FIG. 1 illustrates a general LED driver as a drive device for a known light-emitting element. This LED driver includes a current detecting resistor 2, a voltage detecting unit 3, a comparator 4, a D/A converter 5, and a variable voltage source 6 to drive an LED 1 for light emission.
In the LED driver, the D/A converter 5 receives digital data that indicates a value corresponding to a value of current set to flow through the LED 1. The D/A converter 5 converts the digital data into an analog voltage. This analog voltage is used as a comparison voltage in the comparator 4. In the LED 1, an anode is coupled to one end of the current detecting resistor 2, and a cathode is grounded. The other end of the current detecting resistor 2 is coupled to a positive voltage output of the variable voltage source 6. A negative voltage output of the variable voltage source 6 is grounded. The voltage detecting unit 3 detects a voltage across both terminals of the current detecting resistor 2. The comparator 4 includes one input terminal that receives a detection voltage from the voltage detecting unit 3. The other input terminal of the comparator 4 receives the above-described comparison voltage from the D/A converter 5. The comparator 4 supplies a difference voltage between the detection voltage from the voltage detecting unit 3 and the comparison voltage to the variable voltage source 6. The variable voltage source 6 includes, for example, a switching power supply. The variable voltage source 6 generates a driving voltage, and applies this driving voltage to a series circuit of the current detecting resistor 2 and the LED 1. This allows a current flow through the current detecting resistor 2 and the LED 1. Thus, the LED 1 emits light. The variable voltage source 6 varies the driving voltage corresponding to the difference voltage from the comparator 4.
In the LED driver, a voltage is generated corresponding to a current flowing through the LED 1 across both terminals of the current detecting resistor 2. This voltage is detected as the detection voltage by the voltage detecting unit 3. When the detection voltage is higher than the comparison voltage, the variable voltage source 6 decreases the driving voltage corresponding to the difference voltage. On the other hand, when the detection voltage is lower than the comparison voltage, the variable voltage source 6 increases the driving voltage corresponding to the difference voltage. This operation is continuously performed to control the current flowing through the LED 1 so as to obtain a constant value of current corresponding to the digital data.
The LED driver with this configuration is, for example, described in Patent Literature 1.
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