VDD exceeds the absolute max rating

[sergey_alayev]

Hi,

We are looking for information in regards to the behavior of the microcontroller FS32K148 in the instance when the power supply exceeds the absolute maximum rating.

I do not doubt that it will damage the microcontroller; what we need to predict is what could happen:

Fire, smoke, shorted power line to the ground, cease operation in the “cold” state (fail-safe), or any other scenario.

Does any FMEA exist for that microcontroller?

[jinjingyang]

Hello, @sergey_alayev ,

According to limited experience of some failed part, the overvoltage may cause shorted power line to the GND or open power line to the GND. It's not certain. We don't have statistic data for such cases. If the MCU is accompanied with an SBC, the overvoltage event will be monitored by SBC. And the SBC should put the system in safe state. If SBC is also damanged by the high voltage, the upper level HW should make sure the system is in safe state (for example a fuse is broken). And according to MCU safety manual, it's assumed that MCU in powered down mode should be in a safe state. This should be assured by customer in system level.

[naveenm]

Hi,

As understood from your query that the S32K148 is exposed to overvoltage, which is beyond the absolute maximum ratings, and hence damaged the microcontroller.

The microcontroller will cease operations and the state is unpredictable.

In such scenarios, it is recommended to use an external watchdog which runs on a different power supply and clock. In an event, where the microcontroller fails to service external watchdog, an action will be initiated which triggers a reaction (independent of microcontroller) to push the system into safe state.

FMEA does not exist for any such scenario where Microcontroller is damaged due to external power supply failures.

Hope it helps.

Regards,

NaveenM

[sergey_alayev]

Hello Naveen,

Thank you for your response. We have not yet encountered this failure; however, we would like to anticipate this overvoltage scenario on Vdd. We use a TMR architecture with three identical legs(circuits) where three S32K microcontrollers draw power from independent rails. If one microcontroller receives excessive voltage from the power supply, will the other microcontrollers be damaged through the GPIO/SPI/Etc lines? Please see the attached diagram. 

Regards,

Sergey

[naveenm]

Hi Sergey,

I consulted appropriate Designers and the outcome of discussion is that whenever the part is exposed to a higher voltage than specified in DataSheet, there is no predictability that the part would work seamlessly.

Like in the example you listed. If the K1 microcontroller (middle one) receives a voltage between 9-12 V and damages the part, there is likelihood that the high voltage enters the other two parts (top and bottom) via Digital/analog pads and will eventually damage other two parts as well. The extent of damage cannot be evaluated (damage contributing factors includes the applied voltage, its duration and the current source capability of the supply) There is no predictable scenario that can guarantee proper functioning of other two microcontrollers, hence no certainty that safe state would be achieved.

Certainly there are on-chip mechanisms in place which will protect the microcontroller from overvoltage, but only in case when the applied voltage does NOT exceed the absolute maximum ratings. Beyond which the mechanism themselves may get damaged (due to process technology node limitations). It is almost definite that survival of SoC is not possible when applied voltage is in the range of 9-12V.

Independent system level safety measure, like watchdog, can help here to move the system into safe state.

Hope it helps. 

Regards,

NaveenM