LM3420 Datasheet, PDF (15/26 Pages) Texas Instruments – 8.4-V Li-Ion Battery Charge Controller

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LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
The minimum supply voltage for this charger is approximately 11 V, and the maximum is around 30 V (limited by
the 32-V maximum operating voltage of the LM358). If another op-amp is substituted for the LM358, make sure
that the input common-mode range of the op-amp extends down to ground so that it can accurately sense 50
mV. R1 is included to provide a minimum load for the switching regulator to assure that switch leakage current
does not cause the output to rise when the battery is removed.
Figure 17. High-Efficiency Switching Regulator Constant Current/Constant Voltage 2-Cell Charger
9.2.4.3 Low Dropout Constant Current/Constant Voltage Li-Ion Battery Charger
The circuit in Figure 18 is very similar to Figure 17, except the switching regulator has been replaced with a low
dropout linear regulator, allowing the input voltage to be as low as 10 V. The constant current and constant
voltage control loops are the same as the previous circuit. Diode D2 has been changed to a Schottky diode to
provide a reduction in the overall dropout voltage of this circuit, but Schottky diodes typically have higher leakage
currents than a standard silicon diode. This leakage current could discharge the battery if the input voltage is
removed for an extended period of time.
Figure 18. Low Dropout Constant Current/Constant Voltage Li-Ion Battery Charger
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