JP2010067751A - Light-emitting element driving circuit, and cellphone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light-emitting element driving circuit to efficiently operate a boosting circuit, and to provide a cellphone including the same. <P>SOLUTION: The light-emitting element driving circuit 10 includes a luminance setting part 60 which sets luminance for video images based on a video image information signal from the outside to output luminance setting information for video images and output luminance change information for the video images, a light-emitting element driving part 40 which drives a light-emitting element by a current having a current value corresponding to the luminance setting information from the luminance setting part 60, a detection comparing part 50 which compares the terminal voltage of the light-emitting element with a prescribed voltage, a boost determining part 30 which determines whether the terminal voltage of the light-emitting element is boosted or not based on the luminance change information for images, and a boosting circuit part 20 which boosts the terminal voltage of the light-emitting element when determined to boost it, and does not boost the terminal voltage of the light-emitting element when determined not to boost it. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a light emitting element driving circuit and a mobile phone, and more particularly to a light emitting element driving circuit for changing the luminance of a light emitting element and a mobile phone including the light emitting element driving circuit.

  In recent years, video such as one-segment broadcasting is often viewed on the main liquid crystal of a mobile phone, and a light-emitting element driving circuit that changes the luminance of the backlight of the liquid crystal is sometimes mounted on the mobile phone. In the light emitting element driving circuit, in order to display a clear video with a good contrast, the luminance of the backlight of the liquid crystal is changed depending on the content of the video signal. Specifically, dimming is performed so that bright images are brighter by increasing the luminance of the backlight, and dark images are darker by decreasing the luminance of the backlight. Patent Document 1 describes a drive circuit that adjusts the luminance of a light-emitting element.

JP 2005-310854 A

  However, if the brightness increases during bright images, the forward voltage (Vf) of the backlight increases as the current value flowing through the backlight increases in order to increase the brightness of the backlight. Therefore, the cathode terminal of the LED constituting the backlight The terminal voltage decreases. If the cathode terminal voltage becomes lower than the set voltage, the booster circuit is operated to boost the anode terminal voltage. After the voltage is boosted, for example, when the video is dark, the brightness decreases and the backlight brightness In order to reduce the current, the value of the current flowing through the backlight is reduced, and the forward voltage (Vf) of the backlight may be reduced. In such a case, there is a problem that even if the terminal voltage of the cathode terminal of the LED increases, if the booster circuit is in an operating state, useless current consumption occurs.

  An object of the present invention is to provide a light emitting element driving circuit for efficiently operating a booster circuit and a mobile phone including the light emitting element driving circuit.

  A light emitting element driving circuit according to the present invention sets a luminance for video based on a video information signal from a video processing circuit unit, outputs luminance setting information for video, and outputs luminance change information for video A light emitting element driving unit that drives the light emitting element with a current having a current value corresponding to the luminance setting information for video from the luminance setting unit, and a terminal voltage of one terminal of the light emitting element is detected and compared with a predetermined voltage A detection comparison unit that performs the above, a boost determination unit that determines whether to boost the terminal voltage of the other terminal of the light emitting element based on the luminance setting information for video and the output of the detection comparison unit, and the boost determination unit A boosting circuit unit that boosts the terminal voltage of the other side terminal of the light emitting element when it is determined, and does not boost the terminal voltage of the other side terminal of the light emitting element when it is determined not to boost by the boosting determination unit; With It is characterized in.

  In the light emitting element driving circuit according to the present invention, the booster circuit unit is driven by the power supply circuit and boosts the terminal voltage of the other terminal of the light emitting element, and the power supply circuit and the other terminal of the light emitting element A through connection signal line that enables connection of the power supply circuit and a boost circuit that functions when the boost determination unit determines that the boost is determined, and the power supply circuit and the other terminal of the light emitting element are not connected by the through connection signal line. It is preferable to have a switching circuit that connects the power supply circuit and the other side terminal of the light emitting element by the through connection signal line and does not function the booster circuit when it is determined that the booster does not boost.

  Further, in the light emitting element driving circuit according to the present invention, when the video luminance change information is information in which the video luminance is changed from a high state to a low state, the boost determination unit determines the terminal voltage of the other terminal of the light emitting element The switching circuit connects the power supply circuit and the other side terminal of the light emitting element by the through connection signal line and does not function the boost circuit, and then the other side of the light emission based on the output of the detection comparison unit It is preferable to determine whether or not to boost the terminal voltage of the side terminal.

  A mobile phone according to the present invention includes the light-emitting element driving circuit according to any one of the above, a light-emitting element driven by the light-emitting element driving circuit, and an image display device illuminated by the light-emitting element. To do.

  According to the light emitting element driving circuit and the mobile phone according to the present invention, the voltage is boosted or not boosted based on the luminance change information for video and the output of the detection comparison unit. Thereby, the booster circuit can be operated efficiently.

  Embodiments according to the present invention will be described below in detail with reference to the accompanying drawings. Further, in this description, specific shapes, materials, numerical values, directions, and the like are examples for facilitating understanding of the present invention, and can be appropriately changed according to the use, purpose, specification, and the like.

  FIG. 1 is a diagram showing a liquid crystal backlight luminance changing system 8 including a light emitting element driving circuit 10 according to an embodiment of the present invention. The liquid crystal backlight luminance change system 8 includes a liquid crystal unit 90, a video processing circuit unit 70, and a light emitting element driving circuit 10. The liquid crystal backlight luminance changing system 8 has a function of changing the luminance of the liquid crystal backlight 92 in accordance with the video signal. Although the liquid crystal backlight luminance change system 8 including the light emitting element driving circuit 10 is mounted on a mobile phone, it may be mounted on a terminal having a liquid crystal unit 90 other than the mobile phone. Further, although the video processing circuit unit 70 is not included in the light emitting element driving circuit 10, the video processing circuit unit 70 may be included in the light emitting element driving circuit 10.

  The liquid crystal unit 90 is an image display device incorporating a liquid crystal element. The liquid crystal unit 90 includes a backlight 92, a liquid crystal element (not shown), and a polarizing filter. The liquid crystal unit 90 performs display by blocking or transmitting light emitted from the light source of the backlight 92.

  The backlight 92 is a light emitting element that emits light when a forward voltage is applied to the anode terminal (anode) with respect to the cathode terminal (cathode). The forward voltage (Vf) of the backlight 92 is normally around 3.6 V, but the forward voltage (Vf) varies depending on process variations, current values, and the like. In addition, the luminance can be changed by changing the value of the current flowing through the backlight 92.

  The video processing circuit unit 70 has a function of inputting a signal obtained by processing a video signal such as broadcast to the liquid crystal unit 90. In addition, the video processing circuit unit 70 has a function of generating a luminance adjustment signal corresponding to the content of the video signal and outputting the signal to the luminance setting unit 60.

  The light emitting element drive circuit 10 includes a luminance setting unit 60, a light emitting element drive unit 40, a boost determination unit 30, a detection comparison unit 50, and a boost circuit unit 20. FIG. 2 is a block diagram of the brightness setting unit 60. The luminance setting unit 60 includes a luminance selection unit 601 and a luminance change analysis unit 602. The luminance selection unit 601 receives the luminance adjustment signal from the video processing circuit unit 70 as an input, selects the luminance corresponding to the luminance adjustment signal, and sets the voltage corresponding to the selected luminance to the video luminance setting. It has a function of outputting to the light emitting element driving unit 40 as information.

  The luminance change analysis unit 602 stores the luminance information selected by the luminance selection unit 601, obtains changes in the previous luminance information and the current luminance information, and sends the luminance change information for video to the boost determination unit 30 described later. Output function. Here, the luminance change information for video is a digital signal and is normally set to 0, but is set to 1 when it is detected that the luminance has changed from a high state to a low state.

  FIG. 3 is a diagram illustrating each element of the light emitting element driving unit 40. The light emitting element drive unit 40 is a drive circuit that includes a current circuit unit 42 and a current value setting circuit unit 46. The light emitting element driving unit 40 has a function of controlling the current flowing through the light emitting element to have a predetermined target value according to the luminance set by the luminance setting unit 60.

  The current circuit unit 42 is a current mirror circuit that allows a current value set by a current value setting circuit unit 46, which will be described later, to flow through the backlight 92. The current flowing through the left transistor 42a and the current flowing through the right transistor 42b have the same value. Become. One end of the current circuit unit 42 is electrically connected to the cathode terminal of the backlight 92, and the other end is electrically connected to the current value setting circuit unit 46.

  The current value setting circuit unit 46 has a function of obtaining a current value based on the voltage output by the luminance setting unit 60 and setting the current value for the current circuit unit 42. The current value setting circuit unit 46 includes a current value setting side resistor 462, a current value setting side comparator 463, a current value setting side transistor 464, a current value setting side current mirror circuit 465, a D / A circuit 466, a reference And a current source 468.

  The current value setting side resistor 462 is a circuit element that suppresses the flow of current. A voltage source that supplies a predetermined voltage is connected to one end side of the current value setting side resistor 462, and a current value setting side comparator 463 and a current value setting side transistor 464 are connected to the other end side. Here, a voltage obtained by dividing a predetermined voltage by the current value setting side resistor 462 is input to the current value setting side comparator 463 as a current value setting side reference voltage here. The current value setting side resistor 462 is provided as an external resistance element on the semiconductor substrate on which the light emitting element driving circuit 10 is provided, and the current value flowing through the current value setting side transistor 464 is changed as necessary. It is also possible to do.

  The current value setting side comparator 463 compares the current value setting side reference voltage with the voltage output by the luminance setting unit 60 and outputs the difference. The output of the current value setting side comparator 463 is input to the current value setting side transistor 464.

  Each electrode of the current value setting side transistor 464 is electrically connected to a current value setting side resistor 462, a current value setting side current mirror circuit 465, and a current value setting side comparator 463. The current value setting side transistor 464 flows a current corresponding to the output voltage of the current value setting side comparator 463. That is, a current corresponding to the video brightness setting information flows. Although the bipolar transistor is used as the current value setting side transistor 464, a MOS transistor can also be used.

  In the current value setting side current mirror circuit 465, the current flowing through the left transistor 465a and the current flowing through the right transistor 465b have the same value. In the current value setting side current mirror circuit 465, when the current value setting side transistor 464 is turned on, a current having the same value as the current flowing in the left transistor 465a also flows in the right transistor 465b.

  The reference current source 468 is a current source that allows a predetermined current value to flow. One end of the reference current source 468 is connected to a predetermined power supply voltage, and the other end is electrically connected to the D / A circuit 466 and the DC side current mirror circuit 465.

  The D / A circuit 466 is a circuit that converts a digital signal into an analog signal. The D / A circuit 466 receives the current of the current value subtracted by the current value setting side current mirror circuit 465 from the reference current value flowing by the reference current source 468 and converts the current into an analog signal, and the current circuit unit 42 receives the analog signal. Input the signal.

  FIG. 4 is a diagram illustrating elements and the like of the detection / comparison unit 50, the boost determination unit 30, and the boost circuit unit 20. The detection comparator 50 has a function of detecting the terminal voltage of the cathode terminal of the backlight 92 and comparing it with a predetermined voltage. The detection comparator 50 includes a comparator 501, a first flip-flop 503, a second flip-flop 504, a third flip-flop 505, and an AND circuit 506.

  The comparator 501 is a circuit that compares two input voltages. The reference voltage input to one input terminal of the comparator 501 is supplied by a reference power source 502 having a potential of 0.2 V, for example. The other input terminal of the comparator 501 is connected to the cathode terminal of the backlight 92. The comparator 501 compares the terminal voltage of the cathode terminal of the backlight 92 with a reference voltage. The comparator 501 outputs 1 when the terminal voltage of the cathode terminal of the backlight 92 is lower than the reference voltage, and 0 when the terminal voltage of the cathode terminal of the backlight 92 is higher than the reference voltage. Is output.

  The first flip-flop 503 operates at a predetermined frequency CLK (clock) and receives the output of the comparator 501 as an input. The output of the first flip-flop 503 is connected to the input of the second flip-flop 504 and the input of the AND circuit 506.

  The second flip-flop 504 operates at a predetermined frequency CLK (clock) and receives the output of the first flip-flop 503 as an input. The output of the second flip-flop 504 is connected to the input of the third flip-flop 505 and the input of the AND circuit 506.

  The third flip-flop 505 operates at a predetermined frequency CLK (clock) and receives the output of the second flip-flop 504 as an input. The output of the third flip-flop 505 is connected to the input of the AND circuit 506. The reset terminals of the first flip-flop 503, the second flip-flop 504, and the third flip-flop 505 are controlled by the boost determination unit 30.

  The AND circuit 506 is an AND circuit, and outputs 1 when the outputs of the first flip-flop 503, the second flip-flop 504, and the third flip-flop 505 are all 1, and outputs 0 otherwise. .

  When the output of the AND circuit 506 is 1, the boost determination unit 30 performs control to turn on the switch 201 of the boost circuit unit 20 and turn off the switch 202 of the boost circuit unit 20. The boost determination unit 30 performs control to turn off the switch 201 of the boost circuit unit 20 and turn on the switch 202 of the boost circuit unit 20 when the output of the AND circuit 506 is zero. The boost determination unit 30 is reset to reset the first flip-flop 503, the second flip-flop 504, and the third flip-flop 505 when the video luminance change information signal from the luminance setting unit 60 is 1. It has a function of sending a reset signal to the terminal.

  The step-up circuit unit 20 includes a step-up comparator 22, a step-up PWM circuit 24, a step-up transistor 25, a step-up coil 26, a step-up diode 27, a step-up capacitor 28, and a step-up power supply 29. Is configured as a booster circuit.

  The boosting comparator 22 is a circuit that compares the magnitudes of two input voltages and amplifies and outputs the difference between the input voltages. The reference voltage input to one input terminal of the boosting comparator 22 is supplied by a reference power source 21 having a potential of 4.2 v, for example. The other input terminal of the boosting comparator 22 is connected to the anode terminal of the backlight 92. The boosting comparator 22 compares the terminal voltage of the anode terminal of the backlight 92 with a reference voltage. The output of the boosting comparator 22 is input to the boosting PWM circuit 24.

  The switch 201 is attached between the boosting comparator 22 and the boosting PWM circuit 24. When the switch 201 is on, the boosting comparator 22 and the boosting PWM circuit 24 are electrically connected. When the switch 201 is off, the boosting comparator 22 and the boosting PWM circuit 24 are electrically connected. It is assumed that it is not connected to. The switch 201 is controlled by the boost determination unit 30. The switch 202 will be described later.

  The step-up PWM circuit 24 is one of modulation methods, and is a circuit that modulates by changing the duty ratio of the pulse wave. Specifically, the comparison result by the boosting comparator 22 is received as an input, and the duty ratio of the pulse wave is changed based on the comparison result. And it has the function to perform switching control of the boosting transistor 25 by a pulse wave based on the comparison result.

  The boosting transistor 25 is a MOS transistor that controls the current between the source and drain terminals on the principle that a voltage is applied to the gate terminal and a gate (gate) is provided for the flow of electrons or holes by the electric field of the channel. The boosting transistor 25 is subjected to switching control by applying a pulse wave output from the boosting PWM circuit 24 to the gate terminal. The boosting transistor 25 has a gate terminal electrically connected to the output of the boosting PWM circuit 24, a drain terminal electrically connected to the anode terminals of the boosting coil 26 and the boosting diode 27, and a source terminal grounded. ing.

  The boosting coil 26 has one end connected to the boosting power supply 29 and the other end connected to the drain terminal of the boosting transistor 25 and the anode terminal of the boosting diode 27. The boosting coil 26 is in a state where a voltage from the boosting power supply 29 is applied when the boosting transistor 25 is turned on, and accumulates energy.

  The step-up diode 27 is a circuit element having a rectifying action (an action in which current flows only in a certain direction). In the boosting diode 27, a current flows from the boosting coil 26 in which energy is stored when the boosting transistor 25 is turned off to the load via the boosting diode 27. The anode of the boosting diode 27 is electrically connected to the boosting coil 26 and the boosting transistor 25.

  The step-up capacitor 28 is a circuit element that stores and discharges electric charges (electric energy) by electrostatic capacitance. The step-up capacitor 28 has a function of accumulating charges flowing from the step-up coil 26 when the step-up transistor 25 is turned off. One end of the boosting capacitor 28 is electrically connected to the cathode terminal of the boosting diode 27 and the anode terminal of the backlight 92, and the other end is grounded.

  The switch 202 is attached on the through signal line 200 between the boosting power source 29 and the anode terminal of the backlight 92. When the switch 202 is on, the boosting power supply 29 and the anode terminal of the backlight 92 are electrically connected. When the switch 202 is off, the boosting power supply 29 and the anode terminal of the backlight 92 are connected. It shall be in a state where it is not electrically connected. The switch 202 is controlled by the boost determination unit 30.

  Next, the operation of the light emitting element driving circuit 10 having the above configuration will be described with reference to FIGS. When the terminal voltage of the cathode terminal of the backlight 92 becomes equal to or lower than the reference voltage supplied from the reference power supply 502, the output of the comparator 501 becomes 1. Then, at the next rising edge of CLK (clock), 1 which is the value of the output of the comparator 501 is input to the first flip-flop 503 and the first flip-flop 503 outputs 1. At the next rising edge of CLK (clock), 1 which is the value of the output of the first flip-flop 503 is input to the second flip-flop 504, and the second flip-flop 504 outputs 1. At the next rising edge of CLK (clock), 1 as the output value of the second flip-flop 504 is input as an input to the third flip-flop 505, and the third flip-flop 505 outputs 1. The AND circuit 506 outputs 1 when the anode terminal of the backlight 92 needs to be boosted when the outputs of the first flip-flop 503, the second flip-flop 504, and the third flip-flop 505 are all 1.

  When the output of the AND circuit 506 is 1, the boost determination unit 30 performs control to turn on the switch 201 of the boost circuit unit 20 and turn off the switch 202 of the boost circuit unit 20. At this time, the boosting comparator 22 of the boosting circuit unit 20 compares the terminal voltage of the anode terminal of the backlight 92 with the reference voltage supplied from the reference power supply 21 and outputs the difference, and the boosting PWM circuit 24. Generates a boosting PWM signal which is a boosting PWM signal corresponding to the output.

  The boosting transistor 25 is on / off controlled according to the boosting PWM signal. When the boosting transistor 25 is on, energy is stored in the boosting coil 26. After that, when the boosting transistor 25 is turned off, the energy stored in the boosting coil 26 is charged into the boosting capacitor 28 via the boosting diode 27, and consequently the anode of the backlight 92. Side terminal voltage can be boosted.

  In the above description, the booster circuit unit 20 includes the booster comparator 22, the booster PWM circuit 24, the booster transistor 25, the booster coil 26, the booster diode 27, the booster capacitor 28, and the booster power supply. However, a circuit having a boosting function, such as a charge pump circuit, may be used.

  When the luminance adjustment signal corresponding to the content of the video signal from the video processing circuit unit 70 is received and the luminance changes from a high state to a dark state, the luminance setting unit 60 receives the video luminance change information. 1 is output as the signal. At this time, since the luminance set by the luminance setting unit 60 is set to a low value, the current value of the backlight 92 set by the light emitting element driving unit 40 is a low value. As a result, the forward voltage (Vf) of the backlight 92 is lowered, so that the cathode voltage is increased and there is a case where boosting is not necessary.

  The boost determination unit 30 receives a signal for resetting the reset terminals of the first flip-flop 503, the second flip-flop 504, and the third flip-flop 505 when the video luminance change information signal from the luminance setting unit 60 is 1. Therefore, the output of the AND circuit 506 is zero.

  When the output of the AND circuit 506 is 0, the boost determination unit 30 performs control to turn off the switch 201 of the boost circuit unit 20 and turn on the switch 202 of the boost circuit unit 20. At this time, the boosting power supply 29 and the anode terminal of the backlight 92 are directly connected, but the operation of the boosting circuit is stopped because the switch 201 is turned off. As described above, when the boosting operation is not required according to the video luminance change information, the operation of the boosting circuit is stopped, thereby preventing an unnecessary increase in current consumption. Note that after reset is performed by the reset terminals of the first flip-flop 503, the second flip-flop 504, and the third flip-flop 505, a search is performed again as to whether or not boosting is performed based on the output result from the comparator 501. It becomes.

It is a figure which shows the liquid-crystal backlight luminance change system containing the light emitting element drive circuit of one Embodiment of this invention. It is a block diagram of the brightness | luminance setting part of the light emitting element drive circuit of one Embodiment of this invention. It is a figure which shows each element of the light emitting element drive part of the light emitting element drive circuit of one Embodiment of this invention. It is a figure which shows the element etc. of the detection comparison part of a light emitting element drive circuit of one Embodiment of this invention, a pressure | voltage rise determination part, and a pressure | voltage rise circuit part.

Explanation of symbols

  8 Liquid Crystal Backlight Brightness Change System, 10 Light Emitting Element Drive Circuit, 20 Booster Circuit Unit, 21 Reference Power Supply, 22 Booster Comparator, 24 Booster PWM Circuit, 25 Booster Transistor, 26 Booster Coil, 27 Booster Diode, 28 Capacitor for boosting, 29 Power source for boosting, 30 Boosting determination unit, 40 Light emitting element driving unit, 42 Current circuit unit, 42a Left side transistor, 42b Right side transistor, 46 Current value setting circuit unit, 50 Detection comparison unit, 60 Brightness setting unit, 70 video processing circuit unit, 90 liquid crystal unit, 92 backlight, 200 through signal line, 201, 202 switch, 462 current value setting side resistor, 463 current value setting side comparator, 464 current value setting side transistor, 465b right side transistor, 465a Left transistor, 46 5 Current value setting side current mirror circuit, 466 D / A circuit, 468 reference current source, 501 comparator, 502 reference power supply, 503 first flip-flop, 504 second flip-flop, 505 third flip-flop, 506 AND circuit, 601 Luminance selection unit, 602 Luminance change analysis unit.

Claims (4)

A luminance setting unit configured to set luminance for video based on a video information signal from the video processing circuit unit, output luminance setting information for video, and output luminance change information for video;
A light emitting element driving unit that drives the light emitting element with a current having a current value according to the luminance setting information for video from the luminance setting unit;
A detection comparison unit that detects a terminal voltage of one terminal of the light emitting element and compares it with a predetermined voltage;
A boost determination unit that determines whether or not to boost the terminal voltage of the other terminal of the light emitting element based on the luminance setting information for video and the output of the detection comparison unit;
When the boost determining unit determines that the voltage is boosted, the terminal voltage of the other side terminal of the light emitting element is boosted. When the boost determining unit determines not to boost, the terminal voltage of the other side terminal of the light emitting element is boosted. No boost circuit part,
A light-emitting element driving circuit comprising: In the light emitting element drive circuit according to claim 1,
The booster circuit section
A booster circuit driven by a power supply circuit to boost the terminal voltage of the other terminal of the light emitting element;
A through connection signal line enabling connection between the power supply circuit and the other terminal of the light emitting element;
When the boost determination unit determines to boost, the boost circuit functions, and the power supply circuit and the other terminal of the light emitting element are not connected by the through connection signal line, and the boost determination unit determines that no boost is performed. A light-emitting element driving circuit comprising a switching circuit that connects a power supply circuit and the other terminal of the light-emitting element by a connection signal line and does not function a booster circuit. In the light emitting element drive circuit according to claim 2,
When the luminance change information for video is information that has changed from a state where the luminance for video is high to a low state,
The boost determination unit determines not to boost the terminal voltage of the other terminal of the light emitting element,
The switching circuit connects the power supply circuit and the other side terminal of the light emitting element by the through connection signal line and does not function the booster circuit, and then boosts the terminal voltage of the other side terminal of the light emission based on the output of the detection comparison unit. A light-emitting element driving circuit, characterized by determining whether or not to perform. The light emitting element driving circuit according to any one of claims 1 to 3,
A light emitting element driven by a light emitting element driving circuit;
An image display device illuminated by a light emitting element;
A mobile phone comprising:

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