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  • TI Thinks Resolved

LM5085-Q1: LM5085 damaged after 1 week running

Prodigy 20 points

Replies: 3

Views: 95

Part Number: LM5085-Q1

Hi.

I made a project with the LM5085 to reduce the voltage from 48V to 12V. The output has a minimum current of 10mA and maximum current of 2A.
For the LM5085, I used WEBENCH to define the components around it.
The electrical diagram of the LM5085 can be seen in the attached diagram.


I assembled 12 boards of this project and let them run. After 1 week, 10 boards stopped working.
I made measurements on the 10 boards and all of the LM5085 was damaged. With minimum load the output voltage (on the Cout) was 4V, and without load the output voltage was 48V.
On some boards, I just changed the LM5085 and it ran again.
On other boards, I changed the LM5085 and other components that were plugged in after it, as those components were also damaged.

Today, I downloaded the "LM (2) 5085_Quick_Start" and put all the information about this my project (I put the Quick_Start in annex).
But some Quick_Start results are different from the values in my project.
In addition, Quick_Start recommends a 45mOhm resistor (R4) in series with Cout and in my design it does not have this resistor.

Could anyone check if there is something wrong with my LM5085 project?
Can this lower value of L1 be damaging the LM5085?
Or missing the R4 may be damaging the LM5085?

Thank you.
Marcos Bloedow

3007.LM(2)5085_Quick_Start.xlsLM(2)5085_Quick_Start.pdf

  • Hi Marcos,

    Yes, a resistor in series with the output cap is required. This generates triangular ripple (required for the COT control architecture) that is fed to the FB pin by the feedback resistor divider. The feedforward cap across the upper feedback resistor couples more of this output ripple to FB. I recommend using the component values listed in the quickstart file and also follow the guidelines in the datasheet.

    Regards,

    Tim

  • In reply to Timothy Hegarty:

    Hi, Tim.

    Thanks for your answer. I will add the resistor in series with the output capacitor in my design.
    Was the lack of this resistor the reason for damaging the LM5085 of my boards?

    Regards,
    Marcos

  • In reply to Marcos Bloedow:

    Marcos, most likely yes. Ripple is required at the FB pin to ensure regulator stability.

    --

    Tim

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