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UCC256402: System external shut dowm circuit

Guru 18490 points
Part Number: UCC256402
Other Parts Discussed in Thread: UCC256302

About UCC256402 or same series, I confirming to add external disable circuit based on UCC256302. (please refer UCC256302 datasheet page-20)

Please let me know tow points below;

・What is  the threshold of disable voltage?

 I guess that is lower than BMT_LMIN: 0.2V, correct?

・When trigger the external shut down, is restart function invalid to operate soft start, correct?

 If correct, is there any way to operate soft start? 

Best regards,


  • Hi Satoshi,

    Thanks for reaching out to us. It's not recommended to use the FB pin for system shut down. In other words, The external disable circuit that's been used for UCC256302 should not be used or UCC256402. That's because, when the FBreplica less than BMT, controller simply turns off the switching pulses. There is no way to provide soft start if you use this method. 

    You can implement external shut down using BLK pin voltage. When the BLK voltage less than threshold, controller turns off the pulses. This is safe way to do it since there will be a soft start each time BLK reaches the threshold voltage.

    BMT lower threshold set by the both SS/LL pin and BW pin of the controller. SS/LL pin decides the BMTH whereas BW determines the ratio of BMTH/BMTL. For additional details, please go through sections and of the datasheet.

    Please let me know if you have any further questions. 


    Manikanta P