JP2003133065A - EL element drive circuit - Google Patents

Abstract

To provide an EL element driving circuit capable of protecting an EL element connected to an output and a driving circuit itself. SOLUTION: An oscillator circuit 2, a booster circuit 1 for boosting an input DC voltage by switching control based on an oscillation output thereof, and an operating circuit which operates by supplying an output voltage boosted by the booster circuit 1 EL element 2 based on output
And a switching circuit 14 for driving the switching circuit 3 for detecting the abnormal voltage at the output of the booster circuit 1. The output signal of the oscillation circuit 2 supplied to the booster circuit 1 is output from the detection circuit 16. The operation of the booster circuit 1 is stopped based on the signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an EL element drive circuit and an EL element drive method used for a backlight of a liquid crystal display section or the like.

[0002]

2. Description of the Related Art EL devices are used as backlights for liquid crystal display parts and are widely used in various fields such as mobile phones, watches and mobile terminals. As a driving method of an EL element, a method in which a coil and a switching element are used to boost a DC voltage of a dry cell or the like, which is rectified by a diode, and then converted into an AC using a switching element or the like is mainstream.
The EL element applies a voltage of 100 V or more from 100 Hz to 1
It emits light when applied at a frequency of about 000 Hz. Therefore, a high breakdown voltage switching element is used in the output circuit to the EL element. However, since this step-up circuit usually has a simple configuration including a coil, a diode, and a switching element, when the capacity of the EL element decreases with deterioration of the EL element, a high voltage higher than the withstand voltage of the switching element is applied to the switching element. May be destroyed. Also, when the drive circuit operates without connecting the EL element in the factory assembly process, etc., a high voltage exceeding the withstand voltage is applied to the output circuit to the EL element and the switching element for boosting. , Will be destroyed.

FIG. 4 is a diagram showing a circuit configuration of a conventional EL element drive circuit.

In FIG. 4, 1 is a booster circuit, 2 is an oscillation circuit, 3 is a coil, 4 is a switching element, 6 is a rectifying diode, 7 is a frequency dividing circuit, 9 and 10 are high side switching elements, 11 and Reference numeral 12 is a low side switching element, 13 is a power input terminal, 15 is a capacitor for removing high voltage spikes, 23 is an EL element, and 25 and 26 are electrodes of the EL element 23.

The booster circuit 1 operated by the power supply from the power supply input terminal 13 has a coil 3 for storing electromagnetic energy, a switching element 4 for switching control of the coil 3, an oscillator circuit 2, It is composed of a diode 6 that rectifies the energy emitted from the coil 3. The capacitor 15 smoothes the output rectified by the diode 6 and converts it into a DC voltage. The switching element 4 in the booster circuit 1 is switching-controlled by the oscillation output of the oscillator circuit 2. Then, when the switching element 4 is turned on, electromagnetic energy is accumulated in the coil 3, and the electromagnetic energy is stored in the switching element 4
Is released as a high voltage spike from coil 3 when is turned off. The high voltage spike is rectified by the diode 6 and converted into a high DC voltage by the capacitor 15.

The switching circuit 14 operates when the output voltage of the booster circuit 1 is applied via the power supply terminal 8. The high side switching element 9 and the low side switching element 11 are connected in series between the power supply terminal 8 and the ground point, while the high side switching element 10 and the low side switching element 12 are also connected in series between the power supply terminal 8 and the ground point. As a result, an H-bridge type output circuit is formed, and the electrode 2 of the EL element 23 is used for output.
5, 26 are connected.

The frequency dividing circuit 7 is a means for generating control signals for the switching elements 9 to 12 which form the switching circuit 14, and divides the oscillation output of the oscillation circuit 2 to generate an output signal of a predetermined frequency. is doing.

The high side switching element 9,
Inputs of 10 and low-side switching elements 11 and 12 are connected to respective outputs of the frequency dividing circuit 7 to switch the switching elements 9 to 12 so as to generate an alternating output voltage at a predetermined repetition frequency. E with the alternating output voltage
The L element 23 is driven.

The operation of the EL element drive circuit configured as described above will be described below.

FIG. 5 is a diagram showing voltage waveforms at various parts when the EL element is driven by the drive circuit shown in FIG. When a predetermined DC voltage is input to the power input terminal 13, the oscillation circuit 2
Starts to operate, a high frequency signal of frequency f (cil) is input to the gate terminal of the switching element 4, and the switching element 4 repeats on / off. A current flows through the coil 3 when the switching element 4 is on, and at the moment when the switching element 4 is turned off, a high voltage spike indicated by V (CIL) in FIG. 5 occurs at the drain terminal of the switching element 4. The high voltage spike is rectified by the rectifying diode 6 and supplied to the switching circuit 14. The output signal of the oscillation circuit 2 is frequency-divided by the integration integrated circuit 7 and becomes a low frequency signal f (EL), which is input to each switching element of the switching circuit 14. At this time, the signal V (g1) having the same phase is input to the high-side switching element 9 and the low-side switching element 11, and the signal V (g2) delayed by a half wavelength is input to the high-side switching element 10 and the low-side switching element 12.
Is entered. As a result, the electrodes 25, 2 of the EL element 23
In No. 6, charging and discharging are alternately repeated, and high voltage waveforms such as V (EL1) and V (EL2) in FIG. 5 are obtained, respectively. At this time, the power supply terminal 8 of the switching circuit 14 is
As shown in V (DC) of FIG. 5, V (EL1) and V (E
A voltage waveform that is the sum of L2) is obtained.

[0011]

However, in the conventional configuration, the voltage applied to the EL element 23 is
And the value of the spike removing capacitor 15, the oscillation frequency of the oscillator, the equivalent capacitance of the EL element, and the like. Therefore, when designing the EL element drive circuit, it is necessary to design the EL element drive circuit so that the applied voltage to the EL element is equal to or lower than the withstand voltage of the circuit element in consideration of the respective variations. When the EL element deteriorates after being used for a long time, the equivalent capacitance of the EL element decreases, and the output voltage of the drive circuit becomes higher than the initial value even if the same drive circuit is used. It is necessary to allow for a margin for variations in the equivalent capacitance of the EL element 23, which makes it difficult to design the EL element drive circuit.

The present invention solves such a conventional problem and safely drives an EL element without exceeding the withstand voltage of the EL element drive circuit even when the EL element is deteriorated and its capacity is reduced. An object is to provide an EL element drive circuit.

[0013]

To achieve this object, an EL element drive circuit of the present invention comprises a booster circuit for boosting an input DC voltage by switching control by an oscillation output of an oscillator circuit, A switching circuit that operates by supplying a voltage power source to apply an alternating voltage to the EL element; and a detection circuit that detects a high voltage component of the direct current voltage. When the high voltage component reaches a protection voltage, In this configuration, the supply of the oscillation output is stopped.

With this structure, even if the output voltage of the booster circuit rises due to a decrease in the capacitance value of the EL element or a high voltage component is generated due to abnormal operation of the booster circuit, the oscillation output of the oscillator circuit is output to the booster circuit. Since the supply of voltage is stopped and the output voltage of the booster circuit is lowered, deterioration of the EL element can be prevented.

Further, when the detection circuit is constructed by using the Zener diode, a protection circuit that consumes almost no electric power can be constructed because current does not flow to loads other than the EL element during normal operation.

Further, when the inverting circuit is composed of a CMOS inverter, the threshold voltage of the inverting circuit becomes about 1/2 of the power supply voltage, and the output of the boosting circuit is generated by the resistance voltage division of the series circuit by the first and second resistors. A high voltage component included in the voltage can be detected.

[0017]

1 is a circuit configuration diagram showing the configuration of an EL element drive circuit according to the present invention.

In FIG. 1, 1 is a booster circuit, 2 is an oscillation circuit, 3 is a coil, 4 is a switching element such as a MOS transistor and a bipolar transistor, 6 is a rectifying diode, 7 is a frequency dividing circuit, and 9 and 10 are. P channel M
High-side switching element composed of OS transistor or PNP transistor, 11 and 12 are N-channel M
A low-side switching element such as an OS transistor or an NPN transistor, 13 is a power input terminal, 15 is a capacitor for removing high voltage spikes, 16 is a detection circuit,
17 is a Zener diode, 18 is a resistor, 19 is an inverter, 20 is an AND gate, 23 is an EL element, 25 and 26
Is an electrode of the EL element 23.

The booster circuit 1 operated by the power supply from the power supply input terminal 13 has a coil 3 for storing electromagnetic energy, a switching element 4 for controlling switching of the coil 3, an oscillator circuit 2, It is composed of a diode 6 that rectifies the energy emitted from the coil 3. The capacitor 15 smoothes the output rectified by the diode 6 and converts it into a DC voltage. The switching element 4 in the booster circuit 1 is switching-controlled by the oscillation output of the oscillator circuit 2. Then, when the switching element 4 is turned on, electromagnetic energy is accumulated in the coil 3, and the electromagnetic energy is stored in the switching element 4
Is released as a high voltage spike from coil 3 when is turned off. The high voltage spike is rectified by the diode 6 and converted into a high DC voltage by the capacitor 15.

The switching circuit 14 operates when the output voltage of the booster circuit 1 is applied via the power supply terminal 8. The high side switching element 9 and the low side switching element 11 are connected in series between the power supply terminal 8 and the ground point, while the high side switching element 10 and the low side switching element 12 are also connected in series between the power supply terminal 8 and the ground point. As a result, an H-bridge type output circuit is formed, and the electrode 2 of the EL element 23 is used for output.
5, 26 are connected.

The frequency dividing circuit 7 is a means for generating control signals for the switching elements 9 to 12 constituting the switching circuit 14, and divides the oscillation output of the oscillation circuit 2 to generate an output signal of a predetermined frequency. is doing.

The high side switching element 9,
Inputs of 10 and low-side switching elements 11 and 12 are connected to respective outputs of the frequency dividing circuit 7 to switch the switching elements 9 to 12 so as to generate an alternating output voltage at a predetermined repetition frequency. E with the alternating output voltage
The L element 23 is driven.

The detection circuit 16 includes a Zener diode 17 having a cathode connected to the power supply terminal 8, a resistor 18 having one end grounded, and an inverter having an input end connected to an intermediate connection point of a series circuit of the Zener diode 17 and the resistor 18. And an AND circuit 2 that outputs the output signal of the inverter 19
By inputting 0, the output signal of the oscillation circuit 2 is not input to the input terminal of the switching element 4 when an abnormal high voltage is detected.

In the circuit of FIG. 1 which is the first embodiment of the present invention, the voltage applied to the power supply terminal 8 is applied to the zener diode when the peripheral circuit has variations, the EL element has deteriorated, or the EL element 23 has a poor connection. When the Zener voltage Vz of 17 is reached, a current flows through the resistor 18 and the output of the inverter 19 switches from high level to low level. The output signal of the inverter 19 is input to the AND gate 20, the switching control of the switching element 4 is stopped, and the electric charge supply to the EL element 23 is stopped. As a result, the voltage applied to each electrode 25, 26 of the EL element 23 is V (EL1) in FIG.
The waveform becomes as shown in V (EL2), and the high-side switching elements 9 and 10 and the low-side switching element 1
1, 12 and the switching element 4 are not applied with a high voltage exceeding their withstand voltage.

As described above, according to the present embodiment, as shown in FIG. 2, the applied voltage V (EL1) to the EL element 23,
V (EL2) and the output voltage V (CIL) of the booster circuit 1 can be suppressed to a voltage equal to or lower than a preset voltage. Further, when the detection circuit 16 is operating, the switching operation of the booster circuit 1 is stopped, and Power consumption can be reduced.

FIG. 3 is a circuit diagram of the second embodiment of the present invention, which is different in that the Zener diode 17 in the detection circuit 16 of FIG. 1 is replaced with a resistor 21. In this case, when the CMOS inverter 19 is used, the voltage of 1/2 of the power supply voltage Vcc applied to the inverter 19 becomes substantially equal to the threshold voltage Vth of the inverter 19. Therefore,
Since the voltage divided by the series circuit of the resistor 18 and the resistor 21 is input to the input terminal of the inverter 19, the resistance value of the resistor 18 is R18, the resistance value of the resistor 21 is R21, and the high voltage to be detected is If the pulse level is Vp, Vp ≧ Vth
When the condition of (R18 + R21) / R18 is satisfied, the output of the inverter 19 becomes low level, the switching control of the booster circuit 1 by the oscillation circuit 2 is stopped, and operation is performed so as not to boost further.

More specifically, the second embodiment will be described. When the power supply voltage Vcc of the inverter 19 is 5V, the threshold voltage Vth of the inverter 19 is 2.5V, the resistance value of the resistor 18 is 10KΩ, and the resistance value is 10KΩ. 21 resistance value R21 to 470K
When set to Ω, the switching control of the booster circuit 1 is stopped when the voltage of the power supply line (power supply terminal 8) of the switching circuit 14 exceeds Vp = 120V. If the voltage of the power supply terminal 8 at the normal time is 100V, the same function as that of the above-described first embodiment is performed. However, in the second embodiment, the circuit current is constantly consumed through the resistor 21 and the resistor 18, which is disadvantageous in terms of power consumption.

When the inverter 19 is replaced with an operational amplifier type comparator by applying the second embodiment, the reference voltage Vref = at the non-inverting input terminal of the comparator.
When 2.5 V is applied and the inverting input terminal is connected to the intermediate connection point between the resistor 21 and the resistor 18, the same function as the former of the second embodiment is performed. In the case of this application example, if an accurate reference voltage is used, the operating voltage (detection voltage) can be set with much higher accuracy than in any of the above examples.

[0029]

As described above, the EL element drive circuit according to the present invention detects that the voltage applied to the power supply line of the switching circuit has become abnormally large and stops the operation of the booster circuit. The circuit itself can be protected, and the deterioration of the EL element can be prevented by driving the EL element with an excessive voltage.

Further, when the detection circuit is constructed using the Zener diode, almost no current flows in the detection circuit in the normal operation, and the EL element drive with low power consumption can be realized.

Further, when the inverting circuit is composed of a CMOS inverter, the threshold voltage of the inverting circuit becomes about 1/2 of the power supply voltage, and the output of the boosting circuit is generated by the resistance voltage division of the series circuit by the first and second resistors. A high voltage component included in the voltage can be detected.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a first embodiment of the present invention.

FIG. 2 is a voltage waveform diagram according to the first embodiment of the present invention.

FIG. 3 is a circuit diagram of a second embodiment of the present invention.

FIG. 4 is a circuit diagram of a conventional embodiment.

FIG. 5 is a voltage waveform diagram of a conventional embodiment.

[Explanation of symbols]

1 Booster circuit 2 oscillator circuits 3 coils 4 switching elements 6 Rectification diode 7 frequency divider 8 power terminals 9,10 High side switching element 11,12 Low-side switching element 13 Power input terminal 14 Switching circuit 15 Capacitors for removing high voltage spikes 16 Detection circuit 17 Zener diode 18,21 resistance 19 Inverter 20 AND gate 23 EL element 25,26 electrodes

Claims (4)

[Claims] 1. A booster circuit that boosts an input DC voltage by switching control by an oscillation output of an oscillator circuit, a switching circuit that operates by the power supply of the DC voltage and applies an alternating voltage to an EL element, and the DC voltage. And a detection circuit for detecting the high voltage component, and when the high voltage component reaches a protection voltage, the supply of the oscillation output to the booster circuit is stopped. 2. The detection circuit, a series circuit including a Zener diode having a cathode connected to the output end of the booster circuit and a pull-down resistor having one end grounded, and an inverter having an input end connected to the anode of the Zener diode. The EL element drive circuit according to claim 1, wherein the EL element drive circuit comprises: 3. The detection circuit is input to an intermediate connection point between the first and second resistances connected in series between the output end of the booster circuit and a ground point, and the intermediate connection point between the first and second resistances. The EL element drive circuit according to claim 1, wherein the EL element drive circuit comprises an inverter having its ends connected. 4. The EL element drive circuit according to claim 2, wherein the inverter is a CMOS inverter.