PS_NVVDD_EN_R low - circuit question

[Prott]

Hello,
I have a possibly stupid question which I am, unfortunately, already trying to figure out for some time.

I have a card (reference 1080Ti) that has PS_NVVDD_EN present (3v) while PS_NVVDD_EN_R is low. The schematics doesn't quite make sense to me as both of the resistors that are connecting PS_NVVDD_EN_R line to 1V8 and PS_NVVDD_EN are omitted (R536 and R535).

And now the stupid part - which component is responsible for pulling the PS_NVVDD_EN_R line high?

Thank you.

[Prott]

Re: PS_NVVDD_EN_R low - circuit question

Schematic in question - there's no other connection except those two mosfets. Is there a way the rail is getting powered from EN_Q through them?

My understanding is that once PS_NVVDD_EN is high the first mosfets pulls EN_Q low which then in turn shuts off the other mosfet and enables EN_R to go high (leaves it floating) - EN_R itself needs to be pulled high from something else somewhere. Is this incorrect?

Thank you.

[piernov]

Re: PS_NVVDD_EN_R low - circuit question

Reading the datasheet of the buck controller (uP9511?) can help. EN is not pulled-up externally, external circuit leaves it floating to enable or pulls it to ground to disable. Since there's a resistor to ground which must be inside of a certain range of value, one can assume the chip has an internal current source on that pin, although it may not be active all the time.

[Prott]

Re: PS_NVVDD_EN_R low - circuit question

Thank you! I was reading the datasheet, but somehow skipped the part and it didn't occur to me that the buck controller produces its own enable signal.

Time to check the soldering.

[Prott]

Re: PS_NVVDD_EN_R low - circuit question

Fixed (it was a bad soldering job on the buck controller), thank you piernov!