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I'm checking a schematic with SD Reader, and I have realized that SD Card Reader needs a special 3.3 V through an special circuit, but I can't understand what is this circuit.

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  • \$\begingroup\$ Q21 turns the 3.3V on and off. R116 is DNP (do not place); the blank resistor space allows for the switch to be bypassed if it turns out to be useless. \$\endgroup\$
    – user253751
    Apr 25, 2022 at 14:59

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It’s a circuit to fully discharge the 3.3V line with a reset. Q21 is the power switch. Q22 is turned on (and Q21, off) whenever one of the reset lines is low, discharging the SD card 3.3V pin through the 51 ohm resistor.

Q23 is working as an inverter. D41 and D42 form a wire-OR for the two resets.

This power+discharge setup is often available as part of a load switch IC. The designers chose instead to render it with discretes because they were attempting to reduce cost.

This circuit is optional. The whole thing can be depopulated and R116 installed instead.

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