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LM5149: Internal LDO Issue

Francesco Rizzotto
Prodigy 85 points
Part Number: LM5149


Tool/software:

Hi,

We have developed two boards using the LM5149.

The first one was a development board designed to test the microcontroller (uC). It performs well, successfully regulating 12V (100W) with an input voltage range of 22-36V.

However, upon integrating this circuit into the complete board, we encountered some issues.

The uC is no longer switching and is unable to maintain the 12V output. Additionally, we observe that VCC (pin 9) is stable at 1.25V, while CBOOT (pin 15) remains stable at 0.8V.

We are unable to determine the cause of these problems.

Attached is the schematic for this section.

Thank you for your assistance.

Best regards,

Francesco

.LM5149-Issue.pdf

  • 0
    TI__Guru* 75782 points

    Hi Francesco,

    Go ahead and complete the quickstart calcualtor for this design, as it will provide key information on stability. No need for those large 50V electrolytics on the 12V output (better to use a 100uF/16V polymer and some 22uF/25V ceramics.

    In terms of VCC being low (possibly current limited), can you check the current into the VIN pin (unfortunately, it looks like you haven't got a series resistor to measure its voltage drop). Also, no local cap on VIN. Another option is to disconnect the AEF supply and see if that allows VCC to come up.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 85 points

    Hi Tim,

    Thank you for your response.

    We have completed the calculator sheet for this design, and as I mentioned earlier, the first prototype is functioning well without any issues.

    We removed the AEF resistor, which successfully allowed us to achieve 5V on VCC. However, we encountered an additional problem.

    When we supplied the circuit with 36V input and a 3-ohm load, we noticed that VCC only reached 150mV and the output voltage is 0V.

    We are struggling to understand why the microcontroller is unable to regulate this voltage.

    Thank you for your assistance.

    Best regards,

    Francesco

  • 0 in reply to Francesco Rizzotto
    TI__Guru* 75782 points

    Can you check the loading on VCC, either by measuring current into the VIN pin or the VCC itself? Are you running in PFM at light load where VCC is okay and then transitioning to CCM when the gate drive loss will increase based on the full Fsw?

    PS: It's just a "PWM controller", not a microcontroller.

  • 0 in reply to Timothy Hegarty
    Prodigy 85 points

    Hi Tim,

    Thank you for your response and for the clarification.

    We have replaced the controller, which resolved the initial issue. However, we have encountered another problem.

    The DC-DC converter operates effectively up to approximately 6A output current (Iout). Beyond this point, as we try to increase the power absorbed by the resistive load (by decreasing the load resistance), the output voltage begins to drop.

    While measuring the voltage across the shunt resistor (4mΩ) used for current feedback at Iout = 6A and Vout < 12V, we observed a voltage difference of at least 60mV. This indicates that the converter is entering current limit protection mode.

    I have attached an image illustrating this behavior:

    • CH1: COMP
    • CH2: Iout
    • CH3: Vout
    • CH4: ISNS+

    Given that the DC-DC converter is designed to supply a maximum output current of 10A, it remains unclear why it triggers current limiting at only 6A.

    Thank you for your support

    Francesco

  • +1 in reply to Francesco Rizzotto
    TI__Guru* 75782 points

    Try adding an RC filter on the current sense, as you may be picking up some noise.