JP4669382B2 - LED drive circuit - Google Patents

Description

  The present invention relates to an LED drive circuit that applies a constant current to a plurality of arranged LEDs using a current mirror circuit.

For example, various devices using a plurality of LEDs arranged in a plane as a light source can be seen. Here, as the LED drive circuit, a circuit device is generally known in which a so-called current mirror circuit for supplying a constant current is added to a surface-arranged LED array. FIG. 7 shows an example of a device comprising an LED array and a current mirror circuit. The DC-DC converter circuit 1 obtains a variable output voltage with a control voltage, one LED, or two or more LEDs connected in series. The LED column circuit 2 in which a plurality of LED columns are arranged, and a plurality of columns corresponding to the LED columns are formed, and the transistor bases of the other column are connected in common to the transistor bases of the reference column transistors 31T. 1 shows an LED driving circuit comprising a current mirror circuit 3 for passing a constant current. Each circuit of FIG. 7 will be described more specifically. In the DC-DC converter circuit 1, the voltage at the ground resistance 31R of the transistor 31T of the reference column 31 of the current mirror circuit 3 is applied, and this voltage is used as a control voltage to drive the LED from the input voltage so that the control voltage becomes constant. An output voltage, which is a power supply voltage for use, is obtained. In FIG. 7, the LED string circuit 2 has a configuration in which three LED strings 21, 22, and 23 including two LEDs are arranged, and each of the LED strings 21, 22, and 23 has a DC-DC converter circuit. An output voltage of 1 is applied as a power supply voltage. Further, the current mirror circuit has a plurality of columns 31, 32, 33 corresponding to the LED columns 21, 22, 23 of the LED column circuit 2, and each of the columns 31, 32, 33 includes the LED columns 21, 22, 33, respectively. 23, transistors 31T, 32T, and 33T for controlling energization of the reference circuit transistor 31T, of which the base collector is short-circuited, and the base of the reference column transistor 31T is connected to the transistors 32T of the other columns 32 and 33. 33T is connected in common to the base of the 33T, and in addition, each column has a ground resistance (emitter resistance) 31R, 32R, 33R, and the base voltage / current branched from the collector of the reference column transistor 31T and the other columns 32, The base voltages and currents of 33 transistors 32T and 33T are made equal to allow a constant current to flow through each LED array.
JP 2002-237745 A

Incidentally, in the prior art shown in FIG. 7 described above, the LED column circuit 21 in the reference column may cause an open failure. That is, in the LED row circuit 21 of the reference row, an open circuit failure may occur such as a line break or an LED break. In this case, the collector of the transistor 31T in the reference column 31 is not connected to the power source, and the base voltage / current is not supplied. For this reason, there is a problem in that the base voltages and currents of the transistors 32T and 33T in the other columns 32 and 33 are not supplied, and all the LEDs are turned off.
As a circuit that can solve the problem that all LEDs are turned off due to an open failure of the LED array circuit of the reference array, there is also a circuit as shown in Patent Document 1. Since this circuit includes a current mirror circuit including a reference row that is not connected to the LED row, it is possible to prevent all LEDs from being turned off even if an open failure occurs in the LED row circuit of the reference row. However, since this reference row consumes power that does not energize the LEDs, wasteful power consumption occurs.

  The present invention has been invented in view of the above-described problems, and it is an LED drive circuit that eliminates a reference row that causes unnecessary power consumption without turning off all LEDs even if there is an open failure of the reference row. For the purpose of provision.

  The present invention that achieves the above-described object includes an LED string circuit in which each end of an LED string composed of one LED or two or more LEDs connected in series is connected to each other, and each LED string of the LED string circuit Each of the transistors includes a transistor connected in series with the other end, and one of the transistors is a reference column transistor that connects a connection point of the LED column and a control electrode, and applies a control voltage to the output voltage variable power source. A current mirror circuit in which the control electrode of the reference column transistor is commonly connected to the other control electrodes of the transistor, and is connected to the reference column transistor in the LED driving circuit connected to the output terminal of the output voltage variable power source. Open fault detection circuit for detecting open faults in the LED array, and fault detection output by this open fault detection circuit Answer to is characterized in that and a current switching circuit for switching the current path of the current mirror circuit.

  According to the present invention, the drive of the reference column transistor is ensured by detecting the open failure of the reference row and using the normal LED row as the reference row or switching the normal reference row transistor. The LEDs of all LED rows can be turned on except for the fault row as a failure has occurred.

Embodiments of the present invention will be described below with reference to FIGS. 1 to 6, the same parts as those in FIG. 7 are denoted by the same reference numerals.
[First Embodiment]
The LED drive circuit shown in FIG. 1A includes a DC-DC converter circuit 1, an LED array circuit 2, and a current mirror circuit 3, which are output voltage variable power supplies. The DC-DC converter circuit 1 obtains an output voltage which is a power supply voltage for LED driving from an input voltage. In this case, the control voltage is extracted from the ground resistance 31R of the reference string transistor 31T connected to the LED string 21, so that the current flowing through the LED string 21 is constant, in other words, the voltage drop of the ground resistance 31R is constant. Thus, the output voltage is controlled by this control voltage.

  FIG. 2 shows a simplified example of the internal circuit of the DC-DC converter circuit 1. The DC-DC converter circuit 1 includes a DC-DC converter IC 11, in which the error amplifier 111 amplifies the difference between the reference voltage of the reference voltage generation circuit 112 and the control voltage, and the difference voltage is compared with the comparator 113. To enter. The comparator 113 compares this difference voltage with the triangular wave of the triangular wave generating circuit 114, and the comparison result becomes a PWM waveform, which is output to the FET drive driver 115 to control the FET 12 on and off. By the on / off operation of the FET 12, the output voltage of the DC-DC converter circuit 1 is controlled to be a voltage corresponding to the control voltage. The DC-DC converter circuit 1 operates as a constant current source by using the voltage of the ground resistor 31R in FIG. 1 as a control voltage. The configuration of the DC-DC converter circuit 1 is the same in the DC-DC converter circuit 1 shown in FIG. 3 in the second embodiment described later and in FIG. 4A in the third embodiment.

Returning to FIG. 1A, the LED column circuit 2 in which the output voltage of the DC-DC converter circuit 1 is used as the power supply voltage for LED driving includes three LED columns 21, 22, and 23 in this embodiment. Illustrated, two LEDs are connected in series for each column. Note that the number of LEDs connected in series and the number of LED rows can be increased or decreased as necessary.
The current mirror circuit 3 has a plurality of columns 31, 32, 33 connected corresponding to the LED columns 21, 22, 23 of the LED column circuit 2, and transistors corresponding to the columns 31, 32, 33. 31T, 32T, and 33T, and ground resistors 31R, 32R, and 33R having the same resistance value connected to the emitters of the transistors 31T, 32T, and 33T. Therefore, each LED row 21, 22, 23 is connected to each transistor 31T, 32T, 33T. Here, the transistor 31T is a reference column transistor whose collector base is short-circuited. The base (control electrode) of the reference column transistor 31T is common to the bases of the transistors 32T and 33T in the other columns 32 and 33. Connected. For this reason, the base voltages and currents of the transistors 31T, 32T, and 33T are equalized, the collector currents are equalized, and the LED strings 21, 22, and 23 can be illuminated uniformly. The ground resistor 31R connected to the reference column transistor 31T is connected to the DC-DC converter 1, and the control voltage is taken into the DC-DC converter 1 as described above.

  The LED drive circuit shown in FIG. 1A further includes an open failure detection circuit 4 and a current switching circuit 5. The open fault detection circuit 4 includes a voltage comparator 411, a reference voltage circuit 412, and a state holding circuit 413. A voltage at one input terminal of the voltage comparator 411 is a connection point between the LED column 21 and the reference column transistor 31T. The reference voltage of the reference voltage circuit 412 is applied to the other input terminals. Here, the reference voltage is a voltage commensurate with the voltage drop of the transistor 31T and the ground resistor 31R when a constant current flows through the reference column transistor 31T. When the LED string 21 is normal without failure and a normal voltage is applied to one input terminal of the voltage comparator 411 connected to the LED string 21, the output of the voltage comparator 411 becomes "L", while the other LED string When 21 is an open failure and the voltage at one input terminal decreases, the output of the voltage comparator 411 becomes “H”. When the output of the voltage comparator 411 becomes “H” at the time of an open failure, the state holding circuit 413 holds the “H” state and drives the current switching circuit 5. FIG. 1B shows a specific circuit block of the open fault detection circuit 4 shown in FIG. 1A. The voltage comparator 411 is a comparator circuit, and the state holding circuit 413 is a D-type flip-flop. It is said.

  The current switching circuit 5 is a switching means, and the switch is closed by the “H” output of the state holding circuit 413 described above. The current switching circuit 5 is connected to the output voltage line of the DC-DC converter circuit 1 for supplying current to the LED string circuit 2 on the one hand via the resistor Rc, and on the other hand to the reference string of the current mirror circuit 3. It is connected to a connection point between the transistor 31T and the LED string 21. When the “H” signal from the state holding circuit 413 is input to the current switching circuit 5, the output voltage of the DC-DC converter circuit 1 and the reference column transistor 31T are connected by closing the switch, via the resistor Rc. Since the reference column transistor 31T is energized, the base currents of the transistors 32 and 33 are also supplied, and the other LED columns 22 and 23 remain on even if the LED column 21 is turned off due to an open failure. The resistor Rc bears power consumption instead of the LED array 21. The inside of the current switching circuit 5 is composed of two transistors 51 and 52 in this example, but as a switching means, it may be provided with one junction type transistor, or may be composed of an FET, Various switching means can be applied.

In this way, when an open circuit failure occurs in the LED array 21 of the LED array circuit 2, the reference voltage of the reference voltage circuit 412 becomes relatively large and an "H" output is generated from the voltage comparator 411, and the state holding circuit The current switching circuit 5 is driven through 413, the switching means is closed, the output voltage of the DC-DC converter circuit 1 is applied to the reference column transistor 31T through the resistor Rc, and the energization of the reference column transistor 31T is maintained. Thus, lighting of the other LED rows 22 and 23 can be maintained. When the open failure of the LED array 21 is resolved, the normal LED drive circuit can be restored by turning off the DC-DC converter circuit 1.
[Second Embodiment]
FIG. 3 shows a second embodiment of the LED drive circuit. In FIG. 3, the configurations of the DC-DC converter circuit 1, the LED array circuit 2, and the current mirror circuit 3 are the same as those in the first embodiment, and the configurations of the open fault detection circuit 4 and the current switching circuit 5 are different. In FIG. 3, the open fault detection circuit 4 includes individual open fault detection circuits 41 and 42 and an AND circuit 43. Each of the individual open fault detection circuits 41 and 42 incorporates the voltage comparator 411, the reference voltage circuit 412, and the state holding circuit 413 described in FIGS. 1A and 1B in the first embodiment. The current switching circuit 5 includes switches SW1, SW2, and SW3 corresponding to the LED rows 21, 22, and 23 of the LED row circuit 2 or the rows 31, 32, and 33 of the current mirror circuit 3, and these LED rows. It is interposed between the circuit 2 and the current mirror circuit 3. The switch SW1 is a changeover switch in which a common terminal a is connected to the reference column transistor 31T. The changeover terminal b of the switch SW1 is connected to the LED row 21, and the changeover terminal c is connected to the changeover terminal c of the switch SW2. Yes. The switch SW2 is a changeover switch in which the common terminal a is connected to the LED row 22, and the changeover terminal b of the switch SW2 is connected to the transistor 32T. Further, the switch SW3 is an on / off switch that short-circuits or disconnects the collector and base of the transistor 33T, and the transistor 33T can be a reference column transistor when the switch SW3 is turned on. In this embodiment, the reference column transistor may be 31T or 33T. Therefore, not only one end of the ground resistor 31R but also one end of 33R is connected to the DC-DC converter circuit 1 via the diodes 31D and 33D. Connected to.

  The connection state of the open fault detection circuit 4 and the current switching circuit 5 will be described. In the individual open failure detection circuit 41, one input terminal A of the voltage comparator 411 is connected between the LED array 21 and the switching terminal b of the switch SW1 of the current switching circuit 5, and the output terminal C of the state holding circuit 413. Are connected to the switches SW1 and SW2 to be their control terminals. Although not shown in the figure, the individual open fault detection circuit 42 includes a voltage comparator, a reference voltage circuit, and a state holding circuit. One input terminal B of the voltage comparator in the individual open fault detection circuit 42 is an LED array. 22 and the common terminal “a” of the switch SW 2 of the current switching circuit 5, and the output terminal of the state holding circuit in the individual open failure detection circuit 42 is connected to one input terminal of the AND circuit 43. Here, the other input terminal of the AND circuit 43 is connected to the output terminal C of the state holding circuit 413 of the individual open failure detection circuit 41, and the output terminal D of the AND circuit 43 is connected to the switch SW 3 of the current switching circuit 5. And this control terminal.

  Here, each of the switches SW1 and SW2 outputs an “H” output from the state holding circuit 413 when an open failure occurs in the LED row 21 of the reference row and an abnormal voltage is input to the voltage comparator 411 of the individual open failure detection circuit 41. Are simultaneously switched from the switching terminal b to the switching terminal c to disconnect the LED string 21 in which the open failure has occurred and connect the normal LED string 22 to the reference string transistor 31T. Therefore, in this case, the base current of the reference string transistor 31T and the base current of the transistor 33T connected to the LED string 22 flow through simultaneous switching of the switches SW1 and SW2, and the LED string 22 and the LED string excluding the LED string 21 flow. The lighting of 23 is maintained. In addition, when the LED row 21 is not broken and the LED row 22 is open, the switch is not switched and the LED row 22 is simply turned off.

  In the above state, the LED row 21 having the open failure is disconnected and the normal LED row 22 is connected to the reference row transistor 31T. In FIG. 3, a case where an open fault occurs in the LED string 22 is also dealt with, and an open fault occurs in the LED string 22 of the new reference string, and the voltage comparator of the individual open fault detection circuit 42 is abnormal from the input terminal B. When a voltage is input, the AND circuit 43 outputs an AND output “H” from the output terminal D by the “H” output of the state holding circuit 413 of the individual open fault detection circuit 41 and the state holding circuit of the individual open fault detection circuit 42. Is output and the switch SW3 is turned on. In this state, the transistor 33T becomes a reference column transistor, and FIG. 3 only shows three LED columns and transistors. However, when there are many other LED columns and transistors, the reference column transistor 33T Thus, another transistor (not shown) can be driven.

  In the example of FIG. 3, the LED string 22 is connected to the reference column transistor 31T in the case of an open failure of the LED string 21, and the transistor 33T is used as the reference column transistor in the case of the LED string 22 also having an open failure. In this embodiment, even thicker care is taken, the LED string 22 is connected to the reference string transistor 31T in the case of an open failure of the LED string 21, and another LED string (not shown) is connected to the reference string when the LED string 22 also fails to open. When this LED string is also connected to the transistor 31T and an open failure occurs, the transistor 33T may be a reference string transistor. In this case, the switches and the individual open fault detection circuits are further increased one by one.

In this way, lighting of the other LED rows is ensured except for the LED row that has failed open by maintaining the energization of the reference row transistors and flowing the base current.
[Third Embodiment]
FIG. 4A shows a third embodiment. In the third embodiment, not only an open failure of the LED row 21 of the LED row circuit 2 but also an open failure in the reference row 31 including the reference row transistor 31T is supported. In FIG. 4A, the DC-DC converter circuit 1, the LED array circuit 2, and the current mirror circuit are the same as those in the first and second embodiments. Also, the structure of the open fault detection circuit 4 is the same as that of FIG. 2 showing the second embodiment except for a part. Furthermore, the switches SW1, SW2, and SW3 of the current switching circuit 5 have the same configuration as the SW3 of FIG. 3 showing the second embodiment. Since all the transistors 31T, 32T, and 33T of the current mirror circuit 3 can be reference column transistors, one end of each of the ground resistors 31R, 32R, and 33R is connected to the DC-DC converter circuit 1 via the diodes 31D, 32D, and 33D. Connected to.

  Here, in the open fault detection circuit 4, the voltage comparator 411 and the reference voltage circuit 412 of each of the individual open fault detection circuits 41 and 42 are for distinguishing the open fault on the LED column side from the open fault on the transistor side. The fault voltage range is set. That is, for example, the input voltage of the voltage comparator 411 when an open fault occurs in the LED string 21 is close to the ground voltage, and the input voltage of the voltage comparator 411 when an open fault occurs on the transistor 31T side is the LED string. The voltage drop of 21 disappears and becomes the output voltage of the DC-DC converter circuit 1. Therefore, as the voltage comparator 411, except for the normal voltage range lower than the high output voltage by the voltage drop of the LED string 21, the voltage comparator 411 becomes higher or lower on the output voltage side and approaches the ground voltage. It is necessary to compare and detect the case as an abnormal voltage. For this reason, a high reference voltage or a low reference voltage is set for the voltage of the reference voltage circuit 412 except for the normal voltage range. In this way, the individual open fault detection circuit 41 can detect both the open fault of the LED array 21 and the open fault on the transistor 31T side. FIG. 4B shows a specific circuit block of the individual open failure detection circuits 41 and 42 shown in FIG. 4A, and two reference voltage circuits 412 and two voltage comparators 411 are incorporated. What corresponds to a reference voltage higher or lower than the normal voltage range.

The switches SW1 and SW2 of the current switching circuit 5 are on / off switches for short-circuiting or separating between the collector base of the transistor 31T and the collector base of the transistor 32T, respectively, similarly to the switch SW3. The output terminals of the state holding circuit 413 are connected to the switches SW1 and SW2.
When an open failure occurs on the LED array 21 and the transistor 31T side, an “H” signal indicating failure is output from the voltage comparator 411 of the individual open failure detection circuit 41, and the state holding circuit 413 controls the switches SW1 and SW2 simultaneously. In response to the signal, the switch SW1 is turned off and the switch SW2 is turned on. In this state, when the switch SW2 is turned on, the collector base of the transistor 32T is short-circuited, and the transistor 32T becomes a reference column transistor. In this way, the reference row moves from the LED row 21 and the transistor 31T to the LED row 22 and the transistor 32T.

Further, when an open failure occurs on the LED row 22 and the transistor 32T side, a control signal from the AND circuit 43 to the switch SW3 is output by the control signal from the state holding circuit of the individual open failure detection circuit 42, and the switch SW3. And the reference row is moved to the LED row 23 and the transistor 33T. FIG. 4 (a) only shows three LED rows and transistors, but the number of switches and individual open fault detection circuits can be increased for each LED row.
In this way, it is possible to maintain the lighting of the other LED strings except for the LED string related to the open fault by switching the LED string that has the open fault and the reference string including the transistor.

In the description of the second embodiment and the third embodiment, the AND circuit 43 is used in the open fault detection circuit 4. However, it is also possible to control the switches SW1, SW2 and SW3 by using a ROM 40 as shown in FIG.
Furthermore, in the first, second, and third embodiments, an example is shown in which a junction transistor is used in the current mirror circuit 3, but an FET as shown in FIG. 6 is used instead of the junction transistor. Is also possible. In the claims, the concept of a transistor includes both a junction type and a field effect type.

It is a circuit block diagram which shows 1st Embodiment of this invention. It is an internal circuit block diagram of a DC-DC converter circuit. It is a circuit block diagram which shows 2nd Embodiment of this invention. It is a circuit block diagram which shows 3rd Embodiment of this invention. It is a block diagram which illustrates the open fault detection circuit using ROM. It is a circuit block diagram at the time of using FET for a current mirror circuit. It is a circuit block diagram which illustrates the conventional LED drive circuit.

Claims (8)

An LED array circuit in which one end of an LED array composed of one LED or two or more LEDs connected in series is connected to each other, and a transistor connected in series to the other end of each LED array in the LED array circuit One of these transistors is a reference column transistor that connects a connection point of the LED column and a control electrode and applies a control voltage to an output voltage variable power supply. In the LED drive circuit in which the current mirror circuit commonly connected to the control electrode of the other transistor is connected to the output terminal of the output voltage variable power supply,
An open fault detection circuit for detecting an open fault in the LED string connected to the reference string transistor; and a current switching circuit for switching the current path of the current mirror circuit in response to a fault detection output from the open fault detection circuit. The LED drive circuit characterized by the above-mentioned.   2. The LED driving circuit according to claim 1, wherein the current switching circuit is switching means interposed between a reference column transistor and a connection point of the output voltage variable power source.   2. The LED driving circuit according to claim 1, wherein the current switching circuit is switching means for connecting a normal LED string to the reference string transistor.   2. The LED driving circuit according to claim 1, wherein the current switching circuit is switching means for substantially disconnecting a reference column transistor related to an open failure and switching one of the other transistors as a reference column transistor. The open fault detection circuit and the current switching circuit are provided for each LED row except the one,
In response to the open-circuit failure detection output generated in the fault after switching output is the input of the open-circuit failure detection circuit, according to claim 3, characterized in that for switching the normal LED string in predetermined order LED drive circuit. 4. The LED drive circuit according to claim 2, wherein the open fault detection circuit includes a voltage comparator that detects an open fault in the LED string connected to the reference string transistor.   5. The LED drive circuit according to claim 4, wherein the open fault detection circuit includes a voltage comparator that detects an open fault in the reference column transistor and the LED column connected thereto. The open fault detection circuit and the current switching circuit are provided for each LED row except the one,
5. The LED according to claim 4, wherein the reference column transistor is switched in a predetermined order in response to the open fault detection output generated after the output of the open fault detection circuit is input and the fault detection is switched. Driving circuit.

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