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LM5157EVM-BST: 26Vin, 45Vout evaluation

Taketo Sato
Genius 9475 points
Part Number: LM5157EVM-BST
Other Parts Discussed in Thread: LM5157-Q1

Hello team,

My customer tries to evaluate LM5157-Q1 with 26Vin, 45Vout, 1~2Aout.

They changed R14(160kohm), R17(3.6kohm) and C24(0.047uF) on its EVM and measured 26Vin, 45Vout but it resulted in big noise.

They have following questions. Could you please support below?

  1. Ripple noise is very slower than fsw.(fsw=2MHz and ripple is tens of Hz). Do you come up with any reason?
  2. Should we change also D1 on EVM for 45Vout measurement?
  3. There is no actual Vout waveform in datasheet. Do you have actual Vout waveform for reference?
  4. Is there any other suggestions to modify EVM to evaluate 26Vin to 45Vout?

Best regards,

  • 0
    TI__Expert 7450 points

    Hi Taketo,

    I highly suggest they change to a higher voltage diode as it is now right at the max.

    The reason of the ripple could be different. Do they have the waveform?

    Please refer to the 5.4 steady state waveforms in the user guide. https://www.ti.com/lit/pdf/snvu739

    Another important change is the output capacitors. They are rated for 50V on the board, which have very low capacitance at 45V DC. 100V caps are needed for 45V output cases.

    Thanks,

    Yinsong

  • 0 in reply to Yinsong Cai1
    TI__Genius 9475 points

    Hello Yinsong-san,

    Thank you for your comments.

    Customer changed diode and output caps for high voltage. but it didn't help.

    I got some waveform from customer and looks like the noise has something to do with load condition.

    150 ohm load(300mA) condition looks not bad, but noise get big with 1.2k ohm load(37.5mA) condition. Please see below for your reference.

    1) 150 ohm load: 80mVpp

    2) 1.2k ohm load: 450mVpp

    3) 1.2k ohm load (enlarge): about 4kHz ripple is observed

    Customer wants stable performance also at light load. Could you please advise any thing?

    Should they change also inductance value?

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Genius 9475 points

    Hello Yinsong-san,

    Could you please comment on the behavior?

    Please let me know if you have questions on this.

    Best regards,

  • 0 in reply to Taketo Sato
    TI__Expert 7450 points

    Hi Taketo-san,

    The output waveform looks very noisy. I would try two things first.

    1. check the SW voltage in several cycles and make sure it's working stably.

    2. check their method of measuring Vout. Please try to use the ground tip barrel around the probe to minimize the measure loop as much noise could be coupled and affect the result.

    Thanks,

    Yinsong