Uses two more p-channel MOSFETs to eliminate cross conduction by forming back-to-back diodes with the body diodes of the MOSFETs.
The comparator (LM293) compares the battery voltage with the main power voltage. When the main voltage drops below the battery voltage, the MOSFET is turned on and the LED is lit.
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“Ideal” Power Diode
The protection circuit in the RPI model B+ functions as an ideal power diode.
Primer on PowerPath Controllers, Ideal Diodes & Prioritizers