TPS40422: Disable Temperature Sensing Function

Allan Fan

Part Number: TPS40422

Hi expert,

Customer has a product in production for years used TPS40422 and got some failure returns showing OT fault however it's just room temperature. They found out it was due to the BJT aging cause the Vo become good once a new BJT replaced. Wondering if there is a HW(modify circuit)/FW(change registers) workaround can disable to the temperature sensing function?

Regards,

Allan

over 2 years ago

0 Travis Williams over 2 years ago

TI__Expert 8960 points

Hi Allan,

I wouldn't recommend doing that. The inductor temperature sensing is needed to give accurate output current reporting, as the DCR of the inductor is a function of temperature.

Also, there is no way to turn off temperature sensing in firmware.

I would recommend using a scope with a high impedance probe (>=1MΩ) to check the voltage across the BJT on a failed board and on a good board. While it is certainly possible that BJT aging is the issue, its a strange one. I'd expect the board's caps to fail years before a discrete NPN fails.

If it does turn out to be that the BJT voltage is bad (should be in the 500-700mV range), I'd recommend replacing it with a different model BJT rather than removing temp sense entirely.

Thanks,

Travis

0 Allan Fan over 2 years ago in reply to Travis Williams

TI__Expert 6910 points

Hi Travis,

Customer measured Vbe of TSNS1 and TSNS2 but we would see TSNS1 the delta Vbe become larger, low level of Vbe dropping from 520mV to 400mV after an hour.

May we know what's the relationship between the Vbe and the temperature?

What parameter in the BJT we can go check it to clarify the issue was caused by that?

Regards,

Allan

0 Travis Williams over 2 years ago in reply to Allan Fan

TI__Expert 8960 points

Hi Allan,

Just about any silicon NPN transistor should have a Vbe of around 700mV at room temperature, and that voltage should drop by ~2mV for every degree C that the junction temperature increases by. Have you checked the transistor temperature when its reporting 520mV?

Unless that transistor is very different design of NPN than I'm used to, I'm not sure how those voltage readings are even possible.

Is it possible your customer is using a germanium-based transistor? TPS40422 is designed to exclusively work with silicon NPN, which is the most common type of discrete BJT.

Thanks,

Travis