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PS_POR_B_500 (PG_ALL) signal in ZYBO

joniengr

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Hi,
I have attached a circuit that is used in ZYBO development board. I guess the signal PS_POR_B_500 is an input signal to ZYNQ that should be asserted by the power distribution network when the power is good and stable. The signal PS_POR_B_500 (PG_ALL) is connected to N-channel FET in the attached circuit. I need some help in understanding this circuit. Which pins are Gate, Source and Drain ? I guess Pin 1 is Gate and the other two are Drain and Source. Perhaps Pin 2 is Source and Pin 3 is Drain. Can someone please explain the flow of current when the Source at Pin 2 is connected to GND. What will be the voltage at Gate Pin 1 ?
 

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FvM

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You don't need to guess. NMOS symbol pin assignment is pretty clear https://en.wikipedia.org/wiki/MOSFET#Circuit_symbols

You can also search the schematic pdf document to find all instances of PG_ALL. The signal is driven by the PWRGD pin of IC25 (open drain output) and pulled up to 3.3V by R314. The LED circuit is just an indicator, the most important target pin is ZYNC POR input.
 

joniengr

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Yes, the signal PG_ALL is driven by PWRGD pin of IC25 and is pulled up to 3.3 V by R314.

The Gate (1) is connected to PG_ALL this means that Vgs is is 3.3 V and Vgs control the Ids current from Drain (3) to Source (2) which is essential for LED.

What is the purpose of zener diode D17 in the circuit ?
 

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It's a Schottky diode discharging the 4.7 uF capacitor when the power is turned off.
 

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