STM32WB55VGQ6 wiring without RF ?

[JMARC.11]

Hello,

I plan to use a STM32WB55VGQ6 for a project without RF because of components shortage. 

I read the datasheet and I couldn't find any information about the MCU wiring when there is no antenna connected to it. Do you know if I have to let the RF1 pin floating or if I have to pull it down with a resistor or something like this when I don't use the RF please ?

Thanks.

[Javier1]

I have never done this but:

If i were you i would match the impedance the internal radio is expecting to find at the RF working freq (50ohms??).

Maybe tying it to ground would do the trick also ( i have serious doubts)..... if you leave it floating all the power the radio output power will slap back to its face overheating your chip, in case you accidentally turn it on (which i dont think is an easy thing to do).

[JMARC.11]

Hello Javier,

Thanks to your answer. Yes now that you say this, it makes sense to me to connect a 50ohms impedance (the input impedance is well 50ohms) at the output of the RF pin. I checked the maximum current sinked by the RF pin which is 12.7mA in bluetooth transmission with SMPS in bypass mode.

With a 50ohms resistor and a maximum current of 13mA, it gives a resistance power of 50*0.013²=8.45mW.

A 0201 resistor has a typical power rating of 50mW.

Therefore, I will connect a 0201 resistor of 50ohms at the end of the RF pin.

Thank you very much for your help Javier.

[Javier1]

is that 50ohms resistor going to be still 50ohms at 2,4GHz? (BLE freq band)

Maybe you need to place a very tiny 0201 radiofrequency resistor.

[Remy ISSALYS]

Hello,

You can let the RF1 pin floating when you don't use the RF.

Best Regards