• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 64 subscribers
  • Views 540 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

Need help on TPS54040

micro tca
Prodigy 10 points
Other Parts Discussed in Thread: TPS54040

Hi, I'm using TPS54040 in one design, it worked on the rev1 board, but on the rev2 board, it doesn't work. Rev2 shares the same design, and PCB layout is same as well.

The design is to have 5V in, 4V out, 650KHz switching frequency, and input should also work with 12V input as well. I disconnected the load, measured output, found that output is similar to saw wave with around 8-9Hz change frequently from 2.7 to 3.6, and Pin 10(PH) output the similar wave, 8-9 Hz saw wave. Looks like internal oscillator is not working properly, I checked resistor connected to Pin5(RT), it's correct. I'm wondering is there any possibility could cause this problem.

Thanks.

  • 0
    TI__Guru**** 191445 points

    I f you have 5V in, 4 V out and light load, you have a combination of eco-mode and low headroom drop out operation.  In that circumstance, the BOOT cap is not completely charging above the BOOT UVLO and the converter must shut down while the output discharges to Vin - 2.1 V.  You can add a charge pump or a passive diode block circuit to prevent this.  Ill have to look up the  details next week though.

  • 0 in reply to JohnTucker
    Genius 3685 points

    Hi John,

    I just hit the exact same problem (although for a converter needing to operate from ~6.0V - 8.4V input and with output 5V instead, but I think the problem is pretty similar :-). In case of light/no load and low input voltage I of cause/unfortunately hit the same under voltage shut down condition as described above. I'm therefore interested in hearing if you had and luck finding any ideas on how to circumvent this problem? 

    Could you kindly elaborate a little more on the charge pump and passive diode block circuit approach? Would it be OK just applying 12V (from a charge pump) to the BOOT pin or do you seen any other and better solution? I think  this is the idea of the charge pump solution? But I'm not completely sure I follow what you mean by the "passive diode block circuit". Maybe this is better/preferable?

    Should I take any special precautions supplying this voltage externally (i.e. supplying it through a diode or similar)?

    Best regards and thanks
      Søren 

    PS: The selection of TPS54040 was done mainly based on a suggestion from TI web side wizard and simulations done in Switcher Pro. None of these suggesting/stating any thing about this "problem" with low drop out and low load. I could of cause just had read through the datasheet in details (I fully admit :-( :-), but a note in Switcher Pro or in the wizard would had been beneficial to avoid this problem - Hopefully you can add this for others in the future to avoid this kind of "problem" :-)

  • 0 in reply to Soren Steen Christensen
    Genius 3685 points

    PPS: Forgot to mention, that the under voltage lockout in my setup seems to be around 2.3-2.4V rather than the specified 2.1V. I'm running at 25 degree and at no load, so the circuit shouldn't be heated in any special way. Any other valid reason why it can/should deviate as much as this (~10%) from the datasheet specified value? Is this OK/intentional/as expected?

  • 0 in reply to Soren Steen Christensen
    TI__Guru**** 191445 points

    The charge pump info is contained in SLVA444 application note.  The alternate solution is to use a blocking diode as described in the encloosed ppt.

    0714.TPS54160 low dropout.ppt

  • 0 in reply to JohnTucker
    Genius 3685 points

    Hi John,

    Thanks for the fast reply and two good sources of information for going forward. Meanwhile I experimented connecting the BOOT pin to a 12V supply (I had for other usage in the system) through a Diode in series with a 6K8 resistor for recharging the boot cap (basically roughly the same idea as your charge pump apporach), and I now have the circuit working as expected... :-)

    Best regards and thanks agian for your fast feedback - It has been highly appreciated
      Søren