This is a generic TTL display driver, and each display has a slightly different backlight configuration. For that reason, you may need to make some adjustments to your board.
To make sure you don't accidentally damage your backlight, we make the default backlight current 25mA. Since the backlight driver is a constant-current boost, it will adjust the voltage up to 25V until it gets 25mA of draw.
If you are using a 4.3" diagonal screen, chances are its a 25mA backlight, so you should keep both jumpers clear
If you are using a 5.0" diagonal screen, chances are its a 50mA backlight. You can run it at 25mA but for the best look, close the +25mA jumper
If you are using a 7.0" diagonal screen, chances are its a 125mA backlight - the ones we sell in the Adafruit shop are 125mA. If not using an Adafruit screen please check, as we have also seen some backlights that are 50mA! You can run it at 25mA but it will be very dim. You should close the +100mA jumper
If you want to adjust the backlight brightness, you can feed a PWM signal (1KHz or greater) into the Backlight pin, 3-5V logic level. Or you can just connect it to ground to turn off the backlight driver. This will greatly reduce the power usage