Since there is no point in opening a new thread I will just write an update in this one.
So after burning 3 x Teensy 4.0 I would really like to know what am I doing wrong. Yesterday the 3rd Teensy 4.0 just got the same problem described in the first post. It is a short between the 3V3 and GND rails. When I power up the Teensy with 5V on the VIN pin, the multimeter shows only ~3.0V on the VIN pin. After a few seconds the Teensy starts to “hiss” and the voltage goes up to ~4.2V on the VIN pin that is connected directly to the 5V input.
I get the 5V input from an 11.1V battery pack (3 x 3.7V 18650 cells in series) that goes through a LDO (AMS1117-5.0). Yesterday when the Teensy burned out the voltage in the powerpack was only ~6.5V. Since my 2nd Teensy got fried I made it so that the Teensy does not power any other peripherals via the 3V3 pin (I thought that the first one burned because of the current draw of the connected peripherals). Now the Teensy was powered from the 5V LDO, the peripherals were connected to another 3.3V LDO, and the Teensy only got input or send output on the sensors etc (not powering anything). I know now that the current draw was not the problem because the Teensy burned out exactly at startup all 3 times and everything else works fine + even if I connected the peripherals, the measured current draw does not go over the 250mA max stated for the Teensy 4.0 board.
Things I observed:
1. The current draw is not the problem (the Teensy does not power anything, they have separate power sources).
2. The boards did not show signs of burning out (overheating, “hissing” etc.). They would just not turn on and then the 3V3 short to ground would be present.
3. The LDO seems to give constant 5V (maybe max 5.03V)
4. Now (after burn out) the MKL02Z32VFG4 chip gets hot if I let the board powered for ~10 sec.
5. Now, when powering the board (the burned out ones) with 5V on the VIN pin, the multimeter shows ~3.0V for about 4 seconds and ~4.2V after on the VIN pin (the voltage changes when the board starts “hissing”).
6. I think it is a power source problem but cannot find the problem.
7. The problem (not working and "hissing") persists even if I power the burned boards from the 3V3 pin.
I attached a schematic and the PCB layout I am using. (The top copper layer is GND and the bottom copper layer is 3V3.)
Could anyone point me to what I am doing wrong? I want to upgrade to the Teensy 4.1 (need a few more input pins) but I really do not want to burn any more boards. I have another device that works just fine but the Teensy 4.0 there is powered through the 3V3 pin using a 3.7V LiPo source through a 3.3V LOD (SP6205EM5-L-3.3/TR). You can see both the boards in the link below.
Here is a link where you can see more about my project.
SCHEMATIC
LAYOUT
PCB TOP
BOTTOM
Could I replace something on the burned boards so that they work again?
I know a lot of you are now busy with the Teensy 4.1 launch but I could really use your help here. I can provide any other information you would need regarding my setup.
Thank you very much.