UCC28780: UCC28780 startup issue

George Mattson

Part Number: UCC28780

I'm working on a new design with the UCC28780. Startup is going through the normal sequence until it gets to the step where the RUN pin is supposed to go high. Run doesn't go high and PWML never goes high. VDD continues to drop until it gets low enough (about 8ms), and then it recharges with the SWS pin, and this cycle continues. I checked the Chvg is 2.2nF and it's fall time to 11V is much greater than 25us. Vref is going to 5V. And I couldn't find evidence that any of the other shutdowns were causing this. Do you have any suggestions for troubleshooting this?

6 months ago

0 ROHAN SHAH 6 months ago

TI__Intellectual 2860 points

Hello

I want to have clarification about what is doubt. whether VDD is cycling between VDD(ON) and VDD(OFF) ? and RUN pin status is permanently Low ?

Regards

Rohan

0 George Mattson 6 months ago in reply to ROHAN SHAH

Prodigy 30 points

Yes, VDD is cycling between VDD(ON) and VDD(OFF). And RUN is permanently low. Also, I made a mistake and hit the link to unsubscribe to emails for this issue. I can't see how to turn that back on so I get emails. Can you turn that back on?

0 George Mattson 6 months ago in reply to ROHAN SHAH

Prodigy 30 points

Do you have any suggestions for troubleshooting this?

0 George Mattson 6 months ago in reply to ROHAN SHAH

Prodigy 30 points

I was checking the protections. On my board, RDM goes to 0.6V for about 100us when VDD reaches VDDon. But in my simulation, it goes to about 2.3V and stays there. Both my board and simulation have a 95.3k resistor to GND on this pin. With RTZ, my board matches my simulation with both going to about 3.5V.

0 ROHAN SHAH 6 months ago in reply to George Mattson

TI__Intellectual 2860 points

Hello George,

What is turns ratio of transformer (Np:Ns:Na) ?RCS value, RVS1, RVS2 and LM value ?

Can you check aux waveform and FB pin waveform at startup ?

Also can you check NTC pin waveform at startup along with RUN pin waveform ?

I don't have any idea about what should i do so that you will get mails. I have clicked on "email subscribe to user". Not sure whether this will work or not.

Regards

Rohan

0 George Mattson 6 months ago in reply to ROHAN SHAH

Prodigy 30 points

When I zoomed in to see these waveforms, I noticed that the RUN pin briefly went to about 5V for 4us. PWML went high for about 2us, but this wasn't enough time for the driver to turn on the FET. NTC went to 2V for 4us. Aux was 0V because the FET never turned on. FB goes to 4.2V at startup.

0 George Mattson 6 months ago in reply to ROHAN SHAH

Prodigy 30 points

The transformer turns ration is 1:1:1. RCS is 50mOhms. RVS1 is 36.5k, RVS2 is 8.06k. LM is 5uH.

0 George Mattson 6 months ago in reply to ROHAN SHAH

Prodigy 30 points

I think I just figured out what's going on. It looks like a CS pin fault. The driver IC for the low side FET has two possible pinouts. That driver isn't turning on, and I think the supplier bought the wrong pinout.

0 ROHAN SHAH 6 months ago in reply to George Mattson

TI__Intellectual 2860 points

Hello George,

let me know when you get it corrected IC driver.

Also i would recommend to check the resistors value connected on RVS1 and RVS2 and connect RDM resistor as per equation given in section 7.3.7 in Datasheet of UCC28780.

Regards

Rohan

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UCC28780: UCC28780 startup issue