A LED backlight driving circuit is disclosed. A screw expander and a screw compressor are set up within a common casing, and the exhaust side of a rotating shaft of the screw expander is connected to the discharge side of a rotating shaft of the screw compressor. The weighting coefficients are adjusted based on an error signal produced by comparing the combined output signal with a reference signal. If the rotational time for a particular command is long relative to its seek length, the seek operation is performed in the conservative mode with a very high probability of success within the available time.