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LM3150: Short on turn on

Part Number: LM3150
Other Parts Discussed in Thread: CSD18537NQ5A, CSD18504Q5A

Hi,

I did a webench Design for a LM3150 21-30Volt input and 19V output.

After assembly and turning on the current limit of the Power supply who feed the circuit was maxed out i limited to 0.1A ramped up to 3A just to check. No output on the LM3150 circuit..

So i removed the FET's and checked the VCC and HG/LG for signal. And  VCC was 6volt HG was switching at around 100mv and LG was switching around 5volt. I checked the Schematic and PCB no error found so far.

Use M1 Fet -> CSD18504Q5A and M2 Fet -> CSD18537NQ5A

Seems something happend to HG ?

Check the FET with multimeter diode test Unlimited and 0.5v on the other side, on both FET so seems okay.

Can you help me to understand what could be wrong ? WBDesign19.pdf

many thanks

  • Hi Michael,

    HG high-output is powered by BST capacitor. What is the BST-SW voltage across the capacitor?

    Does the HG problem appear with a different LM3150 IC?

    You can try power up to low VIN like (6-8V/0.1A) to test while minimizing risk of damage. 

    -Orlando

  • Well i am on the second PCB and second new LM3150 exact the same issue. I start a third one and will check the BST-SW voltage. Can i do the whole assembly but not install the HG/LG fets and power then up to check the pins ? How should the switching pattern be ? Last time i had the LG Fet installed and the HG not installed and it was looking ok with the switching and after i installed the HG Fet short again.

  • Hi Orlando,

    ok i did following:

    Soldered a new LM3150 in removed the HG and LG fet.

    Powersupply set to 6v and 0.1amp

    Checked the HG pad nice square wave 0v and around 6v

    same on the LG pad inverted signal

    Voltage between BST and SW 5.4volt

    Installed then the FET and short again

    Removed the HG fet checked HG and LG was looking still ok.

    removed the LG fet and Installed the HG fet. Constant voltage on SW 6volt

    but not square wave signal on the HG pin still signal on the LG

    Removed the HG fet again and HG pin only 2.45volt and the voltage between BST and SW 2.45v

    LG Voltage still 0 - 6volt square wave but only spikes.

    Seems LM3150 again fried.

    What can this be ?

  • Hi Orlando,

    i am an idiot...The FETs have Source and Drain swapped. I had a closer look on the picture of the datasheet of this fets and suddenly i couldsee it...I swapped the pins with some wire on the fet and it works now.

    but one thing i have learned how to troubleshoot LM3150 closer Slight smile

    You got a free beer from me.

    cheers

    Michael

  • Glad you got this working!

    Any other questions let us know.

    cheers

    -Orlando