STM32F429ZI VBAT pin characteristics to resist short-circuit

[Peng]

Hello,

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We use STM32F429ZI on our project now, and the VBAT pin is connected to a button cell as RTC power supply. And we are running a short circuit test on VBAT pin, if there is a short circuit on the VBAT pin, then it means that the positive terminal of the button cell is shorted to ground, which will cause an issue.

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So my question is what are the VBAT pin characteristics to resist short-circuit ?

[Peter BENSCH]

Welcome, @Peng​, to the community!

The short circuit test probably checks for a connection between VBAT and GND, right?

If necessary you can do without the short circuit test, because presence of a voltage at VBAT excludes a short circuit.

Anyway, the VBAT pin would not be disturbed if there was a short to GND, unlike the battery by the way, which would be discharged in a very short time.

Does it answer your question?

Regards

/Peter

[Peng]

Hello Peter,

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Thanks for your reply, this test was done by a laboratory, since the button cell is made by lithium-ion, so the laboratory considered the button cell as a lithium-ion battery. If there is an internal short circuit on the VBAT pin, there will be a potential risk on the battery, because the positive side of the battery is connected directly to the ground.

From the power supply diagram of STM32F42xxx, we see there is a power switch to choose the power supply for RTC. So is there any protection to avoid the short circuit on this power switch or anywhere in the STM32?

[Peter BENSCH]

Oh, I probably know what you mean and intend now.

However, short circuits do not occur within an STM32, since all STM32 are fully and extensively tested for function in the Fab, so that external short circuits must always be assumed: solder bridges, mechanical faults or PCB faults.

As for the power switch, like any normal transistor, it is not explicitly protected, so protection has to be designed externally.

What Jan probably meant below is the reference from RM0090, section 5.1.2:

During the startup phase, if VDD is established in less than 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.

However, it is less about the value of the current, but about the fact that the voltage source connected to VBAT (battery, capacitor, external voltage source, etc) should not be overloaded if its voltage falls below VDD-0.6V. To achieve this, it may be useful to connect a Schottky diode from VBAT (anode) to VDD (cathode).

Hope that helps?

Regards

/Peter

[waclawek.jan]

There's a switch and an internal diode between VDD and VBAT. Switch shorts VDD and VBAT as long as the mcu is operational (i.e. after reset temporization and before powerdown/brownout is detected); however, outside this time, if VDD>VBAT+0.6V, current flows through that internal diode.

I'm not sure what the maximum of that current is, it's not specified in DS. For further details, you should contact ST directly, through FAE or web support form.

JW

[Peng]

Hello Jan,

Thank you for your reply.