1. Field of the Invention
The present invention relates to a light emission control technique and, more particularly, to a light emission control circuit controlling light emission of a plurality of light-emitting elements.
2. Description of the Related Art
Battery-driven portable equipment such as a portable telephone and a personal data assistant (PDA) use light-emitting diodes (LED) for a variety of purposes. For example, LEDs are used to provide backlight for a liquid crystal display (LCD) or an electronic flash light for a charge-coupled device (CCD) camera. LEDs emitting different colors are operated to blink for illumination. Patent Document No. 1 describes the use of blue, green and red LEDs and mixing of colors emitted by these diodes to produce colors other than blue, green and red for illumination. Light emission of an LED is controlled according to the pulse width modulation (PWM) method. Three LEDs are connected to three PWM circuits via three output transistors, respectively.
The drain of the output transistor is connected to the cathode of the LED, the source of the output transistor to a reference voltage, the gate of the transistor to the PWM circuit. When a pulse signal output from the PWM circuit is at a high (H) level so that the output transistor is turned on, the LED emits light. When the duration of H level of the pulse signal is extended so that the duty ratio of the pulse signal is accordingly large, the duration of on period of the output transistor is accordingly extended. As a result of this, the quantity of light emitted by the LED is increased. In order for the three LEDs to provide illumination with a color other than blue, green or red, it is ensured that the quantity of light emitted by the LED differs from color to color by controlling the duty ratio of the pulse signal output from the three PWM circuits accordingly.