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UC2842: minimum duty limit by voltage mode with UCC2842

chingyu Wang
Prodigy 140 points
Other Parts Discussed in Thread: UC2842

rt Number: UC2842

Hi, Ti member

We are developing the buck converter based on UC2842. Instead of current mode, use voltage mode refer datasheet and add additional OPA circuit to limit output current.

there are a compensated circuit cascade a diode to limit Vcomp by internal current source of IC and Isense-Pin injected a Ramp signal by RT,CT. (Vcomp compared to Isense to produce a PWM signal) 

the parameter setup:

1. OSC frequency is 8%

2. Isensn Ramp range: 90mV --> 920mV

 

but the minimum  duty is 8% , it very hard to control output current at short condition. 

Ch1: output current , Ch3: Vds ,Ch4: Vgs

we tune a lot of ways to limit duty :

1. change Isense Range

2. Vcomp value by DC source

3. OSC frequency

above all ,the minimum duty still keep 8% .    Could you tell me how to do can limit PWM below 1%?  

  • 0
    TI__Intellectual 2640 points

    Hi Chingyu! 

    Thanks for reaching out and for your interest in the UC2842! Can you provide waveforms from the COMP, VFB, and ISENSE pins of the UC2842? 

    Thank you again for getting in touch, I look forward to talking more soon! 

    Aidan Davidson

  • 0 in reply to Aidan Davidson1
    Prodigy 140 points

    Hi, Davidson

     

    This is my prototype circuit; modify the UC2842 from current mode to voltage mode.

    1. a compensated amplifier cascade a diode to limit Vcomp value
    2. Isense is Ramp signal followed OSC.

    Here are the waveform:

    l   OSC frequency is 140kHz (period: 7.16us)

    l   first PWM period is 460ns about 6%

     

    Ch2: Vcomp approximately 2.5V à this value is minimum what we can controlled If the value drop than 2.5V, then PWM will be disabled.

    Ch3: PWM from Output pin (range from 200mV to 920mV)

    Ch4: Isense signal

    The area of zoom-in is PWM start-point at soft-start.

    During soft-start, the output of compensated is lower level compared to Isense to produce small PWM.

    Confused why the UC2842 can’t control PWM approximately to 1% ~ 2%, but 6%.

     

    Do I misunderstand something parameter?

    By the way , we had tuned the value of Vea many times.


  • 0 in reply to chingyu Wang
    TI__Intellectual 2640 points
    Hi Chingyu!
    US TI offices are closed Thursday 11/26 and Friday 11/27 due to the Thanksgiving holiday. I will get back to you with an update early next week! 

    Thanks for your patience, I look forward to talking more soon! 
    Aidan
  • 0 in reply to Aidan Davidson1
    TI__Intellectual 2640 points

    Hi Chingyu, 

    Apologies for the wait here, thank you so much for your patience. Do you have another UC2842 device on hand to try dropping in here? Normally I would expect the PWM to disable closer to 1.4V, I am curious to see if the device you're using might be an outlier. 

    How are you inputting your tweaked COMP voltage and where are you measuring it? 

    Thanks, I look forward to talking more soon!

    Aidan