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TPS54917 Design Guidelines

trevdawg122
Prodigy 100 points
Other Parts Discussed in Thread: TPS54910, SWITCHERPRO, TPS54610, TPS54917

The datasheet is rather lacking in explaining many of the component value selections. Usually power supply datasheets have formulas for computing output capacitance/inductance based on allowed output voltage/current ripple. I also don’t see an explanation for the compensation and feedback network. Can TI provide more insight into designing power supplies with this part?

Thanks,

Trevor

  • 0
    TI__Guru**** 191445 points

    that device is a package spin of the TPS54910.  You can use the same design procedure.  for more generic treatment of voltage mode buck converter see:

    http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slva104a&fileType=pdf

    This is a good intro to designing with this series of devices.  Alternately you can use the online switcherpro software to generate your designs.

  • 0 in reply to JohnTucker
    Prodigy 100 points

    Just to verify, did you mean to type TPS54610? That is the part number associated with the link you included in your post, not TPS54910.

    Thanks,
    Trevor

  • 0 in reply to trevdawg122
    TI__Guru**** 191445 points

    The TPS54917 is a repackaged version of the TPS54910.  The TPS54610 is a 6 A version of the TPS54910 (hint: TPS54xyz, x = output current).  These are all voltage mode control buck converters.  The design methodology is identical (although I should also point out there are several possible design methods that will produce valid designs).

  • 0 in reply to JohnTucker
    Prodigy 100 points

    I have used SwitcherPro on this design, but wanted to verify my results. The link you described above should be able to help me do that. Thanks.

    I do have a couple questions about SwitcherPro though. One thing that worried me was SwitcherPro's recommendation for 1500uF of output capacitance. The datasheet application design has 200uF, which is quite a big difference. Do you know why there is a large difference and which solution is more appropriate? I tried to compare to the guide you recommended, but the guide covers the capacitor’s dc voltage rating, the capacitor’s ripple current rating, and the maximum output ripple voltage, but does not list a formula for calculating the output capacitance value.

    Also, I'd like to view the affect that decreasing the operating frequency would have on the efficiency of the device (the datasheets leads me to believe the efficiency would increase), but the tool does not allow me to edit the resistor which sets the adjustable frequency. Do you know if this is possible and if so how?

    Thanks,
    Trevor

  • 0 in reply to trevdawg122
    TI__Guru**** 191445 points

    output capacitiance is generally a function of transient response, and allowable voltage ripple.  In general the transient requirement is more stringent.  During a output current load step, all the additional current must come from energy stored in the output cap.  The capacitance must be sufficient so that the output does not drop too much when that charge is pulled out. after a couple switching cycles the control loop responds and teh output current from the converter ramps up to the new level and re-charges the output capacitor.  Alos there may be lilitations with regard to stability.  in general you probably do not want the L/C corner frequency of the output filter above  20 kHz or so.  Also I usually will not design a supply with less than 22 uF on teh output, although i have seen low current designs with 10 uF or less.  Something in the 10uF range works wel for many designs, but high performance processor rails often require thousands of uFs.  For switcherpro, you cannot directly edit the RT resistor.  ti change frequency, you have to create a new design using the "advanced inputs".  In that form you can select your switching frequency and SP will calculate the RT value.