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Low power consumption design of RS485...

Peter Tseng
Genius 5555 points
Other Parts Discussed in Thread: ISO1500, THVD1400

Dear Sir,

My customer has a question about the power consumption of RS485 while designing it.

Add pull high or pull low resistor will increase power consumption. If the Voc is 3.3V, then there will have 2V crossover the resistor.

Is there any solution of TI that can help customer? Or is there any method can make the PU/PD resistor will not increase the power consumption?

Thanks.

  • 0
    Guru 243820 points

    If all transceivers on the bus have short and idle bus fail-safe inputs, then you do not need external fail-safe resistors. (All modern TI transceivers like the THVD1400 or ISO1500 have this feature.) However, external fail-safe resistors will increase the noise margin.

    In general, the power consumption of the active transmitter(s) is much larger than that of the fail-safe resistors. If the bus is idle most of the time, consider AC termination (see RS-485 basics: When termination is necessary, and how to do it properly). You might even remove the termination resistors, but this will limit the achievable speed.

  • 0
    TI__Mastermind 43426 points

    Hi Peter,

    Thanks for reaching out - Clemens is correct they are not needed (but can be used to increase noise margin) - in general the active drivers on the device will consume much more power than the fail-safe resistors (maybe 10's of mWs worst case). The resource Clemens pointed is a good place to start for terminations.

    Please also see our app note on the fail-safe resistor: https://www.ti.com/lit/an/slyt324/slyt324.pdf 

    Please let me know if you have any other questions!

    Best,

    Parker Dodson