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Original question:

UCC28950: about unused pins

UCC28950: Some question about UCC28950

Gabriel Wang1
Genius 3825 points
Part Number: UCC28950

Dear Expert

1. Under light load, the chip will enter Burst mode, which causes the output voltage ripple to increase obviously.What are the best solution?      

2. Under light load, the chip will enter Burst mode, eg: no load jump to 80% of Max load, output voltage drop is relatively deep, what is the way to quickly exit Burst mode?If a quick exit burst mode, dynamic is helpful to us.

3.Burst mode stops after passing an even number of pulses. What determines the exact number of pulses (e.g. 2 or 4) to be passed?

Tips :The resistance of Tmin to GND is 10K.

  • 0
    TI__Guru 66135 points

    Hello,

    1. You can reduce the Tmin time constant to as small as possible.  You can add a slight amount of preload with a resistor. 

    2. I still think reducing Tmin to the minimum value will help with this.

    3. When the duty cycle demands less on time than Tmin the controller will stop delivering pulses and will not turn-on again until the on time is above Tmin.

    >The even number of pulses is so the transformer does not saturate during burst mode.

    Regards.

  • 0
    Prodigy 130 points

    Hello，  

    1. You can reduce the Tmin time constant to as small as possible.  You can add a slight amount of preload with a resistor. 

    -->The minimum allowed resistor on TMIN, RTMIN is 10 kΩ. now we use  RTMIN  also is 10 kΩ, Any other suggestions ?

         if  add a slight amount of preload with a resistor, Will affect efficiency.

    2. I still think reducing Tmin to the minimum value will help with this.

     -->now we use  RTMIN   is 10 kΩ

    3. When the duty cycle demands less on time than Tmin the controller will stop delivering pulses and will not turn-on again until the on time is above Tmin.

  • 0 in reply to user5032528
    Prodigy 130 points

    Hello TI team，

             Looking forward to your reply.

  • 0 in reply to Mike O'
    Genius 3825 points

    Dear Mike

    Below is our customer's R&D ,they still have some question about your suggestions .Hope u could give some more advice ,many thanks!

  • 0 in reply to Gabriel Wang1
    TI__Guru 66135 points

    You have nothing attached in your response.

  • 0 in reply to Mike O'
    Prodigy 130 points

    question: Under light load, the chip will enter Burst mode, eg: no load jump to 80% of Max load, output voltage drop is relatively deep, what is the way to quickly exit Burst mode?

    Ti reply: I still think reducing Tmin to the minimum value will help with this.

    customer reply: The minimum allowed resistor on TMIN, RTMIN is 10 kΩ. now we use  RTMIN  also is 10 kΩ, Any other suggestions to  quickly exit Burst mode?

  • 0 in reply to user5032528
    TI__Guru 66135 points

    Hello,

    This is not a light load issue but a transient response issue.  The device has OCP shutdown and that may be what you are hitting.  Study the CS signal, SS, Vout and comp durring this load step.  If SS is discharging down to 0.55 V during this transient the device is going into OCP protection.  Section 7.3.14 explains the OCP hiccup protection.

    If this is what is occurring check to make sure your transformer is not saturating.  You may be able to adjust your CS resistor and/or SS capacitor to prevent the device from entering hiccup mode.

    Also double check your voltage loop compensation to make sure there is not and instability.  You can double check this with the excel design tool at the following link.  https://www.ti.com/lit/zip/sluc222

    Regards,