LTM4646 - power outputs shorted to GND net

Hello,

I made a PCB using an LTM4646 chip to generate 1.5V and 3.3V from a 12V supply.

All three of the power nets, 12V, 3.3V and 1.5V are shorted to the GND net in the finished board.The PCB manufacturer checked them out with an X-Ray to verify the issue was not with the manufacturing of the board, so I wanted to see if my design looked correct to this community.

Here is my setup in LTSPice, which is the design I used on my PCB. It simulated the correct voltages just fine.

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  • I just wanted to follow up on this.

    The reason the nets were shorted was that R3 and R6 on my PCB were milliohm values. They were defined with the lower-case 'm' in the LTSPICE demo file, and so I ordered them that way. After I realized those resistors were creating a rather effective short, I removed them from the PCB. LTSpice lets you use a lowercase 'm' for a Megohm resistor. Lesson learned.

    To avoid damaging the chip due to the 6V RUN pins, I also lowered the input voltage of the shown setup to 5.5 VDC. At 12VDC I was blowing a 2 amp fuse on my PCB. 

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  • I just wanted to follow up on this.

    The reason the nets were shorted was that R3 and R6 on my PCB were milliohm values. They were defined with the lower-case 'm' in the LTSPICE demo file, and so I ordered them that way. After I realized those resistors were creating a rather effective short, I removed them from the PCB. LTSpice lets you use a lowercase 'm' for a Megohm resistor. Lesson learned.

    To avoid damaging the chip due to the 6V RUN pins, I also lowered the input voltage of the shown setup to 5.5 VDC. At 12VDC I was blowing a 2 amp fuse on my PCB. 

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