KR101048257B1 - LED driving circuit - Google Patents

Abstract

LED driving circuit of the present invention, the first driver for driving a first LED string; A second driver for driving a second LED string; A first driving control unit outputting a first control signal for controlling an operation of the first driving unit; A second driving controller outputting a second control signal for controlling the operation of the first driving unit; A first time constant circuit for determining a time constant for the first control signal; A second time constant circuit for determining a time constant for the second control signal; And a frequency synchronizer for reflecting the characteristics of the first control signal to the second time constant circuit.

Sync, LED, Drive Circuit, Time Constant

Description

LED driving circuit {LED Driving Circuit}

The present invention relates to a LED driving circuit having a synchronization function between the PWM control IC for the LED lighting device, in particular applied to a plurality of PWM control IC of the LCD backlight to synchronize each drive signal of the plurality of PWM control IC. The present invention relates to an LED driving circuit having a synchronization function between PWM control ICs capable of synchronizing a plurality of PWM control ICs and preventing generation of interference signals and noise generated by asynchronous operation.

LED light source devices consisting of a plurality of strings of light emitting diodes (LEDs) are rapidly spreading for a wide range of applications for lighting applications or backlights of LDC panels.

LEDs with high brightness as a whole can be used in many applications, including backlights of liquid crystal displays (LCDs) based on monitors and televisions (collectively referred to as monitors hereinafter). In large LCD monitors, LEDs typically consist of one or more strings of LEDs connected in series.

To provide backlight for LCD monitors, one of two basic techniques is commonly used. The first technique uses one or more strings of white LEDs, which typically include blue LEDs with phosphors (materials). This phosphor absorbs the blue light generated by the LED and emits white light. In the second technique, one or more individual strings of colored LEDs are placed adjacent to each other so that the light combined with each other looks like white light.

On the other hand, as the market for LCD TVs and monitors is enlarged, LED strings for backlighting LCDs have also been developed in large capacities, which necessitates the use of two or more LED string driving ICs in parallel. In this case, when driving two or more ICs, the phases between the driving signals must be equalized.

1 is a configuration diagram of a conventional LED driving circuit.

Referring to FIG. 1, a conventional LED driving circuit includes a time constant circuit 1 that provides a time constant τ for determining a frequency of a drive signal, and a first constant according to the time constant of the time constant circuit 1. A first PWM control IC IC1 for generating a drive signal Sd1 and a second PWM control IC IC2 for generating a second drive signal Sd2 according to the time constant of the time constant circuit 1. do.

The time constant circuit 1 may include a time constant capacitor CT that determines the time constant, and may further include a time constant resistor, wherein the time constant capacitor CT and the time constant resistance are various. Can be connected in a manner.

In addition, each of the first and second PWM control ICs IC1 and IC2 includes an LED driving circuit for generating the driving signal, which will be described with reference to FIG. 2.

FIG. 2 is a configuration diagram showing the detailed configuration of the PWM control IC of FIG. 1.

Referring to FIG. 2, the conventional PWM control IC includes a fundamental wave generator 10 that generates a triangular wave and a square wave according to the time constant, and a difference signal Sero between a feedback voltage Vfb and a predetermined reference voltage Vref. ) Comparing the triangular wave signal Sosc to generate a PWM signal Spwm, a square wave of the fundamental wave generator 10, and a PWM signal Spwm of the PWM comparator 200. ) Includes a drive signal generator 30 for generating a drive signal Sd.

3 is a main signal timing chart of the LED driving circuit of FIG. 1, in which Vfb is a feedback voltage, Vref is a predetermined reference voltage, and Sd is a driving signal, which is a high signal HO and a low signal ( LO).

Referring to FIG. 3, as described above, the conventional PWM control IC generates the driving signal Sd including the HO signal and the LO signal according to the feedback voltage Vfb and the preset reference voltage Vref.

2 and 3, a process of generating a driving signal in a conventional LED driving circuit includes a square wave of the fundamental wave generator 10 and a PWM signal of the PWM comparator 200. In comparison with Spwm, the HO signal (or LO) is output in the first period and the LO signal (or HO) signal is output in the second signal period. That is, it can be seen that the HO signal and the LO signal appear randomly without having a priority.

Accordingly, when two or more PWM control ICs in a conventional LED driving circuit are driven in parallel, there is no priority between the HO and LO even when using the same triangular wave signal. There is this.

Accordingly, since a plurality of driving signals are randomly output from the plurality of PWM control ICs, synchronization is not performed between the IC and the IC output, and thus interference and noise of light are generated between the plurality of LED strings. There is a problem that a bad phenomenon such as flickering occurs in the LCD backlight.

An object of the present invention is to provide an LED driving circuit capable of preventing the generation of interference signals and noise generated by asynchronous in driving a plurality of LED strings.

An LED driving circuit according to an embodiment of the present invention, the first driver for driving a first LED string; A second driver for driving a second LED string; A first driving control unit outputting a first control signal for controlling an operation of the first driving unit; A second driving controller outputting a second control signal for controlling the operation of the first driving unit; A first time constant circuit for determining a time constant for the first control signal; A second time constant circuit for determining a time constant for the second control signal; And a frequency synchronizer for reflecting the characteristics of the first control signal to the second time constant circuit.

By implementing the LED driving circuit of the present invention according to the above configuration, there is an advantage that can be synchronized to drive a plurality of LED strings at a low cost.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings referred to in the present invention, components having substantially the same configuration and function will use the same reference numerals.

LED driving circuit according to an embodiment of the present invention shown in Figure 4, the first driving unit for driving a first LED string (not shown) (210); A second driver 220 for driving a second LED string (not shown); A first control IC 110 as a first driving controller for outputting a first control signal PWM1 for controlling the operation of the first driver C 210; a second controlling the operation of the second driver 220. A second control IC 120 as a second driving controller to output a second control signal PWM2; a first time constant capacitor 130 as a first time constant circuit to determine a time constant for the first control signal PWM1. A second time constant circuit for determining a time constant for the second control signal PWM2, the second time constant capacitor 140 and a time constant resistor 148; and the first control signal PWM1 A frequency synchronization unit 160 may be included to reflect the characteristic to the second time constant circuit.

The frequency synchronization unit 160 includes: a coupling capacitor 168, one end of which is connected to an output terminal of the first control signal PWM1; A first diode 162 having an anode connected to the other end of the coupling capacitor 168 and a cathode connected to a connection node N1 of the second time constant capacitor 140 and the time constant resistor 148; And a second diode 164 having a cathode connected to the other end of the coupling capacitor 168 and an anode connected to the ground.

The first time constant capacitor 130 may have one end connected to the first control IC 110 and the other end connected to ground.

One end of the second time constant capacitor 140 is connected to the second control IC 120, and one end of the time constant resistor 148 is connected to the other end of the second time constant capacitor 140. The other end may be connected to ground.

The first driver 210 and / or the second driver 220 may include a switch element (eg, a field effect transistor), and the first control signal PWM1 may be a switch element of the first driver 210. The signal is turned on / off, and the second control signal PWM2 is a signal for turning on / off the switch element of the second driver 220.

The first control signal PWM1 is converted into a sawtooth-like waveform as shown at node N0 while passing through the coupling capacitor, and only a forward component of the converted waveform to the first diode 162 is shown. The first diode 162 passes through the first diode 162 and is applied to the connection node N1 of the second time constant capacitor 140 and the time constant resistor 148. As a result, the time constant of the second control IC 120 follows the frequency of the signal applied to the connection node N1 of the second time constant capacitor 140 and the time constant resistor 148. The second diode 164 is for flowing a component that cannot pass through the first diode 162 to ground.

The first driver 210 and / or the second driver 220 may be a circuit for supplying driving power to an LED string, and may include, for example, a charge pump circuit or a switched-mode power supply (SMPS) circuit. . In this case, the switch device of the first driver 210 and / or the second driver 220 shown in the drawings may be a switch device constituting the charge pump or switched-mode power supply (SMPS) circuit.

The first control IC 110 and / or the second control IC 120 may be implemented as having the detailed structure of FIG. 2. That is, the fundamental wave generator for generating a triangular wave and a square wave according to the time constant determined by the first time constant circuit; A PWM comparison unit generating a PWM signal by comparing the feedback voltage with respect to the output signal of the first driver, a difference between a predetermined reference voltage, and the triangle wave; And a driving signal generator configured to generate the first control signal according to the square wave and the PWM signal.

FIG. 5 illustrates a charge pump that may be provided in the first driver 210 and / or the second driver 220.

The illustrated charge pump includes an inductor 10 that stores a current of an input power source; A switching unit 20 for switching the inductor 10 to accumulate current and to emit the accumulated current; A first storage capacitor (30) which accumulates the charge of the current emitted from the inductor (10); A second storage capacitor 40 which accumulates electric charges received from the first storage capacitor 30 and outputs it as an output power source; A first diode (52) forming a one-way current path from the inductor (10) to the first storage capacitor (30); A second diode (54) and a third diode (56) forming a one-way current path from the first storage capacitor (30) to the second storage capacitor (40); And a smoothing capacitor 60 connected between the connection node of the second diode 54 and the third diode 56 and the inductor 10.

The inductor 10 has a characteristic of maintaining a constant current state after exponentially increasing the current for a predetermined delay time (that is, a transient period) from the time when a voltage is applied to the input terminal I. In addition, when the load is connected to the inductor 10 in the constant current state, the inductor 10 transfers the electric energy stored in the constant current form to the load.

To this end, when the switching unit 20 is turned on during the predetermined delay time, when the inductor 10 is in a constant current state, the switching unit 20 is turned off, and the constant current stored in the inductor 10 After the exhaustion, the switching unit 20 is turned on again.

If the switching unit 20 is turned off while the inductor 10 is in the constant current state, the constant current flows to the first storage capacitor 30 for a predetermined time due to the characteristic of the inductor 10 that maintains the constant current. As a result, charge is accumulated in the storage capacitor 30.

The smoothing capacitor 60 bypasses the AC component generated in the switching process of the switching unit 20, and / or in the first storage capacitor 30 during the predetermined delay time (ie, the transition period). A function of suppressing current flow to the second storage capacitor 40 is performed.

When the voltage of the first accumulation capacitor, in which charge is accumulated, becomes high enough to exceed the threshold voltages of the two diodes 54 and 56, current flows from the first accumulation capacitor 30 to the second accumulation capacitor 40. The electric charge is accumulated in the second storage capacitor 40 by the current.

The charge accumulated in the second storage capacitor 40 is transferred to the load connected to the power output terminal O in the form of a current emitted from the power output terminal O.

FIG. 6 illustrates a power supply circuit having a switched-mode power supply (SMPS) that may be provided in the first driver 210 and / or the second driver 220.

The illustrated SMPS circuit includes four FETs S1 to S4 as switching elements for converting direct current into alternating current; And an intermittent switch element (SW1) which intercepts transmission of a driving signal for driving the switching element to the switching element. And transformer (T). The intermittent switch element SW1 may be implemented to be turned on / off by the first control signal PWM1 or the second control signal PWM2.

Although the technical spirit of the present invention has been described in detail according to the above embodiment, it should be noted that the above embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

For example, for convenience of description, the above-described embodiment is described as an LED driving circuit for driving two LED strings, but the idea of the present invention can be easily applied to an LED driving circuit for driving three or more LED strings. In addition, of course, it belongs to the scope of the present invention.

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

2 is a configuration diagram showing the detailed configuration of the PWM control IC of FIG.

3 is a main signal timing chart of the LED driving circuit of FIG. 1;

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

FIG. 5 is a circuit diagram illustrating a charge pump that may be included in the first driver and / or the second driver of FIG. 4.

FIG. 6 is a circuit diagram illustrating a power supply circuit having an SMPS circuit that may be included in the first driver and / or the second driver of FIG. 4.

Claims (9)

A first driver for driving the first LED string; A second driver for driving a second LED string; A first driving control unit outputting a first control signal for controlling an operation of the first driving unit; A second driving control unit outputting a second control signal for controlling an operation of the second driving unit; A first time constant circuit for determining a time constant for the first control signal; A second time constant circuit for determining a time constant for the second control signal; And By applying the signal of the converted waveform of the first control signal to the second time constant circuit, the time constant of the second drive control part of the signal of the converted waveform of the first control signal applied to the second time constant circuit LED driving circuit comprising a frequency synchronizer to follow the frequency. The method of claim 1, The first time constant circuit includes a first time constant capacitor having one end connected to the first driving controller and the other end connected to ground. The second time constant circuit may include a second time constant capacitor having one end connected to the second driving controller, and And a time constant resistor having one end connected to the other end of the second time constant capacitor and the other end connected to the ground. The method of claim 1, The frequency synchronization unit, A coupling capacitor, one end of which is connected to an output terminal of the first control signal; A first diode having an anode connected to the other end of the coupling capacitor and a cathode connected to the second time constant circuit; And A second diode having a cathode connected to the other end of the coupling capacitor and an anode connected to ground LED driving circuit comprising a. The method of claim 1, The first drive control unit, A fundamental wave generator for generating triangular and square waves in accordance with the time constant determined by the first time constant circuit; A PWM comparison unit generating a PWM signal by comparing the feedback voltage with respect to the output signal of the first driver, a difference between a predetermined reference voltage, and the triangle wave; And A driving signal generator configured to generate the first control signal according to the square wave and the PWM signal LED driving circuit comprising a. delete The method of claim 1, The first driving unit, Includes switched-mode power supply (SMPS) circuitry, The first control signal, LED driving circuit for turning on / off the switch element provided in the switched-mode power supply (SMPS) circuit. The method of claim 1, The second drive control unit, A fundamental wave generator configured to generate a triangular wave and a square wave according to the time constant determined by the second time constant circuit; A PWM comparator for generating a PWM signal by comparing the feedback voltage with respect to the output signal of the second driver, a difference between a preset reference voltage, and the triangle wave; And A driving signal generator configured to generate the second control signal according to the square wave and the PWM signal LED driving circuit comprising a. delete The method of claim 1, The second drive unit, Includes switched-mode power supply (SMPS) circuitry, And the second control signal turns on / off a switch element provided in the switched-mode power supply (SMPS) circuit.

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