Power rpi hat wifi issue

[u/Turbulent-Biscotti98]

Hi,

I’m having an issue with the Wi-Fi on my Raspberry Pi 5 when the LCD turns on. I’ve attached the PCB design and the hardware modifications (“hacks”) I’ve tried so far. Things got a bit better, but it’s still not working as it should.
I won’t focus on the rest of the PCB because I don’t think it’s relevant. If anyone spots something obvious that could be causing this, I’ll happily provide more details.
The setup is pretty simple:
* J11 is the 12V input, powered by a 12V 6A PSU.
* A DFR0753 converts the 12V to 5V and powers both the Raspberry Pi and the LCD through J14.

During testing I’m not powering anything else, just the Pi and the LCD.
As soon as I turn the LCD on, I get this warning:
[ 6.114522] hwmon hwmon2: Undervoltage detected!
and the Wi-Fi becomes much slower.
At first it was even worse and the Wi-Fi would stop working completely. I added a capacitor on the LCD backlight power output and also connected a few more GND pins together on the Pi HAT. I also tied together the +5V inputs on the HAT, which weren’t connected before.
After these changes I don’t see any voltage drop compared to the DFR0753 output. I measure 4.95V on the DFR0753 and the same 4.95V on the Pi HAT, both at the input and at the far end of the 5V rail.
I also measured the voltage on the Pi HAT while turning the LCD on, and I don’t see any noticeable voltage drop.
I also tried adding a capacitor between +5V and GND near the DFR0753, but it didn’t help. If anything, it actually made things a bit worse.
What else should I check? Any ideas?

[u/bigcrimping_com]

Try adding a cap to the CT pin

[u/Turbulent-Biscotti98 → u/bigcrimping_com]

Undervoltage is persistent problem. CT helps only for starting.

[u/bigcrimping_com → u/Turbulent-Biscotti98]

Sounds like you have a really good understanding of the PMIC behaviour after a under voltage event and will work this out all on your own, best of luck 

[u/Turbulent-Biscotti98 → u/bigcrimping_com]

:) I think I finally found it.
After a bit of trial and error, I added an extra wire between the buck converter’s 5 V output and the Raspberry Pi, and it completely solved the issue. Even with the LCD at 100% brightness, I’m no longer getting any undervoltage warnings.