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UC3525A: Dead time variation with temp

Dan Andersen
Genius 11270 points
Part Number: UC3525A

My customer is using the UC3525 and is noticing a fair amount of variation of dead time with temp.  They have the following questions:

1.       Is deadtime controlled or characterized in any way? I see a curve in section 6.6 (above) but no measured specs in section 6.5 for deadtime.

2.       Is the correlation between dead time and temp (higher temp is more dead time) a known and “correct” behavior?

3.       Our design uses 470pF for CT. Section 6.3 recommends only down to 1000pF (twice our value), but the Maximum Frequency parameter in section 6.5 uses 470pF so it makes me think this may be fine.

Thanks,

Dan

  • 0
    TI__Expert 6080 points

    Hello Dan,

    Below are my comments:

    1. The dead time control is characterized under section 6.3 as Deadtime resistor range which is over the operating free-air temperature range.

    2. The oscillator discharge should follow the typical characteristic curve. Can I please know their values of Rt and Rd. It is alright to use Ct= 470pF. Can I also ask how much is the noted variation?

    Regards,

    Sonal

  • 0 in reply to Sonal Singh
    TI__Genius 11270 points

    Thanks, Sonal.  My customer had additional feedback:

    We do not see where it tells empirically what dead time we will get for a given resistor value - just a typical curve.  Over a 0-70C range we see dead time change dramatically on the bench.  Is there any area of the datasheet that guarantees a certain limit to the dead time variation over temp?

     

    Rt = 15.0k, Rd = 100, Ct = 470pF. We’re driving an external clock at ~335kHz into the SYNC pin, though, so this isn’t setting the operating freq. We bumped up Rd to 200ohm to give us the margin we needed. However, I don’t understand why we had such variation.

     

    We were seeing around 500ns to 600ns, depending on the IC, with the lowest values causing issues in our SMPS design.

    Is the variation they see over temp expected behavior?

    Thanks,

    Dan

  • 0 in reply to Dan Andersen
    TI__Expert 6080 points
    Hello Dan,

    I have asked the Test team to share this data.
    It might take some days before they get back to my request. I will reply to your question as soon as I hear back from them.

    Regards,
    Sonal