KR100691189B1 - LED array driving apparatus - Google Patents

Abstract

The present invention relates to an LED array driving apparatus capable of preventing damage due to magnetic saturation by limiting the lower limit of the off period of the switching pulse in PWM current controlling the power applied to the LED by the constant current driving, the configuration is the input power supply A driving circuit unit for applying a predetermined driving power to the LED array through a switching transistor for switching on / off of Vin; A comparison unit which receives a voltage corresponding to the driving current applied from the driving circuit unit to the LED array and compares the reference voltage to obtain a deviation thereof; A PWM IC for generating a switching pulse whose on / off duty ratio is adjusted according to a deviation value output from the comparing unit so that the driving circuit unit is driven with a constant current, and applying the same to a switching transistor of the driving circuit unit to perform constant current control; And a duty limit circuit for adjusting the deviation value output from the comparator to a voltage within a predetermined range so that the off time of the switching pulse output from the PWM IC does not become less than or equal to a preset lower limit. .

LED, Constant Current, PWM, Duty Ratio, Magnetic Saturation

Description

LED array driving apparatus

1 is a circuit diagram showing a conventional LED array driving apparatus.

2 is a block diagram showing a detailed configuration of a PWM IC used in a conventional LED array driving apparatus.

3 is a circuit diagram showing an LED array driving apparatus according to the present invention.

4A and 4B are operation timing diagrams comparing the operation of a conventional LED array driving apparatus and an LED array driving apparatus according to the present invention.

Explanation of symbols on the main parts of the drawings

31: driving circuit part

32: PWM IC

33: comparator

34: duty limit circuit

The present invention relates to a device for driving an LED used as a back light source of an LCD panel, and more particularly, to self-saturate by limiting the lower limit of the off period during PWM switching control in PWM current control of the power supply of the LED array. It is to provide an LED array driving apparatus that can prevent device damage by.

Backlight is a device that illuminates a display panel. In the past, a CCFL (Cold Cathode Fluorescent Lamp) was used as a light source.However, the CCFL may cause environmental hazards due to the use of mercury. In recent years, LED (Light Emitting Diode) devices have been in the spotlight as a light source because they have various problems such as 15ms slow, 75% lower color reproducibility than NTSC, and generate a predetermined white light.

Compared with CCFL, LED is eco-friendly, response speed is several nanoseconds, high speed response is possible, impulsive driving is possible, color reproducibility is over 100%, and the amount of red, blue, and green LEDs is adjusted. Therefore, there are various advantages in that the brightness and color temperature of the backlight can be arbitrarily adjusted.

The driving device for turning on such LEDs may be implemented in a buck, boost, or buck-boost type, in which an LED array connects a plurality of LED elements in series. Because there is a forward voltage deviation per connected LED, the LED drive must be driven with a constant current.

Figure 1 shows the LED array driving circuit of the conventional step-down type.

Referring to FIG. 1, a conventional LED array driving apparatus includes a switching element 11 that is provided in series on a power line to which an input power Vin is applied, and switches on / off of the switching element 11. A pin diode D10 and an inductor L10 configured to apply an output voltage to the LED array 10, and a PWM IC to turn on / off the switching element 11 by applying a PWM switching pulse to the switching element 11; And a sensing resistor Rs provided between the switching element 11 and the ground.

As shown in FIG. 2, the PWM IC 12 compares a reference voltage and a feedback voltage to obtain an deviation thereof, and compares an output signal and a sensing voltage of the error amplifier 123 with a comparator ( 126, an oscillator 127 for generating a reference clock, logic circuits 128, 129, 131 for comparing the comparison signal and the output signal of the oscillator 127, and determining an on / off period of a switching pulse; Transistors Q1 and Q2 for outputting a predetermined high level voltage (5V) in an on section and an output low level (0V) in an off section in response to the output signals of the fields 128, 129 and 131. The COMP terminal 121a receiving the input, the FB terminal 121b receiving the feedback signal fed back from the LED array, the CS terminal 121c receiving the sensing voltage, and the RT / CT terminal 121d receiving the reference frequency signal; GND terminal 121e connected to ground and a switch To OUT terminal (121f) and a VDD terminal (121g), and input and output terminals made of a VREF terminal (121h) that are applied with a reference voltage which is a power supply voltage output is called an output pulse is provided.

At this time, the CS terminal 121c is connected to the sensing resistor Rs, and the OUT terminal 121f is connected to the switching element 11.

Accordingly, the PWM IC 12 receives the sensing voltage Vs of the sensing resistor Rs through the sensing voltage input terminal CS, and compares the sensing voltage with the reference voltage to be applied to the switching element 11. Adjust the width of the pulse. By this, the drive current applied to the LED array 10 can be kept constant and the constant current can be controlled.

In the LED array driving apparatus, the duty of the switching pulse applied from the PWM IC 12 to the switching element 11 so that the driving voltage applied to the LED array 10 is not less than twice the input voltage Vin. The ratio should be at least 50%.

On the contrary, the LED driving voltage should be generated high, or an abnormality of the LED driving device may occur, so that the switching pulse applied from the PWM IC 12 to the switching element 11 as shown in FIG. When the ratio is close to the ratio, and the off time of the switching pulse becomes too short, the energy of the inductor L10 cannot be released sufficiently, especially in the boost type driving device, so that the inductor L10 is self-saturated, resulting in a falling edge. The portion is distorted and the peak current rises very high. In addition, due to the above-described magnetic saturation phenomenon, the inductor L10 may be ignited or the switching element 11 may be broken.

The present invention has been proposed to solve the conventional problems, the object of which is to drive a constant current by PWM switching control of the power supply, LED array driving device that can prevent the magnetic saturation phenomenon by limiting the lower limit of the off period during PWM switching To provide.

As a configuration means for realizing the above object, the LED array driving apparatus according to the present invention, the driving circuit unit for applying a predetermined driving power to the LED array through a switching transistor for switching the input power (Vin) on / off; A comparison unit which receives a voltage corresponding to the driving current applied from the driving circuit unit to the LED array and compares the reference voltage to obtain a deviation thereof; A PWM IC for generating a switching pulse whose on / off duty ratio is adjusted according to a deviation value output from the comparing unit so that the driving circuit unit is driven with a constant current, and applying the same to a switching transistor of the driving circuit unit to perform constant current control; And a duty limit circuit for adjusting the deviation value output from the comparator to a voltage within a predetermined range so that the off time of the switching pulse output from the PWM IC is not less than or equal to a preset lower limit. It is characterized by.

Hereinafter, an LED array driving apparatus according to the present invention will be described with reference to the accompanying drawings.

3 is a block diagram showing an LED array driving apparatus according to the present invention.

Referring to FIG. 3, the LED array driving apparatus according to the present invention includes a driving transistor 31 that provides driving power to the LED array 30 including a switching transistor Q30 that switches input power Vin on / off. And a PWM IC 32 which generates a switching pulse whose on / off duty ratio is adjusted to drive a constant current in response to the input deviation voltage, and outputs the switching pulse to the switching transistor Q30 of the driving circuit unit 31. A comparator 33 for receiving a voltage corresponding to the driving current applied to the LED array 30 in 31), comparing the reference voltage, and outputting the deviation voltage, and the deviation voltage output from the comparator 33. After the upper and lower limits are limited, a duty ratio limiting circuit section 34 provided to the PWM IC 32 is included.

In the above description, the driving circuit unit 31 may include an inductor L30 connected to an input power source Vin, a switching transistor Q30 connected to the inductor L30, and between the switching transistor Q30 and ground. And a sensing resistor Rs for detecting a voltage corresponding to a driving current, a pin diode D30 provided between the inductor L30 and the LED power output terminal Vled connected to the LED array 30, And a capacitor C30 provided between the cathode of the pin diode D30 and the ground.

In the driving circuit unit 31, the input power Vin having a constant amplitude is transformed into a square wave pulse by the on / off switching operation of the switching transistor Q30, which is connected to the pin diode D30 and the capacitor C30. Is applied to the LED array 30. In this case, by adjusting the on / off duty ratio of the switching transistor Q30, the driving current applied to the LED array 30 may be adjusted.

The PWM IC 32 is a PWM control circuit in the constant current mode. As shown in FIG. 2, the COMP terminal 121a receiving the comparison result, the FB terminal 121b receiving the feedback signal, and the sensing voltage are input. The CS terminal 121c to be received, the RT / CT terminal 121d to which the reference frequency signal is applied, the GND terminal 121e connected to the ground, the OUT terminal 121f to which the switching pulse is output, and the power supply voltage are outputted. Input to the VDD terminal 121g, the VREF terminal 121h for outputting the reference voltage, the buffer 122 for buffering and outputting the reference voltage, the output signal of the buffer 122, and the FB terminal 121b. Compares the received voltages and outputs the deviations, and the output voltages of the error amplifiers 123 are received through the plurality of pin diodes 124, the resistors R, and the zener diodes 125. In accordance with the comparator 126 comparing the voltage input to the CS terminal 121c and the signal input to the RT / CT terminal 121d. An oscillator 127 for generating a reference clock of a predetermined frequency, an SR flip-flop 128 that operates by receiving an output signal of the oscillator 127 in a set stage with an output signal of the comparator 126 as a reset stage; The OR of the timer 129 for maintaining a constant time of the output signal of the oscillator 127, the output signal of the oscillator 127, the output signal of the timer 129 and the output signal of the SR flip-flop 128 is OR Transistors Q1 and Q2 that operate on / off according to the gate 131 and the output signal of the or gate 131 to apply the high level signal 5V or the low level 0V signal to the OUT terminal 121f. And a comparator 133 for limiting the potential of the GNS 121e to the voltage input to the VDD 121g, and a reference voltage generator 132 for generating a reference voltage by receiving a comparison result of the comparator 133. Include. In addition, reference numeral 130 denotes a check for normal operation according to a reference voltage, and thus a detailed description thereof is omitted because it is not directly related to the operation of the present invention.

In the present invention, in the PWM IC 32 having the above-described configuration, the Comp terminal 121a is connected to the output of the comparator 33 through the duty limit circuit 34, and the VREF terminal 121h is a resistor. (R34) and the ground through the capacitor (C31), RT / CT terminal 121d is grounded through the capacitor (C31), FB terminal 121b and GND terminal 121e is connected to the ground, CS terminal ( 121c is connected to the sensing resistor Rs of the driving circuit 31 to receive the sensing voltage Vs, and the sensing resistor Rs is connected to the comparator 33. The OUT terminal 121f of the PWM IC 32 is connected to the gate of the switching transistor Q30 of the driving circuit unit 31, and the power supply V _DD is connected to the VDD terminal 121g. In other words, the LED array driving apparatus of the present invention does not use the FB terminal 121b of the PWM IC 32 described above, but generates a switching pulse using the Comp terminal 121a. There is a big difference. In addition, the voltage of the RT / CT terminal 121d becomes a sawtooth wave by the action of the resistor R34 and the capacitor C31.

Next, the comparator 33 compares the sensing voltage Vs detected by the sensing resistor Rs of the driving circuit 31 with the reference voltage Vref, and outputs a voltage corresponding to the deviation. In this case, the reference voltage Vref may use a voltage output from the VREF terminal 121h of the PWM IC 32 or may be externally provided.

The duty limiting circuit section 34 has a PNP transistor whose base is connected to the output of the comparing section 33 via a resistor R31, the collector is grounded and the emitter is connected to the Comp terminal 121a of the PWM IC 32. Q31, a resistor R32 connected between the base and the emitter of the PNP transistor Q31, and a resistor R33 provided between the emitter and the ground of the PNP transistor Q31.

Next, the operation of the LED array driving apparatus shown in FIG. 3 will be described.

The LED array driving apparatus of the present invention, in the PWM switching drive of the switching transistor (Q30) of the drive circuit unit 31 through the PWM IC 32, the Comp terminal 121a of the PWM IC 32 within a predetermined range. By limiting the deviation voltage to be input, the off time of the switching pulse is not less than the minimum time, and as a result to ensure the minimum energy release time of the inductor (L30), the magnetic saturation phenomenon does not occur.

In more detail, the driving circuit unit 31 applies an input voltage Vin to the LED array 30 through the switching transistor Q30, in which the on time and the switching time of the switching transistor Q30 are switched. By adjusting the ratio of the off time, the amount of driving current applied to the LED array 30 is adjusted.

As described above, when the driving circuit unit 31 operates to apply the driving current to the LED array 30, the sensing voltage Vs corresponding to the driving current applied by the operation of the switching transistor Q30 becomes the sensing resistor Rs. Is detected by The sensing voltage Vs is input to the comparator 33, and the comparator 33 compares the input sensing voltage Vs with the reference voltage Vref and outputs the deviation. The reference voltage Vref is set to a voltage value corresponding to the desired driving current.

The deviation voltage output from the comparator 33 is input to the PWM IC 32 through the duty limit circuit 34.

In detail, the deviation voltage output from the comparison unit 33 is applied to the base of the PNP transistor Q31 through the resistor R31 of the duty limit circuit unit 34, where the deviation voltage is applied to the PNP transistor Q30. If the threshold voltage (approximately 0.7 V) or less, the PNP transistor Q31 is turned on, and connects the Comp terminal 121a of the PWM IC 32 to ground. Therefore, the deviation voltage input value of the PWM IC 32 becomes zero. When the deviation voltage output from the comparator 33 is equal to or greater than the threshold voltage of the PNP transistor Q30, the PNP transistor Q30 is maintained in an off state, and the deviation voltage of the comparator 33 is a resistance ( The R32 is transmitted to the Comp terminal 121a of the PWM IC 32. In this case, a resistor R33 is provided between the Comp terminal 121a and the ground, so that the maximum input voltage of the Comp terminal 121a is limited to a value proportional to the resistance value of the resistor R33.

Therefore, by changing the resistance value of the resistor R33 arbitrarily, the maximum deviation voltage value input to the Comp terminal 121a can be limited.

At this time, as shown in (b) of FIG. 4, the resistance value of the resistor R33 is set such that the maximum input voltage of the Comp terminal 121a is equal to or less than the peak value of the RT / CT signal as the reference signal. Preferably, more preferably, the off time is set to be larger than the lower limit value (the minimum time at which the inductor L60 can release energy).

By the above, even if the deviation voltage more than the threshold value is output from the comparator 33, the magnitude of the deviation voltage is limited to the threshold value by the resistor R33, and is sent to the Comp terminal 121a of the PWM IC 32. Is entered.

The PWM IC 32 compares the input voltage of the Comp terminal 121a with the RT / CT value which is a reference signal generated from the oscillator 127 by an internal circuit configuration, so that the input voltage of the Comp terminal 121a is increased. A section smaller than the RT / CT is an on section of the switching pulse, and a section in which the input voltage of the Comp terminal 121a is larger than the RT / CT is set to the off section of the switching pulse, and the transistors Q1 and Q2 of FIG. 2 operate. Thus, a switching pulse having an on / off duty ratio corresponding to the set value is output.

In addition, the voltage of the Comp terminal 121a is changed in correspondence with the deviation voltage of the comparator 33 within the set range, so that the duty ratio of the switching pulse is changed in accordance with the deviation voltage. The on / off ratio of the switching transistor Q30 of the driving circuit unit 31 is varied according to the change of the duty ratio of the switching pulse, so that the average value of the driving current applied to the LED array 30 is adjusted.

According to the present invention, as shown in Fig. 4 (b), the deviation voltage within the range set by the Comp terminal 121a of the PWM IC 32 is input, so that the switching pulse output from the PWM IC 32 is The time value of the off section does not become below the minimum off time shown, and thus sufficient energy is released from the inductor L30, so that the inductor current value is changed within a stable range as shown in FIG.

As described above, the LED array driving apparatus according to the present invention, in controlling the LED driving current through the PWM switching control, by limiting the off time during PWM switching not to be less than the predetermined lower limit value, the off time is too short inductor It is possible to prevent the magnetic saturation phenomenon caused by the insufficient energy of the discharge, as a result there is an excellent effect that can prevent the element damage and fire.

Claims (5)

A driving circuit unit for applying a predetermined driving power to the LED array through a switching transistor for switching the input power Vin on / off; A comparison unit which receives a voltage corresponding to the driving current applied from the driving circuit unit to the LED array and compares the reference voltage to obtain a deviation thereof; A PWM IC for generating a switching pulse whose on / off duty ratio is adjusted according to a deviation value output from the comparing unit so that the driving circuit unit is driven with a constant current, and applying the same to a switching transistor of the driving circuit unit to perform constant current control; And a duty limit circuit for adjusting the deviation value output from the comparator to a voltage within a preset range so that the off time of the switching pulse output from the PWM IC is not less than or equal to a predetermined lower limit value and applying it to the PWM IC. , The driving circuit unit, An inductor L30 connected to an input power Vin terminal to which a predetermined level of DC power is applied; A switching transistor Q30 connected to the inductor L30; A sensing resistor Rs provided between the switching transistor Q30 and ground; A pin diode D30 provided between the inductor L30 and the LED power output terminal Vled connected to the LED array 30; It includes a capacitor (C30) provided between the cathode of the pin diode (D30) and the ground, The PWM IC, COMP terminal 121a for receiving a comparison result, FB terminal 121b to which a signal corresponding to the LED driving current fed back from the driving circuit unit is input, CS terminal 121c for receiving the sensing voltage; An RT / CT terminal 121d to which a reference frequency signal is applied; OUT terminal 121f to which a switching pulse is output, A buffer 122 for buffering a reference voltage, An error amplifier 123 for comparing the output signal of the buffer 122 with the voltage input to the FB terminal 121b and outputting a deviation thereof; A comparator 126 that receives the output voltage of the error amplifier 123 through a plurality of pin diodes 124, resistors R and zener diodes 125, and compares the voltage with the voltage input to the CS terminal 121c; , An oscillator 127 for generating a reference clock of a predetermined frequency in accordance with the signal input to the RT / CT terminal 121d; SR flip-flop 128 which operates by receiving the output signal of the comparator 126 to the reset stage and the output signal of the oscillator 127 to the set stage, A timer 129 which maintains an output signal of the oscillator 127 for a predetermined time; An OR gate 131 for ORing the output signal of the oscillator 127, the output signal of the timer 129, and the output signal of the SR flip-flop 128; Transistors Q1 and Q2 which are turned on / off according to the output signal of the or gate 131 to apply a high level signal 5V or a low level 0V signal to the OUT terminal 121f; The Comp terminal (121a) is connected so that the deviation value of the comparator is applied through the duty limiting circuit portion, and the FB terminal (121b) characterized in that the grounding device. delete delete The system of claim 1, wherein the duty limit circuit part A resistor R31 connected to an output terminal of the comparator; A PNP transistor Q31 having a base connected to the output terminal of the comparator through the resistor R31, a collector grounded, and an emitter connected to the Comp terminal 121a of the PWM IC; And a resistor (R32) connected between the base and the emitter of the PNP transistor (Q31). The system of claim 1, wherein the duty limit circuit part And a deviation voltage of the comparator so as not to be greater than an arbitrary reference voltage set below a peak value of the reference signal RT / CT in the PWM IC.

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