Low-current charger provides battery backup from USB to 1-cell applications
Another app note from Maxim Integrated on small current battery management.The circuit provides a 1-cell Li ion backup battery charger for low-current devices performing resistor limited current sharing.
A simple circuit sometimes needs to charge a 1-cell Lithium-ion (Li+) battery from a USB power source and simultaneously provide backup power for low-current-consumption devices. The circuit in Figure 1 shows such a solution.
Today, most handheld devices use USB as the power source. This design circuit includes a linear, 1-cell charger that accepts a DC power source to charge a 1-cell Li+ battery. The power source provides load current while charging the 1-cell battery. The circuit performs resistor-limited current sharing. The maximum current available is fixed by the setting resistor and that current will primarily drive the load. When the load does not require the full maximum current, the remaining current is available to charge the battery. When the DC source is not available and the cell voltage drops to a user-determined level, a voltage monitor activates an ultra-low-resistance switch to disconnect the cell from the load. This operation avoids a deep discharge of the cell.
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