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
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
________________________________________________________________ Maxim Integrated Products 1
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
R
S
CCI
CCV
CCS
VADJ
ISETIN
VL
REF
ISETOUT
INPUT SUPPLY
CELL
DHI
SYSTEM
LOAD
MAX1737
19-1626; Rev 3; 5/05
PART
MAX1737EEI -40°C to +85°C
TEMP RANGE PIN-PACKAGE
28 QSOP
EVALUATION KIT
AVAILABLE
Typical Operating Circuit
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
Pin Configuration
Ordering Information
Notebook Computers
Hand-Held Instruments
Li+ Battery Packs
Desktop Cradle Chargers
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.
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, V
DCIN
= V
CSSN
= V
CSSP
= +18V, SHDN = VL, CELL = GND, V
BATT
= VCS= +4.2V, V
VADJ
= V
REF
/ 2, ISETIN =
ISETOUT = REF, R
THM
= 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.
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
VREF Output Voltage
SWITCHING REGULATOR
SUPPLY AND REFERENCE
With 1% VADJ resistors
-1 +1
Absolute Voltage Accuracy
Not including VADJ resistor tolerances
-0.8 +0.8
%
V
VADJ
= REF
V
VADJ
= GND
4.386 4.421 4.453
PARAMETER CONDITIONS MIN TYP MAX UNITS
6.0V < V
DCIN
< 28V
SHDN = GND, V
BATT
= 19V
LX = V
DCIN
= 28V, SHDN = GND
I
REF
= 0 to 1mA
6V < V
DCIN
< 28V
V
CSSN
= V
CSSP
= V
DCIN
= 28V, SHDN = GND
In dropout f
OSC
/ 4, V
CCV
= 2.4V,
V
BATT
= 15V, CELL = VL
6.0V < V
DCIN
< 28V
V
BATT
= 15V, CELL = VL
IVL= 0 to 15mA
µA
0.1 5
BATT, CS Input Current
µA
0.1 10
LX Leakage
Ω
7
DHI, DLO On-Resistance
µA
210
CSSN + CSSP Off-State Leakage
%
97 98
LX Maximum Duty Cycle
kHz
270 300 330
PWM Oscillator Frequency
mA
57
DCIN Quiescent Supply Current
V
628
DCIN Input Voltage Range
mV
614
REF Load Regulation
mV
26
REF Line Regulation
4.179 4.20 4.221
V
0.05 0.155
DCIN to BATT Undervoltage Threshold,
DCIN Falling
V
0.19 0.40
DCIN to BATT Undervoltage Threshold,
DCIN Rising
V
5.10 5.40 5.70
VL Output Voltage
mV
44 65
VL Output Load Regulation
225 500
CELL = SHDN = VL, V
BATT
= 17V
BATT, CS Input Voltage Range
Battery Regulation Voltage (V
BATTR
) CELL = float, GND, VL, or REF (Note 1)
019
4.167 4.2 4.233
V
V/cell
Battery Regulation Voltage Adjustment
Range
V
CCV
= 2V
3.948 3.979 4.010
V/cell
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, V
DCIN
= V
CSSN
= V
CSSP
= +18V, SHDN = VL, CELL = GND, V
BATT
= VCS= +4.2V, V
VADJ
= V
REF
/ 2, ISETIN =
ISETOUT = REF, R
THM
= 10kΩ, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
Full-Charge Timer
81 90 100
min
Fast-Charge Timer
81 90 100
min
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
V
BATTR
TIMER1, TIMER2 Oscillation Frequency
2.1 2.33 2.6
kHz
Prequalification Timer
6.25 7.5 8.75
min
CCI, CCS Clamp Voltage with Respect
to CCV
25 200
mV
CCV Clamp Voltage with Respect
to CCI, CCS
25 200
mV
THM Trip-Threshold Voltage
1.386 1.4 1.414
V
THM Low-Temperature Current
46.2 49 51.5
µA
THM High-Temperature Current
344 353 362
µA
THM COLD Threshold Resistance (Note 3)
26.92 28.70 30.59
kΩ
THM HOT Threshold Resistance (Note 3)
3.819 3.964 4.115
kΩ
BATT Undervoltage Threshold (Note 4)
2.4 2.5 2.6
V/cell
THM low-temperature or high-temperature
current
V
THM
= 1.4V
V
THM
= 1.4V
Combines THM low-temperature current and
THM rising threshold, V
TRT/ITLTC
Combines THM high-temperature current and
THM rising threshold, V
TRT/ITHTC
CCV Amplifier Transconductance (Note 2)
0.39 0.584 0.80
mS
CCV Amplifier Maximum Output Current
±50
µA
CS to BATT Current-Sense Voltage
30 40 50
mV
CS to BATT Full-Scale Current-Sense
Voltage
185 200 215
mV
CS to BATT Current-Sense Voltage When in
Prequalification State
51015
mV
CS to BATT Hard Current-Limit Voltage
355 385 415
mV
CSSP to CSSN Current-Sense Voltage
10 20 30
mV
CSSP to CSSN Full-Scale
Current-Sense Voltage
90 105 115
mV
CCI Amplifier Transconductance
0.6 1 1.4
mS
CCI Amplifier Output Current
±100
µA
CCS Amplifier Transconductance
1.2 2 2.6
mS
CCS Amplifier Output Current
±100
µA
4.15V < V
BATT
< 4.25V, V
CCV
= 2V
3.5V < V
BATT
< 5V, V
CCV
= 2V
V
ISETOUT
= V
REF
/ 5
V
BATT
= 3V to 17V, CELL = GND or VL
V
BATT
< 2.4V per cell
6V < V
CSSP
< 28V, V
ISETIN
= V
REF
/ 5,
V
CCS
= 2V
6V < V
CSSP
< 28V, V
CCS
= 2V
V
CCI
= 2V
VCS- V
BATT
= 0, 400mV
I
SET
= REF, V
CCS
= 2V
V
CSSP
- V
CSSN
= 0, 200mV
PARAMETER MIN TYP MAX UNITSCONDITIONS
STATE MACHINE
ERROR AMPLIFIERS
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, V
DCIN
= V
CSSN
= V
CSSP
= +18V, SHDN = VL, CELL = GND, V
BATT
= VCS= +4.2V, V
VADJ
= V
REF
/ 2, ISETIN =
ISETOUT = REF, R
THM
= 10kΩ, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
CELL Input Voltage
0 0.5
1.5 2.5
V
REF
- 0.3 V
REF
+ 0.3
V
VVL- 0.4 V
VL
FASTCHG, FULLCHG, FAULT
Output Low Voltage
0.5
V
FASTCHG, FULLCHG, FAULT Output High
Leakage
1
µA
For 1 cell
For 2 cells
For 3 cells
For 4 cells
I
SINK
= 5mA
FASTCHG, FULLCHG, FAULT = 28V;
SHDN = GND
Top-Off Timer
40.5 45 49.8
min
SHDN Input Voltage High
1.4
V
SHDN Input Voltage Low (Note 8)
0.6
V
VADJ, ISETIN, ISETOUT Input Voltage
Range
0V
REF
V
VADJ, ISETIN, ISETOUT
Input Bias Current
nA
-50 50
SHDN Input Bias Current
-1 1
µA
CELL Input Bias Current
-5 5
µA
ISETIN Adjustment Range
V
REF
/ 5 V
REF
V
ISETOUT Adjustment Range
V
REF
/ 5 V
REF
V
ISETOUT Voltage for I
CHG
= 0
150 220 300
mV
V
VADJ
, V
ISETIN
, V
ISETOUT
= 0 or 4.2V
SHDN = GND or VL
PARAMETER MIN TYP MAX UNITSCONDITIONS
Temperature Measurement Frequency
0.98 1.12 1.32
Hz1nF on TIMER1 and TIMER2
CONTROL INPUTS/OUTPUTS
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
_______________________________________________________________________________________ 5
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, V
DCIN
= V
CSSN
= V
CSSP
= +18V, SHDN = VL, CELL = GND, V
BATT
= VCS= +4.2V, V
VADJ
= V
REF
/ 2, ISETIN =
ISETOUT = REF, R
THM
= 10kΩ, TA= -40°C to +85°C, unless otherwise noted.) (Note 9)
PARAMETER CONDITIONS MIN MAX UNITS
DCIN Input Voltage Range
VL Output Voltage
REF Output Voltage
REF Line Regulation
PWM Oscillator Frequency
DHI, DLO On-Resistance
BATT, CS Input Voltage Range
Battery Regulation Voltage (V
BATTR
)
Absolute Voltage Accuracy
CS to BATT Current-Sense Voltage
CS to BATT Full-Scale Current-Sense
Voltage
CS to BATT Current-Sense Voltage When in
Prequalification State
CS to BATT Hard Current-Limit Voltage
CSSP to CSSN Current-Sense Voltage
CSSP to CSSN Full-Scale Current-Sense
Voltage
THM Trip-Threshold Voltage
THM Low-Temperature Current
THM COLD Threshold Resistance (Note 3)
BATT Undervoltage Threshold (Note 4)
BATT Overvoltage Threshold (Note 5)
BATT Charge Current Full-Charge
Termination Threshold, CS-BATT (Note 6)
Temperature Measurement Frequency 1nF on TIMER1 and TIMER2
4.55 4.8 V/cell
35 55 mV
0.93 1.37 Hz
6.0V < V
DCIN
< 28V
6V < V
DCIN
< 28V
V
BATT
= 15V, CELL = VL
CELL = float, GND, VL, or REF
Not including V
ADJ
resistor tolerances
V
ISETOUT
= V
REF
/5
V
BATT
= 3V to 17V, CELL = GND or VL
V
BATT
< 2.4V per cell
6V < V
CSSP
< 28V, V
ISETIN
= V
REF
/5,
V
CCS
= 2V
6V < V
CSSP
< 28V, V
CCS
= 2V
THM low-temperature or high-temperature current
V
THM
= 1.4V
Combines THM low-temperature current and
THM rising threshold, V
TRT/ITLTC
260 340 kHz
6 mV
4.166 4.242 V
7 Ω
019V
4.158 4.242 V/cell
-1 1 %
628V
5.1 5.7 V
25 55 mV
180 220 mV
317mV
350 420 mV
535mV
85 115 mV
1.386 1.414 V
46.2 51.5 µA
26.92 30.59 kΩ
2.4 2.6 V/cell
SUPPLY AND REFERENCE
SWITCHING REGULATOR
ERROR AMPLIFIERS
STATE MACHINE
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
6 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, V
DCIN
= V
CSSN
= V
CSSP
= +18V, SHDN = VL, CELL = GND, V
BATT
= VCS= +4.2V, V
VADJ
= V
REF
/ 2, ISETIN =
ISETOUT = REF, R
THM
= 10kΩ, TA= -40°C to +85°C, unless otherwise noted.) (Note 9)
Note 1: Battery Regulation Voltage = Number of Cells
× (3.979V + 0.10526 × V
VADJ
).
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 I
CHG
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 5µ A ( max), but internal IC bias current remains on.
Note 9: Specifications to -40°C are guaranteed by design and not production tested.
SHDN Input Voltage Low (Note 8)
0.6
V
SHDN Input Voltage High
1.4 V
PARAMETER MIN TYP MAX UNITSCONDITIONS
CONTROL INPUTS/OUTPUTS
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
________________________________________________________________________________________ 7
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
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
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)
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)
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)
50
60
70
80
90
100
81216202428
EFFICIENCY vs. INPUT VOLTAGE
MAX1737 toc06
INPUT VOLTAGE (V)
EFFICIENCY (%)
CELL = FLOAT (2 CELLS)
V
BATT
= 7V
R18 = 0.1Ω (I
BATT
= 2A)
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)
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
1000
1
0.1 1 10
FAST-CHARGE TIMEOUT
vs. TIMER2 CAPACITANCE
10
MAX1737 toc09
CAPACITANCE (nF)
TIMEOUT (MINUTES)
100
Typical Operating Characteristics
(Circuit of Figure 1, V
DCIN
= +18V, ISETIN = ISETOUT = REF, V
VADJ
= V
REF
/ 2, TA= +25°C, unless otherwise noted.)
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
8 _______________________________________________________________________________________
Pin Description
Source Current-Sense Positive Input. See Input Current Regulator section.CSSP27
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.
DCIN28
Synchronous-Rectifier MOSFET Gate-Drive Bias. Bypass VLO to PGND with a 0.1µF capacitor.VLO22
High-Side MOSFET Gate Drive Bias. Connect a 0.1µF or greater capacitor from BST and LX.BST23
Power Inductor Switching Node. Connect LX to the high-side MOSFET source.LX24
High-Side MOSFET Gate-Drive OutputDHI25
Source Current-Sense Negative Input. See Input Current Regulator section.CSSN 26
Shutdown Input. Drive SHDN low to disable charging. Connect SHDN to VL for normal
operation.
SHDN
18
Battery Current-Sense Positive Input. See Charging Current Regulator section.CS19
Power GroundPGND20
Synchronous-Rectifier MOSFET Gate-Drive Output DLO21
Full-Charge Indicator. Open-drain output pulls low when charging with constant voltage in
full-charge state.
FULLCHG
17
Fast-Charge Indicator. Open-drain output pulls low when charging with constant current.
FASTCHG
16
Charge Fault Indicator. Open-drain output pulls low when charging terminates abnormally
(Table 1).
FAULT
15
Timer 2 Adjustment. Connect a capacitor from TIMER2 to GND to set the fast-charge time. See
Timers section.
TIMER214
Voltage Regulation Loop Compensation PointCCV9
Input Source Current Regulation Compensation PointCCS10
Battery-Current Regulation Loop Compensation PointCCI11
Cell-Count Programming Input. See Table 2CELL12
Timer 1 Adjustment. Connect a capacitor from TIMER1 to GND to set the prequalification,
full-charge, and top-off times. See Timers section.
TIMER113
4.2V Reference Voltage Output. Bypass REF to GND with a 1µF or larger ceramic capacitor. REF5
Analog GroundGND6
Battery Voltage-Sense Input and Current-Sense Negative InputBATT7
Voltage Adjust. Use a voltage-divider to set the VADJ voltage between 0 and V
REF
to adjust the
battery regulation voltage by ±5%. See Setting the Voltage Limit section.
VADJ8
Thermistor Input. Connect a thermistor from THM to GND to set a qualification temperature
range. If unused, connect a 10kΩ resistor from THM to ground. See Thermistor section.
THM4
Battery Charging Current Adjust. Use a voltage-divider to set the voltage between 0 and V
REF
.
See Charging Current Regulator section.
ISETOUT3
PIN
Input Current Limit Adjust. Use a voltage-divider to set the voltage between 0 and V
REF
.
See Input Current Regulator section.
ISETIN2
Chip Power Supply. Output of the 5.4V linear regulator from DCIN. Bypass VL to GND with a
2.2µF or larger ceramic capacitor.
VL
1
FUNCTIONNAME
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
_______________________________________________________________________________________ 9
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 stepdown 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
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
4.7µF
C18
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
Figure 1. Typical Application Circuit
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
10 ______________________________________________________________________________________
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 fastcharge 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 (C
TIMER1
). 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 fastcharge 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-
tor (C
TIMER2
). 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 currentsense 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
FAST-
CHARGE
STATE
OPENDRAIN
LOW
OPENDRAIN
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
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
______________________________________________________________________________________ 11
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
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
Figure 3. PWM Controller Block Diagram
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
12 ______________________________________________________________________________________
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.
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
I
CHARGE
< I
MIN
OR
FULL-CHARGE
TIMEOUT
ONCE PER
SECOND
ONCE PER
SECOND
TEMP
QUAL
V
BATT
> 2.5V
V
BATT
< 0.95 × V
BATTR
V
BATT
< 0.95 × V
BATTR
V
DCIN
< BATT
V
BATT
< UNDERVOLTAGE
THRESHOLD
V
BATT
= BATTERY
REGULATION VOLTAGE (V
BATTR
)
FAST-CHARGE
TIMEOUT
PREQUAL
TIMEOUT
TEMP
NOT OK
TEMP
NOT OK
SHUTDOWN IS
ENTERED FROM ALL STATES
WHEN SHDN IS LOW.
SHDN HIGH
V
DCIN
> V
BATT
Figure 4. State Diagram
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
______________________________________________________________________________________ 13
Table 1. Charging State Conditions
From initial power on
or
From done state if battery voltage <
recharge voltage threshold
or
V
DCIN
- V
BATT
< 100mV or V
BATT
> bat-
tery overvoltage threshold
Timers reset, charging current = 0,
FASTCHG = high, FULLCHG = high,
FAULT = high
Reset
Fault
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
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
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
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
(C
TIMER1
= 1nF), FASTCHG = high, 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 (C
TIMER1
= 1nF),
FASTCHG = high, FULLCHG = low, FAULT = high
ENTRY CONDITIONS STATE CONDITIONS
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 (C
TIMER1
= 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 = current limit,
timeout = 90min typ (C
TIMER2
= 1nF),
FASTCHG = low, FULLCHG = high, FAULT = high
STATE
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
14 ______________________________________________________________________________________
The current-mode PWM controller uses the inductor
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 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
capacitor 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 synchronous 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 synchronous 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
charging 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 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 (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 (140103LAG-RB1) work well.
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
4VL
3REF
2
1GND
CELL COUNT (N)CELL
Float
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
_______________________________________________________________________________________ 15
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
internally buffered so that high-value resistors can be
used. Set V
VADJ
by choosing a value less than 100kΩ
for R8 and R9 (Figure 1) from VADJ to GND. The percell 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
determined, the VADJ voltage is calculated by the
equation:
where V
BATTR
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 (V
ISETOUT
). This voltage determines the
charging current during the current-regulation fastcharge mode. The full-scale charging current (I
FSI
) is
set by the current-sense resistor (R18, Figure 1)
between CS and BATT. The full-scale current is I
FSI
=
0.2V / R18.
The charging current I
CHG
is therefore:
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 (V
ISETIN
). This sets the maximum source current allowed at any time during charging. The source
current (I
FSS
) is set by the current-sense resistor (R12,
Figure 1) between CSSP and CSSN. The full-scale
source current is I
FSS
= 0.1V / R12.
The input current limit (IIN) is therefore:
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 charging current (LIR) can be used to
calculate the optimal inductor value:
where f is the switching frequency (300kHz).
The peak inductor current is given by:
Capacitor Selection
The input capacitor absorbs the switching current from
the charger input and prevents that current from circulating 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 approximated by the equation:
where I
CIN
= the input capacitor RMS current, D =
PWM converter duty ratio (typically V
BATT
/ V
DCIN
), and
I
CHG
= battery charging current.
II
CIN CHG
DD
2
≈−
II
LIR
PEAK CHG
=+
1
2
L
VV V
VfILIR
BATT DCIN MAX BATT
DCIN MAX CHG
=
−
×× ×
()
()
()
II
V
V
IN FSS
ISETIN
REF
=
II
V
V
CHG FSI
ISETOUT
REF
=
V
9.5 V
N
(9.0 V )
ADJ
BATTR
REF
=
×
−×
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
16 ______________________________________________________________________________________
The maximum RMS input current occurs at 50%
duty cycle, so the worst-case input ripple current is
0.5 × I
CHG
. 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 preventing 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 effectiveness as a filter and to ensure stability of the PWM circuit. The minimum output capacitance
for stability is:
where C
OUT
is the total output capacitance, V
REF
is the
reference voltage (4.2V), V
BATT
is the maximum battery
voltage (typically 4.2V per cell), and V
DCIN(MIN)
is the
minimum source input voltage.
The maximum output capacitor ESR allowed for stability
is:
where R
ESR
is the output capacitor ESR and RCSis 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 fullcharge 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 corresponding pin. See the Typical
Operating Characteristics.
Compensation
Each of the three regulation loops—the input current
limit, the charging current limit, and the charging voltage 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
charging 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 (R
DS(ON)
) and the gate
charge. In general, these are inversely proportional.
To determine the MOSFET power dissipation, the operating 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:
and the synchronous-rectifier MOSFET duty cycle (D′)
will be 1 - D or:
′
≈−D
VV
V
DCIN BATT
DCIN
D
V
V
BATT
DCIN
≈
R
RV
V
ESR
CS BATT
REF
<
×
C
V
V
V
VfR
OUT
REF
BATT
DCIN MIN
BATT CS
>
+
××
1
()
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
______________________________________________________________________________________ 17
For the high-side switch, the worst-case power dissipation due to on-resistance occurs at the minimum source
voltage V
DCIN(MIN)
and the maximum battery voltage
V
BATT(MAX)
, and can be approximated by the equation:
The transition loss can be approximated by the equation:
where tTRis the MOSFET transition time. So the total
power dissipation of the high-side switch is P
TOT
= P
R
+ PT.
The worst-case synchronous-rectifier power occurs at
the minimum battery voltage V
BATT(MIN)
and the maxi-
mum source voltage V
DC(MAX)
, and can be approxi-
mated by:
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 voltage to ground. During the dead time, the inductor current 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 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 current
and power dissipation of the linear regulator are not
exceeded. The synchronous-rectifier MOSFET gate driver (DLO) is powered 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
internal 4.2V reference is stable. In both cases use a
low-ESR ceramic capacitor.
Chip Information
TRANSISTOR COUNT: 5978
P
VV
V
RI
DL
DCIN MAX BATT MIN
DCIN MAX
DS ON CHG
≈
−
××
() ()
()
()
2
P
VIft
T
DCIN CHG TR
≈
×××
3
P
V
V
RI
R
BATT MAX
DCIN MIN
DS ON CHG
≈××
()
()
()
2
MAX1737
Stand-Alone Switch-Mode
Lithium-Ion Battery-Charger Controller
QSOP.EPS
E
1
1
21-0055
PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH
Note: The MAX1737EEI is a 28-pin QSOP and does not have a heat slug.
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implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
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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
.)
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