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LM5118: Issue

John Smith
Prodigy 40 points
Part Number: LM5118

Hi TI Experts, I am testing a board with two Buck-Boost modules, they are identical circuits using LM5118MHX. They are made to accept +7V to +25V and deliver +24V (1.5A), both. I used Webench in order to generate the schematics and I adjust it with my references, but my issue is they are not correctly working. 

On several cards we can see that the last Buck-Boost is working great but the first one isn't and it's not all the time like this, sometimes both don't work. I copy/paste the schematics and the only difference is I add a Diode (D101) on the first module to prevent current inversion, apart from that components are the same. Can you help me please, we are going crazy here.

First schematic and circuit :

Second schematic and circuit :

Thanks

John

  • 0
    TI__Guru 53781 points

    Hi John,

    thank you for using the E2E forum.

    I could not find the D101 in your schematic.

    Can you also add more details on the failure behavior, e.g.:

    - what means: does not work

    - add some scope plots of 

      - gate signals

      - SS pin, COMP pin 

      - output voltage

    - how did you route the CSG connection to the sense resistor

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 40 points

    Hello Stefan,
    thanks for your response.

    First the D101 is on another sheet, here you can see it : 

    It's complicated for me to describe the behavior, because we soldered like 4 cards and we have differents things on each, one isn't working at all, I mean there is no output voltage. But one common behavior is when we supply the Buck-Boost with +17V (it's the average voltage that we will have in use) the output is below +24V, around +21V and when we modify the IN voltage it changes. But the second Buck-Boost on the same board works perfectly.

    I didn't have the time today to monitor the signals with the scope. I'll do it tomorrow.

    Thanks to you, I look at the CSG connection and unfortunately I tied it to the ground plane directly so I made a modification to add a track directly to the Rsns.

    I also saw a problem on the ground plane, the shorter path between the Input connector and the LM5118's ground goes under a mosfet and I thought it might be causing disturbance so I tried to gently remove it, but nothing changed. You can see the path below, I made it in black :

    Best regards

    John

  • 0 in reply to John Smith
    TI__Guru 53781 points

    Hi John,

    thank you for the update - yes, the layout can have a big influence here.

    Will wait for the scope plots.

    Best regards,

     Stefan