JP2008294232A - Light emitting diode driving device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting diode driving device that can alleviate the stress of a constant current source caused by a ringing influence. <P>SOLUTION: A light emitting diode driving device 1 has a constant current source 5 supplying serially connected light emitting diodes 3a to 3d with a constant current, by-pass switches 4a to 4d connected in parallel with each of the light emitting diodes 3a to 3d, and a control circuit 2 for controlling switch on and off of each of the by-pass switches 4a to 4d, wherein the control circuit 2 controls on/off of each of the by-pass switches 4a to 4d so that one switching timing of the by-pass switches 4a to 4d gets delayed compared to another switching timing of the by-pass switches 4a to 4d. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a light emitting diode driving device that drives a plurality of light emitting diodes (hereinafter also referred to as LEDs) connected in series and a constant current source in series.

  Cold cathode fluorescent lamps (CCFLs) are the mainstream of light sources for flat panel displays (FPD) typified by liquid crystals and plasmas, but CCFLs contain mercury, so LEDs are expected to replace CCFLs in consideration of the environment. It is rare.

  In general, when LEDs are used as a backlight of a liquid crystal TV or the like, a very large number of LEDs are used. For these LEDs, a method of connecting an LED and a constant current source in series is often used in order to maintain constant brightness.

  However, the LED has a variation with respect to the forward voltage Vf, and if the power supply voltage applied to the anode of the LED is constant, the power lost in the constant current source due to the variation of the individual forward voltage Vf of the LED is growing.

  For this reason, in the case of a system using many constant current sources, the loss of the entire system increases. For example, two LEDs are connected in series to one constant current source, and the total number of constant current sources is set to 1. A method of reducing the loss of the entire system by setting to / 2 is often used. Thus, the number of constant current sources can be reduced to 1 / N by connecting N LEDs in series to one constant current source.

  In this case, in order to reduce the power consumption, it is necessary to lower the input voltage or stop the constant current. However, the effect of reducing the power consumption when N LEDs are connected is lower than when one LED is connected. Therefore, in the case where two or more LED devices connected in series have a further power consumption reduction effect, the backlight is controlled in accordance with the video data, etc., thereby reducing the brightness of unnecessary light sources and the entire system. What is necessary is just to take a method of reducing the power consumption.

  In order to realize this in the LEDs connected in series, it is necessary to individually turn on or turn off the individual LEDs. In order to realize this, there is an LED driving device in which a bypass switch is connected in parallel with the LED, and the LED can be turned off or turned on by turning on or off the switch.

  FIG. 13 is a circuit diagram showing a configuration of a conventional light emitting diode driving device 91. The light emitting diode driving device 91 includes four light emitting diodes 93a, 93b, 93c, and 93d connected in series. A constant current source 95 is connected to the opposite side of the light emitting diode 93d to the light emitting diode 93c, and the other end of the constant current source 95 is grounded. The constant current source 95 supplies a signal current A to the light emitting diodes 93a, 93b, 93c, and 93d. A bypass switch 94a is connected in parallel with the light emitting diode 93a, and a bypass switch 94b is connected in parallel with the light emitting diode 93b. A bypass switch 94c is connected in parallel with the light emitting diode 93c, and a bypass switch 94d is connected in parallel with the light emitting diode 93d. The bypass switches 94a, 94b, 94c, and 94d are connected in series. The light emitting diode driving device 91 is provided with a control circuit 92. The control circuit 92 outputs signals B, C, D, and E in synchronization with the clock signal CLK, and controls opening and closing of the bypass switches 94a, 94b, 94c, and 94d.

In this way, when the signal current A is supplied from the constant current source 95 and the LEDs 93a, 93b, 93c, and 93d connected in series are caused to emit light, the LED can be turned off by using a bypass switch. It is necessary to make the light emitted by the flowing current so weak that it cannot be seen by the human eye. The voltage corresponding to the current at that time is Vfa. In order to make the light emitted by the current flowing through the LED so weak that it cannot be seen by the human eye, a voltage is generated by passing a current through the bypass switch and the on-resistance of the bypass switch. Needs to be Vfa or less. When the current from the constant current source 95 is 35 mA and the Vf of the LED when it is not visible is Vfa = 0.7 V,
“0.7V” ÷ “35mA” = 20Ω (1)
Therefore, the on-resistance of the bypass switches 94a, 94b, 94c, and 94d needs to be smaller than 20Ω.

When many LEDs are connected in series, the voltage supplied to the system is
"Voltage required for constant current source" + "LED Vf" x "Number of LEDs" (2)
The above voltage is required.

If the voltage required for the constant current source is 1 V, the LED Vf is 3.5 V, and the number of LEDs connected in series is 10, it is necessary to supply a voltage of 36 V or more from Equation (2). At this time, the loss due to the bypass switch is maximized when all the bypass switches are in the ON state, and the loss (P) is
P = “constant current value” × “constant current value” × “on-resistance of bypass switch” × “number of on-state switches”
It becomes.

FIG. 14 shows a conventional timing waveform. The current signal A, the signal B, the signal C, the signal D, the signal E, and the clock signal CLK coincide with the reference numerals in FIG. The signals B, C, D, and E transition in synchronization with the rising edge of the clock signal CLK. FIG. 14 shows a case where all the signals B, C, D, and E transition at the same timing. Since the voltage transition of the current signal A is large, the constant current source 95 is stressed by the influence of ringing.
JP 2002-43073 A (published on February 8, 2002 (2002.2.8)) Japanese Utility Model Publication No. 5-30888 (published April 23, 1993)

  In the case of the LED driving device 91 given by the constant as described above, the voltage at the connection portion (hereinafter referred to as position P) between the cathode of the LED 93d and the constant current source 95 is a bypass switch when a constant current is flowing. When the current is stopped depending on the state, the voltage is about 36V.

  The LED driving device 91 has a problem that the loss in the bypass switches 94a, 94b, 94c, and 94d is large when the LED is turned off. In order to solve this problem, it is necessary to reduce the on-resistance of the bypass switches 94a, 94b, 94c, and 94d. For this purpose, the size of the bypass switches 94a, 94b, 94c, and 94d is increased. Thus, the reduction in loss and the size of the bypass switches 94a, 94b, 94c, and 94d have a trade-off relationship. Further, in the case of the conventional LED driving device 91 shown in FIG. 13, the voltage transition at the position P on the cathode side of the LED 93d shown in FIG. Transition from about 1V to about 36V. In the case of such a large transition, there is a problem that stress is applied to the constant current source 95 and the like by ringing.

  An object of the present invention is to provide an LED driving device that reduces the loss in the bypass switch, which is the above-described problem, and relieves stress on the constant current source due to the influence of ringing or the like.

  In order to solve the above problems, a light emitting diode driving device according to the present invention includes a constant current source that applies a constant current to two or more light emitting diodes connected in series, and 2 connected in parallel to each light emitting diode. Including at least one bypass switch and a control circuit for controlling opening and closing of each bypass switch, wherein the control circuit delays one switching timing of the bypass switch from another switching timing of the bypass switch. Thus, opening and closing of each bypass switch is controlled.

  According to the above feature, since each bypass switch opens and closes so that one switching timing of the bypass switch is delayed with respect to the other switching timing of the bypass switch, the voltage transition of the constant current source Will be more gradual than if they open and close at the same time. For this reason, the stress with respect to the constant current source by the influence of ringing is relieved.

  In the light emitting diode driving device according to the present invention, it is preferable that the bypass switch bypasses two or more light emitting diodes connected in series.

  According to this configuration, variations in the forward voltage Vf when the light emitting diode is turned on can be absorbed.

  In the light emitting diode driving device according to the present invention, it is preferable that the bypass switch bypasses two or more light emitting diodes connected in parallel to each other.

  According to this configuration, low voltage driving is possible.

  The light emitting diode driving device according to the present invention preferably further includes an auxiliary bypass switch that further bypasses the two or more bypass switches.

  According to this configuration, the combined resistance of the bypass switch can be reduced by turning on the auxiliary bypass switch when all of the two or more bypass switches are turned on. For this reason, since R of the loss P = I × I × R is reduced, the power (loss) inside the IC can be reduced. Thereby, the heat generation of the IC can be reduced.

  In the light emitting diode drive device according to the present invention, it is preferable that the control circuit turns on the auxiliary bypass switch when all of the two or more bypass switches to be bypassed are turned on.

  According to this configuration, the combined resistance of the bypass switch can be reduced, so that the power (loss) inside the IC can be reduced, and thereby the heat generation of the IC can be reduced.

  In the light emitting diode driving device according to the present invention, the bypass switch is configured such that a voltage generated by a current supplied from the constant current source and an ON resistance of the bypass switch is 1 V or less. preferable.

  In the light emitting diode driving device according to the present invention, the light emitting diode is configured to emit red light, and the bypass switch is generated by a current supplied from the constant current source and an on-resistance of the bypass switch. The voltage to be applied is preferably configured to be 0.7V or less.

  In the light-emitting diode driving device according to the present invention, it is preferable that the control circuit switches an on state and an off state of the bypass switch in synchronization with a clock signal.

  In the light emitting diode driving device according to the present invention, it is preferable that the control circuit controls the opening and closing of each bypass switch so that the switching timings of all the bypass switches are different from each other.

  In the light-emitting diode driving device according to the present invention, the control circuit is configured so that the switching timing of the bypass switch to one off state is delayed from the switching timing of the bypass switch to the other one off state. It is preferable to control the opening and closing of the bypass switch.

  In the light emitting diode driving device according to the present invention, it is preferable that the control circuit controls the opening and closing of each bypass switch so that the switching timings of all the bypass switches to the off state are different from each other.

  In the light emitting diode driving device according to the present invention, it is preferable that the light emitting diode driving device further includes a constant current source control circuit that controls a current supplied to the light emitting diode by the constant current source.

  In the light emitting diode driving device according to the present invention, when all of the bypass switches are turned on, and at least one of the bypass switches is turned off, the constant current source is turned off before the bypass switch is turned off. It is preferable to start the electric current.

  In the light emitting diode driving device according to the present invention, all of the bypass switches are turned on, and before the constant current source stops the current, all of the bypass switches are turned on, and the current is delayed. It is preferable to have

  As described above, the light emitting diode driving device according to the present invention opens and closes each bypass switch so that one switching timing of the bypass switch is delayed with respect to the other switching timing of the bypass switch. The voltage transition of is more gradual than when the bypass switches open and close simultaneously. For this reason, the effect that the stress with respect to the constant current source by the influence of ringing is relieved is produced.

  An embodiment of the present invention will be described below with reference to FIGS.

(Embodiment 1)
FIG. 1 is a circuit diagram showing a configuration of a light-emitting diode driving apparatus 1 according to Embodiment 1. The light emitting diode driving device 1 includes four light emitting diodes 3a, 3b, 3c, and 3d connected in series. The constant current source 5 is connected to the opposite side of the light emitting diode 3d to the light emitting diode 3c, and the other end of the constant current source 5 is grounded. The constant current source 5 supplies a signal current A to the light emitting diodes 3a, 3b, 3c, and 3d. A bypass switch 4a is connected in parallel with the light emitting diode 3a, and a bypass switch 4b is connected in parallel with the light emitting diode 3b. A bypass switch 4c is connected in parallel with the light emitting diode 3c, and a bypass switch 4d is connected in parallel with the light emitting diode 3d. The bypass switches 4a, 4b, 4c, and 4d are connected in series.

  The light emitting diode driving device 1 is provided with an auxiliary bypass switch 6a that bypasses the bypass switches 4a and 4b and an auxiliary bypass switch 6b that bypasses the bypass switches 4c and 4d. The auxiliary bypass switches 6a and 6b are connected in series.

  The light emitting diode driving device 1 is provided with a control circuit 2. The control circuit 2 outputs signals B, C, D, and E in synchronization with the clock signal CLK, and controls the opening and closing of the bypass switches 94a, 94b, 94c, and 94d. The control circuit 2 further outputs signals G · H in synchronization with the clock signal CLK to control the auxiliary bypass switches 6a and 6b.

Specifically, the bypass switch is configured by an analog switch (P-type / N-type MOS transistor parallel arrangement or the like), and the size thereof is relatively large. Although the size varies depending on the purpose of use and the manufacturing process, it is considered to be about mm ^{2 in} order. Since the number of bypass switches is approximately the same as the number of LEDs, the entire size of the bypass switch portion requires a considerable area when formed inside the IC. In this embodiment, since the auxiliary bypass switches 6a and 6b are added as shown in FIG. 1, the size of the auxiliary bypass switches 6a and 6b increases. However, the increase rate is shown in FIG. It shows that the increase rate decreases as the number of switches used increases.

  When driving a plurality of LEDs, each corresponding constant current source is connected (that is, connected in parallel), and each constant current source has an individual forward voltage Vf variation. As a total, the loss on the constant current source 5 side increases. In order to prevent (absorb) this, a system in which a plurality of LEDs 3a to 3d are connected in cascade to one constant current source 5 as shown in FIG.

  As shown in FIG. 1, the LEDs 3 a to 3 d are used in cascade connection for the above-mentioned reason. In order to operate the constant current source 5 at that time, a voltage that must be secured at a minimum “necessary for the constant current source” "Necessary voltage" is required. The voltage for driving (emitting) the LEDs 3a to 3d connected at this time is about 3.5V for blue and green LED elements and about 2V for red LED elements. Incidentally, in order to turn off the LED (not visible to the human eye), it is necessary to set the voltage to about 1 V or less for a blue or green element and about 0.7 V or less for a red element. That is, in the case of red, it cannot be turned off unless it is set to a considerably low voltage.

  FIG. 2 is a waveform diagram for explaining timing waveforms of the signal G for controlling the auxiliary bypass switch 6a provided in the light emitting diode driving device 1 and the signals D and E for controlling the bypass switches 4a and 4b. When the bypass switch 4b is turned on by the signal D, the bypass switch 4a is turned on by the signal E, and both the bypass switches 4b and 4b are turned on, the auxiliary bypass switch 6a is turned on by the signal G. . When the signals D, E, and G are in a low state, each switch is turned on.

  FIG. 3 is a waveform diagram for explaining timing waveforms of signals for controlling the bypass switch provided in the light emitting diode driving device 1. When the clock signal CLK rises, the signal B rises from the low state to the high state, the bypass switch 4d changes from the on state to the off state, and the signal current A decreases. Then, the signal C rises from the low state to the high state, the bypass switch 4c changes from the on state to the off state, and the signal current A further decreases. Next, the signal D rises from the low state to the high state, the bypass switch 4b changes from the on state to the off state, and the signal current A further decreases. Thereafter, the signal E rises from the low state to the high state, the bypass switch 4a changes from the on state to the off state, and the signal current A further decreases. Next, the clock signal CLK falls.

  When the clock signal CLK rises, the signal B falls from the high state to the low state, the bypass switch 4d changes from the off state to the on state, and the signal current A increases. Then, the signal C falls from the high state to the low state, the bypass switch 4c changes from the off state to the on state, and the signal current A further increases. Next, the signal D falls from the high state to the low state, the bypass switch 4b changes from the off state to the on state, and the signal current A further increases. Thereafter, the signal E falls from the high state to the low state, the bypass switch 4a changes from the off state to the on state, and the signal current A further increases. Next, the clock signal CLK falls.

  Thus, the signals B, C, D, and E transition in synchronization with the rising edge of the clock signal CLK. In FIG. 3, when all signals transition at the same timing, the signal B, C, D, and E have a delay, so that the voltage transition of the signal current A becomes gentle and ringing (voltage fluctuation b1). And the stress on the constant current source 5 due to the influence of b0) is alleviated.

Here, when the current supplied by the constant current source 5 is 35 mA, the on-resistance of the bypass switch is 20Ω, and the on-resistance of the auxiliary bypass switch is set to a combined resistance that is half the total number of bypass switches, the number of bypass switches If one auxiliary bypass switch is attached to five bypass switches, the resistance value of one auxiliary bypass switch is 100Ω. Of course, the ratio of the number of auxiliary bypass switches to the bypass switch and the on-resistance thereof are not limited to those shown here. FIG. 4 shows the loss reduction effect of the related art and the present embodiment in relation to the loss and the number of bypass switches when one LED is turned on and all other LEDs are turned off with the constants set in this way. Here, the loss Pa according to the prior art (when there are 10 bypass switches)
Pa = “35 mA” × “35 mA” × “20Ω” × “9 in series” = 220.5 mW (3)
Can be expressed as

Similarly, the loss Pb when this embodiment is used is
Pb = “35 mA” × “35 mA” × (1 ÷ ((1 ÷ (“20Ω” × “5 series”)) + (1 ÷ “100Ω”)) + (“20Ω” × “4 series”) )) = 159.25 mW (4)
Can be expressed as

FIG. 5 is a graph showing a rate of increase in the area of the switch unit by the light emitting diode driving device 1. First, when the resistance of the bypass switch is R, the area of the bypass switch is S. When the resistance of the bypass switch is doubled, the area of the bypass switch is 0.5 × S. When the resistance of the bypass switch is ½, the area of the bypass switch is 2 × S. From this, if the area of the bypass switch having an on-resistance of 20Ω is S, the area of one auxiliary bypass switch can be expressed as 1/5 × S. Since there are two auxiliary bypass switches in the above setting, the area of the auxiliary bypass switch is 2/5 × S. At this time, the increase (ΔS) from the area of the conventional method is
ΔS = ([area of this embodiment] − [conventional area]) ÷ [conventional area] = ((10 × S + 2/5 × S) − (10 × S)) / (10 × S) = 4 %
Can be calculated as

  From the graph shown in FIG. 5, the greater the number of bypass switches, the greater the effect of loss reduction, the smaller the increase rate of the area occupied by the switch section, and the greater the effect of this embodiment.

  In the configuration of the prior art, when the bypass switch is switched on and off, the timing is switched in synchronization with the clock signal CLK, and when all the bypass switches connected in series are switched on, all bypass switches are switched on. Although the switching is performed at the same time, in the bypass switch according to the present embodiment, the switching timing of at least one or more bypass switches may be delayed. The same applies when the bypass switches connected in series are switched to the OFF state. In the configuration of the prior art, all bypass switches are switched simultaneously when the bypass switches connected in series are switched to the OFF state. However, the bypass switch according to the present embodiment only needs to have a delay in switching timing of at least one of the bypass switches.

  Moreover, it is also possible to adjust the brightness | luminance of LED by carrying out PWM drive using the on / off state of these said bypass switches. The off state in this case is a light emitting state of the LED that cannot be seen by humans. This is done by setting the voltage Vf, which is the voltage across the anode and cathode of a blue or green LED, to 1 V or less, and for the red LED, the voltage Vf is set to 0.7 V or less. It becomes possible. Based on the current output from the constant current source, the OFF state is made invisible by determining the ON resistance of the bypass switch so that the voltage generated by the ON resistance of the bypass switch is smaller than the above value. Is possible.

  By using the auxiliary bypass switches 6a and 6b simultaneously with the normal bypass switches 4a to 4d, the combined resistance of the bypassed switches can be reduced. When the combined resistance is reduced, R of P = I × I × R, which is a loss here, can be reduced, so that power (loss) inside the IC can be reduced. Thereby, the heat generation of the IC can be reduced.

  The operation timing of the auxiliary bypass switch is preferably set to an on state with a delay after all the switches bypassed by the auxiliary bypass switch are turned on, as a countermeasure against the stress reduction for the ringing.

  For example, after the bypass switch 4b is turned on by the signal D and the bypass switch 4a is turned on by the signal E, the auxiliary bypass switch 6a is turned on by the signal G with a delay.

  It should be noted that the current is constant. The current flowing through the LED as a load has the problem of whether it flows through the bypass switch or the auxiliary bypass switch (divides), but the total current is the same (constant).

  The individual LEDs 3a to 3d are turned off in the configuration shown in FIG. 1 when the corresponding bypass switches 4a to 4d are turned on. At this time, since there is a constant current source 5 as a whole, the current consumption of the entire system is constant. However, since a current flows through the bypass switch that is turned on, if the current (power) consumed here is considered in total, there is a case where waste occurs depending on the drive timing. In order to prevent (reduce) this, the auxiliary bypass switches 6a and 6b are used as in the present embodiment to control the on / off timing of the entire two sets of switches, thereby wasting power consumed on the switch side. Is reduced.

  FIG. 6 is a circuit diagram showing another configuration of the light emitting diode provided in the light emitting diode driving device 1. The bypass switch 4a may be configured to bypass the two light emitting diodes 3a connected in series. Similarly, the bypass switches 4b, 4c, and 4d may be configured to bypass the two light emitting diodes 3b, 3c, and 3d connected in series. FIG. 6 shows a method of connecting a plurality of LEDs in series per bypass switch connected in series, and is a case where a high voltage is applied as a whole (the switch has a high breakdown voltage). In this case, a voltage of 3.5V × 8 = 36V or more is required. The merit of this connection method is that the forward voltage Vf varies individually when the LEDs are lit, but the current can be made equal to the point that this variation can be absorbed.

  FIG. 7 is a circuit diagram showing still another configuration of the light emitting diode provided in the light emitting diode driving device 1. The bypass switch 4a may be configured to bypass the two light emitting diodes 3a connected in parallel to each other. Similarly, the bypass switches 4b, 4c, and 4d may be configured to bypass the two light emitting diodes 3b, 3c, and 3d connected in parallel to each other. FIG. 7 shows a method of connecting a plurality of LEDs in parallel for each bypass switch. In this case, the current varies (is different) for each LED, but it is advantageous in that it can be driven at a low voltage. However, strictly speaking, since the current is different for each LED, the display illuminance varies, but depending on the conditions used as illumination, there may be applications that can absorb the variation.

  In addition, although LED which concerns on this Embodiment considers the object for backlight illuminations as an example, white LED, red, green, and blue LED can also be used as a light source. In that case, in the case of an LED for each color, a total color can be obtained by combining the colors. As for the connection method, red, green, and blue colors can be connected in cascade as shown in FIG. 6, or can be connected in parallel as shown in FIG.

  In the present embodiment, regarding the number of corresponding switches of the bypass switch (first switch) and the auxiliary bypass switch (second switch), when there are ten first switches, the number of second switches is five (that is, , 2 in total). In addition, this may correspond to every number. 10 pieces = 2 pieces + 2 pieces + 2 pieces + 2 pieces + 2 pieces, 4 pieces + 2 pieces + 4 pieces, 3 pieces + 2 pieces + 4 pieces + 1 pieces may be used.

(Embodiment 2)
FIG. 8 is a circuit diagram showing a configuration of the light-emitting diode driving device 1a according to the second embodiment. The same reference numerals are given to the same components as those described above. Therefore, detailed description of these components is omitted. The light emitting diode driving device 1a includes a constant current source 5a and a constant current source control circuit 7. FIG. 9 is a circuit diagram showing configurations of the constant current source control circuit 7 and the constant current source 5a. The constant current source 5a has an N-channel MOSFET 8, the drain of the N-channel MOSFET 8 is coupled to the cathode of the light emitting diode 3d, and the source of the N-channel MOSFET 8 is connected to the ground via a resistor 9. It is connected.

  The constant current source control circuit 7 has an amplifier 10 and a mode changeover switch 11, and the source of the N-channel MOSFET 8 is coupled to the inverting input terminal of the amplifier 10. Either the output of 10 or the ground is connected to the gate of the N-channel MOSFET 8.

  The constant current source control circuit 7 that controls the current output from the constant current source 5a controls the constant current source 5a using the signal F from the control circuit 2a. The signal F becomes low level with a delay when all of the bypass switches 4a to 4d are turned on, and stops the current of the constant current source 5a. Thereby, since LED4a-4d can stop the electric current which flows from the constant current source 5a at the time of all the light extinctions, it becomes possible to reduce the power consumption of a system and to reduce the loss of bypass switches 4a-4d.

  When both the bypass switches 4a and 4b are turned on, the auxiliary bypass switch 6a is turned on. When the signals D, E, and G are low, the bypass switch 6a is turned on.

  FIG. 10 shows a conventional timing waveform. The signal current A, the signals B, C, D, E, and F and the clock signal CLK coincide with the symbols in FIG. The signals B, C, D, E, and F transition in synchronization with the rising edge of the clock signal CLK. FIG. 10 shows a case where all signals transition at the same timing. Since the voltage transition of the signal current A is large, the constant current source 5a is stressed by the influence of ringing (voltage fluctuations c1 and c0).

  FIG. 11 shows the timing waveform of the second embodiment, and the signal current A, the signals B · C · D · E · F, and the clock signal CLK coincide with the reference numerals in FIG. The signals B, C, D, and E transition in synchronization with the rising edge of the clock signal CLK. In FIG. 11, when all signals transition at the same timing, the signal B, C, D, and E are delayed, so that the voltage transition of the signal current A becomes gradual and ringing (voltage The stress on the constant current source 5a due to the influence of the fluctuations d1 and d0) is alleviated. Furthermore, in Embodiment 2, since the current of the constant current source 5a is stopped when all LEDs are turned off, power consumption and loss in the bypass switch can be reduced.

Here, as in the first embodiment, the current flowing from the constant current source 5 is set to 35 mA, the on-resistance of the bypass switch is set to 20Ω, and the on-resistance of the auxiliary bypass switch is set to a combined resistance that is half the total number of bypass switches. To do. If the number of bypass switches is 10 and one auxiliary bypass switch is attached to five bypass switches, the resistance value of one auxiliary bypass switch is 100Ω. Of course, the ratio of the auxiliary bypass switch to the bypass switch and its on-resistance are not limited to those shown here. FIG. 12 shows the loss reduction effect of the related art and the present invention in relation to the loss and the number of bypass switches when all the LEDs are turned off with the constants set in this way. Here, the loss Pc according to the prior art is 10 bypass switches.
Pc = “35 mA” × “35 mA” × “20Ω” × “10 in series” = 245 mW (4)
Can be expressed as

  Similarly, the loss Pd when this embodiment is used does not occur because the current is stopped by the constant current source 5a. From this graph, the greater the number of bypass switches, the greater the effect of loss reduction.

  In the above, the timing of switching the on state and the off state of the bypass switches 4a to 4d connected in series switches the state in synchronization with the clock CLK. When the bypass switches 4a to 4d connected in series are switched to the ON state, a delay is given to the timing at which at least one bypass switch is switched. Further, when the bypass switches 4a to 4d connected in series are switched to the ON state, the respective bypass switches are switched to the ON state at different timings with respect to the clock signal CLK. Furthermore, when the bypass switches 4a to 4d connected in series are switched to the OFF state, at least one of the bypass switches has a delay in switching timing. Further, when the bypass switches 4a to 4d connected in series are switched to the off state, the respective bypass switches 4a to 4d are switched to the off state at different timings with respect to the clock signal CLK.

  In addition to the above, in the second embodiment, the current from the constant current source 5a is stopped by the signal F when all of the bypass switches 4a to 4d are steadily turned on. When all of the bypass switches 4a to 4d are turned on and at least one of the bypass switches is turned off, the current starts to flow from the constant current source 5a before the bypass switch is turned off. All of the bypass switches 4a to 4d are turned on, and before the constant current source 5a stops the current, a delay is given from when all the bypass switches 4a to 4d are turned on until the current stops.

  As described above, according to the first and second embodiments, it is possible to realize an LED driving device that can reduce the loss of the bypass switch using the auxiliary bypass switch and can relieve the stress on the constant current source according to the control timing. .

  Embodiments 1 and 2 are illumination devices that are used for backlight light sources of image display devices, etc., that are connected in cascade and driven at a constant current, and each LED element is turned on individually. When a bypass switch for turning on / off is provided and driven by a control signal to the bypass switch, the loss in the bypass switch is reduced, ringing at the on / off state is reduced, and the adverse effect on the entire system ( The present invention relates to a configuration and a method for solving such problems as noise and lifetime reduction.

  Conventionally, there is one set (first) of bypass switches corresponding to each LED, but in the first and second embodiments, another set of bypass switches (auxiliary bypass switch, second) is provided. In order to reduce the total on-resistance by connecting in parallel, the second group is subjected to switching control according to the driving condition of the first group. Of course, the switching timing is controlled so that no ringing occurs.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  INDUSTRIAL APPLICABILITY The present invention can be applied to a light emitting diode driving device that is driven by connecting a plurality of light emitting diodes (LEDs) connected in series and a constant current source in series, and can be applied to a backlight light source device of a display device. it can.

1 is a circuit diagram illustrating a configuration of a light-emitting diode driving device according to Embodiment 1. FIG. It is a wave form diagram explaining the timing waveform of the signal which controls the auxiliary | assistant bypass switch provided in the said light emitting diode drive device. It is a wave form diagram explaining the timing waveform of the signal which controls the bypass switch provided in the said light emitting diode drive device. It is a graph which shows the loss reduction effect by the said light emitting diode drive device. It is a graph which shows the ratio of the area increase of the switch part by the said light emitting diode drive device. It is a circuit diagram which shows the other structure of the light emitting diode provided in the said light emitting diode drive device. It is a circuit diagram which shows the further another structure of the light emitting diode provided in the said light emitting diode drive device. FIG. 6 is a circuit diagram illustrating a configuration of a light emitting diode driving device according to a second embodiment. It is a circuit diagram which shows the structure of the constant current source control circuit and constant current source which were provided in the said light emitting diode drive device. It is a wave form diagram explaining the comparative example of the timing waveform of the signal which controls a bypass switch. It is a wave form diagram explaining the timing waveform of the signal which controls the bypass switch provided in the said light emitting diode drive device. It is a graph which shows the loss reduction effect by the said light emitting diode drive device. It is a circuit diagram which shows the structure of the conventional light emitting diode drive device. It is a wave form diagram explaining the timing waveform of the signal which controls the bypass switch provided in the said light emitting diode drive device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light emitting diode drive device 2 Control circuit 3a, 3b, 3c, 3d Light emitting diode 4a, 4b, 4c, 4d Bypass switch 5 Constant current source 6a, 6b Auxiliary bypass switch 7 Constant current source control circuit 8 MOSFET
9 Resistance 10 Amplifier 11 Mode selector switch A Signal current B to E, G, H Signal P Position

Claims (26)

A constant current source for supplying a constant current to two or more light emitting diodes connected in series;
Two or more bypass switches connected in parallel to each light emitting diode;
A control circuit for controlling the opening and closing of each bypass switch,
The control circuit controls the opening and closing of each bypass switch so that one switching timing of the bypass switch is delayed from another switching timing of the bypass switch.   The light-emitting diode driving device according to claim 1, wherein the bypass switch bypasses two or more light-emitting diodes connected in series.   The light emitting diode driving apparatus according to claim 1, wherein the bypass switch bypasses two or more light emitting diodes connected in parallel to each other.   The light emitting diode drive device according to claim 1, further comprising an auxiliary bypass switch that further bypasses the two or more bypass switches.   5. The light emitting diode driving device according to claim 4, wherein the control circuit turns on the auxiliary bypass switch when all of the two or more bypass switches to be bypassed are turned on.   2. The light emitting diode driving device according to claim 1, wherein the bypass switch is configured such that a voltage generated by a current supplied from the constant current source and an on-resistance of the bypass switch is 1 V or less. The light emitting diode is configured to emit red light,
2. The light emitting diode driving device according to claim 1, wherein the bypass switch is configured such that a voltage generated by a current supplied from the constant current source and an ON resistance of the bypass switch is 0.7 V or less.   The light emitting diode driving device according to claim 1, wherein the control circuit switches an on state and an off state of the bypass switch in synchronization with a clock signal.   The light emitting diode driving device according to claim 1, wherein the control circuit controls opening and closing of each bypass switch so that switching timings of all the bypass switches are different from each other.   The control circuit controls opening and closing of each bypass switch so that a switching timing of the bypass switch to one off state is delayed from a switching timing of the bypass switch to another one off state. The light emitting diode drive device of description.   The light emitting diode driving device according to claim 1, wherein the control circuit controls opening and closing of each bypass switch so that switching timings of all bypass switches to an off state are different from each other.   The light emitting diode driving device according to claim 1, further comprising a constant current source control circuit that controls a current that the constant current source supplies to the light emitting diode.   13. The light emitting diode driving device according to claim 12, wherein the constant current source control circuit stops the current of the constant current source when all of the bypass switches are turned on. The constant current source has an N-channel MOSFET,
The drain of the N-channel MOSFET is coupled to the cathode of the light emitting diode;
The source of the N-channel MOSFET is connected to the ground through a resistor,
The light emitting diode driving device according to claim 12, wherein the constant current source control circuit is coupled to a source and a gate of the N-channel MOSFET. The constant current source control circuit has an amplifier and a mode switch,
The source of the N-channel MOSFET is coupled to the inverting input terminal of the amplifier,
The light emitting diode driving device according to claim 12, wherein the mode changeover switch connects either the output of the amplifier or the ground to the gate of the N-channel MOSFET.   13. The light emitting diode driving device according to claim 12, wherein the bypass switch bypasses two or more light emitting diodes connected in series.   The light-emitting diode driving device according to claim 12, wherein the bypass switch bypasses two or more light-emitting diodes connected in parallel to each other. An auxiliary bypass switch that further bypasses the two or more bypass switches;
13. The light emitting diode driving device according to claim 12, wherein the control circuit turns on the auxiliary bypass switch when all of the two or more bypass switches to be bypassed are turned on.   13. The light emitting diode driving device according to claim 12, wherein the bypass switch is configured such that a voltage generated by a current supplied from the constant current source and an on-resistance of the bypass switch is 1 V or less. The light emitting diode is configured to emit red light,
The light-emitting diode driving device according to claim 12, wherein the bypass switch is configured such that a voltage generated by a current supplied from the constant current source and an on-resistance of the bypass switch is 0.7 V or less.   The light emitting diode driving device according to claim 12, wherein the control circuit switches an on state and an off state of the bypass switch in synchronization with a clock signal.   The light emitting diode driving device according to claim 12, wherein the control circuit controls opening and closing of each bypass switch so that switching timings of all the bypass switches are different from each other.   The control circuit controls opening and closing of each bypass switch so that a switching timing of the bypass switch to one off state is delayed from a switching timing of the bypass switch to another one off state. The light emitting diode drive device of description.   The light emitting diode driving device according to claim 12, wherein the control circuit controls the opening and closing of each bypass switch so that the switching timings of all the bypass switches to the off state are different from each other.   2. The light emitting device according to claim 1, wherein when all of the bypass switches are turned on and at least one of the bypass switches is turned off, current starts to flow from a constant current source before the bypass switch is turned off. Diode drive device.   2. The light emitting diode according to claim 1, wherein all of the bypass switches are turned on, and a delay is provided until the current stops after all of the bypass switches are turned on before the constant current source stops the current. Drive device.

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