19-2556; Rev 1; 10/02 KIT ATION EVALU E L B A AVAIL Switch-Mode 1-Cell Li+ Chargers Features ♦ Small (4mm ✕ 4mm) Package 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. ♦ Programmable Safety Timer Applications ♦ 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 ♦ Autorestart at Cell = 4V ♦ Thermistor Monitor Input Ordering Information PINPACKAGE INPUT CHARGING RANGE -40°C to +85°C 12 Thin QFN 4mm x 4mm 4.5V to 6.1V -40°C to +85°C 12 Thin QFN 4mm x 4mm 4.25V to 12V PART TEMP RANGE MAX1925ETC MAX1926ETC Digital Cameras Self-Charging Battery Packs PDAs Cradle Chargers Typical Operating Circuit Pin Configuration IN INP EXT 12 11 10 INPUT 4.5V TO 12V IN INP EXT PGND CS CHG 1 EN 2 9 PGND 8 GND LED BATT CHG THRM MAX1925 MAX1926 3 7 CT MAX1925 MAX1926 GND BATT THRM 4 ACON 5 6 CT CS ON OFF EN ACON 1-CELL Li+ BATTERY THIN QFN ________________________________________________________________ Maxim Integrated Products 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. 1 MAX1925/MAX1926 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. MAX1925/MAX1926 Switch-Mode 1-Cell Li+ Chargers 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 = +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 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 = 10kΩ to GND, CCT = 100nF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Supply Voltage Range CONDITIONS MIN TYP 4.30 4.50 4.78 MAX1926 4.00 4.25 4.50 MAX1925 4.17 4.30 4.43 MAX1926 3.90 4.15 4.40 Rising 5.8 6.4 Falling 5.3 5.9 VINP, VIN falling INP, IN Shutdown Threshold MAX1925 EN Input Resistance MAX1926 internally pulled up to 3V EN Leakage Current MAX1925 EN Logic Input High Threshold 125 300 -1 V V 550 kΩ +1 µA 2 V EN Logic Input Low Threshold IN + INP Total Input Current V MAX1925 ACON Trip Point CS Input Current UNITS 12.0 4.5 VINP, VIN rising BATT + CS Input Current (Total Current into BATT and CS) MAX VINP, VIN (MAX1925 does not charge above 6.1V) 0.8 DONE state, VBATT = 4.1V 25 50 EN = GND (Note 1) 2 10 VBATT = VINP = VIN = 4V, shutdown (Note 1) 2 10 V µA Charging 39 EN = GND 5 8 mA VBATT = VINP = VIN = 4V, shutdown 2 10 µA VBATT = 4.1V; charging µA 10 VBATT = 4.3V; done 5 8 4.2000 4.2315 mA VOLTAGE LOOP Voltage Loop Set Point 4.1685 Voltage Loop Hysteresis 15 V mV BATT Prequal1 Voltage Threshold 1.9 2 2.1 V BATT Prequal2 Voltage Threshold 2.85 3 3.15 V 2 _______________________________________________________________________________________ Switch-Mode 1-Cell Li+ Chargers (VPGND = VGND = 0, VINP = VIN = V CHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10kΩ to GND, CCT = 100nF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) MIN TYP MAX UNITS Restart Threshold PARAMETER Charging restarts when BATT falls to this point CONDITIONS 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 132 142 152 CURRENT LOOP CS - BATT Sense Threshold VCS - VBATT, average value Rise/fall hysteresis 30 Prequal1 Charge Current VBATT < 2V Prequal2 CS - BATT Sense Threshold Average value, 2V < VBATT < 3V (charge current is C/10) 14 Rise/fall hysteresis, 2V < VBATT < 3V 12 Current Threshold for Full-Battery Indication ILOAD falling, as percentage of fast charge current 3 6 4 12 6 mV mA mV 20 % DRIVER FUNCTIONS EXT Sink/Source Current 1 EXT On-Resistance EXT high or low Nominal Switching Frequency VBATT = 3.6V, L =10µH 5 A 12 Ω 235 kHz 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 CHG Output Low Current VCHG = 1V CHG Output High Leakage Current VCHG = 12V CHG Blink Rate - Fault Fault state (50% duty cycle), CCT = 100nF ACON High Leakage VACON = 12V ACON Sink Current VACON = 0.4V TIMER FUNCTIONS Full-Time Timeout - tFULL -15 7 10 +15 % 14 mA 1 µA 0.5 0.01 Hz 1.00 2 µA 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 1.386 1.400 1.414 V THRM Sense-Voltage Trip Point (Note 3) _______________________________________________________________________________________ 3 MAX1925/MAX1926 ELECTRICAL CHARACTERISTICS (continued) MAX1925/MAX1926 Switch-Mode 1-Cell Li+ Chargers ELECTRICAL CHARACTERISTICS (VPGND = VGND = 0, VINP = VIN = V CHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10kΩ to GND, CCT = 100nF, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (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 MIN MAX UNITS 4.5 TYP 12.0 V 4.78 MAX1925 4.30 MAX1926 4.0 4.5 MAX1925 4.17 4.43 MAX1926 3.9 4.4 Rising 5.8 6.4 Falling 5.3 5.9 125 550 kΩ -1 +1 µA INP, IN Shutdown Threshold MAX1925 EN Input Resistance MAX1926 internally pulled up to 3V EN Leakage Current MAX1925 EN Logic Input High Threshold 2 EN Logic Input Low Threshold BATT + CS Input Current (Total Current into BATT and CS) IN + INP Total Input Current V V 0.8 DONE State, VBATT = 4.1V V V 50 EN = GND (Note 2) 10 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 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 % 12 Ω DRIVER FUNCTIONS EXT On-Resistance EXT high or low TIMER FUNCTIONS Timer Accuracy CCT = 100nF for 3 hours CHG Output Low Current V CHG = 1V 4 -16 +16 % 7 14 mA _______________________________________________________________________________________ Switch-Mode 1-Cell Li+ Chargers (VPGND = VGND = 0, VINP = VIN = V CHG = 5V, VBATT = VCS = VEN = 4V, THRM = 10kΩ to GND, CCT = 100nF, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 4) PARAMETER CONDITIONS CHG Output High Leakage Current V CHG = 12V ACON High Leakage V ACON = 12V ACON Sink Current V ACON = 0.4V MIN TYP MAX UNITS 1 µA 1 2 µA mA THERMISTOR MONITOR THRM Sense Current for Hot Qualification 342 363 µA THRM Sense Current for Cold Qualification 47.3 50.3 µA 1.379 1.421 V THRM Sense-Voltage Trip Point (Note 3) 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 temperature 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. _______________________________________________________________________________________ 5 MAX1925/MAX1926 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (Circuit of Figure 1, VSUPPLY = 5V, VBATT = 4V, TA = +25°C, unless otherwise noted.) CHARGE-CURRENT ACCURACY vs. SUPPLY VOLTAGE 1.0 5 0 -5 VIN = 10V 0.8 SEE THE DROPOUT BEHAVIOR SECTION 0.4 4.208 0.2 6 7 8 9 10 11 4.204 4.202 4.200 4.198 4.194 3.0 12 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4 5 6 7 BATTERY VOLTAGE (V) VSUPPLY (V) SHUTDOWN BATTERY CURRENT vs. BATTERY VOLTAGE 95 VBATT = 4V EFFICIENCY (%) 3.0 2.5 2.0 1.5 10 11 100 90 95 85 80 VIN = 5V 90 EFFICIENCY (%) 3.5 9 12 EFFICIENCY vs. BATTERY VOLTAGE EFFICIENCY vs. SUPPLY VOLTAGE 100 MAX1925/26 toc04 4.0 8 VSUPPLY (V) MAX1925/26 toc05 5 4.206 4.196 0 -20 1.0 VIN = 8V 85 80 VIN = 12V 75 70 75 0.5 0 65 60 70 3.0 3.2 3.4 3.6 3.8 4.0 4.2 5 6 7 8 9 10 11 3.0 12 3.2 3.4 3.6 3.8 4.0 VSUPPLY (V) BATTERY VOLTAGE (V) SWITCHING FREQUENCY vs. INDUCTANCE SWITCHING FREQUENCY vs. RSENSE SWITCHING FREQUENCY vs. SUPPLY VOLTAGE VIN = 8V 100 VIN = 5V VBATT = 4V 10 1 VIN = 12V VIN = 5V 10 10 INDUCTANCE (µH) 100 VIN = 8V 100 450 400 350 VBATT = 4V 300 250 200 150 100 50 VBATT = 4V 0.01 500 4.2 MAX1925/26 toc09 1000 SWITCHING FREQUENCY (kHz) VIN = 12V SWITCHING FREQUENCY (kHz) 1000 MAX1925/26 toc07 VBATT (V) MAX1925/26 toc08 SHUTDOWN BATTERY CURRENT (µA) VIN = 5V 0.6 -10 -15 6 VIN = 8V 4.210 MAX1925/26 toc06 CHARGE CURRENT (A) 10 VIN = 12V MAX1925/26 toc03 1.2 FULL-BATTERY VOLTAGE (V) VBATT = 4V FULL-BATTERY VOLTAGE vs. SUPPLY VOLTAGE MAX1925/26 toc02 15 CHARGE CURRENT vs. BATTERY VOLTAGE MAX1925/26 toc01 CHARGE-CURRENT ACCURACY (%) 20 SWITCHING FREQUENCY (kHz) MAX1925/MAX1926 Switch-Mode 1-Cell Li+ Chargers 0 0.10 RSENSE (Ω) 1.00 5 6 7 8 9 VSUPPLY (V) _______________________________________________________________________________________ 10 11 12 Switch-Mode 1-Cell Li+ Chargers FREQUENCY vs. BATTERY VOLTAGE 300 250 VIN = 5V 200 tPREQUAL2 10 1 150 100 tPREQUAL1 0.1 MAX1925/26 toc12 1.0 CHARGE CURRENT (A) 350 tFULLCHG 100 TIMER (MINUTES) VIN = 8V 0.8 0.6 0.4 0.2 50 0 0 0.01 3.0 4.0 3.5 10 100 BATTERY VOLTAGE (V) 1000 0 0.5 BATTERY VOLTAGE vs. TIME 1.0 1.5 2.0 2.5 3.0 TIME (h) CT CAPACITANCE (nF) SWITCHING WAVEFORM MAX1925/26 toc14 4.5 MAX1925/26 toc13 2.5 4.4 BATTERY VOLTAGE (A) SWITCHING FREQUENCY (kHz) 400 1.2 MAX1925/26 toc11 VIN = 12V 450 CHARGE CURRENT vs. TIME TIMER vs. CT CAPACITANCE 1000 MAX1925/26 toc10 500 4.3 VIN = 5V 1.2A 1A INDUCTOR CURRENT 0.8A 4.2 10V 4.1 VLX 0V 4.0 VBATT 100mV/ div 3.9 3.8 0 0.5 1.0 1.5 2.0 2.5 4µs/div 3.0 TIME (h) SWITCHING WAVEFORM CHARGE-ENABLE REPSONSE MAX1925/26 toc15 1.4A MAX1925/26 toc16 VIN = 12V 1.2A 5V INDUCTOR CURRENT 1A 0V EN 0.8A VLX 10V 0V 1A VBATT 100mV/ div 0A INDUCTOR CURRENT 1µs/div _______________________________________________________________________________________ 7 MAX1925/MAX1926 Typical Operating Characteristics (continued) (Circuit of Figure 1, VSUPPLY = 5V, VBATT = 4V, TA = +25°C, unless otherwise noted.) Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926 Pin Description PIN 8 NAME FUNCTION 1 CHG 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. 2 EN 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). 4 ACON Power-OK Indicator Output. Open-drain output goes low when AC adapter power is valid. See Table 2 for ACON states. 5 CT 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 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 Power Ground Supply-Sense Input. Connect IN to INP. _______________________________________________________________________________________ Switch-Mode 1-Cell Li+ Chargers MAX1925/MAX1926 D1 POWER SOURCE C3 10µF D3 LED Q1 D2 L1 10µH CHG ACON IN INP EXT 4.5V FOR MAX1926 4.25V FOR MAX1925 10mA ACON PREQUAL1 4mA CS RSET 0.14Ω BATT > 2V 6.1V MAX1925 ONLY BATT MAX1925 MAX1926 INP C2 10µF HV DRIVER REF/8 OSC C2 0.1µF PGND REF/10 I > 12% REF ACON REF BATT > 2V REF STATE MACHINE AND TIMERS EN BATT > 3V REF 353µA TEMP FAULT 49µA REF BATT > 4.35V REF 3V_ANA 3V_DIG ACON THRM 10kΩ IN REGULATOR REF GND PGND Figure 1. Functional Diagram _______________________________________________________________________________________ 9 MAX1925/MAX1926 Switch-Mode 1-Cell Li+ Chargers mode, the charger regulates the peak and valley of the output ripple. 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 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 VIN > VBATT VIN < VBATT SHDN (ACON HIGH Z) VIN < VBATT AUTOMATIC RESTART BATT DROPS TO 4V 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) FULL TOPOFF (CHG LED OFF) VBATT < 4.2V *SEE TABLE 2 FOR ACON WINDOW FOR MAX1925/MAX1926 DONE (CHG LED OFF) ANY STATE (INCLUDING FAULT) VBATT > 4.35V TEMP OUT AND IN WITHIN ACON OF RANGE WINDOW* AND EN HIGH tPREQUAL1 TIMES OUT BEFORE VBATT > 2V F_VB F_PQ1 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 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 CHG LED OFF FULL TOPOFF CVM 4V CCM FASTCHG PREQUAL1 3V PREQUAL2 CELL VOLTAGE 2V 1V 4mA C/10 C/8 C CHARGE CURRENT CVM NORMAL VOLTAGE PROFILE 4V 4.2V CCM C FAULT FASTCHG 2V NORMAL CURRENT PROFILE FAULT CHARGE CURRENT CELL VOLTAGE CHG LED OFF 3V C/8 1V C/10 2mA t tPREQUAL2 = t/16 tPREQUAL1 = t/1024 CHARGE TIME (tFULLCHG = 3 HOURS WITH CCT = 100nF) 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 MAX1925/MAX1926 Switch-Mode 1-Cell Li+ Chargers 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 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/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 = (VIN − 2 × VBATT ) × tIDelay 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. 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 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 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). 2 ×L Re-Initiating a Charging Cycle where ∆ICHG is the charge-current error, and tIDelay is the current-sense comparator delay. 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- Table 1. CHG Output States STATE OFF 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. Blinking. LED on 50% fBLINK (0.5Hz). Can only be cleared by cycling input power, THRM, or EN. FAULT FULL Charge current does not drop to C/8 before FULL CHARGE timeout. FAULT BATT VOLTAGE Battery voltage has exceeded 4.35V. Blinking. LED on 50% fBLINK (0.5Hz). 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 12 CHG CONDITION EN low or no battery or input power Initial power-up or enable with battery not present. MAX1925—High impedance (LED off) Blinking at rapid rate as charger cycles through RESET, PREQUAL1, and DONE. ______________________________________________________________________________________ Switch-Mode 1-Cell Li+ Chargers ACON CHARGING CHG LED LOW YES ON (until charge complete) VIN > VACON threshold and VIN < VBATT (Note: This state should never occur) High Z NO OFF VIN < VACON threshold High Z NO OFF LOW YES ON (until charge complete) VIN > VACON threshold and VIN < VBATT (Note: This state should never occur) High Z NO OFF VIN < VACON Threshold High Z NO OFF PART VIN VIN > VACON threshold (4.5V nom) and < 6.1V, and VIN > VBATT MAX1925 VIN > VACON threshold (4.25V nom) and VIN > VBATT 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. 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. 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 × VBATT ) × tDelay 2 × ∆ICHG 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 × L + tIDelayVBATT fSwitch > + 2 × tIDelay + VIN − VBATT IHYST × L + tIDelay × (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 MAX1925/MAX1926 Table 2. ACON Behavior vs. VIN MAX1925/MAX1926 Switch-Mode 1-Cell Li+ Chargers mode. To ensure stable transition from CCM to CVM choose a capacitor with the following ESR: V RESR > VHIST × 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 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 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. 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. 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. 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 thermistor between THRM and GND. If no temperature qualification is desired, replace the thermistor with a 10kΩ resistor connected through the battery-latch mechanism. The thermistor should be 10kΩ at +25°C and have a negative temperature coefficient, as defined by the expression below: 1 1 − β 298 RT = R25°C × e T + 273 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. Dropout Behavior The MAX1925/MAX1926 regulate charging current by ramping inductor current between upper and lower thresholds, typically 128mV and 158mV across RSET. Chip Information TRANSISTOR COUNT: 5722 PROCESS: BiCMOS Table 3. Fault Temperature for Different Thermistors THERMISTOR BETA Resistance at +25°C 3000 3250 3500 3750 10000Ω 10000Ω 10000Ω 10000Ω Resistance at +50°C 4587.78Ω 4299.35Ω 4029.06Ω 3775.75Ω Resistance at 0°C 25140.55Ω 27148.09Ω 29315.94Ω 31656.90Ω 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 14 ______________________________________________________________________________________ Switch-Mode 1-Cell Li+ Chargers PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm 21-0139 A ______________________________________________________________________________________ 15 MAX1925/MAX1926 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 Switch-Mode 1-Cell Li+ Chargers 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 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.