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LMR51420: Low Freq Ripple of LMR51420XDDCR

Adam
Expert 1145 points
Part Number: LMR51420
Other Parts Discussed in Thread: LMZM23600EVM, LMZM23600

Hello,

I am using LMR51420XDDCR to step down voltage from 21V to 3.3V using 8.2uH inductor and 88uF MLCC at output.

I have light load of 60mA.

With that condition I am getting 20mV low frequency output voltage ripple like in the picture:

To reduce that I can add second LC filter or LDO at output... but looking on the LMZM23600EVM (21Vin,3.3Vout, FPWM) board there is only 69uF at output and there is no low frequency ripple:

So based on LMZM23600EVM dev board there must be the way to reduce Low Frequency Ripple without using LDO or 2nd filter. Why on LMZM2600 this low freq ripple is not visible?

Is there a way for me to have with LMR51420XDDCR similar low freq ripple as on LMZM23600EVM without adding LDO or 2nd filter?

Adam

  • +1
    TI__Mastermind 27040 points

    HI 

    You can use LMR51420XFDDCR, it works at FPWM model same as LMZM23600 which can reduce the no load ripple. 

  • +1 in reply to Daniel Li14
    Expert 1145 points

    Thanks,

    Also question about high frequency ringing..

    When I add 50ohm resistor in series to bootstrap 100nF capacitor I'm getting nice reduction of switching ringing on the output voltage.

    My question is - can this device work with 50ohm resistor in series to bootstrap cap in terms of stability/control? (as small values like 3ohm do not make such a good job in reducing ringing)

    I know it reduces overall efficiency, but that's ok for my design.

    Adam