power to 3.3v on Teensy 3*
- Thread starter manitou
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Constantin
Well-known member
Short Answer: AFAIK, based on my personal experience with Teensy 3.0's and my own Teensy clones, Yes.
Long Answer: You will need to cut the VIN-VUSB connection if you want to use VUSB as a source of power for an external voltage regulator. That is what I did to minimize the heat load on the MK20 chip. On that board I used a simple 3-pin header to determine the source of power between an external source or VUSB. The center pin went to the LDO voltage regulator, the two outside pins went to either VUSB or the external power source.
On one of my Teensy clones, the MCP1825 LDO regulator on my board is fed via a dual-Schottky diode (PMEG4005) - with one leg going to VUSB, the other going to an on-board power supply. That way, the power supply will always auto-switch between sources. Takes up less space than the pin header approach but is also not as certain re: where the power is coming from (i.e. sometimes it's a good thing to be able to isolate power sources definitively).
If you roll your own board (using one of Paul's bootloader chips), remember to hook up the MK20 voltage regulator output (3.3Vout) to the 3.3V bus even if you don't up using the internal MK20 voltage regulator. Somewhat counter-intuitively, the USB connection for the MK20 series depends on 3.3V on that pin to be powered. If you power any of the 3.3V power pins on the Teensy 3.x that happens automagically.
Rolling your own board has its own hazards, as I recently discovered once again when I finally had the time to populate a power board (been busy at work) and then borked $40+ worth of parts because the MK20 MCU was offset 0.25mm on one axis of the chip.
Moral of the story: Look twice before reflowing a expensive board. Visors with magnifying lenses are cheap insurance - if you are smart/humble enough to use them (argh!).
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