JP3966452B2 - LED driving circuit and light emitting device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a driving circuit for an LED (light emitting diode) and further to a technique for driving the LED at a low voltage. For example, the technique is useful when the LED is mounted on a battery-driven device such as a PDA (personal digital assistant). About.
[0002]
[Prior art]
Since the forward voltage of the LED is about 1.7 to 2.2 V, for example, when the LED is mounted on a device that operates with a normal 1.5 V battery, stable driving of the LED cannot be expected only by the electromotive force of the battery. .
[0003]
Conventionally, for example, when driving an LED with a power supply voltage of about 1.5 V, the power supply voltage is boosted by a DC / DC converter to generate a stable voltage of about 3 V, and the LED is driven using this voltage. . When driving an LED, it is common to connect a resistor in series with the LED so that the forward current of the LED is stabilized.
[0004]
[Problems to be solved by the invention]
However, in the above conventional configuration, in order to stabilize the supply voltage, a relatively large capacitor is connected between the output terminals of the DC / DC converter, or a switching regulator circuit for boosting. Etc. had to be added, resulting in an increase in the number of parts and the circuit area, resulting in a higher unit price.
[0005]
Further, the conventional LED driving circuit has a drawback that a large power loss is caused by the resistor connected in series with the LED.
In addition, when the conventional LED driving circuit is mounted on the apparatus, the LED driving circuit is disposed and a configuration like a printed circuit board for wiring connection between the LED and the driving circuit is necessary. There was also a problem that the degree of freedom in doing so was reduced.
[0006]
An object of the present invention is to provide an LED driving circuit that can drive an LED with an electromotive force of one battery and that is inexpensive and can reduce resistance loss.
Another object of the present invention is to provide a light-emitting device that can be driven at a low voltage and does not reduce the degree of freedom in mounting.
The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.
[0007]
[Means for Solving the Problems]
Outlines of representative ones of the inventions disclosed in the present application will be described as follows.
That is, a drain terminal is connected to the first power supply terminal, a source terminal is connected to the first electrode of the LED, a substrate potential is connected to the second power supply terminal, and the source terminal of the first N channel MOSFET A capacitor to which one electrode is connected, a second N-channel MOSFET having a drain terminal connected to the other electrode of the LED, a source terminal and a substrate potential connected to a second power supply terminal, and the first N-channel An oscillation circuit for supplying an oscillation signal to the gate terminal of the MOSFET, the other electrode of the capacitor and the gate terminal of the second N-channel MOSFET, and a CMOS inverter for inverting the output of the oscillation circuit, Depending on the output and the output of the CMOS inverter, the gate terminal of the first N-channel MOSFET and the other of the capacitor Oscillation signal of opposite phase relation to each other to the gate terminal of the pole and the second N-channel MOSFET is an LED drive circuit configured to be supplied.
In addition, the LED driving circuit and the LED element are a light emitting device configured integrally.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a circuit diagram showing an embodiment of a suitable LED driving circuit to which the present invention is applied.
The LED driving circuit 1 is a driving circuit for driving an LED having a forward voltage of about 1.7 to 2.2 V with a power supply voltage of about 1.5 to 3 V. For example, one LED such as single crystal silicon is used. Formed on the semiconductor chip. The drive circuit 1 includes a first oscillation circuit 11 such as a ring oscillator that oscillates at about 10 MHz, a charge pump capacitor C1, and a switching operation based on an oscillation signal from the oscillation circuit 11 to charge the capacitor C1. N-channel MOSFET (hereinafter referred to as NMOS) MN1, a second NMOS MN2 that is turned on / off by an oscillation signal to prevent a through current flowing through LED 2, and the other electrode of capacitor C1 from the ground potential GND. The CMOS inverter INV1 is composed of a PMOS QP1 and an NMOS QN1 that are boosted to the voltage VDD, and a CMOS inverter INV2 that inverts the output of the oscillation circuit 11 and supplies it to the gate terminal of the second NMOS MN2.
[0009]
The connection node n1 of one electrode of the capacitor C1 and the drain terminal of the second NMOS MN2 are connected to the external terminals T1 and T2, respectively. LED2 is connected between the external terminals T1 and T2.
[0010]
Further, the LED drive circuit 1 is provided with a power supply terminal to which a power supply voltage VDD is supplied, a power supply terminal to which a ground potential GND is supplied, and an input terminal for a control signal CONT that activates the oscillation circuit 11. . Here, the oscillation circuit 11 can be controlled to be active / inactive by connecting the input terminal of the control signal CONT and the power supply terminal of the power supply voltage VDD and supplying / cutting off the power supply voltage VDD. That is, on / off switching of the LED drive circuit 1 is automatically performed by supplying / cutting off the power supply voltage VDD.
[0011]
The first NMOS MN1 has a source follower connection in which the drain terminal is connected to the power supply voltage VDD, the substrate potential is connected to the ground potential GND, and the source terminal is connected to the output side electrode of the capacitor C1. As described above, by using the N-channel MOSFET in the source follower connection, even when the output-side electrode of the capacitor C1 is boosted and becomes higher than the power supply voltage VDD, the first NMOS MN1 is sufficiently turned off and the power supply voltage VDD side The current does not flow backward.
The capacitor C1 has a capacity of about several pF, for example, and can be formed on a semiconductor chip with a relatively small area.
[0012]
The second NMOS MN2 prevents a through current from flowing from the power supply voltage VDD to the ground potential GND via the LED2 when a voltage larger than the forward voltage VF of the LED2 is applied as the power supply voltage VDD. is there. That is, when the first NMOS MN1 is on, the second NMOS MN2 is off, so that no current flows through the LED 2 even if the power supply voltage VDD increases. On the other hand, when the first NMOS MN1 is in the off state, the second NMOS MN2 is in the on state and passes the current charged in the capacitor C1 to the ground GND side through the LED2.
Note that the second NMOS MN2 and the inverter INV2 can be omitted if there is a guarantee that the power supply voltage VDD is equal to or lower than the LED forward voltage VF.
[0013]
In this embodiment, it is conceivable that the second NMOS MN2 is not provided on the cathode side of the LED 2 but is provided on the anode side of the LED 2 so as to prevent a through current. When the voltage is lower than the forward voltage VF, the source potential of the second MOS MN2 rises and the second MOS MN2 cannot be turned on. Therefore, the second NMOS MN2 is provided on the cathode side of the LED2.
[0014]
According to the configuration as described above, the oscillation signal output from the oscillation circuit 11 is input to the gate terminal of the first NMOS MN1, the capacitor C1 via the inverters INV1 and INV2, respectively, and the gate terminal of the second NMOS MN2. Is done. The inputs of the first NMOS MN1, the capacitor C1, and the second NMOS MN2 are in reverse phase.
[0015]
Therefore, when the oscillation signal is at the high level, the first NMOS MN1 is turned on and the output of the inverter INV1 is set to the low level. Therefore, a potential difference is generated between both electrodes of the capacitor C1, and the capacitor C1 is charged. Here, the electrode on the output side of the capacitor C1 becomes a potential lower than the power supply voltage VDD by the threshold voltage Vth of the first NMOS MN1, and the other electrode becomes the ground potential GND.
At this time, since the second NMOS MN2 is turned off, no through current flows through the LED 2 even when the power supply voltage VDD is very high.
[0016]
Next, when the oscillation signal becomes low level, the first NMOS MN1 is turned off and the output of the inverter INV1 is set to high level to push up the potential of one electrode of the capacitor C1. Since this potential change is steep, the potential on the opposite side of the capacitor C1 is also increased by the power supply voltage VDD and exceeds the forward voltage VF of the LED2. Furthermore, since the second NMOS MN2 is turned on at this time, a current flows from the capacitor C1 through the LED2, and the LED2 emits light.
[0017]
The LED element has a risk of destruction when an excessive current flows due to a through current or the like. In this embodiment, the current flowing to the LED 2 is due to the electric charge charged in the capacitor C1, and the electrode on the output side of the capacitor C1. Even when a relatively high voltage is generated in the LED 2, no excessive current flows through the LED 2. That is, according to the drive circuit 1 of this embodiment, the voltage applied to the LED 2 is not stable, but the current flowing through the LED 2 is stable, and relatively stable light emission is obtained.
[0018]
FIG. 2 is a plan view showing an LED light emitting device including the LED driving circuit of the embodiment.
The LED light-emitting device of this embodiment is an LED formed by fixing an LED light-emitting body 210 formed by joining a p-type semiconductor and an n-type semiconductor and providing an electrode on a chip 230 provided with electrodes 221 to 224. The LED chip 200 as a light emitting element and the semiconductor chip 100 on which the LED driving circuit 1 is formed are integrated with a resin mold. Further, each electrode is connected by a gold wire WR or the like and the top thereof is also sealed with resin.
[0019]
In FIG. 3, the graph which compared the characteristic of the applied voltage-light quantity of said LED light-emitting device with the conventional LED is shown. In this figure, the solid line indicates the characteristics of the LED light emitting device of the embodiment, and the dotted line indicates the characteristics of the conventional LED.
As shown in FIG. 3, according to the LED light emitting device of the embodiment, it is possible to obtain a characteristic that light is emitted when the power supply voltage VDD is about 0.9 V or more and the light emission amount is gradually increased to about 3 V. From this, it can be seen that the conventional LED requires a voltage of 1.7 V or more for light emission and cannot be driven by a single battery, whereas the light emitting device of this embodiment can be driven by a single battery.
[0020]
Furthermore, since the conventional LED has a characteristic that the current increases rapidly when the voltage at which light emission starts is exceeded, it is necessary to stabilize the current by connecting a resistor or the like in series with the LED in an actual circuit. However, in the LED light emitting device of this embodiment, the through current does not flow from the power supply voltage VDD to the ground GND through the LED, and no rapid current flows even if the power supply voltage VDD increases. Stable light emission can be obtained without a large resistance loss.
[0021]
Further, according to the LED light emitting device having the structure as shown in FIG. 2, since the size and shape can be made almost the same as those of the conventional LED light emitting element, it can be mounted by the same handling as the conventional LED light emitting element. In addition, it does not require a high voltage as in the prior art to emit light and can be driven at a low voltage. However, since a printed wiring or the like for disposing a drive circuit is unnecessary, it is as if an LED with a low forward voltage is used. The degree of freedom of design for mounting the light emitting device is also increased.
[0022]
FIG. 4 shows a modification of the LED drive circuit according to the present invention.
In this modified example of the LED driving circuit, a voltage whose polarity is reversed from that of the driving circuit 1 in FIG. 1 is generated at the external terminals T1 and T2, and a negative voltage is generated in the capacitor C1 for the charge pump. A voltage is generated. Therefore, the cathode-side electrode of the LED 2 is connected to the external terminal T4 to which one electrode of the capacitor C1 is connected.
[0023]
In the LED drive circuit 1B of this modification, P-channel type first and second PMOS MP1 and MP2 are used instead of the first and second NMOS MN1 and MN2 in FIG. Also, according to the change in polarity, the output of the oscillation circuit 11 is directly input to the gate of the first PMOS, and the signal obtained by inverting the oscillation output by the inverter INV2 is input to the gate of the capacitor C1 and the second PMOS. I am letting.
[0024]
According to the drive circuit 1B of this modified example, the portion using the N-channel MOSFET in the embodiment of FIG. 1 is changed to the P-channel MOSFET. Although the area becomes large, the same effect as the LED drive circuit 1 of FIG. 1 can be obtained.
[0025]
As another modification of the LED driving circuit according to the present invention, for example, when driving an LED having a high forward voltage VF such as a blue LED or driving an LED with a lower power supply voltage, the following circuit is provided. Is also possible. That is, a plurality of stages including the first NMOS MN1, the charge pump capacitor C1, and the inverter INV1 in FIG. 1 are provided, and each of them is cascade-connected to constitute a drive circuit. According to such a configuration, a voltage that is three or four times the power supply voltage VDD can be generated in the final stage capacitor. Therefore, an LED having a high forward voltage VF can be driven by this voltage, or the power supply voltage can be increased. The LED can be driven even when it is low.
[0026]
As described above, according to the LED driving circuit of the above embodiment, the LED can be made to emit light with one battery, and it is not necessary to connect a resistor so that an excessive current does not flow through the LED. Reduction can also be achieved.
[0027]
Further, according to the LED light-emitting device shown in FIG. 2, while the same effect as that obtained by the LED drive circuit can be obtained in terms of electrical characteristics, it can be handled in the same manner as a conventional LED element during mounting. There is an effect that the degree of freedom of design does not become low in mounting the LED. Therefore, for example, when it is mounted on a battery-driven portable electronic device such as a PDA, light emission of the LED can be obtained using the power supply voltage as it is and the power consumption can be suppressed to be low, which is particularly effective.
[0028]
The invention made by the present inventor has been specifically described based on the embodiments. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Nor.
For example, in the embodiment, the configuration in which the oscillation circuit 11 is built in the drive circuit is shown. However, instead of providing the drive circuit inside, an oscillation signal generated outside may be supplied to the drive circuit. . In addition, the charge pump capacitor may be connected as an external terminal instead of being provided on the same chip as the other components.
[0029]
In the embodiment, an inverter is provided in the preceding stage of the capacitor C1 in order to make the signals supplied to the first NMOS MN1 and the capacitor C1 have opposite phases, but the present invention is not limited to such a configuration. In addition, by providing an inverter in front of the first NMOS MN1 so that the output from the oscillation circuit 11 is directly input to the capacitor C1, these signals can be in opposite phase to each other.
[0030]
Further, the method of integrating the LED element and the LED driving circuit is not limited to the example of FIG. 2. For example, a semiconductor chip of the LED driving circuit is provided on the back side of the LED chip sealed with the LED element. Various modifications may be possible, such as fixing the mold to the LED chip directly on the semiconductor chip on which the drive circuit is formed and connecting each electrode by wire bonding or the like. is there.
[0031]
【The invention's effect】
The effects obtained by the representative ones of the inventions disclosed in the present application will be briefly described as follows.
That is, according to the present invention, the LED can be made to emit light with one battery, and there is an effect that it is not necessary to connect a resistor in series with the LED when driving, and power consumption can be reduced.
[0032]
Further, according to the light emitting device of the present invention, it can be driven at a low voltage and power consumption can be reduced, and since it can be handled as one light emitting element in mounting, the degree of freedom in mounting is increased. effective.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of an LED driving circuit suitable for applying the present invention.
FIG. 2 is a plan view showing an LED light emitting device including an LED drive circuit according to an embodiment.
FIG. 3 is a graph comparing the relationship between the applied voltage and the amount of light of the LED light emitting device of the example and a conventional LED.
FIG. 4 is a circuit diagram showing a modification of the LED drive circuit.
[Explanation of symbols]
1 LED drive circuit 1B LED drive circuit 2 LED
11 Oscillator circuit 100 Semiconductor chip 200 LED chip 210 LED luminous body C1 Capacitor MN1 First NMOS
MN2 second NMOS
INV1, INV2 Inverter MP1 First PMOS
MP2 Second PMOS

Claims (5)

The drain terminal is connected to the first power supply terminal, the source terminal is connected to the first electrode of the LED, the substrate potential is connected to the second power supply terminal, and the source terminal of the first N-channel MOSFET is one A capacitor connected to the other electrode, an oscillation circuit that supplies an oscillation signal to the other electrode of the capacitor, the gate terminal of the first N-channel MOSFET, and a CMOS inverter that inverts the output of the oscillation circuit, Based on the output of the oscillation circuit and the output of the CMOS inverter, oscillation signals having opposite phases are supplied to the other electrode of the capacitor and the gate terminal of the first N-channel MOSFET. LED driving circuit. A second N-channel MOSFET having a drain terminal connected to the other electrode of the LED, a source terminal and a substrate potential connected to a second power supply terminal, and the other terminal of the capacitor at the gate terminal of the second N-channel MOSFET. 2. The LED driving circuit according to claim 1, wherein an oscillation signal having the same phase as that of a signal supplied to the electrode is supplied. 3. The first N-channel MOSFET, the capacitor, the oscillation circuit, the CMOS inverter, and the second N-channel MOSFET are formed on one semiconductor chip. The LED driving circuit described. The LED driving circuit according to claim 1, wherein the capacitor is connected as an external element. The LED drive circuit according to claim 3 and an LED element driven by the LED drive circuit are integrally configured.

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