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TPS40170 not keeping volts up

Tom Walker
Prodigy 60 points
Other Parts Discussed in Thread: TPS40170

Hi, I'm using a TPS40170 to switch 28V nominal down to 5V for running a USB hub...

(hope that image is readable)

At idle it works fine, but as soon as any appreciable load is put on (over 700mA) then the voltage drops to 4.7V or lower and looks dirty (lots of switching noise).  At this point the USB hub tends to lose the will to live and loses connection with the PC.

I need to be able to run up to 3A off the supply, so what am I doing wrong?

Thanks in advance, Tom

  • 0
    TI__Mastermind 35165 points

    Hi Tom,

    Reviewing your schematic everything looks ok. Can you post some screenshots of the switching node and the output voltage below and above 700mA load? Also feel free to share any other screenshots of signals you find are not as you expected.

    Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 60 points

    Hi Anthony, thanks for looking.  First trace is PSU with very little load...

    CH1 in red is the SW node - we are running from 28V. The spikes on the yellow trace only appear when I probe the SW terminal, so this could be bad earthing on my scope - I'm not sure.  Although I'm going to try a snubber on here to see what happens.

    I also notice that the SW pulses are alternating between a big a small pulse, but once the supply has been on for a short while it settles down to this...

    Which is at half the frequency, but steady. ??

    Next, this trace is with 450mA load and you can see the spikes have got worse and the output trace (yellow) has dropped to 5.7V already.

    I then did the same at about 1A...

    And it's not changed much but the output voltage has dropped a little further.

    Now I thought I would get a close up of the ringing, and [accidentally] discovered that if I quickly connect and disconnect the load (at about 400mA), occasionally it would drop into a steady mode where it would switch without any ringing and the voltage would remain at 5V.  Is this a clue??

    I appreciate your help.  Tom

  • 0 in reply to Tom Walker
    TI__Mastermind 35165 points

    Hi Tom,

    Sorry the holidays added some delay in my response.

    The variation in the duty cycle and screenshot with half the switching frequency is very unusual. Can you show the gate drive signals in these conditions?

    Double checking the component selection and schematic I haven't seen anything which looks incorrect. Can you also share the layout?

    Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 60 points

    Hello again Anthony,

    I think I've actually made some progress myself.  I first tried a snubber (430pF + 12R) on the SW node which helped a bit, but more successful was adding a small resistor (10R) in series with the bootstrap capacitor.  This has eliminated most of the ringing on the switching line.

    Also, I increased my output capacitance (a lot) and this combination of changes seems to have made the difference.  I can now pull over 2A from the supply and it doesn't drop below 2.85V.

    The board is undergoing a major re-lay now (for mechanical reasons) - so I'll probably have to review my values on the new layup, but generally speaking I think I am there.

    Regards,
    Tom