Patent Application
20110018906
The present invention relates to an electroluminescent (EL) backlight driving circuit, and more particularly to a driving circuit for driving an EL panel to emit light for use as a backlight source of a display.
Most of various currently available high-tech products, such as computer display screens, tablet television sets, mobile phones, cameras and digital photo frames, have adopted a liquid crystal display (LCD) as a human-machine interface. All the LCD displays require backlight to show the screen.
Currently, among others, cold cathode fluorescent lamp (CCFL) and light emitting diode (LED) are the most common backlight sources. A new trend of the backlight source is an electroluminescent (EL) panel, which is an area light source providing more uniform light than the linear light source of the CCFL and the point light source of the LED. The light from the EL panel is soft and not glaring. What is most important is the EL panel does not produce heat, has low power consumption, and is ultra-thin and elastic to enable wider applications than the CCFL and LED. Further, the EL panel is environment-friendly and requires lower manufacturing cost than the LED. Therefore, the EL panel has very good competing potential in the market.
Generally speaking, it is not difficult to drive the EL panel to emit light. The EL panel can be driven to emit light so long as a working voltage ranged between 50 and 250 Vrms, a working frequency about 1 KHz, and an output current about 0.14 mA/cm2 are output to the EL panel. However, to use the EL panel as the backlight source of a display device, it is not only necessary to drive the EL panel to emit light, but also give the EL panel with a protection function to meet related safety codes. The EL panel must also be compatible with the software or firmware currently used by the LCD display, and have the functions of controlling the on/off and adjusting the brightness thereof. Therefore, it is the most important issue to design a driving circuit for stably controlling and driving the EL panel to emit light, so that the EL panel can be more popularly applied in the display devices to serve as a backlight source.
A primary object of the present invention is to provide an EL backlight driving circuit that is compatible with the control software and firmware as well as the power supply, on/off and brightness control signals of the current display for driving an EL panel to emit light, so that the EL panel can be popularly applied in the display devices to serve as a backlight source thereof
To achieve the above and other objects, the EL backlight driving circuit according to the present invention includes a power on/off unit electrically connected to a DC voltage of a display device and controlled by an ON signal of the display to output the DC voltage as a working voltage; a boost unit electrically connected to the working voltage of the power on/off unit for boosting and converting the working voltage into a modulation working voltage; a resonance unit electrically connected to the modulation working voltage of the boost unit and controlled by the ON signal of the display for oscillating the modulation working voltage to generate and output an AC working voltage to drive an electroluminescent (EL) panel to emit light; and a voltage feedback unit for comparing an output feedback voltage of the resonance unit with a brightness signal of the display and then comparing the amplified output feedback voltage with a feedback control voltage of the boost unit to generate a voltage regulation signal, which is fed back to the boost unit for regulating the size of the modulation working voltage and accordingly the size of the AC working voltage to thereby control the brightness of the EL panel. Therefore, the EL panel can maintain stable brightness and have prolonged service life.
To achieve the above and other objects, the EL backlight driving circuit according to the present invention further includes a protection unit, which includes an over-voltage protection circuit electrically connected to the AC working voltage of the resonance unit for detecting whether the AC working voltage exceeds a preset high-threshold voltage; and the protection unit outputs an OFF signal to the resonance unit when the AC working voltage is detected as being higher than the preset high-threshold voltage, so as to stop the resonance unit from oscillating.
To achieve the above and other objects, the protection unit in the EL backlight driving circuit of the present invention further includes an open lamp protection circuit for detecting whether the AC working voltage is lower than a preset low-threshold voltage, and the protection unit outputs the OFF signal to the resonance unit when the AC working voltage is detected as being lower than the low-threshold voltage, so as to stop the resonance unit from oscillating. And, the protection unit further includes a time-delay circuit for delaying the detection by the open lamp protection circuit, so that the OFF signal is not generated when an under-voltage condition occurs at booting and normal booting can be ensured.
The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein
Please refer to
The EL backlight driving circuit 3 of the present invention includes a power on/off unit 10, a boost unit 20, a resonance unit 30, a voltage feedback unit 40, and a protection unit 50. The on/off signal (ENA) generated by the display 2 includes an ON signal and an OFF signal. Preferably, the ON signal is a high-level voltage (high) and the OFF signal is a low-level voltage (low).
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Since the energy-storing voltage size of the Inductor L1 is determined by the pulse on width (time), the output voltage is also a modulation working voltage (Vin) boosted from the working voltage (Vcc). Meanwhile, since the pulse on width is also controlled by a voltage regulation signal (Vinv) of the voltage feedback unit 40, the size of the modulation voltage value can also be controlled by the voltage regulation signal (Vinv).
The output modulation working voltage (Vin) is further divided by two resistors R13 and R14 to generate a feedback control voltage (Vcon) to the voltage feedback unit 40 for voltage comparison, from which the voltage regulation signal (Vinv) is obtained.
Please refer to
The circuit operation of the resonance unit 30 is now described with reference to
Further, through the on/off of the transistor Q6, an output feedback voltage (Vout) can be generated to the voltage feedback unit 40 for comparison and to serve as a basis of regulating the size of the AC working voltage.
Please refer to
The EL panel generally has the drawback of optical attenuation. That is, the brightness of the EL panel will reduce when the EL panel has been used over a long time. However, the voltage feedback unit 40 in the EL backlight driving circuit according to the present invention can change the output voltage regulation signal (Vinv) according to actual changes in the brightness of the EL panel, so as to regulate the size of the modulation working voltage (Vin), allowing the EL panel to maintain stable brightness and have prolonged service life.
Please refer to
The over-voltage protection circuit is shown at the upper part of
The open lamp protection circuit is shown at the lower part of
However, the condition of under-voltage would also occur during booting. Therefore, the protection unit 50 uses a set of transistors Q10 and Q11 and a capacitor C16 and a resistor R42 to form a time-delay circuit for delaying the detection by the open lamp protection circuit. When booting, the transistors Q11 and Q10 will be temporarily turned on to bypass the cutoff of the transistor Q8, so that the transistor Q9 is cut off, and the point A is connected to the on/off signal (ENA) to keep the ON signal until the booting is completed. Thus, the condition of generating the OFF signal due to the under-voltage at booting can be avoided to ensure normal booting.
