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LT1769 Datasheet(PDF) 11 Page - Linear Technology |
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LT1769 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LT1769 APPLICATIONS INFORMATION When power is on, there is about 200 µA of current flowing out of the BAT and SENSE pins. If the battery is removed during charging, and total load including R3 and R4 is less than 200 µA, VBAT could float up to VCC even though the loop has turned switching off. To keep VBAT regulated to the battery voltage in this condition, R3 and R4 can be chosen to draw 0.5mA and Q3 can be added to disconnect them when power is off (Figure 4). R5 isolates the OVP pin from any high frequency noise on VIN. An alternative method is to use a Zener diode with a breakdown voltage two or three volts higher than battery voltage to clamp the VBAT voltage. Lithium-Ion Charging The 2A Lithium-Ion Battery Charger (Figure 1) charges at a constant 2A until battery voltage reaches a limit set by R3 and R4. The charger will then automatically go into a constant-voltage mode with current decreasing to 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. Battery Voltage Sense Resistors Selection To minimize battery drain when the charger is off, current through the R3/R4 divider is set at 15 µA. The input current to the OVP pin is 3nA and the error can be neglected. With divider current set at 15 µA, VBAT = 8.4V, R4 = 2.465/15 µA = 162k and, R3 R4 V 2.465 2.465 162k 8.4 2.465 2.465 390k BAT = () − () = − () = Li-Ion batteries typically require float voltage accuracy of 1% to 2%. Accuracy of the LT1769 OVP voltage is ±0.5% at 25 °C and ±1% over full temperature. This leads to the possibility that very accurate (0.1%) resistors might be needed for R3 and R4. Actually, the temperature of the LT1769 will rarely exceed 50 °C in float mode because charging currents have tapered off to a low level, so 0.25% resistors will normally provide the required level of overall accuracy. Figure 4. Disconnecting Voltage Divider PWM RPROG 4.7k 300 Ω PROG CPROG 1 µF Q1 VN2222 5V 0V LT1769 1769 F03 IBAT = (DC)(2A) Figure 3. PWM Current Programming R3 12k 0.25% R4 4.99k 0.25% OVP VIN + 8.4V VBAT Q3 VN2222 LT1769 1769 F04 R5 220k Some battery manufacturers recommend terminating the constant-voltage float mode after charge current has dropped below a specified level (typically around 10% of the full current) and a further time out period of 30 to 90 minutes has elapsed. This may extend battery life, so check with the manufacturer for details. The circuit in Figure 5 will detect when charge current has dropped below 270mA. This logic signal is used to initiate a timeout period, after which the LT1769 can be shut down by pulling the VC pin low with an open collector or drain. Some external means must be used to detect the need for additional charging or the charger may be turned on periodically to complete a short float-voltage cycle. Current trip level is determined by the battery voltage, R1 through R3 and the sense resistor (RS1). D2 generates hysteresis in the trip level to avoid multiple comparator transitions. |
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