Light emitting elements such as LEDs are used as, for example, multicolor display elements of a cellular phone for displaying data received, back light sources of an LCD (liquid crystal display), or flash light sources of a camera. For these purposes, red (R), green (G), and blue (B) LEDs are often used simultaneously.
To activate an LED for emission of light, a driving voltage is required that amounts to a voltage drop across the LED for emission of a required amount of light plus a control voltage for controlling the driver that controls the luminance of the LED. In a case where LEDs of a portable apparatus for example are driven by a battery voltage, the battery voltage is stepped up to a high voltage by a step-up circuit.
It is noted that different operating voltages are required for LEDs of different colors. For example, a white, a green, and a blue LED requires an operating voltage in the range from about 3.5 to 4.0 V, but a red LED requires a lower operating voltage in the range from about 2.8 to 3.0 V.
When for example a battery having rated voltage of about 3 V is used to drive multicolor LEDs, the battery voltage is stepped up to a higher voltage to drive green and blue LEDs while the battery voltage is used to directly drive red LED, since green and blue LEDs have a higher operating voltage than the battery voltage while red LED have a lower operating voltage, as disclosed in Japanese Patent Application Laid Open 2002-214677.
However, such prior art system as disclosed in the literature cited above cannot operate a red LED when the battery voltage has lowered below the operating voltage of the LED. Thus, the usable range of battery voltage is limited by the operating voltage of a red LED, which presents a problem that for this reason the battery energy cannot be fully exploited.