A light source driving device includes a transformer, a rectifying diode, a filter, and an open loop preventing circuit. The transformer has a primary winding part including first and second external input terminals configured to receive external power from a ballast stabilizer and a coil having an impedance level set to allow the ballast stabilizer to output a normal amount of power, and a secondary winding part electromagnetically coupled to the primary winding part to transform received external power. The rectifying diode rectifies power from the secondary winding part, and the filter filters the rectified power from the rectifying diode. The open loop preventing circuit provides a closed loop to the filter such that power stored in the filter is applied to the output terminal when the rectifying diode is turned off.