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LMR62014: Cf design

Roy Chou1
Genius 13085 points

Part Number: LMR62014

Hi team,

We would like to use LMR62014 to replace APW7137. We removed the R31.

 

The original C38(Cf)=0.1uF I measure the Vout and sw node. It's ok to me.

But I look feed forward compensation design in datasheet.  Cf = 1/(2*pi*33k*6k)=0.8nF

So I replace 0.1uF with 820pF but there is low frequency noise in no load. And noise could be eliminated when adding small load(20mA)

no load

20mA load

 

Could you let me know why?

Regards,

Roy

  • 0
    TI__Mastermind 34201 points

    Hi Roy,

    Thanks for reaching out.

    LMR62014 is a current-mode boost converter operating at fixed frequency of 1.6MHz. At no load condition, because there is a minimum on time limit, it will switch for some time, Vout increases until the FB voltage is higher than reference voltage. So it stops for some time until Vout decreases. This is normal behavior for old fixed frequency boost converter parts.

    LMR62014 maximum output voltage is 20V. The Vout of this application is 19V so there is little room. LM27313 is more suitable for this application.

  • 0 in reply to Zack Liu
    TI__Genius 13085 points

    Hi Zack,

    1. But there is frequently noise in steady state.

    2. If we modify the Cf cap from 820pF to 0.1uF, the issue could be fixed.

    3. if fixed frequency is root cause, I think LM27313 also has the issue.

    Roy

  • 0 in reply to Roy Chou1
    TI__Mastermind 34201 points

    Hi Roy,

    Thanks for the update.

    You can run a load transient test to see if the chosen Cf is correct and phase margin is enough.

    An application report explains how to choose the correct Cf. Below picture helps evaluate the phase margin.