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LM26001: Ouput voltage oscillating behaviour upon touching (LM26001 related) components surrounding the Buck Converter IC

Part Number: LM26001

Dear all,

I've designed an embedded system which roughly consist of an microcontroller + External Flash, CAN transciever and the LM26001 buck converter from TI.

During testing of the first prototypes of the embedded system i have discovered that sometimes my microcontroller seems to crash and result in a dead microcontroller that i cannot program again or use in it any other form.

This event seems to happen when the embedded system is powered on and when I hold the PCB within my hands. Digging into the problem I've discovered that I'm causing some kind of interference of the LM26001 IC what causes a oscillation on the output of the regulator. When my fingers touch the PCB or components that belong to the LM26001 the output starts to oscillate. This oscillating produces a high enough voltage that kills my microcontroller. 

The Buck converter is configured to produces a 3.3V output and powers the microcontroller, CAN transciever logic section and Flash memory. The input voltage is 24V, during testing a linear lab power supply is used to power the device.

The following screenshot shows the oscillating behavior of the buck converter when i'm touching the PCB/components around the LM26001 with my fingers. It can be seen that a too high voltage is generated which causes my IC's to fail.

The following image shows the schematic design of the power supply section, the design is based on a TiWebbench simulation. 

The following image shows the PCB layout section of the LM26001. The components that are visible are placed on the bottom layer (L6) of the PCB. Layer 5 is a solid GND plane which covers the entire PCB.

below: PCB Layer 6

Below : Layer 5 thru 1

My question is if there is anything i could to in order to improve the immunity of the LM26001 IC in order to make it more resilient against accidental touches of fingers while operating.

My goal is to make sure the microcontroller doesn't get fried.

Whats I have already tried:

  • Reducing the feedback network (R38 + (R39 // R40)) to a series impedance of max 3k2 Ohm in order to evaluate if the high input impedance of the feedback network could be the cause.
  • Recalculated the feedback compensation network by hand with the actual parameters of the used components (ESR, Cout) -> C67 = 10nF, C68 = 16pF and R37 = 44k. This gave no measurable improvement.
  • increased the SS capacitor from 2.2nF to 100nF. ( no improvement)

Thanks for your effort, any suggestion is more than welcome.

Arjan

  • Arjan ,

    The design is marginal stable . Here are the changes you will ned to do in order to a stable closed loop for this design :

    1. Add 2 * 22uF 10V cap at the output .
    2. Reduce the Rcomp to 5k from 10K ie R37 .


    Further here is the feedback on the design :

    1. Connect a bulk capacitor at the input parallel to C59 . Use 22uF or higher bulk capacitor .
    2. Why is Bias connected to GND ?

    --_Ambreesh
  • Hi Ambreesh,

    Thanks for your response to my question. I will add the 2x 22uF Caps on the output of the regulator and change R37 to 5k.

    In response to your feedback:

    1. There is an 100uF bulk capacitor at the input of the regulator. Although this is a shared bulk capacitor with a second 5V buck regulator. In the schematic file I posted, this bulk capacitance is not visible, however it is placed in front of FB4 between the VCC24 Rail and ground.

    2. VBIAS is grounded due to the information I interpreted from the datasheet: "Connect to an external 3-V or greater supply to bypass the internal regulator for improved efficiency. If not used, VBIAS should be tied to GND." I've chosen to ground this signal because the improved efficiency is not critical in my design.

    With respect to your statement that my design is marginal stable. Could you elaborate on that? I'm trying to get the hang of closed loop designs but am still struggling to fully understand all the details with regards to control loop stability.  How did you evaluate that my design is marginal stable? I'm curious just for educational purposes.

    Arjan

  • Arjan ,

    For VBias , you could have simply connected output of this DC/DC converter to the Bias pin . That way your device would have been running little cooler .
    Here is the paper you can use as an reference for loop calculation .
    www.ti.com/.../slup340.pdf

    ---Ambreesh