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UCC256404: Value of VFB Clamp Voltage

Jongyoon Chae
Prodigy 30 points
Part Number: UCC256404

Tool/software:

Hi This is Jongyoon, 

I am currently designing a LLC converter with UCC256404 

I have a question regarding the value of VFB Clamp Voltage. 

In the datasheet of UCC256404, it says that VFB voltage is clamped when it reaches the "FB pin clamp voltage threshold"

I would like to know the value of the clamp voltage. 

Thanks and best regards. 

Jongyoon. 

  • 0
    TI__Guru 69675 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 69675 points

    Hello,

    Based on the block diagram and the VFB specified when sinking 50 uA indicates the clamp is 5.6 V.

    Regards,

  • 0
    Prodigy 30 points

    Hello, Thanks for your reply but I have some concerns. 

    I'm designing LLC converter therefore isolated feedback network is constructed utilizing the optocoupler (which is connected to the FB pin) 

    In UCC256404 datasheet 7.3.3.2 FB Pin Voltage clamp it says as follows. 

    "When FB pin voltage drops below the FB pin clamp voltage threshold, an extra current source is turned on to clamp the FB voltage" 

    So is the 5.6 V is the FB pin clamp voltage threshold ? 

    In other words, when optocoupler sinks current of IFB (82uA), the voltage clamped on the FB pin is 5.6 V ?

    It seems a bit too high, can you please check it again? 

    Thanks. 

  • 0 in reply to Jongyoon Chae
    TI__Guru 69675 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 69675 points

    Hello,

    I checked the data sheet and the internal clamp voltage is VFB = 5.6 V.  The clamp voltage does vary based on the current drawn out of the FB pin but not much.  Please see the information below from the data sheet.

    The internal source IFB is what controls the FB voltage which is (IFB-Iopto)*RFB.  Iopto = 0 the design is operating at the minimum switching frequency.

    Iopto = 82 uA the design is operating at the maximum switching frequency.  Under all these conditions the FB pin should be roughly clamped at 5.6 V.  This is according to the block diagram and data sheet parameters.

    Regards,