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LM2840/LM2841/LM2842 Startup

Clay Weston
Prodigy 200 points
Other Parts Discussed in Thread: LM2840

Greetings,

I have essentially duplicated Figure 15 of the LM2840 Datasheet. I'm using the LM2841X. I did a careful layout heeding the guidelines. Above a certain input voltage, the output does not regulate when started using the Enable pin unless it is loaded. As I increase the input voltage, the output fails to regulate on startup even when loaded. I tried a soft-start circuit (100K ohms and 0.1uF) but it didn't seem to make any difference. Any suggestions?

Thank you,

Clay

  • 0
    TI__Genius 15810 points

    Hello Clay,

    It looks like your query fell through the cracks. I apologize for the delay in getting back to you. To start debugging the issue, would you please send us your layout and the design requirements? If you've changed any BOM component from the original schematic, it would be helpful to know those too.

    Further, please send us some scope shots of the SW, EN, BOOT voltages and the inductor current. What is the input voltage above which you cannot maintain regulation unless loaded?

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 200 points

    Akshay,

    Thank you for the response. I found the problem with my implementation. While I was very careful about the layout of current paths, I failed to properly route the ground pin of the IC. I ran a single via from pin 2 into the ground plane. All of the other components were on a substantial layer 1 ground. I only had a single via from this layer 1 ground near the output capacitor. 

    I don't normally make this kind of rookie mistake, which is why I wasn't looking for it immediately. Anyway, I used a piece of bus wire to connect pin 2 of the IC to the layer 1 ground plane and the problem completely went away. I guess that is a great illustration of the importance of careful layout.

    Regards,

    Clay

  • 0 in reply to Clay Weston
    TI__Genius 15810 points

    Hello Clay,

    I'm glad you were able to solve the problem. Layout is indeed very important for the proper working of the switching regulators. please feel free to write us if you need assistance with anything else. 

    Regards,

    Akshay