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LM61460: Vout ripple

Takashi Yamaguchi
Genius 11610 points
Part Number: LM61460

Hi team,

My customer measured Vout ripple on LM61460EVM-AS-400K. Could you check and comment for each condition 1 and 2? I'd like to know that these waveforms are normal or not.

Condition.1

Vin=24V

Vout=3.308V (Rfbt=300kohm, Rfbb=130kohm)

Fsw=490kHz (Rrt=27kohm)

Iout=0A to 6A (24v-0A in the file means output current is 0A)

/cfs-file/__key/communityserver-discussions-components-files/196/LM61460EVM-Vout-ripple.zip

Looking at the zoomed waveform, the output ripple voltage does not seem to be normal. Have you seen similar Vout ripple on the LM61460EVM? Is that normal operation?

Customer modified the L, Cout, and Vout value on condition.2. Could you also check the Vout ripple voltage waveforms in the below link? In the zoomed waveform of 24 V, the ripple voltage contains a square wave, which seems to be an effect of Cout ESL.

Condition.2

Vin=24V / 7V

Vout=3.375V (Rfbt=285kohm, Rfbb=120kohm)

Iout=0A to 6A

L=3.3uH (1267AY-3R3N)

Cout=added 22uF as CO5, so total cout=22uFx5=110uF

Fsw=490kHz (Rrt=27kohm)

/cfs-file/__key/communityserver-discussions-components-files/196/LM61460-Vout-ripple-with-condition.2.zip

Customer added 22uF(CO5) in the red circle.

Regards,

Yamaguchi

  • 0
    TI__Guru 54912 points

    Hello Yamaguchi-san,

    Where on the board is the output ripple being measured?

    The square wave behavior is likely an inductive divide of the switch-node voltage based on the output cap ESL, as you mentioned. In terms of the low-frequency behavior, can you try a different input source supply with lower common-mode noise.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Genius 11610 points

    Hello Tim-san,

    Thank you for your support.

    I will check to customer where on the board is the output ripple being measured.

    For the condition.1, isn't this waveform(24V, 4A) normal? If possible, could you share the waveform which is similar condition with the Condition.1? I'd like to have the reference waveform.

    Does the meaning of "low-frequency behavior" mean the following 125Khz?

    Regards,

    Yamaguchi

  • 0 in reply to Takashi Yamaguchi
    TI__Guru 54912 points

    Hi Yamaguchi-san,

    Take a look in the LM61460 datasheet for ripple waveforms.

    I doubt this is instability, but it could be some bumpy behavior in the input supply (125kHz is above the loop rejection bandwidth of the DC-DC converter). Please ensure that the input supply voltage doesn't have a similar ripple signature. If so, it could be attenuated by an input LC filter. For input filter design, see part 15 here: http://www.how2power.com/other/EMI_Guide.php  

    Regards,

    Tim