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UC28025: Output A starts 400 us before Output B

Part Number: UC28025

Regarding the UC28025, we are experiencing cases where Output A starts sooner than Output B at power up, as much as 400 us sooner.  This causes transformer problems and the transistors fail.  Does anyone know why one output might start sooner than the other?  We have tried to hold SD high, and hold the SS low for a long time even after the power supplies come up, and the Output A still starts sooner than Output B.  any ideas would be appreciated.  thanks.

  • Hi Bruce,

    Thank you for your interest in UC28025. Would you be willing to share your schematic with me? I will review it over the weekend and get back to you early next week.

    Best Regards,

    Ben Lough

  • thanks Ben. I would rather not publicize the schematic, can you send your email address to mine? I also have some scope screen shots. You should know that this is a short duration device, the device is triggered, powers up and operates within 10 seconds. We did verify a couple of things at startup: 1) Vref appears stable for 100s of milliseconds before soft start allows the output pulses, 2) we manually held soft start low and shutdown high for up to 10 seconds or so to make sure the supplies were up and happy before releasing them. We still get random startups where Output A typically starts before Output B by up to 100s of microseconds. The internal IC schematic appears to show that the Output A and Output B should always run together. there was another question on this forum that was very similar to this, but I could not find that this question was answered. This is the link, which appears to be from August 2017. e2e.ti.com/.../2267485 outputs not starting at same time#2267485
  • Hi Bruce,

    If you could send me your schematic and the scope captures you mentioned that would be helpful: ben.lough@ti.com. Also does this problem only appear during startup? 

    Best Regards,

    Ben Lough