UCC25600: External discharge resistor on pin SS

Mason Noh

Expert 2490 points

Part Number: UCC25600

Hi TI,

My customer is working on designing SMPS board using UCC25600.

Sometimes, it has ZCS operation when they have input chattering test.

And they solved this issue using external discharge resistor for the soft start capacitor on SS pin.

Could you recommend the range of this resistor value? SS cap is 0.1uF.

over 6 years ago

0 Benjamin Lough over 6 years ago

TI__Mastermind 27145 points

Hi Mason,

Typically the RT pin resistors are sized to provide a minimum switching frequency that is above the capacitive region to avoid hard switching. How does your customer have the discharge resistor connected? Is it just in parallel with the soft start capacitor?

Best Regards,
Ben Lough

0 Mason Noh over 6 years ago in reply to Benjamin Lough

Expert 2490 points

Hi Benjamin,
Yes, 1.2Mohm is just in parallel with the SS cap.

0 Benjamin Lough over 6 years ago in reply to Mason Noh

TI__Mastermind 27145 points

Hi Mason,

Thanks for the additional information. Limiting the lowest switching frequency would be the preferred solution for avoiding hard switching. If your customer would prefer to use a resistor on the SS pin, then I would recommend 1.2MΩ as the maximum allowable resistance based on the internal voltage rail powering that circuit and the value of the internal current source.

Best Regards,
Ben Lough

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UCC25600: External discharge resistor on pin SS