Comments on VBAT source switching circuit

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VBAT source switching circuit

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This is VBAT switching circuit installed in one of my PCB for Timekeeping when VCC is not present.

Original Switching circuit

The switching circuit is working as expected - when VCC is present, the current flows from D3 to VBAT and when VCC is not present, the current flows from J2 to Q6 to VBAT. After a month, we randomly checked the RTC coin cell battery (3V) and it was discharged to 1.6V and that's not normal because these batteries are supposed to work for years without changing.

Then I measured the current VBAT with VCC present = 3.117 and current is -2.251mA Without VCC, the VBAT is 2.8927V and current is 0.015mA (16uA)

In my observation, the negative current is flowing back to the RTC battery and hence degrading it. May be Q6 is not turned OFF properly.

I am not sure if that's what causing the issue. I tried simulating another circuit online. Can I use this circuit and replace the existing one ?

Without VCC With VCC

**Update - ** Here is the new circuit which is on the PCB and this circuit is causing battery drain.(ignore the top circuit) Revised circuit

battery switching rtc

posted 12 months ago

CC BY-SA 4.0

12mo ago

Embedded_geek‭

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[moderator note] I have restored the original version of your question to which you have already received answers. Don't edit your post to the point that the answers you already got become invalid. (2 comments)

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−0

Olin already mentioned that Q6 is always on because R39 keeps the gate permanently low.

Power MOSFETs have a parasitic body diode. The way the MOSFET if drawn in your schematic, it can never fully cut off the battery from the VCC node, because the battery has a discharge path through the MOSFET body diode of Q6.

illustration showing an N-channel and P-channel MOSFETs with body diodes