WO2016157494A1

WO2016157494A1 - Automatic luminance adjustment circuit for digital display device

Info

Publication number: WO2016157494A1
Application number: PCT/JP2015/060462
Authority: WO
Inventors: 隆一小池; 渉西岡
Original assignee: 理化工業株式会社
Priority date: 2015-04-02
Filing date: 2015-04-02
Publication date: 2016-10-06
Also published as: JPWO2016157494A1

Abstract

The purpose of the present invention is to obtain an automatic luminance adjustment circuit for a display device that allows the luminance among a plurality of display units to be equalized. Provided is an automatic luminance adjustment circuit for a display device, comprising an FET (24) that sequentially selects a display unit to be illuminated from among three display units (11, 12, 13) in accordance with a control signal Drv output from a controller (30) and connects, from among segment LEDs (11a - 11h, 12a - 12h, 13a - 13h) constituting the selected display unit, a segment LED to which a power supply voltage Vdd is applied by a transistor (22) to ground (25) so that a forward current flows through the segment LED. The controller calculates a lighting period (TD1, TD2, TD3), for each display unit, during which a forward current is made to flow through the segment LEDs constituting the display unit in order to adjust the luminance in the display unit within a prescribed range. For each display unit, the controller controls the connection to ground via the FET so that the forward current flows through the segment LEDs constituting the display unit during the calculated lighting period.

Description

Automatic brightness adjustment circuit for digital display

The present invention relates to an automatic luminance adjustment circuit for a digital display device including a plurality of display units including a plurality of light emitting elements, and more particularly, to an automatic luminance adjustment circuit for a digital display device in a temperature controller including a plurality of display units including a plurality of LED segments. It is about.

Conventionally, for example, in a digital display device of a temperature controller, a plurality of display units each including a plurality of segment LEDs such as a 7-segment LED and a 16-segment LED as a light emitting element are used. For example, in the case of 7 segment LED, the display unit is provided with 7 segment LED. As an electrical characteristic of the segment LED constituting the 7 segment LED, there is a “forward voltage” which is a voltage drop generated when a forward current is passed between the anode and the cathode of the segment LED. This forward voltage generally has a certain degree of variation due to differences in raw materials and production processes of each segment LED, and is regarded as an element specification.
When the voltages applied to the plurality of segment LEDs constituting the display unit are equal, if there is a variation in the forward voltage of the plurality of segment LEDs, the forward current flowing through the plurality of segment LEDs also varies accordingly. This variation becomes more conspicuous as the drive voltage is lowered, which is an obstacle to power saving.
Since the brightness of the segment LEDs is proportional to the forward current, if the forward current flowing through the plurality of segment LEDs varies, the brightness of the plurality of segment LEDs also varies.
Therefore, the display unit manufacturer generally suppresses variations in luminance between the segment LEDs constituting the display unit by performing rank designation by luminance selection or using segment LEDs of the same lot. I am doing so. Alternatively, there is a method of keeping the forward current constant by adding a constant current circuit, but there is a tendency that the components are expensive or the circuit becomes complicated.

Patent Document 1 below discloses a luminance adjustment circuit that varies the luminance of a plurality of light emitting elements so that the number of pulses commonly applied to the electrodes of the plurality of light emitting elements within a predetermined period can be changed. It is disclosed.

Japanese Patent No. 2758840

Since the electrical characteristics of the plurality of segment LEDs constituting the display unit are substantially uniform, the variation in the brightness of the plurality of segment LEDs constituting the display unit is suppressed within a certain range. However, when manufacturing a plurality of display devices composed of a plurality of display units, a plurality of segment LEDs are required for each display unit constituting the display device, and as a result, a large number of segment LEDs must be prepared. For this reason, in order to reduce the variation in the electrical characteristics among a plurality of display units, the manufacturing restriction that the segment LED of the same lot is used and the rank by luminance selection are designated. It is necessary to impose restrictions, which makes it practically difficult to operate within limited manufacturing costs. Therefore, the variation in electrical characteristics of the segment LEDs mounted on each display unit is not always small between the plurality of display units constituting the display device, and thus, variation in luminance occurs between the display units. Sometimes.
When the luminance adjustment circuit disclosed in Patent Document 1 is applied to a display device in which N display units are arranged, the display unit is composed of 8 segment LEDs including a segment LED representing a decimal point. If it is segment LED, the brightness | luminance of (Nx8) segment LED can be adjusted simultaneously. However, since the pulses applied to the electrodes of (N × 8) segment LEDs are common and the brightness is not adjusted for each segment LED, the electrical characteristics of the segment LEDs mounted on the N display units When the values are different from each other, there is a problem in that variation in luminance among the N display units cannot be suppressed.

The present invention has been made to solve the above-described problems, and provides an automatic luminance adjustment circuit for a digital display device capable of making the luminance uniform among a plurality of display units constituting the digital display device. With the goal.

An automatic brightness adjustment circuit for a digital display device according to the present invention includes a voltage application unit that applies a power supply voltage to a plurality of segment LEDs in a plurality of display units each composed of a plurality of segment LEDs, and a plurality of displays. A display unit to be lit is sequentially selected from the units, and among the plurality of segment LEDs constituting the selected display unit, the segment LED to which the power supply voltage is applied by the voltage application unit is set to the ground. By connecting, a selection switch that allows forward current to flow through the segment LED, a current measurement unit that measures forward current flowing through the segment LED connected to the ground by the selection switch for each display unit, and a plurality of displays In order to keep the brightness of the unit within a certain range, the forward current measured by the current measurement unit and multiple A period calculation unit that calculates a period in which a forward current flows to the segment LED that constitutes the display unit from a common luminance setting value for the display unit, and the selection switch control unit is provided for each display unit. In addition, during the period calculated by the period calculation unit, the connection with the ground by the selection switch is controlled so that the forward current flows through the segment LED constituting the display unit.

The automatic luminance adjusting circuit of the digital display device according to the present invention includes a lighting pattern storage unit that stores lighting patterns indicating periods in which a plurality of display units can be lit, and the period calculation unit includes the luminances of the plurality of display units. In order to keep the value within a certain range, from the forward current measured by the current measurement unit and the common brightness setting value for the plurality of display units, for each display unit, the display unit indicated by the lighting pattern can be turned on. In addition, a period during which a forward current is supplied to the segment LED constituting the display unit is calculated.

In the automatic brightness adjusting circuit of the digital display device according to the present invention, the lighting pattern stored in the lighting pattern storage unit indicates that the turn-off period of all the display units is before and after the turn-on period of each display unit. The selection switch control unit controls the connection with the ground by the selection switch so that forward current does not flow due to interference with the segment LEDs constituting the plurality of display units during the extinguishing period indicated by the lighting pattern. It is what you do.

According to the present invention, there is an effect that the luminance of the plurality of display units can be made uniform.

It is a block diagram which shows the digital display apparatus by Embodiment 1 of this invention. It is a block diagram which shows the controller 30 of the digital display apparatus by Embodiment 1 of this invention. It is explanatory drawing which shows an example of the dynamic lighting pattern stored in the lighting pattern storage part.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a digital display device according to Embodiment 1 of the present invention.
In FIG. 1, the display 1 which is a digital display device has three

display units

11, 12, and 13 arranged.
The display unit 11 is a seven-segment LED including eight

segment LEDs

11a, 11b, 11c, 11d, 11e, 11f, 11g, and 11h including a segment LED 11h that represents a decimal point.
The display unit 12 is a seven-segment LED including eight

segment LEDs

12a, 12b, 12c, 12d, 12e, 12f, 12g, and 12h, including a segment LED 12h that represents a decimal point.
The display unit 13 is a seven-segment LED including eight

segment LEDs

13a, 13b, 13c, 13d, 13e, 13f, 13g, and 13h including a segment LED 13h that represents a decimal point.

In the example of FIG. 1, the

display units

11, 12, and 13 are 7-segment LEDs, but are not limited to 7-segment LEDs. For example, 17 segment LEDs including a segment LED that represents a decimal point are included. 16 segment LED comprised from these may be sufficient.
In the example of FIG. 1, the display device 1 includes three display units. However, this is only an example, and it is only necessary to include two or more display units.
It is assumed that the electrical characteristics of the eight segment LEDs constituting each display unit are aligned in advance. Therefore, if attention is paid to one display unit, the variation in luminance of the eight segment LEDs is suppressed within a certain range.
However, since the electrical characteristics of the mounted segment LEDs are not necessarily the same between the

display units

11, 12, and 13, there may be variations in luminance.

A power supply voltage Vdd output from a power supply device (not shown) is applied to the power supply voltage line 21. Since the forward voltage of the segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h constituting the

display units

11, 12, and 13 is 1.7 to 2.5V, the power supply voltage Vdd is a voltage that can be driven at a higher voltage. It is.
The transistor 22 that is a switching element switches the connection state between the power supply voltage line 21 and the resistor 23 in accordance with a control signal Seg output from a controller 30 described later. The power supply voltage line 21 and the transistor 22 constitute a voltage application unit.
The resistor 23 is a current measuring resistor inserted between the transistor 22 and the display 1.

In FIG. 1, only one transistor 22 and resistor 23 are drawn for simplicity of explanation, but the transistor 22 and resistor 23 are connected to each segment LED constituting the

display units

11, 12, and 13. ing. In the example of FIG. 1, the

display units

11, 12, and 13 are each composed of eight segment LEDs, so that eight transistors 22 and eight resistors 23 are mounted.
Therefore, the control signal Seg output from the controller 30 instructs to apply the power supply voltage Vdd to the anodes of the

segment LEDs

11a, 12a, 13a constituting the

display units

11, 12, 13, for example. Then, of the eight transistors 22, only the transistor 22 connected to the

segment LEDs

11a, 12a, and 13a is turned on, and the transistor 22 connected to the segment LEDs 11b to 11h, 12b to 12h, and 13b to 13h. Becomes OFF. The ON state is a state where the power supply voltage line 21 and the resistor 23 are connected, and the OFF state is a state where the power supply voltage line 21 and the resistor 23 are disconnected.

A FET (Field Effect Transistor) 24 selects a display unit to be lit in order from the three

display units

11, 12, and 13 in accordance with the control signal Drv output from the controller 30. The FET 24 is connected to the ground LED 25 by connecting the cathode of the segment LED to which the power supply voltage Vdd is applied by the transistor 22 among the segment LEDs constituting the selected display unit. It is a selection switch that allows current to flow.

The controller 30 includes, for example, an MCU (Micro Control Unit) that is a micro control unit, a semiconductor integrated circuit on which a CPU (Central Processing Unit) is mounted, and the like.
The controller 30 identifies the lighting period and the extinguishing period with reference to the dynamic lighting pattern of FIG. 3 in which the lighting period and the extinguishing period are repeated.
The controller 30 controls the transistor 22 to be in an ON state during the lighting period so that the power supply voltage Vdd is applied to the anodes of the segment LEDs constituting the

display units

11, 12, and 13.
The controller 30 controls the transistor 22 to be in an OFF state during the extinguishing period so that the power supply voltage Vdd is not applied to the anodes of the segment LEDs constituting the

display units

11, 12, and 13.
In addition, the controller 30 controls the segment LEDs 11a to 11h and 12a to constitute the display unit for each of the

display units

11, 12, and 13 in order to keep the luminance of the

display units

11, 12, and 13 within a certain range. 12h and 13a to 13h, the period during which the forward current flows is calculated. Then, the controller 30 causes each of the

display units

11, 12, and 13 so that forward current flows through the segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h constituting the display unit during the calculated period. The connection to the ground 25 by the FET 24 is controlled.

FIG. 2 is a block diagram showing the controller 30 of the digital display device according to Embodiment 1 of the present invention.
In FIG. 2, the lighting pattern storage unit 31 is a storage medium such as a RAM that stores a dynamic lighting pattern indicating the lighting order and lighting periods of the

display units

11, 12, and 13.
The transistor control unit 32 turns on the transistor 22 during the time of each drive, and lights up the plurality of segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h in each of the

display units

11, 12, and 13. Control is performed so that the power supply voltage Vdd is applied to the anode of the segment LED that is desired to be generated.
The transistor control unit 32 turns off the transistor 22 during the extinguishing period, and the power supply voltage Vdd is the anode of all the segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h constituting the

display units

11, 12, and 13. It controls so that it may not be applied to.

The current measuring unit 33 is a voltage V applied to both ends of the resistor 23 connected to the segment LED (for example, 11a, 12a, 13a) connected to the ground 25 by the FET 24 for each of the

display units

11, 12, 13. ₁ and V ₂ are measured, and the potential difference ΔV between the voltage V ₁ and the voltage V ₂ is divided by the resistance value R of the resistor 23, whereby the segment LED connected to the ground 25 by the FET 24 (for example, 11a, 12a, 13a ) To calculate the forward current I (I _D1 , I _D2 , I _D3 ).
The period calculation unit 34 sets the forward current I measured by the current measurement unit 33 and the common luminance setting value L for the

display units

11, 12, 13 in order to keep the luminance of the

display units

11, 12, 13 within a certain range. From each of the

display units

11, 12, and 13, the forward current is supplied to the segment LEDs 11 a to 11 h, 12 a to 12 h, and 13 a to 13 h constituting the display unit during the lighting period of the display unit indicated by the lighting pattern. A lighting period T (T _D1 , T _D2 , T _D3 ) is calculated.

The period storage unit 35 is a storage medium such as a non-volatile memory that stores a lighting period T in which a forward current is supplied to the segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h calculated by the period calculation unit 34.
The selection switch control unit 36 refers to the dynamic lighting pattern stored in the lighting pattern storage unit 31 and identifies the lighting period and the extinguishing period.
Further, the selection switch control unit 36 is provided for each of the

display units

11, 12, 13 during the lighting period T stored in the period storage unit 35, and the segment LEDs 11 a to 11 h, 12 a to 12 h, constituting the display unit. The connection of the FET 24 to the ground 25 is controlled so that a forward current flows through 13a to 13h.
Here, an example is shown in which the period storage unit 35 is provided. However, when the period storage unit 35 is not provided, the segment LEDs 11a to 11h and 12a calculated by the period calculation unit 34 by the selection switch control unit 36 are shown. It is also possible to acquire and hold a lighting period T in which a forward current is passed through to 12h and 13a to 13h.

FIG. 3 is an explanatory diagram illustrating an example of a dynamic lighting pattern stored in the lighting pattern storage unit 31.
In the example of FIG. 3, a dynamic lighting pattern with a lighting cycle of 16 msec is shown, and Drv0 is a period during which the

display units

11, 12, and 13 are turned off for 2 msec.
Drv1 indicates a period during which the display unit 11 can be turned on (4 msec), and indicates that the display unit 11 is lit for 1 to 4 msec and the display unit 11 is turned off for 3 to 0 msec.
Drv2 indicates a period during which the display unit 12 can be turned on (4 msec). The display unit 12 is turned on for 1 to 4 msec and the display unit 12 is turned off for 3 to 0 msec.
Drv3 indicates a period during which the display unit 13 can be turned on (4 msec), and indicates that the display unit 13 is lit for 1 to 4 msec and the display unit 13 is turned off for 3 to 0 msec.
Note that “turn off” before and after Drv1, Drv2, and Drv3 means that the

display units

11, 12, and 13 are turned off during a period of 0.5 msec.
By providing the “light-off” period, it is possible to prevent flickering by preventing interference due to a delay in switching the control signal Drv between the

display units

11, 12, and 13.
In the example of FIG. 3, the lighting cycle of the dynamic lighting pattern is 16 msec. Generally, however, the lighting of the

display units

11, 12, and 13 is intermittent when the lighting cycle is 20 msec or less. However, since it seems to be continuously lit by human eyes, the lighting cycle of the dynamic lighting pattern is set to 16 msec from a value lower than that and the number of display units.

Next, the operation will be described.
Since the electrical characteristics of the eight segment LEDs 11a to 11h constituting the display unit 11 are uniform, the luminance of any one of the eight segment LEDs 11a to 11h (for example, 11a) It can be seen that the luminance of the other seven segment LEDs (for example, 11b to 11h) is the same.
As for the

display units

12 and 13, if the luminance of any one segment LED is known, the luminance of the other seven segment LEDs is the same.
When the segment LED is repeatedly turned on and off in a short cycle, the human eye feels that the lighting state continues, and the brightness of the segment LED is proportional to the time and amount of current that the forward current flows. If the amount of current converted per unit time is the same, the brightness of the LEDs is the same.
Therefore, in the first embodiment, each of the forward currents flowing through any one of the segment LEDs is measured in the

display units

11, 12, and 13, and based on the measurement result, per unit time flowing through each segment LED. By adjusting the lighting time of the segment LED so that the converted current amounts are the same, the luminance variation between the

display units

11, 12, and 13 is suppressed.
Specifically, it is as follows.

First, processing contents when shipping the digital display device of FIG. 1 will be described.
The transistor control unit 32 of the controller 30 refers to the dynamic lighting pattern stored in the lighting pattern storage unit 31 and turns on the transistor 22 during the periods of Drv1, Drv2, and Drv3 that are lighting possible periods. The power supply voltage Vdd is controlled to be applied to the anodes of the segment LEDs constituting the

display units

11, 12, and 13.
As described above, the electrical characteristics of the eight segment LEDs constituting the

individual display units

11, 12, and 13 are aligned in advance by the manufacturer. For this reason, if the luminance of any one segment LED (for example, 11a) is known in each of the

display units

11, 12, and 13, it is determined that the luminance of the other segment LEDs (for example, 11b to 11h) is comparable. it can.
For this reason, in the first embodiment, for convenience of explanation, the transistor control unit 32 selects the

segment LEDs

11a, 11a, 11a, 13a, 13a, 13a, 13h, 13a, 13h, 13a, 13h, Control is performed so that the power supply voltage Vdd is applied to the anodes of 12a and 13a.
Further, the transistor control unit 32 turns off the transistor 22 during periods other than Drv1, Drv2, and Drv3, which are extinguishing periods, and the segment LEDs 11a to 11h in which the power supply voltage Vdd constitutes the

display units

11, 12, and 13 are displayed. , 12a to 12h and 13a to 13h are controlled so as not to be applied.

The selection switch control unit 36 of the controller 30 refers to the dynamic lighting pattern stored in the lighting pattern storage unit 31, and during the lighting period (1 to 4 msec) in Drv1, the

display unit

11, 12, 13 Then, the display unit 11 is selected.
In the first embodiment, since the power supply voltage Vdd is applied to the anodes of the

segment LEDs

11a, 12a, and 13a, the selection switch control unit 36 grounds the cathode of the segment LED 11a that constitutes the selected display unit 11. A control signal Drv indicating connection to 25 is output to the FET 24.

The selection switch control unit 36 of the controller 30 selects the display unit 12 from the

display units

11, 12, and 13 during the lighting period (1 to 4 msec) in Drv2, and configures the selected display unit 12. A control signal Drv indicating that the cathode of the segment LED 12 a connected to the ground 25 is connected is output to the FET 24.
The selection switch control unit 36 of the controller 30 selects the display unit 13 from the

display units

11, 12, and 13 during the lighting period (1 to 4 msec) in Drv 3, and configures the selected display unit 13. A control signal Drv indicating that the cathode of the segmented LED 13 a is connected to the ground 25 is output to the FET 24.

During the lighting period (4 msec) in Drv1, the current measurement unit 33 of the controller 30 controls the cathode of the segment LED 11a that constitutes the display unit 11 according to the control signal Drv output from the selection switch control unit 36 by the FET 24. When connected to the ground 25, the voltages V ₁ -D1 and V _2-D1 applied to both ends of the resistor 23 connected to the segment LED 11a are measured.
Current measuring unit 33 during the lighting periods within Drv1, when measuring the voltage _{_{V 1-D1, V 2-}} D1, the resistance value of the voltage _{V 1-D1} and the voltage _{V 2-D1} of the potential difference [Delta] V _D1 a resistor 23 R in by dividing to calculate a forward current _{I D1} flowing into segments LED11a constituting the display unit 11.

During the lighting period (4 msec) in Drv2, the current measuring unit 33 of the controller 30 controls the cathode of the segment LED 12a constituting the display unit 12 according to the control signal Drv output from the selection switch control unit 36 by the FET 24. When connected to the ground 25, the voltages V _{1 -D2} and V _2-D2 applied to both ends of the resistor 23 connected to the segment LED 12a are measured.
Current measuring unit 33 during the lighting periods within Drv2, when measuring the voltage _{_{V 1-D2, V 2-}} D2, the resistance value of the voltage _{V 1-D2} and the voltage _{V 2-D2} of the potential difference [Delta] V _D2 and resistor 23 R The forward current _ID2 flowing in the segment LED 12a constituting the display unit 12 is calculated by dividing by.

The current measuring unit 33 of the controller 30 controls the cathode of the segment LED 13a constituting the display unit 13 according to the control signal Drv output from the selection switch control unit 36 by the FET 24 during the lighting period (4 msec) in Drv3. When connected to the ground 25, the voltages V ₁ -D3 and V _2-D3 applied to both ends of the resistor 23 connected to the segment LED 13a are measured.
Current measuring unit 33 during the lighting periods within Drv3, when measuring the voltage _{_{V 1-D3, V 2-}} D3, the resistance value of the voltage _{V 1-D3} and the voltage _{V 2-D3} potential difference [Delta] V _D3 resistor 23 R in by dividing to calculate a forward current _{I D3} flowing into segments LED13a constituting the display unit 13.

When the current measurement unit 33 calculates the forward currents I _D1 , I _D2 , and I _D3 , the period calculation unit 34 of the controller 30 includes the segment LEDs 11a to 11h, 12a to 12h, which constitute the

display units

11, 12, and 13; In order to make the amount of current converted per unit time flowing through 13a to 13h constant, as shown in the following formulas (1) to (3), the forward current I _D1 , The

display units

11, 12, and 13 are configured during the lighting period (1 to 4 msec) in Drv1, Drv2, and Drv3 from the common luminance setting value L for I _D2 and I _D3 and the

display units

11, 12, and 13. The lighting periods T _D1 , T _D2 , and T _D3 for flowing a forward current to the segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h are calculated. As the common luminance setting value L for the

display units

11, 12, and 13, for example, the luminance when a forward current equivalent to 17 mA per unit time flows to the segment LED can be considered.

For example, when L = 50, and I _D1 = 20, I _D2 = 18, and I _D3 = 16, T _D1 , T _D2 , and T _D3 are as follows.
T _D1 = 50/20 = 2.5 msec
T _D2 = 50/18 = 2.77 msec
T _D3 = 50/16 = 3.12 msec

The lighting periods T _D1 , T _D2 , and T _D3 through which the forward current calculated by the period calculation unit 34 flows are stored in the period storage unit 35.
Since the period storage unit 35 is composed of a nonvolatile memory, the storage of the lighting periods T _D1 , T _D2 , and T _D3 is retained even when the digital display device of FIG. 1 is turned off.
This completes the process for shipping the digital display device of FIG.

Next, the processing content after turning on the power of the digital display device that has been shipped is described.
When the power is turned on, the selection switch control unit 36 of the controller 30 acquires the lighting periods T _D1 , T _D2 , and T _D3 stored in the period storage unit 35.
When the selection switch control unit 36 acquires the lighting periods T _D1 , T _D2 , and T _D3 , the selection switch control unit 36 refers to the dynamic lighting patterns stored in the lighting pattern storage unit 31 and sets the periods of Drv1, Drv2, and Drv3 (lighting and extinguishing) The total duration). For example, the time-up time of the internal clock is set to the lighting cycle (16 msec) indicated by the dynamic lighting pattern, and every time the internal clock times up, the Drv0 turn-off time (2 msec) → light-off time ( 0.5 msec) → Drv1 time (4 msec) → light-out time (0.5 msec) → Drv2 time (4 msec) → light-out time (0.5 msec) → Drv3 time (4 msec) → light-out time (0. 5msec), the period of Drv1, Drv2, Drv3 is specified.
That is, after the internal clock times up, the selection switch control unit 36 specifies that the 4 msec period between 2.5 and 6.5 msec is the period of Drv1, and after the internal clock times up, A period of 4 msec between ˜11 msec is specified as a period of Drv2.
Further, after the internal clock times out, the selection switch control unit 36 specifies that the 4 msec period between 11.5 and 15.5 msec is the Drv3 period.

The selection switch control unit 36, between the T _D1 is the lighting periods within the period of Drv1 identified as forward current flows through the segment LED constituting the display unit 11, to constitute a display unit 11 A control signal Drv _D1 indicating that the segment LED is connected to the ground 25 is output to the FET 24.
However, the segment LED connected to the ground 25 varies depending on the characters displayed on the display unit 11. Specifically, if the character displayed on the display unit 11 is the numeral “8”, the segment LEDs 11a to 11g are connected to the ground 25 so that forward current flows through the segment LEDs 11a to 11g constituting the display unit 11. A control signal Drv _D1 indicating connection is output to the FET 24. For example, if the character displayed on the display unit 11 is the number “1”, only the two

segment LEDs

11b and 11c constituting the rightmost vertical line are displayed. A control signal Drv _D1 indicating that the two

segment LEDs

11b and 11c constituting the rightmost vertical line are connected to the ground 25 is output to the FET 24 so that a forward current flows through the FET 24.

The selection switch control unit 36, between the T _D2 is a lighting period in the period Drv2 identified as forward current flows through the segment LED constituting the display unit 12, to constitute a display unit 12 A control signal Drv _D2 indicating that the segment LED connected to the ground 25 is connected is output to the FET 24.
The selection switch control unit 36, between the T _D3 is a lighting period in the period Drv3 identified as forward current flows through the segment LED constituting the display unit 13, to constitute a display unit 13 A control signal Drv _D3 indicating that the segment LED is connected to the ground 25 is output to the FET 24.
In the period of Drv2 and Drv3, as in the period of Drv1, the segment LED connected to the ground 25 varies depending on the characters displayed on the

display units

12 and 13.

FET24 receives the signal _{Drv D1} from the selection switch control unit 36 of the controller 30, during the lighting period _{T D1} of Drv1, ground 25 segment LED display unit 11 to which the control signal _{Drv D1} is instructed to connect Connect with. In the previous example, for lighting period _{T D1} is 2.5 msec, in the period Drv1 (4 msec), while the 2.5 msec, connect segment LED display unit 11 and the ground 25, the remainder of 1.5msec In the meantime, the segment LED of the display unit 11 and the ground 25 are disconnected.

Moreover, FET 24 receives a signal _{Drv D2} from the selection switch control unit 36 of the controller 30, during the lighting period _{T D2} of Drv2, segment LED display unit 12 to which the control signal _{Drv D2} is instructed to connect Connect to ground 25. In the previous example, since the lighting period T _D2 is 2.77 msec, the segment LED of the display unit 12 is connected to the ground 25 for 2.77 msec in the Drv2 period (4 msec), and the remaining 1.23 msec In the meantime, the segment LED of the display unit 12 and the ground 25 are disconnected.

Moreover, FET 24 receives a signal _{Drv D3} from the selection switch control unit 36 of the controller 30, during the lighting period _{T D3} of Drv3, segment LED display unit 13 to which the control signal _{Drv D3} is instructed to connect Connect to ground 25. In the previous example, for lighting period _{T D3} is 3.12Msec, in the period Drv3 (4 msec), during 3.12Msec, connect segment LED display unit 13 and the ground 25, the remainder of 0.88msec In the meantime, the segment LED of display unit 13 and ground 25 are not connected.
Accordingly, since the forward current flows through each segment LED constituting the

display units

11, 12, and 13 only during the lighting periods T _D1 , T _D2 , and T _D3 calculated by the period calculation unit 34, the

display unit

11 , 12 and 13, the amount of current converted per unit time flowing to each segment LED becomes constant, and the luminance between the

display units

11, 12 and 13 becomes uniform.

As apparent from the above, according to the first embodiment, the display units to be turned on are sequentially selected from the three

display units

11, 12, and 13 in accordance with the control signal Drv output from the controller 30. And the segment LED to which the power supply voltage Vdd is applied by the transistor 22 is connected to the ground 25 among the segment LEDs constituting the selected display unit, so that a forward current flows through the segment LED. In order to make the brightness of the

display units

11, 12, 13 within a certain range, the controller 30, the segment LED 11 a constituting the display unit is provided for each

display unit

11, 12, 13. calculated ~ 11h, 12a ~ 12h, the lighting period _{_T _D1,} _T _D2, _T _D3 passing a forward current to 13a ~ 13h Each

display unit

11, 12, 13, in the lighting period _{_T _D1,} _T _D2, _T _D3 that the calculated, as the forward current segment LED constituting the display unit flows, connected to the ground 25 by FET24 Therefore, the luminance of the

display units

11, 12, and 13 can be made uniform.
Further, when a plurality of display devices 1 including the

display units

11, 12, and 13 are arranged, the luminance of the plurality of display devices 1 can be made uniform.

As already described, the variation in the brightness of the plurality of segment LEDs constituting the display unit is suppressed by the manufacturer of the display unit.
In recent years, the power supply voltage Vdd has been lowered, and the segment LED drive voltage may be lowered. In such an environment where the segment LED drive voltage is lowered, a plurality of segment LEDs are used. Variations in brightness may become apparent.
In the first embodiment, control is performed so that the amount of current converted per unit time flowing through the plurality of segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h constituting the

display units

11, 12, and 13 is constant. Therefore, in an environment where the drive voltages of the segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h are lowered and used, the variation in luminance of the plurality of segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h becomes obvious. Even in this case, it is possible to suppress variations in luminance of the plurality of segment LEDs 11a to 11h, 12a to 12h, and 13a to 13h.

1 display, 11, 12, 13 display unit, 11a to 11h, 12a to 12h, 13a to 13h segment LED, 21 power supply voltage line (voltage application unit), 22 transistor (voltage application unit), 23 resistance, 24 FET ( Selection switch), 25 ground, 30 controller, 31 lighting pattern storage unit, 32 transistor control unit, 33 current measurement unit, 34 period calculation unit, 35 period storage unit, 36 selection switch control unit.

Claims

A plurality of display units each composed of a plurality of segment LEDs;
In each of the plurality of display units, a voltage application unit that applies a power supply voltage to the plurality of segment LEDs;
A display unit to be lit is sequentially selected from the plurality of display units, and a power supply voltage is applied by the voltage application unit among the plurality of segment LEDs constituting the selected display unit. A selection switch that allows a forward current to flow through the segment LED by connecting the segment LED to the ground;
For each display unit, a current measuring unit that measures a forward current flowing in a segment LED connected to the ground by the selection switch;
In order to keep the luminance of the plurality of display units within a certain range, the display unit is determined for each display unit from the forward current measured by the current measuring unit and the common luminance setting value for the plurality of display units. A period calculation unit for calculating a period for flowing a forward current to the segmented LED,
Selection switch control for controlling the connection of the selection switch to the ground so that a forward current flows through the segment LEDs constituting the display unit during the period calculated by the period calculation unit for each display unit. And an automatic brightness adjustment circuit for a digital display device.
A lighting pattern storage unit that stores lighting patterns indicating periods in which the plurality of display units can be lit is provided.
The period calculation unit is configured to determine, based on a forward current measured by the current measurement unit and a common luminance setting value for the plurality of display units, for each display unit in order to bring the luminances of the plurality of display units within a certain range. 2. The digital display device according to claim 1, wherein a period during which a forward current is passed through the segment LEDs constituting the display unit is calculated during a period in which the display unit indicated by the lighting pattern can be turned on. Automatic brightness adjustment circuit.
In the lighting pattern stored in the lighting pattern storage unit, the turn-off period of all the display units is shown before and after the turn-on period of each display unit,
The selection switch control unit is connected to the ground by the selection switch so that a forward current does not flow by interfering with the segment LEDs constituting the plurality of display units during the light-off period indicated by the lighting pattern. 3. The automatic brightness adjustment circuit for a digital display device according to claim 2, wherein: