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LTC4002ES8-4.2 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4002ES8-4.2 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC4002-4.2 11 400242i Gate Drive The LTC4002-4.2 gate driver can provide high transient currents to drive the external pass transistor. The rise and fall times are typically 20ns and 50ns respectively when driving a 2000pF load, which is typical for a P-channel MOSFET with RDS(ON) in the range of 50mΩ. A voltage clamp is added to limit the gate drive to 8V below VCC. For example, if VCC is 10V then the GATE output will pull down to 2V max. This allows low voltage P-channel MOSFETs with superior RDS(ON) to be used as the pass transistor thus increasing efficiency. Stability Both the current loop and the voltage loop share a com- mon, high impedance, compensation node (COMP pin). A series capacitor and resistor on this pin compensates both loops. The resistor is included to provide a zero in the loop response and boost the phase margin. The compensation capacitor also provides a soft-start function for the charger. Upon start-up, the COMP pin voltage will quickly rise to 0.05V, due to the 2.2k series resistor, then ramp at a rate set by the internal 100 µA pull- up current source and the external capacitor. Battery charge current starts ramping up when the COMP pin voltage reaches 0.8V and full current is achieved with the COMP pin at 1.3V. With a 0.47 µF capacitor, time to reach full charge current is about 235ms. Capacitance can be increased up to 1 µF if a longer start-up time is needed. Automatic Battery Recharge After the 3 hour charge cycle is completed and both the battery and the input power supply (wall adapter) are still connected, a new charge cycle will begin if the battery voltage drops below 4.05V due to self-discharge or exter- nal loading. This will keep the battery capacity at more than 80% at all times without manually restarting the charge cycle. Battery Temperature Detection A negative temperature coefficient (NTC) thermistor located close to the battery pack can be used to monitor battery temperature and will not allow charging unless the battery temperature is within an acceptable range. Connect a 10k Ω thermistor (DALE NTHS-1206N02) from the NTC pin to ground. If the temperature rises to 50 °C,the resistance of the NTC will be approximately 4.1k Ω. With the 85 µA pull-up current source, the Hot temperature voltage threshold is 350mV. For Cold temperature, the voltage threshold is set at 2.456V which is equal to 0 °C (RNTC ≅ 28.4kΩ) with 85µA of pull-up current. If the temperature is outside the window, the GATE pin will be pulled up to VCC and the timer frozen while the output status at the CHRG pin remains the same. The charge cycle begins or resumes once the temperature is within the acceptable range. Short the NTC pin to ground to disable the temperature qualification feature. APPLICATIO S I FOR ATIO |
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