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LM25011: Doesn't work properly

Przemek Szymanski
Prodigy 30 points
Part Number: LM25011

Hello

I have problem with LM25011. I have 5 boards with this converter and all boards have the same issue. 2,4V on output (when input voltage is 8V). No PG signal. RT pin works as described in datasheet. D2 is B260A-13-F. VFB 1,1V.

One board have 3,1V output and 0,025V VFB

My schematic:

Layout:

Wave on SS capacitor:

Wave on SW pin:

Waveform on BST capacitor:

  • 0
    TI__Prodigy 700 points
    Hi Przemek.

    the ceramic capacitor C1 is supposed to be the high frequency decoupling cap going from Vin pin to GND. In your schematic it is connected to the RT pin. Another thing is in the layout that the DAP (Die Attach Pad) same as the EP (Exposed Pad) is not connected to GND and isolated floating that will cause thermal issues.

    Best Regards,
    Robert Loke
  • 0 in reply to Robert Loke
    Prodigy 30 points

    Hi Robert!

    Oh, I made mistake with this decoupling cap. I changed this but this not fixed my problem. For DAP it may cause only thermal issues or something else? In this circuit I will have small currents so it shouldn't be a problem. If this pad has impact for functional mode I can try to connect it to GND.

    Another thing which confusing me is signal on SS pin is like saw wave but from datasheet (Figure 17 startup sequence)  I expected continuous high value.

    Best regards
    Przemek

  • 0 in reply to Przemek Szymanski
    TI__Prodigy 700 points
    Hi Przmek,

    first, yes please connect the DAP to GND for proper operation.
    Second, the CSG pin has to be connected to GND as well.

    Please make sure you are using the LM25011 IC without the letter A at the end.
    In case you are using the LM25011A version we would need to add components for external ripple generation.

    Best Regards,
    Robert Loke
  • 0 in reply to Robert Loke
    Prodigy 30 points

    Hi Robert,

    Missing GND on CSG pin cause problem. I don't know why I missed this connections :(

    I noticed that 2 ICs were damaged and I had to replace it. 3 ICs were ok and connecting CSG to GND fixes it.

    1st broken IC had 80 ohms between feedback pin and GND

    2nd broken IC had strange behavior on output (should be 5V):