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Original question:

TPS40009: TPS40009DGQR output is Abnormal

TPS40009: TPS40009: Problems replacing with TPS40005 and TPS40003.

Jonatan Arroyo
Prodigy 10 points
Part Number: TPS40009
Other Parts Discussed in Thread: TPS40005, , TPS40003, TPSM82866A

Dear TI power team,
At the company, we have a product that uses the TPS40009 in a buck regulator designed to deliver 3.3V / 5A. Currently, there is no stock available from suppliers for this IC. For this reason, we are trying to replace it with the TPS40003 or TPS40005 models since both have similar characteristics (similar pinout and operating modes) and operate at a frequency of approximately 600 KHz. we tested both replacements but they do not behave as expected. When our board is turned on with load, both ICs provide a voltage of less than 3.3V (approximately 3.15V), causing our board to work incorrectly. We also probed the SW pin voltage and we noticed an unstable behavior. In the original IC the SW signal is stable at 63% duty cycle while in replacement IC's the SW signal is unstable arround 97%. We want to know if direct replacement is possible, or if some values need to be recalculated. In case we have to recalculate values, do you have any software tool that allows you to select the IC TPS40003/5 and simplify the calculation of components?
Attached is a file with the tests we performed.
Looking forward hearing from you.
Best Regards.

tps40005 tests_V01.pdf

  • 0
    TI__Guru*** 147360 points

    Hi Jonatan,

    First, I recommend that you redesign your board with a newer device.  This controller family is 20 years old and you are using NRND components as a short term solution.  The TPSM82866A provides the same functionality and integrates most of your passives for a much simpler and smaller design.

    Yes, these devices do provide similar functionality.  The main difference is in how they startup with a pre-bias.  In your first waveform, I see that you have a pre-bias.

    If the IC was installed by hand, I recommend replacing all of the small size Rs and Cs in the circuit--nearby components like these are sometimes damaged when replacing the IC.   You could also check if you get the same behavior on all boards or just on this 1 board that you tested.

    Chris

  • 0 in reply to Chris Glaser
    Prodigy 10 points

    Hi Chris,

    Thanks for your answer.

    At this time a redesign of the board is not possible. We will keep it in mind for the future.

    We already tested on 2 boards. One of the boards is the one from which we obtained the osciloscope captures included in the TPS40005 test_V01.pdf file.

    On the second board we also mounted a TPS40005 and as previously mentioned the board did not work correctly. On that same board we mounted back the TPS40009 and it started working fine again. That is why we discarded other components being damaged.

    Any other suggestions?

  • 0 in reply to Jonatan Arroyo
    TI__Guru*** 147360 points

    Ah, thanks for explaining.

    Are you measuring the output voltage with a meter or the scope?  I recommend measuring with a meter at the same location on the same PCB for the TPS40009 and TPS40005 circuits.  In this way, the measurement setup and passives are the same.

    Usually, probing the FB pin introduces some offset or noise to the output voltage, so it is not recommended.  I have also not had much success with measuring the voltage across a resistor in series with the inductor.  (The best way to measure the inductor current is by using a current probe around a wire, added where you added the resistor.)  I would retest without these items and see if the behavior is improved.

    There may also be something caused by the PCB layout.  I can check it, if you are able to share the portion around the IC.

    Chris