What happens at Vbat when Vdda is not present?

michael.m · ‎2018-12-11

In my system (STM32L471) I want to periodically measure Vbat. Can I do this through the Vbat pin if Vdda may not always be present? Obviously I turn on Vdda whenever I want to measure Vbat but are there any leakage currents or other problems in case Vbat is present but Vdda is not?

Uwe Bonnes · ‎2018-12-11

Your question is not clear. I guess you provide VBAT to some ADCx_INy. Most ADCx_INy are FT pins, so as long as VBAT is well below 3.6 volt no damage will happen. TT pins, e.g. PA3 may not be powered without the chip powered. For FT pin, Table 58 tells about up to 0.65 uA

michael.m · ‎2018-12-11

Sorry, I'll clarify my question.

I tried feeding the battery voltage through a FET circuit to a voltage divider I then sampled on an ADC_IN pin. This didn't work as intended because this circuits current consumption was too large when Vdda is disabled (28uA).

Now I want to attach Vbat to the "Vbat" pin and use the integrated option to measure this.

waclawek.jan · ‎2018-12-11

> This didn't work as intended because this circuits current consumption was too large when Vdda is disabled (28uA).

I don't see a reason why the STM32 would be the cause for the excessive current in this case (i.e. your FET switch is supposed to prevent any leakage, regardless of whether VDDA is connected to STM32 or not).

I see no explicit guarantee in the public data (datasheet, RM) that there's no leakage from VBAT to VDDA, so for that you might need to contact ST directly.

JW