JP4722344B2 - LED lighting circuit of portable terminal device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an LED (light emitting diode) lighting circuit of a mobile terminal device suitable for use in a mobile terminal such as a mobile phone and a PHS.
[0002]
[Prior art]
In the cellular phone, it is necessary to operate with a voltage as low as about 3.3 V because a lithium ion battery is used. This is to use a voltage as low as possible in order to lengthen the call / standby time.
[0003]
By the way, with the recent spread of color liquid crystal displays, white LEDs using indium gallium nitride materials are used for backlight illumination. Recently, the surface of the color LCD has become larger due to the spread of e-mails and the like. For this reason, the color liquid crystal display requires about three white LEDs. Further, it is necessary to pass a current of about 15 mA to each white LED in order to make the color LCD easy to see by backlight illumination. However, since the forward drop voltage Vf of the LED using the indium gallium nitride material is as high as about 3.3 V, the battery voltage is 3 in consideration of the series resistance of the white LED and the voltage drop of the switch means for on / off control. When .3v, a sufficient current cannot be supplied to the white LED. For this reason, it is necessary to boost the voltage.
[0004]
On the other hand, when a mail is created in a dark room, a key illumination that brightly illuminates characters printed on the key is necessary. Since the key illumination needs to illuminate the entire key surface, about 10 LEDs are required. In general, inexpensive gallium phosphorus-based yellow-green LEDs are often used for key illumination. The forward drop voltage Vf of the gallium phosphide LED is about 2.2 V, and a booster circuit is unnecessary. When a current of about 5 mA is passed through each LED, a total current of about 50 mA is obtained.
[0005]
FIG. 3 is a circuit diagram showing a configuration of a conventional LED lighting circuit. In this figure, reference numeral 1 is a battery, and the minimum voltage is about 3.3V. Reference numeral 2 denotes a voltage doubler circuit that boosts the voltage of the battery 1 twice. Reference numeral 3 denotes a white LED for a backlight of a liquid crystal display, and its forward drop voltage Vf is 3.3V. Reference numeral 4 denotes a current limiting resistor for the white LED. Reference numeral 5 denotes a key LED for key illumination, and its forward drop voltage Vf is 2.2V. Reference numeral 6 denotes a current limiting resistor for the key LED 5. Here, the white LED 3 is supplied with current from the voltage doubler circuit 2, and the key LED 4 is supplied with current directly from the battery 1. SW1 and SW2 are changeover switches (semiconductor switches).
[0006]
In such a configuration, when a liquid crystal display is used, the switch SW1 is turned on, a current of 15 mA flows through each of the white LEDs 3, and a total current of 45 mA flows. As a result, a current of at least 90 mA flows on the input side of the voltage doubler circuit 2. When the key is used at night or the like, the switch SW2 is turned on, a current of 5 mA flows through each key LED 5, and a total current of 50 mA flows. That is, when both the backlight of the liquid crystal display and the key illumination are used, a battery consumption current of at least 140 mA flows.
[0007]
[Problems to be solved by the invention]
As described above, in a mobile phone, boosting is necessary to light an LED. Conventionally, a power supply device using a transformer is used for boosting. However, if a transformer is used, the device becomes large and cannot be used for a mobile phone. On the other hand, the voltage doubler circuit is simple and inexpensive to design, and is suitable for use in a mobile phone.
However, particularly when both the backlight of the liquid crystal display and the key illumination are used at the same time, a large current consumption of 140 mA or more flows as described above. Therefore, the battery 1 is consumed quickly and the usage time is shortened. There was a problem.
The present invention has been made in view of the above-described circumstances, and an object thereof is an LED lighting circuit capable of reducing current consumption in a portable terminal device that lights LEDs having different forward voltages using a voltage doubler circuit. Is to provide.
[0008]
[Means for Solving the Problems]
The present invention has been made to solve the above-described problems, and has adopted the following means.
The LED lighting circuit of the portable terminal device according to the present invention includes a battery, a first LED having a forward voltage substantially equal to or higher than the battery voltage, and a forward voltage of the battery voltage. a lower second LED than, and a voltage doubler circuit for boosting the voltage of the battery series connecting the first LED and the second LED between the ground and the output terminal of該倍pressure circuit It is characterized by that.
[0009]
A plurality of the first LEDs are connected in parallel, a plurality of the second LEDs are connected in parallel, the first LED is used as a backlight of a liquid crystal display, and the second LED is used as a key. It is used for lighting.
Characterized in that a first switching means for grounding between the series connected first LED and the second the LED.
[0010]
Characterized in that a second switching means for opening and closing between said series connected first LED and the second the LED.
The output terminal of the voltage doubler circuit is connected to the anode side of the first LED, the cathode side of the first LED is connected to the anode side of the second LED, and the cathode side of the second LED is connected to the anode side of the second LED. It is connected to the ground.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing a configuration of an LED lighting circuit for a cellular phone according to a first embodiment of the present invention. In this figure, 1 is a battery and 2 is a voltage doubler circuit. 3 is a white LED for a backlight of the liquid crystal display (first LED: indium gallium nitride material), 4 is a resistor connected in series to the white LED 3, and three series connection circuits of the white LED 3 and the resistor 4 are provided. An LED unit HU is configured by being connected in parallel. The anode of the white LED 3 is connected to the voltage doubler circuit 2, and one end of the resistor 4 is connected to one end of each of the switches SW1 and SW2. A resistor 7 is connected in parallel with the LED unit HU. The resistor 7 has a resistance value such that a current of 5 mA flows when a current of 15 mA flows through the white LED 3. A resistor 8 is connected between the other end of the switch SW1 and the ground.
[0012]
Reference numeral 5 is a key LED for key illumination (second LED: gallium phosphide-based material), 6 is a series resistor of the key LED 5, and 10 sets of series connection circuits of the key LED 5 and the resistor 6 are connected in parallel. An LED unit KU is configured. The anode of each key LED 5 is connected to the other end of the switch SW2, and the other end of each resistor 6 is grounded.
[0013]
In such a configuration, when the backlight of the liquid crystal display and the key illumination are used simultaneously, the switch SW1 is turned off and the switch SW2 is turned on. As a result, the LED unit HU and the LED unit KU are connected in series between the output terminal of the voltage doubler circuit 2 and the ground, and each white LED 3 and each key LED 5 are driven by a current of 50 mA output from the voltage doubler circuit 2. . That is, a current of 15 mA flows through the three white LEDs 3, and a current of 5 mA flows through the resistor 7. In addition, a current of 5 mA flows through each of the ten key LEDs 5. At this time, the output current of the battery 1 is 100 mA. Of course, the resistors 4 and 6 are selected to have resistance values that allow the currents of the white LED 3 and the key LED 5 to flow, respectively.
[0014]
On the other hand, when only the backlight of the liquid crystal display is used, the switch SW1 is turned on and the switch SW2 is turned off. As a result, the voltage to be applied to the LED unit KU in the above-described case is applied to both ends of the resistor 8, and a current of 15 mA flows through each white LED 3 as described above.
[0015]
The above embodiment is an example in which a transmissive color liquid crystal display is used. In the transmissive color liquid crystal display, since the display of the color display cannot be seen unless the backlight using the white LED 3 is turned on even in a bright place, the white LED 3 is turned on first when the luminance is high by an optical sensor or the like. There is a need. For this reason, this is an embodiment in which only the white LED 3 can be turned on. According to this embodiment, when the white LED 3 and the key LED 5 are turned on at the same time, the battery current of 140 mA, which has been conventionally required, can be reduced to 100 mA.
[0016]
Next, a second embodiment of the present invention will be described. FIG. 2 is a circuit diagram showing the configuration of the second embodiment of the present invention. In this figure, the same members as those in FIG. 1 are denoted by the same reference numerals. In the embodiment shown in this figure, one end (LED anode side) of the LED unit for key illumination is connected to the output terminal of the voltage doubler circuit 2, and the other end (resistor side) of the LED unit KU is connected to the switches SW1 and SW2. Is connected to one end of the switch SW1, a resistor 9 is connected between the other end of the switch SW1 and the ground, and one end of the LED unit HU for backlight (the anode side of the LED) is connected to the other end of the switch SW2. The other end (resistance side) is grounded, and a resistor 7 is inserted between the other end of the switch SW2 and the ground.
[0017]
In such a configuration, when the backlight of the liquid crystal display and the key illumination are used simultaneously, the switch SW1 is turned off and the switch SW2 is turned on. As a result, the LED unit KU and the LED unit HU are connected in series between the voltage doubler circuit 2 and the ground, and each white LED 3 is output by the current of 50 mA output from the voltage doubler circuit 2 as in the first embodiment described above. Each of the key LEDs 5 is driven, a current of 15 mA flows through the three white LEDs 3, a current of 5 mA flows through the resistor 7, and a current of 5 mA flows through the ten key LEDs 5. At this time, the output current of the battery 1 is 100 mA.
[0018]
On the other hand, when only the key illumination is used, the switch SW1 is turned on and the switch SW2 is turned off. As a result, the voltage to be applied to the LED unit HU in the above-described case is applied to both ends of the resistor 9, and a current of 5 mA flows through each key LED 5 in the same manner as described above.
[0019]
The above embodiment is an example in the case of using a reflective color liquid crystal display. The reflective color liquid crystal display is an embodiment in the case where only the key LED 5 is to be turned on first because it is not necessary to turn on the backlight using the white LED 3 even in a considerably dark place. Also in this embodiment, as in the first embodiment, when the white LED 3 and the key LED 5 are turned on at the same time, the battery current can be reduced from 140 mA, which has been conventionally required, to 100 mA.
[0020]
【The invention's effect】
As described above, according to the present invention, the following effects can be obtained.
(1) A power supply circuit can be configured without using a transformer.
(2) Larger equipment and higher costs can be solved.
(3) The current consumption of the battery can be greatly reduced compared to the conventional case.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a configuration of an LED lighting circuit according to a first embodiment of the present invention.
FIG. 2 is a circuit diagram showing a configuration of an LED lighting circuit according to a second embodiment of the present invention.
FIG. 3 is a block diagram illustrating a configuration example of a conventional LED lighting circuit.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Battery 2 ... Voltage doubler circuit 3 ... White LED
4 ... Resistance 5 ... Key LED
6,7,8,9 ... resistance

Claims (5)

In a portable terminal device that can be driven by a battery,
Battery,
A first LED having a forward voltage approximately equal to or greater than the battery voltage;
A second LED having a forward voltage lower than the battery voltage;
And a voltage doubler circuit for boosting the voltage of said battery,
LED lighting circuit of the portable terminal device of the first LED and the second LED, characterized in that connected in series between the output end of該倍pressure circuit and the ground.   A plurality of the first LEDs are connected in parallel, a plurality of the second LEDs are connected in parallel, the first LED is used as a backlight of a liquid crystal display, and the second LED is used as a key. The LED lighting circuit of the portable terminal device according to claim 1, wherein the LED lighting circuit is used for illumination. LED lighting circuit of the portable terminal device according to claim 1, characterized in that a first switching means for grounding between the series connected first LED and the second the LED. LED lighting circuit of the portable terminal device according to claim 3, characterized in that a second switching means for opening and closing between said series connected first LED and the second the LED. The output terminal of the voltage doubler circuit is connected to the anode side of the first LED;
The cathode side of the first LED is connected to the anode side of the second LED;
The LED lighting circuit of the portable terminal device according to any one of claims 1 to 4, wherein a cathode side of the second LED is connected to the ground.

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