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UCC28C55-Q1: soft start time of UCC28C55-Q1

Karthik karri
Prodigy 235 points
Part Number: UCC28C55-Q1
Other Parts Discussed in Thread: UC2842

Tool/software:

Hi TI team,

i am working on  UCC28C55-Q1 and I have query regarding how to calculate Rss,Css based on soft start time .It is mentioned in the datasheet as 1mA of the Error amplifier output current will charge the Css capacitor thought the PNP transistor.

By taking 1mA current metioned in the datasheet below and using  formula, 

Css * Vcomp =i *tss  

By taking Vcomp pin voltage is 5V and reuired soft start time of 20ms ,can i calculate  Css * Vcomp =i *tss   ==>  Css= (1mA* 20mA)/ 5V   = => 4uF.Is this correct??

It will not charge from Vref??

Because in the Ti forum for UC2842 soft start time,it was metioned as ,

can i use like above picture to calcualte the soft start time as Vref will charge the Css capacitor .Got confused after seeing this post whether Vref will charge the Css or  E/A output (Comp pin) through PNP transistor will charge the Css capacitor.

Can you clarify this and kindly suggest correct way to calculate the soft start time with Rssa and Css.

Thanks,

  • 0
    Prodigy 235 points

    Hi Ti team,

    any update on the requested query ?

  • 0 in reply to Karthik karri
    TI__Guru 74115 points

    Hi,

    You can use the same approach of UCC28C4x to UCC28C5x. The adjust Rss on the bench to get the tss you need. The approach is the same between these two devices..

  • 0 in reply to Hong Huang
    Prodigy 235 points

    Hi Hing

    in  UCC28C4x to UCC28C5x datasheet they given only description.Not given calculation formula..

    can you confirm the below formula and approach is correct for UCC28C4x to UCC28C5x??

    It is mentioned in the datasheet as 1mA of the Error amplifier output current will charge the Css capacitor thought the PNP transistor.

    By taking 1mA current metioned in the datasheet below and using  formula, 

    Css * Vcomp =i *tss  

    By taking Vcomp pin voltage is 5V and required soft start time of 20ms ,can i calculate  Css * Vcomp =i *tss   ==>  Css= (1mA* 20mA)/ 5V   = => 4uF.Is this correct??

    It will not charge from Vref??

     

  • 0 in reply to Karthik karri
    TI__Guru 74115 points

    Hi,

    COMP pin effective range is between 1V and 4.5V. After Css voltage larger than 0.7V, the bipolar emitter to collector voltage drop Vec < Veb, so Iec is COMP pin major current to force COMP voltage or Vec to follow Css voltage increase. After COMP > 0.7V, Ieb becomes much smaller than Iec due to beta of the transistor, then Css charging current is from VREF through Rss.

    The effective time for soft start is after COMP > 1V, until it reaches 4.5V. So tss is calculated fromCOLP  = 1V to COMP = 4.5V

    Vss = 5V x [1-exp(-t/(RssxCss)]

    Vss1 = 5V x [1-exp(-t1/RssxCss)], which is = 0.3V, to find t1

    Vss2 = 5V x [1-exp(t2/RssxCss)], which is = 4.5V, to find t2

    tss = t2 - t1