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LM5148: Schematic review

Minghao Yin
Prodigy 365 points
Part Number: LM5148

Hi Team,

This is our customer schematic about LM5148.

And they found output noise is a little aloud. With a 6A output there is a 240mV noise. There is a 180mV noise without load.

And I found 18pin PFM/SYNC is pulled up. They want have a high efficiency with a lower noise in PFM mode.

Is there any defects about customer's schematic?

  • 0
    TI__Mastermind 22301 points

    Hi Minghao,

    The ripple in PFM mode at low current is expected yes. See datasheet figure 9-21.

    However at 6A it should enter PWM mode, and the ripple should be reduced.

    The schematic looks OK, do you have a quickstart calculator?

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Prodigy 365 points

    Hi Orlando,

    Our customer have a test about LM5148, their design for 6V output,.and they found when LM5148 SW is ok. While customer change output to 7.4V(by software control a DAC)  in 4 A.

    The waveform is strange.

    And it will continued for a long time.

    And following is a common wavefrom.

    Customer afriad that is a sympol that device is broken, although the device is normal now.

  • 0 in reply to Minghao Yin
    TI__Mastermind 22301 points

    Hi Minghao,

    It looks like the device is entering dropout, where VOUT is close to VIN.

    From SW node it looks like VIN is close to 8V.

    In this dropout condition the ON-time can be extended, to keep duty cycle and reduce drop-out voltage.

    See figure 8-1 in the datasheet.

    Hope this helps,

    -Orlando