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UCC28C44: How to setup and optimize the current mode control of UCC28C44

Justin Schools
Prodigy 80 points
Part Number: UCC28C44

Tool/software:

In the Datasheet of the UCC28C44 the example shows a blocking capacitor to remove DC offset when generating a ramp for slope compensation. 

The Cramp removes the DC offset and the signal appearing at the CS pin should be a scaled version of the RC waveform of the RT/CT pin. However, when I try to probe this pin
1) the transformer starts squealing when probing the CS pin.

2) what if the load is low and the peak current doesn't turn off the comparator before the output voltage is met?

At low currents the Ramp provided by the current sense resistor is so small it appears as noise. Therefore, how does the controller regulate anything since the CS pin is the only source of a ramp to generate

the PWM? That is, if there is virtually no signal from the current sense resistor, then adding noise to the decoupled ramp yields a negative value does it not?

I hear in materials that there isn't enough ramp or there is too much, how then would I adjust this? Is the ratio purely the ratio between Rramp and Rcsf ?

If the current does go to low does the PWM converter try and operate in the Voltage Mode vs Current Mode?

Currently, I have set teh Cramp at 220nF, Rramp at 15K, Rcsf at 10K. Csf as 220pC and Rsense is 0.75ohm. The timing Resistor Rrt is 10K and Cct is 1nF.

Any information helps clarify things for me.

Thanks,

Justin

  • 0
    Prodigy 80 points


    This is a screenshot of the CS pin (Yellow) at about 350mA Load. The Drain Voltage of the Switching Mosfet is in Blue.

  • 0 in reply to Justin Schools
    TI__Guru 73805 points

    Hi,

    That circuit needs to correct like the below. So Cramp needs to discharge each switching cycle to make operation repeatable and stable.

  • 0 in reply to Hong Huang
    Prodigy 80 points

    This is interesting. The datasheet I got from TI website shows the Diagram I originally posted. Is it just outdated? I will definitely give this a try.

  • 0 in reply to Justin Schools
    TI__Guru 73805 points

    Hi,

    That particular circuit needs to add the discharge resistor.

  • 0 in reply to Hong Huang
    Prodigy 80 points

    So I grabbed another screenshot of the CS Pin using a spring clip. Much cleaner signal. I then added a resistor as you proposed and the resulting signal is more of a triangle wave as expected. Im a bit confused why the data sheet wants to use a capacitor though because it makes the triangle wave negative. WHat is the impact of this? When the current ramp is added to this wouldn't the negative portion null out the current ramp signal for that portion of the wave form. Can I just remove the Capacitor and let it DC biased?

    I don't have the scope AC coupled either just in case it was a concern.

    Thanks, 

    Justin

  • +1 in reply to Justin Schools
    TI__Guru 73805 points

    Hi,

    That capacitor is not a must when max voltage in the connection to CS pin < 6.3V. Datasheet Fig 7-8 does not have this capacitor.

    That capacitor is to help isolate the fault to isolate voltage if either side can have a voltage to damage the other side when in a fault. If this is not a concern, or not possible, then that capacitor is not a must.

    CS pin is ok with negative voltage up to -0.3V. The negative voltage is adjustable by the capacitor value.