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Original question:

UC2843B question

UC2843: PWM output missing

Xiao Zhu2
Expert 3215 points
Part Number: UC2843

Hello Team

The customer is using UC2843 in their product. Sometime, the device will work at open load mode. and the current only 10mA. At this mode, the device PWM duty cycle pulse width output  will reduce low than minimum of UC2843. And the device sometime will no PWM output. But the output voltage is stable. Please see the following scope. So they want to know if used in this way, is there any risk? And if have risk, what the risk is? thanks.

  • 0
    TI__Genius 13660 points

    Hi Xiao Zhu,

    The PWM duty depends on Isense pin voltage and COMP pin voltage and there is no CS blank time feature (minimum duty cycle). 

    First, please attach the  waveform of VCOMP and Isense Voltage.   and try to compare the difference between the period with pwm output and the period with duty loss.

    If the output voltage is stable, the comp voltage should also be a stable voltage. When Isense voltage positive exceeds the inverting input of PWM comparator (proportional to comp volatge), pwm is terminated.

    Regards,

    Teng





     

  • 0 in reply to Teng Feng
    TI__Expert 3215 points

    Thanks for your reply.

  • 0 in reply to Xiao Zhu2
    TI__Genius 13660 points

    Hi Xiao Zhu,

    As we discussed on the email. 

    At no load, COMP pin may be pulled to ground  then it will command zero duty cycle (no pulse).

    Allowing zero-duty cycle (pulse skip) ensures that the output voltage is always under control and won't rise above the setpoint.

    Thus this part is operating normally

    Regards,

    Teng