TIDA-00540: Multiprotocol Transceiver Reference Design with THVD15xx

Hi Ben,

You can find a detailed description of the operation of this circuit here:

www.ti.com/.../TIDA-01238

Basically, a dual FET (CSD87502Q2) is used to enable the 120-Ohm termination when the FETs are on and disable it when the FETs are off. (Two FETs are used so that there is not conduction through the FET body diodes when the polarity of the differential signal is reversed.) The other FET (CSD17483) is just used to control the gate of the dual FET. This is because a higher-voltage gate control is useful, since the RS-485 signals can be within a relatively wide voltage range and the circuit needs to ensure that the gate is always higher than the RS-485 signals when termination is enabled and always lower when the termination is disabled. So, the single FET just serves to amplify a low-voltage logic control (provided to its gate) to a higher-voltage signal (referenced to "Vbias" in this circuit) to control the gate of the dual FET.

I hope this is all clear - please let me know if not.

Regards,
Max