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LM25116: Vout signal

Andres Gomez
Intellectual 450 points
Part Number: LM25116
Other Parts Discussed in Thread: LM5116

Hi, I designed a DC/DC converter using a LM25116 and everything is fine. I measure the Frequency and it is 336Khz, Vout 5.1V @Iout 6A; however, checking the transient response, Vout is not a straight "ramp" as the DS but it more like different steps. is this might be bad soldering? Because I solder everything by myself

 PS: The first measurement was ok as the picture below, but when I change the Iout from 6A to 4A, the Vout started behaving like the above picture in all Iout (1A, 2A,4A, 6A). 


  • 0
    TI__Guru 58761 points

    Hello Andres

    1. Check if your load is DC. VOUT cannot increase smoothly if the load current is not DC. 

    2. Measure/verify V(SS-AGND) at the device pins. V(SS-AGND) is the VOUT regulation target during soft-start. If V(SS-AGND) is ugly, VOUT becomes ugly. 

    3. Measure/verify V(COMP-AGND) at the device pins. V(COMP-AGND) is the inductor current regulation target. If V(COMP-AGND) is ugly, VOUT becomes ugly. 

    4. Monitor inductor current. If any malfunction, the inductor current will be distorted and the VOUT will become ugly as a result.

    -Eric Lee

  • 0 in reply to EricLee
    Intellectual 450 points

    The load is DC (I'm using a DC Electronic series)

    V(SS-AGND) Look smooth.


    Tomorrow I check V(COMP-AGND) and Inductor current

  • 0 in reply to Andres Gomez
    TI__Guru 58761 points

    Hello Andres

    Thanks for the waveforms. 

    -Eric Lee

  • 0 in reply to EricLee
    Intellectual 450 points

    Hi Eric, sorry for the delay. So here are the waveforms

    This is without Load (Vout 5V1 and Iout 0A)

    And this is with a resistance load Iout 6A

    So I see some noise in the Vcomp and Vss with maximum load. However, the output waveform during the start-up is almost the same than it is load

  • +1 in reply to Andres Gomez
    TI__Guru 58761 points

    Hello Andres

    Thanks for the waveforms.

    I believe the issue happens because the soft-start time is relatively shorter than the minimum duty cycle which LM5116 can provide at 336kHz. 

    Ideally, the minimum duty cycle should be zero in order to provide a smooth start-up waveform, but it is not.

    All buck converters have this issue, but you can find the issue more easily when the switching frequency is high, the soft-start time is short, COUT is small or the load is light.  

    - Eric Lee

  • 0 in reply to EricLee
    Intellectual 450 points

    Hi Eric. So checking what you said, I noticed a mistake on my circuit. I put in the Css 1.8nF capacitor instead of 18nF, giving the former a soft-start of 210uS.

    I changed the capacitor to 20nF since I don't have right now 18nF ceramic capacitor (I need to wait few days to get it from DIgikey) and the output is smoothly. Additionally I need to change Chf (loop compensation) because I made the same mistake as in Css and I put a 1.8nF capacitor instead of 18nF (in this time I replace it for a 10nF capacitor).

    Now my question is, the soft-start is fine but there is some noise in the SS and Comp pin

    Without Load

    Full load (Iout 6A)


    why does it show a lot of noise in Vcomp and Vss? I might made a mistake because I measure the signal with GND (Vout) instead of AGND.

  • +1 in reply to Andres Gomez
    TI__Guru 58761 points

    Hello Andres

    I think you are talking about switching noise. You will be able to see  better waveforms if you measure COMP-AGND and SS-AGND at the device pin using a probe with short ground lead 

    ...

    -Eric Lee