JP2003151784A - Light-emitting diode drive circuit, method of control thereof, and electronic equipment therewith - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an LED drive circuit stably supplying an optimal current to LEDs. SOLUTION: Output voltage Vg is produced based on the electromotive force Vf of a chemical battery 31. A current I corresponding to the output voltage Vg flows in a load circuit 20. The current I is converted to a voltage Vm by a current-limiting resistance 22. A comparator 45 compares the voltage Vm to a reference voltage Vr; if the voltage Vm is less than the reference voltage Vr, an output signal Vh becomes 'L'. Then the output signal Vk of a comparator 42 depends on an output signal Vj of an oscillator 43. Thus, a transistor 41 is turned on/off, approximately in synchronism with the output signal Vj. When the transistor 41 is on, the energy of the chemical battery 31 is stored in an inductor 49. When the transistor 41 is off, currents flow into the inductor 49 via a diode 48 to maintain a certain output voltage Vg.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LED (light emitting diode) drive circuit, a control method thereof, and the LE.
An LED suitable for use in driving an LED for illuminating a display unit of a battery-powered electronic device such as a mobile phone, which relates to an electronic device including a D drive circuit
The present invention relates to a drive circuit, a control method thereof, and an electronic device including the LED drive circuit.

[0002]

2. Description of the Related Art In a portable electronic device such as a mobile phone, an LED is often used as a lighting means for a display section. In a portable electronic device, since a chemical battery is used as a power source, there is a limit to the operable time. For this reason, there is a demand for an LED drive circuit in which the power consumed by the LED is reduced as much as possible and the operation time of the portable electronic device is extended.

This type of LED drive circuit is conventionally used, for example, as shown in FIG.
/ DC) converter 10. DC /
The load circuit 20 is connected to the output side of the DC converter 10. The DC / DC converter 10 includes a pnp type transistor 11, a comparator 12, and an oscillator 13.
, Resistor 14, comparator 15, resistor 16, reference voltage source 17, diode 18, inductor 19
And a capacitor 1A. Load circuit 2
0 is composed of an LED 21 and a current limiting resistor 22.

The transistor 11 is on / off controlled based on the output signal Ve of the comparator 12. The comparator 12 compares the output signal Vd of the oscillator 13 with the output signal Vc of the comparator 15, and outputs the output signal Ve as a comparison result. The oscillator 13 outputs an output signal Vd having a predetermined frequency. The resistor 14 is a comparator 15
It is a feedback resistor connected from the output side to the non-inverting input side. The comparator 15 outputs the output voltage Vb and the reference voltage V
r is compared, and the output signal Vc is output as the comparison result. The resistor 16 is an input resistor for the non-inverting input side of the comparator 15. The reference voltage source 17 is composed of, for example, a reference voltage IC or the like, and the reference voltage Vr is set. The diode 18 acts as a flywheel diode. The inductor 19 stores energy supplied from the chemical battery 1. The capacitor 1A smoothes the output voltage Vb. The LED 21 emits a constant amount of light when a predetermined current is supplied in the forward direction. The current limiting resistor 22 sets the current flowing through the LED 21.

In this LED drive circuit, the output voltage Vb is generated based on the electromotive force Va of the chemical battery 1. Then, the output voltage Vb is compared with the reference voltage Vr by the comparator 15, and when the output voltage Vb is smaller than the reference voltage Vr, the output signal Vc of the comparator 15 becomes a low level (hereinafter referred to as “L”). At this time, the output signal Ve of the comparator 12 depends on the output signal Vd of the oscillator 13. Therefore, the transistor 11 outputs the output signal Vd
Turns on and off almost in synchronization with. Then, when the transistor 11 is in the ON state, a current flows from the chemical cell 1 to the inductor 19, and energy is stored in the inductor 19. When the transistor 11 is in the off state, a current flows into the inductor 19 via the diode 18 and the constant output voltage Vb is maintained. Further, the output voltage Vb is smoothed by the capacitor 1A.

When the output voltage Vb is higher than the reference voltage Vr, the output signal Vc of the comparator 15 becomes high level (hereinafter referred to as "H"). At this time, the output signal Ve of the comparator 12 becomes "H", and the transistor 11 is turned off. Then, when the output voltage Vb decreases and the output voltage Vb becomes smaller than the reference voltage Vr, the output signal Vc of the comparator 15 becomes “L”, and the output signal Ve of the comparator 12 outputs the oscillator signal similarly to the above. 13 depends on the output signal Vd. In this way, the stable output voltage Vb is supplied to the load circuit 20,
The constant current limited by the current limiting resistor 22 is the LED
It flows to 21.

[0007]

However, the above-mentioned conventional LED drive circuit has the following problems.
That is, since the voltage drop inside the LED 21 varies depending on the current flowing through the LED 21, the temperature, and the like, it is difficult to set the current with high accuracy. To improve the accuracy of the current setting, the output voltage of the DC / DC converter 10 is increased. It is necessary to reduce the influence of the voltage drop inside the LED 21 by setting Vb high and increasing the value of the current limiting resistor 22. However, if the value of the current setting resistor 22 is increased, the output voltage Vb of the DC / DC converter 10 needs to be set higher. In this case, since more power is consumed by the LED 21 and the current limiting resistor 22,
There has been a problem that the use efficiency of the chemical battery 1 deteriorates drastically.

In order to solve this problem, the load circuit 20
Instead of this, a load circuit 20A as shown in FIG. 5 may be used. The load circuit 20A includes an LED 21 and
It is composed of a resistor 23, a Zener diode 24, a transistor 25, and a resistor 26. In this load circuit 20A, the base-emitter voltage of the transistor 25 and the voltage of the resistor 26 are balanced with the zener voltage of the zener diode 24, so that the transistor 2
The base current of 5 is controlled to a constant value, and a constant current flows through the LED 21.

However, in the load circuit 20A, a voltage drop occurs between the collector and the emitter of the transistor 25, and the transistor 25 consumes power. Also, D
The C / DC converter 10 has a fixed output voltage Vb,
It does not have a function for varying the output voltage Vb so that an optimum drive current flows through the LED 21. Therefore, from the output voltage Vb of the DC / DC converter 10 to LE
There is a problem that reactive power is consumed by the voltage excluding the voltage drop at D21.

The present invention has been made in view of the above circumstances, and an LED drive circuit for stably supplying an optimum current to an LED, an electronic device including the LED drive circuit,
Another object of the present invention is to provide a method for controlling the LED drive circuit.

[0011]

In order to solve the above-mentioned problems, the present invention according to claim 1 provides an LED and a load circuit having a current-limiting resistor connected in series to the LED, of a set value. An LED drive circuit for supplying and driving a constant current, which generates an output voltage based on a chemical battery and an electromotive force of the chemical battery, and based on a fluctuation of the voltage generated from both ends of the current limiting resistor. It is characterized by comprising a DC / DC converter for performing negative feedback control of the output voltage.

The invention according to claim 2 is the L according to claim 1.
Regarding the ED drive circuit, the DC / DC converter compares a voltage generated from the current limiting resistor with a reference voltage, and stores / discharges an electromotive force of the chemical battery at a timing based on the comparison result. Thus, the output voltage is configured to be negatively feedback controlled.

The invention according to claim 3 is the L according to claim 2.
The ED drive circuit is characterized in that the reference voltage is set to a product value of the set constant current value and the set current limiting resistance value.

A fourth aspect of the present invention relates to the LED drive circuit according to the first or second aspect, characterized in that the DC / DC converter is a step-down DC / DC converter.

A fifth aspect of the present invention relates to the LED drive circuit according to the first or second aspect, wherein the DC / DC converter is a step-up DC / DC converter.

The invention according to claim 6 relates to an electronic device,
An LED drive circuit according to any one of claims 1 to 5 is provided.

A seventh aspect of the present invention relates to a method for controlling an LED drive circuit, which supplies a constant current of a set value to an LED and a load circuit having a current limiting resistor connected in series to the LED. A chemical battery and a DC / DC converter are provided in advance for the LED drive circuit driven by
A C / DC converter generates an output voltage based on the electromotive force of the chemical battery, and negatively controls the output voltage based on a change in the voltage generated from both ends of the current limiting resistor. .

The invention according to claim 8 is the L according to claim 7.
According to a control method of an ED drive circuit, the DC / DC converter compares a voltage generated from the current limiting resistor with a reference voltage, and stores / discharges an electromotive force of the chemical battery at a timing based on the comparison result. By doing so, the output voltage is negatively feedback controlled.

The invention according to claim 9 is the L according to claim 8.
According to the control method of the ED drive circuit, the reference voltage is set to a product value of the set constant current value and the current limiting resistance value.

[0020]

DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described with reference to the drawings.
The embodiment will be described.First embodiment FIG. 1 shows an LED drive circuit according to a first embodiment of the present invention.
It is a schematic circuit diagram which shows the electric constitution of a path. Of this form
The LED drive circuit is, as shown in FIG.
And a DC / DC converter 40.
The load circuit 20 is provided on the output side of the DC / DC converter 40.
Are connected. DC / DC converter 40 steps down
Type DC / DC converter, pnp type transformer
Resistor 41, comparator 42, oscillator 43,
Anti-44, comparator 45, resistor 46, reference voltage
Source 47, diode 48, inductor 49,
It is composed of a dancer 4A. The load circuit 20 is L
It is composed of an ED 21 and a current limiting resistor 22.
It

The transistor 41 is on / off controlled based on the output signal Vk of the comparator 42. The comparator 42 compares the output signal Vj of the oscillator 43 with the output signal Vh of the comparator 45, and outputs the output signal Vk as a comparison result. The oscillator 43 outputs an output signal Vj having a predetermined frequency. The resistor 44 is a comparator 45.
It is a feedback resistor connected from the output side to the non-inverting input side. The comparator 45 compares the voltage Vm with the reference voltage Vr and outputs an output signal Vh as a comparison result. The resistor 46 is an input resistor for the non-inverting input side of the comparator 45. The reference voltage source 47 is, for example, the reference voltage I.
The reference voltage Vr is set by C and the like. The diode 48 acts as a flywheel diode. The inductor 49 stores the energy supplied from the chemical battery 31. The capacitor 4A smoothes the output voltage Vg. The LED 21 emits a constant amount of light when a predetermined current is supplied in the forward direction. The current limiting resistor 22 sets the current flowing through the LED 21 to an optimum value.

The DC / DC converter 40 is used for the chemical battery 3
The output voltage Vg is generated based on the electromotive force Vf of 1, and the current limiting resistor 22 is used as a current / voltage converting unit for converting the current flowing through the LED 21 into a voltage. Voltage Vm generated from the reference voltage V
r and the electromotive force of the chemical battery 31 is changed to the inductor 49 and the capacitor 4 at a timing based on the comparison result.
By accumulating / discharging at A, the output voltage Vg is negatively feedback controlled. The reference voltage Vr is set to the value of the product of the value of the constant current optimally set for the LED 21 and the value of the current limiting resistor 22.

Next, a method of controlling the LED drive circuit of this embodiment will be described. In this LED drive circuit, the output voltage Vg is generated based on the electromotive force Vf of the chemical battery 31. The output voltage Vg is applied to the load circuit 20, and a current I corresponding to the output voltage Vg flows. The current I is converted into the voltage Vm by the current limiting resistor 22. And the voltage Vm
Is compared with the reference voltage Vr by the comparator 45, and when the voltage Vm is smaller than the reference voltage Vr, the output signal Vh of the comparator 45 becomes "L". At this time, the output signal Vk of the comparator 42 is the output signal Vj of the oscillator 43.
Depends on. Therefore, the transistor 41 is turned on / off substantially in synchronization with the output signal Vj. Then, when the transistor 41 is in the ON state, a current flows from the chemical battery 31 to the inductor 49, and energy is stored in the inductor 4.
Accumulated in 9. When the transistor 41 is off,
A current flows into the inductor 49 via the diode 48, and a constant output voltage Vg is maintained. Further, the output voltage Vg is smoothed by the capacitor 4A.

When the voltage Vm is higher than the reference voltage Vr, the output signal Vh of the comparator 45 becomes "H". At this time, the output signal Vk of the comparator 42 becomes "H", and the transistor 41 is turned off. Then, the output voltage Vg decreases and the voltage Vm changes to the reference voltage Vr.
When it becomes smaller than the above, the output signal Vh of the comparator 45 becomes "L", and similarly to the above, the comparator 42
Of the output signal Vk depends on the output signal Vj of the oscillator 43. In this way, the output voltage Vg of the DC / DC converter 40 is controlled so that the voltage Vm is kept constant (that is, the value of the current I flowing through the LED 21 is kept constant).

As described above, in the first embodiment,
The current limiting resistor 22 serially connected to the LED 21 of the load circuit 20 is used as a current / voltage converting means, the voltage Vm generated from the current limiting resistor 22 is compared with the reference voltage Vr, and the output is made based on this comparison result. Since the voltage Vg is negatively feedback-controlled, the output voltage Vg is controlled to a value necessary and sufficient for allowing an optimum current to flow through the LED 21. For this reason,
There is no ineffective power consumption in the load circuit 20, and the electromotive force Vf of the chemical battery 31 is used with high efficiency.

[0026]Second embodiment FIG. 2 shows an LED drive circuit according to a second embodiment of the present invention.
1 is a schematic circuit diagram showing an electrical configuration of a path, which is a first embodiment.
Elements common to the elements in FIG.
It is attached. In the LED drive circuit of this form, as shown in FIG.
As shown, instead of the DC / DC converter 40 in FIG.
Thus, a DC / DC converter 50 is provided. DC
The DC / DC converter 50 is a step-up DC / DC converter.
And an npn-type transistor 51 and a comparator.
Data 52, oscillator 53, resistor 54, comparator
55, a resistor 56, a reference voltage source 57, and a diode 5
8, an inductor 59, and a capacitor 5A
Has been.

The transistor 51 is on / off controlled based on the output signal Vk of the comparator 52. The comparator 52 compares the output signal Vj of the oscillator 53 with the output signal Vh of the comparator 55, and outputs the output signal Vk as a comparison result. The oscillator 53 outputs an output signal Vj having a predetermined frequency. The resistor 54 is a comparator 55.
It is a feedback resistor connected from the output side to the non-inverting input side. The comparator 55 compares the voltage Vm with the reference voltage Vr and outputs an output signal Vh as a comparison result. The resistor 56 is an input resistor for the non-inverting input side of the comparator 55. The reference voltage source 57 is, for example, the reference voltage I.
The reference voltage Vr is set by C and the like. The diode 58 acts as a flywheel diode. The inductor 59 stores the energy supplied from the chemical battery 31. The capacitor 5A smoothes the output voltage Vg.

The DC / DC converter 50 is used for the chemical battery 3
The output voltage Vg is generated based on the electromotive force Vf of 1, and the current limiting resistor 22 is used as a current / voltage converting unit for converting the current flowing through the LED 21 into a voltage. Voltage Vm generated from the reference voltage V
r and the electromotive force of the chemical battery 31 is changed to the inductor 59 and the capacitor 5 at a timing based on the comparison result.
By accumulating / discharging at A, the output voltage Vg is negatively feedback controlled. The reference voltage Vr is set to the value of the product of the value of the constant current optimally set for the LED 21 and the value of the current limiting resistor 22.

Next, a method of controlling the LED drive circuit of this embodiment will be described. In this LED drive circuit, the output voltage Vg is generated based on the electromotive force Vf of the chemical battery 31. The output voltage Vg is applied to the load circuit 20, and a current I corresponding to the output voltage Vg flows. The current I is converted into the voltage Vm by the current limiting resistor 22. And the voltage Vm
Is compared with the reference voltage Vr by the comparator 55. When the voltage Vm is higher than the reference voltage Vr, the output signal Vh of the comparator 45 becomes "H". At this time, the output signal Vk of the comparator 52 is the output signal Vj of the oscillator 53.
Depends on. Therefore, the transistor 51 is turned on / off in almost synchronization with the output signal Vj. Then, when the transistor 51 is in the ON state, a current flows from the chemical cell 31 to the inductor 59, and energy is stored in the inductor 5.
Accumulated in 9. When the transistor 51 is off,
Load circuit 2 from inductor 59 via diode 58
A current flows into 0, and a constant output voltage Vg is maintained. Further, the output voltage Vg is smoothed by the capacitor 5A.

When the voltage Vm is lower than the reference voltage Vr, the output signal Vh of the comparator 55 becomes "L". At this time, the output signal Vk of the comparator 52 becomes "L", and the transistor 51 is turned off. Then, the output voltage Vg increases and the voltage Vm changes to the reference voltage Vr.
Output signal Vh of the comparator 55 becomes “H”, and the comparator 52
Of the output signal Vk depends on the output signal Vj of the oscillator 53. In this way, the output voltage Vg of the DC / DC converter 50 is controlled so that the voltage Vm is kept constant (that is, the value of the current I flowing through the LED 21 is kept constant).

As described above, in the second embodiment,
There are advantages similar to those of the first embodiment.

[0032]Third embodiment FIG. 3 is a circuit diagram of an electronic device according to a third embodiment of the present invention.
It is a schematic block showing a physical structure. This form of electron
The device is a mobile phone 60, as shown in FIG.
Antenna 61, power amplifier 62, transceiver 63, controller
64, driver 65, display 66, microphone speaker
Marker 67, power supply circuit 68, regulator 69, DC / D
The C converter 6A and the load circuit 6B are provided. Pa
The word amplifier 62 is for transmission output from the transmitter / receiver 63.
The signal is transmitted via the antenna 61 to the TDMA (Time Division
Transmitted as a radio wave transmitted by the Multiple Access method
It In Japan, the TDMA system is a PDC (Personal D
egital Cellular) method is used in Europe etc.
Is GSM (Global System for Mobile communications)
 ) Method is used.

The transmission / reception unit 63 transmits / receives a radio signal via the antenna 61. The control unit 64 includes a CPU (central processing unit) and the like, and controls the overall operation of the mobile phone 40 based on a control program. The driver 65 converts the audio signal blown from the microphone / speaker unit 67 into a digital signal, converts the received digital signal into an audio signal, and sends the audio signal to the microphone / speaker unit 67. Further, the driver 65 sends a display signal to the display 66. The display 66 is an LCD (Liqu
id Crystal Display, liquid crystal display device, etc.,
Display information such as various messages to the user. The power supply circuit 68 has a chemical battery 68 a and supplies an output voltage to the power amplifier 62. The regulator 69 inputs the output voltage of the power supply circuit 68 and outputs a constant voltage of a predetermined value,
The data is supplied to the transmission / reception unit 63, the control unit 64, the driver 65, and the microphone / speaker 67. DC / DC converter 6A
Is configured similarly to the DC / DC converter 40 in FIG. 1 showing the first embodiment. In addition, the load circuit 6B
Is configured similarly to the load circuit 20 in FIG. 1, and the LED in the load circuit 6B is used for illuminating the display 66.

Next, the operation of the electronic device of this embodiment will be described. In this mobile phone, the transmission radio wave is transmitted by the TDMA method, and at this time, the power supply circuit 68 transmits the same radio wave.
A pulsed load current having a frequency based on the DMA method is taken out. Further, an optimum current is supplied from the chemical battery 68a to the load circuit 6B via the DC / DC converter 6A, and the display 6 is displayed by the LED in the load circuit 6B.
Illumination light is supplied to 6.

As described above, in the third embodiment,
The DC / DC converter 6A and the load circuit 6B are the DC / DC converter 40 and the load circuit 20 of the first embodiment.
Since the same load circuit 6B has no ineffective power consumption, the electromotive force of the chemical battery 68a is used with high efficiency.

The embodiment of the present invention has been described in detail above with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like within the scope not departing from the gist of the present invention. Also included in the present invention. For example, the transistor 41 is a p-channel MO transistor having a low on-resistance.
It may be SFET. Further, in the first embodiment, the transistor 41 is turned on / off substantially in synchronization with the output signal Vj of the oscillator 43, but the transistor 41 is PWM (Pulse Width Modulation) controlled according to the level of the voltage Vm. The DC / DC converter 40 may be configured so that. The reference voltage sources 47 and 57 may be Zener diodes or the like in addition to the reference voltage IC. The DC / DC converters 40 and 50 may have any circuit configuration as long as they are configured to take in the voltage Vm generated from the current limiting resistor 22 and control the output voltage Vg by negative feedback.

[0037]

As described above, according to the configuration of the present invention, the voltage generated from both ends of the current limiting resistor connected in series to the LED of the load circuit is compared with the reference voltage, and based on the comparison result. Output voltage is controlled by negative feedback,
The output voltage is controlled to a value that is necessary and sufficient for passing an optimum current through the LED. For this reason, it is possible to realize an LED drive circuit in which there is no ineffective power consumption in the load circuit and the electromotive force Vf of the chemical battery is used with high efficiency. Furthermore, L of this invention
Since the ED drive circuit is used in the electronic device, there is no ineffective power consumption in the load circuit, and the electromotive force of the chemical battery can be used with high efficiency.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an electrical configuration of an LED drive circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing an electrical configuration of an LED drive circuit according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing an electrical configuration of an electronic device according to a third embodiment of the present invention.

FIG. 4 is a circuit diagram showing an electrical configuration of a conventional LED drive circuit.

FIG. 5 is a circuit diagram of a load circuit 20A.

[Explanation of symbols]

20 load circuit 21 LED 22 Current limiting resistor 31 chemical battery 40,50 DC / DC converter 41,51 transistors 42, 45, 52, 55 comparator 43,53 oscillator 44,46,54,56 resistance 47,57 Reference voltage source 48,58 diode 49,59 inductor 4A, 5A capacitors

Claims (9)

[Claims] 1. A light emitting diode drive circuit for supplying and driving a constant current of a set value to a load circuit having a light emitting diode and a current limiting resistor connected in series to the light emitting diode, which is a chemical battery. And a DC / DC converter that generates an output voltage based on the electromotive force of the chemical battery and negatively feedback controls the output voltage based on fluctuations in the voltage generated from both ends of the current limiting resistor. A light emitting diode drive circuit characterized by the following. 2. The DC / DC converter compares a voltage generated from the current limiting resistor with a reference voltage, and stores / discharges an electromotive force of the chemical battery at a timing based on the comparison result. 2. The light emitting diode drive circuit according to claim 1, wherein the output voltage is configured to be negatively feedback controlled. 3. The light emitting diode drive circuit according to claim 2, wherein the reference voltage is set to a value of a product of a value of the set constant current and a value of the current limiting resistor. . 4. The light emitting diode drive circuit according to claim 1, wherein the DC / DC converter is composed of a step-down DC / DC converter. 5. The light emitting diode drive circuit according to claim 1, wherein the DC / DC converter is a step-up DC / DC converter. 6. An electronic device comprising the light emitting diode drive circuit according to claim 1. 7. A light emitting diode drive circuit for driving a load circuit having a light emitting diode and a current limiting resistor connected in series with the light emitting diode by supplying a constant current of a set value, wherein a chemical battery and a direct current are used. / DC converter is provided, the DC / DC converter generates an output voltage based on the electromotive force of the chemical battery, and the output voltage is generated based on the fluctuation of the voltage generated from both ends of the current limiting resistor. A method for controlling a light emitting diode drive circuit, characterized in that negative feedback control is performed. 8. The DC / DC converter compares a voltage generated from the current limiting resistor with a reference voltage, and accumulates / discharges an electromotive force of the chemical battery at a timing based on the comparison result. 8. The method of controlling a light emitting diode drive circuit according to claim 7, wherein the output voltage is negatively feedback controlled. 9. The light emitting diode drive circuit according to claim 8, wherein the reference voltage is set to a value of a product of a value of the set constant current and a value of the current limiting resistor. Control method.