• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 408 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC256403: About malfunction when noise enters BW pin

user6230314
Prodigy 220 points
Part Number: UCC256403

Hi,


I have connected between the BW pin and GND with a resistor, but when noise enters the BW pin, OVP malfunctions.

I would like to connect a capacitor between the BW pin and GND.

Is the time constant of CR affected by the programming at startup?

What is the recommended capacity?

Thanks in advance.

  • 0
    TI__Guru 65965 points

    Hello,

    I have someone on our applications team looking into this for you.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 65965 points

    Hello,

    The BW pin programs burst mode and needs resistance to ground to program it before switching starts.  It is programed with 52 uA of current being source out of the BW pin so adding a lot of capacitance on this pin may prevent the correct burst mode from being set up.

    In the data sheet go to page 9 and look for RWMPrgm1..7.  During power up the resistance seen from this pin to ground needs to be equivalent in the data sheet.   For example if you wanted to disable burst this would RBWMPrgm6.  The resistance to ground needs to be between 2422 ohms and 3038 ohms.  If you were using the BW divider off the aux this would be the impedance of the BW high and low resistor in parallel.  In your case you are not using the BW off an aux winding or using OVP.  So you would just have to tie a resistor to ground and choose the option of burst mode that you want.

    Please not if you are not using burst mode under no load conditions the output could climb and run away because the gain can not be controlled low enough.  This can be easily solve with a pre-load resistor.

    Adding the resistance from the BW pin to ground should help resolve some noise issues.  If you still have noise issue and want to add a capacitor, I would suggest using a small capacitor 22 to 100 pF. 

    Regards,

  • 0 in reply to Mike O'
    Prodigy 220 points

    Thank you for your reply

    For example, what are the recommended capacitances for 2500Ω and 9500Ω?


    I would like to know the detailed recommended capacitance for each resistor.
  • 0 in reply to user6230314
    TI__Mastermind 26215 points

    Hello,

    The way the controller determines the BW pin resistance is by sourcing a 54uA current out of the pin and measuring the resulting voltage. You would want to make sure the RC time constant is small enough that the BW pin voltage settles within the 2ms programming time where R is 2.5kOhm||9.5kOhm and C is the filtering capacitor. If I keep C small enough that 5*R*C < 2ms, this would make the largest capacitor you would want to use is 200pF.

    Best Regards,

    Ben Lough