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SN65HVD74: SN65HVD74 questions

Kai Nomiyama
Expert 5335 points
Part Number: SN65HVD74

Hi Team,

I have questions about SN65HVD74 spec.

Could you help below?

Question 1

In the datasheet figure 1. we can see "Driver output voltage vs Driver output current" curve.

Why does Vol output voltage is increased when Driver output current is increased?

Why does driver output voltage draw that curve?

Question 2

when they tested with 8Mbps data rate, they found propagation delay(IN50% -> OUT 50%) is 22ns.

in the datasheet, the propagation delay spec is changed depending on the data rate(400k/20M/50Mpbs).

could you let me know the reason why if the propagation delay time become longer when data rate is decreased?

Thanks,

Kai

  • 0
    TI__Mastermind 25705 points
    Kai,
    You can read the figure 1 this way. When the output current increases, it means the load becomes bigger (smaller resistance). Let's consider the TX keep its driving strength (constant output resistance). Therefore by resistor divider, the voltage drop on the load is smaller and on the driver (Pside or Nside) are bigger. The voltage drop on Pside is Vcc-Voh, and on Nside it's Vol.
    Which propagation delay did you refer to, TX or RX? Generally speaking, the design is tweaked to make the different devices optimized for a certain speed. For example, HVD70/HVD71 are designed for 400kbps data rate, HVD76/HVD77 for 50Mbps. You can think like the circuit responses more slowly for lower data rate device.
    Please let me know if you have more questions.
    Regards,
    Hao
  • 0 in reply to Hao L
    TI__Expert 5335 points

    Hi Hao,

    Please let me supplement/correct "Question 2".
    this question is related to SN65HVD74 Driver propagation delay.
    in the datasheet, the driver propagation delay @20Mbps is 20ns_max.
    When my customer tested with 8Mbps, the driver propagation delay was 22ns.

    They found if data rate decreases(20Mbps to 8Mbps), the driver propagation delay increases(20ns_max to 22ns).

    Is this normal behavior of SN65HVD74?

    I don't have test condition(RL/CL) but is this only related to test condition?

    Thanks,
    Kai

  • 0 in reply to Kai Nomiyama
    TI__Mastermind 25705 points
    Kai,
    I'm sorry I didn't fully understand your question 2. The device's propagation delay should not be dependent on the data rate. But the factors like the thresholds and common mode voltage do impact the result. If you could share the test condition and the way of measurement, we can help debug to see what makes the difference.
    Regards,
    Hao
  • 0 in reply to Hao L
    TI__Expert 5335 points

    Hi Hao,

    I'm asking their test condition and the way of measurement so once I get it, I will get back to you.

    Thanks,

    Kai