LPC55S28 VBAT_PMU Current Draw

[guitardenver]

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? 

[guitardenver]

Thank you for your reply!

I have code that will trigger the Deep power down on a brownout interrupt, which works great. But there is a huge problem with this method. If the ROM Bootloader is executing, and power is removed, the brownout interrupt is not setup and does not have code to put it into power down.

Is there a good way to guarantee that I can get into the deep power down mode? If there is any state that leaves the MCU in normal mode, the battery will get drained. Maybe there is something I am missing. If not, I may have to put an external RTC chip on the board. 

[Harry_Zhang]

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.

BR

Harry