JP2008205276A - Led driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an LED driving circuit capable of controlling lighting/putting out lights at a high speed by using a current pulse with a narrow width and of supplying a current-carrying current of high precision to an LED series connection circuit. <P>SOLUTION: The LED driving circuit includes: an LED series connection circuit 12 which serially connects many LEDs; a first switching device 13 which controls a current-carrying current of the LED series connection circuit 12; a second switching device 14 in which a cascade connection is carried out between the LED series connection circuit 12 and the first switching device 13; a current detecting resistor 15 which is connected between the first switching device 13 and a ground terminal; a buffer amplifier 19 which is connected to a base terminal of the first switching device 13; a multiplexer 20 which is connected to an input of the buffer amplifier 19 and switches an LED ON signal and an LED OFF signal; and a lighting time control circuit 24 which forms time durations of the LED ON signal and the LED OFF signal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an LED drive circuit that performs on / off control of an energization current of an LED series connection circuit in which a large number of LEDs are connected in series, and turns on and off a large number of LEDs at once.

Conventionally, in lighting devices and the like, LED panels in which a large number of LEDs are arranged in a matrix on a plane are used. In such an LED panel, a large number of LEDs are connected in series, and the ON / OFF control of the energization current using a switching element (transistor) of the drive circuit controls the lighting / extinction of the large number of LEDs collectively. Things have been done.
JP-A-6-77532 JP 2004-241908 A

  FIG. 6 is an example of a conventional general LED drive circuit. A large number (n) of LEDs are connected in series to form an LED series connection circuit 12, and a DC power supply 11 and a switching element (transistor) 13 are connected in series thereto. An inverter circuit 13a is connected to the control terminal (base) of the switching element 13 and a control signal for controlling on / off of the switching element 13 is supplied. When the ON signal voltage is supplied to the switching element 13, the collector-emitter of the switching element 13 is turned on, an energizing current flows from the power supply 11, and a large number of LEDs are turned on collectively. When the off signal voltage is supplied to the switching element 13, the collector-emitter between the switching elements 13 is turned off, the energization current from the power source 11 is cut off, and a large number of LEDs are turned off collectively.

  However, in the conventional LED drive circuit, since the control voltage input to the control terminal of the transistor 13 uses the inverter circuit 13a, the control voltage cannot be set in a wide range due to this restriction, and the control accuracy is not good. There is a problem. In addition, in the LED driving circuit, since a large number of LEDs are connected in series, the wiring length becomes long, and the stray inductance and stray capacitance are large. For example, the lighting / extinguishing control can be performed at high speed using a narrow current pulse of 10 nS unit. There was a problem that was difficult.

  The present invention has been made based on the above-mentioned circumstances. For example, it is possible to control lighting / extinguishing at high speed using a narrow current pulse in units of 10 nS, and to supply a highly accurate energization current to the LED series connection circuit. It is an object of the present invention to provide an LED driving circuit that can be used.

  The LED drive circuit of the present invention includes an LED series connection circuit in which a large number of LEDs are connected in series, a first switching element that controls an energization current of the LED series connection circuit, and the LED series connection circuit and the switching element. A second switching element cascode-connected to the first switching element, a current detection resistor connected between the first switching element and a ground terminal, a buffer amplifier connected to a base terminal of the switching element, and a buffer amplifier A multiplexer is connected to the input and switches between an LED on signal and an off signal, and a lighting time control circuit that forms a time between the LED on signal and the off signal.

  According to the LED drive circuit of the present invention, since the LED on signal and the off signal are switched using the multiplexer, the high-speed multiplexer and the buffer amplifier having a wide frequency band are used, so that the first switching element can be quickly turned on and off. Is possible. Then, the so-called Miller effect is prevented by cascode-connecting the second switching element having a DC bias voltage applied to the base terminal between the LED series connection circuit and the first switching element, and the LED series connection circuit. The energization current can be turned on / off at high speed.

  Furthermore, by connecting a capacitor in parallel with the current detection resistor, the charge of the stray capacitance of the LED series connection circuit can be released to the capacitor, thereby enabling a high-speed switching operation. Further, the lighting time control circuit includes a circuit for setting the LED on time and the LED off time, and a counter for forming a variable length pulse of the on time and the off time, so that an arbitrary time interval of, for example, 10 nS can be provided. The length on / off time can be set. In addition, by connecting the output of the D / A converter to the ON signal terminal of the multiplexer, it is possible to control the energizing current over a wide range and with high precision, and by connecting the ground voltage to the OFF signal terminal of the multiplexer, high speed Can be turned off.

  Therefore, according to the LED drive circuit of the present invention, the LED series connection circuit can be turned on and off at high speed, and a highly accurate energization current can be supplied to the LED series connection circuit.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows an LED driving circuit according to an embodiment of the present invention.

  The LED drive circuit includes a DC power source 11, an LED series connection circuit 12 in which a large number of LEDs are connected in series, a first switching element (transistor) 13 that controls the energization current of the LED series connection circuit 12, and the LED series connection circuit 12. The second switching element (transistor) 14 that is cascode-connected between the switching element 13 and the switching element 13 is connected between the emitter of the transistor 13 and the ground terminal (GND), and the energization current of the LED series connection circuit 12 is detected. And a current detection resistor 15 for generating a detection voltage of the energization current.

  In this embodiment, the LED series connection circuit 12 is a two-terminal circuit in which m columns of circuits in which n LEDs are connected in series are connected in parallel, and a total of n × m LEDs are connected in series and parallel. It is preferable to insert a resistor r in each circuit in which LEDs are connected in series. As a result, even if there is a variation in the LED forward voltage Vf of each LED series connection circuit, it is possible to pass a substantially uniform energization current to each LED series connection circuit, ensuring uniformity of brightness as a surface light source. can do. The number n of LEDs connected in series and the number m of parallel connections thereof may be natural numbers (1, 2, 3,...), Respectively. For example, when m = 1, only one circuit row is driven.

  Since the LED series connection circuit 12 is connected in series to the DC power supply 11 and the switching element (transistor) 13, when the transistor 13 is turned on, an energization current flows through each LED series connection circuit almost evenly, and all the LEDs are connected. When the transistor 13 is turned on and the transistor 13 is turned off, the energization current is cut off and all the LEDs are turned off. Therefore, all the LEDs can be turned on / off collectively by controlling the on / off of the transistor 13, and all the LEDs can be brightened all at once by controlling the magnitude of the energization current when turned on. Can be controlled.

  For this reason, the power supply voltage Vcc of the DC power supply 11 includes the lighting voltage (forward voltage Vf × n) of n LEDs for one column connected in series, the ON voltage of the switching elements 13 and 14, and the current detection resistor. This is the total voltage with the voltage formed in the vessel 15.

Next, a driving circuit for the transistor 13 will be described. The output of a wideband buffer amplifier 19 having a bandwidth of about 350 MHz is connected to the base terminal of the transistor 13, and power supplies of + V _DD and −V _DD are supplied to the buffer amplifier 19, and the analog voltage is approximately within this voltage range. Can be output. The output of the high speed multiplexer 20 having 10 nS switching and 250 MHz bandwidth is connected to the input of the buffer amplifier 19. The multiplexer 20 switches the LED on (lighting) signal of the input terminal 20a and the LED off (lights off) signal of the input terminal 20b under the control of the controller 20c and outputs them.

  An 8-bit D / A converter 21 and an 8-bit brightness setting circuit 22 are connected to the input terminal 20 a of the multiplexer 20. Therefore, 256 levels of analog voltage can be output from the D / A converter 21 by a combination of 8-bit digital signals from the brightness setting circuit 22. A ground terminal is connected to the other input terminal 20b of the multiplexer 20, and a GND voltage is output. Note that a negative voltage is connected to the input terminal 20b, and a current can be extracted from the base terminal of the transistor 13, whereby a faster light-off operation can be performed.

  A counter 24 is connected to the controller 20c to control the on (lighting) and off (light extinguishing) time of the LED series connection circuit. That is, when an ON signal is output from the controller 20c, the output of the multiplexer 20 is switched to the input terminal 20a, and the analog voltage output from the D / A converter 21 is applied to the base terminal of the switching element 13 via the buffer amplifier 19. The supplied current corresponding to the base voltage flows to the LED series connection circuit 12 during the ON signal period. When the off signal is output from the controller 20c, the output of the multiplexer 20 is switched to the input terminal 20b, the GND voltage is supplied to the base terminal of the transistor 13 via the buffer amplifier 19, the transistor 13 is turned off, and the off signal During the period, the energization current flowing through the LED series connection circuit 12 is interrupted.

  That is, the LED lighting cycle time and duty ratio are set in the on / off time setting circuits 23a and 23b, and the counter 24 counts, for example, a unit time of 10 nS pulses from the clock source 25, and sets it in the on / off time setting circuit. The variable on-time and off-time pulses of the LED are formed and output to the controller 20c. Therefore, the controller 20c switches the input terminal of the multiplexer at the timing set by the LED on-time setting circuit 23a and the off-time setting circuit 23b, and outputs the LED on signal and the off signal.

  As a result, the ON time and OFF time of the LED can be set in an integer multiple (N times) of 10 nS in the range of 0 to 48H, and the blinking period can be set in units of 10 nS in the range of 20 nS to 48H. Therefore, the duty ratio, which is the ratio between the blinking period and the lighting time, can also be changed. However, the integer multiple N is about 0 to 2 ^ 48.

Next, the operation of the current detection resistor 15 will be described. FIG. 2 shows an equivalent circuit around the first switching element (transistor) 13 when the resistance of the resistor 15 is R. The relationship among the base voltage Vb, the emitter voltage Ve, the collector current Ic, the emitter current Ie, and the base current Ib of the transistor 13 is as shown in equations (1)-(3).
Vb = Vbe + R × Ie (1)
Where Vbe is the base-emitter voltage of the transistor, Ve = R × Ie
Ie = Ib + Ic (2)
Ic = h _FE × Ib (3)
Where h _FE is the transistor current gain

Therefore, from (1)
Ie = (Vb−Vbe) / R (4)
(2) From (3)
Ie = (1 / h _FE + 1) × Ic (5)
Incidentally, for example, the transistor _(2SC5610), since _{h FE} is 150 to 300, a _{(1 / h FE +1) ≒} 1,
Ie≈Ic (6)
Therefore,
Ic≈ (Vb−Vbe) / R (7)
It becomes.

  For example, in the transistor (2SC5610), Vbe is 0.7-1.0V, Vb can be finely adjusted in the range of 0-4.5V, and (Vb-Vbe) is constant, the collector current Ic is the resistance R Is approximately inversely proportional to For example, if (Vb−Vbe) is adjusted to 3V and the resistance R is 1Ω, the collector current Ic is 3A, and if the resistance R is 10Ω, the collector current Ic is 0.3A and the base voltage Vb is constant. A constant current circuit is formed.

However, the base voltage Vb can be adjusted as follows. That is, an 8-bit D / A converter 21 and an 8-bit brightness setting circuit 22 are connected to the input terminal 20a of the multiplexer 20, and the combination of the 8-bit digital signals of the brightness setting circuit 22 determines D An analog voltage output in 256 steps from the / A converter 21 at equal intervals is output to the base terminal of the transistor 13 via the buffer amplifier 19. Therefore, in this embodiment, the 8-bit D / A converter 21 and the 8-bit brightness setting circuit 22 are based on 256 steps at equal intervals in the range of the power supply + V _DD and −V _DD of the buffer amplifier 19. It is possible to set the voltage Vb.

  As an example, the base voltage Vb can be adjusted in the range of 0 to 4.5 V, and accordingly, the energization current (collector current Ic) of the LED series connection circuit 12 can be adjusted according to the equation (7). For example, if the resistance R is 1Ω and (Vb−Vbe) is 1V, an energization current of 1A is obtained, and if (Vb−Vbe) is 3V, an energization current of 3A is obtained. Thus, by providing the D / A converter 21, it is possible to adjust the energization current with high accuracy and in a wide range.

  Next, the operation of the cascode-connected transistor 14 will be described. As shown in FIG. 3A, in the conventional circuit configuration, the mirror effect is generated by the parasitic capacitance Cbc1 between the collector and the base of the transistor 13, the cut-off frequency is lowered in the frequency characteristics of the circuit, and the switching speed is lowered. There is a problem.

That is, an equivalent circuit of a conventional transistor grounding circuit with a common emitter is as shown in FIG. Therefore, the apparent input capacitance Ci of the transistor 13 is
Ci = Cbc1 × (1 + Av)
It becomes.
Where Cbc 1 is the collector-base capacitance of the transistor 13, Av is the voltage gain of the transistor 13.
Therefore, the voltage gain A1 of the equivalent amplifier circuit shown in FIG.
A1 = Vo / Vs = Av / (1 + 2πf × Ci × Rs × j)
It is represented by Where f: frequency, Rs: signal source internal resistance, j: imaginary number.

As shown in FIG. 1, the LED drive circuit includes a second switching element (transistor) 14 that is cascode-connected between the LED series connection circuit 12 and the switching element 13. For this reason, since the collector voltage Vc1 of the transistor 13 is cascode-connected to the transistor 14,
Vc1 = Vbi-Vbe2
And is fixed at a constant value. Where Vbi is the base bias voltage of the transistor 14, and Vbe2 is the base-emitter voltage of the transistor 14.

The voltage between the collector terminal and the base terminal of the transistor 13 is Vc1-Vb1, and the mirror effect does not occur. Therefore, the apparent input capacitance at the base terminal is also Cbc1, and its equivalent circuit is as shown in FIG.
Therefore, the voltage gain A2 of the equivalent amplifier circuit is
A2 = Vo / Vs = Av / (1 + 2πf × Cbc1 × Rs × j)
It is represented by Where f: frequency, Rs: signal source internal resistance, j: imaginary number.

The cutoff frequency is fc1, and the cascode connection example is fc2, respectively.
fc1 = 1 / (2π × Ci × Rs)
fc2 = 1 / (2π × Cbc1 × Rs)
It becomes.

Therefore, when the ratio between the cutoff frequency fc2 of this example and the cutoff frequency fc1 of the conventional example is calculated,
fc2 / fc1 = Ci / Cbc1 = 1 + A
Thus, the cutoff frequency is improved by an amount corresponding to the voltage gain A. In other words, a narrow pulse current can be applied, and the LED can be turned on / off at high speed.

Next, the diode 26 in FIG. 1 will be described. In this LED drive circuit, a diode 26 is connected in parallel to the LED series connection circuit 12. Generally, there is a parasitic inductance in the wiring, but the LED series connection circuit 12 is a circuit in which a large number of LEDs are connected in series, the wiring length is particularly long, and there is a large parasitic inductance. For this reason, an equivalent circuit as shown in FIG. 4A is obtained. When the equivalent parasitic inductance of the LED series connection circuit 12 is L, the counter electromotive voltage Vr caused by the parasitic inductance L when the LED is turned off. Will occur.
Vr = L × (ΔIc / Δt)

This counter electromotive voltage Vr becomes high particularly during high-speed switching, and the voltage Vsw applied to the collector of the transistor 13 is as follows.
Vsw = Vcc + Vr−Vf × n
However, Vcc: power source voltage, Vf: forward voltage of the LED, n: LED number Therefore, the voltage Vsw applied to the collector of the transistor 13 exceeds the absolute maximum rated voltage _{V CEO} between the collector and the emitter of the transistor 13 The transistor 13 is damaged.

  As shown in FIG. 4B, by connecting the diode 26 in parallel to the LED series connection circuit 12, even if the back electromotive force Vr is generated, it can be released as a circulating current through the diode 26, The back electromotive force Vr is not applied between the collector and emitter of the transistor 13. Therefore, by connecting the diode 26 in parallel to the LED series connection circuit 12, the diode 26 constitutes a bypass circuit for the high-frequency component of the energization current, which is considered to contribute to speeding up the LED drive circuit. .

  Next, the capacitor 27 in FIG. 1 will be described. In general, the wiring has a stray capacitance between GND and the like, but the LED series connection circuit 12 is a circuit in which a large number of LEDs are connected in series, and the wiring length is particularly long, and a large stray capacitance exists. For this reason, even if the switching element is turned on / off to turn on / off the LED, a delay time occurs when the LED is actually turned on / off, and cannot be turned on / off at high speed (short time). There is a problem. In other words, the charge time of the stray capacitance is the delay time.

An equivalent circuit of the conventional example is shown in FIG. If the stray capacitance is Cf, the energization current (collector current) is Ic, the voltage change of the stray capacitance Cf that turns the LED from the unlit state to the lit state is ΔV, and the current flowing through the LED is ignored,
Ton = ΔV × Cf / Ic
This is the delay time. For example, when ΔV = 5 V, Cf = 1000 pF, and Ic = 10 mA, the delay time Ton is 5 × 10 ⁻⁷ (sec).

In the present invention, in order to shorten this delay time, a capacitor 27 (capacitance C) is connected in parallel to the LED series connection circuit 12. This equivalent circuit is shown in FIGS. The transient response when the LED is switched from the OFF state to the ON state is as shown in FIG. 5C from the additional capacitor C via the transistor 13 (ON resistance Ron) in which the charge (initial voltage V1) of the stray capacitance Cf is ON. It is considered to flow in and can be expressed by the following equation.
Cf × Vf = Cf × V1 × (1 + exp (−t × 2 / Ron / C)) / 2

For simplicity, if C = Cf and the current flowing through the LED is ignored,
Vf = V1 × (1 + exp (−t × 2 / Ron / C)) / 2
Solving this in t (seconds)
t = Ron × C × ln (V1 / (2 × Vf−V1)) / 2
It becomes.

For example, assuming that Ron = 100 mΩ, C = 1000 pF, Cf change voltage ΔV = V1 / 3, and the time from the turn-off state to the lighting state is Ton,
Ton = 5.5 × 10 ⁻¹¹ (seconds)
It becomes.
Therefore, by connecting the capacitor C in parallel with the current detection resistor, it becomes possible to perform switching approximately 9000 times faster.

  On the other hand, when the LED is turned off, since the capacitor 27 is in a charged state, when a low voltage (for example, a GND voltage) is applied to the base of the transistor 13, the voltage across the terminal (VC) of the capacitor 27 is Since the reverse bias voltage is set, the transistor 13 can be switched to the OFF state at high speed. Therefore, the LED can be turned off in a short time (high speed).

  Although one embodiment of the present invention has been described so far, it is needless to say that the present invention is not limited to the above-described embodiment, and may be implemented in various forms within the scope of the technical idea.

It is a circuit diagram of the LED drive circuit of one Embodiment of this invention. It is an equivalent circuit diagram around the resistor for current detection. It is a circuit diagram explaining the operation of a cascode-connected transistor. It is a circuit diagram explaining the operation | movement of the diode connected in parallel with the LED serial connection circuit. It is a circuit diagram explaining operation | movement of the capacitor | condenser connected in parallel with the resistor for electric current detection. It is a circuit diagram which shows an example of the conventional LED drive circuit.

Explanation of symbols

11 DC power supply 12 LED series connection circuit (LED array)
13 Switching elements (transistors)
13a Inverter circuit 14 Switching element (transistor)
DESCRIPTION OF SYMBOLS 15 Current detection resistor 19 Buffer amplifier 20 Multiplexer 20a, 20b Input terminal 20c Controller 21 D / A converter 22 Brightness setting circuit 23a, 23b On / off time setting circuit 24 Counter (lighting time control circuit)
25 Clock source 26 Diode 27 Capacitor

Claims (6)

An LED series connection circuit in which a large number of LEDs are connected in series;
A first switching element that controls an energization current of the LED series connection circuit;
A second switching element cascode-connected between the LED series connection circuit and the switching element;
A current detection resistor connected between the first switching element and a ground terminal;
A buffer amplifier connected to a base terminal of the switching element;
A multiplexer connected to the input of the buffer amplifier for switching between an LED on signal and an off signal;
A lighting time control circuit for forming a time between the LED on signal and the off signal;
An LED driving circuit comprising:   The LED driving circuit according to claim 1, wherein a capacitor is connected in parallel with the current detection resistor. The lighting time control circuit includes a circuit for setting an LED on time and an LED off time;
A counter that forms a variable length pulse of the on-time and off-time,
2. The LED driving circuit according to claim 1, wherein the multiplexer outputs an ON signal to the buffer amplifier during the ON time, and the multiplexer outputs an OFF signal to the buffer amplifier during the OFF time. The output of the D / A converter is connected to the ON signal terminal of the multiplexer.
2. The LED driving circuit according to claim 1, wherein a ground voltage is connected to each off signal terminal of the multiplexer. An LED series connection circuit in which a large number of LEDs are connected in series;
A first switching element that controls an energization current of the LED series connection circuit;
A current detection resistor connected between the first switching element and a ground terminal;
A capacitor connected in parallel with the current detection resistor;
An LED driving circuit comprising:   2. The LED drive circuit according to claim 1, wherein a diode is connected in parallel with the LED series connection circuit.

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