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TPS65251: switching stops

Hirotaka Matsumoto
Guru 27585 points
Other Parts Discussed in Thread: TPS65251

Hi all

Would you mind if we ask TPS65251?
Please refer to an attachment file which our customer measured.
20170213_TPS65251.pdf

There are no switching terms during startup.
The condtion is as follows;
<The condition>
Vin : 12V
Vout : 1.2V / 300mA
Cout : 22uF + 1nF
L : 4.7uH

So, as your opnion, could you let us know the condition which switching stops in case of startup?

Of cource, we consider that we have to check inductor's current.
If OCP occurs, we will add the cap to SS.(CSS=4700pF)

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Guru**** 191445 points
    The device is not switching because the output voltage is too high. I have seen this before with other devices. Typically it can be due to one of two reasons:

    1. At start up, the minimum on time may be higher than the on time to convert form your input voltage to the very low output voltage at the beginning of the slow start . This causes the output voltage to ram up to faster than what is set by the SS capacitor. After some time the control loop responds to this over voltage and turns of switching. After a time, the control loop starts switching again Noe the ramping up output voltage requires a larger on time above the minimum on time and the output voltage ramps up smoothly with continuous switching. I have seen several cycles of this operation on other devices,\.

    2. You have a pre-bias leakage someplace in your system. A higher voltage is impressed on to your output. Switching may stop until the SS voltage reaches the pre-bias voltage.

    There may be other possibilities as well. but those two come to mind quickly, as they have been commented on many times in the past.

    Let us know if one of those are your issue.
  • 0 in reply to JohnTucker
    Guru 27585 points

    John san

    Thank you very much for your reply always!
    We got that it is normal operation.
    In this case <1.>, we explain to our customer.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    Guru 27585 points

    John san

    Just in case, we would like to confirm one point.
    And then, as our recognition, we guess that it doesn't causes of failure for startup(device is normal operation).
    Is our recognition correct?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru**** 191445 points
    If you have verified that it is in fact due to < 1 > above, then that would be normal operation. I have not observed this directly with TPS65251, but my answer is based on similar performance with several other devices. Let me see if I can get the designated apps engineer to look at this as well.
  • 0 in reply to JohnTucker
    Guru 27585 points

    John san

    Thank you very much for your reply!
    OK, we got it.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Genius 9550 points

    Thanks John.

    Hirotaka,

    you mentioned that "If OCP occurs, we will add the cap to SS.(CSS=4700pF)", so i suppose you didn't put Css on board. Please add Css(4700pF), and test again. 

    Jason