I've recently purchased the MCP1700 LDO to power my arduino via the 3.3V pin with a battery. However, I'd like to know if I am still allowed to plug my arduino in via usb (to upload a new sketch) while not powered by battery; battery will be removed. Will it damage the MCP1700 if there is 3.3V on the output but nothing on the input? This will determine whether I solder everything together or have some sort of disconnect for the arduino.
That's not an Arduino you're showing in that image. I was already wondering which model you used that runs on USB and runs on 3.3V...
On those ESP32 based boards the USB voltage is stepped down to 3.3V by an on board regulator.
I'm not sure what will happen. Probably the regulator which supplies the highest voltage (of course nether is exactly 3.3V) ends up being the one supplying the power to the board.
The data sheet gives as absolute maximum ratings Vss-0.3 to Vin+0.3V for all input/output pins. As long as your battery is connected and supplies at least 3.1V to Vin you should be safe. If your battery is disconnected but the regulator remains connected the regulator will be killed (as in that case Vout = 3.3V and Vin = 0V).
MCP1700 is OK for an ESP8266, but not if your using WiFi, for that you need a regulator with a higher current capability.
For some years I have been powering boards direct from the USB-Serial interface when writing or debugging programs. These boards have an MCP1700 and there is mostly no external battery connected.
Never had a problem with damage to the MCP1700s, so they survive when perhaps the datasheet suggests they should not.
