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LM25118 Cramp

Seishin
Genius 4135 points
Other Parts Discussed in Thread: LM25118, LM25118EVAL

Hello,

We design the power supply board using LM25118.

Required specifications : Vin=8-28V,Vout=12V/Iout=Max.5A

When Cramp is 1nF with our custom board,MOSFET Switching waveforms are disturbed.(Please see the attached file)

LM25118_seishin.pdf

Our custom board 　　　　Cramp = 1nF, Rs=10mΩ, L=15uH
Webench 　　　　　　　  Cramp = 1nF, Rs=9mΩ, L=15uH,Ambient Temp=40℃
LM25118EVAL/NOPB    　 Cramp = 330pF, Rs=15mΩ, L=10uH

What is the recommended value of Cramp for normal operation?

Best regards,
Seishin

  • 0
    TI__Mastermind 21720 points

    Seishin,

    The ramp capacitor value can be calculated by equation 28 found in the LM25118 datasheet.

    -Garrett

  • 0 in reply to Garrett Roecker
    Genius 4135 points

    Hello Garrett,

    Thank you for your prompt reply.

    I understand it.

    Since It is L=15uF,Rsense=10mΩ in our application,result of the calculation is 750pF.(apprximate:680pF)

    Regards,
    Seishin

  • 0 in reply to Seishin
    Genius 4135 points

    Hi,

    Do you have any suggestions to reduce the switch-node ringing other than board layout?

    A boot resistor, a high-side gate resistor, or a snubber?

    I am using the MOS-FET of Si7148DP.(same as EVM)

    Best regards,
    Seishin

  • 0 in reply to Seishin
    TI__Mastermind 21720 points
    Seishin,

    Yes any of those could be implemented.

    The gate resistor or boot resistor will be the easiest if there is a place holder for the resistor. These will slow the turn-on rate of the high-side switch which will result in less switch node ringing.

    The simplest way to design a snubber is thinking of it as a low pass filter. Select a cut off frequency below the switch node ringing frequency and select the resistor and capacitor that meet that cut off frequency. Capacitors are usually selected to be in the 100's pico-farad range. Then the resistor power rating can be estimated by the below equation.

    Presistor = Csnub*(Vin^2)*fsw*(1+M)

    Where

    Presistor = Snubber resistor power rating

    Csnub = selected snubber resistor

    Vin = Input voltage

    fsw = Switching frequency

    M = Margin (Usually give a minimum of 30%)

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Genius 4135 points
    Dear Garrett,

    Thank you for the information.
    This is quite useful.
    I will try it!

    Best regards,
    Seishin