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Original question:

THVD1550: Short Circuit Current Limit?

THVD1550: Stub constraints?

Antonio Faggio
Expert 8050 points
Part Number: THVD1550
Other Parts Discussed in Thread: THVD1429, , THVD1520, THVD2450

Hi,
Back to this post for THVD1550 for practical reasons, customer is unable to reduce the stub below about 18 cm. Value much greater than the 1.9 cm that this transceiver would like based on the minimum value of its rise time (1 nsec). On the other hand, an alternative part number could be THVD1429 which, being 20 Mbps, has "slower" fronts; however it has a worse common-mode (+/- 12 V), although still acceptable for customer application.
In addition, THVD1429 has internal voltage suppressors which we fear may introduce excessive mode capacity common in multi-point applications like ours. We are helped by the fact that our baud-rate (max 12 Mbps, but typically not higher than 8 Mbps) is much lower to the 50 Mbps "plate" of the THVD1550, therefore any reflections due to the stub could be acceptable given the "Width" of the bit. We will do further tests and, if there are any problems, in essence I see two ways: 1) "Slow down" the THVD1550 (with low value resistors in series to pins A and B or any other methods?) 2) Change part-number?
What do you think about? Any solutions or improvements.
Thank you,
Antonio
  • 0
    TI__Mastermind 25705 points

    Antonio,

    Can you elaborate what internal voltage suppressors are in THVD1429? Are you worried about the parasitic capacitor to load the bus too much?

    If the stub is too long, the reflected signal will take time to settle down. However I guess 100ns bit width might be long enough for the settling, unfortunately which can only be verified by testing. Sometimes several long stubs on the bus could make the situation more complicated. Other than slowing down THVD1550's signal, you might try adding some big termination at the end of the stub like 10kOhm to filter out the high frequency components due to reflection.

    If it turns out that you need to change the part, another part you can check out is THVD1520 (10Mbps).  

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 50 points

    Hello, we can reduce the stub to about 12 cm. THVD1429 has built-in TVS diodes and the differential capacitance between A and B is 220 pF. The common mode capability introduced by TVS is not declared but I wonder if it could be a problem in multi-point applications (maximum about 16 nodes) if the value is close to the differential one. THVD1520 has common mode -7 / 12V instead of -15/15 V of the THVD1550 and also seems to inhibit the possibility of running at 12 Mbps. Thanks for the suggestion of the 10k termination. We can test all these part numbers but I was wondering if putting low value resistors (<5 Ohm approx) in series with pins A and B could slow down THVD1550 and solve the problem.

    Thank you,

    Paolo

  • 0 in reply to Paolo Brentegani
    TI__Mastermind 25705 points

    Paolo,

    The impact of 'TVS diodes' in THVD1429 on the common mode has been taken into account in the datasheet. With the diodes, the common mode is from -12V to 12V. I forgot to mention THVD2450 (50Mbps part) has +/-25V common mode, which you might be interested.

    Adding small serial resistors might slow down THVD1550. However the effect depends on the parasitic capacitor, which works with the resistor to generate a first order RC low pass filter.

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 50 points

    Hi, thanks for the reply. I apologize for the mistake: not "capability" but "The common mode capacitance introduced by TVS is not declared but I wonder if it could be a problem in multi-point applications (maximum about 16 nodes) if the value is close to the differential one".

    Thank you!

    Paolo

  • 0 in reply to Paolo Brentegani
    TI__Mastermind 25705 points

    Paolo,

    My understanding is that the differential capacitor value matters since the signal is differential. However 220pF*16=3.52nF is not a small value for 10s of Mbps communication.

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 50 points

    Thanks again for the information, I'll proceed with some tests.

    Regards,

    Paolo