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TCA9517: subsequent question of "Asking for the TCA9517 design application"

Toshiro Imi
Expert 7220 points
Part Number: TCA9517

Hello,

Since our customer faced on DDC capacitance violation when he tested by connecting B-side to HDMI port, I shared  "TCA9517: asking for the TCA9517 design application"  thread for best understanding why B-side connection is not recommended. Then he has subsequent question on the answer of this thread.

1) what is "voltage offset" in this thread.

my understanding of "voltage offset" stands for "VOL-VILC :Low-level input voltage below low-level output voltage" in the datasheet. Please correct me if my understanding is incorrect.

2) would you mind letting us know if there is any limitation VCCA, VCCB voltage e.g. is it available to use in both VCCA>VCCB case and VCCA<VCCB case. 

3) would you please let us know why B-side capacitor is much larger than A-side, although the datasheet specify 8pF typ input capacitor for both SCLA, SCLB, SDAA, SDAB.

does it comes form bias voltage ? The datasheet is specified VCCB=3.3V or 0V case only not specified in VCCA.

best regards,

  • 0
    TI__Genius 15345 points

    Hello,

    I believe the thread you are referring to is this one correct?

    Toshiro Imi said:
    1) what is "voltage offset" in this thread.

    I believe the voltage offset is the value you talked about. I will check with the original Apps Engr to see what he meant by offset voltage.

    Toshiro Imi said:
    2) would you mind letting us know if there is any limitation VCCA, VCCB voltage e.g. is it available to use in both VCCA>VCCB case and VCCA<VCCB case. 

    The supply restrictions for this device is that VCCA <= VCCB.

    Toshiro Imi said:
    3) would you please let us know why B-side capacitor is much larger than A-side, although the datasheet specify 8pF typ input capacitor for both SCLA, SCLB, SDAA, SDAB.

    Can you point me to what you are referencing? Where does it say that the B-side capacitor is much larger than the A-side?

    Best,

    Chris

  • 0
    TI__Expert 7220 points

    Thank you for your feedback.

    2) would you mind letting us know if there is any limitation VCCA, VCCB voltage e.g. is it available to use in both VCCA>VCCB case and VCCA<VCCB case. 

    The supply restrictions for this device is that VCCA <= VCCB.

    since the customer plans to use in VCCA=5V, VCCB=3.3V, I understand that TCA9517 does not use in this application. Could you please let us know other appropriate device for this use case of  VCCA=5V, VCCB=3.3V, guaranteeing HDMI DDC standard.

      

    3) would you please let us know why B-side capacitor is much larger than A-side, although the datasheet specify 8pF typ input capacitor for both SCLA, SCLB, SDAA, SDAB.

    Can you point me to what you are referencing? Where does it say that the B-side capacitor is much larger than the A-side?

    please look at the datasheet page-6 CI : capacitance, CIO : Input/output capacitance specification. it defines SCLB SDAB input , input/output capacitance of 8pF in typical but the customer could see 400pF capacitor, that violate HDMI DDC max capacitance spec of 50pF.

    best regards,

  • 0 in reply to Toshiro Imi
    TI__Genius 15345 points

    Toshiro,

    I checked with the original Apps Engr and they said that by "offset voltage" they meant just the VOL on the B-side. The B-side VOL is much higher than the A-side VOL and therefore is "offset" higher.

    Is there any reason why the 5V has to be on the A side of the device? Can the customer swap the voltages so that the 5V is on the B side?

    Toshiro Imi said:
    please look at the datasheet page-6 CI : capacitance, CIO : Input/output capacitance specification. it defines SCLB SDAB input , input/output capacitance of 8pF in typical but the customer could see 400pF capacitor, that violate HDMI DDC max capacitance spec of 50pF.

    I saw on the datasheet the capacitance values for the B-side and A-side. You are saying that the customer measured the capacitance themselves correct? Would you be able to provide the test setup for measuring the capacitance?

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    TI__Expert 7220 points

    Hello,

    The customer test set up is as shown in below that is following to HDMICTS. The DC bias voltage is 2.5V.

    Can you also tell us your test condition.

    best regards,

  • 0 in reply to Toshiro Imi
    TI__Genius 15345 points

    Hello,

    Was the device disabled when this test was run? The HDMI test will fail if it is run on the B-side due to the offset voltage. So it should be done on the A side of the device or you can disable the device by pulling EN low and then run the test.

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    TI__Expert 7220 points

    Thank you for your feedback.

    Although our customer tested in EN=low and measured 25pF from B-Side, HDMI DDC/CEC Line capacitance test has to be done in EN=High condition. We also understood B-side VOL offset voltage is the root cause of input capacitor enlarged.

    Based on this, our customer has some questions in below.

    - Can you tell us what is the major portion to show measured 400pF capacitance on B-Side.

    - Can you also tell us how offset voltage link to the capacitance value.

    As the result, TCA9517 may not be able to support this application. Could you tell us alternative product for his application.

     Best regards,

  • +1 in reply to Toshiro Imi
    TI__Genius 15345 points

    Hi Toshiro,

    Toshiro Imi said:
    - Can you tell us what is the major portion to show measured 400pF capacitance on B-Side.

    Can you elaborate on what you mean by major portion?

    Toshiro Imi said:
    - Can you also tell us how offset voltage link to the capacitance value.

    I am going to quote a post from Jonathan Valdez. You can read more about it from this E2E thread.

    "I would recommend having the HDMI connector be on the A side to avoid the compliance testing issues that I have seen in the past. The reason this device can have issues if you put the B side to the connector is because of the testing procedure used, it is not good for any buffers with static voltage offset:

    1) Input signal from compliance tester swings between 80% and 20% VCC to test Capacitance. The test also requests that the device be powered, this means that the TCA9517 has VCCA and VCCB, and is enabled.

    2) When the input on B side (connector side) goes low, it will trip the TCA9517, which will think an external slave is pulling low (voltage is below 30% VCC), and so the A side will pull low to propagate this signal

    3) WHen the A side goes low, it will also trip the A-side input low threshold (Because A side is now below 30% of VCCA). This will cause the B side internal pull down (of the TCA9517) to activate to propagate a signal back through.

    4) When the compliance tester releases the pin to see how long it takes for the voltage to rise (my measuring this rise time, it can calculate capacitance), the TCA9517 B side pull down is still active, and holding the B side low. However, the voltage will be above the VILC value, so the A side pull down will be released and start rising. Once the A side voltage rises above 30%, the B side pull-down will be released and then the B side will rise.

    5) The compliance tester will see this additional propagation delay as capacitance load, and it estimates the capacitance on the line to be ~20 nF (even though this is not even remotely true).

    You can avoid this entire wrap-around delay if you put the A side on the HDMI connector side."

    The TCA9517 can work for this application. After doing some more reading it is actually possible to have the VCCA > VCCB. This was actually a restriction for the TCA6517 not the TCA9517.

    If you absolutely need to have the HDMI connector connected to the B side of the device this is another post from that same thread.

    "If TCA9517 is used and B side is connected to HDMI connector, you will need to ensure that EN is pulled to ground during capacitance compliance testing, otherwise the measurement equipment could get an invalid measurement due to the reasons I stated above. If the tester is more intelligent (and uses small signal analysis to do capacitance measurement instead of 20% to 80% VCC swing), then this is not a concern and will pass capacitance test."

    Hopefully this answered your question. Let me know if you have any more.

    Best,

    Chris 

  • 0 in reply to Chris Ayoub
    TI__Expert 7220 points

    Hi Chris,

    Thank you very much for your detailed explanation of why TCA9517 B-side is so large capacitance. Eventually our customer continue to design his HDMI circuit by taking A-side to HDMI connector in VCCA=5V, B-side to internal circuit in VCCB=3.3V.

    Best regards,