JP2008277496A - Led drive circuit and light-emitting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an LED drive circuit which keeps an LED driving current constant in a plurality of LEDs connected in series irrespective of a light emitting state of individual LEDs, and drives so as to avoid a mutual interference between the LEDs. <P>SOLUTION: The LED drive circuit comprises bypass circuits 51 to 5n connected in parallel between an anode electrode and a cathode electrode of each LED for a plurality of LEDs 11 to 1n, separately. Each of the bypass circuits comprises: a first input terminal T1 connected to an anode terminal of the LED; a first control input terminal T3 inputting LED control signals S01 to S0n which control a light emitting state of the LED, separately; and a first output terminal T3 connected to a cathode terminal of the LED. According to this structure, a bypass input current inputting the first input terminal can be controlled based on the LED control signals. Further, according to this structure, a current fluctuation component increasing/decreasing with the control of the light emitting state compensates for the bypass input current, and a bypass output current which is the same current with the bypass input current can be output from the first output terminal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a light emitting device, and more particularly to an LED drive circuit that drives a plurality of light emitting diode elements (hereinafter abbreviated as LEDs) connected in series constituting the light emitting device with constant current, and in particular, LED. It is related with the LED drive circuit which can adjust the light quantity of each LED uniformly, and can adjust the light quantity of each LED separately.

  As an LED drive circuit for driving a plurality of LEDs connected in series at a constant current, for example, there is a circuit disclosed in Patent Document 1 below. FIG. 4 is a circuit diagram showing an example of an LED drive circuit disclosed in Patent Document 1. As shown in FIG. As shown in FIG. 4, in the conventional circuit, bypass switches 21 to 2n are connected in parallel to the individual LEDs 11 to 1n, and the energization of the bypass switches 21 to 2n is individually controlled, whereby the LEDs 11 to 1n. Is configured to be able to individually control blinking. In FIG. 4, bypass switch transistors 21 to 2n are PNP bipolar transistors (hereinafter referred to as “PNP transistors”), and one end of base resistors 31 to 3n is connected to each PNP transistor 21 to 2n. Then, LED control signals CS1 to CSn for controlling blinking of the LEDs are input from the other end. The constant current source 4 is connected to the plurality of LEDs 11 to 1n connected in series, and the LEDs 11 to 1n are driven with a constant current.

  The conventional LED drive circuit is configured as described above, and the operation of the conventional circuit when the LED is blinked by energization control of the bypass switch transistor will be described below.

  When all the LEDs 11 to 1n are lit, the LED control signals CS1 to CSn are all open or H (high) level (the collector of the PNP transistor) with respect to the base resistors 31 to 3n of the bypass switch transistors 21 to 2n. When the potential is output, all the PNP transistors 21 to 2n for the bypass switch are turned off. As a result, since a constant current supplied from the constant current source 4 flows to the LEDs 11 to 1n, all the LEDs 11 to 1n can be lit with a uniform and stable light amount without any variation in the light amount between them. Can do.

  Further, when turning off all the LEDs 11 to 1n, the energization of the constant current source 4 is stopped, and the supply of the drive current supplied to all the LEDs 11 to 1n is stopped. Alternatively, when the LED control signals are all output to the ground potential or L (low) level state with respect to the base resistors 31 to 3n of the bypass switch transistors 21 to 2n, the bypass switch transistors 21 to 2n are all turned on. Become. As a result, the driving current supplied from the constant current source 4 is not supplied to the LEDs 11 to 1n, and all the LEDs 11 to 1n flow through the bypass line (series circuit of the bypass switch transistors 21 to 2n). .

  Further, when only a part of the LEDs 11 to 1n is turned off, for example, when only the LED 12 is turned off and other LEDs are turned on, the LED control signal CS2 corresponding to the LED 12 is used as the base resistance of the bypass switch transistor 22. 32, the ground potential or the L level state is output, and the LED control signals CS1, CS3 to CSn are respectively opened to the base resistances 31 and 33 to 3n of the other bypass switch transistors 21, 23 to 2n, respectively, in the open state or the H level. Output in a state. As a result, the base current flows only to the PNP transistor 22 for the bypass switch and is turned on, and the constant current for driving for turning on the LED 12 flows to the bypass line side and is not supplied to the LED 12, so the LED 12 is turned off. To do. Even when only the LED 12 is extinguished, the driving constant current supplied from the constant current source 4 flows only to the bypass switch transistor 22 connected in parallel to the LED 12 and bypasses, so that the other LEDs 11 and 13 to 1n are driven. Electric current flows and lights up.

JP-A-5-129665

  As shown in FIG. 4, when driving a plurality of LEDs connected in series with a constant current source, if at least one LED fails and becomes open, all other LEDs are also extinguished. A bypass switch transistor is added in parallel with the LED. However, since a part of the constant current for driving the LED is consumed as a base current in order to operate the bipolar transistor which is a bypass switch transistor, it depends on the position and number of the bypass switch transistors to be operated. The driving current flowing through the LED changes. If the current for driving the LED changes, there is a possibility of affecting the light quantity variation of the LED.

  The present invention has been made in view of the above problems, and its purpose is to maintain an LED driving current constant and avoid interference between LEDs even if the light quantity of each LED is controlled. The driving circuit is provided.

  In order to achieve the above object, an LED drive circuit according to the present invention is an LED drive circuit that drives a plurality of light emitting diode elements connected in series at a constant current regardless of the light emission state of each of the light emitting diode elements. A bypass circuit connected in parallel between the anode electrode and the cathode electrode of the light emitting diode element is provided for each of the plurality of light emitting diode elements, each of the bypass circuits being connected to the anode terminal of the light emitting diode element. A first input terminal to be connected; a first control input terminal for inputting an LED control signal for individually controlling a light emission state of the light emitting diode element; and a first output terminal connected to a cathode terminal of the light emitting diode element; The bypass input current input from the first input terminal is based on the input current or input voltage of the LED control signal. And configured to compensate for a current fluctuation that increases or decreases with the control of the light emission state with respect to the bypass input current, so that a bypass output current having the same current as the bypass input current is supplied to the first output. The first feature is that the terminal is configured to be capable of outputting.

  According to the LED drive circuit of the first feature, the bypass input current input to the bypass circuit connected in parallel from any one anode terminal of the plurality of LEDs connected in series is the same regardless of the amount of current. Since the current bypass output current is output to the cathode terminal, the current input to the parallel circuit of the LED and bypass circuit in the next stage is constant regardless of the position (order of serial connection) and the amount of current flowing through the bypass circuit ( As will be described later, the light quantity of the LED becomes constant if the bypass input current is constant, and variation in the light quantity can be suppressed. Furthermore, not only the on / off control of energization of the bypass circuit but also the amount of bypass input current can be controlled based on the input current or input voltage of the LED control signal, so that the light amount of any LED can be changed to the light amount of other LEDs. It can be adjusted continuously or stepwise without affecting it.

  In addition to the first feature, the LED drive circuit according to the present invention further includes a bypass switch circuit unit and a bypass control circuit unit, and the bypass switch circuit unit includes the first feature. A second input terminal connected to one input terminal, a second control input terminal for inputting an internal LED control signal for individually controlling the light emitting state of the light emitting diode element, and a second output terminal connected to the first output terminal The internal LED control that can control a bypass input current that is input from the second input terminal based on the input current of the internal LED control signal and that is the amount of current fluctuation with respect to the bypass input current A current obtained by adding or subtracting a signal input current is configured to be output from the second output terminal, and the bypass control circuit unit is connected to the second control input terminal. The LED input to the first control input terminal, comprising: a control output terminal to be connected; a third control input terminal connected to the first control input terminal; and a third output terminal connected to the first output terminal. An input current of the internal LED control signal controlled based on a control input current or a control input voltage of the control signal can be output from the control output terminal to the second control input terminal, and the internal LED control signal is input. A second feature is that a compensation current for compensating the current can be output from the third output terminal.

  According to the LED drive circuit of the second feature, the sum of the current output from the bypass switch circuit unit and the compensation current output from the bypass control circuit unit is the same as the bypass input current input to the bypass switch circuit unit. Therefore, the LED drive circuit of the first feature can be specifically realized, and the same operational effects can be achieved.

Note that even if the bypass output current is not completely the same as the bypass input current, the variation in the amount of light of the LED can be sufficiently suppressed, and an error within a range that can be considered substantially the same as the bypass input current is allowable. . For example, as an example for explanation, assuming that the current fluctuation of the bypass input current is 5% or less with respect to the bypass input current and the error of the compensation current is 5% or less with respect to the current fluctuation, the result As a result, the current fluctuation of the bypass input current is greatly suppressed to 0.25% or less with respect to the bypass input current. When a plurality of LEDs are connected in series, for example, when one LED closest to the constant current source is lit and when one LED farthest from the constant current source is lit, variation in the amount of light of the LEDs is in series. As the number of stages increases, and the rate of current fluctuation (current decrease) is x%, in the n-stage series connection, the light quantity variation is converted into current {(1-x / 100) ^{− (n−1)} − 1}. If the ratio x% of the current fluctuation to the bypass input current is 5% and the number of series stages is 10, the current flowing to the LED farthest from the constant current source is 1.587 times the current flowing to the LED closest to the constant current source. Thus, the current increase rate is 58.7%. However, when the error of the compensation current with respect to the current fluctuation is within ± 5% and the bypass output current is substantially the same as the bypass input current, the current ratio is 1.587 times to 1.023 times. Alternatively, it becomes 0.978 times, and the current increase rate is greatly reduced from 58.7% to + 2.3% or -2.2%. Furthermore, if the ratio x% of the current fluctuation to the bypass input current is 1% and the number of series stages is 20, the current flowing to the LED farthest from the constant current source is 1. It becomes 210 times, and the current increase rate is 21.0%. However, when the error of the compensation current with respect to the current fluctuation is within 1% and the bypass output current is substantially the same as the bypass input current, the current ratio is 1.210 times to 1.002 times or 0.999 times, the current increase rate is greatly reduced from 21.0% to + 0.2% or -0.19%. Both examples show that even if the bypass output current is not completely the same as the bypass input current, the effect of the LED drive circuit of the first feature can be obtained and the light quantity variation of the LED can be sufficiently suppressed. .

  In addition to the second feature, the LED driving circuit according to the present invention further includes a bipolar transistor in the bypass switch circuit unit, and one of the second input terminal and the second output terminal is an emitter of the bipolar transistor. And the second input terminal and the second output terminal are connected to the collector terminal of the bipolar transistor, and the second control input terminal is connected to the base terminal of the bipolar transistor. This is the third feature.

  In the LED drive circuit according to the present invention, in addition to the third feature, the bipolar transistor of the bypass switch circuit unit is a PNP bipolar transistor, and the second input terminal is connected to the emitter terminal of the bipolar transistor. The fourth feature is that the second output terminal is connected to the collector terminal of the bipolar transistor.

  According to the LED drive circuit of the third or fourth feature, a large current drive capability can be obtained as the bypass switch circuit unit. In this case, since the increase / decrease in the base current of the bipolar transistor is compensated by the compensation current output from the bypass control circuit unit, the LED drive circuit having the first feature can be specifically realized, and the same effect can be achieved. Can do.

  In addition to any one of the second to fourth features, the LED drive circuit according to the present invention further includes a control input of the LED control signal input to the first control input terminal by the bypass control circuit unit. The control transistor that controls the input current of the internal LED control signal based on the current or the control input voltage, and the current fluctuation due to the control with respect to the input current of the internal LED control signal or the input current of the internal LED control signal increased or decreased A first current mirror circuit that outputs a current after being mirrored with a constant first mirror ratio, and an output current of the first current mirror circuit with a second mirror ratio that is a reciprocal of the first mirror ratio. And a second current mirror circuit that outputs from the third output terminal.

  In the LED drive circuit according to the present invention, in addition to the fifth feature, the control transistor, the first current mirror circuit, and the second current mirror circuit are configured by a bipolar transistor or a MOS transistor. Is a sixth feature.

  In addition to the sixth feature, the LED drive circuit according to the present invention further includes the PNP bipolar transistor in the bypass switch circuit unit, and the control transistor and the first current in the bypass control circuit unit. A seventh feature is that the mirror circuit is composed of an NPN bipolar transistor, and the second current mirror circuit of the bypass control circuit section is composed of a PNP bipolar transistor.

  In addition to the sixth feature, the LED drive circuit according to the present invention further includes the PNP bipolar transistor in the bypass switch circuit unit, and the control transistor and the first current in the bypass control circuit unit. An eighth feature is that the mirror circuit is formed of an N-type MOS transistor, and the second current mirror circuit of the bypass control circuit unit is formed of a P-type MOS transistor.

  According to the LED driving circuits of the fifth to eighth features, it is possible to specifically realize a bypass control circuit unit capable of outputting a compensation current for compensating the input current of the internal LED control signal from the third output terminal. The same effect as the LED drive circuit having the first feature can be obtained.

  In addition to any of the above features, the LED drive circuit according to the present invention is further configured such that each of the bypass circuits provided separately for each of the plurality of light emitting diode elements is configured as a semiconductor integrated circuit on the same semiconductor substrate. This is the ninth feature.

  According to the LED drive circuit of the ninth feature, the bypass circuit at each stage can be formed so as to have equal electrical characteristics, so that the performance of the LED drive circuit can be improved.

  A light emitting device according to the present invention includes a plurality of light emitting diode elements connected in series, a constant current source connected in series to the plurality of light emitting diode elements, and an LED driving circuit having any one of the above characteristics. It is characterized by becoming.

  According to the light-emitting device having the above characteristics, the light-emitting device includes a plurality of LEDs connected in series, the variation in the light amount among the plurality of LEDs is suppressed, and the light amount of an arbitrary LED is changed to that of another LED. A light-emitting device that can be individually adjusted without affecting the amount of light can be realized.

  Embodiments of an LED drive circuit and a light emitting device according to the present invention (hereinafter abbreviated as “the present circuit” and “the present device” as appropriate) will be described below with reference to the drawings. In the following description, for the sake of simplicity, the same reference numerals are given to the same circuit components as those of the light emitting device having the conventional LED driving circuit shown in FIG.

  FIG. 1 shows an example of the circuit configuration of the device of the present invention, which is configured using the circuit 50 of the present invention. As shown in FIG. 1, the device of the present invention includes a plurality of light emitting diode elements (LEDs) 11 to 1n connected in series, a constant current source 4 connected in series to the plurality of LEDs 11 to 1n, and a circuit 50 of the present invention. It is configured with. FIG. 2 shows a detailed circuit configuration example of a main part of the circuit 50 of the present invention in any one stage of the plurality of LEDs 11 to 1n.

  Further, as shown in FIGS. 1 and 2, the circuit 50 of the present invention is configured so that each of the LEDs 11 to 1n is provided with bypass circuits 51 to 5n connected in parallel between the respective anode electrodes and cathode electrodes. Is done. As shown in FIG. 2, each bypass circuit 5i (i = 1 to n) includes a bypass switch circuit unit 6i (i = 1 to n) and a bypass control circuit unit 7i (i = 1 to n). Here, the subscript i in FIG. 2 represents the serial order of the plurality of LEDs connected in series, and increases by 1 from 1 to n from the side far from the constant current source 4. N indicates the number of series stages of the plurality of LEDs. Hereinafter, unless there is a particular confusion, the suffix i or (i = 1 to n) will be omitted as appropriate.

  Each bypass circuit 5 has a first input terminal T1 connected to the anode terminal of each LED1, and a first control input terminal to which an LED control signal S0i (i = 1 to n) for controlling the light emission state of each LED1 is input. T3 and a first output terminal T2 connected to the cathode terminal of each LED 1i. Each bypass switch circuit unit 6 receives a second input terminal T4 connected to the first input terminal T1 and an internal LED control signal S1i (i = 1 to n) for controlling the light emission state of each LED 1i separately. The second control input terminal T6 includes a second output terminal T5 connected to the first output terminal T2, and the bypass control circuit unit 7 further includes a control output terminal T7 connected to the second control input terminal T6, and a first output terminal T6. A third control input terminal T9 connected to the control input terminal T3 and a third output terminal T8 connected to the first output terminal T2 are provided. The terminals T1 to T9 are for the purpose of explaining the connection relationship between the circuit components for convenience, and are connected to each other (T1 and T4, T2 and T5 and T8, T6 and T7, T3. And T9) constitute a common circuit contact.

  Each bypass switch circuit section 6 includes a bypass switch PNP transistor 2i (i = 1 to n), a base resistor 3i (i = 1 to n), and a pull-up resistor 8i (i = 1 to n). Composed. The emitter terminal, the collector terminal, and the base terminal of the PNP transistor 2 are connected to one end of the second input terminal T4, the second output terminal T5, and the base resistor 3, respectively, and the other end of the base resistor 3 is the second terminal. Connected to the control input terminal T6, the pull-up resistor 8 is connected between the emitter terminal and the base terminal of the PNP transistor 2. With each configuration, each bypass switch circuit unit 6 has a bypass input current Iin input from the second input terminal T4, a base current Ib flowing from the emitter terminal of the PNP transistor 2 to the base terminal, and an emitter terminal of the PNP transistor 2 to the collector terminal. Collector current Ic flowing through the second input terminal T4 through the pull-up resistor 8 to the resistance current Ir flowing through the second control input terminal T6 (Iin = Ic + Ib + Ir), and the sum of the base current Ib and the resistance current Ir ( Ib + Ir) becomes the input current Is1 of the internal LED control signal S1i, and the collector current Ic is output from the second output terminal. Here, the input current Is1 of the internal LED control signal S1i is the amount of current fluctuation associated with the current control with respect to the bypass input current Iin. Note that when the internal LED control signal S1i is not driven from each bypass control circuit unit 7 and is in a high impedance state (open state), the pull-up resistor 8 sets the potential of the second control input terminal T6 to the second input terminal. It is provided to raise the potential to T4 and to surely turn off the PNP transistor 2. Therefore, it is sufficient if the potential of the second control input terminal T6 can be determined when the internal LED control signal S1i has a high impedance, and the resistance is set to be higher than that of the base resistor 3.

  Each bypass control circuit unit 7 receives the input current Is1 (= Ib + Ir) of the internal LED control signal S1 based on the control input current of the LED control signal S0 input to the third control input terminal T9 via the first control input terminal T3. ) And an emitter current Ie1 of the NPN transistor 10 obtained by adding the base current Ib1 of the NPN transistor 10 to the input current of the internal LED control signal. The first current mirror circuit 11 that outputs (= Is1 + Ib1) as a current mirror at a constant first mirror ratio M, and the output current of the first current mirror circuit 11 is the second mirror ratio that is the reciprocal of the first mirror ratio M. A second current mirror circuit 12 that outputs current from the third output terminal T8 after current mirroring at 1 / M. With configured. More specifically, the first current mirror circuit 11 includes two NPN transistors 11a and 11b, and the second current mirror circuit 12 includes two PNP transistors 12a and 12b. Here, the collector terminal of the NPN transistor 11a is connected to the emitter terminal of the NPN transistor 10 and the base terminals of the NPN transistors 11a and 11b, the emitter terminals of the NPN transistors 11a and 11b are grounded, and the collector of the NPN transistor 11b. The terminals are connected to the collector terminal of the PNP transistor 12b and the base terminals of the PNP transistors 12a and 12b, the emitter terminals of the PNP transistors 12a and 12b are respectively connected to the power supply terminals, and the collector terminal of the PNP transistor 12a is the third output terminal. Connected to T8.

  Accordingly, each bypass control circuit unit 7 has a current output from the third output terminal T8 as the emitter current Ie1 when the first mirror ratio M and the second mirror ratio 1 / M are accurately set by the above configuration. (= Is1 + Ib1). Since the base current Ib1 of the NPN transistor 10 is negligible as 1 / β (β is the current amplification factor of the NPN transistor 10) compared to the input current Is1 of the internal LED control signal, the control input current of the LED control signal S0 ( The input current Is1 of the internal LED control signal S1 controlled based on the base current Ib1 of the NPN transistor 10) can be output from the control output terminal T7 to the second control input terminal T6, and the internal LED control signal S1 The compensation current Ie1 for compensating the input current Is1 can be output from the third output terminal. The input current Is1 is actually drawn from the second control input terminal T6 to the control output terminal T7. However, since the control is performed by the NPN transistor 10 in the bypass control circuit unit 7, the controlled input current is controlled. Is1 is output from the control output terminal T7 to the second control input terminal T6 and transmitted to the bypass switch circuit unit 6.

  Next, the operation of the circuit of the present invention will be described. Hereinafter, for the sake of simplicity of explanation, a case will be described in which lighting or extinguishing of any one LED 1i shown in FIG. 2 is controlled. In the following description, the notation of the subscript i is omitted.

  When the LED 1 is turned on, the LED control signal S0 is input to the third control input terminal T9 of the bypass control circuit unit 7 as a ground state or an L level state. Since the third control input terminal T9 is connected to the base terminal of the NPN transistor 10 for controlling the input current Is1 of the internal LED control signal S1, the ground state or L level state of the LED control signal is output to the base terminal. As a result, the NPN transistor 10 is turned off, and the input current Is1 of the internal LED control signal S1 is cut off (Is1 = 0). Since the input current Is1 is the sum (Ib + Ir) of the base current Ib and the resistance current Ir of the PNP transistor 2 of the bypass switch circuit section 6, the base current of the PNP transistor 2 cannot be drawn, and the PNP transistor 8 is pulled out by the pull-up resistor 8. Since the base terminal 2 is pulled up to the H level, the PNP transistor 2 is turned off. As a result, the bypass input current Iin does not flow (Iin = 0), and the LED driving current is not bypassed by the PNP transistor 2 but flows through the LED 1 and the LED 1 is lit.

  Next, when the LED 1 is turned off, the LED control signal S0 is input to the third control input terminal T9 of the bypass control circuit unit 7 as an H level state. Since the third control input terminal T9 is connected to the base terminal of the NPN transistor 10 for controlling the input current Is1 of the internal LED control signal S1, the H level state of the LED control signal is output to the base terminal. The NPN transistor 10 is turned on. When the NPN transistor 10 is turned on, the input current Is1 of the internal LED control signal S1 flows, and the base current of the PNP transistor 2 of the bypass switch circuit unit 6 via the second control input terminal T6 and the control output terminal T7. Ib is extracted, and the PNP transistor 2 is turned on. As a result, the LED drive current does not flow through the LED 1 but flows as the bypass input current Iin to the bypass switch circuit unit 6 side, so that the LED 1 is turned off.

  At this time, the collector current Ic of the PNP transistor 2 is a current obtained by subtracting the input current Is1 of the internal LED control signal S1i which is the sum (Ib + Ir) of the base current Ib and the resistance current Ir from the bypass input current Iin.

  However, in the circuit 50 of the present invention, the compensation current Ie1 (current equivalent to the emitter current Ie1 of the NPN transistor 10) for compensating the input current Is1 of the internal LED control signal S1i from the bypass control circuit unit 7 is output from the third output terminal T8. It is output to the first output terminal T2. The compensation current Ie1 is substantially the same as the input current Is1 although there is an error in the base current Ib1 of the NPN transistor 10. Therefore, the compensation current Ie1 is connected to the collector current Ic of the PNP transistor 2 from the first output terminal T2 of the bypass circuit 5. A bypass output current Iout that is the sum of the compensation current Ie1 (= Iin−Is1 + Ie1≈Iin) flows to the cathode terminal of the LED1. As a result, the bypass input current Iin input from the first input terminal T1 of the bypass circuit 5 is output from the first output terminal T2 as the bypass output current Iout having substantially the same current, and returns to the series circuit side of the plurality of LEDs 11 to 1n. .

  For this reason, even when any LED 1 is turned off, the constant current characteristics of the LED drive current flowing through the front and rear stages of the LED 1 can be maintained, so that it is possible to suppress variations in the light amount of the LED between the front and rear stages of the LED 1. it can.

  Next, the effect of the compensation current Ie1 by the bypass control circuit unit 7 will be described in comparison with a case where there is no compensation current Ie1.

Assuming that there is no compensation current Ie1, in order to extinguish any LED 1i, the current from the previous stage is input as the bypass input current Iin from the first input terminal T1 of the bypass circuit 5, and the input current of the internal LED control signal S1i A bypass output current Iout ′ decreased by Is1 is output from the first output terminal T2. When this current reduction rate is x%, the current input to the anode terminal side (first input terminal T1 of the bypass circuit 5) of the LED 1i is 1 / (1 when viewed from the cathode terminal side (first output terminal T2). −x / 100) times. Accordingly, when the LED 1n closest to the constant current source 4 is turned on, the current flowing through the LED 1n is the same as the constant current Icnt of the constant current source 4, but when only the LED 11 farthest from the constant current source is turned on, Since the constant current Icnt of the constant current source 4 bypasses the LEDs 12 to 1n and flows through the PNP transistors 22 to 2n of the bypass switch circuit units 62 to 6n, the current is 1 / () at each stage of the PNP transistors 22 to 2n. The current flowing through the LED 11 farthest from the constant current source is (1−x / 100) ^{− (n−1)} times. Therefore, the light quantity variation is {(1-x / 100) ^{− (n−1)} −1} in terms of current.

  If the current reduction rate x% is 5% and the number of series stages is 10, the current flowing to the LED farthest from the constant current source is 1.587 times the current flowing closest to the constant current source, which is 58.7%. Current increase rate. If the current reduction rate x% is 1% and the number of series stages is 20, the current flowing through the LED farthest from the constant current source is 1.210 times the current flowing closest to the constant current source, and 21.0 % Current increase rate.

  Here, assuming that the compensation current Ie1 is increased by y% by increasing the base current Ib1 of the NPN transistor 10 from the input current Is1 of the internal LED control signal S1i (when the current amplification factor of the NPN transistor 10 is 100). The bypass output current Iout is expressed by the following formula 1.

[Equation 1]
Iout = Iin−Is1 + Ie1
= Iin-Is1 + Is1 (1 + y / 100)
= Iin (1-x / 100 + x / 100 (1 + y / 100))
= Iin (1 + xy / 10000)

Accordingly, the ratio of the current input to the anode terminal side of each LED 1 as viewed from the cathode terminal side is 1 / (1 + xy / 10000) from Equation 1, which is 1 / (1-x−1) when there is no compensation current Ie1. / 100), the error rate is greatly reduced. Further, the light quantity variation is {(1 + xy / 10000) ^{− (n−1)} −1} in terms of current. Specifically, in the case of x = 5, y = 1, and n = 10, it is −0.45%, which is greatly suppressed from 58.7%. Further, when x = 1, y = 1, and n = 20, it is −0.19%, which is significantly suppressed from 21.0%.

  As described above, the case where the input level H / L of the LED control signal S0 is switched to control the lighting or extinguishing of any one LED 1i has been described. However, the input level of the LED control signal S0 is limited to the H / L level. However, by adjusting to the intermediate level, it is possible to adjust the light quantity of the LED 1 continuously or stepwise while maintaining the constant current property of the LED drive current in the same manner as described above. Specifically, by adjusting the input level of the LED control signal S0 to an intermediate level, the input current Is1 of the internal LED control signal S1 is not completely cut off, but the current value of the input current Is1 when the LED 1 is turned off. The current amount of the base current Ib of the PNP transistor 2 in the bypass switch circuit unit 6 is adjusted to be smaller. As a result, the collector current Ic of the PNP transistor 2 can be adjusted, and the bypass input current Iin flowing to the bypass switch circuit unit 6 side can be adjusted. Therefore, among the constant LED drive currents, the amount of LED drive current flowing on the LED1 side is shunted. It becomes possible to adjust the light quantity of the LED 1 by changing the ratio. Thus, even when adjusting the halftone of the light quantity of the LED 1, compensation for compensating the input current Is 1 of the internal LED control signal S 1 i from the bypass control circuit unit 7, similarly to the control of turning on / off any one LED 1 i. Since the current Ie1 is output from the third output terminal T8 to the first output terminal T2, the constant current characteristic of the sum of the LED drive current amount shunted to the LED1 side and the bypass input current Iin shunted to the bypass switch circuit unit 6 side is secured. Is done.

  Further, in the above description, the operations of the bypass switch circuit unit 6 and the bypass control circuit unit 7 related to the lighting / light-off control of any one LED 1i have been described, but the position of the LED 1i related to the lighting / light-off control (series connection) The order) and the number can be arbitrarily changed. That is, in each stage, the constant current characteristic of the sum of the LED drive current amount shunted to the LED 1 side and the bypass input current Iin shunted to the bypass switch circuit unit 6 side is ensured, so the influence of the control of one stage is another Therefore, the variation in the amount of light can be suppressed, and individual light amount adjustment of the LED 1i can be performed independently. Furthermore, even if there is a variation in the light emission characteristics of the individual LEDs 1i, the variation can be corrected by individually adjusting the input level of the LED control signal S0i. The circuit 50 of the present invention can also be applied to the case where each LED 1 is a light emitting element capable of adjusting color development in addition to the amount of light depending on the amount of drive current, and can be applied to light emitting devices such as colored illumination. Become.

  Next, another embodiment of the circuit of the present invention will be described.

  <1> First, another embodiment of the bypass control circuit unit 7 will be described with reference to FIG. In the above embodiment, the control transistor 10 of the bypass control circuit unit 7 and the two transistor pairs 11a and 11b of the first current mirror circuit 11 are configured by NPN transistors, and the two transistor pairs 12a and 12b of the second current mirror circuit 12 are configured. However, these transistors are not limited to the bipolar transistors, and may be other types of active elements such as MOS transistors. For example, as shown in FIG. 3, the two transistor pairs 11 a and 11 b of the control transistor 10 of the bypass control circuit unit 7 and the first current mirror circuit 11 are configured by N-type MOS transistors, and 2 of the second current mirror circuit 12. It is also a preferred embodiment that the two transistor pairs 12a and 12b are constituted by P-type MOS transistors.

  When the control transistor 10 is a bipolar transistor, the emitter current is the sum of the collector current and the base current, and the inflow and outflow amounts of current between the collector and the emitter are not completely the same, that is, the internal LED control signal S1 Although a slight error occurs between the input current Is1 and the compensation current Ie1 (current equivalent to the emitter current Ie1 of the NPN transistor 10), when a MOS transistor is used as the control transistor 10, as shown in FIG. The source current is the same except for the leak current to the substrate side of the drain and the source, and the input current Is1 of the internal LED control signal S1 and the compensation current substantially coincide with each other. The constant current property of the drive current becomes higher. However, in the case of a MOS transistor, the element size to be used may be increased depending on the current value and voltage value used. Therefore, for example, only the control transistor 10 of the bypass control circuit unit 7 shown in FIG. 2 may be configured by an N-type MOS transistor.

  <2> Further, in each of the above embodiments, the device of the present invention includes a series circuit of a plurality of LEDs 11 to 1n and a constant current source 4 connected in series between a positive power supply potential and a ground potential (0 V). Although the configuration is such that the constant current source 4 is provided on the ground potential side, the position of the constant current source 4 may be provided on the power supply potential side. Further, the power supply potential is positive, but it may be negative. In this case, the bypass control circuit unit 7 has a circuit configuration in which a positive power supply terminal shown in FIGS. 2 and 3 is a negative power supply terminal, and a PNP transistor and an NPN transistor, or a P-type MOS transistor and an N-type MOS transistor, What is necessary is just to replace each other. Further, the bypass switch transistor 2 of the bypass switch circuit unit 6 may use an NPN transistor instead of the PNP transistor.

  <3> Further, in each of the above embodiments, the connection relationship between each transistor and resistor constituting the circuit 50 of the present invention has been particularly explained, and a specific structure has not been mentioned. The semiconductor integrated circuit is preferably configured as a semiconductor integrated circuit on the same semiconductor substrate. As a result, the bypass circuits 51 to 5n at the respective stages can be formed so as to have the same electrical characteristics, so that the performance of the circuit 50 of the present invention can be improved. Of the circuit 50 according to the present invention, only the bypass control circuit unit 7 may be configured as a semiconductor integrated circuit.

  INDUSTRIAL APPLICABILITY The present invention can be used for a light-emitting device. In particular, when a plurality of LEDs connected in series constituting the light-emitting device are driven with a constant current, the light amount of the LED can be made uniform, and the light amount of each LED can be uniformed. It is useful for LED driving circuits that can be individually adjusted.

FIG. 1 is a circuit block diagram showing a circuit configuration in an embodiment of an LED drive circuit and a light emitting device according to the present invention. 1 is a circuit diagram showing a detailed circuit configuration example of a main part in an arbitrary one stage of the LED driving circuit according to the present invention shown in FIG. The circuit diagram which shows the circuit structure in another embodiment of the bypass control circuit part of the LED drive circuit which concerns on this invention. Circuit diagram showing a circuit example of a conventional LED driving circuit for driving a plurality of LEDs connected in series with constant current

Explanation of symbols

1, 11-1n: Light emitting diode element (LED)
2, 21 to 2n: Bypass switch transistor 3, 31 to 3n: Base resistance 4: Constant current source 50: LED drive circuit according to the present invention 5, 51 to 5n: Bypass circuit 6, 61 to 6n: Bypass switch circuit section 7, 71-7n: Bypass control circuit unit 8, 81-8n: Pull-up resistor 10: Control transistor 11: First current mirror circuit 11a, 11b: Transistor pair of first current mirror circuit 12: Second current mirror circuit 12a 12b: transistor pair of the second current mirror circuit S0, S01 to S0n: LED control signal S1, S11 to S1n: internal LED control signal CS1 to CSn: LED control signal T1: first input terminal T2: first output terminal T3 : First control input terminal T4: Second input terminal T5: Second output terminal T6: Second control input terminal T7: Control output terminal T8: Third output terminal T9: Third control input terminal

Claims (10)

An LED driving circuit that drives a plurality of light emitting diode elements connected in series at a constant current regardless of the light emitting state of each of the light emitting diode elements,
A bypass circuit connected in parallel between the anode electrode and the cathode electrode of the light emitting diode element is provided for each of the plurality of light emitting diode elements,
Each of the bypass circuits includes a first input terminal connected to the anode terminal of the light emitting diode element, a first control input terminal for inputting an LED control signal for controlling the light emission state of the light emitting diode element, and the light emission. A first output terminal connected to the cathode terminal of the diode element, the bypass input current input from the first input terminal is configured to be controllable based on an input current or an input voltage of the LED control signal, and Compensating for a current fluctuation that increases or decreases with control of the light emission state with respect to the bypass input current, a bypass output current having the same current as the bypass input current can be output from the first output terminal. An LED drive circuit characterized by that. Each of the bypass circuits comprises a bypass switch circuit unit and a bypass control circuit unit,
The bypass switch circuit unit has a second input terminal connected to the first input terminal, a second control input terminal to which an internal LED control signal for controlling the light emitting state of the light emitting diode element is input, and the first A second output terminal connected to the output terminal, the bypass input current input from the second input terminal can be controlled based on the input current of the internal LED control signal, and the bypass input current A current obtained by adding or subtracting the input current of the internal LED control signal as a current fluctuation component can be output from the second output terminal,
The bypass control circuit unit includes a control output terminal connected to the second control input terminal, a third control input terminal connected to the first control input terminal, and a third output terminal connected to the first output terminal. An input current of the internal LED control signal controlled based on a control input current or a control input voltage of the LED control signal input to the first control input terminal from the control output terminal to the second control input 2. The LED drive circuit according to claim 1, wherein the LED drive circuit is configured to be able to output to a terminal and to output a compensation current for compensating an input current of the internal LED control signal from the third output terminal.   The bypass switch circuit unit includes a bipolar transistor, one of the second input terminal and the second output terminal is connected to an emitter terminal of the bipolar transistor, and the other of the second input terminal and the second output terminal is the 3. The LED drive circuit according to claim 2, wherein the LED drive circuit is connected to a collector terminal of a bipolar transistor, and the second control input terminal is connected to a base terminal of the bipolar transistor.   The bipolar transistor of the bypass switch circuit section is a PNP type bipolar transistor, the second input terminal is connected to the emitter terminal of the bipolar transistor, and the second output terminal is connected to the collector terminal of the bipolar transistor. The LED driving circuit according to claim 3. The bypass control circuit unit is
A control transistor for controlling an input current of the internal LED control signal based on a control input current or a control input voltage of the LED control signal input to the first control input terminal;
A first current mirror circuit for current mirroring and outputting a current after an increase / decrease in current fluctuation accompanying control of the input current of the internal LED control signal or the input current of the internal LED control signal at a constant first mirror ratio; ,
A second current mirror circuit that current mirrors the output current of the first current mirror circuit at a second mirror ratio that is a reciprocal of the first mirror ratio and outputs the current from the third output terminal;
The LED drive circuit according to any one of claims 2 to 4, wherein the LED drive circuit is provided.   6. The LED driving circuit according to claim 5, wherein the control transistor, the first current mirror circuit, and the second current mirror circuit are configured by bipolar transistors or MOS transistors. The bypass switch circuit unit is configured to include a PNP-type bipolar transistor,
The control transistor of the bypass control circuit unit and the first current mirror circuit are composed of NPN bipolar transistors,
The LED driving circuit according to claim 6, wherein the second current mirror circuit of the bypass control circuit unit is configured by a PNP-type bipolar transistor. The bypass switch circuit unit is configured to include a PNP-type bipolar transistor,
The control transistor and the first current mirror circuit of the bypass control circuit unit are configured by N-type MOS transistors,
The LED drive circuit according to claim 6, wherein the second current mirror circuit of the bypass control circuit unit is configured by a P-type MOS transistor.   9. The semiconductor integrated circuit according to claim 1, wherein each of the bypass circuits provided in each of the plurality of light emitting diode elements is configured as a semiconductor integrated circuit on the same semiconductor substrate. LED drive circuit. A plurality of light emitting diode elements connected in series;
A constant current source connected in series to the plurality of light emitting diode elements;
The LED drive circuit according to any one of claims 1 to 9,
A light-emitting device comprising:

Images