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LT1769IGN Datasheet(PDF) 11 Page - Linear Integrated Systems |
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LT1769IGN Datasheet(HTML) 11 Page - Linear Integrated Systems |
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11 / 16 page  11 LT1769 1769fa APPLICATIONS INFORMATION 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%. 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. 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 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 is to use a Zener diode with a breakdown voltage two or three volts higher than battery voltage to clamp the VBAT voltage. Battery manufacturers recommend terminating the con- stant-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 Figure 5. Current Comparator for Initiating Float Time Out NEGATIVE EDGE TO TIMER 1769 F04 3.3V OR 5V ADAPTER OUTPUT 3 8 7 1 4 2 D1 1N4148 C1 0.1 µF BAT SENSE R1* 1.6k RS1 0.05 Ω R4 470k R3 430k R2 560k LT1011 D2 1N4148 * TRIP CURRENT = = ≈ 270mA R1(VBAT) (R2 + R3)(RS1) (1.6k)(8.4V) (560k + 430k)(0.05 Ω) + – VBAT BAT RS3 200 Ω RS2 200 Ω LT1769 IBAT |
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