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TL432: VREF value drift

Maxime Bougouin
Prodigy 10 points
Part Number: TL432

Hi,

I have a POE supply design based on the evalboard . The TL432 is used in the feedback loop of the converter.

NCP1083QBCGEVB_SCHEMATIC.PDF

The design seemed fine until I noticed some drifting in the outpout voltage of the converter. Target voltage was 12.3V and I measured some design at 15.9V.

When investigating the issue, I noticed when changing the TL432, the output voltage was drifting a lot : from 13.2V to 15.9V (with a target voltage of 12.3V).

From the original design provided by OnSemi, the values of the following resistor have changed :

  • Rfb3 = 8.2KOhms
  • Rbias2 = 1MOhms

Even with changing these values, I should have a current > 1mA on the TL432's cathode, hence i should be in the Recommended operating conditions [1mA ; 100mA], and don't understand why I have so much drifting in my output voltage.

Is this a known issue ?

Is there a mean to minimize the drift from one component to another ?

Regards,

Maxime

  • +1
    TI__Expert 6660 points

    Hello Maxime,

    Is there a reason why Rfb3 is connected before the inductor L1 and Rbias2 is connected after L1? Usually Rfb3 is connected to the same node as Rfb1 and Rbias2 is connected between pins 1 and 2 of the optocoupler. 

    With the new values of Rfb3 and Rbias2, the device is not receiving enough current. The current through Rbias2 is only 9.5uA and Rfb3 might be too large for the optocoupler to have enough gain. I suggest lowering Rfb3 to the original value of 820 ohms and lower Rbias2 to 9.5K or lower.

    If this does not solve the issue the device might be unstable and Cfb1 will need to be adjusted.

    Thanks,

    Walter