1. Field of the Invention
The present invention is related to a driving circuit for light emitting diodes, dimmer and method thereof, and particularly to a driving circuit for light emitting diodes, dimmer and method thereof by adjusting a duty cycle of driving current to dim accurately.
2. Description of the Prior Art
Generally speaking, dimming methods of light emitting diodes (LEDs) backlights have two modes, a burst mode and a continuous mode, where the continuous mode is used for adjusting a peak of a driving current of the light emitting diodes. But, operation in the continuous mode may influence optical characteristics of a display, so the burst mode is the main dimming method for light emitting diodes.
The burst mode dimming method controls turning-on or turning-off of driving current of light emitting diodes according to a pulse width modulation (PWM) dimming signal. Luminance of the LED is proportional to an average value of the driving current, which is adjusted linearly by adjusting a duty cycle of the PWM dimming signal. Due to original circuit conditions and design concerns regarding electromagnetic interference, when the driving current is turned on and turned off, the driving current for driving the light emitting diodes exhibits a delay time and a rising time before stabilizing. Please refer to
An embodiment provides a driving circuit for light emitting diodes. The driving circuit for light emitting diodes includes a power supply unit, a series of light emitting diodes, a current sink, and a dimming unit. The power supply unit has an input terminal for receiving an alternating current (AC) voltage/a first direct current (DC) voltage, and an output terminal for supplying a second DC voltage and a driving current. The series of light emitting diodes includes at least one light emitting diode, wherein the series of light emitting diodes has a first terminal coupled to the output terminal of the power supply unit for receiving the second DC voltage and the driving current, and a second terminal. The current sink is coupled to the second terminal of the series of light emitting diodes, wherein the current sink has a dimming control terminal for receiving a dimming signal. And the dimming unit is used for adjusting a duty cycle of an original dimming signal to generate the dimming signal according to the driving current and an ideal current, wherein the dimming unit has a first input terminal coupled to the second terminal of the series of light emitting diodes for sensing the driving current, a second input terminal for receiving the original dimming signal, a third input terminal coupled to the current sink for sensing the ideal current, and an output terminal for outputting the dimming signal.
Another embodiment provides a driving circuit for light emitting diodes. The driving circuit for light emitting diodes includes a power supply unit, a plurality of series of light emitting diodes, a current sink, and a dimming unit. The power supply unit has an input terminal for receiving an alternating current (AC) voltage/a first direct current (DC) voltage, and an output terminal for supplying a second DC voltage and a driving current. The plurality of series of light emitting diodes, each series of light emitting diodes includes at least one light emitting diode, wherein each series of light emitting diodes has a first terminal coupled to the output terminal of the power supply unit for receiving the second DC voltage and the driving current, and a second terminal. The current sink is coupled to the second terminals of the plurality of series of light emitting diodes, wherein the current sink has a dimming control terminal for receiving a dimming signal. And the dimming unit is used for adjusting a duty cycle of an original dimming signal to generate the dimming signal according to the driving current and an ideal current, wherein the dimming unit has a first input terminal coupled to the second terminal of the series of light emitting diodes for sensing the driving current, a second input terminal for receiving the original dimming signal, a third input terminal coupled to the current sink for sensing the ideal current, and an output terminal for outputting the dimming signal.
Another embodiment provides a dimmer. The dimmer includes a first sensing circuit, a second sensing circuit, an adder, a dimming compensator, and a PWM dimming signal generator. The first sensing circuit is used for generating a practical average voltage according to a driving current. The second sensing circuit is used for generating an ideal average voltage according to an ideal current and an original dimming signal. The adder is coupled to the first sensing circuit and the second sensing circuit for generating a difference between the ideal average voltage and the practical average voltage. The dimming compensator is coupled to the adder for generating a direct current (DC) compensation value according to the difference. The pulse width modulation (PWM) dimming signal generator is coupled to the dimming compensator and the current sink for generating a dimming signal according to the DC compensation value.
Another embodiment provides a method of dimming. The method includes generating a practical average voltage according to a driving current; generating an ideal average voltage according to an ideal current and an original dimming signal; generating a difference between the ideal average voltage and the practical average voltage; generating a DC compensation value according to the difference; generating a dimming signal according to the DC compensation value; and adjusting a duty cycle of the driving current according to the dimming signal; wherein the driving current is used for driving a series of light emitting diodes.
A driving circuit for light emitting diodes, dimmer and method thereof provided by the present invention adjust the duty cycle of the original dimming signal to generate the dimming signal according to the driving current of the series of light emitting diodes and the ideal current. Then, the switch of the current sink can adjust the duty cycle of the driving current according to the dimming signal. Therefore, the driving circuit for light emitting diodes, dimmer and method thereof provided by the present invention can control accurately luminance of at least one series of light emitting diodes by adjusting the duty cycle of the driving current to improve a display quality.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
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The dimming unit 208 includes a first sensing circuit 2082, a second sensing circuit 2084, an adder 2086, a dimming compensator 2088, and a PWM dimming signal generator 2090. The first sensing circuit 2082 includes a first resistor 20822 and a first average circuit 20824, where the first average circuit 20824 is coupled to the first resistor 20822. After the first sensing circuit 2082 senses the driving current ILED through the first input terminal of the dimming unit 208, the first resistor 20822 generates a practical voltage VLED (pulse width modulation voltage) according to the driving current ILED, and the first average circuit 20824 generates a practical average voltage
The adder 2086 is coupled to the first sensing circuit 2082 and the second sensing circuit 2084 for generating a difference diff between the ideal average voltage
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Step 70: Start.
Step 72: The first sensing circuit 2082 generates the practical average voltage
Step 74: The second sensing circuit 2084 generates the ideal average voltage
Step 76: The adder 2086 generates the difference diff between the ideal average voltage
Step 78: The dimming compensator 2088 generates the DC compensation value CPWM according to the difference diff.
Step 80: The PWM dimming signal generator 2090 generates the dimming signal DLED according to the DC compensation value CPWM.
Step 82: The switch 2064 of the current sink 206 adjusts the duty cycle of the driving current ILED according to the dimming signal DLED.
Step 84: End.
In Step 72, the driving current ILED is used for driving the series of light emitting diodes 204. In Step 72, the first resistor 20822 generates the practical voltage VLED (pulse width modulation voltage) according to the driving current ILED, and the first average circuit 20824 generates the practical average voltage
To sum up, the driving circuit for light emitting diodes, dimmer and method thereof described above adjust the duty cycle of the original dimming signal to generate the dimming signal according to the driving current of the light emitting diodes and the ideal current. Then, the switch of the current sink can adjust the duty cycle of the driving current according to the dimming signal. Therefore, the driving circuit for light emitting diodes, dimmer and method thereof described above can accurately control luminance of the light emitting diodes by adjusting the duty cycle of the driving current to improve on the disadvantages of not controlling luminance of the light emitting diodes accurately and poorer display quality in the prior art.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.
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