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TPS56628: TPS56628 light load ripple

Derek Xie
Genius 12255 points
Part Number: TPS56628

Hi Team:

My customer using TPS56628 in their project, but they find the TPS56628's output ripple is about 100mV when the output voltage is 5V.

Could you help me to verify whether this is the best ripple performnace of our part? Any way to improve it? Thanks.

  • 0
    TI__Prodigy 575 points

    Hi, Derek,

    could you show us snapshot of the output ripple waveform?

    how do you test the output ripple? 

    Best Regards,

    Nick

  • 0 in reply to Nick Chen
    TI__Genius 12255 points

    Nick:

    Please find the ripple wave.

    The customer have tried different inductor value, the result is similar. May I understand the 100mV ripple is the best performance? Any othe way to improve it?

  • 0 in reply to Derek Xie
    TI__Genius 12255 points
    For normal status like high current mode, the ripple will be below 50mV. Their ripple test use very short GND loop.
  • 0 in reply to Derek Xie
    TI__Mastermind 18510 points
    Hi Derek,

    Yes, light loading will make chip enter e-co mode, which has a little bigger ripple.
    BTW, the test is based on osc bandwith limited at 20Mhz, right?
    and what's the output LC value? Cout should be MLCC, right? would you please upload the schematic? let's my teammate review and double check then.

    BR,
    Yuchang
  • 0 in reply to Yuchang Zhang
    TI__Genius 12255 points

    Hi Yuchang:

    We have limited the bandwidth to 20MHz. The output inductor is 1uH and 50uF total. But we try different inductor and capacitor. No much improvment.

    We have verify the EVM, the ripple is also around 100mV(Vpp). Could you please optimize the EVM to see what's best ripple at the light load? Thanks.

  • 0 in reply to Derek Xie
    TI__Mastermind 18510 points
    Hi Derek,

    OK, I will ask my teammate to double check, and by the way, 1uH should be smaller, have you tried 4.7uH and 3x22uF(>16V, X5R or X7R better) MLCC?
    what's your 50uf? 2X22UF MLCC+.. or...??

    BR,
    Yuchang
  • 0 in reply to Yuchang Zhang
    TI__Genius 9550 points

    Derek,

    description from datasheet page 7: (red words)

    Auto-Skip Eco-mode™ Control
    The TPS56628 is designed with Auto-Skip Eco-mode™ to increase light load efficiency. As the output current
    decreases from heavy load condition, the inductor current is also reduced and eventually comes to a point where
    its rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous
    conduction modes. The rectifying MOSFET is turned off when its zero inductor current is detected. As the load
    current further decreases the converters runs in discontinuous conduction mode. The on-time is kept almost the
    same as it was in the continuous conduction mode

    so, if the VIN and Vout is fixed, so the on-time is fixed too.

    to decrease the ripple, two approach: increase inductor value; increase Cout value. But for this device, inductor and Cout has limited according to the Vout, refer datasheet page 13, table 1.

     

    Jason Wang