt-
age to ground. During the dead time, the inductor cur-
rent will turn on the synchronous-rectifier MOSFET body
diode, which may degrade efficiency. To prevent this,
connect a Schottky rectifier across the drain source of
the synchronous rectifier to stop the body diode from
conducting. The Schottky rectifier may be omitted,
typically degrading the efficiency by approximately 1%
to 2%, causing a corresponding increase in the low-side
synchronous-rectifier power dissipation.
VL and REF Bypassing
The MAX1737 uses an internal linear regulator to drop the
input voltage down to 5.4V, which powers the internal cir-
cuitry. The output of the linear regulator is the VL pin. The
internal linear regulator may also be used to power exter-
nal circuitry as long as the maximum current and power
dissipation of the linear regulator are not exceeded. The
synchronous-rectifier MOSFET gate driver (DLO) is pow-
ered from VLO. An internal 12Ω resistor from VL to VLO
provides the DC current to power the gate driver. Bypass
VLO to PGND with a 0.1µF or greater capacitor.
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.
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
17
Chip Information
TRANSISTOR COUNT: 5978
Note: The MAX1737EEI is a 28-pin QSOP and does not have a heat slug.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
28 QSOP E28+1 21-0055 90-0173
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
www.maximintegrated.com Maxim Integrated
│
18
Package Information
For the latest package outline information and land patterns (foot-
prints), go to www.maximintegrated.com/packages. Note that
a “+”, “#”, or “-” in the package code indicates RoHS status only.
Package drawings may show a different suffix character, but the
drawing pertains to the package regardless of RoHS status.
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
4 5/09 Initial release 1, 9, 18
5 7/17 Replaced Figure 4 and added information to Thermistor section 12, 14
Revision History
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
MAX1737 Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
© 2017 Maxim Integrated Products, Inc.
│
19
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
