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UCC28951: equations seem to give wrong values?

Adam Daluga
Expert 6385 points
Part Number: UCC28951

Tool/software:

Hello,

I’m still having difficulty setting the timing that I require using the UCC28951. The equations do not seem to give me the correct values. Instead, I must experiment with values in the lab to get reasonable delay times. The functional diagram in the datasheet does not detail the dependencies between, say, R_A and the voltage at ADEL. Similar problem with R_EF and ADELEF. Can you provide some guidance on how these are related so I may proceed in a more formal manner?

Thanks,

Adam

  • 0
    TI__Guru* 85616 points

    Hello,

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

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 85616 points

    Hello Adam,

    The data sheet timing equations were recently updated and can be calculated as below.

    KA is the resistor divider from CS to ADEL and ground.

    KEF is the resistor divider from CS to ADELEF to ground. 

    ADELEF and ADEL were design to try to use an adaptive delay approach.  Roughly 4 to 5 % of the population will try to use this approach to remove body diode conduction.  It is very trial and error.  Most customer will use a fixed delay approach which does not require adaptive delay.  This technique is discuss in application note slua560 that can be found at the following link. To use this approach you need to either set ADEL and ADELF to ground or a set reference voltage with resistor dividers off the VREF pin.

    The application note gives estimate initial estimated values for TABSET, TCDSET, TAFSET, TABSET based on estimated switch node capacitance, Lr and 1/4 the tank frequency.  Please not these are starting points and should load the design to 10% and fine tune the delay times.  Once this is set then you will set the TAFSET and TBESET based on fining tuning at 10 % load. 

    https://www.ti.com/lit/pdf/slua560

    Regards,