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Sync problems with TPS54550

Manfred Hager
Prodigy 50 points
Other Parts Discussed in Thread: TPS54550

Hello guys,

We have some problems with a TPS54550 step-down converter in a 3.3V design. The case is as follows:

  •  Several years ago we developed a design that worked fine. The product went into production and no problems were reported.
  •  The product is manufactured and tested from our supplier with test equipment we delivered. Several 100 devices were tested during the last years.
  •  Recently the supplier observed problems with the product during testing. A whole new lot of devices did not work.
  •  The developers looked at the devices and identified a problem with a TPS54550. The converter did not lock to the delivered sync signal.
  •  We had a look at all relevant surrounding components. Everything is fine on the PCB.
  •  In the attachment below the circuit is shown and some measurements we performed.
  •  From our development work some years ago we still had some samples of the TPS54550. We put one of these onto the PCB and it worked.
  •  Several questions came to our mind:
    •  Did we design our circuit at the edge of a device specification?
    • Is there a problem in a recent IC batch?

·         We tested lots of configurations (see attachment), but now we have the feeling that we can’t get any further without help.

We appreciate if someone is able help us.

1830.Power_Problem_eng.doc


Best regards,

Manfred Hager

 

 

  • 0
    TI__Guru**** 191445 points

    It sounds like you may have an IC problem if older parts work and new ones do not.  There have been some revisions to the IC to prevent damage due to pre-bias voltage at start up, but am not sure how that may affect your circuit.  I would suggest that you submit some of the failing units for failure analysis.  In the mean time can you provide the PH waveform and SYNC signal in the same screen shot?

  • 0 in reply to JohnTucker
    Prodigy 50 points

    Thanks John for the answer.

    I will provide you the screen shot when I am back in the office on Tuesday. To whom I should send the failing units for analysis? We are located in Munich, Germany. TI Germany in Freising is not far from here.

    Regards,

    Manfred Hager

  • 0 in reply to Manfred Hager
    TI__Guru**** 191445 points

    Your local AFA or FAE should be able to handle the paperwork.  I am pretty sure the actual FA will be performed in Dallas.  If you do not have a contact person, let me know and I'll see what I can find out.

  • 0 in reply to JohnTucker
    Prodigy 50 points
    Movie1.wmv

    Attached find the measurements you requested. We recorded two movies showing the scope display.

    Movie 1:

    System parameters: Rt = 160kOhm, measured free running frequency: 328 kHz, sync frequency: 336 kHz

    Upper trace: Sync frequency waveform

    Lower trace: PH waveform

    At the beginning and at the end of the movie (t < 6 s, t > 48 s) the load was low. In between (6 s < t < 48 s) a high load was applied to the circuit. The problem occurs with high loads as you can see.

    We observed that the delay between the falling edge of Sync and the rising edge of PH amounts to about 500 ns, whereas in the datasheet it is specified with typ. 360 ns (but for Rt = 100kOhm). We do not know if this observation is relevant.

    Additional Comment: for a sync frequency of 330 kHz the circuit worked even for high loads for some seconds. Then (we think the components heated up) the behavior got unstable again.

    Movie 2 (see next reply):

    System parameters: Rt = 160kOhm, measured free running frequency: 328 kHz, sync frequency: variable, load: low

    Upper trace: Sync frequency waveform

    Lower trace: PH waveform

    With low load the circuit runs synchronously in a wide frequency range around the free running frequency.

     

    By the way. We don’t have a contact person at TI. Would you be so kind to find that out. Thanks.

     

    Regards,

    Manfred

  • 0 in reply to Manfred Hager
    TI__Guru**** 191445 points

    I'll take a look at these later today.  As far as a contact person, where are you located and what is your company name? Can you tell me about your business opportunity?

  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points

    I looked at movie 2.  I did not see naything wrong with it.  Movie 1 did not seem to be posted.  If you post your contact information, I will get someone in touch with you.

  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points

    I still do not see movie 1 in the first post.  I removed your prior post so your personal info is not visible to the external forum.  See if you can re-post movie 1. 

  • 0 in reply to JohnTucker
    Prodigy 50 points
    Movie1.wmv

    I attachted it again. Hope it works.

  • 0 in reply to Manfred Hager
    TI__Guru**** 191445 points

    I do not see any issues at all with movie 2, but movie 1 is definitely dropping out. So far as I know there is no relationship to output current and SYNC capability.  It may be over current or possibly a stability issue.  Can you provide the inductor current waveform as well as PH?  As you can guess, troubleshooting via the forum will take some time.  I am looking to see if there is local support for your concern.  If you don't mind my asking, how many k units per month is this opportunity?