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Battery reverse current

Question asked by Mike Forster on Apr 20, 2017

We have a custom STM32F769 board with a Lithium Manganese Dioxide CR1220 non-rechargable coin cell connected to Vbat. We are concerned that there may be an issue with this as there is no reverse charge protection circuitry. On the next revision I will add a diode but the current revision is going for certification.


The reference manual says  -

"During tRSTTEMPO (temporization at VDDstartup) or after a PDR is detected, the power switch between VBAT and VDD remains connected to VBAT.
During the startup phase, if VDD is established in less than tRSTTEMPO (Refer to the datasheet for the value of tRSTTEMPO) and VDD> VBAT+ 0.6 V, a current may be injected into VBAT through an internal diode connected between VDD and the power switch (VBAT).
If the power supply/battery connected to the VBAT pin cannot support this current injection, it is strongly recommended to connect an external low-drop diode between this power supply and the VBAT pin."


I don't understand this. Figure 8 of the datasheet shows tRSTTEMPO starts once VDD is above VPDR so VDD is always established in less than tRSTTEMPO. I am using a VDD of 3.3V.

Can someone please explain this current injection mechanism in more detail so I can determine if this could be a problem.

Given that tRSTTEMPO max is 3ms I suspect not but a clearer explanation would help me convince our certification guy.