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TPS5450 Current-limit

Kevin Spahr
Prodigy 210 points
Other Parts Discussed in Thread: TPS5450

I am using the TPS5450 in a 15V-to-1.8V (ish) design, for a current-source in a transconductance amplifier. I am finding that at ~1.7V out, I can only get ~2.8A out and at 2.1V out I can get ~3.4A. The TCA is supposed to regulate to 4.00A in this design and I can't get there. Are there any published curves for the current-limit on the TPS5450 (internally determined) versus both output voltage and temperature? It seems to me that for low-voltage outputs the internal mosfet is driven almost entirely off the charge pump (since the bootstrap is only partially helpful at low output voltages). Since the output current is inferred from the Vds mosfet drop, I would expect the current-limit to have strong dependencies on both temperature (reflected in data sheet parameters (but no curves)) and on gate-drive directly (and output voltage indirectly). Any Help?

  • 0
    TI__Guru**** 191445 points

    The TPS5450 is a voltage mode control device.  It is internally compensated and is expecting to regulate voltage rather than current.  The current limit is a minimum 6 A over voltage and temperature.  So I doubt that you are hitting teh current limit unless you are using a small value inductor (the current limit is peak detect not average load current).  Can you provide your schematic and wave forms that show the problem you are having?

  • 0 in reply to JohnTucker
    Prodigy 210 points

    John -

    Actually according to the data sheet, the minimum current-limit, over temperature, is 4.4A, and the maximum is 11.7A - this is for the EP part. Yes, I understand it is voltage-mode - the regulated current load comes from the transconductance amp. I definitely am hitting the current-limit, because I can take my electronic load, in constant-current mode and dial up the load on the 2V output until the output falls (significantly). Part of the problem I've found (so far) is noise - input decoupling cap got placed much further away from the IC than it should of. I was (and still am to some degree) getting a lot of ringing on pin 8 (sw output) but I've tamed that now (mostly). Looks like I have reverse-recovery ringing from my schottky as well...

    Kevin

  • 0 in reply to Kevin Spahr
    TI__Guru**** 191445 points

    The EP parts may have different specifications.  I don't support those, so I don't keep track of teh exact differences.  I can tell you also, that the placement of the input decoupling capacitor is fairly critical for correct operation.  Also, the thermal capabilities of teh TPS5450 are at the limit for tht package.  Proper heat sinking is important.  Another thing to consider is we usually recommnd that electronic loads be used only in "constant resistance" mode.  I can't say too much more without looking at the schematic and waveforms, but it is very likely that the input cpacitor placement is your problem.  Also if you are very far from your input source, you may need an additional bulk capacitor on the input.  These devices are usually used in POL applications with teh input voltage source located within a few inches of the converter.