nPM1300 BUCK2 not regulating to 3.3 V in standalone mode

[tarunkt]

Hi Nordic team,

I'm debugging a custom board using the nPM1300 and I'm seeing unexpected behavior from BUCK2. I'd appreciate any guidance on whether I'm misunderstanding the VSET configuration or if there is another startup/configuration requirement that I'm missing.

Hardware Configuration

• PMIC: nPM1300

• Operating mode: Standalone (no PMIC configuration over I²C)

• BUCK2 desired output: 3.3 V

• VSET2 resistor: 300 kΩ to GND

• According to the nPM1300 Product Specification v1.3, the VSET2 table shows:

  • 250–500 kΩ → 3.3 V

• NTC pin connected to GND through a 0 Ω resistor

• No intentional connection between BUCK2 output and VSYS

• Resistance from VSET2 to GND measures approximately 300 kΩ on the board

Observed Behavior (Board Under Test)

With USB power connected:

Signal	Measured Voltage
VBUS	5.1 V
VSYS	5.0 V
BUCK2 Output (VOUT2)	4.45 V

The expected BUCK2 output is 3.3 V, but instead it sits around 4.45 V.

Battery-only operation

When running from battery:

• One board measures ~3.3 V at BUCK2 output

• Another board measures ~3.5 V at BUCK2 output

The output appears to track the battery voltage rather than regulate to a fixed 3.3 V.

Questions

1. Is 300 kΩ a valid VSET2 value for selecting 3.3 V on nPM1300 Product Specification v1.3?

2. In standalone mode, should BUCK2 automatically enable and regulate to the VSET-selected voltage at startup?

3. Could grounding the NTC pin through 0 Ω affect BUCK2 startup or operation?

4. Is it expected that VSYS measures almost the same as VBUS (5.0 V vs 5.1 V) in this configuration?

5. Are there any recommended measurements or status registers that would help determine whether BUCK2 is actually enabled?

Any suggestions for further debugging would be greatly appreciated.

Thank you.

[Szabolcs]

Hi,

This sound like an issue, the nPM should start the Buck automatically with the configuration you stated. Can you please post your schematic?

Regarding your questions:

1. Yes,
2. Yes.
3. I think it shouldn't, but you can try replacing it with a 10K resistor just for debugging purposes. If there is no NTC for the battery, this feature should be disabled in the register, but I think it only affects battery charging, and not the Bucks.
4. Yes, there is no regulator behind VSYS, it is only a current-limited output.
5. Yes, see BUCKSTATUS and BUCK2VOUTSTATUS.

[tarunkt]

Dear Szabolcs,

Thank you for your response and for clarifying the questions.

Please find the complete schematic attached to this ticket for your review.

https://devzone.nordicsemi.com/support/359762

Kindly review the schematic and let me know if you notice anything that could prevent BUCK2 from starting or regulating correctly?

Thank you for your support.

Regards,

Tarun

[tarunkt]

Additional information from testing:

I am able to communicate with the nPM1300 successfully over I²C.

Using the regulator shell commands, I can query the PMIC and obtain the following:

regulator vget BUCK1
3.300000 V

regulator vget BUCK2
3.300000 V

So there appears to be a discrepancy between the PMIC-reported BUCK2 voltage setting (3.3 V) and the physical voltage measured on the BUCK2 output (3.33 V) when LiPo Battery only connected state.

Then I connected USB to give the VBUS supply when battery connected, immediately the voltage on VBUCK2 changed to 4.2V. Still I could see the I2C communication active.

Current consumption shows: 90-110mA in the USB power-meter

Please let me know any insights.

Thank you