Datasheet

LM3420
www.ti.com SNVS116D –MAY 1998–REVISED MAY 2013
Figure 37. MOSFET Low Dropout Charger
Figure 37 shows a low dropout constant voltage charger using a MOSFET as the pass element, but this circuit
does not include current limiting. This circuit uses Q3 and a Schottky diode to isolate the battery from the
charging circuitry when the input voltage is removed, to prevent the battery from discharging. Q2 should be a
high current (0.2Ω) FET, while Q3 can be a low current (2Ω) device.
Note: Although the application circuits shown here have been built and tested, they should be
thoroughly evaluated with the same type of battery the charger will eventually be used with.
Different battery manufacturers may use a slightly different battery chemistry which may require different
charging characteristics. Always consult the battery manufacturer for information on charging
specifications and battery details, and always observe the manufacturers precautions when using their
batteries. Avoid overcharging or shorting Lithium-Ion batteries.
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