| 摘要 |
PROBLEM TO BE SOLVED: To provide an illumination LED element driving circuit for a display device in a portable apparatus, which has a small size and little variation of luminance on every product, while being capable of easily driving an LED element having high Vf with small power consumption and suppressing the change in luminance even if the voltage of a battery drops. SOLUTION: A booster circuit by n/m voltage rectifier switching using first and second capacitors is used, and a constant current circuit having a MOS transistor at an output stage is used as the load of the booster circuit by disposing it in series to the LED element, to drive the LED element with a constant current. Thereby, electromagnetic noise is not generated, and even if a voltage ripples are produced in the output of the booster circuit by boosting the n/m voltage by switching, the ripples are received by the constant current circuit by adopting a high switching frequency, and the terminal voltage of the LED element can be practically kept constant in accordance with its diode characteristics. Since the ripples are absorbed on the constant current side, the LED element is practically driven with a constant current and a constant voltage to suppress variation in luminance. |
