Power ManagementのトピックPF8121 adjusting LDO voltages in U-boot/Linux

PF8121 adjusting LDO voltages in U-boot/Linux

dsmith — Tue, 03 Feb 2026 23:33:44 GMT

I am trying to adjust the voltage output of LDOOUT1 over I2C from 3.3V (the value flashed in the OTP memory) to 1.8V. However when I changed the value of VLDO1_RUN, the PMIC resets. I suspect I am trigger an under voltage event as when I turn LDO1VMON_EN off, the PMIC does not reset, but I can't figure out what register the voltage is being compared against. My guess is that it is being compared to OTP_VLDO1, but I can't seem to read the value of that register so I am not sure.

u-boot=> i2c md 0x08 0xCE 0x3
00ce: 00 00 00 ...
u-boot=>

Having to disable the voltage monitor to be able to change the value of the rail seems a bit extreme and I wanted to ask if I am missing something about how to properly swap the voltage of the LDOOUT1 rail over I2C?

Re: PF8121 adjusting LDO voltages in U-boot/Linux

guoweisun — Wed, 04 Feb 2026 04:01:55 GMT

Can you help and share with your schematic here?

Please also let me know the PF8121 full part number?

Re: PF8121 adjusting LDO voltages in U-boot/Linux

dsmith — Wed, 04 Feb 2026 16:42:35 GMT

The full PMIC number is PC32PF8121A0EP. I have also included images of the PMIC in our design.

Re: PF8121 adjusting LDO voltages in U-boot/Linux

guoweisun — Thu, 05 Feb 2026 07:11:24 GMT

1:PC32PF8121A0EP is the blank part which mean we can't identify the actual OTP part, because I assume it had done OTP.

2:I don't have PF8121 in hand now, but I have PF8100 I tested it by enable monitor and UV/OV State then adjust the LDO1 value ,there is no reset happen.

So you can directly change the LDO1 value at least in my hand there is no more issue directly change the LDO1 value.

Re: PF8121 adjusting LDO voltages in U-boot/Linux

guoweisun — Thu, 05 Feb 2026 08:04:59 GMT

For your schematic:

1:Pin47 need 2.2uF decouple capacitor

2:The input capacitors for LDOX are 1uF

3:Pin 48 connect it to ground.

4:Pin21 pull up to VDDIO with 100K

5:You can try to read below registers for confirm the fault.

I will close your case 00958417 and please neglect its survey request.

At last please let me know the read registers result after update your schematic.