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TPS54327: About the rise of Vout

Y.Ottey
Mastermind 6145 points
Part Number: TPS54327

Dear all,

When operated by the following circuit by the customer, a phenomenon like the following measured waveform was confirmed.

〇No load
The customer confirmed that Vout started rising 100 μs before switching EN from Low to High as shown by red circles.

Switching is not started during the rising of this red circle. (See the measurement waveform below)

〇When a 1.6kΩ load is connected
As with no load, Vout rises as shown by red circle before 100 μs when EN is switched from low to high.

Then, it was confirmed that Vout fell at the timing when EN was switched to High (VREG5 rises).

It should be noted that an offset of 30 mV was also confirmed when a load of 60 mV and 1.6 kΩ was connected with no load when EN was low.

Please answer the following questions about the above phenomenon.

Is the above phenomenon caused by "Prebiased Soft-Start" of TPS54327? (It is not possible to explain that there is a change in Vout when a load is connected due to this characteristic, but I would like to confirm it just in case.)


If another factor seems to be possible, I would appreciate advice.

Best Regards,

Y.Ottey

  • 0
    TI__Prodigy 450 points

    Hi Ottey,

    Let me do some investigation, and I will get back to you.  Thanks!

    Regards,

    jenson

  • 0 in reply to Jenson Lam
    Mastermind 6145 points

    Hi Jenson

    Thank you for your reply.


    We have received a request from our customer for an immediate response,

    so we would be grateful if you could tell us the results as soon as possible.

    Regards,

    Y.Ottey

  • 0 in reply to Y.Ottey
    TI__Prodigy 450 points

    Hi Y. Ottey,

    Yes, you are correct.  What you are seeing is the pre-biased soft start.

    Regards,

    jenson

  • 0 in reply to Jenson Lam
    Mastermind 6145 points

    Hi Jenson

    Thank you for your reply.

    I think this phenomenon is also affected by pre-biased soft start. 

    However, I do not believe that Vout rises before switching to EN = Hign or that the rise occurs when a load is connected is not the effect of the pre-biased soft start.


    Could you answer the following questions?
    (1) When a load (1.6 kΩ) is connected, the voltage drops to 0 V when EN goes high. Is this a recognition that the low-side FET inside the TPS54327 is pulled out?
    (2) I think that Vout rises before EN = High this time because the current flows from the board connected to the output, which charges the output capacitor, and Vout rises. If Vout is EN = Low with no load (no load or board connected to output), can the phenomenon occur?

    Best Regards,

    Y.Ottey

  • 0 in reply to Y.Ottey
    TI__Prodigy 450 points

    Hi Ottey,

    Sorry for the delay, and I should have answers for you soon.

    regards,

    jenson

  • 0 in reply to Y.Ottey
    TI__Prodigy 450 points

    Hi Y Ottey,

    For Ques 1, at what point of time is the 1.6kohm connected?  Before power up?

    For Ques 2, I have already verified it on the bench.  If there is no load or board connected at the output, this scenario should not happen.

    regards,

    jenson