Electrical questions

Denis Bélières

Denis Bélières

Active member
Just to be sure ... I provide a common external +3.3V supply to my teensy 4.0 board and to others components of my system because total current is to high for the small on board regulator U4=TLV7533. So probably it is better to open the small strap connecting VUSB and +5V VIN, thus disconnecting this poor U4. I know that TLV7533 can withstand an applied voltage on its output while input is open. But what happens if USB is connected to a computer while external power is off, or switched off while USB running : there may be some voltage applied on USB input pin without power on processor chip, is it dangerous for it ?
 
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Probably a requirement somewhere in the USB specification that locally powered devices play nice if their power fails.
 
Kinda following your question.

1. Do not power the Teensy 4.x by applying 3v3 power to the 3v3 pin. While it will work, there are some internal bootup proceedures that will be skipped. Power with 3v7 to 5v5 to the Vin pin.
2. If powering via the external Vin AND you connect the USB. You could damage the USB device in the event of power fluctuations. Best to cut th VIN from USB trace on the back of the Teensy
1. if powered from the Vin, 3v3 can be used to power devices total <= 100 mA. I never use 3v3 pin, i have a separate external 3v3 rail that all devices are powered with--just tie all grounds.
 
KrisKasprzak said:
Kinda following your question.

1. Do not power the Teensy 4.x by applying 3v3 power to the 3v3 pin. While it will work, there are some internal bootup proceedures that will be skipped. Power with 3v7 to 5v5 to the Vin pin.
2. If powering via the external Vin AND you connect the USB. You could damage the USB device in the event of power fluctuations. Best to cut th VIN from USB trace on the back of the Teensy
1. if powered from the Vin, 3v3 can be used to power devices total <= 100 mA. I never use 3v3 pin, i have a separate external 3v3 rail that all devices are powered with--just tie all grounds.
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Yes, thanks. Things are clear now with replies I had on this other thread. I dropped my idea of 3V3. I will use the 3V3 as output power to some peripherals, just low frequency CMOS circuits. The greatest current was for an OLED display, but it can be powered on +5 will still accepting 3V logic level.
 
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