LPC55S28 VBAT_PMU Current Draw

guitardenver — Mon, 22 Jun 2026 15:50:38 GMT

I have a PCB with the schematic below. I have a coin cell battery for backup when the PCB main power is removed. From my understanding, the VBAT_PMU (Pin 51) on the MCU is dedicated for the Always‑On (PD_AON) domain.

When I measure the current draw of the battery, with no main power (only the battery), I get around 102uA. Which is far more than expected for the RTC to consume. Event if D12 is removed. Vbat only is connected to the VBAT_PMU pin and nothing else.

What is the expected behavior here? I would expect the VBAT_PMU to only draw enough current for the RTC to keep time while power is down. 102uA would drain a CR1632 coin cell battery pretty quickly.

How do I power the MCU so the battery only powers the RTC and draws less than 1uA when main power is removed?

Re: LPC55S28 VBAT_PMU Current Draw

Harry_Zhang — Tue, 23 Jun 2026 03:24:31 GMT

Hi @guitardenver

On LPC55S28, VBAT_PMU is the supply for the always-on domain , which includes the PMC, RTC, and OS Event Timer, and that domain remains powered as long as a valid VBAT supply is present. The RTC can continue running in deep power-down from that domain.

VBAT_PMU by itself does not guarantee “RTC-only current.” To get sub-µA backup current, the device must be in deep power-down mode.

So your measured ~102 µA with main power removed is not consistent with RTC-only deep-power-down operation .

For LPC55S2x/LPC552x, the datasheet shows typical total supply current of 110 µA in deep-sleep mode at 25 °C and 3.0 V, while deep power-down with RTC oscillator disabled is 590 nA , and with the RTC running from an external crystal is 790 nA .

To get less than 1 µA from the coin cell on LPC55S28, power only VBAT_PMU from the battery and make firmware enter Deep Power-Down before main power is removed.

Harry

LPC MicrocontrollersのトピックLPC55S28 VBAT_PMU Current Draw

LPC55S28 VBAT_PMU Current Draw

Re: LPC55S28 VBAT_PMU Current Draw