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Original question:

LMR33620: LMR33620 ：Is it normal for voltage output to appear like this

LMR33620: Vout large overshoot is observed when the output load is changed.

Kazuya Nakai59
Guru 21645 points
Part Number: LMR33620

 Hello guys,

 One of my customers is evaluating LMR33620.

  In their evaluation, they observed that Vout large overshoot (about 100mV) is observed when the output load is changed from 0.5A to 0A.

 Is it normal operation as the device?

 Your reply would be much appreciated.

 Best regards,

 Kazuya.

 

  • 0
    TI__Mastermind 23845 points

    Hi Kazuya,

    This sounds normal, depending on how it looks. I am comparing it with this picture

    http://www.ti.com/lit/ds/symlink/lmr33620.pdf#page30

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Guru 21645 points

     Hi Arief,

     Thank you very much for your reply.

      Could I ask you a few additional questions?

     Q1. Why is the large Vo overshoot generated? Could you plese tell me the mechanizm?

     Q2. Is there any way to reduce the overshoot?

     Thank you and best regards,

     Kazuya.

     

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 23845 points

    Hi,

    The overshoot is caused by the reaction when the part drops a current from a certain load to no load. Therefore the extra energy from the inductor will be absorbed by the output capacitor.

     One way to reduce the overshoot could be adding more output capacitor. 

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Guru 21645 points

     Hi Arief,

     Thank you very much for your reply and I'm sorry for my late response because I took summer vacation last week.

     Could I ask you an aditional question as the below?

     Q. I think LMR33620A goes in PWM mode when the load current is full and goes in PFM mode when the load current is almost 0mA.

         In the both mode, the internal reference voltage of LMR33620A not changed?

     Thank you very much again and best regards,

     Kazuya Nakai.

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 35210 points

    Nakai-san,

    We guarantee by datasheet the feedback (FB) voltage min and max across operating conditions.

    Are you trying to understand how the part will regulate in PFM mode? Please keep in mind, PFM while increases efficiency at light loads comes at the cost of increased output ripple.

  • 0 in reply to Marshall_Beck
    Guru 21645 points

     Hello Beck,

     Thank you very much for your reply.

     Yes. I'm trying to understand how LMR33620A will regulate in PFM mode because I think the difference voltage of output between in PWM mode and in PFM mode is bigger a little than other devices which have PFM mode.

     Alief said that one way to reduce the overshoot could be adding more output capacitor. 

     Do you know other way to reduce the difference voltage? Is changing inductor value effective?

     Thank you again and best regards,

     Kazuya.   

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 35210 points

    Are you talking about a steady-state difference from the set output voltage or

    Are you talking a transient in the output voltage?

    Thanks

  • 0 in reply to Marshall_Beck
    Guru 21645 points

     Hello Beck,

     I'm sorry to confuse you.

     I'm taking about a transient in the output voltage.

     I'm trying to understand how LMR33620A will regulate at transient timing from PWM mode (Full load) to PFM mode (No load) because I think the difference voltage of output between in PWM mode and in PFM mode is bigger a little than other devices which have PFM mode.

     Also I'd like to know whether changing inductor value to smaller is effective or not?

     Thank you and best regards,

     Kazuya.

     

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 35210 points

    The "extra" energy that is stored in an inductor is E = LI^2. "I" increases with reduced inductance because ripple current gets larger.

    This would be a good experiement to try based on your change in inductance you may a change in overshoots.

    The most cost effective way is to improve control loop by getting appropriate phase margin through appropriate CFF and crossover.