In the past, there have been proposed a lighting apparatus, which comprises a light emitting diode (LED) as a light source, and transmits a signal by modulating intensity of an illuminating light. Because such an illuminating light communication device transmits the signal by modulating the illuminating light itself, a special device such as an infrared-ray communication device is not required. Then, because the light emitting diode is used as the light source for illuminating, electric power saving can be implemented. Therefore, it has been studied that the illuminating light communication device is used for the ubiquitous information system in underground malls.
FIG. 44 is a circuit diagram of the conventional illuminating light communication device. Then, a constant current circuit 52, three light emitting diodes 53, and a switching element Q1 are connected in series between both ends of a DC power supply 51. The switching element Q1 is switched on/off in accordance with high/low of an optical communication signal S1, and thereby a load current I1 flowing to the light emitting diodes 53 is modulated while keeping a constant current property (see FIGS. 45A and 45B).
In this circuit, the constant current circuit 52 is located to make the light emitting diode 53, having a small operating resistance, turn on stably, and then circuit loss of the constant current circuit 52 increases. For example, when the constant current circuit 52 comprises a constant current diode, assuming that the load current I1 is 500 [mA], the estimated circuit loss becomes about 2.3 [W]. Therefore, even as the light emitting diode needs little electricity, the benefit of the light emitting diode is diminished.
Thus, as shown in FIG. 46A, a DC-DC converter is located in substitution for the constant current circuit 52, and then it is conceivable that the DC-DC converter is controlled by PWM (pulse-width modulation) and thereby the circuit loss decreases. In this circuit, a current sensing resistor R3, three light emitting diodes 53, an inductor L1, and a switching element Q1 are connected in series between both ends of a DC power supply 51. Then, on/off operation of the switching element Q1 is controlled by a control circuit 54. Then, a smoothing capacitor C3 and a rectifier diode D2 are connected between both ends of a series circuit comprising the three light emitting diodes 53 and the inductor L1, and constitute the DC-DC converter, together with the inductor L1 and the switching element Q1. A feedback signal is inputted from a constant current feedback circuit 55 into the control circuit 54, and thereby an output current of the DC-DC converter is controlled to be kept generally constant. In addition, the optical communication signal S1 is inputted into the control circuit 54, and the switching element Q1 is switched on/off with a high frequency during a period of a high value of the optical communication signal S1 and thereby the load current I1 is modulated (see FIGS. 47A-47C).
Now, FIG. 46B shows a specific example of the constant current feedback circuit 55, and then an error amplifier A1 is configured to compare a voltage drop of a resistor R3, to which the load current I1 flows, to a reference voltage E1, and amplifies to output its partial difference into the control circuit 54. A series circuit comprises a resistor R4 and a capacitor C2, which are connected between an inverting input terminal and an output terminal of the error amplifier A1, and constitutes a phase compensation circuit to secure stability of the above-mentioned feedback system. For such a phase compensation circuit, a compensation circuit including an integral element is generally used to adjust a gain and a phase in a loop transfer function, and has been known as PI (Proportional-Integral) control or PID (Proportional-Integral-Derivative) control of a classic information theory. For example, FIG. 46C shows a circuit diagram of a mean current detecting circuit disclosed in Japanese Patent Application Laid-Open No. 2006-120910. An integral circuit 56 (comprising a resistor R5 and a capacitor C3) is connected between both ends of a current sensing resistor R3, and is configured to use the above-mentioned PI control as a means for averaging output.
In the illuminating light communication device shown in the above-mentioned FIG. 46A, the load current I1 is modulated by operating the DC-DC converter intermittently in accordance with the optical communication signal S1, and then the following two conditions are needed to reproduce the optical communication signal S1 faithfully as a current waveform. One of the conditions is that an operating frequency of the DC-DC converter is higher than a frequency of the optical communication signal (condition 1), and the other is that the load current I1 is not smoothed (condition 2).
For satisfying the condition 1, for example, if a communication speed is 9.6 [kbps], it is necessary that the operating frequency of the DC-DC converter is equal to or more than 100 [kHz] (that is, about 10 times of the communication speed), preferably equal to or more than 1 [MHz] (that is, about equal to or more than 100 times of that). However, if the operating frequency of the DC-DC converter is relatively high, loss of the switching element Q1 in the DC-DC converter increases, and thus there is a problem that measures of noise-reduction are needed.
Then, in the case of the condition 2, when a smoothing capacitor is connected in parallel to the light emitting diode 53, the load current I1 is not intermitted even if the switching element Q1 in the DC-DC converter is intermitted. Therefore, it becomes difficult to modulate the load current I1 in accordance with the optical communication signal. On the other hand, if the load current I1 is not smoothed, a ripple current occurs in the load current I1. The ripple current depends on the operating frequency component of the DC-DC converter. Thus, there is a problem that measures of noise from electric wiring are needed. It is desirable to increase the operating frequency of the DC-DC converter to inhibit the ripple of the operating frequency component while keeping a capacity of the smoothing capacitor as small as possible so as not to smooth the load current I1. However, when the operating frequency increases, there is a problem that loss of the switching element Q1 also increases.
Then, in the past, there have been also proposed an illuminating light communication device, having a circuit configuration shown in FIG. 48, and using a DC-DC converter to which a constant current feedback circuit is added. In this circuit, a DC-DC converter 62 is operated by an input from a DC power supply 61, and its output is converted to a DC voltage having a desired voltage value by a rectifier circuit 63 and a smoothing capacitor C4. Three light emitting diodes 64 and a current sensing resistor R4 are connected in series between both ends of the smoothing capacitor C4. When the load current I1 flows, the voltage drop occurred in the resistor R4 is compared to the reference voltage E1 by an error amplifier A2. Then, its partial difference is amplified and is fed back to an output controller 65 in the DC-DC converter 62 by the error amplifier A2, and then the load current I1 is controlled so as to be a constant current. In addition, a series circuit, comprising a resistor R5 and a capacitor C5, is connected between an inverting input terminal and an output terminal of the error amplifier A2, and the series circuit constitutes a phase compensation circuit, and adjusts a phase of a feedback signal while increasing the gain in a low frequency domain and inhibiting the gain in a high frequency domain.
FIG. 49A is Bode diagram showing an output property of this circuit, and then the gain (L1 in FIG. 49A) linearly decreases with increasing of the frequency. Then, the phase angle (L2 in FIG. 49A) maintains around 90 degrees to the frequency of about 10 [kHz] and phase margin is secured. Thus, stability of the feedback system is good.
In the circuit shown in FIG. 48, a reference voltage E1 or E2 is connected to a non-inverting input terminal of the error amplifier A2 through an electric switch SW1, and the electric switch SW1 is switched in accordance with the optical communication signal S1, and thereby the load current I1 can be modulated. That is, one reference voltage E1 is set so that the load current I1 becomes a current value for normal illumination, and the other reference voltage E2 is set so that the load current I1 becomes a current value smaller than the current value for normal illumination. Then, the switch SW1 is switched in accordance with the optical communication signal S1, and thereby the load current is modulated. Here, when the load current in the reference voltage E1 is 500 [mA] and the load current in the reference voltage E2 is 100 [mA] and the switch SW1 is switched in accordance with the optical communication signal of 10 [kHz], FIG. 49B shows a simulation result of the load current I1. In this simulation result, the load current I1 is averaged into a value (about 300 [mA]) intermediate between the current value (500 [mA]) in the reference voltage E1 and the current value (100 [mA]) in the reference voltage E2, and thus the load current I1 is not modulated in accordance with the optical communication signal.
As can be expected from Bode diagram in FIG. 49A, when the optical communication signal has a frequency of about 10 [kHz], the frequency domain is a domain where the gain of the error amplifier A2 can not be expected and output control can not follow. Therefore, when the load currents in the reference voltages E1, E2 are averaged, only the averaged load current can be provided. Here, if the phase compensation circuit is removed from the circuit shown in FIG. 48, the gain of the error amplifier A2 can be secured even in a high frequency domain, and it can be expected that the load current is modulated in accordance with the optical communication signal. However, the feedback system becomes unstable and thereby abnormality oscillation may occur. Therefore, there is a problem that it is difficult to modulate the load current I1 by changing a circuit constant of the phase compensation circuit added to the error amplifier A2.