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UCC28C50-Q1: Pulse skipping in UCC28C50-Q1 Controller

Hen Ben David
Prodigy 30 points
Part Number: UCC28C50-Q1
Other Parts Discussed in Thread: UCC28C50

Tool/software:

I am using the UCC28C50-Q1 in the Flyback circuit, while in low load, I see that PWM of the controller is skipping some cycles here and there.

There is nothing in the DS that say that this controller has pulse skipping mode but I know from my experience that those family of controllers have these function.

Furthermore, if this controller do have this function, what is the reason to not to deliver a really small duty cycle pulse and instead skip a period ? What is the internal mechanism that allow this ?

Thanks in advance.

  • 0
    TI__Guru 74115 points

    Hi,

    At light load, when feedback loop causes COMP pin voltage close to 1.1V, the internal PWM comparator threshold is very close to 0 or reached to 0, this will make OUT pin pulse very narrow or no pulse width can present. That is the reason and the mechanism.

  • 0 in reply to Hong Huang
    Prodigy 30 points

    Hi,

    Thank you for your answer.

    I would like to mention that I am monitoring the COMP pin and it is on 1.8V, which indicates that it is not the case you are talking about - what else can be the problem ?

  • 0 in reply to Hen Ben David
    TI__Guru 74115 points

    Hi,

    Please provide your waveforms of COMP, OUT, CS, and RTCT to help understand your question and your description. Note if you observe COMP voltage using a volt-meter as average COMP voltage you could see 1.8V but you cannot see instant COMP voltage is it drops to below 1.1V or further lower so you need to provide the waveforms to help check instant voltage on these pins to decide. 

  • 0 in reply to Hong Huang
    Prodigy 30 points

    Hi,

    I can not provide the waveforms because it is confidential.

    I will add that in low current (DCM mode), there is  lag between the input and the output (It can be up to 2us).

    In addition, is it possible that there is a problem while working in really low input voltages? is less than 10mV is a problem for the INA to amplify?

  • 0 in reply to Hen Ben David
    TI__Guru 74115 points

    Hi,

    We need to analyze the waveforms to understand how to resolve.

    What is INA?

  • 0 in reply to Hong Huang
    Prodigy 30 points

    Hi,

    I am using the INA180A1 to amplify my current sense voltage to the level I need on the CS pin.

    If the only way it is possible to proceed is with the waveforms, I guess it will not happen, thanks anyway.

  • 0 in reply to Hen Ben David
    TI__Guru 74115 points

    Hi,

    We are not familiar with INA180A1. Please create a new thread to ask support related to INA180A1.

    Without waveforms, we cannot support UCC28C50 as we do not know where is likely wrong so not able to provide suggestions.