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UCC256403: UCC256403 maximum switching frequency during startup

Part Number: UCC256403

Hi Team,

Have a good day!

I have a question on device UCC256403. I noticed that in datasheet, SS initial voltage provides a freedom to limit the maximum switching frequency during startup. This helps to prevent hard switching due to excessively high switching frequency. So my question is how and where to set the SS initial voltage? And what's the relationship between voltage level and maximum switching frequency?


BRs, Sherman

  • Hi,

    The switching frequency during startup will be decided purely based on the VTH and VTL thresholds and the VCR pin capacitors values. These VTH and VTL thresholds are determined by the soft start initial voltage (3rd point in below picture) and the soft start capacitor value (which decides how quickly frequency will be ramped down).


    To change the startup frequency (at the start of the startup), Vssinit can be used. The above picture shows how the startup frequency can be changed with the help of Vssinit. This value can be modified using Resistor values at the LL/Ss pin. Please use the design calculator for finding these values.


    Neglecting the contribution to the VCR waveform from charge control and considering only the internal ramp current, the initial switching frequency will be approximated as  2mA/(Vssinit*C_vcrlowercap).




    Manikanta P