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TPS54318: We want to check the Vo phenomenon(waveform) if the pull-down feedback resistor is short to gnd

Frank Xiao
Genius 12055 points
Part Number: TPS54318
Other Parts Discussed in Thread: TPS62826, TPS54618

Hi team,

My customer is using the TPS54318 now for 3.3v Vin, 2.5v Vo. We want to check the Vo phenomenon(waveform) if the pull-down feedback resistor is short to gnd.

Could you please help provide it? Thanks.

  • 0
    TI__Guru*** 142750 points

    Hi Frank,

    You can order the EVM and measure whatever is important in your application.  What is important for this case and why do you use this older device?  The TPS62826 is much smaller and simpler, for example.

    For any buck converter, when the FB pin is below the target voltage the IC will increase the duty cycle to try and bring FB up.  With FB grounded, the IC will operate near 100% mode and Vout will be nearly Vin.

  • 0 in reply to Chris Glaser
    TI__Genius 12055 points

    Hi Chris,

    I have ordered a EVM,but it will take 2 weeks to receive it. I am not sure whether product line can do the test or not. The customer has used the TPS54318 for many years. So they also use the device. Thanks. 

  • 0 in reply to Frank Xiao
    TI__Guru*** 142750 points

    Hi Frank,

    Here is the waveform, which shows the basic operation.  You may have more questions, so it is good that you ordered the EVM to test per your requirements.

    Yellow is 2.5Vout

    Green is 3.3Vin

    Red is PH

    Yes, they are welcome to keep using the older TPS54318.  This is ok.  But I ask: why is the customer working on a new design?  Why is their existing system (the previous design) not enough?  I think it is because they want to make some improvement in their design in order to make it better.  TI does this too in our ICs.  After many years, we have better ICs than we did before.  So, you should consider moving to a newer, better IC.

  • 0 in reply to Chris Glaser
    TI__Genius 12055 points

    Hi Chris,

    Good proposal. Below is the customer's requirement. Please share your recommendations and did a simple test to prove it(especially for the item 6). Thanks.

    basic requirements are listed as below:

    1.       Input range: 2.8~5.5V.

    2.       Output range: 0.6~3.3V, current should be 3A/6A, better p2p.

    3.       Selectable soft start time.

    4.       Pre-bias startup.

    5.       Small solution size, easy layout.

    6.       Strictly monotonic rise in full load startup.

     

  • 0 in reply to Frank Xiao
    TI__Guru*** 142750 points

    Thanks for confirming your requirements Frank.  Is the TPS54618/318 sufficient for you?

    Your requested Vin and Vout ranges are a bit lower than what that family supports, so I am unsure if this is a new need or just margin added to the existing TPS54618/318 designs?

    If you need up to 6A in a p2p with 3A, TPS54618/318 is a good family.  The TPS62826 is p2p up to 4A and supports the lower Vin and Vout ranges.  The SS time is fixed however.

  • 0 in reply to Chris Glaser
    TI__Genius 12055 points

    Hi Chris,

    Thanks  for your support. After short the low side resistor, the device will operate like your picture then keep stable, correct? Thanks.

  • 0 in reply to Frank Xiao
    TI__Guru*** 142750 points

    Hi Frank,

    I don't know what you mean by 'stable' so this is another reason to measure the EVM and see how it behaves.  Then, you can judge the performance.  I am also traveling this week, so I cannot take any more lab data until later.

    This app note (figure 4) shows operation in 100% mode, where the boot cap needs to be recharged: http://www.ti.com/lit/an/slyt747/slyt747.pdf