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LM5117 WEBENCH design not working at all

idelcastillo
Prodigy 20 points
Other Parts Discussed in Thread: LM5117

Hi,

I have created a design for the LM5117 using WEBENCH online application.
VinMin: 10V VinMax: 50V Vout: 9V Iout: 3A

I mounted the current circuit and it appear to works fine. Without any load connected, the voltage at the output is 8.99V.
All the components used for the mounting are the recommended by the WEBENCH simulator, exept some resistors, that are the same value, power handling and tolerance, but from another manufacturer.

The problem appears when I connect a load to the output.
In this case, when I connect a load of 5 Ohm at the output(9V/5Ohm=1.8A) and selecting an input voltage approximately below 25V, the buck appear to works, checking that the output voltage drops from 8.99V to 8.65V.
If I increase the input voltage, after exceeding aproximately 25V, the output voltage drops drastically to 1.35V, and the buck stops its regulation. At this point, the input current consumption is near zero.
Once I decrease the input voltage at about 24V, the circuit starts to regulate the voltage again, watching at its output 8.65V.

I have made the test with several loads (10 Ohm, 20 Ohm, etc...) and the behaviour is the same.

Can anyone know wich could be the reason of this behaviour?.
Maybe the main problem could be the compensating network, that is working partially good at the first.

Regards,
Ignacio

  • 0
    TI__Intellectual 2930 points

    Hi Ignacio,

    Thank you for you interest in TI power solutions, I'm sorry you are having these issues with your design. I'd be glad to take a look and see why this could be happening if you could provide me with more information. If you could give me your WEBENCH design and any schematic and layout files you have it would be helpful. You can either post things to the forum or if you'd like to take this offline you can email me at Tommy.Jewell@ti.com.

    Regards,
    Tommy Jewell
    WEBENCH Apps, TI
  • 0 in reply to Tommy Jewell - WEBENCH Design Center
    Prodigy 20 points

    Hi Tommy,

    7506.DCDC_LM5117_components.pdf

    5700.LM5117_sch.pdf

    I attach in this post the WEBENCH design and the schematic. I have also made the calculation manually using the application design example aviable on the datasheet, where there are the expresions for calculate the value of the important components. I have checked that all the values calculated by WEBENCH are right, but I have obtained different components value for the compensation network. I think that is because the WEBENCH selects the fcross at 1/12 the switching frequency, and the design example uses 1/10 of the switching frecuency for a first approximation.
    I have mounted the two compensation networks, obtaining the same behaviour.

    From the WEBENCH calculator:
    Ccom1= 3.3nF
    Rcomp= 27.4K
    Ccomp2= 68pF

    From the manual calcs based on datasheet example:
    Ccom1= 9nF (mounted 10nF because I haven´t got this exact value)
    Rcomp= 33.3K (mounted 33K)
    Ccomp2= 72pF(mounted 68pF)

    Any ideas about what could be happening?...

    Thanks a lot in advance,

    Regards,
    Ignacio

  • 0 in reply to idelcastillo
    TI__Intellectual 2930 points

    Hi Ignacio,

    I saw this post and also received your email. I will take a look at what's attached and try to get back to you promptly.

    Regards,

    Tommy