Adding Schottky diode in PGood

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engr_joni_ee

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Hi, I have attached a schematic in which they have connected LEDs to PGood signals.

LED DS21: VCCINT PGOOD
LED DS22: VCCAUX PGOOD
LED DS23: VCCBRAM PGOOD

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LED DS25: MGTVCCAUX PGOOD
LED DS14: UTIL_3V3 PGOOD

They have used two bi-direction tri state buffers SN74AVC8T245 to drive the LEDs which is fine. All the PGood signals are pulled-up to UTIL_3V3 using resistors RP1, RP2 and RP3 but what is the purpose of Schottky diodes BAT54T1G ?

and also input and output at the uni-directional tri state buffer NC7SV125 is the same, UTIL_3V3 ?
 

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The diodes form a diode OR gate, any one of the signals can be pulled LOW without affecting the others
 

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Hi,

It's a wired AND.
All inputs need to be HIGH for the output to become HIGH.
If one (or more) input is LOW, the output is LOW, too.

Klaus
 
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If it is And of all the pgood signals from different power rails then I am getting it that will end up to a single LED on the board showing all power rails are stable, right?
 

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Correct. The weak pull-up after the diodes makes the COMBINED_PGOOD signal high but if one or more diodes sees a low voltage on its cathode, the current through the diode will pull that signal low. The diodes are to isolate the individual power good signal from each other but ensure that only when ALL the inputs are high, the COMBINED_PGOOD signal is high. Although it works passively, it effectively makes a nine input AND gate.

Brian.
 

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