Teensy 4.0 design and usage notes - TeensyUser/doc GitHub Wiki

Quoted from PJRC Forum by Paul Stoffregen

Teensy 4.0 is meant to be powered from VUSB or VIN, but not directly from the 3.3V pin.

3.3V can be used without VIN, but only in certain ways. Here's 2 things to try:

1: Apply 3.3V power first to VBAT (a coin cell works great), so the RTC & power management hardware is powered first. Then apply 3.3V to the 3.3V power pin.

2: Use a physical switch or power supply with very fast startup. If your power supply as a "soft start" circuit or a large capacitor, the 3.3V power can rise up too slowly. If the rest of the chip sees power before the RTC & power management, the chip can get "stuck" and not boot up properly.

Again, the normal intended way to power Teensy 4.0 is from the VIN or VUSB pins. When done that way, the chip's power management controls the 3.3V regulator enable pin. That way guarantees proper startup.