Help identifying a SOT23 part

[Virgo]

Hi,

Can some one help me identify a SOT23 part on a sensor board?
The Part has code AF4C on it and pin connections are as below.

Pin1 is connected to Vcc(3.3V)
Pin 2 is connected to a processor GPIO pin that can also be configured as ADC.
Pin 3 is connected to ground.

I was thinking this is a protection device, but realized that the pin 2 is connected only to processor pin, so guess it may be a voltage reference?
Appreciate any help identifying this part.

 

[(*steve*)]

It could also be a voltage detector chip, but they're more common on the reset pin of a microcontroller.

 

[kellys_eye]

Got any more details on the board, its schematic and/or its function?

 

[(*steve*)]

I've looked through my SMD codebooks and I can't find it.

 

[Virgo]

@kellys_eye , The processor is STM32F072C8T6 (48LQFP) and the SOT23 pin2 is connected to Procesor pin 18 - Port PB0 that can also be used as ADC_in8.

 

[kellys_eye]

Port PB0 that can also be used as ADC_in8.

That's the problem - if it was programmed as an input (ADC) port then it's possible it's a clamping zener to protect the input. If it was an output port I'd expect the SOT device to have a load resistor connected to Vcc in its collector (or in the emitter to ground) - indeed it would have a base current limiting resistor too. If there is no resistor to Vcc or a base resistor from the port pin then assume its a zener clamp diode.