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TPS79901-EP: Regulator enable input does not work if input voltage derivate is "to fast".

johan olofsson
Prodigy 80 points
Part Number: TPS79901-EP

I have some report regarding your part  TPS79901. This applies to all fixed models to..

In our application we have short risetime for the (IN) input voltage. 3,3V @ 200us.

When the input voltage rises this fast, the (EN) enable input does not work properly.

See oscilloscope picture attached.

 

Ch2 (Green)  is the input voltage.

Ch3 (Blue) is the output voltage.

EN-pin voltage threshold is reached at the X1 marker.

 

Before the enable goes high there is an unwanted output voltage behavior.

Similar behavior is seen even when the enable is hard gronded to GND.

 

These measures have been verified with your eval-board.

 

Here is shown typical device connection in our application.

Likewise, the same issue occurs in your evaluation board, if input voltage rises fast like 3,3V during 200us.

 

 

 

  • 0
    TI__Mastermind 29270 points

    Hello Johan,

    I am reviewing your question and also our archives on this part.  Please allow 2 business days for a response.

    Thanks,

    - Stephen

  • 0
    TI__Mastermind 29270 points

    Hello Johan,

    I have reviewed our archives and we have seen this before.  If you can increase the turn on ramp rate to 50us or faster, the problem should go away based on our archives on this issue.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 80 points

    I cant change the derivate of the input voltage rail, it is what it is. If I could I would increase the turn on time, then it has shown to be working properly.. not the opposite. Did you really understand my issue? My investigation shows that the device is not working according to specification when the supply input voltage rate is to fast. If this is a known issue this should be clearly informed in errata for the device datasheet. Where is this errata?

  • 0 in reply to johan olofsson
    TI__Mastermind 29270 points

    Hi Johan,

    I understand the challenge here and that you are unable to speed up the ramp rate of Vin.  The issue being described has been documented in our datasheet correction log for update during the next revision of the datasheet.  Just so we are clear, can you confirm that you can or cannot slow down the ramp rate of Vin?  The issue does not exist in the datasheet turn on plot (figure 26) which indicates a slow ramp rate would not be an issue.  If you can slow down the ramp rate, let me know and I can look into what rate you would need to prevent this issue in your design.

    If you absolutely cannot change the ramp rate of Vin (speed up or slow down), we can also explore alternative options if you provide a list of key requirements if you would like.

    Thanks,

    - Stephen