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UC3823A: UC3823A duty cycle

WILLIAM CONNOLLY
Prodigy 40 points
Part Number: UC3823A
Other Parts Discussed in Thread: UC1825, UC1823

Short description: Unable to obtain PWM duty cycle of less than 30% with UC3823A.Case details or comments:

When error amplifier output voltage (pin 3) is lowered, the duty cycle reduces smoothly from 95% to about 30%. When it reaches 30% further reduction of the error amp output voltage causes the IC to skip pulses and then go completely off. When placing a UC1825 in the same socket for comparison, duty cycle can be varied smoothly from 50% to about 7% duty cycle as expected. Frequency is 250kHz and the ramp is a clean sawtooth.

  • 0
    TI__Guru 54515 points

    Hi,

    UC3823A, check your RAMP and EAOUT,  PWM is produced by comparing these two pins. Also check your RT and CT, their values should be from Fig 2 on page 6. If you use numbers outside of Fig 2, unexpected results can be observed, including what you saw from other people's observations.

    UC1825 max duty 50%, half of UC1823. 7% might be just because of possibilities of some devices can do outside of Fig 2. So check your numbers to make sure within Fig 2.

    Further questions, please provide your circuit and each component numbers.

  • 0
    Prodigy 40 points

    the values are within the proper range so that is not the problem. I will prepare a PowerPoint file with more data. How do I send that to you?

  • 0 in reply to WILLIAM CONNOLLY
    TI__Guru 54515 points

    Hi,

    hong_huang@ti.com

    Let's continue on email. I will close this thread.