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.

PMP21639: 8V output unless load added

Part Number: PMP21639
Other Parts Discussed in Thread: ATL431, UCC28780

I duplicated the PMP21639 reference design exactly and I'm checking the Vo (without USB-C cable attached). I matched every component used as well.

The issue is Vo is always 8VDC with no load and occurring on every board tested. If I add a small resistive load between Vo and SGND then Vo drops to the expected 5VDC. One of the tests I tried, I replaced R45 with a potentiometer that feeds the ATL431 REF pin and noticed that with no load the Vo does not change from potentiometer. But with a small load applied Vo starts to work as expected and changes with potentiometer. It seems the standby mode in UCC28780RTER is not working. 

Also, is it possible to purchase a working PMP21639 that I can compare signals to?

  • Hi Eugene,

    The test report shows it was able to power up at no load so this is interesting. Can you still forward your schematic, bom, and layout files? Can you double check your connections on the PCB and component value/placement as well. It may be that the schematic is correct but maybe there is something misplaced in the assembly? 

  • Enclosed files.

    That is correct, the 5V must be measured with no load. Here is the power and interface section that is under test. You'll need to note that CTL1, CTL2, and CTL3 are left floating for this 5V testing, this is following the Table 4 from TPS25740BRGER datasheet (see enclosed picture of table). So CTL1, CTL2, and CTL3 are not needed for this test.

    Please note, I compared the Gerbers from the Reference Design to my work by overlaying the images of Top, Inners, Bottom and it all shows same component pad placement and same wire connections.

    The component values are harder to check in-circuit as well as some IC's have markings that their datasheets do not define. The board was assembled by our CM as well as the purchasing of the parts. I've looked over many IC's and components and it looks okay so far.

    What I think will help me isolate the issue is finding out what condition(s) would stop the UCC28780RTER chip from paying attention to its FB pin?

  • Here's what my testing has shown.

    I noticed that U4 is measuring the low current load draw and when load current increased past a certain threshold (I think >50mA) it disables Q101 and in turn removes the voltage/current divider R18 from the circuit that feeds the CS pin of UCC28780RTER.

    So that Q101 and R18 divider circuit is active when no load is applied causing the current detection into the CS pin of UCC28780RTER to drop by a considerable amount. I also noted that this divider circuit is not used or mentioned anywhere in any other reference designs, except the one I used PMP21639. 

    I removed R18 to disable the divider circuit and now everything works as expected. Am I missing something here?

  • Hi Eurgene, thanks for confirming this behavior on your board. My first suspicion was UCC28780 has features that enable light load efficiency and no load performance. Features such as burst mode directly affect the RUN pin which directly correlates to the R18 pull down on CS. I believe your measurements have confirmed it since the behavior goes away when R18 is removed.

  • Unfortunately, not all of the boards cooperated with the R18 change. The final fix was to reduce R21 (Rfb) from 20K to 10K and now everything works as advertised.