1.  Field of the Invention
The present invention generally relates to pulse width modulation signal generating circuits and, more particularly, to a pulse width modulation signal generating circuit which generates a pulse width modulation signal supplied to a light-emitting device such as a light-emitting diode to cause the light-emitting device to emit a light. 
2.  Description of the Related Art
FIG.  1A is a block diagram showing a structure of a portable telephone A2 provided with a luminescence circuit C4 of a light-emitting diode 102, which uses a conventional pulse width modulation (PWM) signal generating circuit 200.  The portable telephone A2 comprises, in addition to the luminescent circuit C4, a central processing unit (CPU) 150, a read only memory (ROM) 151 which stores a control program of the luminescent circuit C4, a random access memory (RAM) 152 which is used as a work memory when executing each program, a transmission and reception part 153 and an operation part 154 including a liquid crystal panel, a ten-key, a microphone and a speaker. 
The CPU 150 controls the light-emitting diode 102, which is an incoming call lamp, provided in the luminescence circuit C4 to emit a wavy light at a time of reception of a telephone call signal from a remote terminal.  Here, the emission of a wavy light refers to a light-emitting method in which an intensity of light is increased and decreased gradually in a wavelike manner. 
Specifically, the CPU 150 outputs a 4-bit digital signal increasing and decreasing with a predetermined cycle to the luminescence circuit C4.  A PWM signal generating circuit 200, which constitutes the luminescence circuit C4, applies a PWM signal determined according to a value of the digital signal output from the CPU 150 to a gate of an N-channel type MOSFET 101.  When the PWM signal supplied is active, i.e., at a high level in the luminescence circuit C4, the MOSFET 101 turns on so as to cause the light-emitting diode 102, to which a power supply voltage Vcc is supplied, to emit a light. 
A digital-to-analog converter (DAC) 201 provided in the PWM signal generating circuit 200 is driven by a voltage of 3V, a voltage of 2V being input as a top voltage VRT and a voltage of 1V being input as a bottom voltage VRB.  The DAC 201 outputs an analog signal Va (1V-2V) corresponding to the value (0000-1111) of the 4-bit digital signal supplied from the CPU 150 of the portable telephone, as shown in FIG.  1B. A triangular-wave generator 202 outputs a triangular wave TW of a predetermined frequency.  As shown in FIG.  1C, a comparator 203 outputs the PWM signal of a high level or a low level according to the sign (positive or negative) of the difference that is obtained by subtracting the analog signal Va output from the DAC 201 from the triangular-wave signal TW output from the triangular-wave generator 202. 
It should be noted that the luminescence circuit C4 mentioned above does not relate to an invention which has become publicly known. 
In the luminescence circuit C4 having the above-mentioned structure, in order to cause the light-emitting diode 102 to emit a wavy light, that is, in order to gradually turn on and turn off the light-emitting diode 102 within a period T, the CPU 150 is required to execute a control program to gradually increase the value of the 4-bit digital signal within one half (2/T) of the period T and then gradually decrease the digital signal within another half (2/T) of the period T. In this case, the CPU 150 is required to execute the above-mentioned control program parallel to other processes such as an incoming call reception processes (a display control of a liquid crystal panel constituting the operation part 154), thereby increasing a load to the CPU 150.  For this reason, in order to achieve a smooth incoming call reception process, a CPU is required which can operate at a higher speed but more expensive than a CPU of a portable telephone provided with a luminescence circuit which does not cause a light-emitting diode to emit the above mentioned wavy light. 
In the PWM signal generating circuit 200 used in the above-mentioned luminescence circuit C4, even when there is no arrival of an incoming call and there is no need to cause the light-emitting diode 102 to emit a light, an electric power of the voltage of 3V is consumed unless a power supply is turned due to an energy saving function, etc.  Moreover, since there is a part which processes an analog signal in the circuit, a fluctuation tends to be generated in the output PWM signal.