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DS90UB933-Q1: ESD selection for 933

Part Number: DS90UB933-Q1

Dear team,

My customer choose ESD for our 933, and we have some questions as below,

1. They choose SESD0402X1UN, and input capacitance is 0.3pF, breakdown voltage is 8.8V;

2. What is DOUT pin ratings, V(VDD_n)+0.3? If so, the maximum rating of DOUT is 2.1V, then ESD's breakdown voltage should be smaller than 2.1V, right? 

3. Is it good to put ESD between DOUT pin and AC capacitor? What is our recommendation?

Thanks & Best Regards,

Sherry

  • Hello Sherry,

    I think the part you have selected should work. Is there going to be PoC as well? If that's the case, then you might have to take that into consideration. Either the breakdown voltage of the ESD diode needs to be larger than the PoC voltage or it needs to be placed between the pin and the AC capacitor. 

    Yes, the ESD diode would be between the DOUT pin and AC capacitor.

    Overall, you need to select an ESD diode that has very less capacitance (0.5pF or lower) so that it does not add capacitance to the FPD-Link line. 

    Best Regards,

    Shruti More

  • Hi Shruti,

    Thanks for your help!

    1. 933 datasheet doesn't list DOUT maximum rating. In the 935 datasheet, this value is clear, and the maximum value is 1.21V. I want to know this value because this value is very important for selecting the breakdown voltage in my understanding. Our device will be damaged if this DOUT pin has a voltage which is larger than 1.21V. When doing ESD test, the breakdown voltage is 8.8V which is larger than 1.21V, then our device will be damaged in my understanding. But you said that this ESD diode is ok, I am confused for this point.

    2. Could you please help clarify "it needs to be placed between the pin and the AC capacitor, as close to the connector as possible."? You mean this ESD diode should be closed to the AC capacitor?

    Thanks & Best Regards,

    Sherry

  • Hello Sherry,

    The absolute maximum table says what conditions will permanently damage the part. 

    The voltage at the DOUT pins should not exceed the breakdown voltage of the diode. If this is the case, then the diode should be placed between the connector on the board and the AC cap; as close to the connector as possible. 

    Is there going to be PoC as well? If the breakdown voltage of the ESD diode is less than the PoC voltage, it needs to be placed between the device pin and the AC capacitor.

    Make sure to select an ESD diode that has very less capacitance (0.5pF or lower) so that it does not add capacitance to the FPD-Link line. 

    Best Regards,

    Shruti More

  • Hi Shruti,

    Thanks for your reply! 

    Yes, the customer uses POC. Considering large DC bias, it is better place the TVS between DOUT pin and AC capacitor. When we choose the TVS, we need to consider two voltage specs, VRWM and Vclamp. Firstly, the normal working voltage should be smaller than VRWM; secondly, DOUT pin’s maximum rating should be larger than Vclamp. In this way, SESD0402X1UN is not appropriate because Vclamp=8.8V which is larger than 1.21V(DOUT pin’s absolute maximum rating), right?

    If the customer place the TVS between AC capacitor and connector, then VRWM and Vclamp should be both larger than Vpoc, right?

    In addition, our device includes the ESD protection as below. In this way, the external TVS is not necessary, right?

    Thanks & Best Regards,

    sherry

  • Hello Sherry,

    Vrwm should be selected to ensure that it is above the expected maximum operating voltage. If the applied voltage rises above VRWM, there is a chance to see diode leakage increase significantly. For example, if the protected line operates at 5 V nominal with a maximum variance up to 7 V, ensure that the VRWM is 7 V or greater. The TVS Vrwm should be chosen somewhere in the zone between the highest "expected" supply voltage and the voltage that causes damage. Vclamp determines the voltage that the system will be exposed to during a surge; it is the voltage that the TVS diode will regulate to when exposed to a transient current.

    Yes, you are correct, Vclamp should not be larger than the absolute maximum voltage rating of the DOUT pin. In this case, Vclamp is similar to an AC spec. The absolute max ratings are a DC spec. 

    If the customer places the TVS between the AC capacitor and connector, then Vrwm should be larger than Vpoc and Vclamp should be close to Vpoc.

    Yes, our devices have an internal diode for ESD protection. I would suggest following what the datasheet recommends in the connection diagram and there is no external diode shown there. 

    Thank you,

    Best Regards,

    Shruti