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LTC4002ES8-4.2 Datasheet(PDF) 10 Page - Linear Technology |
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LTC4002ES8-4.2 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page LTC4002-4.2 10 400242i 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 4.2V and at least 250mV above the battery voltage. To prevent oscillation around the threshold voltage, the UVLO circuit has 200mV of built-in hysteresis. Trickle Charge and Defective Battery Detection At the beginning of a charge cycle, if the battery voltage is below 2.9V, the charger goes into trickle charge mode with the charge current reduced to 10% of the full-scale cur- rent. If the low-battery voltage persists for 30 minutes, the battery is considered defective, the charge cycle is termi- nated and the CHRG pin is forced to be high impedance. Shutdown The LTC4002 can be shut down by pulling the COMP pin to ground which pulls the GATE pin high and turns 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 (4.2V), the LTC4002-4.2 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 to 25% of the full-scale current for more than 120 µs,theN-channel MOSFET turns off and a weak 25 µA current source to ground is connected to the CHRG pin. This weak 25 µA 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 go into high impedance. By using two different value resis- tors, a microprocessor can detect three states from this pin (charging, end-of-charge and charging stopped) see Figure 2. 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 to 25% of the full-scale current, the N-channel MOSFET is turned off and a 25 µA 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 impedance state, the current source will pull the pin low through the 400k resistor. When the internal timer has expired, the CHRG pin changes to a high impedance state and the 400k resistor will then pull the pin high to indicate the charging has stopped. 2k 400k VDD VCC OUT CHRG µPROCESSOR LTC4002-4.2 IN 400242 F02 Figure 2. Microprocessor Interface |
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