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UC2843: What is relation between V-sense and Ouput Duty cycle ?

Sachin Godse
Prodigy 30 points
Part Number: UC2843


Hello,

I m using UC2843 as PWM generator for PWM is from 0% to 100% duty cycle.

I assumed there is following relation between input voltage at I-sense and Output duty cycle 

Input voltage at pin3 of UC2843D8 Output Duty cycle
   
1.00 0%
0.90 10%
0.80 20%
0.70 30%
0.60 40%
0.50 50%
0.40 60%
0.30 70%
0.20 80%
0.10 90%
0.00 100%

But is not the case when I observed practical result ,It was different than I my assumption.

I don't found any relative formula in datasheet as well , which derive the output PWM duty cycle with respective input analog voltage at I-sense pin .

Please give suggestion what is formula ? where I find it ?

  • 0
    TI__Genius 14580 points

    Hello,

    I guess the deadtime is large and it makes Dmax is less than 100%. See the below formula. 

    You can reduce CT, and it can help.

    Regards,

    Sean

  • 0 in reply to Sean Xu
    Prodigy 30 points

    Hello ,

    Thank you for quick reply,

    Actuall I have used CT=100nf and RT=22K   My frequency is about 1kHz

    Even I calculate Dmax using above formula we get value is 97% max duty cycle

    but practically we get upto 80%

    We want to know the relation between Isense input analog voltage and Output PWM duty cycle

    Is there any formula which tell us the relation between Input(Analog voltage at Isense pin) and Output PWM duty cycle? 

  • 0 in reply to Sachin Godse
    TI__Genius 14580 points

    Hello,

    Max duty is determined by RT and CT. Isense just chop the the on time. With very low input voltage, it will gert max duty.

    Can you try smaller CT, such as 10nF and higher RT,  Please let me know how it works.

    Thanks

  • 0 in reply to Sean Xu
    Prodigy 30 points

    Hello Sean Xu,

    Actually my requirement is to get output PWM  with 5% to 95% duty cycle.

    So for that, I need to define the input I-sense voltage range which is responsible to generate expected PWM at output.

    So we need the relation between I-sense and PWM duty cycle.

    Can give us proper formula to derive PWM duty cycle from input I-sense voltage. 

  • 0 in reply to Sachin Godse
    Prodigy 30 points

    Hello Sean,

    I want to know how much I-sense voltage I need to apply when I required 5% duty cycle at output. 

    wise-versa

    I want to know how much I-sense voltage I need to apply when I required 95% duty cycle at output

     I-sense pin voltage  ?v = 5% duty cycle.

    I-sense pin voltage  ?v  = 95% duty cycle.

    I want to know the relation between I-sense input voltage and Output PWM duty cycle.

    is there any proportional formula which calculate output duty cycle with respective analog voltage given to I-sense pin as input.

  • 0 in reply to Sachin Godse
    TI__Genius 14580 points

    Hello, Sachin,

    In the voltage mode control, a ramp waveform should be applied on ISENSE pin, The ramp is compared to internal COMP voltage to generate PWM signal.. I recommend to apply a fixed ramp on  ISENSE, then apply different DC voltage on COMP. The max duty is still decide  by 

    Regards,

    Sean