High Efficiency Lithium-Ion Battery Charger Design Note 111 Chiawei Liao Lithium-Ion Battery Charger The circuit in Figure 1 uses the 16-lead LT1510 to charge lithium-ion batteries at a constant 1.3A until battery voltage reaches 8.4V set by R3 and R4. The charger will then automatically go into a constant voltage mode with current decreasing toward near zero over time as the battery reaches full charge. This is the normal regimen for lithium-ion charging, with the charger holding the battery at “float” voltage indefinitely. In this case, no external sensing of full charge is needed. Figure 2 shows typical charging characteristics. The battery DC charging current is programmed by a resistor RPROG ( or a DAC output current) at the PROG pin. High DC accuracy is achieved with averaging capacitor CPROG. The basic formula for full charging current is: IBAT = (IPROG)(2000) = (2.465/RPROG)(2000) = (2.465/3.83k)(2000) = 1.3A Approximately 0.25mA flows out of the BAT pin at all times when adapter power is applied. Therefore, 09/95/111_conv SW VCC + 0.22μF BOOST L1 33μH D2 1N914 PROG VC 0.1μF + – 47k 1μF CPROG 300Ω LT1510 GND CIN 10μF VIN 11V TO 25V RPROG 3.83k 1k OVP BAT SENSE + CB 22μF TANT + Q3 VN2222 4.2V + R3 12k 0.25% 4.2V R4 4.99k 0.25% COMPLETE LITHIUM-ION CHARGER, NO TERMINATION REQUIRED CIN: TOKIN 25V CERAMIC SURFACE MOUNT 1E106ZY5U-C205 L1: COILTRONICS CTX33-2 DN111 F01 Figure 1. Charging Lithium-Ion Batteries (Efficiency at 1.3A = 86%) 1400 8.6 BATTERY VOLTAGE 8.4 1200 BATTERY CURRENT (mA) The LT1510 can charge batteries ranging from 1V to 20V. A blocking diode is not required between the chip and the battery because the chip goes into sleep mode and drains only 3μA when the wall adaptor is unplugged. Soft start and shutdown features are also provided. D3 1N5819 D1 1N5819 BATTERY VOLTAGE (V) The LT®1510 current mode PWM battery charger is the simplest, most efficient solution for fast charging modern rechargeable batteries including lithium-ion (Li-Ion), nickel-metal-hydride (NiMH) and nickelcadmium (NiCd) that require constant current and/ or constant voltage charging. The internal switch is capable of delivering 1.5A DC current (2A peak current). The onboard current sense resistor (0.1Ω) makes the charge current programming very simple. One resistor (or a programming current from a DAC) is used to set the charging current to within 5% accuracy. With 0.5% reference voltage accuracy, the LT1510 16-lead S package meets the critical constant voltage charging requirement for lithium cells. 1000 8.2 800 8.0 BATTERY CURRENT 7.8 600 7.6 400 7.4 200 7.2 0 0 25 50 100 75 TIME (MINUTES) 125 DN111 • F02 Figure 2. Battery Charging Characteristics to ensure a regulated output even when the battery is removed, the voltage divider current should be set at 0.5mA. Q3 is used to eliminate this current drain when adapter power is off, with a 47k resistor to pull its gate low. With divider current set as 0.5mA, R4 = 2.465/0.5mA = 4.93k, let R4 = 4.99k: VBAT 8.4 R3 = R4 2.465 – 1 = 4.99k – 1 = 12k 2.465 VIN has to be at least 3V higher than battery voltage and between 8.5V to 25V. Lithium-ion batteries typically require float voltage accuracy of 1% to 2%. The LT1510 OVP voltage has 0.5% accuracy at 25°C and 1% over full temperature. This may suggest that very accurate (0.1%) resistors are needed for R3 and R4. Actually, in float mode the charging currents have tapered off to a low value and the LT1510 will rarely heat up past 50°C, so 0.25% resistors will provide the required level of overall accuracy. Thermal Calculations Although the battery charger achieves efficiency of approximately 86% at 1.3A, a thermal calculation should be done to ensure that junction temperature will not exceed 125°C. Power dissipation in the IC is caused by bias and driver current, switch resistance, switch transition losses and the current sense resistor. The 16-lead SO, with a thermal resistance of 50°C/W, can provide a full 1.5A charging current in many situations. Figure 3 shows the efficiency for charging currents up to 1.5A. 100 VCC = 16V VBAT = 8.4V EFFICIENCY INCLUDES LOSS IN DIODE D3 98 96 EFFICIENCY (%) 94 92 90 88 86 84 82 80 0.1 0.3 0.5 0.7 0.9 IBAT (A) 1.1 1.3 1.5 DN111 • F03 Figure 3. Efficiency of Figure 1 Circuit PBIAS = (3.5mA)(VCC) + (1.5mA)(VBAT) (V )2(7.5mA + 0.012 • IBAT) + BAT VCC (I )(V )2 PDRIVE = BAT BAT 50 (VCC) Example: VIN = 16V, VBAT = 8.4V, IBAT = 1.3A PBIAS = (3.5mA)(15.6) + (1.5mA)(8.4) (8.4)2(7.5mA + 0.012 • 1.3) = 0.10W + 15.6 PDRIVE = 50 (15.6) Total power in the IC is 0.1 + 0.12 + 0.36 + 0.30 = 0.88W Temperature rise in the IC will be: (50°C/W)(0.88W) = 44°C Some battery manufacturers recommend termination of constant voltage float mode 30 to 90 minutes after charging current has dropped below a specified level (typically 50mA to 100mA). Check with the manufacturers for details. The circuit in Figure 4 will detect when charging current has dropped below 75mA. This logic signal is used to initiate a timeout period, after which the LT1510 can be shut down by pulling the VC pin low with an open collector or drain. Some external means may be used to detect the need for additional charging or the charger may be turned on periodically to complete a short float voltage cycle. The current trip level is determined by the battery voltage, R1 through R3, and the internal LT1510 sense resistor (≈ 0.18Ω pin-to-pin). D2 generates hysteresis in the trip level to avoid multiple comparator transitions. R2 and R3 are chosen to total about 1M to minimize battery loading. D2 is assumed to be off during high current charging when the comparator output is high. To ensure this, the ratio of R2 to R3 is chosen to make the center node voltage less than the logic supply. R4 is somewhat arbitrary and does not affect trip point. R1 is adjusted to set the trip level: (I )(R2 + R3)(0.18<) R1 = TRIP VBAT (75mA)(560k + 430k)(0.18) = = 1.6k 8.4V INTERNAL SENSE RESISTOR 0.18Ω BAT ADAPTER OUTPUT SENSE R1* 1.6k )2 RSW = Switch on resistance ≈ 0.35< TOL = Effective switch overlap time ≈ 10ns VCC = VIN – 0.4V (1.3)(8.4)2 PSENSE = (0.18)(1.3)2 = 0.30W LT1510 (I )2(RSW)(VBAT) PSWITCH = BAT + (TOL)(VCC)(IBAT) VCC PSENSE = (0.18<)(IBAT (1.3)2(0.35)(8.4) 15.6 + (10– 8)(15.6)(1.3)(200kHz) = 0.36W PSWITCH = D1 1N4148 C1 0.1μF 3 *TRIP CURRENT = R1 (VBAT) (R2 + R3) (0.18Ω) 2 R2 560k D2 1N4148 – + 3.3V OR 5V 8 7 LT1011 R4 470k NEGATIVE EDGE TO TIMER 4 1 R3 430k DN111 • F04 Figure 4. Current Comparator for Initiating Float Timeout = 0.12W Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn111f_conv LT/GP 0995 190K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1995