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BUF634A: Any estimate of minimum short circuit current limit?

Michael Steffes
Guru 47545 points
Part Number: BUF634A
Other Parts Discussed in Thread: OPA1656, , BUF634, OPA2810

Working with this composite circuit shown in the OPA1656 PDS. If you parse through 5Vpp input at gain of 2 in the curves, that is +/-5V output into a min load of 16ohms. Which of course will require +/-5V/16ohm = 313mA (and a little more for the 1kohm load in the feedback path. 

Pretty good THD, so apparently this works even with the typ continuous output current spec of +/-250mA. That is not unreasonable as the peak output current is always greater than continuous - however, this kind of undefined region can run into short circuit current non-linearity - that spec. shows typ and max, but perhaps the min number would be needed for these peak current designs exceeding the continuous? 

  • 0
    TI__Genius 15555 points

    Hello Michael,

    No specific number that I can provide here, however the following curves may help provide more insight:

    1. 

    The application curve you provided was taken without a Powerpad package of the BUF634A since the timestamp for the figure is 2019.

    2. 

    Figure 9-8 from the BUF634A datasheet seems to suggest similar distortion results as the figure you provided at 16-ohm load & 20kHz signal. For 16-ohm curve, distortion improves or is the same up until around 10Vpp or +/- 313mA. For 32-ohm load this seems to occur somewhere around 18.5Vpp to 19Vpp or around +/- 290mA to 300mA. 

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Guru 47545 points

    Hey thanks Hasan, 

    Kind of interesting the short ckt current does not really reduce very much with that power tune feature - the BW does reduce a lot, I really don't think the BUF634A should be used inside the loop like that OPA1656 app shows except at full power - the old BUF634 model had the power adjust in it and the phase pulls back pretty dramatically with lower quiescent, Since the new BUF634A model does not support that feature anyway, no way to test that in sim for loop phase margin. I would have just shorted that bias adjust pin to the negative supply in the OPA1656 composite examples. 

    Incidentally, the two composites I am looking at - with no added compensation and using the 2020 BUF634A model, closed loop gain of +2

    OPA2810, phase margin is 35deg - mainly because the LG crossover is up around 29Mhz

    OPA1656, phase margin is 65deg - mainly because of that phase lead in the Aol and the xover is more in the 9Mhz region (bigger spread between xover and the BUF634A speed meaning its phase shift added at xover is less). 

    For production, some min short current current would be needed but not currently provided if you want to really run +/-5V into 16ohm. And, as the plot you provided shows, definitely try to keep the Tj lower with the thermal pad.