FS26 VPRE, VBOS malfunction

[__markus__]

Hi,

we made a design for a customer with the FS26 and the S32K344. Everything worked and we passed the EMI tests. But now during tests in the field, we have FS26 PMICs that have a malfunction and as a result, the uC doesn't work anymore. We use it in a 24V System and have a TVS Diode and an Ideal Diode Controller to protect the input from spikes.

On several boards we see that the VPRE regulator has a malfunction and the voltage just reaches between 4,2V to 4,7V, but it must be 6V. Also VBOS has a too low voltage. In the picture you can see the startup of the VPRE buck converter at 24V, which looks very different to a good PMIC, and the resulting voltage of about 4,2V at the end.

We thought that the damage comes from voltage spikes on the input voltage, but after some measurements it looks like no spike passes the protection circuits.

It could also be that the VPRE voltage is too low, because the internal LDO for VBOS (4,1V) is destroyed and is the result of the defect VBOS LDO.

We have a 1uF capacitor on the VBOS output, which is smaller than the recommended size (4,7uF), but 1uF is also used in some reference designs.

I will also attach a part of the PMIC circuit, maybe we made something wrong there.

Do you have any idea what could be the cause for the issue or have you seen any comparable issues?

We also replaced the PMIC on one board, to be sure that the error comes from the PMIC and afterwards the system booted normal.

Thanks for your help!

[RafaR]

Hello __markus__

Good day!      

I apologize for the delay.

Are you using the FS26 at 24V system with only a TVS protection and without a DCDC in front, right ?  If yes, this is not recommended, please see below in AN13850, a DCDC is must for 24V application as FS26 is a 12V SBC, whose nominal voltage range is 18V max. it is not recommended to use FS26 working at 24V for a long time.

VBOS normal voltage output is 5V, 4.1V is too low. I suppose VBOS has external overstress. 4.7uF VBOS output capacitor is must for VBOS. 

In addition, can you check boot_pre capacitor used in the board ? 100nF is preferred. 

I hope this information has helped you, please let me know if you need help with anything else.

Have a great day and best of luck.

[RafaR]

Hello __markus__

Good day!

I'm working on finding the error; I'll contact you soon as I determine the reason for the failure.

Have a great day and best of luck.