KR101304436B1 - LED Driving System and Driving Control Method therefor - Google Patents

Abstract

An LED drive system and a drive control method thereof are disclosed. An LED driving system according to an embodiment of the present invention, the LED driving system for driving at least one LED (Light Emitting Diode), DC / DC converter for supplying a constant driving voltage to the LED; A current source unit including an operating amplifier (OP), a transistor, and a resistor and supplying an LED driving current to the LED; And a DC / DC controller for controlling the duty ratio of the DC / DC converter based on the lowest voltage among the gate voltages of the transistors.

Description

LED driving system and driving control method {LED Driving System and Driving Control Method therefor}

An embodiment of the present invention relates to an LED drive system and a drive control method thereof. More particularly, the present invention relates to an LED driving apparatus and a driving control method thereof capable of minimizing the number of pins of an integrated circuit required per LED driving current source when the LED driving current source is implemented outside the integrated circuit.

The contents described in this section merely provide background information on the embodiment of the present invention and do not constitute the prior art.

Liquid crystal displays (LCDs) are widely used as display devices for TVs, monitors, medical devices, laptops, and mobile phones. The LCD cannot emit light on its own, so in order to display the screen on the LCD, a backlight is required to shine a light from the back of the LCD. Cold backlights (CCFL: Cold Cathode Florescent Lamp) and LED (Light Emitting Diode) have been mainly used as the backlight.

In the case of mobile phones, the LED backlight is used all over, and in notebooks and small portable devices, LED cathodes are used in certain parts, but in the case of monitors and TVs, the cold cathode tube backlight has been mainly used until recently. Recently, however, LED backlights, which are environmentally friendly and consume less energy because they do not contain environmental harmful elements such as mercury, have been applied to monitors and TVs, and are rapidly replacing cold cathode tube backlights.

The brightness of the LCD screen, that is, the brightness, is determined by the brightness of the backlight, and as the LCD panel increases, the brightness of the required backlight must also increase. Accordingly, the number of LEDs used in the LED backlight must also increase. LED brightness generally increases with larger LEDs, which in turn increases the drive current required to drive the LEDs.

In LED backlight systems of LCD monitors or LCD TVs, the LED driver is a single or multiple current sources for supplying a constant current to the LED and a DC / DC converter for negative feedback control such that the minimum voltage among the voltages applied to the current sources becomes a constant voltage. It consists of

In the LED backlight, the LED configuration is commonly used in a series of parallel and parallel configuration in which a plurality of LEDs are connected in series to form a single LED string, which is connected in parallel. An LED string is often called a channel, and a number of channels come together to form an LED backlight. The configuration method of the series-parallel connection has an advantage of lowering the LED driving voltage compared to the method of configuring all the LEDs in one string.

DC / DC converter of LED driver receives DC power from Switching Mode Power Supply (SMPS) or Adapter to supply the driving voltage required to drive the LED. Pulse-width modulation (PWM) control of the output voltage of the DC / DC converter to be equal to the voltage is commonly used with boosted DC / DC converters.

Such a driving method does not have a big problem in realizing the LED driving current source in the integrated circuit when the driving current is small. However, in the case where the driving current is large, heat generation may occur in a portion of the element constituting the LED driving current source to handle a large current, and thus, the LED driving current source should be implemented outside the integrated circuit. In this case, if the existing driving method is applied as it is, there is a problem in that the number of pins (PIN) of the integrated circuit required per LED channel is greatly increased.

An embodiment of the present invention has been devised to solve the above-described problem, LED drive device that can minimize the number of pins of the integrated circuit required per LED drive current source when the LED drive current source is implemented outside the integrated circuit and It is an object to provide a driving control method.

LED driving system according to an embodiment of the present invention for achieving the above object, in the LED driving system for driving at least one LED (Light Emitting Diode), DC / DC converter for supplying a constant driving voltage to the LED ; A current source unit including an operating amplifier (OP), a transistor, and a resistor and supplying an LED driving current to the LED; And a DC / DC controller for controlling the duty ratio of the DC / DC converter based on the lowest voltage among the gate voltages of the transistors.

Here, the current source unit may be installed outside the integrated circuit.

In addition, the DC / DC control unit, the minimum voltage selection circuit for receiving the output voltage of the current source unit and outputs the minimum of the voltage; An error amplifier for generating a duty ratio control voltage of the DC / DC converter based on the minimum voltage and the error amplifier reference voltage; And a current mode PWM controller configured to receive a sensing voltage based on the output voltage of the error amplifier and the inductor current of the DC / DC converter and output a pulse width modulation (PWM) control signal.

In addition, the DC / DC controller may set the error amplifier reference voltage so that the gate voltage of the transistor does not exceed the maximum output voltage of the OP amplifier.

In addition, the DC / DC controller responds to LEDs for each channel when the output voltage of the DC / DC converter drops due to the occurrence of a shorted LED and the gate voltage of the current source that does not cause a short circuit exceeds the set maximum value. It can be detected that a short has occurred in the LED channel having the minimum gate voltage among the gate voltages of the transistor.

LED drive control method according to an embodiment of the present invention for achieving the above object, the DC / DC converter for supplying a constant driving voltage to the LED, the current source for supplying the LED driving current to the LED, and the duty of the DC / DC converter An LED driving control method for controlling driving of at least one LED using an LED driving system including a DC / DC control unit for controlling a ratio, the method comprising: receiving an output voltage of a current source unit and outputting a minimum voltage among them; Generating a duty ratio control voltage of the DC / DC converter based on the minimum voltage and the error amplifier reference voltage; And outputting a PWM control signal by receiving a sensing voltage based on the output voltage of the error amplifier and the inductor current of the DC / DC converter, wherein the current source includes an OP amplifier, a transistor, and a resistor, the gate voltage of the transistor being the most The duty ratio control voltage of the DC / DC converter is generated based on the low voltage.

Here, the LED driving control method detects that a short circuit occurs in the LED channel having the minimum gate voltage among the gate voltages of the transistors corresponding to the LEDs when the output voltage of the DC / DC converter drops due to the shorted LED. It may further comprise the step.

According to the exemplary embodiment of the present invention, the LED driving system and the driving control method thereof may minimize the number of pins of the integrated circuit required per LED driving current source when the LED driving current source is implemented outside the integrated circuit.

1 is a view schematically showing an example of an LED driving device.
2 is a diagram showing an example of a drive current source.
3 is a diagram illustrating an embodiment of the present invention.
4 is a diagram illustrating a relationship between a voltage applied to LEDs of respective channels and a gate voltage of a current source transistor.
FIG. 5 is a diagram illustrating a method of detecting a short-circuit LED when controlling LED driving according to an exemplary embodiment of the present invention.

Hereinafter, some embodiments of the present invention will be described in detail with reference to exemplary drawings. In adding reference numerals to the components of each drawing, it should be noted that the same reference numerals are assigned to the same components as much as possible even though they are shown in different drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

In addition, in describing the component of this invention, terms, such as 1st, 2nd, A, B, (a), (b), can be used. These terms are intended to distinguish the constituent elements from other constituent elements, and the terms do not limit the nature, order or order of the constituent elements. When a component is described as being "connected", "coupled", or "connected" to another component, the component may be directly connected or connected to the other component, Quot; may be "connected," "coupled," or "connected. &Quot;

1 is a view schematically showing an example of an LED driving device.

Referring to FIG. 1, the LED driving system 100 according to the first embodiment includes an LED load 110, a DC / DC converter 120 for supplying a constant driving voltage to the LED load 110, and an LED driving IC. 130. In FIG. 1, the DC / DC converter 120 is illustrated as a boost type DC / DC converter that controls the output voltage Vo to be higher than the input voltage VIN, but may be configured as a step-down DC / DC converter depending on input / output voltage conditions. . In addition, although the LED driving system 100 of FIG. 1 is shown as driving an LED string of 8 channels, this is only for illustrative purposes and may be configured with any other number of channels.

In FIG. 1, the LED driving IC 130 pin configuration shows only the basic pins for controlling the DC / DC converter 120 and supplying the driving current to the LED load 110, but in the actual IC, PWM frequency setting and dimming are performed. Pins such as control may be further included. Referring to FIG. 1, the pins of the LED driving IC 130 are briefly described. VCC is a power input pin, and an input voltage VIN of the DC / DC converter 120 may be directly connected or may be input from a regulator according to a circuit structure in the IC. have. The EN pin is a pin for turning on / off the operation of the LED driving IC 130. When the EN pin is turned on, the operation of the LED driving system 100 starts. The COMP pin is a pin for stabilizing the negative feedback loop of the DC / DC converter 120, and the RISET pin is a pin for setting a driving current value of the LED load 110. The OVP pin is a pin for protecting the system from malfunction or damage of the LED driving system 100 due to the overvoltage of the output voltage Vo of the DC / DC converter 120. The NDRV is a pin for driving the switch M1 of the DC / DC converter 120. In FIG. 1, the switch M1 is implemented as an external to the IC. However, in some cases, the NDRV is implemented as an internal to the LED driving IC 130. May be The CS pin is a pin for controlling a current mode. The CS pin senses a current flowing through the inductor L1 when it is switched on, is converted into a voltage at the sensing resistor Rs, and is input to the LED driving IC 130.

Next, the operation of the LED driving IC 130 of FIG. 1 will be described in detail. The LED driving IC 130 of FIG. 1 includes a current source unit 140 and a DC / DC control unit 150. Here, the current source unit 140 includes a reference current generator 142 and a drive current source 144. The reference current generator 142 generates a voltage Vcon for setting the current value of the driving current source 144 by connecting an external resistor RSET between the RISET pin and ground. Therefore, the current value of the drive current source 144 can be set by adjusting the external resistance RSET value.

The DC / DC controller 150 controls the output voltage of the DC / DC converter 120 so that the LED driving current is normally supplied to the LED load 110. The output voltages CS1 to CS8 of the driving current source 144 are provided. And the output voltage Vmin of the minimum voltage selection circuit 152 and the output voltage Vmin of the minimum voltage selection circuit 152 and the error amplifier reference voltage Vr, err are received from the DC / DC converter 120. An error amplifier 154 that generates a duty ratio control voltage, a current mode PWM controller 156 that receives the output voltage and the sensing voltage CS of the error amplifier 154 as inputs and outputs a PWM control signal NDRV, and a DC / DC converter 120 And an overvoltage comparator 158 for detecting an overvoltage state of the output voltage Vo. Here, the error amplifier 154 controls the output voltage Vmin of the minimum voltage selection circuit 152 to be equal to the error amplifier reference voltages Vr and err by negative feedback. The resistor Rc and the capacitor Cc connected to the output pin COMP of the error amplifier 154 are for stabilizing the negative feedback loop of the DC / DC conversion, and the DC / DC converter 120 reaches a steady state. The capacitor Cc is charged with an error voltage for controlling the duty ratio such that the output voltage Vmin of the minimum voltage selection circuit 152 and the reference voltage Vr, err of the error amplifier 154 are the same. In FIG. 1, the resistor Rc and the capacitor Cc for stabilizing the negative feedback loop are compensated by externally connecting to the COMP pin. However, the resistor Rc and the capacitor Cc may be implemented inside the IC and may be stabilized by another modified configuration. The current mode PWM controller 156 senses the current of the inductor L1 when the switch M1 is turned on to control the current mode of the DC / DC converter 120 and receives a value converted from the sensing resistor Rs into a voltage from the CS pin. The switch M1 is turned off when the CS pin voltage is multiplied by some gain and the slope compensation signal becomes greater than the error amplifier 154 output voltage. The overvoltage comparator 158 protects the system from malfunction or damage of the LED driving system 100 due to the overvoltage of the DC / DC converter 120 output voltage Vo. The overvoltage comparator 158 divides Vo into resistors R1 and R2 to the OVP pin. The overvoltage condition of Vo is detected by comparing this divided voltage with the overvoltage comparison reference voltages Vr, ovp in the overvoltage comparator 158. When the overvoltage occurs, the output DOWN of the overvoltage comparator 158 becomes high and stops switching of the DC / DC converter 120 until the overvoltage condition is resolved, and simultaneously removes the channels causing the overvoltage from the negative feedback control loop. The overvoltage protection plays a key role in the OLP (Open LED Protection) function of the LED driving system 100. When one or more of the LED strings are open, the voltage VCSx (one of the voltages applied to pins CS1 to CS8) applied to the corresponding current source becomes zero, so the output voltage Vim of the minimum voltage selection circuit 152 becomes zero. Accordingly, the output voltage COMP of the error amplifier 154 of the LED driving IC 130 continues to increase to saturate to the maximum voltage. The DC / DC converter 120 thus operates at the maximum duty ratio and the output voltage Vo continues to increase. When Vo reaches the overvoltage state, the PWM switching operation stops and the output voltages of the current sources driving the open LED strings at the same time do not participate in the negative feedback control of the DC / DC converter 120 and only operate normally with the unopened LED strings. To help.

Such a driving method does not have a big problem in implementing the driving current source 144 together in the integrated circuit 130 when the driving current is small. However, when the driving current is large, heat generation may occur in a portion of the element constituting the driving current source 144 that handles a large current.

2 is a diagram showing an example of a drive current source.

As shown in FIG. 2A, the driving current source 200 may include an operating amplifier (OP), a transistor Mx, and a resistor Rs. In the terminology of current source, it means to supply a constant current irrespective of the voltage applied to the current source. However, in an actual semiconductor circuit, the current source normally operates as a current source only when the voltage applied to the current source is above a certain minimum voltage. FIG. 2 (b) shows the voltage and current characteristics of a current source in a general semiconductor circuit. When the voltage Vds applied to the current source exceeds the minimum voltage ΔV for operating as a current source, the current current operates as a constant current Io but when Vds becomes smaller than ΔV. The current value Ids of the current source also decreases gradually. In the semiconductor circuit, the minimum voltage ΔV of the current source is determined by the circuit configuration of the current source and the transistor size. In order to increase the size Io of the current source at any arbitrary value ΔV, the size of the transistor Mx constituting the current source must be large.

At this time, the Vds voltage must be increased as the LED current increases, so the power consumed by the current source also increases as the LED current increases. When the transistors constituting the current source are all implemented in the integrated circuit, as the LED current and the number of current sources increase, the power consumed in the integrated circuit increases, which inevitably increases the operating temperature of the integrated circuit. Therefore, when the LED current is large or the number of channels is large, the transistor Mx is externally mounted among the components of the current source, and in some cases, the current source resistor Rs is also externally mounted.

Meanwhile, in order to apply the LED driving method of the LED driving apparatus of FIG. 1 to the LED driver using an external transistor as it is, three pins are required per channel, and due to the limitation on the number of pins of the integrated circuit, the maximum number of channels that can be implemented is severe. Bring a constraint For example, in the case of 12-channel LED driver, which is commonly used, 36 pins are necessary for the current source configuration, and at least 50 Pin is required considering other pins to configure the LED driver. Done.

Three pins per channel are required. First, the output of the DC / DC converter is controlled to minimize the voltage applied to the current source (drain of Mx). Second, one or more LEDs may be shorted. In order to protect the current source, the current source should be turned off because the drain voltage of the transistor Mx is directly detected to detect the current source.

In order to minimize the number of pins of the integrated circuit required per LED current source when the LED current source is installed outside the integrated circuit, in the embodiment of the present invention, the current source transistor is used to control the DC / DC converter 120 of FIG. A method of using the gate voltage without directly using the drain voltage of (Mx) is proposed. By doing this, it is possible to reduce the number of pins constituting the current source to 2/3 of the conventional level.

3 is a diagram illustrating an embodiment of the present invention. For convenience, the description will be made using three channels. However, the operation principle for the embodiment of the present invention is independent of the number of channels.

Unlike the LED driving method according to the LED driving apparatus of FIG. 1, the LED driving apparatus according to the embodiment of the present invention is provided with a driving current source on the outside of the integrated circuit as shown in FIG. 3, and the DC / DC converter ( The output voltage of 120 is controlled so that the lowest voltage among the gate voltages Vgx of the current source transistor is a set value Vr, err. At this time, the driving current source of each LED may have the same structure as the driving current source as shown in FIG.

When the voltages applied to the LEDs for each channel are Vstr1, Vstr2, and Vstr3, respectively, and Vstr1 <Vstr2 <Vstr3, the gate voltage of each current source transistor is Vg1 <Vg2 <Vg3. The relationship between the voltage Vstr applied to each LED of each channel and the gate voltage Vg of the current source transistor is shown in FIG. 4.

In FIG. 4, Vg1 becomes Vr, err which is a set minimum value because Vstr1 < Vstr2 < Vstr3. If Vstr2 <Vstr1 <Vstr3, Vg2 is controlled to be Vr, err and satisfies the relationship of Vg2 <Vg1 <Vg3.

In Fig. 4, Vg, max is the maximum output voltage of the OP amplifier constituting the current source. If Vstr3 increases and Vg3 exceeds Vg, max, the current flowing to M3 decreases. Therefore, the voltage of Vr, err must be set so that the gate voltage of each current source transistor does not exceed Vg, max. (Vstrx) should be managed.

5 illustrates a method of detecting a short LED when the LED driving method according to the exemplary embodiment of the present invention is adopted. The drain voltage of the current source transistor of the channel where the short LED is generated is increased. In the driving method of the LED driver of FIG. 1, the LED channel in which the short is generated is easily detected by turning off the channel by directly monitoring the drain voltage. It could protect against excessive power consumption.

However, the LED driving apparatus according to the embodiment of the present invention does not directly monitor the drain voltage of the current source transistor, and may indirectly detect / off the channel where the short LED occurs by using the following principle.

Assume that a short occurs in the third LED channel of FIG. 3.

Since the LED short has occurred, the drain voltage of M3 rises, so that the current flowing through M3 also rises momentarily. However, the feedback current of the op amp constituting the current source lowers the Vg3 voltage, causing the set current to flow. At this time, the Vg3 voltage is the lowest voltage among the various channels because the high voltage is applied to the drain. Therefore, the output voltage of the DC / DC converter is controlled so that the Vg3 voltage becomes the Vr, err voltage. The voltages of Vg1 and Vg2 rise to compensate for the current decrease as the output voltage of the DC / DC converter decreases. Since there was a large voltage drop due to the Short LED, the voltages of Vg1 and Vg2 exceed all or one of Vg, max. do. In other words, when the output voltage of DC / DC converter due to short LED drops, the gate voltage of the remaining LED channels exceeds one or more Vg, max, so in this case, the short LED is generated in the LED channel with the minimum gate voltage. Can be detected.

The above description is merely illustrative of the technical idea of the present invention, and those skilled in the art to which the present invention pertains may make various modifications and variations without departing from the essential characteristics of the present invention. It should be noted that the embodiments disclosed in the present invention are not intended to limit the scope of the present invention and are not intended to limit the scope of the present invention. Therefore, the scope of protection of the present invention should be construed according to the following claims, and all technical ideas within the scope of the same shall be construed as being included in the scope of the present invention.

Claims (7)

In the LED driving system for driving at least one LED (Light Emitting Diode),
A DC / DC converter for supplying a constant driving voltage to the LED;
A current source unit including an operating amplifier (OP), a transistor, and a resistor and supplying an LED driving current to the LED; And
A DC / DC controller which controls the duty ratio of the DC / DC converter based on the lowest voltage among the gate voltages of the transistors
Including but not limited to:
The DC / DC control unit,
A minimum voltage selection circuit which receives an output voltage of the current source unit and outputs a minimum voltage among them;
An error amplifier generating a duty ratio control voltage of the DC / DC converter based on the minimum voltage and the error amplifier reference voltage; And
A current mode PWM controller that receives a sensing voltage based on the output voltage of the error amplifier and the inductor current of the DC / DC converter and outputs a PWM (Pulse Width Modulation) control signal.
LED drive system comprising a.
The method of claim 1, wherein the current source unit,
LED drive system, characterized in that installed on the outside of the integrated circuit.
delete The method of claim 1,
The DC / DC control unit,
And setting the error amplifier reference voltage such that the gate voltage of the transistor does not exceed the maximum output voltage of the OP amplifier.
The method of claim 1,
The DC / DC control unit,
When the output voltage of the DC / DC converter drops due to the LED short-circuit, detecting that the short (Short) has occurred in the LED channel having the minimum gate voltage of the gate voltage of the transistor corresponding to the LED for each channel further includes LED drive system, characterized in that.
By using a LED driving system including a DC / DC converter for supplying a constant driving voltage to the LED, a current source for supplying the LED driving current to the LED, and a DC / DC control unit for controlling the duty ratio of the DC / DC converter In the LED drive control method for controlling the driving of at least one of the LED,
Receiving an output voltage of the current source unit and outputting a minimum voltage thereof;
Generating a duty ratio control voltage of the DC / DC converter based on the minimum voltage and the error amplifier reference voltage; And
Outputting a PWM control signal by receiving a sensing voltage based on an output voltage of the error amplifier and an inductor current of the DC / DC converter;
Including but not limited to:
The current source unit includes an OP amplifier, a transistor, and a resistor, and generates a duty ratio control voltage of the DC / DC converter based on the lowest voltage among the gate voltage of the transistor.
The method according to claim 6,
If the output voltage of the DC / DC converter drops due to the shorted LED, detecting that the short circuit occurs in the LED channel having the minimum gate voltage among the gate voltages of the transistors corresponding to the LEDs for each channel. LED drive control method, characterized in that.

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