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UC3825: minimum duty cycle limit

user5823347
Prodigy 10 points
Part Number: UC3825

Uc3825 has the minimum duty cycle limit, such as 5%. If the duty cycle is less than 5%, the duty ratio of the chip output may become 0. I want to know the principle.

  • +1
    TI__Guru 58015 points

    Hi,

    The PWM is produced by comparing the ramp and the ref. When the ref is < RAMP bottom, there will be no pulses present so to become zero duty. In theory, the duty should be ok to be < 5% become jump to 0%. But in practice on reason 0 < duty < 5% may not be achieved, because propagation delay, the ref and the ramp noise, the comparator input offset needed, are all among why. UC3825 is an old device so 5% is already very good.

    A newer device may have better min duty on the improvement to the mentioned,

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Why do different RT and CT correspond to different minimum duty cycles?

  • 0 in reply to user5823347
    TI__Guru 58015 points

    Hi,

    Different RT and CT give different speed and noise level which interact with the mentioned factors. So different min duty can be resulted.