LTC4002_15 Datasheet, PDF (12/16 Pages) Linear Technology – Standalone Li-Ion Switch Mode Battery Charger

LTC4002
APPLICATIO S I FOR ATIO
impedance state, the current source will pull the pin low
through the 390k resistor. When the internal timer has
expired, the CHRG pin changes to a high impedance state
and the 390k resistor will then pull the pin high to indicate
charging has stopped.
Gate Drive
The LTC4002 gate driver can provide high transient cur-
rents 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 R_DS(ON) in the range of 50mΩ.
A voltage clamp is added to limit the gate drive to 8V below
V_CC. For example, if V_CC is 10V then the GATE output will
pull down to 2V max. This allows low voltage P-channel
MOSFETs with superior R_DS(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 0.22V, due to the 2.2k series
resistor, then ramp at a rate set by the internal 100µA 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 0.47µF capacitor, time to reach
full charge current is about 2.35ms. Capacitance can be
increased up to 1µF 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 per cell due to self-discharge
or external 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 10kΩ 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 4.1kΩ. With
the 85µA pull-up current source, the Hot temperature
voltage threshold is 350mV. For Cold temperature, the
voltage threshold is set at 2.465V which is equal to 0°C
(R_NTC ≅ 28.4kΩ) with 85µA of pull-up current. If the
temperature is outside the window, the GATE pin will be
pulled up to V_CC 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.
4002f
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