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TPS53126: Unable to Get Buck Controller to Work

MARK IVEY
Prodigy 40 points
Part Number: TPS53126

Hello, 

I am trying to make a dual buck design for a project, but I am unable to get an output voltage from either of the two outputs. After reflowing the design, verifying solder joints on the TPS53126, the two SiZF920DT's and doing a check for shorts, I get no output voltage when I apply 8-12V. I see 5.06V on the two 4.7 uF and 1.0 uF caps for the internal 5V LDO, and I see 4.56V across the two boost caps. I also measured ~5 mV across the two trip resistors. However, I don't see any switching going on, and I think I would see that on the boost cap to ground. 

The schematic is here, and the top layer for reference is here.

Thanks in advance.

  • 0
    TI__Genius 17905 points

    HI ,

    Thanks for sharing the schematic and layout. Have you verified the voltages at EN1 and EN2?

    Regards,
    Kris

  • 0 in reply to Kris F
    Prodigy 40 points

    Yes, I verified that the pins are tied to VIN this after making the mistake of leaving the enable floating on a previous rev.

    I also looked at the bootstrapping cap on both halves of the converter with my oscope and did not see any switching. 

  • 0 in reply to MARK IVEY
    TI__Genius 17905 points

    Mark,

    What are the voltages you measure at FB1, FB2, VO1, and VO2 -- are these all showing 0V? Just want to make sure the outputs are not pre-biased.

    Regards,
    Kris 

  • 0 in reply to Kris F
    Prodigy 40 points

    Yes, they are all at 0V. I verified this and the resistance by measuring from the capacitor to the FB pins, and from the buck converters FB pins to ground.

    The only thing I can think is I missed something very obvious in the datasheet and was hoping a different pair of eyes could catch the mistake, as my oscope shows no switching on any of the pins

  • 0 in reply to MARK IVEY
    TI__Genius 17905 points

    Mark, I didn't see any obvious issues in your design. Is it just the two layers on your board? If you can share all the layers I can take a deeper look. If you don't feel like sharing on the forum, you can send to bsr-mvhc-support  _at_ ti.com. Please make sure to include the part number in the email subject.

  • 0 in reply to Kris F
    Prodigy 40 points

    Not exactly optimal layout, but It's supposed to be a test board for a larger project. I've also included the gerber files if that helps.

    -MarkRev 32 clock.zip

  • 0 in reply to MARK IVEY
    TI__Genius 17905 points

    Hi Mark,

    I reviewed the layout further and, yes, while I agree probably not an optimal layout, I do not see any obvious reason for the part to not be switching at all.

    I do see that V5FILT cap and TRIP resistors are tied to PGND instead of to SGND (GND pin 3 and where you have FB dividers connected) as the datasheet recommends, but I do not think that would be enough to cause the part to not switch.

    I have to go back to suspecting something related to EN pins. You said you verified the EN pins are tied to VIN - do you mean you measured the voltages on the pins? Or just checked for resistance from a VIN node to each EN pin?

    Can you try measuring the resistance from each VO to PGND? I wonder if you would measure the 40Ω output discharge resistance mentioned in Section 8.3.8 which would indicate that ENx is low or that some other protection mechanism has triggered.

    Have you checked with oscilloscope if there is any switching activity when you first toggle on your input supply? e.g. trigger on rising VIN or EN edge, and capture SW and VO of one channel or both, if possible.

    Do you have multiple boards and do they all show the same behavior (VREG, V5FILT are 5V, but no signs of switching)?

  • 0 in reply to Kris F
    TI__Genius 17905 points

    Hi Mark,

    Just wanted to check in and see if you were able to look into some of the questions/suggestions I had in my previous reply?

    Kris