Driver for driving LED backlight source, LED backlight source and LCD device

A driver for driving an LED backlight source is disclosed. The driver includes a boost converter for boosting inputted DC voltage and outputting boosted DC voltage to an LED series circuit having LEDs connected and a resistor in series, a feedback circuit for feeding back voltage across the resistor to a backlight driving circuit, and a control switch controlled by output of the backlight driving circuit depending on feedback voltage of the voltage across the resistor for keeping current flowing through the LED series circuit in a constant current. The driver drives the LED series circuit with a constant current, thereby prolonging lifetime of each LED of the LED series circuit. Furthermore, since a time period which the current flowing through the LED series circuit approaches to constant value is shorter, the driver has advantages over lower power consumption, faster response time and better operating efficiency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) field, more particularly, to a driver for rapidly adjusting current flowing through a light emitting diode (LED) series circuit of a LED backlight source to a constant current, an related LED backlight source using the driver and an LCD device using the LED backlight source.

2. Description of the Prior Art

As the progress of display technology, backlighting used in the LCD device has been developed. A cold cathode fluorescent lamp (CCFL) for use in a backlight source of a traditional LCD device presents disadvantages over poor color recovery, low lighting efficiency, high discharging voltage, poor discharging characteristic under low temperature and long time period to be heated to emit light with steady grey level. Nowadays, many LED backlight sources are developed.

The LED backlight source are disposed near the LCD panel in the LCD device, so that the LCD panel shows images by using light emitted by the LED backlight source. The LED backlight source includes an LED series circuit having multiple LEDs connected in serial. A specific driver is designed for supplying driving voltage to the LED series circuit. For prolonging lifetime of each LED, driving the LED series circuit with a constant current is required. However, the conventional driver needs a longer time period which the current flowing through the LED series circuit approaches to constant value, and causes higher power consumption, slower response time and worse operating efficiency.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention is to provide a driver for driving LED backlight source, an LED backlight source and an LCD device drives the LED series circuit with a constant current to solve the problem as presented in the prior art.

In one aspect of the present invention, a driver for driving a light emitting diode backlight source comprises a boost converter, for boosting inputted direct current (DC) voltage and outputting boosted DC voltage to a light emitting diode (LED) series circuit having a plurality of LEDs connected and a resistor in series, a feedback circuit, for feeding back voltage across the resistor to a backlight driving circuit, and a control switch controlled by output of the backlight driving circuit depending on feedback voltage of the voltage across the resistor, for keeping current flowing through the LED series circuit in a constant current.

In another aspect of the present invention, an LED backlight source for use in an LCD device, comprises a boost converter, for boosting inputted direct current (DC) voltage and outputting boosted DC voltage, an light emitting diode (LED) series circuit having a plurality of LEDs connected and a resistor connected in series, a feedback circuit, for feeding back voltage across the resistor to a backlight driving circuit, and a control switch controlled by output of the backlight driving circuit depending on feedback voltage of the voltage across the resistor, for keeping current flowing through the LED series circuit in a constant current.

In another aspect of the present invention, an LCD device comprising an LED backlight source and an LCD panel disposed near the LED backlight source to display image by using light emitted from the LED backlight source is provided. The LED backlight source comprises a boost converter, for boosting inputted direct current (DC) voltage and outputting boosted DC voltage, an light emitting diode (LED) series circuit having a plurality of LEDs connected and a resistor connected in series, a feedback circuit, for feeding back voltage across the resistor to a backlight driving circuit, and a control switch controlled by output of the backlight driving circuit depending on feedback voltage of the voltage across the resistor, for keeping current flowing through the LED series circuit in a constant current.

Furthermore, the backlight driving circuit comprise a control module, for adjusting a duty cycle of a driving signal fed to the boost converter based on voltage applied on a negative end of the LED series circuit, to control magnitude of the boosted DC voltage; and an operational amplifier having a negative end coupled to the feedback voltage of the voltage across the resistor through the feedback circuit and a positive end coupled to a reference voltage, for outputting different voltage level signals to control the control switch according to a comparison between the reference voltage and the feedback voltage of the voltage across the resistor through the feedback circuit.

Furthermore, the boosting converter comprises a diode comprising a positive and a negative end coupled to the LED series circuit, an inductor, coupled between the inputted DC voltage and the negative of the diode, a metal oxide semiconductor (MOS) transistor comprising a drain coupled the positive end of the diode, a source coupled to ground, and a gate coupled to the control module, and a capacitor coupled between the negative end of the diode and the ground.

Furthermore, the feedback circuit comprises a differential amplifier for feeding back the voltage across the resistor to the negative end of the operational amplifier.

Furthermore, the control switch is a bipolar junction transistor comprising a collector coupled to the resistor, an emitter coupled to the ground, and a base coupled to an output of the operational amplifier of the backlight driving circuit.

In contrast to the prior art, the driver for driving LED backlight source, the LED backlight source and the LCD device drives the LED series circuit with a constant current, thereby prolonging lifetime of each LED of the LED series circuit. Furthermore, since a time period which the current flowing through the LED series circuit approaches to constant value is shorter, the driver for driving the LED backlight source has advantages over lower power consumption, faster response time and better operating efficiency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, a light emitting diode (LED) backlight source for use in a liquid crystal display (LCD) device is shown according to a preferred embodiment of the present invention.

As shown inFIG. 1, the LED backlight source for use in the LCD device comprises a boost converter110, an LED series circuit120, a feedback circuit130, a control switch140, and a backlight driving circuit150.

The boost converter110is used for boosting inputted direct current (DC) voltage and outputting boosted DC voltage.

The LED series circuit120having a plurality of LEDs121connected and a resistor122in series is used to emit light to a LCD panel. The LED series circuit120receives the boosted DC voltage from the boost converter110.

A number N (N is a positive integer) of the LEDs121in the LED series circuit120is determined by a formula as follow:
N×Vd≦Vs,

Where Vd indicates driving voltage of each LED121, and Vs indicates an output voltage of the boost converter110.

For example, upon the condition that Vd=6.5V, and Vs=48V, N≦7.

The feedback circuit130is used for feeding back voltage across the resistor122to the backlight driving circuit150.

The control switch140, controlled by output of the backlight driving circuit150depending on feedback voltage of the voltage across the resistor122, is used for keeping current flowing through the LED series circuit120in a constant current. Since the control switch140present advantages over faster switching, shorter time period of adjusting current flowing through the LED series circuit in a constant current, and lower power consumption, the driver for use in the LED backlight source may drive the LED series circuit faster and upgrade efficiency accordingly.

The backlight driving circuit150can be integrated within a backlight driving IC chip. The backlight driving circuit150is used for adjusting a duty cycle of a driving signal fed to the boost converter110, and for controlling magnitude of the boosted DC voltage. The control switch140is controlled by output of the backlight driving circuit150depending on feedback voltage of the voltage across the resistor122, so that the control switch140is used for keeping current flowing through the LED series circuit120in a constant current.

Referring toFIG. 2, a LED backlight source comprises a boost converter, a feedback circuit, a control switch, and a backlight driving circuit according to a preferred embodiment of the present invention.

As shown inFIG. 2, the boost converter110comprises an inductor111, a metal oxide semiconductor (MOS) transistor112, a diode113, and a capacitor114.

The inductor111is coupled between the inputted DC voltage Vin and a positive end of the diode112. A negative end of the diode112is coupled a positive end of the LED series circuit120. A drain D, a source S, and a gate G of the MOS transistor113are coupled to a positive end of the diode112, ground, and control module151of the backlight driving circuit150, respectively. The capacitor114is coupled between a negative end of the diode112and the ground.

A control module151of the backlight driving module150can adjust a duty cycle of a driving signal fed to the gate G of the MOS transistor113based on voltage applied on a negative end of the LED series circuit120, so as to control magnitude of the boosted DC voltage. For example, when the driver used in the LED backlight source begins to work, a magnitude of the boosted DC voltage outputted from the boost converter110is insufficient to light up the LED series circuit120, causing that no current flows through the LED series circuit120, voltage drop across each LED121is smaller, and voltage applied on the negative end of the LED series circuit120is greater. Upon detecting the greater voltage applied on the negative end of the LED series circuit120, the control module151outputs the driving signal with a greater duty cycle to the gate G of the MOS transistor113to raise the magnitude of the boosted DC voltage from the boost converter110until the LED series circuit120lights up normally. At this moment, the control module151outputs the driving signal with a constant duty cycle to steadily supply power to the LED series circuit120.

In another embodiment, the control switch140of the LED backlight source can be a Bipolar Junction Transistor (BJT). When the BJT turns on, a voltage drop between a base and a emitter is approximately 0.7V, and current flowing through the collector c is hundred times as much as current flowing through the base b (the hundred times indicates an amplifying number of the BJT). For example, when current flowing through the base b is one mA, the current flowing through the collector c is one hundred mA. Moreover, the BJT presents advantages over easy control, short response time period and low power consumption.

In this embodiment, the BJT is connected to the LED series circuit120in serial. A collector c of the BJT is coupled to the resistor122, an emitter e of the BJT is grounded, and a base b of the BJT is coupled to an output of the operational amplifier152of the backlight driving circuit150. In this way, the current flowing through the collector c of the BJT equals to that flowing through the LED series circuit120.

In this embodiment, the feedback circuit130of the LED backlight source comprises a differential amplifier131which is used for measuring a precise voltage across the resistor122. Additionally, the differential amplifier131may be replaced by other electrical elements capable of measuring precise voltage across the resistor122, such as a subtractor.

A positive end of the differential amplifier131is coupled between the negative end of the multiple LEDs121and the resistor122, and a negative end of the differential amplifier131is coupled between the collector c of the BJT and the resistor122, an output end of the differential amplifier131is coupled to a negative end of the operational amplifier152of the backlight driving circuit150.

The voltage across the resistor122is feedback to the negative end of the operational amplifier152through the differential amplifier131.

In this embodiment, the backlight driving circuit150comprises a control module151and the operational amplifier152.

The control module151is coupled between the gate G of MOS transistor113of the boost converter110and the negative end of the LED series circuit120.

In addition to the above function, the control module151protects the LED series circuit120from malfunction upon the LED series circuit120lights up and emits light. For example, when the LED series circuit120is open-circuited or short-circuited, the control module151detects abnormal voltage applied on the negative end of the LED series circuit120and thus stops outputting driving signal to the gate G of the MOS transistor113so as to shut down the driver.

The negative end of the operational amplifier152is coupled to the output of the differential amplifier131of the feedback circuit130, the positive end of the operational amplifier152is coupled to a reference voltage Vc generated by the backlight driving circuit150, and the output of the operational amplifier152is coupled to the base b of the BJT. The operational amplifier152having a negative end coupled to the feedback voltage of the voltage across the resistor122through the output of the differential amplifier131and a positive end coupled to a reference voltage Vc. The operational amplifier152compares the feedback voltage of the voltage across the resistor122with the reference voltage Vc.

When the feedback voltage of the voltage across the resistor122is less than the reference voltage Vc, the operational amplifier152outputs high voltage level to raise the current flowing through the base b of the BJT. Since the current flowing through the base b is proportional to that flowing through the collector c, the current flowing through the LED series circuit120increases. When the feedback voltage of the voltage across the resistor122is greater than the reference voltage Vc, the operational amplifier152outputs low voltage level to reduce the current flowing through the base b of the BJT. Since the current flowing through the base b is proportional to that flowing through the collector c, the current flowing through the LED series circuit120decreases. Upon the condition that the feedback voltage of the voltage across the resistor122equals to the reference voltage Vc, the current flowing through the base b of the BJT becomes a constant current, as well as the current flowing through the LED series circuit120becomes a constant current.

In this embodiment, the control switch comprises but is not limited to the BJT. In another embodiment, the control switch may be other switches with short response time period and low power consumption.

In addition, the above LED backlight source can be applied in an LCD device, as depicted inFIG. 3.

FIG. 3shows an LCD device according to a preferred embodiment of the present invention.

As shown inFIG. 3, the LCD device comprises an LED backlight source310and an LCD panel320disposed near the LED backlight source310. Since the LCD panel320does not emit light, the LCD panel320displays image by using light emitted from the LED backlight source310. Preferably, the LED backlight source310can be the LED backlight source as shown inFIG. 2.

According to the present invention, the driver for driving LED backlight source, the LED backlight source and the LCD device drives the LED series circuit with a constant current, thereby prolonging lifetime of each LED of the LED series circuit. Furthermore, since a time period which the current flowing through the LED series circuit approaches to constant value is shorter, the driver for driving the LED backlight source has advantages over lower power consumption, faster response time and better operating efficiency.