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Original question:

LM5025C: Unexpected behavior

LM5025C: The driving waveform is abnormal in the non-monotonic region of the output waveform

Gabriel Wang1
Genius 4715 points
Part Number: LM5025C
Other Parts Discussed in Thread: LM5025

Dear Expert

Our company used the active embedded chip LM5025 to make a 150W power supply from 110DC to 15V. The customer proposed that the output rise waveform was not monotonous (there were steps). For specific problems, please see the attachment.

1. The peripheral schematic diagram of the chip is shown in the attached screenshot. At present, our company has tested that only the driving waveform is abnormal in the non-monotonic region of the output waveform, and no abnormality has been found in other waveforms!

2. The phenomenon that the capacitance value of the soft start capacitor C33 is changed from 0.47uF to 0.1uF is not monotonous can be improved by improving it. At present, the customer needs a detailed analysis report, please help to provide analysis, thank you!

Attached customer's analyse report , customer use chinese ，if you need me translate to English ,pls ask me .

LM5025 Test Wave form.xlsx

  • 0
    TI__Guru 73085 points

    Hi,

    Please provide your full schematics for analysis. Usually the startup (non monotonic) is related to the handover from SS to COMP not smooth. You need to look at the handover to find solution. Also, usually reduce SS capacitor value helps to handover from SS to COMP at early stage so help to make smooth for the rest and also early stage Vo non-monotonic usually small and ignorable.  

  • 0 in reply to Hong Huang
    Genius 4715 points

    Dear Hong

    See the following waveforms: the first is the output monotonic waveform, the second is the non-monotonic waveform,

    the blue is the output, the purple is the SS pin, the green is the COMP pin.

    Attached full schemetic:

    CD110T170.pdf

  • +1 in reply to Gabriel Wang1
    TI__Guru 73085 points

    Hi,

    It looks on your non-monotonic Vout capture, COMP and SS have a long overlap, meaning, the control is switching back and forth between SS and COMP, mainly due to the noise on each signal, SS or COMP. As noise level and characteristics can be different on SS and on COMP, the control will generate different pulse widths, then cause Vout non monotonic shape. It looks you need either do the fast SS like your monotonic or change COMP ramp so to avoid long overlap between COMP and SS.

  • 0 in reply to Hong Huang
    Genius 4715 points

    Dear Expert Huang Hong

    Thanks for your replying!

    From the comparison of monotonic and non-monotonic waveforms, the waveforms of COMP pin and SS pin have overlapping parts (the amplitude of COMP pin in Figure 2 is smaller than that in the figure, so if the amplitude is the same, they all have overlapping parts). How to distinguish the non-monotonic output?

  • 0 in reply to Gabriel Wang1
    TI__Guru 73085 points

    Hi, 

    It is difficult to tell as the control changing back and forth between SS and COMP on noise and on each own trends. The way to do is to make the two signals cross instead of overlap.

  • 0 in reply to Hong Huang
    TI__Guru 73085 points

    Hi,

    The waveforms on the left without non-monotonic has less overlap time, SS rising speed much faster, it looks less noisy. You may need to zoom in the area nearby SS and COMP to check. The lower voltage of SS and COMP takes control.

  • 0 in reply to Hong Huang
    Genius 4715 points

    Dear Huang Hong

    When the output waveform is monotonous and non-monotonous, the overlap time of the waveforms of COMP and SS pins is indeed different. However, in the non-monotonous region, SS and COMP are overlapped. This cannot be clearly explained because the slow soft start causes the output to be non-monotonous.

    See the following waveforms. The first one is the monotonic waveform, and the second chapter is the non-monotonic waveform, CH1:SS, CH2: VOUT, CH3:COMP.

  • 0 in reply to Gabriel Wang1
    TI__Guru 73085 points

    Hi,

    You need to zoom in the overlap area to make analysis to decide which signal is in control. The noise from two signals is not the same so if the control back and forth the PWM will follow and possible with pulse width not always increase. But these need to zoom in that overlap to check. Your current resolution cannot find these details.