Running VBAT off of a coin cell and draining battery quickly

[johnblackburn]

Currently have a board with a K61 processor that uses a coin cell battery on the VBAT input to monitor the tamper pins during power down. The battery is 17mAh and is draining overnight. We are also connecting a differential ADC input to monitor battery voltage while powered up. Someone here mentioned hearing of an issue with the differential ADC inputs causing a high drain on the battery. Has anyone heard of this and have any data as to the root cause? Is it a function of the differential inputs? Or is it a particular ADC? If not the ADC, then any other ideas as to why the battery would drain so fast?

[Hui_Ma]

Hi John,

When the main power off, only with VBAT power on, the VBAT power consumption is about 0.91uA(Includes 32kHz oscillator current and RTC operation). The tamper detection power consumption is about 1uA.

While, there with dramatic different if the 32KHz oscillator working mode: low power mode or high gain mode.

If 32KHz oscillator works at low power mode, the power consumption is about 0.5uA for oscillator;

While, if works at hign-gain mode, the power consumption is up to 25uA.

And I also checked the ADC pin leakage current is about 2nA (Max. 0.5uA).

Please check the 32KHz oscillator working mode, if it works at high gain mode (You could use scope to measure the oscillator signal and check the voltage level).

With it helps.

Please be attention that: Due to service metrics, this service request will be closed automatically after 7 days without any updated feedback.

Have a great day,

Ma Hui

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[johnblackburn]

So what voltage levels would indicate that the oscillator is in High Gain mode vs Low Power mode? If I probe the oscillator I get about 1V P-P. Section 27.4 of the Reference Manual discusses the RTC_GP_DATA_REG, but does not give the actual bit definitions for the register. How do I switch the clock modes?

[Hui_Ma]

Hi John,

Sorry for the later reply.

I measure the TWR-K60D100M board, the 32KHz crystal with 800mV voltage.

I checked the K61 reference manual, the RTC oscillator with the automatic gain control.

There without to set the work mode.

My previous mentioned is about system oscillator, which also could connects with 32KHz crystal.

Besides of K61 chip, any other components on your board will be powered with battery?

Have a great day,
Ma Hui

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[johnblackburn]

The only other thing that is being powered off the battery is a Microchip MCP79410, which pulls about 1uA during battery operation.

[Hui_Ma]

Hi John,

Sorry for the so later reply.

Do you have chance to measure the K61 related power consumption when main power is off (just using VBAT battery power)?

Then we can compare the power consumption if as our expected.

Thank you for the attention.

Have a great day,
Ma Hui

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