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THVD1450: Driver Capacitance

Raghu Tumati
Intellectual 720 points
Part Number: THVD1450

Hello 

I have a question regarding the loading of the THVD1450. Now I understand that is a 1/8 UL. So that would be 96KOhm resistance for one THVD1450. 

But what would its capacitance be? I just wanted this value for simulating its load capacitance before building 256 boards with THVD1450 parts :) 

Thanks

Raghu

  • 0
    TI__Mastermind 25705 points

    Raghu,

    It's a good question. You can use 25pF as a nominal number to evaluate the differential driver output capacitance of THVD1450. Please let me know if you have more questions.

    Regards,

    Hao 

  • 0 in reply to Hao L
    Intellectual 720 points

    Hi Hao

     Thanks for the reply. I have 128 nodes on my RS485 bus. If I am trying to simulate a the load capacitance on a single driver when it is transmitting to another node in the bus,

    a) Would I have to simulate it with a capacitive load of 128x25pF = 3.2nF?

    or

    b) Would it be just 25+25 = 50pF because the other 126 nodes would be turned off? 

    I'm not sure if a nodes capacitance would load the bus even if it is off or HiZ? 

    Thanks

    Raghu

  • 0 in reply to Raghu Tumati
    TI__Guru 73761 points

    Hey Raghu,

    I figured I would chime in. If you have devices sitting on your RS485 bus, then the capacitance of each one would be there regardless of if the device were disabled/HI-Z/OFF. The capacitance is due to parasitics (essentially 2 parallel plates and a dielectric inbetween by some distance) and because the drivers/receivers are made up of transistors they will come with some trace amounts of capacitance which Hao estimates to be around 25pF as a typical value. 

    This would mean you would have 128x25pF due to the transceivers loading the bus (This excludes the parasitic capacitance of the cables).

    -Bobby

  • 0 in reply to BOBBY
    Intellectual 720 points

    Yep. That makes sense. I just wanted to confirm my thoughts. Thanks so much Bobby.