ST75MM output saturation

GBosc.1 · ‎2021-07-20

Hi,

we made a board with the same components as the ST75MM demoboard, but we have a strange behavoir. Without signal on the IN+ of PA, we have a saturation (square wave) at the output of PA. Is there anyone who can help us understand why there is this problem?

Thanks

SBEN .2 · ‎2021-07-20

Hello @GBosc.1 ,

Without being expert on the manner, I have a hunch about your issue. What is the frequency of the square signal? If it is 50Hz?

If it is so, it means that the pin is floating and detecting the ambient electromagnetic noise. You can this on a scope by touching the lead, you'll notice a sine wave with a relatively high amplitude, which when staurated might look like a square signal.

Best regards,

@SBEN .2

GBosc.1 · ‎2021-07-20

Hi @SBEN .2,

the signal is at the working frequency of the chip, about 130kHz.

SBEN .2 · ‎2021-07-21

Hello @GBosc.1 ,

That seems within the nominal range of the switching power supply:

Perhaps this is an EMC issue due to your board's layout, provided the BOM and the schematic are the same. See section 5.1.1 and 5.1.2 of the EMC design guide for STM8, STM32 and Legacy MCUs - Application note (reading the whole document and adhering to the provided guidelines is advised in case you need to certify your device).

Best regards,

@SBEN .2