TPS51225CRUKR Voltage limit

[Bradbeat]

Hi,
I have a PGOOD pin down on this chip and I’m wondering if it could be because there is 5,2 v on VREG5 ( the high limit according to the datasheet is 5,1v ).
if yes, why is it to high?
the motherboard board is LA-G881P.
thank you.

here is some measurements :

• VIN = 16,3 V
• VREG3 = 3,299 V
• VREG5 = 5,203 V
• SW2 (repos) = 3,423 V, pikes at ~16,4 V
• VBST2 = 8,592 V → VBST2 − SW2 = ≈ 5,17 V
• VO1 = 5,28 V (stable)
• EN1 = 3,08 V, EN2 = 3,04 V
• VFB1/VFB2 ≈ 2,1 V

[mon2]

No. Pgood is open drain so it must feature an external pull up resistor to park the line when the signal is released. When Vout is 95% reached, the line should float.

[Bradbeat]

Thank you for your answer,
I can't see any pull up on the board so I assume it is in the super I/O chip?
If it is, is the SIO burnt or could it be un SIO firmware issue?

[mon2]

Which product / motherboard are you reviewing? Link the details (here) / schematics (inside the schematic forum).

[Bradbeat]

hi, it's the LA-G881P from an alienware area 51m r1 if i understood what you asked?

[mon2]

Are DE2 and DE3 stuffed on your board?

[Bradbeat]

No, they aren't

[mon2]

Are there pcb trace shorts @ DE2 or D3? If yes, then the EC is monitoring this POK line.

Otherwise, I do not see why POK signal is important. It is not bonded to anything if these parts are missing. It appears to be a floating signal.

[Bradbeat]

no,they're are not.
It's possible that signal is not important for the ec chip to turn EC_RSMRST# on?
I assume my problem is elsewhere 😅

[Bradbeat]

Here are 2 pictures of 5VALW and 3VALW signals, probed on the coils.
Are thoses pikes at 17 v normals?
Shouldn't they be almost flat?

[mon2]

No. These are dangerous IMHO. I recall a similar case when I was on the XMOS (audio) forum in the UK. The XMOS devices are like a FPGA that offers multiple cores of independent logic to run in parallel. One of their reference design was to run the USB engine for which they use a USB connector. The VBUS line would have similar spikes and the root cause was linked to the USB cable due to inductive inrush currents then hitting a high capacitance on the board. That is, the voltage literally multiplied - like your case.

What is the resistance to ground of these rails? What is the capacitance to ground of the same rails?

[Bradbeat]

I'm new to this, i really want to learn so thank you for your help!
What could cause that?

3.3v rail : ~20Kohms 580uF
5v rail : ~20Kohms 470uF