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LM5116: Output capacitors for LM5116

Mihai Vintila
Intellectual 590 points
Part Number: LM5116
Other Parts Discussed in Thread: LM3150

Hello, I m working on a design with LM5116 for a DC-DC converter from 9-28V to 7V and 8A.

In Webench I got a 68uF output capacitor which seem a bit low, the LM5116 eval board is using 5 X 100uF which is almost 10 times higher !

What is the reason of this major differences in the output capacitors ?

I am planning to add 1 or 2 optional output capacitors as backup but I would like to know the reasoning behind these values.

The datasheet does not provide details about how to calculate the output caps.

And another question regarding Rsense - as far as I know this resistor needs Kelvin connection, right ?  It is not very clear how it is done in the eval board.

Are there some other special points to watch out for in the layout ?  I have read the layout guidelines in the datasheet and I looked at the eval board and I have some good knowledge about the layout of DC-DC converters but I had some issues with another layout (with LM3150) and I want to avoid further troubles.

  • 0
    TI__Guru**** 191445 points
    I do not support LM5116 directly, but I can tell you that Webench almost always tries to pick low value output capacitors, I think it is marketing to keep BOM cost and PCB area as small as possible.
  • 0
    Intellectual 590 points
    I saw in the datasheet that the VCCX pin should be connected to GND if it is not used, but in Webench it is left unconnected.
    I looked over some posts here regarding LM5116 and found also a recommendation to connect VCCX to GND.
    So I understand it should be indeed connected to GND, is that correct ?
  • 0 in reply to Mihai Vintila
    TI__Prodigy 700 points

    Hi Mihai,

    Yes you are right, if no external VCCX supply is used then we recommend to connect the VCCX pin to GND.

    Best Regards,

    Robert Loke

     

  • 0 in reply to JohnTucker
    Intellectual 590 points
    OK, thanks, in this case how can I choose a good value to be on the safe side but without overdoing it ?
    Maybe 2X Webench value or 3X would sound like a good choice ?
  • 0 in reply to Robert Loke
    Intellectual 590 points
    Hi Robert, thanks, I will connect it to GND.

    Mihai
  • 0 in reply to Mihai Vintila
    TI__Prodigy 700 points
    Hi Mihai,

    the output capacitor selection depends strongly on the applications power requirement.
    1. there is the output ripple. These selections are usually ending up in the lower Cout values you see like in Webench.
    2. there is the load transient requirement. These are ending up typically with higher Couts to buffer the output like on the EVM.

    here is a nice paper that explains the capacitor selection for DC/DC applications.
    www.ti.com/.../slta055.pdf

    The calculation 8.2.2.6 Output Capacitors in the datasheet for the LM5116 reflects only the need for output ripple noise.

    Best Regards,
    Robert Loke
  • 0 in reply to Robert Loke
    Intellectual 590 points

    Hello Robert, thanks a lot, that paper is very good indeed !

    However I saw in that paper that there should be also a maximum capacitance that should be mentioned in the IC datasheet, but I did not find such info at LM5116. If I remember correctly at the LM3150 that I used before there was indeed a maximum value mentioned in the datasheet but for LM5116 it is not written.

    How can I determine the maximum output capacitance ?

    Thank you,

    Mihai

  • 0 in reply to Mihai Vintila
    TI__Guru**** 191445 points

    If I read that app note correctly, the maximum capacitance mentioned is limitedly the internal current limit at start up.  Most converters will attempt to ramp up the output voltage linearly at start up.  Therefore the required charge current I = Cout * Vout / tss where tss is the slow start time.  that current plus the load current should be less than the current limit.  This is not necessarily a hard limit.  If your converter has hiccup mode current limit, then the converter will fail to start up.  and continually recycle.  However if it is simple plus by pulse current limit, the output ramp up will complete, but the time will be extended beyond the expected SS time.  LM5116 does have hiccup mode.

  • 0 in reply to JohnTucker
    Intellectual 590 points

    Hello, where can I find the PCB file or gerbers for the LM5116 evaluation board ?

    I would like to use it as reference for the layout as the PDF doc for the eval board is difficult to follow.

    Thanks,

    Mihai

  • 0 in reply to Mihai Vintila
    Intellectual 590 points

    Hello, please let me know how can I get the PCB or gerbers for the eval board ?

    Thanks,

    Mihai