LM3420 Datasheet, PDF (17/26 Pages) Texas Instruments – 8.4-V Li-Ion Battery Charge Controller

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LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
Figure 20. (Fast) Pulsed Constant Current 2-Cell Charger
9.2.4.6 MOSFET Low Dropout Charger
Figure 21 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 (0.2-Ω) FET, while Q3 can be a low-current (2-Ω) device.
Figure 21. MOSFET Low Dropout Charger
10 Power Supply Recommendations
The LM3420 is designed to operated from up to 20-V input voltage supply. This input supply must be well
regulated. If the input supply is noisy, additional input capacitors with low ESR can help to improve the output
noise performance.
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