Load driving device, and lighting apparatus and liquid crystal display device using the same
A load driving device disclosed in the specification includes a power supply circuit for supplying to a load an output voltage converted from an input voltage, a detection voltage generation circuit for generating a detection voltage which varies depending on a magnitude of a voltage drop which across the load, and a control circuit for controlling the power supply circuit so that it performs output feedback control of the output voltage, on the basis of the detection voltage.
1. A load driving device comprising:
a power supply circuit for supplying to a load an output voltage converted from an input voltage;
a detection voltage generation circuit that is connected between the load and a ground, for generating a detection voltage that varies depending on a magnitude of a voltage drop across the load; and
a control circuit for controlling the power supply circuit so that output feedback control of the output voltage is performed based on the detection voltage, wherein
the detection voltage generation circuit includes a constant-current source connected in series with the load and flowing constant current that can be adjusted, the detection voltage generation circuit outputting a node voltage at a connection node between the load and the constant-current source as the detection voltage,
the control circuit generates a control signal of the power supply circuit so that the detection voltage of a connecting point of the load and the constant-current source becomes a constant voltage, and
the constant-current source includes:
a transistor having a first terminal connected in series with the load;
a resistor connected between a second terminal of the transistor and a ground; and
an error amplifier for generating a control signal to a control terminal of the transistor, and
wherein the power supply circuit includes:
an isolation transformer, the input voltage being applied to a primary side of the isolation transformer, and the output voltage being supplied from a secondary side of the isolation transformer to the load, and
the power supply circuit feeds back, via a photocoupler to the primary side, a signal corresponding to the detection voltage, in order to control the output voltage according to an output signal of the photocoupler such that the node voltage remains constant.
2. The load driving device according to claim 1 , wherein the power supply circuit further includes:
a diode provided on the secondary side of the isolation transformer, the diode being connected in a current supply path from the isolation transformer to the load, with an anode of the diode on the isolation transformer side and a cathode of the diode on the load side.
3. The load driving device according to claim 2 , wherein the power supply circuit further includes:
a capacitor of which a first terminal is connected to the cathode of the diode, and of which a second terminal is connected to a ground.
4. The load driving device according to claim 1 , wherein a value of the constant current supplied from the constant-current source is adjustable.
5. The load driving device according to claim 1 , wherein the load is an LED.
6. The load driving device according to claim 1 , wherein the isolation transformer is driven by a forward method.