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AN1957 - APPLICATION NOTE

BATTERY SWITCHOVER

A common task of battery switchover devices is to provide an uninterrupted power supply to external de-

vices in the event of voltage drops and brownouts.

Battery switchover devices can also be useful in portable devices. When the external power supply (such

as the AC power supply adapter) is disconnected, the battery switchover device switches to the internal

supply (such as a battery).

The use of a battery switchover has the following advantages:

■

providing continuous and reliable service, even if the external supply fails

■

extending the battery lifetime

■

debouncing the power spikes occurring while connecting and disconnecting the AC adapter.

Battery switchover devices can be used as a main power supply backup for MCUs, memories and other

peripherals, and to prevent system failures (see Figure 10.).

Diode-OR connections are often used as an equivalent solution. However the diode voltage drop repre-

sents a large percentage of the battery voltage, and power supply spikes are not filtered. With an ST Su-

pervisor, the device is supplied from the main power supply as long as the voltage is high enough, even

if the battery voltage is greater than the power supply voltage, which saves the battery and extends its

lifetime.

Instead of a backup battery it is also possible to use a backup capacitor. Recommended capacitor values

start from 0.1F.

Figure 10. Block Diagram of a Battery switchover Device

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