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Original question:

RS422/RS485 tranceiver power consumption

SN65HVD1780-Q1: RS485 transceiver output short circuit power dissipation

Umesh Patil10
Intellectual 760 points
Part Number: SN65HVD1780-Q1
Other Parts Discussed in Thread: SN65HVD1780

Hello team,

I am using sn65hvd1780-q1. 

If I use loading circuit with 4.7 Kohm as pull up/down and 47 Ohm series resistors for both outputs A and B, then how to calculate power dissipation of 47Ohm resistors during output short circut to battery voltage 16V.

Driver short-circuit output current is 200 mA.

My VCC voltage is 5V.

  • 0
    Intellectual 760 points

    This is my application circuit.

  • 0 in reply to Umesh Patil10
    TI__Guru 50795 points

    Hi Umesh,

    In the fault case of 16v on the B pin (with pull-down) while the device is driving B low (D and DE are high), the current through the driver is going to much more significant than the current through the pull-down (16v / 4.7k-ohm ~= 3mA), so we can focus on this. Assuming the driver allows 200mA through the B line - in addition to around 10mA through 120-ohm termination to A and 4.7k-ohm pulldown - the power dissipated by the 47-ohm resistor can be estimated to be (210mA)^2 * 47-ohm ~=  2.07 Watts. 

    This is a rough estimate, but should be in the range that you're looking for. Using these calculations, the required power dissipation can be reduced by using smaller value resistors which would still provide some transient protection. This would require the SN65HVD1780 to dissipate more power instead.

    Let me know if you have any more questions.

    Regards,

    Eric

  • 0 in reply to Eric Schott1
    Intellectual 760 points

    Hello Eric,

    Thank you for your explanation, now my query is whether driver short circuit output current is internally limited to 200 mA or its maximum capacity given as 200 mA? 

    If internally not limited then for 16V short circuit condition greater than 200 mA may flow. 

    in datasheet no where mentioned that it is internally limited. Please have a look on this and help me out. 

    Again thanks for your support.

  • 0 in reply to Umesh Patil10
    TI__Guru 50795 points

    Hi Umesh, 

    Driver short-circuit output current is internally limited by the device. Slight increases in current with increasing voltages may be expected. Once the driving FET is saturated, the IV curve is mostly flat above the specified limit. 

    Regards,

    Eric