KR101028860B1 - Parallel light emitting diode driving circuit - Google Patents

Abstract

PURPOSE: A parallel light emitting diode driving circuit is provided to suppress a ripple voltage and lowering a drain voltage necessary for driving an LED string, thereby reducing unnecessary power consumption and protecting an LED driving circuit device and an LED. CONSTITUTION: An LED driving circuit comprises a DC-DC convertor unit(310), a feedback unit(320), and a current mirror unit(330). A DC-DC converter(312) comprises an inductor(LDC), a transistor(QDC), a diode(DDC), and a capacitor(CDC). The inductor, the diode, and the transistor operate as a power converting circuit and convert an input voltage into a voltage necessary for driving an LED string unit. The feedback unit comprises a plurality of diodes. The feedback unit is connected between the DC-DC converter and the contact point of the LED string unit and the current mirror unit.

Description

PARALLEL LIGHT EMITTING DIODE DRIVING CIRCUIT}

The present invention relates to a parallel light emitting diode driving circuit, and more particularly, to a LED driving circuit which can reduce unnecessary power consumption by lowering a drain voltage required for driving an LED string by performing feedback control with respect to an LED string having a high forward voltage. will be.

Backlight is a device that illuminates a display panel. Conventionally, a cold cathode fluorescent lamp (CCFL) is used as a light source. However, when CCFL is used, environmental hazards may occur due to mercury use. The response speed is about 15ms and the color reproducibility is about 75% lower than that of NTSC. In addition, since various problems, such as generating a predetermined white light occurs, in recent years, light emitting diode (LED) devices have been in the spotlight as a light source.

Compared with CCFL, LIGHT EMITTING DIODE (LED) is environmentally friendly, has a response speed of several nanoseconds, high speed response, impulse driving, and color reproducibility from 80% to 100%. .

In addition, there is an advantage that the brightness and color temperature of the backlight can be arbitrarily adjusted by adjusting the amount of light of the light emitting diodes emitting red (R), green (G), and blue (B).

The driving device for lighting the light emitting diode includes an LED string formed by connecting a plurality of light emitting diode elements in series. In addition, since there is a forward voltage deviation for each LED connected in series, the LED driving device must be driven with a constant current.

Since the light characteristic of the light emitting diode is determined by the current passing through the light emitting diode, it is preferable to adjust the voltage across the light emitting diode by sensing and feeding back the current of the light emitting diode for effective control of the light emitting diode.

1 is a circuit diagram showing a LED driving circuit according to the prior art.

As shown in FIG. 1, the LED driving circuit 10 may include a DC-DC converter 11, a DC-DC controller 12, and a sensing resistor Rs.

The DC-DC converter 11 converts the supply power supply Vcc into a DC voltage Vo required for driving the LED and outputs it. The DC-DC converter 11 includes an inductor L, a transistor Q, a diode D, and a capacitor C. The inductor L, the transistor Q, and the diode D operate as a power conversion circuit, converting the input voltage Vcc into a voltage required for driving the LED and outputting it. The capacitor C smoothes the output voltage passing through the diode D to a DC voltage.

The DC-DC controller 12 controls the DC-DC converter 11 according to the voltage fed back from the sensing resistor Rs and the LED string 20.

Meanwhile, a plurality of light emitting diodes are connected in series to the output terminal of the DC-DC converter 11 to form the LED string 20.

As shown in FIG. 1, it is relatively easy to control the current flowing through the LEDs in the LED string 20 in which a plurality of LEDs are connected in series, but it is almost impossible to serially connect the LEDs constituting a backlight composed of a large number of LEDs. impossible. In addition, since the backlight cannot be subdivided into blocks when connected in series, it is necessary to connect a plurality of LED arrays in parallel.

However, the LED does not match the characteristics of the forward voltage and the forward current. In general, since a voltage drop of about 2 mV occurs every time the temperature rises by 1 ° C. in the constant current state, the characteristics of the forward voltage and the forward current become unstable as the temperature increases.

Therefore, when a plurality of LED strings connected in parallel are driven at the same output voltage, different currents flow in different parallel circuits according to characteristic values of the light emitting diodes, resulting in uneven brightness.

In addition, a light emitting diode having a high current flows in a large amount of forward current as the temperature increases, resulting in a short lifespan of the light emitting diode device.

In order to solve this problem, a separate DC-DC converter may be driven for each LED string, but the price competitiveness becomes a problem due to the use of an excessive number of DC-DC converters.

Accordingly, in order to reduce the number of DC-DC converters, an LED driving circuit using a current mirror as shown in FIG. 2 is proposed.

2 is a circuit diagram showing another LED driving circuit according to the prior art.

As shown in FIG. 2, a conventional LED driving circuit for driving a plurality of LED strings 20a,..., 20n includes a DC-DC converter 11, a DC-DC controller 12, a feedback unit 13, and the like. And a current mirror portion 14.

The DC-DC converter 11 and the DC-DC controller 12 operate in the same manner as in the LED driving circuit of FIG.

The current mirror unit 14 maintains a constant current flowing through the LED strings 20a, ..., 20n by adjusting the drain-source voltages of the field effect transistors (QC1, ..., QCN).

The feedback unit 13 is connected between the DC-DC controller 12 and the current mirror unit 14 to feed the drain voltages of the FETs QC1, QC2,..., QCN to the feedback FB of the DC-DC controller 12. ) Feedback to the terminal.

However, in the LED driving circuit of FIG. 2, the operation switching of the FET is properly performed when driven at a current of 20 to 30 mA per LED string, but when 60 mA or more flows, the DC-DC feedback is reduced due to the switching ringing of the FET. I'm shaking. The result is an inaccurate constant current flow for each LED string.

On the other hand, each of the LED FETs QC1, QC2, ..., QCN connected to each LED string 20a, ..., 20n and the operational amplifiers U1, U2, ..., QN driving the FETs QC1, QC2, ..., QCN The switching behavior causes high ripple in the smoothing capacitor (C _DC ). In other words, by independently switching a plurality of FETs, a high ripple phenomenon occurs in a capacitor (C _DC ) that smoothes direct current (DC) input to the anode terminal of the LED string, thereby adversely affecting the life of the LED driving circuit. It can be crazy and generate noise.

In addition, the LED driving circuit of FIG. 2 operates in a manner of detecting and feeding back a voltage based on the LED string having the highest terminal voltage of the cathode among the plurality of LED strings 20a,. That is, since the voltage is detected and fed back based on the LED string having the lowest forward voltage among the plurality of LED strings 20a, ..., and 20n, the LED string 20n having the high forward voltage receives the voltage Vds of the FET QCN. If it is not set above a certain value, there may be a problem that constant current cannot flow. To prevent this, the voltage Vds of the FET QCN may be set to compensate for the deviation of the forward voltage by adjusting the resistance values of the resistors RF1 and RF2. However, in this case, since the drain voltage Vds is set higher than necessary for other LED strings having a low forward voltage, the heat is consumed as much as voltage Vds × LED current.

Therefore, the technical problem to be achieved by the present invention is to reduce the power consumption by reducing the drain voltage required for driving the LED string, and to protect the LED driving circuit elements and the LED by suppressing the ripple voltage generation.

According to an embodiment of the present invention, an LED driving circuit for driving an LED string unit in which a plurality of strings of LEDs (Light Emitting Diode) are connected in parallel, converts a power supply into a DC output voltage according to a feedback voltage. A DC-DC converter unit outputting to the anodes of the plurality of LED strings, a current mirror unit including a plurality of first transistors respectively connected to the cathodes of the plurality of LED strings, and allowing the plurality of LED strings to flow a constant current; And a feedback unit for feeding back the lowest voltage among the voltages of the cathodes of the plurality of LED strings and the voltages of the contacts of the plurality of first transistors to the DC-DC converter.

The feedback unit may include a plurality of first diodes having an anode connected to a feedback terminal of the DC-DC converter and a cathode connected to a cathode of the plurality of LED strings and a contact of the plurality of first transistors.

The plurality of first transistors may be Bipolar Junction Transistors (BJTs).

The current mirror unit further includes a first comparator configured to apply the same voltage to the bases of the plurality of first transistors, and a non-inverting terminal of the first comparator receives a first reference voltage and inverts the first comparator. The terminal may receive an emitter voltage of one of the plurality of first transistors, and the emitter of the one transistor may be connected to a ground power source through a first resistor.

And a shutdown circuit unit configured to detect disconnection of the plurality of LED strings, wherein the shutdown circuit unit includes: a non-inverting terminal receiving the second reference voltage divided by the first reference voltage through a second resistor and a third resistor; And a second comparator including an inverting terminal to which the emitter voltage of the one transistor is applied.

A voltage generator for outputting the first reference voltage, a PWM generator for outputting a PWM (pluse width modulation) signal at a DC level, and an ON / OFF operation according to the PWM signal output from the PWM generator to adjust the magnitude of the first reference voltage. The apparatus may further include a dimming circuit unit including a variable second transistor.

As described above, according to the present invention, the drain voltage required for driving the LED string can be lowered to reduce unnecessary power consumption. In addition, ripple voltage generation can be suppressed to protect the LED driving circuit elements and the LEDs.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and like reference numerals designate like parts throughout the specification.

Throughout the specification, when a part is "connected" to another part, this includes not only "directly connected" but also "electrically connected" with another element in between. . In addition, when a part is said to "include" a certain component, which means that it may further include other components, except to exclude other components unless otherwise stated.

Hereinafter, an LED driving circuit according to an embodiment of the present invention will be described in detail with reference to FIGS. 3 to 5.

3 is a circuit diagram showing an LED driving circuit according to a first embodiment of the present invention.

As shown in FIG. 3, the LED driving circuit includes a DC-DC converter 310, a feedback unit 320, and a current mirror unit 330.

First, the DC-DC converter 310 converts a supply power supply (Vcc) into a DC output voltage required for driving an LED driving circuit and outputs the DC-DC converter 311 and a DC-DC converter using the feedback voltage. And a DC-DC controller 312 that controls the output voltage of 311.

The DC-DC converter 312 includes an inductor (LDC), a transistor (QDC), a diode (DDC), and a capacitor (CDC).

The first end of the inductor LDC is connected to the supply power supply Vcc, and the second end of the inductor LDC is connected to the anode of the diode DDC. The drain of the transistor QDC may be connected to the second terminal of the inductor LDC and the anode of the diode DDC, and the source may be connected to a ground power source.

In addition, a first end of the capacitor CDC may be connected to the cathode of the diode DDC, and a second end of the capacitor CDC may be connected to a ground power source.

In this case, although the DC-DC converter 311 is shown in a boost type, it may be implemented in a flyback, buck, or buck-boost method. And, the design change for this can be easily implemented by those skilled in the art, detailed description thereof will be omitted.

DC-DC controller 312 includes a feedback (FB) terminal and a gate (Gate) terminal, the feedback (FB) terminal is connected to the anode of the diode (D1, D2, ..., DN), the gate (Gate) terminal Is connected to the gate of transistor QDC.

Here, the inductor LDC, the diode DDC, and the transistor QDC operate as a power conversion circuit, converting the input voltage Vcc into a voltage required for driving the LED string unit 400 and outputting the same. The capacitor CDC outputs the voltage Vo to the LED string unit 420 by smoothing the voltage passing through the diode DDC to a DC voltage.

In addition, since the LED driving circuit is normally driven when the output voltage Vo of the DC-DC converter 310 is greater than the forward voltage of the light emitting diode, the DC-DC converter 310 adjusts the output voltage Vo. A rated voltage may be applied to the anodes of the plurality of LED strings constituting the LED string unit 400.

The LED string unit 400 is mounted on a backlight of a display such as a liquid crystal display (LCD), and includes a plurality of LED strings STR1, STR2,..., And STRN: N LED strings in FIG. 3. These LEDs are connected in parallel, and each LED string STR1, STR2, ..., STRN is connected in series with a plurality of light emitting diodes (in FIG. 3, N LEDs constitute each string). The number of LED strings and the number of light emitting diodes constituting the LED string may be appropriately added or subtracted according to the embodiment.

The feedback unit 320 includes a plurality of diodes DF1, DF2,..., DFN, and the contacts P1, P2,..., PN of the LED string unit 400 and the current mirror unit 330 and the DC-DC. It is connected between the converter 310.

The anodes of the diodes DF1, DF2,..., DFN are connected to the feedback terminal FB side of the DC-DC controller 312. The cathodes of the diodes DF1, DF2,..., DFN are connected to the contacts P1, P2,..., PN of the LED string unit 400 and the current mirror unit 330.

By connecting the diodes DF1, DF2,..., And DFN, the feedback unit 320 receives the lowest voltage among the voltages of the contacts P1, P2,..., PN and the feedback (FB) terminal of the DC-DC controller 312. Feedback. According to the configuration of the feedback unit 320 according to the present invention, the string having the highest forward voltage among the LED strings STR1, STR2, ..., STRN, that is, the collector voltage of the transistors QC1, QC2, ..., QCN The feedback voltage can be set based on the lowest string. Therefore, the collector-emitter voltage of the transistors QC1, QC2, ..., QCN may allow only the minimum voltage required for driving the LED string unit 400 to be applied. As a result, the loss power consumed by the transistors QC1, QC2, ..., QCN can be minimized, and the heat generation problem can be solved.

The current mirror unit 330 includes a plurality of transistors QC1, QC2,..., QCN respectively connected to the LED strings STR1, STR2,..., STRN. The collectors of the transistors QC1, QC2, ..., QCN are connected to the cathode side of the LED strings STR1, STR2, ..., STRN. The emitters of the transistors QC1, QC2, ..., QCN may be connected to the ground power source through the resistors RS1, RS2, ..., RSN. The emitter of transistor QC1 is connected to the inverting terminal (-) of comparator U1.

In the comparator U1, the reference voltage V _REF1 is input to the non-inverting terminal +, and the emitter voltage of the transistor QC1 is input to the non-inverting terminal +.

The operation of the current mirror unit 330 will be described.

First, when the output voltage of the comparator U1 is equally applied to the bases of the transistors QC1, QC2, ..., QCN, and the resistance values of the resistors RS1, RS2, ..., RSN are the same, each LED string STR1, STR2 , ..., STRN) the same current flows.

The comparator U1 adjusts the base voltage of the transistors QC1, QC2, ..., QCN according to the voltage applied from the emitter of the transistor QC1 to the inverting terminal (-) at the contact of the resistor RS1. The current flowing in is kept constant. For example, when the forward voltage decreases while the LED strings STR1, STR2,..., STRN are driven, the current flowing through the LED strings STR1, STR2,..., STRN increases. Then, the emitter current of the transistor QC1 increases and the voltage applied to the inverting terminal (-) increases, so that the comparator U1 adjusts the base voltage of the transistors QC1, QC2, ..., QCN to a low level so that the current flowing in the LED string is reduced. Adjust so that is lowered.

In the present embodiment, the bipolar junction transistor (BJT) may be used for the transistors QC1, QC2, ..., QCN instead of the FET. This allows the LED string current to be kept constant by voltage level adjustment rather than conventional FET switching, thereby eliminating switching riging, and all LED strings (STR1, STR2, ..., STRN) are the same. By operating in a period can also reduce the ripple (ripple) generated in the capacitor (CDC) of the DC-DC converter 310. As a result, the lifespan of the smoothing capacitor (CDC) and the LED driving circuit can be improved.

4 is a circuit diagram showing an LED driving circuit according to a second embodiment of the present invention.

Referring to FIG. 4, the DC-DC converter 310, the feedback unit 320, and the current mirror unit 330 operate in the same manner as the components having the same reference numerals in the first embodiment of FIG. 3.

In this embodiment, the LED driving circuit further includes a shutdown function.

To this end, the LED driving circuit of the present embodiment further includes a shutdown circuit unit 340.

The shutdown circuit unit 340 includes a comparator U2 and resistors R _shut1 and R _shut2 .

The comparator U2 receives the reference voltage V _REF2 as the non-inverting terminal + and the emitter voltage of the transistor QC1 is input to the inverting terminal −.

If one or more LED strings of the LED strings STR1, STR2, ..., STRN are disconnected and an error occurs, the output voltage of the comparator U1 is connected to the disconnected LED strings STR1, STR2, ..., STRN. It is pulled down to the ground power supply GND via transistors QC1, QC2, ..., QCN. Therefore, the comparator U2 outputs a shutdown signal since the voltage applied to the inverting terminal (-) is lower than the reference voltage V _REF2 . In FIG. 4, the reference voltage V _REF2 is represented by R _shut2 / (R _shut1 + R _shut2 ) × V _REF1 , but may be implemented so that a reference voltage having an appropriate magnitude is input.

5 is a circuit diagram illustrating an LED driving circuit according to a third embodiment of the present invention.

Referring to FIG. 4, the DC-DC converter 310, the feedback unit 320, the current mirror unit 330, and the shutdown circuit unit 340 are the same reference numerals in the first and second embodiments of FIG. 3. It works the same as a component with.

In this embodiment, the LED driving circuit further includes a dimming function.

To this end, the LED driving circuit of the present embodiment further includes a dimming circuit unit 350.

The dimming circuit unit 350 further includes a PWM generator 351, resistors RD1, RD2, RD3, RD4, RD5, and RD6, a diode DD1, and a transistor QD1.

The dimming circuit 350 makes a supply voltage Vcc to a predetermined level by using a diode DD1 and resistors RD1, RD2, and RD3, which are implemented as constant voltage elements, such as a zener diode, and then resistors RD4 and RD5. And a voltage source outputting the reference voltage Vref1 generated by dividing the voltage to the current mirror unit 330.

On the other hand, the transistor QD1 adjusts the base to a DC level so that the resistor RD6 connected to the emitter is connected in parallel with the resistor RD5 so that the reference voltage Vref1 is variable during the operation of the transistor QD1. have.

The PWM generator 351 may output a plus width modulation (PWM) signal at a DC level. Accordingly, the PWM generator 351 outputs a plus width modulation (PWM) signal at a direct current level to the base of the transistor QD1 when necessary, thereby changing the reference voltage Vref1 by turning the transistor QD1 on and off to change the LED string portion ( Dimming control for 400 may be performed.

Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present invention defined in the following claims are also provided. It belongs to the scope of rights.

1 is a circuit diagram showing a LED driving circuit according to the prior art.

2 is a circuit diagram showing another LED driving circuit according to the prior art.

3 is a circuit diagram showing an LED driving circuit according to a first embodiment of the present invention.

4 is a circuit diagram showing an LED driving circuit according to a second embodiment of the present invention.

5 is a circuit diagram illustrating an LED driving circuit according to a third embodiment of the present invention.

Claims (6)

In the LED driving circuit for driving a plurality of LED (Light Emitting Diode) string LED string unit connected in parallel, A DC-DC converter unit converting a supply power supply into a DC output voltage according to a feedback voltage and outputting the output power to an anode of the plurality of LED strings; A current mirror unit including a plurality of first transistors respectively connected to cathodes of the plurality of LED strings, and allowing the plurality of LED strings to flow a constant current; And, A feedback unit for feeding back the lowest voltage among the voltages of the cathodes of the plurality of LED strings and the contacts of the plurality of first transistors to the DC-DC converter unit as the feedback voltage; LED driving circuit comprising a. The method of claim 1, The feedback unit, And a plurality of first diodes having an anode connected to a feedback terminal of the DC-DC converter and having a cathode connected to a cathode of the plurality of LED strings and a contact of the plurality of first transistors. . The method of claim 2, And the plurality of first transistors are Bipolar Junction Transistors (BJTs). The method of claim 3, wherein The current mirror unit, Further comprising a first comparator for applying the same voltage to the base of the plurality of first transistors, A first reference voltage is applied to the non-inverting terminal of the first comparator, and an emitter voltage of one of the plurality of first transistors is applied to the inverting terminal of the first comparator. And the emitter of the one transistor is connected to a ground power source through a first resistor. The method of claim 4, wherein A shutdown circuit unit for detecting disconnection of the plurality of LED strings; More, The shutdown circuit unit, A second comparator including a non-inverting terminal receiving a second reference voltage divided by the second resistor and a third resistor, and an inverting terminal receiving an emitter voltage of the one transistor; LED drive circuit, characterized in that. The method of claim 5, A voltage source for outputting the first reference voltage; A PWM generator for outputting a PWM (pluse width modulation) signal at a DC level, A dimming circuit unit including a second transistor configured to vary the magnitude of the first reference voltage by being turned on and off according to a PWM signal output from the PWM generator; LED driving circuit further comprises.

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