1. Technical Field
The present invention relates to a semiconductor circuit and a semiconductor apparatus used for supplying a predetermined current to a load.
2. Description of Related Art
In recent years, there are increased demands for low power consumption of a liquid crystal display, and thus an LED (Light Emitting Diode) has been used in a backlight instead of a CCFL (Cold Cathode Fluorescent Lamp). Moreover, in order to suppress average power consumption, so-called PWM (Pulse Width Modulation) control has been extensively applied to change a duty ratio of on time to off time by turning on and off a current flowing through the LED in response to the brightness of a picture to be displayed or the environment lightness, and to adjust the luminance of the backlight. In the PWM control, it is necessary to provide a constant current circuit with good controllability.
FIG. 1 illustrates an example of a conventional generally used constant current circuit 102 that supplies a predetermined current to an LED array 100.
The LED array 100 includes a plurality of LEDs 1201 to 120n serially connected to one another. The constant current circuit 102 includes a switch 104, an operational amplifier (OP AMP) 106, an N channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 108, and a resistor 110.
The LED array 100, the MOSFET 108, and the resistor 110 are serially connected to one another. Specifically, one end, on a cathode terminal side, of the LED array 100 is connected to the drain terminal of the MOSFET 108, and the other end, on an anode terminal side, of the LED array 100 is connected to a power source VLED. The source terminal of the MOSFET 108 is connected to one end of the resistor 110, and the other end of the resistor 110 is connected to the ground.
The non-inverting input terminal of the operational amplifier 106 is connected to the switch 104. The switch 104 is switched to one of a first state of supplying a reference voltage VR to the non-inverting input terminal of the operational amplifier 106, and a second state of connecting the non-inverting input terminal of the operational amplifier 106 to the ground, in response to a PWM signal input from outside.
The inverting input terminal of the operational amplifier 106 is connected to a node N between the source terminal of the MOSFET 108 and the resistor 110, and the output terminal of the operational amplifier 106 is connected to the gate terminal of the MOSFET 108. In addition, the circuit includes a connection terminal LGO for connecting the output terminal of the operational amplifier 106 to the gate terminal of the MOSFET 108, and a connection terminal LCM for connecting the inverting input terminal of the operational amplifier 106 to the node N.
In the constant current circuit 102, the MOSFET 108 is turned on, so that a predetermined current (a driving current ILED) flows through the LEDs 1201 to 120n, which are connected to the drain terminal of the MOSFET 108, and the resistor 110 connected to the source terminal thereof, thereby turning on the LEDs 1201 to 120n.
FIG. 2A to FIG. 2E illustrate an example of voltage and current waveforms of each terminal of the constant current circuit 102 shown in FIG. 1. Hereinafter, the following conditions are assumed.                Reference voltage VR: 0.4 V        Driving current ILED when LEDs 1201 to 120n are turned on (driven): 20 mA        Gate-source voltage when MOSFET 108 is turned on to allow a driving current ILED to flow through LED array 100 and resistor 110: 2.1 V        Resistance value of resistor 110: 2Ω        
In an interval in which the PWM signal is low (L) (refer to FIG. 2A), the switch 104 enters the second state and the non-inverting input terminal of the operational amplifier 106 is connected to the ground 0V (refer to FIG. 2B). Furthermore, the inverting input terminal of the operational amplifier 106 is connected to the node N. Consequently, an input voltage of the inverting input terminal of the operational amplifier 106 and an output voltage (refer to a voltage waveform of the LGO of FIG. 2C) VO of the operational amplifier 106 are approximately 0 V (refer to FIG. 2C and FIG. 2D). Since the MOSFET 108 is turned off and the current flowing through the LED array 100 and the resistor 110 is approximately 0 mA (refer to FIG. 2E), each of the LEDs 1201 to 120n of the LED array 100 is turned off.
In an interval in which the PWM signal is high (H) (refer to FIG. 2A), the switch 104 enters the first state and the reference voltage VR (0.4 V) is supplied to the non-inverting input terminal of the operational amplifier 106 (refer to FIG. 2B). The operational amplifier 106 increases the output voltage VO to 2.5 V such that the voltage of the inverting input terminal is 0.4 V (refer to FIG. 2C). A voltage for turning on the MOSFET 108 is applied to the gate terminal from the output terminal of the operational amplifier 106, so that a current of 20 mA flows through the LED array 100 and the resistor 110 (refer to FIG. 2E), a voltage input to the inverting input terminal is 0.4 V (refer to FIG. 2D), and the LEDs 1201 to 120n are turned on.
In addition, as a technology related to the constant current circuit, there has been known an LED driving device including a constant current driving circuit provided with a driving transistor, a plurality of current control transistors, a current control circuit, and a driving transistor control circuit (for example, refer to Japanese Patent Application Laid-Open No. 2010-135379). The driving transistor has a drain terminal connected to a cathode terminal side end of an LED array. Each of the current control transistors has a gate terminal for receiving one of a plurality of first current control signals, a source terminal for receiving a ground voltage, and a drain terminal connected to a source terminal of the driving transistor. The current control circuit switches the plurality of first current control signals, switches the number of current control transistors in an on state, and controls the amount of a current flowing through the LED array. The driving transistor control circuit uses a voltage of the drain terminal of the current control transistor as a feedback signal, amplifies a difference voltage between a voltage value of the feedback signal and a reference voltage such that a change in the voltage value of the feedback signal is suppressed, and outputs the amplified voltage as a gate voltage of a gate terminal of the driving transistor, thereby controlling a current driving ability of the driving transistor to be changed in response to a current amount controlled by the current control circuit.
Furthermore, there has also been known an LED driving circuit including an oscillator, an up/down counter, a digital-to-analog converter, and a transistor (for example, refer to Japanese Patent Application Laid-Open No. 2009-135138). The oscillator generates a clock with a desired frequency. The up/down counter receives the clock from the oscillator, performs a count-up operation in response to the clock when turning on an LED driving current, and performs a count-down operation in response to the clock when turning off the LED driving current. The digital-to-analog converter converts a digital count value output from the up/down counter into an analog signal. The transistor is serially connected to an LED with respect to a direct current power source, and operates in response to an output signal of the digital-to-analog converter.