JP6291290B2 - LED driving device, control method, and portable terminal - Google Patents

Description

  The present invention relates to a technique for driving and controlling an LED that emits light as a backlight on a display unit of a portable terminal.

A liquid crystal display (LCD) is widely used in portable terminals and the like. A backlight is disposed on the back surface of the liquid crystal display device to increase the luminance of the liquid crystal display device. As a light source of the backlight, a white LED (Light Emitting Diode) is generally used.
The LED used for the backlight is generally a high-luminance type, and requires a forward voltage (Vf) of about 3.3 V or more. On the other hand, a battery mounted on a portable terminal has a characteristic that its voltage decreases with use. For this reason, a voltage is not directly supplied from the battery to the LED, and a booster circuit that boosts the battery voltage is used (Patent Document 1).

International Publication No. 2008/050402

However, as described above, there is a problem that the cost of the device increases by providing the booster circuit.
On the other hand, when the booster circuit is not provided, as shown below, the battery voltage fluctuates depending on the use conditions, and this fluctuation causes fluctuation of the current flowing through the LED, and the luminous intensity of the LED fluctuates.

Assume that the Vf-If (forward voltage-forward current) characteristics of the LED are as shown in FIG. It is assumed that the current flowing through the LED is 20 mA, and the battery voltage becomes 3.4 V due to a decrease in the remaining battery level (302). In this case, according to the Vf-If characteristic of FIG. 7, the current that can be passed through the LED is 27 mA (303, 304). Therefore, there is no problem in this state.
Here, if the battery voltage decreases by 300 mV due to a change in the usage conditions of the mobile terminal, for example, due to a change in the wireless environment or a change in the application being used, the battery voltage becomes 3.1 V (305). At this time, the current that may flow through the LED is 10 mA from the Vf-If characteristic (306, 307). That is, only a current of 10 mA can flow instead of the set current of 20 mA. When a current of 10 mA is passed, the luminous intensity of the LED is reduced as compared with the case of 20 mA. Next, when the battery voltage returns to the original state (3.4 V) (302), the current that can be passed through the LED is 27 mA (303, 304). At this time, when a current of 20 mA is supplied to the LED, the luminous intensity of the LED returns to the original state.

As a result, there is a problem that the user sees the display unit of the mobile terminal flickering.
SUMMARY OF THE INVENTION An object of the present invention is to provide an LED driving device, a control method, and a mobile terminal that can solve the above-described problems and prevent flickering of a display unit without providing a booster circuit.

In order to achieve the above object, one aspect of the present invention is an LED driving device that emits light as a backlight to a display unit of a mobile terminal, and the battery voltage value range in a no-load state of the mobile terminal; In the forward voltage forward current characteristics of the LED, a storage unit storing a current value smaller than a characteristic current value corresponding to the voltage value range, and a voltage value of the battery after a voltage drop due to a current load is obtained. An acquisition unit that performs determination, a determination unit that determines whether or not the acquired voltage value is within the voltage value range, and if the voltage value is within the voltage value range, corresponds to the voltage value range from the storage unit A supply controller that controls the LED to supply a constant current indicated by the read current value, and a constant current that supplies the LED with the constant current indicated by the current value. And a part, the current value stored in the storage unit, the voltage value excluding the maximum value of the voltage drop which occurs when the lower limit value of the voltage value range of the battery under load to the mobile terminal The current value is smaller than the corresponding characteristic current value .

Another embodiment of the present invention is a mobile terminal, a battery, a display unit, an LED that irradiates light as a backlight to the display unit, a voltage value range in an unloaded state of the battery, In the forward voltage forward current characteristics of the LED, a storage unit storing a current value smaller than a characteristic current value corresponding to the voltage value range, and a voltage value of the battery after a voltage drop due to a current load is obtained. An acquisition unit that performs determination, a determination unit that determines whether or not the acquired voltage value is within the voltage value range, and if the voltage value is within the voltage value range, corresponds to the voltage value range from the storage unit A supply control unit that controls to read the current value and supply a constant current indicated by the read current value to the LED, and a constant current unit that supplies a constant current indicated by the current value to the LED. Preparation The current value stored in the storage unit, the characteristic current corresponding to the voltage value excluding the maximum value of the voltage drop which occurs when the lower limit value of the voltage value range of the battery under load to the mobile terminal The current value is smaller than the value .

  According to these aspects, it is possible to prevent the display unit from flickering without providing a booster circuit.

It is an external view of the portable terminal 100 as embodiment of this invention. 2 is a block diagram showing a configuration of a mobile terminal 100. FIG. An example of the data structure of the control table 151 is shown. It is the data table 201 for demonstrating the control table 151. FIG. 5 is a flowchart showing the operation of the mobile terminal 100. It is a flowchart which shows operation | movement of the portable terminal 100 as a modification. An example of the Vf-If characteristic of white LED is shown.

1. Embodiment A portable terminal 100 according to an embodiment of the present invention will be described.
The mobile terminal 100 is configured to be able to make a call, send / receive e-mail, or send / receive data to / from another mobile terminal or mobile phone via a radio base station and a mobile phone network. Moreover, the portable terminal 100 can fulfill the functions of the application program by executing various application programs. Examples of the application program include a web browser, a music playback program, and a camera shooting program.

As shown in FIG. 1, in the portable terminal 100, the liquid crystal panel unit 110 is disposed so that the display surface is exposed. The liquid crystal panel unit 110 includes a backlight 114, and the backlight 114 irradiates light on the back surface of the display surface of the liquid crystal panel unit 110.
1.1 Mobile terminal 100
As shown in FIG. 2, the mobile terminal 100 includes an antenna 101, a wireless communication unit 102, a control unit 103, an input control unit 104, a key input unit 105, an audio processing unit 106, a speaker 107, a microphone 108, and an input / output control unit 109. , A liquid crystal panel unit 110, a battery 111, a power source unit 112, an LED drive circuit 113, a storage unit 115, and a timer 116. The LED drive circuit 113 includes constant current circuits 133, 134, 135, and 136. The control unit 103 includes an acquisition unit 137, a determination unit 138, and a supply control unit 139. The liquid crystal panel unit 110 includes a touch pad unit 110a, a liquid crystal display unit 110b, and a backlight 114. The backlight 114 includes LEDs 141, 142, 143, and 144. The power supply unit 112 includes a voltage detection unit 121.

Specifically, the mobile terminal 100 is a computer system including a microprocessor, a digital signal processor, a ROM, a RAM, and the like. A computer program is stored in the RAM. Each processor operates according to the computer program, whereby the portable terminal 100 achieves its functions.
(1) Storage unit 115
The memory | storage part 115 is comprised from the non-volatile semiconductor memory as an example.

The storage unit 115 stores a control table 151, other data, and computer programs such as application programs.
The control table 151 is used to set a current value that flows through the LEDs 141, 142, 143, and 144. As an example, the control table 151 includes a plurality of control data as shown in FIG. Each control data includes an identification number, a voltage value range, and a current value in association with each other.

The identification number is identification information for uniquely identifying the corresponding control data.
The voltage value range indicates a voltage value range of the battery 111. The unit is V (volt).
The current value indicates the value of the current passed through the LEDs 141, 142, 143 and 144. The unit is mA (milliampere).
When the voltage value of the battery 111 (the voltage value after the voltage drop due to the load at the present time) exists in any voltage value range of the control table 151, the current values corresponding to the voltage value range are the LEDs 141, 142, 143. And 144 are set as the values of the currents to be passed through.

As shown in FIG. 3, the control table 151 stores a plurality of control data 161, 171, 181 and 191 as an example.
The control data 161 includes an identification number 162 “1”, a voltage value range 163 “3.768 or more”, and a current value 164 “20”. The identification number 162 “1” uniquely identifies the control data 161. The voltage value range 163 “3.768 or more” indicates that the voltage value range is 3.768 V or more. The current value 164 “20” indicates that the current flowing through the LEDs 141, 142, 143, and 144 is 20 mA. According to the control data 161, when the voltage value of the battery 111 (the voltage value after the voltage drop due to the current load) is 3.768V or more, the current value of 20 mA is the current value that flows through the LEDs 141, 142, 143, and 144. Is set.

  The control data 171 includes an identification number 172 “2”, a voltage value range 173 “3.707 to 3.767”, and a current value 174 “15”. The identification number 172 “2” uniquely identifies the control data 171. The voltage value range 173 “3.707 to 3.767” indicates that the voltage value range is 3.707V or more and 3.767V or less. The current value 174 “15” indicates that the current flowing through the LEDs 141, 142, 143, and 144 is 15 mA. According to the control data 171, when the voltage value of the battery 111 (the voltage value after the voltage drop due to the current load) is 3.707V or more and 3.767V or less, the current value of 15mA is the LED 141, 142, 143 and 144. It is set as a current value to be passed through.

  The control data 181 includes an identification number 182 “3”, a voltage value range 183 “3.656 to 3.706”, and a current value 184 “13”. The identification number 182 “3” uniquely identifies the control data 181. The voltage value range 183 “3.656 to 3.706” indicates that the voltage value range is 3.656V or more and 3.706V or less. The current value 184 “13” indicates that the current passed through the LEDs 141, 142, 143, and 144 is 13 mA. According to the control data 181, when the voltage value of the battery 111 (the voltage value after the voltage drop due to the current load) is 3.656 V or more and 3.706 V or less, the current value of 13 mA is the LED 141, 142, 143 and 144. It is set as a current value to be passed through.

  The control data 191 includes an identification number 192 “4”, a voltage value range 193 “3.655 to 3.400”, and a current value 194 “10”. The identification number 192 “4” uniquely identifies the control data 191. The voltage value range 193 “3.655 to 3.400” indicates that the voltage value range is 3.655V or more and 3.400V or less. A current value 194 “10” indicates that the current flowing through the LEDs 141, 142, 143, and 144 is 10 mA. According to the control data 191, when the voltage value of the battery 111 (the voltage value after the voltage drop due to the load at the present time) is 3.655V or more and 3.400V or less, the current value of 10mA is the LED 141, 142, 143 and 144. It is set as a current value to be passed through.

Next, the reason why the voltage value range and the current value are set as shown in the control table 151 will be described with reference to the data table 201 shown in FIG.
The data table 201 includes seven rows of data columns 211, 212,.
Each data in the data column 211 in the first row indicates the remaining battery level as a percentage.

Each data in the data column 212 in the second row is an actual measurement value, corresponding to the remaining battery level in the data column 211 in the first row, and indicating the battery voltage in a state where there is no load that consumes power. The unit is mV (millivolt).
Each data in the data row 213 in the third row is an actual measurement value, corresponding to the remaining battery level in the data row 211 in the first row, and the battery voltage when a load that consumes the maximum power is applied to the mobile terminal 100 Indicates. The unit is mV (millivolt). Here, an example of the load is a call, transmission / reception of e-mail, transmission / reception of data, web browser, music playback, camera shooting, and the like. The load that consumes the maximum power is the power consumption that is the largest among the combinations of loads that can be used simultaneously in the mobile terminal 100.

Each data in the data column 214 in the fourth row has the same value as the corresponding data in the data column in the third row. Each data in the data column 214 in the fourth row is shown as a forward voltage of the LED.
Each data in the data column 215 in the fifth row indicates the forward current of the LED based on the forward voltage-forward current characteristic of the LED. Each forward current in the data column 215 in the fifth row corresponds to a corresponding forward voltage in the data column 214 in the fourth row.

Each data in the data column 216 in the sixth row indicates a current value set for the LED. Here, the difference between the data in the data column 215 in the fifth row and the data in the corresponding data column 216 in the sixth row is smaller than the threshold value. The threshold is 5.0 as an example.
Each data in the data column 217 of the seventh row indicates an identification number of the control table 151 in FIG.
As shown in FIG. 4, when the remaining battery level is 30%, the battery voltage in the no load state is 3768 mV, and the battery voltage in the state where the load that consumes the maximum power is applied is 3562 mV. Based on the forward voltage-forward current characteristic of the LED, the forward current is 20.2 mA for the battery voltage (3562 mV) under a load that consumes the maximum power. Therefore, 20.0 mA smaller than 20.2 mA is set as the set current value of the LED. Here, the difference between 20.2 mA and 20.0 mA is smaller than the threshold value.

  When the remaining battery level is 25%, the battery voltage in the no load state is 3756 mV, and the battery voltage in the state with the load that consumes the maximum power is 3546 mV. Based on the forward voltage-forward current characteristics of the LED, the forward current with respect to the battery voltage (3546 mV) in a state where a load that consumes the maximum power is applied is 19.8 mA. Therefore, 15.0 mA, which is smaller than 19.8 mA, is set as the set current value of the LED. Here, the difference between 19.8 mA and 15.0 mA is smaller than the threshold value.

Further, when the battery remaining amount is 20% and 15%, 15.0 mA is set as the set current value of the LED in the same manner as described above.
When the remaining battery level is 10%, the battery voltage in the no load state is 3673 mV, and the battery voltage in the state where a load that consumes the maximum power is applied is 3474 mV. Based on the forward voltage-forward current characteristics of the LED, the forward current is 15.4 mA for the battery voltage (3474 mV) under a load that consumes the maximum power. Therefore, 13.0 mA, which is smaller than 15.4 mA, is set as the set current value of the LED. Here, the difference between 15.4 mA and 13.0 mA is smaller than the threshold value.

Further, when the battery remaining amount is 5%, 13.0 mA is set as the set current value of the LED in the same manner as described above.
When the remaining battery level is 0%, the battery voltage without load is 3400 mV, and the battery voltage with a load that consumes the maximum power is 3300 mV. Based on the forward voltage-forward current characteristics of the LED, the forward current with respect to the battery voltage (3300 mV) in a state where a load that consumes the maximum power is applied is 10.2 mA. Therefore, 10.0 mA, which is smaller than 10.2 mA, is set as the set current value of the LED. Here, the difference between 10.2 mA and 10.0 mA is smaller than the threshold value.

As described above, by determining the smallest current value for each voltage value range, when there is a voltage variation within each voltage value range, the luminous intensity of the LED is minimized, and as a result, the liquid crystal It is possible to prevent the display unit 110b from flickering.
In this manner, the set current value is determined for each voltage value range, and the control table 151 that stores the current value for each voltage value range shown in FIG. 3 is generated.

(2) Antenna 101 and wireless communication unit 102
The antenna 101 transmits and receives radio signals to and from the radio base station via a radio line. The wireless communication unit 102 performs frequency selection, frequency conversion, and the like of a wireless signal transmitted and received by the antenna 101.
(3) Speaker 107, microphone 108, voice processing unit 106, key input unit 105, input control unit 104, and timer 116
The speaker 107 outputs sound such as voice.

The microphone 108 inputs sound such as voice.
The audio processing unit 106 demodulates the audio signal received by the wireless communication unit 102 and outputs it to the speaker 107 as an acoustic signal. In addition, the audio signal converted into an electrical signal corresponding to the acoustic signal input from the microphone 108 is modulated and transmitted by the wireless communication unit 102.
The key input unit 105 includes a power key as a power switch for turning the mobile terminal 100 ON or OFF, a numeric keypad, and other keys and buttons. When these keys and buttons are operated, the key input unit 105 generates operation information corresponding to the operated keys and buttons. Next, the key input unit 105 outputs the generated operation information to the control unit 103 via the input control unit 104.

The input control unit 104 relays operation information between the key input unit 105 and the control unit 103.
The timer 116 measures the passage of time.
(4) Liquid crystal panel unit 110 and input / output control unit 109
(LCD panel 110)
The liquid crystal panel unit 110 includes a touch pad unit 110a, a liquid crystal display unit 110b, and a backlight 114. The liquid crystal display unit 110b has a rectangular display surface, and a touch pad unit 110a is attached to the display surface. The liquid crystal display unit 110b displays a screen including icons and other objects received from the control unit 103 via the input / output control unit 109. The touch pad unit 110a detects the contact position of the operating body with respect to the display surface of the liquid crystal panel unit 110, and outputs the detected contact position to the input / output control unit 109 as an input signal.

The backlight 114 is installed on the back surface of the liquid crystal display unit 110b. The backlight 114 includes four LEDs 141, 142, 143, and 144 so that light is emitted toward the back surface of the liquid crystal display unit 110b.
The LEDs 141, 142, 143, and 144 are supplied with constant forward current from the corresponding constant current circuits 133, 134, 135, and 136, respectively, and emit light by the supplied forward current.

(Input / output control unit 109)
The input / output control unit 109 relays input / output of information between the liquid crystal panel unit 110 and the control unit 103.
(5) LED drive circuit 113
The LED drive circuit 113 is composed of constant current circuits 133, 134, 135 and 136.

Each of the constant current circuits 133, 134, 135, and 136 is supplied with power from the power supply unit 112. Also, the current value designation is received from the supply control unit 139, a constant current indicated by the received current value is generated, and the generated constant current is supplied to the LEDs 141, 142, 143, and 144, respectively.
(6) Battery 111 and power supply unit 112
The battery 111 is a rechargeable secondary battery.

The power supply unit 112 supplies the power output from the battery 111 to each component of the mobile terminal 100. In addition, the power supply unit 112 includes a voltage detection unit 121 that detects a voltage of the battery 111 after a voltage drop due to a load at the present time. The voltage detection unit 121 outputs the detected voltage value to the control unit 103.
(7) Control unit 103
The control unit 103 controls the wireless communication unit 102, the input control unit 104, the voice processing unit 106, and the input / output control unit 109 that are components of the mobile terminal 100.

The control unit 103 includes an acquisition unit 137, a determination unit 138, and a supply control unit 139.
The acquisition unit 137 acquires the voltage value of the battery 111 after the voltage drop due to the current load from the voltage detection unit 121 of the power supply unit 112. Next, the acquired voltage value is output to the determination unit 138.

The determination unit 138 determines whether or not the acquired voltage value is within any voltage value range of the control table 151 stored in the storage unit 115.
When the determination unit 138 determines that the acquired voltage value is in any voltage value range, the supply control unit 139 reads a current value corresponding to the voltage value range from the storage unit 115. Next, the constant current circuits 133, 134, 135, and 136 are controlled so that a constant current indicated by the read current value is supplied to the LEDs 141, 142, 143, and 144.

1.2 Operation of Mobile Terminal 100 The operation of the mobile terminal 100 when a constant current is supplied to the LEDs 141, 142, 143, and 144 will be described with reference to the flowchart shown in FIG.
The supply control unit 139 sets the constant current supplied to the LEDs 141, 142, 143, and 144 as an initial value to a current value (20 mA) corresponding to the identification number “1” of the control table 151. Next, the supply control unit 139 instructs the constant current circuits 133, 134, 135, and 136 to supply a constant current of 20 mA. The constant current circuits 133, 134, 135, and 136 supply a constant current of 20 mA to the LEDs 141, 142, 143, and 144, respectively. The LEDs 141, 142, 143, and 144 each emit light with a constant current of 20 mA (step S101).

The control unit 103 uses the timer 116 to count 20 seconds (step S102). If 20 seconds have not elapsed ("NO" in step S102), the process returns to step S102 and the process is repeated.
When 20 seconds have elapsed (“YES” in step S102), the acquisition unit 137 acquires the voltage value of the battery 111 after the voltage drop due to the current load from the voltage detection unit 121 and reads it (step S103). .

Next, the determination unit 138 searches the control table 151 stored in the storage unit 115 for a voltage value range including the read voltage value (step S104).
Next, when a voltage value range including a voltage value exists, the supply control unit 139 reads a current value corresponding to the voltage value range from the control table 151 (step S105).
Next, the supply control unit 139 sets the constant current supplied to the LEDs 141, 142, 143, and 144 to the read current value. Next, the supply control unit 139 instructs the constant current circuits 133, 134, 135, and 136 to supply a constant current indicated by the read current value. The constant current circuits 133, 134, 135, and 136 supply constant currents indicated by the read current values to the LEDs 141, 142, 143, and 144, respectively. Each of the LEDs 141, 142, 143, and 144 emits light with a constant current indicated by the read current value (step S106).

Next, it returns to step S102 and repeats a process.
1.3 Summary As described above, the current value is constant and does not fluctuate if the fluctuation of the voltage value excluding the voltage drop due to the current load of the battery 111 is within the selected voltage value range. For this reason, the light intensity of each LED does not change, and the liquid crystal display unit 110b does not flicker.

2. Other Modifications Although the present invention has been described based on the above embodiment, it is not limited to the above embodiment. You may make it show below.
(1) A modification of the above embodiment will be described.
In the above embodiment, the voltage value of the battery 111 is acquired at regular intervals, for example, every 20 seconds. Next, based on the acquired voltage value, the control table 151 is used to determine the value of the current flowing through the LED. However, the method is not limited.

For example, the voltage value after the voltage drop due to the current load of the battery 111 is acquired three times every 20 seconds. Next, when the three acquired voltage values are within the same voltage value range, the current value to be passed through the LED may be determined based on the current value corresponding to the voltage value range of the control table 151.
Hereinafter, the operation of the mobile terminal 100 when a constant current is supplied to the LEDs 141, 142, 143, and 144 will be described with reference to the flowchart shown in FIG.

Here, an identification number X, an identification number Y, and an identification number Z are provided in the storage unit 115 as three variables. The identification number X, the identification number Y, and the identification number Z are used to store the current identification number, the previous identification number, and the previous identification number, respectively.
The supply control unit 139 sets the constant current supplied to the LEDs 141, 142, 143, and 144 as an initial value to the current value (20 mA) indicated by the identification number “1” of the control table 151. Next, the supply control unit 139 instructs the constant current circuits 133, 134, 135, and 136 to supply a constant current of 20 mA. The constant current circuits 133, 134, 135, and 136 supply a constant current of 20 mA to the LEDs 141, 142, 143, and 144, respectively. The LEDs 141, 142, 143, and 144 each emit light with a constant current of 20 mA (step S121).

  The supply control unit 139 sets “7”, “8”, and “9” to the identification number X, the identification number Y, and the identification number Z, respectively, as initial values. “7”, “8”, and “9” are values that do not exist in the control table 151 and cannot be taken as identification numbers. “7”, “8”, and “9” are different values, and different values are set for the identification number X, the identification number Y, and the identification number Z, respectively (step S122).

The control unit 103 uses the timer 116 to count 20 seconds (step S123). If 20 seconds have not elapsed ("NO" in step S123), the process returns to step S123 and the process is repeated.
When 20 seconds have elapsed (“YES” in step S123), the acquisition unit 137 acquires the voltage value of the battery 111 from the voltage detection unit 121 and reads it (step S124).

Next, the determination unit 138 searches the control table 151 stored in the storage unit 115 for a voltage value range including the acquired voltage value (step S125).
Next, the supply controller 139 overwrites the identification number Z with the value stored in the identification number Y (step S126). In addition, the value stored in the identification number X is overwritten on the identification number Y (step S127).

Next, when there is a voltage value range including a voltage value, the supply control unit 139 reads an identification number corresponding to the voltage value range from the control table 151 (step S128). Next, the read identification number is overwritten on the identification number X (step S129).
Next, the supply control unit 139 determines whether the identification number X, the identification number Y, and the identification number Z are the same (step S130).

If they are not the same ("NO" in step S130), the process returns to step S123 and the process is repeated.
When the identification number X, the identification number Y, and the identification number Z are the same (“YES” in step S130), the supply control unit 139 reads the current value corresponding to the voltage value range from the control table 151 (step S131). .

  Next, the supply control unit 139 sets the constant current supplied to the LEDs 141, 142, 143, and 144 to the read current value. Next, the supply control unit 139 instructs the constant current circuits 133, 134, 135, and 136 to supply a constant current indicated by the read current value. The constant current circuits 133, 134, 135, and 136 supply constant currents indicated by the read current values to the LEDs 141, 142, 143, and 144, respectively. Each of the LEDs 141, 142, 143, and 144 emits light with a constant current indicated by the read current value (step S132).

Next, it returns to step S123 and repeats a process.
As described above, when the voltage value of the battery 111 obtained three times in succession is within the same voltage value range, the current value corresponding to the voltage value range of the control table 151 is set as the current value flowing through the LED. Has been decided.
By the method described above, when the battery voltage is temporarily changed, the current value of the LED can be prevented from being changed. The temporary battery voltage fluctuation occurs, for example, due to temporary use of a web browser. On the other hand, when the battery voltage continuously decreases, the LED current value is changed. In this way, it is possible to prevent the luminance of the liquid crystal display unit 110b from switching over and over.

Note that, as described above, it is not limited to acquiring the voltage value three times in succession.
The voltage value of the battery 111 may be acquired twice, four times, or five times continuously. In this case, when all the voltage values are within the same voltage value range, the current value corresponding to the voltage value range of the control table 151 may be determined as the current value flowing through the LED.
(2) In the above embodiment, the voltage value of the battery 111 is acquired every 20 seconds. However, this is not a limitation. The voltage value of the battery 111 may be acquired every 10 seconds, every 30 seconds, or every minute.

(3) In the above embodiment, the control table 151 has four voltage value ranges and current values. However, it is not limited to this. The control table 151 may have a larger number of sets of voltage value ranges and current values.
(4) In the above embodiment, as shown in FIG. 2, the LEDs 141, 142, 143, and 144 are arranged in parallel as viewed from the LED drive circuit 113.

In the portable terminal 100, the LEDs 141, 142, 143, and 144 are not connected in series. For example, when four LEDs are connected in series, if each LED requires a voltage of 3.3 V, a total voltage of 13.2 V (3.3 V × 4) is required. This exceeds the voltage that the battery 111 can output.
(5) The voltage detection unit 121 may detect the voltage value of the battery 111 before a voltage drop due to the load at the present time. In this case, the determination unit 138 adds the voltage difference value due to the load according to the current usage state to the voltage value detected by the voltage detection unit 121 to calculate the voltage value. Thereby, the voltage value after the voltage drop by the load at the present time is calculated. Next, the determination unit 138 searches the control table 151 stored in the storage unit 115 for a voltage value range including the calculated voltage value.

(6) In the above embodiment, the backlight 114 is installed on the back surface of the liquid crystal display unit 110b. However, the arrangement is not limited.
A light guide plate, a light guide housing, a light guide tube, and the like may be disposed on the back surface of the liquid crystal display unit 110b. The backlight 114 irradiates light to the back surface of the liquid crystal display unit 110b through a light guide plate, a light guide housing, a light guide tube, and the like.

(7) As described above, the portable terminal of the present invention is a computer system that includes a microprocessor and a memory. The memory stores a computer program, and the microprocessor operates according to the computer program.
Here, the computer program is configured by combining a plurality of instruction codes indicating instructions for the computer in order to achieve a predetermined function.

The computer program is recorded on a computer-readable recording medium, for example, a flexible disk, a hard disk, a CD-ROM, an MO, a DVD, a DVD-ROM, a DVD-RAM, a Blu-ray disk, a semiconductor memory, etc. Also good.
The computer program may be transmitted via an electric communication line, a wireless or wired communication line, a network represented by the Internet, a data broadcast, or the like.

The computer program may be recorded on the recording medium and transferred. Alternatively, the computer program may be transferred via a network or the like. Thus, the computer program is executed by another independent computer system.
(8) The above embodiment and the above modifications may be combined.

  The LED driving apparatus according to the present invention can prevent flickering of the display portion without providing a booster circuit. For this reason, it is useful as a technique for driving and controlling an LED that emits light as a backlight to the display unit of the portable terminal.

DESCRIPTION OF SYMBOLS 100 Mobile terminal 101 Antenna 102 Wireless communication part 103 Control part 104 Input control part 105 Key input part 106 Voice processing part 107 Speaker 108 Microphone 109 Input / output control part 110 Liquid crystal panel part 110a Touch pad part 110b Liquid crystal display part 111 Battery 112 Power supply part 113 LED Drive Circuit 114 Backlight 115 Storage Unit 116 Timer 121 Voltage Detection Unit 133, 134, 135, 136 Constant Current Circuit 137 Acquisition Unit 138 Judgment Unit 139 Supply Control Unit 141, 142, 143, 144 LED

Claims (5)

An LED driving device that emits light as a backlight on a display unit of a portable terminal,
A storage unit storing a voltage value range of the battery in a no-load state of the mobile terminal and a current value smaller than a characteristic current value corresponding to the voltage value range in the forward voltage forward current characteristics of the LED;
An acquisition unit for acquiring a voltage value of the battery after a voltage drop due to a load at a current time;
A determination unit that determines whether the acquired voltage value is within the voltage value range;
A supply control unit configured to read out the current value corresponding to the voltage value range from the storage unit and to supply a constant current indicated by the read current value to the LED when the voltage value is within the voltage value range; ,
A constant current unit for supplying a constant current indicated by the current value to the LED;
With
The current value stored in the storage unit is a characteristic current value corresponding to a voltage value excluding a maximum value of a voltage drop generated when a load is applied to the mobile terminal from a lower limit value of a voltage value range of the battery. L ED of drive you being a smaller current value. The acquisition unit periodically acquires the voltage value,
The LED determination device according to claim 1 , wherein the determination unit performs the determination each time the voltage value is acquired. The acquisition unit continuously acquires a plurality of voltage values,
The determination unit determines whether or not the plurality of acquired voltage values are within the voltage value range;
The supply control unit, the plurality of voltage values, when present in the voltage value range, wherein the constant current shown by the current to claim 1, wherein the controller controls so as to supply to the LED LED drive device. A control method used in an LED driving device that irradiates light as a backlight to a display unit of a portable terminal,
The drive device stores a voltage value range of the battery in a no-load state of the mobile terminal and a current value smaller than a characteristic current value corresponding to the voltage value range in the forward voltage forward current characteristic of the LED. A storage unit, and a constant current unit that supplies a constant current indicated by the current value to the LED,
The control method is:
Obtain the voltage value of the battery after the voltage drop due to the current load,
Determining whether the acquired voltage value is within the voltage value range;
When present in the voltage value range, the current value corresponding to the voltage value range is read from the storage unit, and a constant current indicated by the read current value is controlled to be supplied to the LED ,
The current value stored in the storage unit is a characteristic current value corresponding to a voltage value excluding a maximum value of a voltage drop generated when a load is applied to the mobile terminal from a lower limit value of a voltage value range of the battery. A control method characterized by having a smaller current value . Battery,
A display unit;
An LED that emits light as a backlight to the display unit;
A storage unit storing a voltage value range in a no-load state of the battery and a current value smaller than a characteristic current value corresponding to the voltage value range in the forward voltage forward current characteristics of the LED;
An acquisition unit that acquires a voltage value of the battery after a voltage drop due to a load at a current time; a determination unit that determines whether or not the acquired voltage value is within the voltage value range;
A supply control unit configured to read out the current value corresponding to the voltage value range from the storage unit and to supply a constant current indicated by the read current value to the LED when the voltage value is within the voltage value range; ,
A constant current unit for supplying a constant current indicated by the current value to the LED ,
The current value stored in the storage unit is a characteristic current value corresponding to a voltage value excluding a maximum value of a voltage drop generated when a load is applied to the mobile terminal from a lower limit value of a voltage value range of the battery. A portable terminal having a smaller current value .

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