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SN65HVD37: How to get output impedance for Y, Z pin?

Hung Ching Hsu
Expert 3086 points
Part Number: SN65HVD37

Hi Team

My customer have 2 questions about SN65HVD37. 

Please kindly help with these. It will be appreciate. 

1. Datasheet says VOH(min) is 2.4V on Pin 2(Receiver) when IOH=-8mA. 

Customer wants to connect Pin 2 to another device, which VIH(min) is 2.64V. So here's the problem. 

IOH was actually not -8mA but only several uA in their application, is there any graph showing IOH for y-axis and VOH for x-axis? 

Customer wants to conclude that, "when IOH is several uA, VOH is above 2.64V", so they can use SN65HVD37 in this case. 

2. What is output impedance(ohm) for Y and Z pin and how can customer calculate it?

There is no VOL, VOH for Pin Y and Z in datasheet. 

Please advise us. 

Thanks.

Regards,

Jo 

  • 0
    TI__Guru 53575 points

    Hi Jo,

    With a small load current from the R pin, you can expect voltage levels much closer to the typical specified value. With only a few uA draw, I don't anticipate any issue achieving the input requirements for this secondary device. 

    The output impedance for the driver can be estimated using the I-V graph in Figure 10 of the datasheet. The actual impedance value you use will depend on your expected operating conditions. 

    Let me know if you have any more questions.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 3086 points

    Hi Eric 

    Thanks for the reply. 

    Regarding output impedance for the driver, figure 10 shows for RL:Differential load resistance. 

    Does it mean the impedance serial connect to pin Y or pin Z ?

    Since there was "Differential" for RL, shouldn't it be the resistance between pin Y and pin Z? 

     Also want to confirm with you about the meaning of "Differential Output Current".

    Does it mean the current difference flowing to Pin Y and Pin Z?

    Please see more detail about this application in "Thread Tracking Toolkit"

    Thanks.

    Regards, 

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Guru 53575 points

    Hi Jo,

    When active, the output driver will be pulling one output pin towards Vcc and the other towards ground to create a voltage differential between the two bus lines. This differential voltage is described as the voltage at pin Y minus the voltage at pin Z. Ideally this differential voltage would be the same as Vcc to GND, but with output impedance, we see a smaller differential due to the voltage drop across the driving FETs. The impedance of the driver can be estimated by looking at the voltage drop across both pins (different between driven differential voltage and Vcc - GND potential) and the current that flows through the driver. Figure 10 shows this relationship, but it does not follow the typical V = IR curve as the impedance of the driver is complex and changes with output current. This is why we provide a graph of the output characteristics rather than a fixed value for the impedance so real behavior can be estimated based on the system conditions. 

    Hung Ching Hsu said:

    the meaning of "Differential Output Current".

    Does it mean the current difference flowing to Pin Y and Pin Z?

    Yes, differential output current refers to the flow out of one driver pin and into the opposite polarity pin. This current path is ultimately from Vcc, through one driver pin, bus termination(s), second driver pin, and to ground. 

    Regards,
    Eric Schott