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UCC256403: FB pin internal bias current source - value?

Zion79
Prodigy 50 points
Part Number: UCC256403

Hello, 

I am trying to determine a suitable dc bias current of optocoupler in the feedback loop. To do so, I need to know the FB pin internal bias current source IFB in fig 28 of datasheet. Can you tell me the value of IFB? I couldn't find it in the ucc256403 datasheet. 

Thanks,

Zion

  • 0
    TI__Mastermind 26735 points

    Hi Zion79,

    Thanks for reaching out. The IFB current source is 164uA nominal.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Prodigy 50 points

    Hi Ben,

    164uA IFB is too large. 164uA through RFB=100k ohm is 16.4V, which can be larger than the recommended 15V Vcc. Also the voltage on RFB can be larger than the voltage at FB pin, 5.2V, which means the PMOSFET in fig 28 datasheet isn't working properly and FB pin can't remain at 5.2V. This doesn't look right. Can you double check that IFB is indeed 164uA?

    Thanks,

    Zion

  • 0 in reply to Zion79
    TI__Mastermind 26735 points

    Hi Zion,

    IFB is 164uA, this is listed on page 3 of the datasheet. 

    The voltage on the internal RFB resistor is internally limited to 6V. This is the maximum value FBreplica is allowed to have (FBreplica is the voltage across the internal RFB resistor). If for example the opto-coupler draws no current out of the FB pin, the FB pin voltage will float up to the internal AVDD rail of 7V and FBreplica will increase to the maximum allowable value of 6V. This will make the switching thresholds on VCR go their maximum values (VTH = 6V, VTL = 0V). I apologize that this detail is not clear in the datasheet but I want to stress that the device is designed to operate and properly protect its internal nodes even if the opto-coupler were to draw no current out of the FB pin.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    TI__Mastermind 26735 points

    Hi Zion,

    If I could provide some more detail regarding the opto-coupler DC biasing question, an opto-coupler current range of 105uA to 164uA would cover the full range of FBreplica.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Prodigy 50 points

    Hi Ben,

    Thank you for the explain. It is clear to me now. I get a draft simulation running today. The performance looks very good. 

    Best regards,

    Zion