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LTC4002 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4002 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LTC4002 11 4002f APPLICATIO S I FOR ATIO Undervoltage Lockout (UVLO) An undervoltage lockout circuit monitors the input voltage and keeps the charger off until VCC rises above the UVLO threshold (4.2V for the 4.2 version, 7.5V for the 8.4 version) and at least 250mV above the battery voltage. To prevent oscillation around the threshold voltage, the UVLO circuit has 200mV per cell of built-in hysteresis. When specifying minimum input voltage requirements, the volt- age drop across the input blocking diode must be added to the minimum VCC supply voltage specification. Trickle Charge and Defective Battery Detection At the beginning of a charge cycle, if the battery voltage is below the trickle charge threshold, the charger goes into trickle charge mode with the charge current reduced to 10% of the full-scale current. If the low-battery voltage persists for 30 minutes, the battery is considered defec- tive, the charge cycle is terminated and the CHRG pin is forced high impedance. Shutdown The LTC4002 can be shut down by pulling the COMP pin to ground which pulls the GATE pin high turning off the external P-channel MOSFET. When the COMP pin is re- leased, the internal timer is reset and a new charge cycle starts. In shutdown, the output of the CHRG pin is high impedance and the quiescent current remains at 3mA. Removing the input power supply will put the charger into sleep mode. If the voltage at the VCC pin drops below (VBAT + 250mV) or below the UVLO level, the LTC4002 goes into a low current (ICC= 10µA) sleep mode, reducing the battery drain current. CHRG Status Output Pin When a charge cycle starts, the CHRG pin is pulled to ground by an internal N-channel MOSFET which is capable of driving an LED. When the charge current drops below the End-of-Charge threshold for more than 120µs, the N-channel MOSFET turns off and a weak 25µA current source to ground is connected to the CHRG pin. This weak 25µA pull-down remains until the timer ends the charge cycle, or the charger is in manual shutdown or sleep mode. After a time out occurs (charge cycle ends), the pin will become high impedance. By using two different value re- sistors, a microprocessor can detect three states from this pin (charging, end-of-charge and charging stopped) see Figure 1. VCC 2k 390k VDD OUT CHRG µPROCESSOR LTC4002 IN 4002 F02 Figure 1. Microprocessor Interface OPERATIO mode, dropping ICC to 10µA. This will greatly reduce the current drain on the battery and increase the standby time. A 10kΩ NTC (negative temperature coefficient) thermistor can be connected from the NTC pin to ground for battery temperature qualification. The charge cycle is suspended when the temperature is outside of the 0°C to 50°C window (with DALE NTHS-1206N02). To detect the charge mode, force the digital output pin, OUT, high and measure the voltage at the CHRG pin. The N-channel MOSFET will pull the pin low even with a 2k pull-up resistor. Once the charge current drops below the End-of-Charge threshold, the N-channel MOSFET is turned off and a 25µA current source is connected to the CHRG pin. The IN pin will then be pulled high by the 2k resistor connected to OUT. Now force the OUT pin into a high |
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Similar Description - LTC4002 |
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