1. A back light device comprising:a first light emitting block comprising a first plurality of light emitting modules connected in series to each other;
a power supply module configured to apply a driving voltage to the first light emitting block;
a first power switch connected to the first light emitting block and configured to control the driving voltage on or off; and
a control module configured to turn on or off the first power switch such that a constant current is supplied to the first light emitting block and to control an on/off of the first plurality of light emitting modules based on a dimming signal,
wherein the control module is configured to, in response to a ripple value of the constant current being different from a certain ripple value, change 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.