MAX1737EEI+TGC1

EVALUATION KIT AVAILABLE MAX1737 Stand-Alone Switch-Mode Lithium-Ion Battery-Charger Controller General Description The MAX1737 is a switch-mode lithium-ion (Li+) battery charger that charges one to four cells. It provides a regulated charging current and a regulated voltage with only a ±0.8% total voltage error at the battery terminals. The external N-channel switch and synchronous rectifier provide high efficiency over a wide input voltage range. A built-in safety timer automatically terminates charging once the adjustable time limit has been reached. The MAX1737 regulates the voltage set point and charging current using two loops that work together to transition smoothly between voltage and current regulation. An additional control loop monitors the total current drawn from the input source to prevent overload of the input supply, allowing the use of a low-cost wall adapter. The per-cell battery voltage regulation limit is set between +4.0V and +4.4V and can be set from one to four by pin strapping. Battery temperature is monitored by an external thermistor to prevent charging if the battery temperature is outside the acceptable range. The MAX1737 is available in a space-saving 28-pin QSOP package. Use the evaluation kit (MAX1737EVKIT) to help reduce design time. Applications Features ●● Stand-Alone Charger for Up to Four Li+ Cells ●● ±0.8% Accurate Battery Regulation Voltage ●● Low Dropout: 98% Duty Cycle ●● Safely Precharges Near-Dead Cells ●● Continuous Voltage and Temperature Monitoring ●● VBATT PREQUAL FAULT PREQUAL TIMEOUT FASTCHG = LOW FULLCHG = HIGH FAULT = HIGH FASTCHG = HIGH FULLCHG = HIGH FAULT = LOW VBATT > 2.5V TEMP NOT OK TEMP OK TEMP NOT OK TEMP OK FAST-CHARGE TIMEOUT FAST CHARGE ONCE PER SECOND TEMP QUAL DHI AND DLO HELD LOW FOR 800µs SHUTDOWN IS ENTERED FROM ALL STATES WHEN SHDN IS LOW. FASTCHG = HIGH FULLCHG = HIGH FAULT = HIGH FASTCHG = LOW FULLCHG = HIGH FAULT = HIGH TEMP OK VBATT < 0.95 × VBATTR VBATT = BATTERY REGULATION VOLTAGE (VBATTR) ONCE PER SECOND FULL CHARGE TEMP OK FASTCHG = HIGH FULLCHG = LOW FAULT = HIGH TEMP NOT OK VBATT < 0.95 × VBATTR ICHARGE < IMIN OR FULL-CHARGE TIMEOUT TOP-OFF FASTCHG = HIGH FULLCHG = HIGH FAULT = HIGH TOP-OFF TIMEOUT DONE FASTCHG = HIGH FULLCHG = HIGH FAULT = HIGH Figure 4. State Diagram current regulation, the input source must be able to supply the maximum system current plus the maximum charger input current. By using the input current limiter, the current capability of the AC wall adapter may be lowered, reducing system cost. Input current is measured through an external sense resistor at CSSP and CSSN. The voltage at ISETIN also adjusts the input current limit. Full-scale input current is achieved when ISETIN is connected to REF, setting the full-scale current-sense voltage to 100mV. When choosing the input current-sense resistor, note that the voltage drop across this resistor adds to the power loss, reducing efficiency. Reducing the voltage across www.maximintegrated.com the current-sense resistor may degrade input current limit accuracy due to the input offset of the input current-sense amplifier. The input current error amplifier (GMS) is compensated at CCS. A 47nF capacitor at CCS provides suitable performance for most applications. PWM Controller The PWM controller drives the external MOSFETs to control the charging current or voltage. The input to the PWM controller is the lowest of CCI, CCV, or CCS. An internal clamp limits the noncontrolling signals to within 200mV of the controlling signal to prevent delay when switching between regulation loops. Maxim Integrated │  12 MAX1737 Stand-Alone Switch-Mode Lithium-Ion Battery-Charger Controller Table 1. Charging State Conditions STATE ENTRY CONDITIONS STATE CONDITIONS Reset From initial power on or From done state if battery voltage < recharge voltage threshold or VDCIN - VBATT < 100mV or VBATT > battery overvoltage threshold Timers reset, charging current = 0, FASTCHG = high, FULLCHG = high, FAULT = high Prequalification From reset state if input power, reference, and internal bias are within limits Battery voltage ≤ undervoltage threshold, charging current = C/20, timeout = 7.5min typ (CTIMER1 = 1nF), FASTCHG = low, FULLCHG = high, FAULT = high Fast Charge (Constant Current) From prequalification state if battery voltage > undervoltage threshold Undervoltage threshold ≤ battery voltage ≤ battery regulation voltage, charging current = current limit, timeout = 90min typ (CTIMER2 = 1nF), FASTCHG = low, FULLCHG = high, FAULT = high Full Charge (Constant Voltage) From fast-charge state if battery Voltage = battery regulation voltage Battery voltage = battery regulation voltage, charging current ≤ current limit, timeout = 90min typ (CTIMER1 = 1nF), FASTCHG = high, FULLCHG = low, FAULT = high Top-Off (Constant Voltage) From full-charge state if full-charge timer expires or charging current ≤ 10% of current limit Battery voltage = battery regulation voltage, charging current ≤ 10% of current limit, timeout = 45min typ (CTIMER1 = 1nF), FASTCHG = high, FULLCHG = high, FAULT = high Done From top-off state if top-off timer expires Recharge voltage threshold ≤ battery voltage ≤ battery regulation voltage, charging current = 0, FASTCHG = high, FULLCHG = high, FAULT = high Over/Under Temperature From fast-charge state or full-charge state if battery temperature is outside of limits Charge current = 0, timers suspended, FASTCHG = no change, FULLCHG = no change, FAULT = no change Fault From prequalification state if prequalification timer expires or From fast-charge state if fast-charge timer expires Charging current = 0, FASTCHG = high, FULLCHG = high, FAULT = low www.maximintegrated.com Maxim Integrated │  13 MAX1737 The current-mode PWM controller uses the inductor current to regulate the output voltage or current, simplifying stabilization of the regulation loops. Separate compensation of the regulation circuits allows each to be optimally stabilized. Internal slope compensation is included, ensuring stable operation over a wide range of duty cycles. The controller drives an external N-channel MOSFET switch and a synchronous rectifier to step the input voltage down to the battery voltage. A bootstrap capacitor drives the high-side MOSFET gate to a voltage higher than the input source voltage. This capacitor (between BST and LX) is charged through a diode from VLO when the synchronous rectifier is on. The high-side MOSFET gate is driven from BST, supplying sufficient voltage to fully drive the MOSFET gate even when its source is near the input voltage. The synchronous rectifier is driven from DLO to behave like a diode, but with a smaller voltage drop for improved efficiency. A built-in dead time (50ns typ) between switch and synchronous rectifier turn-on and turn-off prevents crowbar currents (currents that flow from the input voltage to ground due to both the MOSFET switch and synchronous rectifier being on simultaneously). This dead time may allow the body diode of the synchronous rectifier to conduct. If this happens, the resulting forward voltage and diode recovery time will cause a small loss of efficiency and increased power dissipation in the synchronous rectifier. To prevent the body diode from conducting, place an optional Schottky rectifier in parallel with the drain and source of the synchronous rectifier. The internal current-sense circuit turns off the synchronous rectifier when the inductor current drops to zero. Timers The MAX1737 includes safety timers to terminate charging and to ensure that faulty batteries are not charged indefinitely. TIMER1 and TIMER2 set the timeout periods. TIMER1 controls the maximum prequalification time, maximum full-charge time, and the top-off time. TIMER2 controls the maximum fast-charge time. The timers are set by external capacitors. The typical times of 7.5 minutes for prequalification, 90 minutes for full charge, 45 minutes for top-off, and 90 minutes for fast charge are set by using a 1nF capacitor on TIMER1 and TIMER2 (Figure 1). The timers cannot be disabled. Charge Monitoring Outputs FASTCHG, FULLCHG, and FAULT are open-drain outputs that can be used as LED drivers. FASTCHG indicates the battery is being fast charged. FULLCHG www.maximintegrated.com Stand-Alone Switch-Mode Lithium-Ion Battery-Charger Controller indicates the charger has completed the fast-charge cycle (approximately 85% charge) and is operating in voltage mode. The FASTCHG and FULLCHG outputs can be tied together to indicate charging (see Figure 2). FAULT indicates the charger has detected a charging fault and that charging has terminated. The charger can be brought out of the FAULT condition by removing and reapplying the input power, or by pulling SHDN low. Thermistor The intent of THM is to inhibit fast-charging the cell when it is too cold or too hot (+2.5°C ≤ TOK ≤ +47.5°C), using an external thermistor. THM time multiplexes two sense currents to test for both hot and cold qualification. The thermistor should be 10kΩ at +25°C and have a negative temperature coefficient (NTC); the THM pin expects 3.97kΩ at +47.5°C and 28.7kΩ at +2.5°C. Connect the thermistor between THM and GND. If no temperature qualification is desired, replace the thermistor with a 10kΩ resistor. Thermistors by Philips/BCcomponents (2322-640-63103), Cornerstone Sensors (T101D103-CA), and Fenwal Electronics (140-103LAG-RB1) work well. Charging pauses briefly (DHI and DLO are held Low for 800µs) to allow accurate temperature measurement . The battery voltage will have an undershoot when DHI and DLO stop switching during the temperature measurement. The battery voltage will have an overshoot when DHI and DLO resume switching after the temperature measurement. Shutdown When SHDN is pulled low, the MAX1737 enters the shutdown mode and charging is stopped. In shutdown, the internal resistive voltage-divider is removed from BATT to reduce the current drain on the battery to less than 1µA. DHI and DLO are low. However, the internal linear regulator (VLO) and the reference (REF) remain on. The status outputs FASTCHG, FULLCHG, and FAULT are high impedance. When exiting shutdown mode, the MAX1737 goes back to the power-on reset state, which resets the timers and begins a new charge cycle. Source Undervoltage Shutdown (Dropout) If the voltage on DCIN drops within 100mV of the voltage on BATT, the charger resets. Table 2. Cell-Count Programming CELL GND Float REF VL CELL COUNT (N) 1 2 3 4 Maxim Integrated │  14 MAX1737 Stand-Alone Switch-Mode Lithium-Ion Battery-Charger Controller Design Procedure Setting the Battery Regulation Voltage VADJ sets the per-cell voltage limit. To set the VADJ voltage, use a resistor-divider from REF to GND. A GND-to-VREF change at VADJ results in a ±5% change in the battery limit voltage. Since the full VADJ range results in only a 10% change on the battery regulation voltage, the resistor-divider’s accuracy need not be as high as the output voltage accuracy. Using 1% resistors for the voltage-dividers results in no more than 0.1% degradation in output voltage accuracy. VADJ is internally buffered so that high-value resistors can be used. Set VVADJ by choosing a value less than 100kΩ for R8 and R9 (Figure 1) from VADJ to GND. The per-cell battery termination voltage is a function of the battery chemistry and construction; thus, consult the battery manufacturer to determine this voltage. Once the percell voltage limit battery regulation voltage is determined, the VADJ voltage is calculated by the equation:  9.5 × VBATTR  = V ADJ   − (9.0 × VREF ) N   where VBATTR is N x the cell voltage. CELL is the programming input for selecting cell count N. Table 2 shows how CELL is connected to charge one to four cells. Setting the Charging Current Limit A resistor-divider from REF to GND sets the voltage at ISETOUT (VISETOUT). This voltage determines the charging current during the current-regulation fast-charge mode. The full-scale charging current (IFSI) is set by the current-sense resistor (R18, Figure 1) between CS and BATT. The full-scale current is IFSI = 0.2V / R18. The charging current ICHG is therefore: V I CHG = I FSI ISETOUT VREF In choosing the current-sense resistor, note that the drop across this resistor causes further power loss, reducing efficiency. However, too low a value may degrade the accuracy of the charging current. Setting the Input Current Limit A resistor-divider from REF to GND can set the voltage at ISETIN (VISETIN). This sets the maximum source current allowed at any time during charging. The source current (IFSS) is set by the current-sense resistor (R12, Figure 1) between CSSP and CSSN. The full-scale source current is IFSS = 0.1V / R12. The input current limit (IIN) is therefore: V I IN = I FSS ISETIN VREF Set ISETIN to REF to get the full-scale current limit. Short CSSP and CSSN to DCIN if the input source current limit is not used. In choosing the current-sense resistor, note that the drop across this resistor causes further power loss, reducing efficiency. However, too low a resistor value may degrade input current limit accuracy. Inductor Selection The inductor value may be changed to achieve more or less ripple current. The higher the inductance, the lower the ripple current will be; however, as the physical size is kept the same, higher inductance typically will result in higher series resistance and lower saturation current. A good trade-off is to choose the inductor so that the ripple current is approximately 30% to 50% of the DC average charging current. The ratio of ripple current to DC charging current (LIR) can be used to calculate the optimal inductor value: L= VBATT (VDCIN(MAX) − VBATT ) VDCIN(MAX) × f × I CHG × LIR where f is the switching frequency (300kHz). The peak inductor current is given by: I PEAK =  LIR  I CHG 1 + 2   Capacitor Selection The input capacitor absorbs the switching current from the charger input and prevents that current from circulating through the source, typically an AC wall cube. Thus, the input capacitor must be able to handle the input RMS current. Typically, at high charging currents, the converter will operate in continuous conduction (the inductor current does not go to 0). In this case, the RMS current of the input capacitor may be approximated by the equation: I CIN ≈ I CHG D − D 2 where ICIN = the input capacitor RMS current, D = PWM converter duty ratio (typically VBATT/VDCIN), and ICHG = battery charging current. www.maximintegrated.com Maxim Integrated │  15 MAX1737 The maximum RMS input current occurs at 50% duty cycle, so the worst-case input ripple current is 0.5 × ICHG. If the input to output voltage ratio is such that the PWM controller will never work at 50% duty cycle, then the worst-case capacitor current will occur where the duty cycle is nearest 50%. The impedance of the input capacitor is critical to preventing AC currents from flowing back into the wall cube. This requirement varies depending on the wall cube’s impedance and the requirements of any conducted or radiated EMI specifications that must be met. Aluminum electrolytic capacitors are generally the least costly, but are usually a poor choice for portable devices due to their large size and low equivalent series resistance (ESR). Tantalum capacitors are better in most cases, as are highvalue ceramic capacitors. For equivalent size and voltage rating, tantalum capacitors will have higher capacitance and ESR than ceramic capacitors. This makes it more critical to consider RMS current and power dissipation when using tantalum capacitors. The output filter capacitor is used to absorb the inductor ripple current. The output capacitor impedance must be significantly less than that of the battery to ensure that it will absorb the ripple current. Both the capacitance and ESR rating of the capacitor are important for its effectiveness as a filter and to ensure stability of the PWM circuit. The minimum output capacitance for stability is:  VBATT  VREF 1 +   VDCIN(MIN)    C OUT > VBATT × f × R CS where COUT is the total output capacitance, VREF is the reference voltage (4.2V), VBATT is the maximum battery voltage (typically 4.2V per cell), and VDCIN(MIN) is the minimum source input voltage. The maximum output capacitor ESR allowed for stability is: R × VBATT R ESR < CS VREF where RESR is the output capacitor ESR and RCS is the current-sense resistor from CS to BATT. Setting the Timers The MAX1737 contains four timers: a prequalification timer, fast-charge timer, full-charge timer, and top-off timer. Connecting a capacitor from TIMER1 to GND and TIMER2 to GND sets the timer periods. The TIMER1 input controls the prequalification, full-charge, and top-off times, while TIMER2 controls fast-charge www.maximintegrated.com Stand-Alone Switch-Mode Lithium-Ion Battery-Charger Controller timeout. The typical timeouts for a 1C charge rate are set to 7.5 minutes for the prequalification timer, 90 minutes for the fast-charge timer, 90 minutes for the full-charge timer, and 45 minutes for the top-off timer by connecting a 1nF capacitor to TIMER1 and TIMER2. Each timer period is directly proportional to the capacitance at the corresponding pin. See the Typical Operating Characteristics. Compensation Each of the three regulation loops—the input current limit, the charging current limit, and the charging voltage limit—can be compensated separately using the CCS, CCI, and CCV pins, respectively. The charge-current loop error amp output is brought out at CCI. Likewise, the source-current error amplifier output is brought out at CCS; 47nF capacitors to ground at CCI and CCS compensate the current loops in most charger designs. Raising the value of these capacitors reduces the bandwidth of these loops. The voltage-regulating loop error amp output is brought out at CCV. Compensate this loop by connecting a capacitor in parallel with a series resistor-capacitor (RC) from CCV to GND. Recommended values are shown in Figure 1. Applications Information MOSFET Selection The MAX1737 uses a dual N-channel external power MOSFET switch to convert the input voltage to the charging current or voltage. The MOSFET must be selected to meet the efficiency and power-dissipation requirements of the charging circuit, as well as the temperature rise of the MOSFETs. The MOSFET characteristics that affect the power dissipation are the drain-source on-resistance (RDS(ON)) and the gate charge. In general, these are inversely proportional. To determine the MOSFET power dissipation, the operating duty cycle must first be calculated. When the charger is operating at higher currents, the inductor current will be continuous (the inductor current will not drop to 0A) and, in this case, the high-side MOSFET duty cycle (D) can be approximated by the equation: D≈ VBATT VDCIN and the synchronous-rectifier MOSFET duty cycle (D′) will be 1 - D or: V − VBATT D′ ≈ DCIN VDCIN Maxim Integrated │  16 MAX1737 Stand-Alone Switch-Mode Lithium-Ion Battery-Charger Controller For the high-side switch, the worst-case power dissipation due to on-resistance occurs at the minimum source voltage VDCIN(MIN) and the maximum battery voltage VBATT(MAX), and can be approximated by the equation: PR ≈ VBATT(MAX) VDCIN(MIN) × R DS(ON) × I CHG 2 The transition loss can be approximated by the equation: VDCIN × I CHG × f × t TR 3 where tTR is the MOSFET transition time. So the total power dissipation of the high-side switch is PTOT = PR + PT. PT ≈ The worst-case synchronous-rectifier power occurs at the minimum battery voltage VBATT(MIN) and the maximum source voltage VDC(MAX), and can be approximated by: PDL ≈ VDCIN(MAX) − VBATT(MIN) VDCIN(MAX) × R DS(ON) × I CHG 2 There is a brief dead time where both the high-side switch and synchronous rectifier are off. This prevents crowbar currents that flow directly from the source voltage to ground. During the dead time, the inductor current will turn on the synchronous-rectifier MOSFET body diode, which may degrade efficiency. To prevent this, www.maximintegrated.com connect a Schottky rectifier across the drain source of the synchronous rectifier to stop the body diode from conducting. The Schottky rectifier may be omitted, typically degrading the efficiency by approximately 1% to 2%, causing a corresponding increase in the low-side synchronous-rectifier power dissipation. VL and REF Bypassing The MAX1737 uses an internal linear regulator to drop the input voltage down to 5.4V, which powers the internal circuitry. The output of the linear regulator is the VL pin. The internal linear regulator may also be used to power external circuitry as long as the maximum current and power dissipation of the linear regulator are not exceeded. The synchronous-rectifier MOSFET gate driver (DLO) is powered from VLO. An internal 12Ω resistor from VL to VLO provides the DC current to power the gate driver. Bypass VLO to PGND with a 0.1µF or greater capacitor. A 4.7µF bypass capacitor is required at VL to ensure that the regulator is stable. A 1µF bypass capacitor is also required between REF and GND to ensure that the internal 4.2V reference is stable. In both cases use a low-ESR ceramic capacitor. Chip Information TRANSISTOR COUNT: 5978 Maxim Integrated │  17 MAX1737 Stand-Alone Switch-Mode Lithium-Ion Battery-Charger Controller Package Information For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO. 28 QSOP E28+1 21-0055 90-0173 Note: The MAX1737EEI is a 28-pin QSOP and does not have a heat slug. www.maximintegrated.com Maxim Integrated │  18 MAX1737 Stand-Alone Switch-Mode Lithium-Ion Battery-Charger Controller Revision History PAGES REVISION NUMBER REVISION DATE 4 5/09 Initial release 5 7/17 Replaced Figure 4 and added information to Thermistor section DESCRIPTION CHANGED 1, 9, 18 12, 14 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com. Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance. Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc. ©  2017 Maxim Integrated Products, Inc. │  19