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Dn111 - lt1510 high efficiency lithium-ion battery chargerLT1510 High Efficiency Lithium-Ion Battery ChargerDesign Note 111 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 nickel-cadmium (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 COMPLETE LITHIUM-ION CHARGER, NO TERMINATION REQUIRED CIN: TOKIN 25V CERAMIC SURFACE MOUNT 1E106ZY5U-C205 The LT1510 can charge batteries ranging from 1V to 20V.
Figure 1. Charging Lithium-Ion Batteries
A blocking diode is not required between the chip and the (Efficiency at 1.3A = 86%)
battery because the chip goes into sleep mode and drainsonly 3µA when the wall adaptor is unplugged. Soft start and shutdown features are also provided.
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 Figure 2. Battery Charging Characteristics
charge is needed. Figure 2 shows typical charging charac-teristics.
voltage divider current should be set at 0.5mA. Q3 is usedto eliminate this current drain when adapter power is off, The battery DC charging current is programmed by a with a 47k resistor to pull its gate low.
resistor RPROG ( or a DAC output current) at the PROG pin.
High DC accuracy is achieved with averaging capacitor With divider current set as 0.5mA, R4 = 2.465/0.5mA = PROG. The basic formula for full charging current is: IBAT = (IPROG)(2000) = (2.465/RPROG)(2000) Approximately 0.25mA flows out of the BAT pin at all times VIN has to be at least 3V higher than battery voltage and when adapter power is applied. Therefore, to ensure a regulated output even when the battery is removed, the , LTC and LT are registered trademarks of Linear Technology Corporation. Lithium-ion batteries typically require float voltage accu- racy 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 Total power in the IC is 0.1 + 0.12 + 0.36 + 0.30 = 0.88W have tapered off to a low value and the LT1510 will rarelyheat up past 50°C, so 0.25% resistors will provide the Temperature rise in the IC will be: (50°C/W)(0.88W) = 44°C Some battery manufacturers recommend termination ofconstant voltage float mode 30 to 90 minutes after charg- Thermal Calculations
ing current has dropped below a specified level (typically Although the battery charger achieves efficiency of ap- 50mA to 100mA). Check with the manufacturers for de- proximately 86% at 1.3A, a thermal calculation should be tails. The circuit in Figure 4 will detect when charging done to ensure that junction temperature will not exceed current has dropped below 75mA. This logic signal is used 125°C. Power dissipation in the IC is caused by bias and to initiate a timeout period, after which the LT1510 can be driver current, switch resistance, switch transition losses shut down by pulling the VC pin low with an open collector and the current sense resistor. The 16-lead SO, with a or drain. Some external means may be used to detect the thermal resistance of 50°C/W, can provide a full 1.5A need for additional charging or the charger may be turned charging current in many situations. Figure 3 shows the on periodically to complete a short float voltage cycle. The efficiency for charging currents up to 1.5A.
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: Figure 3. Efficiency of Figure 1 Circuit
TOL = Effective switch overlap time ≈ 10ns Example: VIN = 16V, VBAT = 8.4V, IBAT = 1.3A Figure 4. Current Comparator for Initiating
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