General Description
The MAX1925/MAX1926 single-cell lithium-ion (Li+)
switch-mode battery chargers use an external PMOS
pass element step-down configuration. Charge current
is programmable, and an external capacitor sets the
maximum charge time.
Additional features include automatic input power
detection (ACON output), logic-controlled enable, and
temperature monitoring with an external thermistor. The
MAX1925 disables charging for inputs greater than
6.1V, while the MAX1926 charges for inputs between
4.25V and 12V.
The MAX1925/MAX1926 feature two precondition levels
to restore near-dead cells. The devices source 4mA to
a cell that is below 2V while sourcing C/10 to a cell
between 2V and 3V. Full charge current is then applied
above 3V. A CHG output drives an LED to indicate
charging (LED on) and fault conditions (LED blinking).
The MAX1925/MAX1926 are available in a 12-pin
4mm ✕4mm thin QFN package and are specified over
the extended temperature range (-40°C to +85°C). An
evaluation kit is available to speed design.
Applications
Digital Cameras
Self-Charging Battery Packs
PDAs
Cradle Chargers
Features
♦Small (4mm ✕4mm) Package
♦4.25V to 12V Input Range (MAX1926)
♦Overvoltage Lockout at 6.1V (MAX1925)
♦±0.75% Battery Regulation Voltage
♦Set Charge Current with One Resistor
♦Automatic Input Power Sense
♦LED (or Logic-Out) Charge Status and Fault
Indicator
♦Programmable Safety Timer
♦Autorestart at Cell = 4V
♦Thermistor Monitor Input
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
MAX1925
MAX1926
IN
EN
INP EXT PGND
INPUT
4.5V TO 12V
LED
CT
CHG
ON
OFF ACON
THRM
GND
BATT
CS
1-CELL
Li+
BATTERY
Typical Operating Circuit
19-2556; Rev 1; 10/02
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
PART TEMP
RANGE
PIN-
PACKAGE
INPUT
CHARGING
RANGE
MAX1925ETC -40°C to +85°C12 Thin QFN
4mm x 4mm 4.5V to 6.1V
MAX1926ETC -40°C to +85°C12 Thin QFN
4mm x 4mm 4.25V to 12V
12
IN
11
INP
10
EXT
45
CT
6
CS
1
2EN
3
9
8
7THRM
PGND
GND
BATT
MAX1925
MAX1926
CHG
ACON
THIN QFN
Pin Configuration
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VPGND = VGND = 0, VINP = VIN = VCHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10kΩto GND, CCT = 100nF, 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.
IN, INP, ACON to GND...........................................-0.3V to +14V
CHG, EXT to PGND ...................................-0.3V to (VINP + 0.3V)
CS, BATT, EN, THRM to GND ..................................-0.3V to +6V
CT to GND ................................................................-0.3V to +4V
EN, THRM, CT to IN................................................-14V to +0.3V
INP to IN ................................................................-0.3V to +0.3V
PGND to GND .......................................................-0.3V to +0.3V
CS to BATT ............................................................-0.3V to +0.3V
EXT Continuous RMS Current.........................................±100mA
Continuous Power Dissipation (TA= +70°C)
Exposed Paddle Soldered to Board
(derate 16.9mW/°C above +70°C).............................1349mW
Exposed Paddle Unsoldered
(derate 9mW/°C above +70°C)....................................721mW
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
PARAMETER CONDITIONS MIN TYP MAX UNITS
Supply Voltage Range VINP, VIN (MAX1925 does not charge above 6.1V) 4.5 12.0 V
MAX1925 4.30 4.50 4.78
VINP, VIN rising MAX1926 4.00 4.25 4.50
MAX1925 4.17 4.30 4.43
ACON Trip Point
VINP, VIN falling MAX1926 3.90 4.15 4.40
V
Rising 5.8 6.4
INP, IN Shutdown Threshold MAX1925 Falling 5.3 5.9 V
EN Input Resistance MAX1926 internally pulled up to 3V 125 300 550 kΩ
EN Leakage Current MAX1925 -1 +1 µA
EN Logic Input High Threshold 2 V
EN Logic Input Low Threshold 0.8 V
DONE state, VBATT = 4.1V 25 50
EN = GND (Note 1) 2 10
BATT + CS Input Current
(Total Current into BATT and CS) VBATT = VINP = VIN = 4V, shutdown (Note 1) 2 10
µA
CS Input Current Charging 39 µA
EN = GND 5 8 mA
VBATT = VINP = VIN = 4V, shutdown 2 10 µA
VBATT = 4.1V; charging 10
IN + INP Total Input Current
VBATT = 4.3V; done 5 8 mA
VOLTAGE LOOP
Voltage Loop Set Point 4.1685 4.2000 4.2315 V
Voltage Loop Hysteresis 15 mV
BATT Prequal1 Voltage Threshold 1.9 2 2.1 V
BATT Prequal2 Voltage Threshold 2.85 3 3.15 V
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VPGND = VGND = 0, VINP = VIN = VCHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10kΩto GND, CCT = 100nF, TA= 0°C to +85°C,
unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Restart Threshold Charging restarts when BATT falls to this point 3.92 4.00 4.08 V
BATT Voltage Fault Threshold If BATT exceeds this threshold, EXT is high
(external MOSFET is off) and CHG blinks 4.275 4.350 4.425 V
CURRENT LOOP
VCS - VBATT, average value 132 142 152
CS - BATT Sense Threshold Rise/fall hysteresis 30 mV
Prequal1 Charge Current VBATT < 2V 3 4 6 mA
Average value, 2V < VBATT < 3V (charge current is C/10) 14
Prequal2 CS - BATT Sense
Threshold Rise/fall hysteresis, 2V < VBATT < 3V 12 mV
Current Threshold for
Full-Battery Indication ILOAD falling, as percentage of fast charge current 6 12 20 %
DRIVER FUNCTIONS
EXT Sink/Source Current 1 A
EXT On-Resistance EXT high or low 5 12 Ω
Nominal Switching Frequency VBATT = 3.6V, L =10µH 235 kHz
TIMER FUNCTIONS
Full-Time Timeout - tFULL CCT = 100nF 3.02 hours
Prequal1 Timeout CCT = 100nF tFULL/1088
(10s) s
Prequal2 Timeout CCT = 100nF tFULL/17
(10.67 min) min
Timer Accuracy CCT = 100nF for 3 hours -15 +15 %
CHG Output Low Current VCHG = 1V 7 10 14 mA
CHG Output High Leakage
Current VCHG = 12V 1 µA
CHG Blink Rate - Fault Fault state (50% duty cycle), CCT = 100nF 0.5 Hz
ACON High Leakage VACON = 12V 0.01 1.00 µA
ACON Sink Current VACON = 0.4V 2 mA
THERMISTOR MONITOR (Note 2)
THRM Sense Current for Hot
Qualification 344.1 352.9 361.7 µA
THRM Sense Current for Cold
Qualification 47.58 48.80 50.02 µA
THRM Sense-Voltage Trip Point (Note 3) 1.386 1.400 1.414 V
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(VPGND = VGND = 0, VINP = VIN = VCHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10kΩto GND, CCT = 100nF, TA= -40°C to +85°C,
unless otherwise noted. Typical values are at TA= +25°C.) (Note 4)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Supply Voltage Range VINP, VIN (MAX1925 does not charge above 6.1V) 4.5 12.0 V
MAX1925 4.30 4.78
VINP, VIN rising MAX1926 4.0 4.5
MAX1925 4.17 4.43
ACON Trip point
VINP, VIN falling MAX1926 3.9 4.4
V
Rising 5.8 6.4
INP, IN Shutdown Threshold MAX1925 Falling 5.3 5.9 V
EN Input Resistance MAX1926 internally pulled up to 3V 125 550 kΩ
EN Leakage Current MAX1925 -1 +1 µA
EN Logic Input High Threshold 2 V
EN Logic Input Low Threshold 0.8 V
DONE State, VBATT = 4.1V 50
EN = GND (Note 2) 10
BATT + CS Input Current
(Total Current into BATT and CS) VBATT = VINP = VIN = 4V, shutdown (Note 1) 10
µA
EN = GND 8 mA
VBATT = VINP = VIN = 4V, shutdown 10 µA
VBATT = 4.1V, charging 10
IN + INP Total Input Current
VBATT = 4.3V, done 8 mA
VOLTAGE LOOP
Voltage Loop Set Point 4.158 4.242 V
BATT Prequal1 Voltage Threshold 1.9 2.1 V
BATT Prequal2 Voltage Threshold 2.85 3.15 V
Restart Threshold Charging restarts when BATT falls to this point 3.92 4.08 V
BATT Voltage Fault Threshold If BATT exceeds this threshold, EXT is high (external
MOSFET is off) and CHG blinks 4.275 4.425 V
CURRENT LOOP
CS - BATT Sense Threshold VCS - VBATT, average value 127 157 mV
Prequal1 Charge Current VBATT < 2V 3 6 mA
Current Threshold for Full-Battery
Indication ILOAD falling, as percentage of fast charge current 4 20 %
DRIVER FUNCTIONS
EXT On-Resistance EXT high or low 12 Ω
TIMER FUNCTIONS
Timer Accuracy CCT = 100nF for 3 hours -16 +16 %
CHG Output Low Current V
CHG = 1V 7 14 mA
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
_______________________________________________________________________________________ 5
Note 1: When the AC adapter is unplugged or if the charger is shut down, BATT drain is less than 10µA.
Note 2: These specifications guarantee the thermistor interface detects a fault at the correct temperature (0°C to +5°C cold temper-
ature and 45°C to +50°C hot temperature) with Philips NTC Thermistor Series 640-6, 2322-640-63103, 10.0K at +25°C, ±5%
(or equivalent).
Note 3: A fault is generated if VTHRM lower than 1.4V during the cold test or higher than 1.4V during the hot test. Hot and cold tests
occur on alternate CT clock transitions.
Note 4: Specifications to -40°C are guaranteed by design and not production tested.
ELECTRICAL CHARACTERISTICS (continued)
(VPGND = VGND = 0, VINP = VIN = VCHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10kΩto GND, CCT = 100nF, TA= -40°C to +85°C,
unless otherwise noted. Typical values are at TA= +25°C.) (Note 4)
PARAMETER CONDITIONS MIN TYP MAX UNITS
CHG Output High Leakage
Current V
CHG = 12V 1 µA
ACON High Leakage V
ACON = 12V 1 µA
ACON Sink Current V
ACON = 0.4V 2 mA
THERMISTOR MONITOR
THRM Sense Current for Hot
Qualification 342 363 µA
THRM Sense Current for Cold
Qualification 47.3 50.3 µA
THRM Sense-Voltage Trip Point (Note 3) 1.379 1.421 V
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
6 _______________________________________________________________________________________
Typical Operating Characteristics
(Circuit of Figure 1, VSUPPLY = 5V, VBATT = 4V, TA = +25°C, unless otherwise noted.)
CHARGE-CURRENT ACCURACY
vs. SUPPLY VOLTAGE
MAX1925/26 toc01
VSUPPLY (V)
CHARGE-CURRENT ACCURACY (%)
11106 7 8 9
-15
-10
-5
0
5
10
15
20
-20
512
VBATT = 4V
CHARGE CURRENT
vs. BATTERY VOLTAGE
MAX1925/26 toc02
BATTERY VOLTAGE (V)
CHARGE CURRENT (A)
4.24.03.83.63.43.2
0.2
0.4
0.6
0.8
1.0
1.2
0
3.0 4.4
VIN = 12V
VIN = 10V VIN = 8V VIN = 5V
SEE THE DROPOUT
BEHAVIOR SECTION
FULL-BATTERY VOLTAGE
vs. SUPPLY VOLTAGE
MAX1925/26 toc03
VSUPPLY (V)
FULL-BATTERY VOLTAGE (V)
11105 6 7 8 9
4.196
4.198
4.200
4.202
4.204
4.206
4.208
4.210
4.194
412
SHUTDOWN BATTERY CURRENT
vs. BATTERY VOLTAGE
MAX1925/26 toc04
VBATT (V)
SHUTDOWN BATTERY CURRENT (µA)
4.03.83.63.43.2
1.5
2.0
2.5
3.0
3.5
4.0
0
1.0
0.5
3.0 4.2
EFFICIENCY vs. SUPPLY VOLTAGE
MAX1925/26 toc05
VSUPPLY (V)
EFFICIENCY (%)
11109876
75
80
85
90
95
100
70
512
VBATT = 4V
EFFICIENCY vs. BATTERY VOLTAGE
MAX1925/26 toc06
BATTERY VOLTAGE (V)
EFFICIENCY (%)
4.03.83.2 3.4 3.6
65
70
75
80
85
90
95
100
60
3.0 4.2
VIN = 12V
VIN = 8V
VIN = 5V
SWITCHING FREQUENCY vs. INDUCTANCE
MAX1925/26 toc07
INDUCTANCE (µH)
SWITCHING FREQUENCY (kHz)
10
100
1000
10
1001
VIN = 12V
VIN = 8V
VIN = 5V
VBATT = 4V
SWITCHING FREQUENCY vs. RSENSE
MAX1925/26 toc08
RSENSE (Ω)
SWITCHING FREQUENCY (kHz)
0.10
100
1000
10
1.000.01
VIN = 12V
VIN = 8V
VIN = 5V
VBATT = 4V
SWITCHING FREQUENCY
vs. SUPPLY VOLTAGE
MAX1925/26 toc09
VSUPPLY (V)
SWITCHING FREQUENCY (kHz)
11108 976
50
100
150
200
250
300
350
400
450
500
0
512
VBATT = 4V
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
_______________________________________________________________________________________ 7
FREQUENCY vs. BATTERY VOLTAGE
MAX1925/26 toc10
BATTERY VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
3.53.0
50
100
150
200
250
300
350
400
450
500
0
2.5 4.0
VIN = 12V
VIN = 8V
VIN = 5V
TIMER vs. CT CAPACITANCE
MAX1925/26 toc11
CT CAPACITANCE (nF)
TIMER (MINUTES)
100
0.1
1
10
100
1000
0.01
10 1000
tFULLCHG
tPREQUAL2
tPREQUAL1
CHARGE CURRENT vs. TIME
MAX1925/26 toc12
TIME (h)
CHARGE CURRENT (A)
2.52.01.51.00.5
0.2
0.4
0.6
0.8
1.0
1.2
0
0 3.0
BATTERY VOLTAGE vs. TIME
MAX1925/26 toc13
TIME (h)
BATTERY VOLTAGE (A)
2.52.01.51.00.5
3.9
4.0
4.1
4.2
4.3
4.4
4.5
3.8
0 3.0
SWITCHING WAVEFORM
MAX1925/26 toc14
4µs/div
1.2A
1A
0.8A
10V
0V
100mV/
div
INDUCTOR
CURRENT
VLX
VBATT
VIN = 5V
SWITCHING WAVEFORM
MAX1925/26 toc15
1µs/div
1.2A
1.4A
1A
0.8A
10V
0V
100mV/
div
INDUCTOR
CURRENT
VLX
VBATT
VIN = 12V
CHARGE-ENABLE REPSONSE
MAX1925/26 toc16
EN
INDUCTOR
CURRENT
5V
0V
1A
0A
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VSUPPLY = 5V, VBATT = 4V, TA = +25°C, unless otherwise noted.)
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
8 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1CHG
Charge Status LED Driver. Open-drain LED driver sinks 10mA when the MAX1925/MAX1926 are charging.
CHG also blinks at a 0.5Hz rate during fault states (see the Timing section). High impedance when charger is
in shutdown. See Tables 1 and 2.
2EN
Enable. Drive EN high to enable charger. Logic level input for normal ON/OFF control. In the MAX1926 EN is
internally pulled up to 3V with a 300kΩ resistor.
3 THRM
Thermistor Input. Monitors external thermistor (10kΩ at +25°C). When external temperature is lower than 0°C
or above +50°C, charging stops and the charger enters fault mode. Charging resumes when the temperature
returns to normal. During a temperature fault the MAX1926 blinks the CHG output, while MAX1925 CHG
remains off (high).
4ACON Power-OK Indicator Output. Open-drain output goes low when AC adapter power is valid. See Table 2 for
ACON states.
5CT
Timing Capacitor Connection. Connect timer cap to program full-charge safety timeout interval and
prequalification fault times. Timeouts with CCT = 100nF are:
Full Timer (tFULLCHG): 3 hours—If FASTCHG is not completed within this time a fault is asserted.
Prequal2 Timer: Full Timer/17 (10.67 min)
Prequal1 Timer: Full Timer/1088 (10s)
6 CS Charge-Current Sense Input. 142mV nominal regulation threshold. CS is high impedance during shutdown.
7 BATT Battery-Sense Input. Also negative side of charge-current sense. BATT is high impedance during shutdown.
8 GND Ground
9 PGND Power Ground
10 EXT PMOS Gate-Driver Output. Drives gate of external PMOS switching transistor from IN to GND. When using the
MAX1926, ensure that the MOSFET VGS rating is greater than VIN.
11 INP Supply Voltage Input
12 IN Supply-Sense Input. Connect IN to INP.
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
_______________________________________________________________________________________ 9
HV
DRIVER
10mA
4.5V FOR MAX1926
4.25V FOR MAX1925
6.1V
INP
PGND
REF
REF/10
D3
LED
POWER
SOURCE
REF/8
PREQUAL1
4mA
I > 12%
REF
REF
REF
REF
REF
BATT > 4.35V
BATT > 3V
BATT > 2V
ACON
RSET
0.14Ω
L1
10µH
REGULATOR
3V_ANA
3V_DIG
IN
ACON
STATE
MACHINE AND
TIMERS
OSC
ACON
IN INP EXT
CS
PGND
GND
EN
BATT
BATT > 2V
TEMP FAULT
REF
353µA49µA
THRM
10kΩ
MAX1925 ONLY
CHG ACON
D1
C3
10µFQ1 D2
C2
10µF
C2
0.1µF
MAX1925
MAX1926
Figure 1. Functional Diagram
MAX1925/MAX1926
Detailed Description
The MAX1925/MAX1926 switch-mode battery chargers
form a complete solution for a single-cell Li+ battery. The
devices include battery undervoltage/overvoltage fault
protection. The MAX1925/MAX1926 use EN and THRM
for shutdown, battery detection, and temperature monitor-
ing. The devices provide outputs to indicate charge sta-
tus (CHG) and presence of input power (ACON).
The MAX1925/MAX1926 include two prequalification
modes that must be passed before the charger enters
the fast-charge state. During fast charge, the charger
operates initially in constant-current mode until the bat-
tery voltage reaches 4.2V. When the battery voltage
has reached 4.2V, the charger operates in constant-
voltage mode. In constant-current mode, the charger
acts as a hysteretic current source, controlling the
inductor’s peak and valley currents. In constant-voltage
mode, the charger regulates the peak and valley of the
output ripple.
Charge Cycle
The MAX1925/MAX1926 initiate PREQUAL when one of
the following occurs:
•When an external power source is connected
•The cell voltage falls to 4V after charging is finished
•EB is toggled
•Input power is cycled
Some Li+ cells can be damaged when fast-charged
from a completely dead state. Moreover, an over-dis-
charged cell may indicate a dangerous abnormal cell
condition. As a built-in safety feature, the MAX1925/
MAX1926 use a two-level prequalification charge to
determine if it is safe to charge. When the cell voltage is
less than 2V, the cell is charged from an internal
Switch-Mode 1-Cell Li+ Chargers
10 ______________________________________________________________________________________
RESET
(ACON IS LOW)
OFF
(ACON HIGH Z)
SHDN
(ACON HIGH Z)
ANY STATE
(INCLUDING FAULT)
EN HIGH AND
IN WITHIN ACON
WINDOW*
EN LOW OR
IN OUTSIDE ACON
WINDOW*
VBATT > 2V
(DEBOUNCED)
VBATT > 3V
(DEBOUNCED)
FULL TOPOFF
(CHG LED OFF)
DONE
(CHG LED OFF)
KELVIN
(CHG LED OFF)
F_PQ1
F_VB
tPREQUAL1 TIMES OUT
BEFORE VBATT > 2V
F_PQ2
F_FULL
tPREQUAL2 TIMES OUT
BEFORE VBATT > 3V
tFULLCHG TIMES OUT
BEFORE ENTERING
FULL TOP-OFF
STATE
10-SECOND
TIMER
FULL-CHARGE
TIMER TIMES OUT
VBATT < 4.2V
*SEE TABLE 2 FOR ACON WINDOW
FOR MAX1925/MAX1926
VBATT > 4.35V
AND IN WITHIN ACON
WINDOW*
AND EN HIGH
VIN WITHIN ACON
WINDOW*
VIN < VBATT
VIN > VBATT
VIN < VBATT
VIN > VBATT AND VIN
OUTSIDE ACON WINDOW*
AUTOMATIC RESTART
BATT DROPS TO 4V
VBATT > 4.2V
F-TEMP
TEMP
IN RANGE TEMP OUT
OF RANGE
VIN OUTSIDE ACON WINDOW*
PREQUAL1
(ICHG = 4mA)
(CHG LED ON)
PREQUAL2
(ICHG = C/10)
(CHG LED ON)
FASTCHG
(ICHG = C)
(CHG LED ON)
ICHG < C/8
(DEBOUNCED)
FAULT
(CHG LED BLINK AT 0.5Hz)
(EXT HIGH FET OFF)
Figure 2. MAX1925/MAX1926 State Diagram
linear 4mA current source (PREQUAL1). When the cell
voltage exceeds 2V, the cell is charged with 10% of the
programmed fast-charge current (IFASTCHG) until it
reaches 3V. When the cell voltage is above 3V, fast
charging occurs at the full set current. If the cell fails to
reach the next prequalification threshold before a set
time (see tPREQUAL1 and tPREQUAL2 in the Timer
Capacitor and Fault Modes section), charging stops, a
fault alarm is set, and the CHG output blinks.
Figures 2 and 3 show charging behavior typical Li+ cell.
The MAX1925/MAX1926 remain in fast-charge mode
until the battery voltage reaches regulation and the
charge current drops below 1/8th of IFASTCHG. The
charger then enters full topoff mode and the CHG LED
is turned off. In full topoff mode, the controller continues
to operate as in fast-charge mode, except that it
remains in constant-voltage mode (CVM) unless the bat-
tery voltage falls. After every tPREQUAL1 (see the Timer
Capacitor and Fault Modes section) the charger enters
the Kelvin state (for 2 CT clock cycles, 60ms with CCT =
100nF) where charge current is interrupted so that the
battery voltage can be accurately measured.
The MAX1925/MAX1926 do not enter done mode until
tFULLCHG has been reached. If the battery is removed
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
______________________________________________________________________________________ 11
CVM
1V
2V
3V
4V
4mA C/10 CC/8
PREQUAL1
CHARGE CURRENT
CELL VOLTAGE
1V
2V
3V
4V
CHARGE CURRENT
2mA
C/10
C
CELL VOLTAGE
t
tPREQUAL2 = t/16
4.2V
C/8
FAULT
NORMAL VOLTAGE
PROFILE
NORMAL
CURRENT
PROFILE
FASTCHG
FULL TOPOFF
CCM
CVM
CCM
FASTCHG
PREQUAL2
FAULT
tPREQUAL1 = t/1024 CHARGE TIME (tFULLCHG = 3 HOURS WITH CCT = 100nF)
CHG LED OFF
CHG LED OFF
Figure 3. Charging Current and Voltage Timing Diagrams
MAX1925/MAX1926
and a new battery is connected during either fast-charge
or full topoff modes, the charger begins with full charge
current without prequalification unless the part is reset.
Detect battery insertion by connecting THRM to a ther-
mistor on the battery, if a thermistor is used, or to a 10kΩ
resistor linked to a battery door mechanism.
Constant-Current Mode (CCM)
When the battery voltage is below 4.2V, the
MAX1925/MAX1926 regulate the charging current by
controlling the peak and valley inductor currents. When
the inductor current exceeds the 158mV/RSET, the
MAX1925/MAX1926 turn the external PFET off. When
the inductor current falls below 128mV/RSET, the
MAX1925/MAX1926 turns the external PFET on, but
only if the battery voltage is below regulation. The maxi-
mum cell charging current is programmed by selecting
the external RSET (see Figure 1) resistor connected
between BATT and CS. Select the external resistor
value using RSET = 142mV/IFASTCHG.
The accuracy of the charge current is a function of
input voltage, battery voltage, inductance, and com-
parator delay (300ns typ). Determine the charge-cur-
rent error according to the following equation:
where ∆ICHG is the charge-current error, and tIDelay is
the current-sense comparator delay.
For this reason choose L for an on-time and off-time greater
than 2 ✕tIDelay to minimize error in the charging current.
Constant-Voltage Mode (CVM)
In constant-voltage mode (CVM), the controller regu-
lates the peak and valley of the output ripple. The maxi-
mum cell voltage is regulated to 4.2V. If, for any reason,
the cell voltage exceeds 4.35V, a fault alarm is set, the
CHG output blinks, and the PFET power switch is held
off. The charger can then be restarted only by cycling
input power or the EN input.
Indication of Charge Completion (
CHG
)
The CHG output is a 10mA current-sink output that indi-
cates the cell’s charging status. Connect an LED from
IN to CHG for a visible indicator. Alternatively, a pullup
resistor (typically 200kΩ) from a logic supply to CHG
provides a logic-level output. Table 1 relates the status
of the LED to the condition of the charger and battery.
ACON
Output
The ACON open-drain output indicates when usable
power is applied to IN. In the MAX1926 when VIN
exceeds ACON threshold (nominally 4.25V with IN ris-
ing—see the Electrical Characteristics table), ACON
goes low. In the MAX1925, ACON goes low when the
input voltage is between 4.5V and 6.1V (see Table 2).
Re-Initiating a Charging Cycle
The MAX1925/MAX1926 feature automatic restart that
resumes charging when the cell voltage drops to 4V and
tFULL_CHG is completed. By automatically resuming charg-
∆=
−×
()
×
×
IVV t
L
CHG IN BATT IDelay
2
2
Switch-Mode 1-Cell Li+ Chargers
12 ______________________________________________________________________________________
STATE CONDITION CHG
OFF EN low or no battery or input power High impedance (LED off)
PREQUAL1 Charge current = 4mA until BATT reaches 2V. Low (LED on)
PREQUAL2 Charge current = C/10 until BATT reaches 3V. Low (LED on)
FAST CHARGE Charge current = C = 142mV/RSET. Low (LED on)
FULL CHARGE Charge current has fallen to C/8. High impedance (LED off)
FAULT PREQUAL1 BATT does not reach 2V before PREQUAL1 timeout.
FAULT PREQUAL2 BATT does not reach 3V before PREQUAL2 timeout.
FAULT FULL Charge current does not drop to C/8 before FULL CHARGE
timeout.
Blinking. LED on 50% fBLINK (0.5Hz). Can
only be cleared by cycling input power,
THRM, or EN.
FAULT BATT
VOLTAGE Battery voltage has exceeded 4.35V. Blinking. LED on 50% fBLINK (0.5Hz).
MAX1925—High impedance (LED off)
FAULT TEMP Temperature has risen above +50°C or fallen below 0°C.
Temp fault clears by itself. MAX1926—Blinking (LED on 50% 0.5Hz)
NONE Initial power-up or enable with battery not present. Blinking at rapid rate as charger cycles
through RESET, PREQUAL1, and DONE.
Table 1. CHG Output States
ing when the battery voltage drops, the MAX1925/MAX1926
ensure that the cell does not remain partially charged after
use when charger power is available.
Charging also restarts if input power is cycled or if the
charger is restarted by the EN or THRM input. If a new
battery is inserted, the charger must be restarted. If the
THRM functionality is used, the charger is automatically
restarted upon battery insertion. When THRM is not
used, toggle EN or connect THRM through a resistor to
be grounded with a battery-door latch switch.
Applications Information
Timer Capacitor and Fault Modes
The on-chip timer checks charge progress and issues
an alarm signal through a blinking CHG output when one
of the safety timers times out (see Table 1). All timers are
set by one external capacitor at CT. A 100nF value sets
the full-charge timer (tFULLCHG) to 3 hours, the
tPREQUAL1 timer to (tFULLCHG)/1088 (10s), and the
tPREQUAL2 timer at (tFULLCHG)/17 (10.67 minutes).
If the charger enters full-charge state (after the charging
current has fallen below C/8) before the full-charge timer
expires, no fault occurs, but if the timer expires before full
charge is reached, a fault is indicated (see Table 1).
A fault is also indicated if the battery voltage exceeds
4.35V. When the cell voltage exceeds 4.35V a fault alarm
is set, the CHG output blinks, and the PFET turns off.
To restart the charger after a fault occurs, the fault state
must be cleared by toggling EN, or by cycling input power
at IN (see Figure 1). Temperature faults do not need to be
cleared by EN. The MAX1925/MAX1926 resume charging
after the temperature returns to within the set window.
Inductor Selection
Because the MAX1925/MAX1926 is hysteretic, the con-
stant-current mode switching frequency is a function of
the inductance, sense resistance, and current-sense
hysteresis (30mV, from the Electrical Characteristics).
To minimize charge-current error:
where ∆ICHG is the acceptable charge-current error
and should usually be less than 1/4th the full charge
current. tIDelay is the current-sense comparator delay
(300ns typical). Calculate L for VIN = VIN,MAX, VBATT =
VBATT,MIN, with positive ∆ICHG and VIN = VIN,MIN,
VBATT = VBATT,MAX, with negative ∆ICHG. Use the larg-
er calculated value for L.
The resulting switching frequency in CCM is:
Choose an inductor with an RMS and saturation current
rating according to the following equation:
where VIPK is the peak current-sense threshold (158mV typ).
Output Capacitor Selection
The ESR of the output capacitor influences the switch-
ing frequency of the charger during voltage regulation
IV
R
VV t
L
SAT RMS IPK
SET
IN BATT IDelay
/>+
()
−
fILtV
VV t
ILt VV
V
Switch HYST IDelay BATT
IN BATT IDelay
HYST IDelay IN BATT
BATT
>×+ +× +
×+ ×
−
−−
2
1
()
LVV t
I
IN BATT Delay
CHG
>×
()
×
×∆
−2
2
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
______________________________________________________________________________________________________ 13
PART VIN ACON CHARGING CHG LED
VIN > VACON threshold (4.5V nom) and < 6.1V, and
VIN > VBATT LOW YES ON
(until charge complete)
VIN > VACON threshold and VIN < VBATT
(Note: This state should never occur) High Z NO OFF
MAX1925
VIN < VACON threshold High Z NO OFF
VIN > VACON threshold (4.25V nom) and VIN > VBATT LOW YES ON
(until charge complete)
VIN > VACON threshold and VIN < VBATT
(Note: This state should never occur) High Z NO OFF
MAX1926
VIN < VACON Threshold High Z NO OFF
Table 2. ACON Behavior vs. VIN
mode. To ensure stable transition from CCM to CVM
choose a capacitor with the following ESR:
where VVHIST is the voltage hysteresis (15mV typ) and
VIHIST is the current-sense threshold hysteresis (typi-
cally 30mV). Tantalum capacitors are recommended.
However a ceramic capacitor (typically 10µF) with a
series resistor can also be used.
MOSFET Selection
The MAX1925/MAX1926 drive an external P-channel
MOSFET’s gate from IN to GND. Choose a P-channel
MOSFET with a |VDS,MAX| > VIN. Since EXT drives from
rail to rail the MOSFET must also be rated for |VGS,MAX|
> VIN. At the lower operating frequencies and currents
for typical MAX1925/MAX1926 applications resistive
and diode losses dominate switching losses. For this
reason choose a MOSFET with a low RDSON. The resis-
tive losses are:
PResistive_losses ≅D ✕ICHG2✕RDSON + ICHG2✕
(RSET + RL)
where D is the operating duty cycle (VOUT/VIN) and RL
is the inductor resistance. The MOSFET’s power dissi-
pation must exceed D ✕ICHG2✕RDSON.
Diode Selection
In the event of a short-circuited source, the body diode
inherent in the external PFET allows the cell to dis-
charge. To prevent this and to protect against negative
input voltages, add a Schottky or silicon diode between
the power source and IN.
The MAX1925/MAX1926 use a diode for catching the
inductor current during the off cycle. Select a Schottky
diode with a current rating greater than VIPK/RSET and
a voltage rating greater than VIN.
Dropout Behavior
The MAX1925/MAX1926 regulate charging current by
ramping inductor current between upper and lower
thresholds, typically 128mV and 158mV across RSET.
This results in an average current of 142mV/RSET. At
input voltages near dropout (4.6V at IN for the typical
circuit), the inductor current ramp waveform becomes
somewhat flattened as inductor, MOSFET, input diode,
and battery resistance limit inductor current. When the
inductor current waveform flattens, it’s average value
rises with respect to the upper and lower current
thresholds. This creates a slight peak (about 5%) in
charging current at high battery voltages as seen in the
Charging Current vs. Battery Voltage plot in the Typical
Operating Characteristics. Charging current is still con-
trolled in dropout and the charger operates normally.
The dropout current peak can be minimized by reduc-
ing MOSFET and inductor resistance, as well as for-
ward voltage in the input diode.
Thermistor Interface
An external thermistor inhibits charging by setting a
fault flag when the cell is cold (<0°C) or hot (>+50°C).
The THRM time-multiplexes two sense currents to test
for both hot and cold qualification. Connect the thermis-
tor between THRM and GND. If no temperature qualifi-
cation is desired, replace the thermistor with a 10kΩ
resistor connected through the battery-latch mecha-
nism. The thermistor should be 10kΩat +25°C and
have a negative temperature coefficient, as defined by
the expression below:
Table 3 shows nominal fault detection temperatures
that result from a wide range of available thermistor
temperature curves.
For a given thermistor characteristic, it is possible to
adjust the fault-detection temperatures by adding a
resistor in series with the thermistor or a parallel resistor
from THRM to GND.
Chip Information
TRANSISTOR COUNT: 5722
PROCESS: BiCMOS
RR T
TCe
=+
°×
−
25
1
273
1
298
β
RV
VR
ESR VHIST
IHIST SET
>×
THERMISTOR BETA 3000 3250 3500 3750
Resistance at +25°C 10000Ω10000Ω10000Ω10000Ω
Resistance at +50°C 4587.78Ω4299.35Ω4029.06Ω3775.75Ω
Resistance at 0°C 25140.55Ω27148.09Ω29315.94Ω31656.90Ω
Nominal Hot Trip Temperature 55.14°C 52.60°C 50.46°C 48.63°C
Nominal Cold Trip Temperature -3.24°C -1.26°C 0.46°C 1.97°C
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
14 ______________________________________________________________________________________
Table 3. Fault Temperature for Different Thermistors
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
______________________________________________________________________________________ 15
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.)
21-0139 A
PACKAGE OUTLINE
12,16,20,24L QFN THIN, 4x4x0.8 mm
MAX1925/MAX1926
Switch-Mode 1-Cell Li+ Chargers
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information (continued)
(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.)
A21-0139
PACKAGE OUTLINE
12,16,20,24L QFN THIN, 4x4x0.8 mm
