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UCC2895: UCC2895 Soft start, unstable early operation

Part Number: UCC2895

We encountered a problem, however, that I would like to get some feedback on. We are using the UCC2895 by using the SS/DISB pin for disabling in the following manner: 

To achieve a more precise output voltage, we are using an external error amplifier, and only using the internal opamp as a follower amplifier. 
We have examined, that if disabled in such a manner, in the first 200us after start-up, the soft start function does not pull down the EAOUT output actively, but puts it into a large impedance state. Therefore, the EAUT voltage is not starting from 0V, so the phase shift initially is not 0°, but close to maximal, resulting in large current transients in some of the cases. 
Is this a nominal operation, that the soft start is not actively pulling town in the beginning (and, therefore, it is slowly charged in the circuit above through the 10k+13k resistors?
Yellow-green: PWM outputs
Blue: SS pin
Pink: EAOUT pin
Applying an external pull-down resistor of course solves the issue (that's what we did) so that the EAUT starts from 0V. However, before figuring this out, the IC gave us a few unwelcome surprises and destructive failures,  as the error was not present in all the cases - the high impedance state means, that the lack of the resistor was not fatal at all start-ups. If this is a nominal behaviour, we would like to request to add this information to the datasheet's next version (if it is not added already - it is possible that we have simply overlooked it). 

  • Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • Hello,

     

    The waveforms you have do not look correct.  The SS pin should adjust the rail of the output voltage amplifier based on the SS voltage.

    The EAOUT is already at 1.8V when the SS is at ground.  This should not be the case.

     

    The VA output is around 1.8V while the is roughly 0V.  Not sure why you don’t have the SS at the same volts/div as the VA out.  However from this waveform it looks like SS only charges up to only 2 V.  It should charge  up to VREF and does not.   Also when SS is below 0.5 V the part should be disabled.  Could you double check there waveforms to make sure they are correct?

      

    I reviewed there schematic and don’t see anything off.

     

    Could you double check pins your are evaluating on the UCC2895.  Also could you double check the schematic?  From the waveforms presented it look like something might be pulling up on the EAOUT output high.

     

    Regards,