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About the actual measured value of slow start setting.

hisashi yoda
Intellectual 640 points
Part Number: LMZ31506

Tool/software:

Previously, I received an answer regarding the variation regarding slow starts.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1322516/lmz31506-about-slow-start-charging-current-of-lmz31506/5067414?tisearch=e2e-quicksearch

 

Parts variation

・Vref 0.6V ∓ 1%

・Iss：2.26uA - 2.32uA (rough estimate).

・Internal capacitor：3300pF ∓ 10%

 

Capacitor connected to SS terminal：4700pF ∓ 10%

 

I calculated it using a formula.

tss=1.84ms - 2.36ms

 

The actual measured value is not within the calculated range.

Please let me know the reason why it doesn't work.

For example,

・The cursor placement is not good.（Measured at 90% voltage rise）

・The information on the variation is different.

Other causes

Actual values：2.4354ms（white frame）
Voltage output by LMZ31506：+3.3V（green line）

The information you provide is very important to us.
Please respond with all questions by May 27, 2024.

  • 0
    Intellectual 640 points

    We apologize for the inconvenience, but could you give us some comments regarding the question?

    Regards

    Yoda

  • 0 in reply to hisashi yoda
    TI__Expert 5328 points

    Hi Hisashi,

    I have measured soft start at VOUT using Css=4700pF, same as you and my results are quite similar. However I placed the second cursor bit before as compared to your measurement, with which I measured about ~2.2ms which is correct with you calculation of a range Tss = 1.84ms - 2.36ms when accounting for tolerances. 

    Your measurement is correct and in range with the SS equation, it's just the cursor placement is a bit different than our measurement. Hope this answers your question.

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Intellectual 640 points

    Hi Eileen

    Thank you for your answer.

    Please tell me how to place the second cursor bit (position b).

    You said that you tilted it before, but at what percentage of the Vout potential was it measured?

    For example, Vout x 95%

    The information you provide is very important to us.
    Please respond with all questions by May 28, 2024.

    Regards
    Yoda

  • 0 in reply to hisashi yoda
    TI__Expert 5328 points

    Hi Hisashi,

    I measured the second cursor at ~98% of Vout. However, my scale was small, so it 's possible that the value could be measured between 95%-100%.

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Intellectual 640 points
    Hi Eileen
    Thank you for your reply.
    We would like to perform quantitative measurements to compare with the design values.
    We will change the way I ask.
    When measuring tss, please tell me at what percentage of Vout you measure it.
    If you cannot indicate the percentage, is the following idea valid?

    The +3.3VD waveform (green line) is rounded when it reaches 3.3V. (White circle)

    The formula below is a linear function of time T and voltage V.

    The formula does not take into account the rounding (white circle) when +3.3V reaches 100%.

    How about the approach for the cursor position?

    1. Extend the wiring as shown (red line).

    2. Indicate the position of 100% potential (white line).

    3. The intersection of 1. and 2. is the cursor (yellow line) position.

    4. tss time (orange line)

    The information you provide is very important to us.
    Please respond with all questions by May 30, 2024.

    Regards
    Yoda

  • 0 in reply to hisashi yoda
    TI__Expert 5328 points

    Hi Hisashi,

    I measured Tss at about ~98% of vout, which would equate to ~3.24Vout (for an output of 3.3Vout), that typically is where the curve ends and vout finishes ramping up. Because there's a SS range due to caps tolerance, you could most likely be able to measure SS at 100% as well.

    The rounding observed at the white circle in VOUT is quite common across most buck converters. The equation you attached refers only to the linear portion of the voltage ramping up. 

    I don't see any issues on your cursor approach in your last image, that should help you measure soft start time between 1.84ms - 2.36ms.

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Intellectual 640 points

    Hi Eileen

    Thank you for your comment on my suggestion.

    The answer was that it is possible to measure at 100%, taking into account the error of the capacitor.

    However, tss=1.84ms - 2.36ms is a result that includes the following errors:

    ・Vref 0.6V ∓ 1%

    ・Iss：2.26uA - 2.32uA (rough estimate).

    ・Internal capacitor：3300pF ∓ 10%

    Capacitor connected to SS terminal：4700pF ∓ 10%

    Is it TI's standard to measure at 98% of Vout?

     

    Regards

    Yoda

  • 0 in reply to hisashi yoda
    TI__Expert 5328 points

    Hi Hisashi,

    The soft start standard varies part by part. I couldn't find the % for this part however we typically do 90% or 100% of Vout.

    Regards,

    Eileen