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TPS2377: Load Capacitor Selection for PD

Matthew Milewski
Prodigy 40 points
Part Number: TPS2377
Other Parts Discussed in Thread: TPS23755

Hello,

I was looking for assistance in choosing a load capacitor for the TPS2377 PD.  Looking on page 16 of the data sheet it provides a formula for determining the right capacitor value.  Any value I look at comes to about an order or two of magnitude larger than the maximum that is recommended (470uF).  Is there an updated formula or am I just calculating it wrong?  Downstream of the capacitor will have a 15 W DC-DC converter which will probably draw around 5W of power.

Thank you for your help!

Matt

  • 0
    TI__Expert 5615 points
    Matt, You have reached out to us over a holiday break for Texas Instruments' employees based in the US. Everyone is off until next Monday (4/22). We will get to your request ASAP next week.

    Thank you for your understanding,
  • 0 in reply to Thomas Lewis
    TI__Intellectual 1005 points

    Matt, 

    I assume that you're referring to equation 2 on page 16 of the TPS2377 datasheet. That's actually an equation for the maximum cap not recommended cap. For your power level you will usually wind up with a much smaller capacitor than 470uF.  Note that the input cap depends on the DC/DC. 

    For example our TPS23755 device integrates an type 1 PoE PD and a flyback converter. It's EVM uses a ~15uF of capacitance.  

    http://www.ti.com/lit/ug/slvubg8/slvubg8.pdf

    Not sure how far along you're into your design, but TPS23755 is a great option for designs <13W.  It offers high integration and competitive total solution cost. 

    http://www.ti.com/product/TPS23755

    -Artem 

  • 0 in reply to Artem Rogachev
    Prodigy 40 points

    Hi Artem,

    That is the formula I was looking at.  The equation calls for the Load Current (in mA) which (when looking at our DC-DC converter) is drawing a maximum of 15 Watts (54V * ~277mA).  Plugging the 277mA into the equation referenced under load capacitance comes up with almost a 5 mF cap.  Nominally we are drawing around 5 Watts (54V * ~92.5mA) which drops the capacitor to requiring 1.67 mF cap....much greater than the maximum recommended capacitor of 470uF.

    Thanks,

    Matt

  • 0 in reply to Matthew Milewski
    TI__Intellectual 1005 points

    Matt, 

    Take a look at the snapshot from the TPS2377 datasheet. The formula is for MAX  capacitance not "recommended cap".  You do not have to use 1.67mF. It just needs to be lower than that and greater than 5uF. 

    Let us know if anything else is unclear. 

    Thanks, 

    Artem 

  • 0 in reply to Artem Rogachev
    Prodigy 40 points

    Hi Artem,

    Ahh I understand now.  What capacitior would you reocmmend then for my application of a nominal 5W load?  I understand between 5uF and 470 uF but would there be a benefit of using a larger vice smaller?

    Thanks,

    Matt

  • 0 in reply to Matthew Milewski
    TI__Intellectual 1005 points
    Matt,

    Smaller cap gives you a smaller solution size and lower BOM cost. A larger cap will reduce the input current ripple and provide a stable supply for your DC/DC. In this cap is a part of DC/DC design.

    For your reference we typically use something in the 10uF - 47 uF for our PD+DC/DC designs. For example TPS23755 EVM uses a 10uF cap. Any reason you're not considering an integrated PW + PWM IC such as TPS23755? You can just copy paste our EVM for a fast design. :)

    -Artem
  • 0 in reply to Artem Rogachev
    Prodigy 40 points

    Artem,

    Thank you for your help on this issue.  I really appreciated the quick responses!

    Have a good day,

    Matt