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UCC256402: Highest switching frequency and Ton at startup

Sheldon He
Expert 8585 points
Part Number: UCC256402

Hi expert,

My customer need to know how to calculate the highest switching frequency and Ton at startup. Could you offer any help on this?

Thanks

Sheldon  

  • 0
    TI__Guru 73595 points

    We will reply to you soon.

  • 0 in reply to Hong Huang
    TI__Mastermind 27115 points

    Hi Sheldon,

    The highest switching frequency will depend on the soft start offset programmed on LL/SS as well as the VCR capacitance. Neglecting the contribution to the VCR waveform from charge control and considering only the internal ramp current, the initial switching frequency will be 2mA/(V_offset*C_vcrlowercap)

    In real application, the charge control contribution will be non-zero as there is some inrush current and also the calculation does not account for propagation delay from crossing the VTH/VTL thresholds and the gate turning off. To get a more accurate estimate, I would suggest using  the simplis model to study the effect of inrush, propagation delays, etc on the initial switching frequency. 

    In my experience, the soft start profile is normally fine tuned on bench by adjusting the VCR caps, LL/SS offset and LL/SS soft start capacitance.

    Best Regards,

    Ben Lough