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THVD1426: Suggestion for termination resistor wattage

sonali zodage
Intellectual 375 points

Part Number: THVD1426

Tool/software:

Hello,

We are using the THVD1426DRLR RS-485 transceivers for our application with a 5V supply voltage and a 120Ω, 1/10W termination resistor. Could you please confirm if the 1/10W termination resistor is sufficient, or should we increase the wattage?

We have attached the schematic of the THVD1426DRLR for your reference. Please review the schematic and advise on the appropriate wattage for the termination resistor.

Thanks,
Sonali

  • 0
    Intellectual 375 points

    Additionally, please let us know what is the voltage level for RS485 signals?

  • 0
    Guru 322000 points

    The typical power dissipation during normal operation is 75 mW. There is not much of a safety margin to 100 mW. Do you want to be able to survive a short to another voltage in the cable?

    The driver's output signals are somewhere between 0 V and 5 V. If there can be a ground shift between the nodes, then the bus voltages might be in a wider range.

  • 0 in reply to sonali zodage
    TI__Guru* 79271 points

    Clemens is correct though the typical power dissipation is probably double his estimate if two 120 ohm resistors are used. 

    The typical VoD for this device is 3V. With two termination resistors of 120 ohms (120//120 = 60 ohms equivalent) the current through the resistor will be 50mA. Power = V x I so 3V x 50mA = 150mW. This doesn't include transients where inductance from the cable could increase the voltage during transitions. For margin, you could double this number to survive normal operation. (This assumes a 50% overshoot in voltage and current).

    For any kind of short to a Vbus rail, you probably need to know what max voltage the Vbus rail will be and assume that voltage will be dropped across the equivalent resistor.

    -Bobby