VDD is unstable. because of VBAT??

[JPark.11]

Hi there.

I'm using a STM32F071 series.

I want to use a RTC with power backup.

So, I added a super-cap on VBAT.

when I power off and on again,

VDD becames a saw blade.... what???

After I discharged the super-capacitor and power on again, VDD is good.

I think the saw blade's bottom (2.98V) is voltage of VBAT.

VBAT is lower than VDD. Because the forward voltage of diode dropped VBAT voltage.

But, What happened to this. It looks like a dancing between VDD and VBAT.

Please help me.

Thank you.

[waclawek.jan]

Some voltage regulators don't cope well with capacitive load, so check your 3.3V voltage regulator first.

JW

[JPark.11]

It doesn't look like resonance problem.

I removed super-capacitor on VBAT circuit. same problem.. T_T

I think VBAT rise up earlier than VDD, a problem occurs.​

[JPark.11]

Should I use the PVD or PWR CTL something register? Actually.. I didn't set that at all. Is that a reason?