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DS90UB948-Q1: DS90UB948 POC Short circuit

Tomas Redelius
Prodigy 40 points
Part Number: DS90UB948-Q1

Hi,

we use POC with the devices DS90UB948 and DS90UB941.

The POC voltage is 35V and the capacitors to the serielizer and deserializer was in the test 22nF/50V.

When shorting the POC (35V side) at least the Deserielizer and possibly also the serielizer was damaged.

Soo:

The energy in the cap is U^2xC/2.

When shorting the 35V, the cap will discarge by a current from the FPD interface.

1) Does tit has a protection? and

2) How much energy / voltage transient will it withstand.

(The TE FAE has declared that this is not a problem with 100nF caps rated at least 35V.)

  • 0
    TI__Expert 3390 points

    Hi Tomas,

    These parts are not typically used with PoC but the pins are not any different from the parts that do use PoC. If you exceed the Absolute Max rating of the pins, then the pins/parts will get damaged. The caps you are discussing (22nF, 100nF), are these AC-coupling caps? If so, they should block any DC. However, we typically recommend having them 2x-3x the voltage rating of the voltage being used. In our systems, we typically use 9V and the having the AC coupling caps at 50V is more than enough. If you are running 35V and the voltage rating for the caps is 35V, they can get damaged, which could damage the parts. So, to answer your questions, the voltage the pins can withstand is absolute max, there's no other protection besides what you place externally.

    Regards,
    Mandeep Singh

  • 0 in reply to Mandeep S
    Prodigy 40 points

    Hi,

    Yes it is the ac coupling cap on the RIN pin.

    This was not a question on voltage rating for the capacitors....

    The problem is when you turn on/off the POC voltage or make a short circuit!

    The capacitor on the FPD link pin (RIN0 /RIN1) has to charge up to the voltage difference between the RIN pin and the POC voltage. To do this,  charge current has to pass the the RIN pin.

    The Maximum rating in the datasheet does only say -0.3 - 2.75V. There is no maximum current specified.

    Howerver they have an ESD rating so they seem to  have a protection for this. 

     BR

      Tomas

  • 0 in reply to Tomas Redelius
    TI__Intellectual 2930 points

    Tomas

         How are you coupling power into the data lines? Coupling needs to be done through an Inductor network that will block quick supply transients. Yes, if you suddenly turn ON/OFF the supply, then the AC coupling caps will act as a short and the full brunt of transient voltage will be seen by the Internal device pins. Now the internal device pins (Rin+ and Rin-) are built to withstand upto 8KV of transients as long as it follows IEC61000-4-2 standard , given in Datasheet but it can only withstand this type of energy for a very short period. So a scopeshot of the energy pulse you are applying seen at the pins Rin would be helpful. I believe the pulse is ON for a while, say in mS and hence the internal devices are getting hot (due to Joule heating,effects of excessive current in metal over excessive time) and hence is getting damaged

    Thanks

    Vijay