Hello there,
I am in the finishing stages of a board design where I am using an FT232 from FTDI to provide a serial interface to the outside world for communication.
The board has a DC barrel jack to plug in a 5V center-positive wall wart to power the board, but kind of on a lark I decided to add a jumper to allow the use of the buss power from the USB jack when there is no wall wart plugged in.
The +5V from USB and the center pin of the barrel jack goes to a 3-pin jumper connector, where you can select the 5V from either (but not both!) to go to an ADP1706 3.3V LDO regulator, which provides all of the power to the rest of the board and any offboard experiments I might design.
This is all fine and good, but I am wondering if I should provide overcurrent limiting with a PTC from the USB, or will the USB hub limit the current itself? The regulator is a 1A device, and has sufficient thermal relief via copper pours to handle the full current, but the USB port certainly can't provide that, even though my wall wart can. If my board/experiements draw more than 500mA, will the USB port clamp it to 500mA, or will I damage my laptop/computer?
I am aware that devices are required to start with a 100mA limit, the FTDI chip allows you configure it's EEPROM to request the upgrade to 500mA. Also I have the "sleep mode" pin hooked up to the microcontroller, so if it is being powered from USB it can go to sleep when the USB bus does, but that is neither here nor there I suppose.
Really I am curious as to whether I should current limit the USB buss power on my board, or am I just duplicating protection that already exists in the computer?
EDIT: Adding the schematic.
I am in the finishing stages of a board design where I am using an FT232 from FTDI to provide a serial interface to the outside world for communication.
The board has a DC barrel jack to plug in a 5V center-positive wall wart to power the board, but kind of on a lark I decided to add a jumper to allow the use of the buss power from the USB jack when there is no wall wart plugged in.
The +5V from USB and the center pin of the barrel jack goes to a 3-pin jumper connector, where you can select the 5V from either (but not both!) to go to an ADP1706 3.3V LDO regulator, which provides all of the power to the rest of the board and any offboard experiments I might design.
This is all fine and good, but I am wondering if I should provide overcurrent limiting with a PTC from the USB, or will the USB hub limit the current itself? The regulator is a 1A device, and has sufficient thermal relief via copper pours to handle the full current, but the USB port certainly can't provide that, even though my wall wart can. If my board/experiements draw more than 500mA, will the USB port clamp it to 500mA, or will I damage my laptop/computer?
I am aware that devices are required to start with a 100mA limit, the FTDI chip allows you configure it's EEPROM to request the upgrade to 500mA. Also I have the "sleep mode" pin hooked up to the microcontroller, so if it is being powered from USB it can go to sleep when the USB bus does, but that is neither here nor there I suppose.
Really I am curious as to whether I should current limit the USB buss power on my board, or am I just duplicating protection that already exists in the computer?
EDIT: Adding the schematic.
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