Single Buffer Gate With 3-State Output

eetech00

Joined Jun 8, 2013
3,418
What is the purpose of the 74AHC1G125 (SINGLE BUFFER GATE WITH 3-STATE OUTPUT) in the schematic and how does it works ?
The 74AHC1G125 is a non-inverting tristate buffer with an active low enable. So the output will follow the input only when the enable pin is low. It looks like the circuit will apply 5v from the USB port unless "VIN" has a voltage applied to it. If VIN has a positive voltage on it, it will prevent the PMOS transistor from turning on.
 

dl324

Joined Mar 30, 2015
15,423
What is the purpose of the 74AHC1G125 (SINGLE BUFFER GATE WITH 3-STATE OUTPUT) in the schematic and how does it works ?
At first glance, it would appear that it was being used to enable power from the USB input. On closer examination, the P channel MOSFET is wired backwards, so it's always enabled (with an undesirable diode drop).
1653421458809.png

This is from Sparkfun.com, so either the schematic is wrong, or it doesn't work as advertised.
https://www.sparkfun.com/products/13672
1653421908621.png
 
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