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LM5006 input capacitors

David Dean
Expert 2305 points
Other Parts Discussed in Thread: LM5006

I am using the LM5006 in a very, very space constrained application.  The datasheet shows two input capacitors - C1 and C5.  C5's state purpose is to "help avoid supply voltage transients and ringing due to long lead inductance at Vin".  C5 should be placed as close to the pin as possible. 

If C1 is a low ESR X7R ceramic capacitors and is already placed very close to the pin, can C5 be removed?

  • 0
    TI__Guru**** 191445 points

    what is your input voltage range?

  • 0 in reply to JohnTucker
    Expert 2305 points

    Input voltage range is 6V - 55V.

    Vout = 5V

  • 0 in reply to David Dean
    TI__Guru**** 191445 points

    I was thinking if your input was less than 50 V, you could find a 0.1 uF in a 0402 package.  You could most likely squeeze that into your layout.  For other converters I sometimes leave that high frequency bypass capacitor out of the design, but I have not tried it with LM5006.

  • 0 in reply to JohnTucker
    Expert 2305 points

    Hmm.  There's always the chance that I could find room for the current 0603, it's just that I'd rather save the space completely if possible.

    I'll often go by the rule of thumb with decaded capacitors in a case like this, but I've also learned that, given the low resistance and inductance of an SMT ceramic capacitor, it doesn't actually buy you much - that basically, if it's close to the pin, an 11uF capacitor is just as good as separate 10uF and 1uF caps.

    With that in mind I'm tempted to remove the 0.1uF and ensure that the 1uF is very close to the pin.  I do have a current design that includes the 0.1uF, as well as the LM5006 eval kit.  Any good way to test the effect of leaving off the 0.1uF?  High-frequency noise on the output?  As long as the 1uF is present, you wouldn't expect the regulator to fall out of regulation or anything serious like that, would you?

  • 0 in reply to David Dean
    TI__Guru**** 191445 points

    In general I agree with your assessment.  I can tell you that I have worked with certain parts where the 0.1 uF high frequency bypass is necessary for proper current limit operation ( the internal current detection is sensitive to that HF noise), but that is not necessarily applicable to LM5006.  As I stated previously, I have not used LM5006 personally, so my comments are just general case.  Maybe someone with detailed knowledge of LM5006 can comment.

  • 0 in reply to JohnTucker
    Expert 2305 points

    Sounds reasonable.  Do I go find an LM5006 expert or will one chime in on this thread? 

  • 0 in reply to David Dean
    TI__Guru**** 191445 points

    LM5006 is supported by a different business unit than mine.  I comment a lot on the forum if I have something to contribute, even if it is not a device I directly support.  If no one else responds, I'll see if I can direct you question to the proper support person on Monday.

  • 0 in reply to JohnTucker
    Expert 2305 points

    Hi John,

    Any chance of an LM5006 specialist taking a look at this question?

    Thanks,

    Dave

  • 0 in reply to David Dean
    TI__Guru**** 191445 points

    LM5006 is supported by an entirely different business unit.  I do not know any of them personally.  Let me ask around.

  • 0
    TI__Genius 11490 points

    Hi David,

    What is the value of C1 you are using. In general if C1 or C5 are 0.47uF/(0805 or smaller) and placed close to the IC in a small loop consisting of VIN pin, SW, Diode, RTN then the other bigger capacitor can be removed.

    The purpose of the small cap is to close the high di/dt loop carring switching element of the current.

    The purpose of the bigger cap is to maintain overall stability of the VIN bus. If you have other caps upstream or the VIN rail does not have lot of ringing or spikes then you can removed it. Usually with low load currents 0.47uF cap can take care of VIN ripple because of the converter current.

    Hope this helps.

    Thanks and regards, 

     

  • 0 in reply to Vijay86929
    Expert 2305 points

    Hi Vijay,

      My C1 is 1uF 0805, and is as close to the pin as possible.  I have about 300uF upstream in aluminum capacitors. 

      If I understand your post correctly, this means that I can remove C5 in my design?

    Thanks,

    Dave