A back light device includes a first light emitting block including a plurality of light emitting modules connected in series to each other; a power supply module that applies a driving voltage to the first light emitting block; a first power switch connected to the first light emitting block and controls the driving voltage on or off; and a control module that turns on or off the first power switch such that a constant current is supplied to the first light emitting block and controls an on/off of the plurality of light emitting modules based on a dimming signal. The control module, in response to a ripple value of the constant current being different from a certain ripple value, changes at least one of a turn-on period and a turn-off period of the first power switch to calibrate the ripple value of the constant current to the certain ripple value.