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UCC2808A-1Q1: UCC2808A-1Q1 usage problem

Eagle.Miao
Intellectual 910 points
Part Number: UCC2808A-1Q1

My design is open loop (OUTA and OUTB duty cycle is constant 50%), the starting current peak value is small (soft start time 1ms and above), the current soft start time can only be 400us, how can we improve the soft start time?

Can you help me see the schematic diagram?

The cs pin is connected to the resistor, which will cause the power supply efficiency to decrease and some troubles later. Does the CS pin need no resistance?

thank you！

  • 0
    TI__Mastermind 31580 points

    Hi Eagle,

    You could add an external soft start circuit to the FB pin which gradually pulls the FB to ground to drive the COMP pin high. Select the R and C value to give you the desired SS time.

    Regards

    PeterUCC2808A SS.docx

  • 0 in reply to Peter Meaney
    Intellectual 910 points

    thank you very much! I found that current start and soft start time have nothing to do. My application is cs directly grounded, no current sampling, the waveform is as follows  a: the starting current and the peak value are large; the CS connection resistance refers to the datesheet   b starting current and the peak value is small; cs can be directly grounded?

    a: the starting current and the peak value are large                                                                               b starting current and the peak value is small

  • 0 in reply to Eagle.Miao
    TI__Mastermind 31580 points
    Hi Eagle

    During start-up the power stage must charge up the capacitors on the output of the power stage as well as deliver current to the load. I suggest you capture the output voltage and the primary current at the same time. I suspect that the current reduces once the output voltage comes into regulation.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Intellectual 910 points

    Thank you very much!

    What is the maximum voltage allowable for UCC2808A COMP PIN? Thank you!

  • 0 in reply to Eagle.Miao
    TI__Mastermind 31580 points

    Hi Eagle,

    COMP is an output and if the FB is pulled to ground the COMP voltage will go to the voltage level of the internal bias rail, something like 5V to 6V approximately.

    There is no need to force COMP high is the circuit are around the internal error amplifier is set up correctly.

    Regards

    Peter