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LM34927: Output voltage drifts a lot or unstable with different Cr and Ron Value

Shenhua Zhang
Prodigy 640 points
Part Number: LM34927

Hi Sir,

My customer is using LM34927 as aux power controller of their DC/DC bricks. The testing input voltage is 50V and other parameters/output loads are shown below.

They want the VOUT1 to be 10.5V, so they choose the feedback resistors R4/R5 as below ,15k and 2k, according to the calculation.

However, they found that VOUT1 is not 10.5V, and is changing a lot with different R3 and C7. The testing results are shown in the chart below.

Could you please tell us what's going wrong here?

  • 0
    TI__Guru 65557 points

    Hi Shenhua,

     

    Use the quickstart file (available by download from the product folder) to calculate the ripple injection component values.

     

    The ripple injected into the FB node should be 20mV pk-pk at min Vin. Higher ripple amplitude (by decreasing the cap) causes a corresponding offset on the output voltage equal to half the ripple amplitude gained up by the feedback divider ratio.

     

    See app note snva874 for more detail.

     

    Regards,

    Tim