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19-2556; Rev 1; 10/02
KIT ATION EVALU ABLE AVAIL
Switch-Mode 1-Cell Li+ Chargers
General Description Features
o Small (4mm 4mm) Package o 4.25V to 12V Input Range (MAX1926) o Overvoltage Lockout at 6.1V (MAX1925) o 0.75% Battery Regulation Voltage o Set Charge Current with One Resistor o Automatic Input Power Sense o LED (or Logic-Out) Charge Status and Fault Indicator o Programmable Safety Timer o Autorestart at Cell = 4V o Thermistor Monitor Input
MAX1925/MAX1926
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 (-40C to +85C). An evaluation kit is available to speed design.
Ordering Information
PART TEMP RANGE -40C to +85C -40C to +85C PINPACKAGE 12 Thin QFN 4mm x 4mm 12 Thin QFN 4mm x 4mm INPUT CHARGING RANGE 4.5V to 6.1V 4.25V to 12V
Applications
Digital Cameras Self-Charging Battery Packs PDAs Cradle Chargers
MAX1925ETC MAX1926ETC
Typical Operating Circuit Pin Configuration
IN 12 INP 11 EXT 10
IN INP EXT PGND CS INPUT 4.5V TO 12V
CHG EN THRM
1 2 3
9
PGND GND BATT
LED CHG BATT CT
MAX1925 MAX1926
8 7
MAX1925 MAX1926
GND
THRM
4 ACON
5 CT
6 CS
ON OFF
EN
ACON
1-CELL Li+ BATTERY
THIN QFN
________________________________________________________________ Maxim Integrated Products
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Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926
ABSOLUTE MAXIMUM RATINGS
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 = +70C) Exposed Paddle Soldered to Board (derate 16.9mW/C above +70C) .............................1349mW Exposed Paddle Unsoldered (derate 9mW/C above +70C) ....................................721mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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.
ELECTRICAL CHARACTERISTICS
(VPGND = VGND = 0, VINP = VIN = V CHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10k to GND, CCT = 100nF, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Supply Voltage Range CONDITIONS VINP, VIN (MAX1925 does not charge above 6.1V) VINP, VIN rising ACON Trip Point VINP, VIN falling INP, IN Shutdown Threshold EN Input Resistance EN Leakage Current EN Logic Input High Threshold EN Logic Input Low Threshold BATT + CS Input Current (Total Current into BATT and CS) CS Input Current DONE state, VBATT = 4.1V EN = GND (Note 1) VBATT = VINP = VIN = 4V, shutdown (Note 1) Charging EN = GND IN + INP Total Input Current VBATT = VINP = VIN = 4V, shutdown VBATT = 4.1V; charging VBATT = 4.3V; done VOLTAGE LOOP Voltage Loop Set Point Voltage Loop Hysteresis BATT Prequal1 Voltage Threshold BATT Prequal2 Voltage Threshold 1.9 2.85 4.1685 4.2000 15 2 3 2.1 3.15 4.2315 V mV V V 5 25 2 2 39 5 2 8 10 10 8 MAX1925 MAX1925 MAX1926 MAX1925 MAX1926 Rising Falling MIN 4.5 4.30 4.00 4.17 3.90 5.8 5.3 125 -1 2 0.8 50 10 10 A mA A mA A 300 4.50 4.25 4.30 4.15 TYP MAX 12.0 4.78 4.50 4.43 4.40 6.4 5.9 550 +1 V k A V V V UNITS V
MAX1926 internally pulled up to 3V MAX1925
2
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Switch-Mode 1-Cell Li+ Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VPGND = VGND = 0, VINP = VIN = V CHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10k to GND, CCT = 100nF, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Restart Threshold BATT Voltage Fault Threshold CURRENT LOOP CS - BATT Sense Threshold Prequal1 Charge Current Prequal2 CS - BATT Sense Threshold Current Threshold for Full-Battery Indication DRIVER FUNCTIONS EXT Sink/Source Current EXT On-Resistance Nominal Switching Frequency TIMER FUNCTIONS Full-Time Timeout - tFULL Prequal1 Timeout Prequal2 Timeout Timer Accuracy CHG Output Low Current CHG Output High Leakage Current CHG Blink Rate - Fault ACON High Leakage ACON Sink Current THERMISTOR MONITOR (Note 2) THRM Sense Current for Hot Qualification THRM Sense Current for Cold Qualification THRM Sense-Voltage Trip Point (Note 3) 344.1 47.58 1.386 352.9 48.80 1.400 361.7 50.02 1.414 A A V CCT = 100nF CCT = 100nF CCT = 100nF CCT = 100nF for 3 hours VCHG = 1V VCHG = 12V Fault state (50% duty cycle), CCT = 100nF VACON = 12V VACON = 0.4V 2 0.5 0.01 1.00 -15 7 10 3.02 tFULL/1088 (10s) tFULL/17 (10.67 min) +15 14 1 hours s min % mA A Hz A mA EXT high or low VBATT = 3.6V, L =10H 1 5 235 12 A kHz VCS - VBATT, average value Rise/fall hysteresis VBATT < 2V Average value, 2V < VBATT < 3V (charge current is C/10) Rise/fall hysteresis, 2V < VBATT < 3V ILOAD falling, as percentage of fast charge current 6 3 132 142 30 4 14 12 12 20 6 152 mV mA mV % CONDITIONS Charging restarts when BATT falls to this point If BATT exceeds this threshold, EXT is high (external MOSFET is off) and CHG blinks MIN 3.92 4.275 TYP 4.00 4.350 MAX 4.08 4.425 UNITS V V
MAX1925/MAX1926
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Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926
ELECTRICAL CHARACTERISTICS
(VPGND = VGND = 0, VINP = VIN = V CHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10k to GND, CCT = 100nF, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 4)
PARAMETER Supply Voltage Range CONDITIONS VINP, VIN (MAX1925 does not charge above 6.1V) VINP, VIN rising ACON Trip point VINP, VIN falling INP, IN Shutdown Threshold EN Input Resistance EN Leakage Current EN Logic Input High Threshold EN Logic Input Low Threshold BATT + CS Input Current (Total Current into BATT and CS) DONE State, VBATT = 4.1V EN = GND (Note 2) VBATT = VINP = VIN = 4V, shutdown (Note 1) EN = GND IN + INP Total Input Current VBATT = VINP = VIN = 4V, shutdown VBATT = 4.1V, charging VBATT = 4.3V, done VOLTAGE LOOP Voltage Loop Set Point BATT Prequal1 Voltage Threshold BATT Prequal2 Voltage Threshold Restart Threshold BATT Voltage Fault Threshold CURRENT LOOP CS - BATT Sense Threshold Prequal1 Charge Current Current Threshold for Full-Battery Indication DRIVER FUNCTIONS EXT On-Resistance TIMER FUNCTIONS Timer Accuracy CHG Output Low Current CCT = 100nF for 3 hours V CHG = 1V -16 7 +16 14 % mA EXT high or low 12 VCS - VBATT, average value VBATT < 2V ILOAD falling, as percentage of fast charge current 127 3 4 157 6 20 mV mA % Charging restarts when BATT falls to this point If BATT exceeds this threshold, EXT is high (external MOSFET is off) and CHG blinks 4.158 1.9 2.85 3.92 4.275 4.242 2.1 3.15 4.08 4.425 V V V V V MAX1925 MAX1925 MAX1926 MAX1925 MAX1926 Rising Falling MIN 4.5 4.30 4.0 4.17 3.9 5.8 5.3 125 -1 2 0.8 50 10 10 8 10 10 8 mA A mA A TYP MAX 12.0 4.78 4.5 4.43 4.4 6.4 5.9 550 +1 V k A V V V UNITS V
MAX1926 internally pulled up to 3V MAX1925
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Switch-Mode 1-Cell Li+ Chargers
ELECTRICAL CHARACTERISTICS (continued)
(VPGND = VGND = 0, VINP = VIN = V CHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10k to GND, CCT = 100nF, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 4)
PARAMETER CHG Output High Leakage Current ACON High Leakage ACON Sink Current THERMISTOR MONITOR THRM Sense Current for Hot Qualification THRM Sense Current for Cold Qualification THRM Sense-Voltage Trip Point (Note 3) 342 47.3 1.379 363 50.3 1.421 A A V V CHG = 12V V ACON = 12V V ACON = 0.4V 2 CONDITIONS MIN TYP MAX 1 1 UNITS A A mA
MAX1925/MAX1926
Note 1: When the AC adapter is unplugged or if the charger is shut down, BATT drain is less than 10A. Note 2: These specifications guarantee the thermistor interface detects a fault at the correct temperature (0C to +5C cold temperature and 45C to +50C hot temperature) with Philips NTC Thermistor Series 640-6, 2322-640-63103, 10.0K at +25C, 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 -40C are guaranteed by design and not production tested.
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Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926
Typical Operating Characteristics
(Circuit of Figure 1, VSUPPLY = 5V, VBATT = 4V, TA = +25C, unless otherwise noted.)
CHARGE-CURRENT ACCURACY vs. SUPPLY VOLTAGE
MAX1925/26 toc01
CHARGE CURRENT vs. BATTERY VOLTAGE
MAX1925/26 toc02
FULL-BATTERY VOLTAGE vs. SUPPLY VOLTAGE
4.208 FULL-BATTERY VOLTAGE (V) 4.206 4.204 4.202 4.200 4.198 4.196 4.194
MAX1925/26 toc03
20 CHARGE-CURRENT ACCURACY (%) 15 10 5 0 -5 -10 -15 -20 5 6 7 8 9 10 11 VBATT = 4V
1.2 VIN = 12V 1.0 CHARGE CURRENT (A) 0.8 0.6 0.4 0.2 0 SEE THE DROPOUT BEHAVIOR SECTION VIN = 10V
4.210
VIN = 8V
VIN = 5V
12
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4
5
6
7
8
9
10
11
12
VSUPPLY (V)
BATTERY VOLTAGE (V)
VSUPPLY (V)
SHUTDOWN BATTERY CURRENT vs. BATTERY VOLTAGE
MAX1925/26 toc04
EFFICIENCY vs. SUPPLY VOLTAGE
MAX1925/26 toc05
EFFICIENCY vs. BATTERY VOLTAGE
95 90 EFFICIENCY (%) 85 80 75 70 VIN = 12V VIN = 5V VIN = 8V
MAX1925/26 toc06
4.0 SHUTDOWN BATTERY CURRENT (A) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 3.0 3.2 3.4 3.6 VBATT (V) 3.8 4.0
100 95 VBATT = 4V EFFICIENCY (%) 90 85 80 75 70
100
65 60 5 6 7 8 9 10 11 12 3.0 3.2 3.4 3.6 3.8 4.0 4.2 VSUPPLY (V) BATTERY VOLTAGE (V)
4.2
SWITCHING FREQUENCY vs. INDUCTANCE
MAX1925/26 toc07
SWITCHING FREQUENCY vs. RSENSE
MAX1925/26 toc08
SWITCHING FREQUENCY vs. SUPPLY VOLTAGE
450 SWITCHING FREQUENCY (kHz) 400 350 300 250 200 150 100 50 0 VBATT = 4V
MAX1925/26 toc09
1000 VIN = 12V SWITCHING FREQUENCY (kHz)
1000
500
SWITCHING FREQUENCY (kHz)
VIN = 12V
VIN = 8V 100 VIN = 5V
100
VIN = 8V VIN = 5V
10 1
VBATT = 4V 10 INDUCTANCE (H) 100
10
VBATT = 4V 0.01 0.10 RSENSE () 1.00
5
6
7
8
9
10
11
12
VSUPPLY (V)
6
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Switch-Mode 1-Cell Li+ Chargers
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VSUPPLY = 5V, VBATT = 4V, TA = +25C, unless otherwise noted.)
MAX1925/MAX1926
FREQUENCY vs. BATTERY VOLTAGE
MAX1925/26 toc10
TIMER vs. CT CAPACITANCE
MAX1925/26 toc11
CHARGE CURRENT vs. TIME
MAX1925/26 toc12
500 450 SWITCHING FREQUENCY (kHz) 400 350 300 250 200 150 100 50 0 2.5 3.0 3.5 VIN = 5V VIN = 12V VIN = 8V
1000 tFULLCHG tPREQUAL2
1.2 1.0 CHARGE CURRENT (A) 0.8 0.6 0.4 0.2 0
100 TIMER (MINUTES)
10
1 tPREQUAL1
0.1
0.01 4.0 10 100 CT CAPACITANCE (nF) 1000 BATTERY VOLTAGE (V)
0
0.5
1.0
1.5 TIME (h)
2.0
2.5
3.0
BATTERY VOLTAGE vs. TIME
MAX1925/26 toc13
SWITCHING WAVEFORM
MAX1925/26 toc14
4.5 4.4 BATTERY VOLTAGE (A) 4.3 4.2
1.2A 1A 0.8A 10V
VIN = 5V INDUCTOR CURRENT
4.1 4.0 3.9 3.8 0 0.5 1.0 1.5 TIME (h) 2.0 2.5 3.0
0V
VLX
100mV/ div 4s/div
VBATT
SWITCHING WAVEFORM
1.4A 1.2A 1A 0.8A 10V 0V VLX 1A VBATT INDUCTOR CURRENT
MAX1925/26 toc15
CHARGE-ENABLE REPSONSE
MAX1925/26 toc16
VIN = 12V 5V 0V EN
100mV/ div 1s/div
0A
INDUCTOR CURRENT
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Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926
Pin Description
PIN 1 NAME CHG FUNCTION 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. 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 300k resistor. Thermistor Input. Monitors external thermistor (10k at +25C). When external temperature is lower than 0C or above +50C, 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). Power-OK Indicator Output. Open-drain output goes low when AC adapter power is valid. See Table 2 for ACON states. 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) Charge-Current Sense Input. 142mV nominal regulation threshold. CS is high impedance during shutdown. Battery-Sense Input. Also negative side of charge-current sense. BATT is high impedance during shutdown. Ground Power Ground 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. Supply Voltage Input Supply-Sense Input. Connect IN to INP.
2
EN
3
THRM
4
ACON
5
CT
6 7 8 9 10 11 12
CS BATT GND PGND EXT INP IN
8
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Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926
D1 POWER SOURCE D3 LED C3 10F Q1 D2 L1 10H IN 4.5V FOR MAX1926 4.25V FOR MAX1925 10mA ACON PREQUAL1 4mA CS 6.1V MAX1925 ONLY INP HV DRIVER REF/8 C2 0.1F OSC PGND REF/10 REF ACON REF BATT > 2V EN STATE MACHINE AND TIMERS REF BATT > 3V REF TEMP FAULT 49A I > 12% BATT > 2V BATT RSET 0.14 ACON INP EXT
CHG
MAX1925 MAX1926
C2 10F
353A
REF BATT > 4.35V REF 3V_ANA ACON 3V_DIG REGULATOR
THRM 10k
IN REF
GND
PGND
Figure 1. Functional Diagram
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9
Switch-Mode 1-Cell Li+ Chargers 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 monitoring. The devices provide outputs to indicate charge status (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 battery voltage reaches 4.2V. When the battery voltage has reached 4.2V, the charger operates in constantvoltage 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-discharged 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
VIN OUTSIDE ACON WINDOW*
VIN > VBATT AND VIN OUTSIDE ACON WINDOW*
VIN WITHIN ACON WINDOW*
RESET (ACON IS LOW) EN HIGH AND IN WITHIN ACON WINDOW*
OFF (ACON HIGH Z)
EN LOW OR IN OUTSIDE ACON WINDOW* TEMP IN RANGE F-TEMP
ANY STATE (INCLUDING FAULT)
VBATT > 4.35V TEMP OUT AND IN WITHIN ACON OF RANGE WINDOW* AND EN HIGH
VIN > VBATT SHDN (ACON HIGH Z)
VIN < VBATT
PREQUAL1 (ICHG = 4mA) (CHG LED ON) VBATT > 2V (DEBOUNCED) PREQUAL2 (ICHG = C/10) (CHG LED ON) VBATT > 3V (DEBOUNCED) FASTCHG (ICHG = C) (CHG LED ON) ICHG < C/8 (DEBOUNCED)
tPREQUAL1 TIMES OUT BEFORE VBATT > 2V F_PQ1
F_VB
VIN < VBATT
FAULT (CHG LED BLINK AT 0.5Hz) (EXT HIGH FET OFF) tPREQUAL2 TIMES OUT BEFORE VBATT > 3V F_PQ2 F_FULL tFULLCHG TIMES OUT BEFORE ENTERING FULL TOP-OFF STATE
AUTOMATIC RESTART BATT DROPS TO 4V
FULL TOPOFF (CHG LED OFF) VBATT < 4.2V *SEE TABLE 2 FOR ACON WINDOW FOR MAX1925/MAX1926 DONE (CHG LED OFF)
10-SECOND TIMER KELVIN (CHG LED OFF)
VBATT > 4.2V
FULL-CHARGE TIMER TIMES OUT
Figure 2. MAX1925/MAX1926 State Diagram 10 ______________________________________________________________________________________
Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926
FULL TOPOFF 4V
CHG LED OFF CVM
PREQUAL1 3V PREQUAL2
FASTCHG
CCM
2V
CELL VOLTAGE
1V
4mA
C/10
C/8 CHARGE CURRENT
C
CVM 4V NORMAL VOLTAGE PROFILE
4.2V
CHG LED OFF
CHARGE CURRENT
CELL VOLTAGE
3V CCM C 2V FAULT FASTCHG NORMAL CURRENT PROFILE FAULT
C/8 C/10
1V
2mA tPREQUAL2 = t/16 tPREQUAL1 = t/1024 CHARGE TIME (tFULLCHG = 3 HOURS WITH CCT = 100nF) t
Figure 3. Charging Current and Voltage Timing Diagrams
linear 4mA current source (PREQUAL1). When the cell voltage exceeds 2V, the cell is charged with 10% of the programmed fast-charge current (I FASTCHG) 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 t PREQUAL1 and t PREQUAL2 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 battery 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
11
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Switch-Mode 1-Cell Li+ Chargers 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 thermistor on the battery, if a thermistor is used, or to a 10k resistor linked to a battery door mechanism. 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 regulates the peak and valley of the output ripple. The maximum 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.
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/R SET , the MAX1925/MAX1926 turn the external PFET off. When the inductor current falls below 128mV/R SET , the MAX1925/MAX1926 turns the external PFET on, but only if the battery voltage is below regulation. The maximum 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 comparator delay (300ns typ). Determine the charge-current error according to the following equation: ICHG =
Indication of Charge Completion (CHG)
The CHG output is a 10mA current-sink output that indicates the cell's charging status. Connect an LED from IN to CHG for a visible indicator. Alternatively, a pullup resistor (typically 200k) 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 V IN exceeds ACON threshold (nominally 4.25V with IN rising--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).
(VIN - 2 x VBATT ) x tIDelay
2 xL
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-
where ICHG is the charge-current error, and tIDelay is the current-sense comparator delay.
Table 1. CHG Output States
STATE OFF PREQUAL1 PREQUAL2 FAST CHARGE FULL CHARGE FAULT PREQUAL1 FAULT PREQUAL2 FAULT FULL FAULT BATT VOLTAGE FAULT TEMP NONE CONDITION EN low or no battery or input power Charge current = 4mA until BATT reaches 2V. Charge current = C/10 until BATT reaches 3V. Charge current = C = 142mV/RSET. Charge current has fallen to C/8. BATT does not reach 2V before PREQUAL1 timeout. BATT does not reach 3V before PREQUAL2 timeout. Charge current does not drop to C/8 before FULL CHARGE timeout. Battery voltage has exceeded 4.35V. Temperature has risen above +50C or fallen below 0C. Temp fault clears by itself. Initial power-up or enable with battery not present. Blinking. LED on 50% fBLINK (0.5Hz). Can only be cleared by cycling input power, THRM, or EN. Low (LED on) Low (LED on) Low (LED on) High impedance (LED off) CHG High impedance (LED off)
Blinking. LED on 50% fBLINK (0.5Hz). MAX1925--High impedance (LED off) MAX1926--Blinking (LED on 50% 0.5Hz) Blinking at rapid rate as charger cycles through RESET, PREQUAL1, and DONE.
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Switch-Mode 1-Cell Li+ Chargers
Table 2. ACON Behavior vs. VIN
PART VIN VIN > VACON threshold (4.5V nom) and < 6.1V, and VIN > VBATT MAX1925 VIN > VACON threshold and VIN < VBATT (Note: This state should never occur) VIN < VACON threshold VIN > VACON threshold (4.25V nom) and VIN > VBATT MAX1926 VIN > VACON threshold and VIN < VBATT (Note: This state should never occur) VIN < VACON Threshold ACON LOW High Z High Z LOW High Z High Z CHARGING YES NO NO YES NO NO CHG LED ON (until charge complete) OFF OFF ON (until charge complete) OFF OFF
MAX1925/MAX1926
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.
Inductor Selection
Because the MAX1925/MAX1926 is hysteretic, the constant-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: L>
(VIN - 2 x VBATT ) x tDelay
2 x ICHG
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 (t FULLCHG ) to 3 hours, the t PREQUAL1 timer to (t FULLCHG )/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.
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 = V BATT,MIN , with positive I CHG and V IN = V IN,MIN , VBATT = VBATT,MAX, with negative ICHG. Use the larger calculated value for L. The resulting switching frequency in CCM is: IHYST x L + tIDelayVBATT fSwitch > + 2 x tIDelay + VIN - VBATT IHYST x L + tIDelay x (VIN - VBATT ) -1 VBATT Choose an inductor with an RMS and saturation current rating according to the following equation:
(VIN - VBATT )tIDelay V ISAT / RMS > IPK + RSET L
where VIPK is the peak current-sense threshold (158mV typ).
Output Capacitor Selection
The ESR of the output capacitor influences the switching frequency of the charger during voltage regulation
______________________________________________________________________________________________________ 13
Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926
mode. To ensure stable transition from CCM to CVM choose a capacitor with the following ESR: V RESR > VHIST x RSET VIHIST where VVHIST is the voltage hysteresis (15mV typ) and VIHIST is the current-sense threshold hysteresis (typically 30mV). Tantalum capacitors are recommended. However a ceramic capacitor (typically 10F) with a series resistor can also be used. 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 controlled in dropout and the charger operates normally. The dropout current peak can be minimized by reducing MOSFET and inductor resistance, as well as forward voltage in the input diode.
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 resistive 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 dissipation must exceed D ICHG2 RDSON.
Thermistor Interface
An external thermistor inhibits charging by setting a fault flag when the cell is cold (<0C) or hot (>+50C). The THRM time-multiplexes two sense currents to test for both hot and cold qualification. Connect the thermistor between THRM and GND. If no temperature qualification is desired, replace the thermistor with a 10k resistor connected through the battery-latch mechanism. The thermistor should be 10k at +25C and have a negative temperature coefficient, as defined by the expression below: 1 1 - T + 273 298 RT = R25C x e 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.
Diode Selection
In the event of a short-circuited source, the body diode inherent in the external PFET allows the cell to discharge. 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. TRANSISTOR COUNT: 5722 PROCESS: BiCMOS
Chip Information
Table 3. Fault Temperature for Different Thermistors
THERMISTOR BETA Resistance at +25C Resistance at +50C Resistance at 0C Nominal Hot Trip Temperature Nominal Cold Trip Temperature 3000 10000 4587.78 25140.55 55.14C -3.24C 3250 10000 4299.35 27148.09 52.60C -1.26C 3500 10000 4029.06 29315.94 50.46C 0.46C 3750 10000 3775.75 31656.90 48.63C 1.97C
14
______________________________________________________________________________________
Switch-Mode 1-Cell Li+ Chargers
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.)
MAX1925/MAX1926
PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm
21-0139
A
______________________________________________________________________________________
15
Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926
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.)
PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm
21-0139
A
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 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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