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UCC2808-2 failure

Peter Schulz1
Prodigy 40 points
Other Parts Discussed in Thread: UCC2808-2

Hi,

we're using the UCC2808-2 in a design that is running in thousand of units since several years.

The part is used to drive two Fets without current sense. FB and CS are tied to ground while COMP is left open.

The frequency is set to approx. 500kHz with 18K and 150pF on RC pin.

In the last years our customers returned us several units where this part is faulty. We're pretty sure that this has something to do with our design because we use the same circuit in many designs and have the problem only with one of them.

The faulty parts don't drive the outputs anymore but some of them still show the sawtooth at the RC pin.
All of them output 6,5V at the COMP pin whilst a good device outputs approx. 3,5V.

Allthough the COMP pin is left open we have a short trace (approx. 15mm) to an ICT testpoint on it. One suggestion was that the part is destroyed because of a disturbtion (ESD?) via this antenna.
The other suggestion was that there are some strange circumstances when the supply for the UCC2808 raises slightly above 13V and destroys the part.

Questions:
- Is it allowed to leave the COMP pin open?
- from the above described behaviour is it possible to draw a conclusion what destroyed the part?

Thanks in advance for any suggestion.

Regards, Peter

  • 0
    TI__Expert 5900 points

    Hi Peter,

    - Is it allowed to leave the COMP pin open?

    Yes, it is allowed.


    - from the above described behaviour is it possible to draw a conclusion what destroyed the part?

    Can you share the schematic? It is difficult to know the exact cause without decap and analysis the failure parts.

  • 0 in reply to Kening Gao
    Prodigy 40 points

    Hi Kening,

    thanks for your response.

    I'm not allowed to post the schematics.

    At the moment we think that the UCC2808 gets an overvoltage on it's supply pin caused by a circuit break at the following transformator. Because of this the part get's not completly destroyed since the output stages don't work anymore but the saw tooth is still visible at the RC pin.

    We try to double check this with further tests.

    Regards, Peter