JP2009069023A - Illuminance sensor and light receiving module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an illuminance sensor and a light receiving module capable of measuring the temperature of an LCD panel and the ambient light, or the illuminance of liquid crystal backlight with a decreased mounting area of the illuminance sensor. <P>SOLUTION: An illuminance sensor IC 13 includes a light receiving element 2 which generates photocurrent according to ambient brightness, an I-V conversion circuit 3 which converts the photocurrent from the light receiving element 2 into a log-compressed voltage, and a thermal voltage generation circuit 4 which generates a thermal voltage proportional to the temperature. The illuminance sensor further includes an A/D conversion part 5 which converts the log-compressed voltage and the thermal voltage into digital data of the illuminance and the temperature, an I-V conversion circuit 3, and a switch SW1 which is connected between the thermal voltage generation circuit 4 and the A/D conversion part 5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an illuminance sensor and a light receiving module that convert brightness into an electrical signal, and in particular, an illuminance sensor having spectral sensitivity characteristics close to human visual sensitivity characteristics, and the illuminance sensor and temperature to electrical signals. The present invention relates to a light receiving module including a temperature sensor.

  As a sensor for visible light, two sensors of a silicon photodiode and a CdS (cadmium sulfide) cell are typical. Silicon photodiodes are small, have high response speed and high stability, and are widely used in optical communications, optical disk light receiving elements, and optical sensors. However, the spectral sensitivity characteristics of silicon photodiodes are very different from human visual sensitivity and have sensitivity in the infrared region. In contrast, CdS cells have spectral sensitivity characteristics close to human vision, and have long been used as camera exposure meters and visible light sensors. However, the use of CdS cells mainly composed of cadmium sulfide is being restricted due to environmentally hazardous substances. In Europe, products using cadmium, lead, hexavalent chromium, and mercury are prohibited from being brought in since July 2006. Has been. Therefore, there is an increasing demand for a visible light sensor having a spectral sensitivity characteristic close to human visibility using a silicon photodiode having a small environmental load.

  Also, in recent years, the brightness of the backlight of mobile phones and LCD TVs is automatically adjusted according to the surrounding brightness, thereby reducing battery consumption of mobile phones and improving the visibility of liquid crystals. As an application to do so, the demand for illuminance sensors that are close to human visual sensitivity is increasing rapidly. Furthermore, by monitoring the brightness of the liquid crystal backlight and the temperature of the liquid crystal panel, the image contrast displayed on the liquid crystal panel and the brightness of the liquid crystal backlight are adjusted to realize a higher-quality liquid crystal display device. can do.

  Currently, there is a need for an illuminance sensor for a liquid crystal backlight automatic dimming system that is easier to use and lower in cost due to higher functionality through digitization. In addition, there is an increasing demand for further downsizing of the illuminance sensor including a temperature sensor for monitoring the temperature of the liquid crystal panel.

  Here, FIG. 13 shows a conventional liquid crystal display device 100 with automatic liquid crystal backlight dimming. The liquid crystal display device 100 of FIG. 13 includes an illuminance sensor 102 that detects ambient brightness or the brightness of the backlight 101, and an A / D that converts the illuminance value detected by the illuminance sensor 102 from an analog amount to a digital amount. The conversion part 103 is provided. In addition, a temperature sensor 105 that detects the temperature of the LCD panel (liquid crystal display panel) 104 and an A / D conversion unit 106 that converts the temperature value detected by the temperature sensor 105 from an analog amount to a digital amount are provided. Furthermore, the illuminance detection unit 107 includes an illuminance sensor 102 and an A / D conversion unit 103, and the temperature detection unit 108 includes a temperature sensor 105 and an A / D conversion unit 106.

  The controller 109 generates an optimum liquid crystal driving voltage so as to correct a change due to the temperature of the contrast of the LCD panel 104 based on the illuminance and temperature digital data detected by the illuminance detection unit 107 and the temperature detection unit 108, Supply to the LCD panel 104. Further, by detecting the ambient brightness or the brightness of the backlight 101, control is performed so that the brightness of the backlight is optimized.

  As having a configuration similar to that of the conventional liquid crystal display device with automatic liquid crystal backlight dimming in FIG. 13, Patent Document 2 discloses an illuminance sensor that detects an illuminance value and a temperature sensor that detects a temperature value. And a liquid crystal contrast adjusting device for adjusting a contrast of a liquid crystal display based on output values of the illuminance sensor and the temperature sensor.

On the other hand, the technique of measuring the temperature of the photoelectric conversion part which has an illuminance sensor or an optical sensor with a temperature sensor is also disclosed. In patent document 1, the illuminating device provided with the temperature detection means to detect the temperature of an illumination intensity sensor is disclosed. Moreover, in the optical measuring device currently disclosed by patent document 3 and patent document 4, a temperature sensor is attached to the photoelectric conversion part which has an optical sensor, and the temperature of the said photoelectric conversion part is measured.
JP 10-294271 A (published on November 4, 1998) JP-A-6-230343 (released on August 19, 1994) JP-A-3-214029 (published on September 19, 1991) Japanese Patent Laid-Open No. 3-44525 (published February 26, 1991)

  However, in the conventional liquid crystal display device with automatic liquid crystal backlight dimming shown in FIG. 13, it is necessary to connect an A / D converter to each of the illuminance sensor and the temperature sensor in order to detect the illuminance and the temperature. When a large number of illuminance sensors and temperature sensors are arranged in each part of the liquid crystal panel to finely control the backlight, the mounting area becomes large, which obstructs the thinning and miniaturization of the liquid crystal panel.

  The present invention has been made in view of the above-described conventional problems, and its purpose is to measure the temperature of the LCD panel and ambient ambient light or the illuminance of the liquid crystal backlight while reducing the mounting area. Another object of the present invention is to provide an illuminance sensor and a light receiving module.

  In order to solve the above problems, the illuminance sensor of the present invention is a light-receiving element that generates a photocurrent according to ambient brightness, and a current-voltage conversion that converts the photocurrent from the light-receiving element into a logarithmically compressed voltage. And an A / D conversion means for converting the logarithmically compressed voltage and the thermal voltage into digital data of illuminance and temperature, in an illuminance sensor comprising means and a thermal voltage generation means for generating a thermal voltage proportional to temperature. In the temperature measurement mode in which the temperature is measured by the thermal voltage generated by the thermal voltage generation means, the thermal voltage generation means and the A / D conversion means are connected, and the ambient current is generated by the photocurrent generated by the light receiving element. In the illuminance measurement mode for measuring brightness, a switch circuit for connecting the current-voltage conversion means and the A / D conversion means is provided.

  According to the invention, in the illuminance measurement mode, the switch circuit outputs data proportional to the illuminance in the illuminance measurement mode, and the A / D conversion means outputs data proportional to the temperature in the temperature measurement mode. As described above, the connection between the current / voltage converting means and the thermal voltage generating means and the A / D converting means is switched.

  Therefore, since the A / D conversion means can be used in both the illuminance measurement mode and the temperature measurement mode, electrical measurement in the illuminance measurement mode and the temperature measurement mode can be realized with a small mounting area, and the liquid crystal panel can be thinned. And downsizing of the equipment becomes possible.

  The illuminance sensor may further include a temperature / illuminance mode switching register that stores a bit for switching between the temperature measurement mode and the illuminance measurement mode.

  As a result, the signal line for transmitting the switch switching signal SW for switching the measurement mode of the A / D conversion means between the temperature measurement mode and the illuminance measurement mode can be omitted.

  In the illuminance sensor, a temperature / illuminance mode switching time register that stores time information for switching between the temperature measurement mode and the illuminance measurement mode at regular intervals, and the temperature measurement mode and the illuminance measurement mode are constant based on the time information. And a switching means for switching every time.

  Thereby, temperature and illuminance can be measured alternately by setting the switching time via the controller. Therefore, the backlight and contrast of the LCD panel can be controlled to the optimum state in real time according to the external temperature and illuminance environment.

  In the illuminance sensor, when the illuminance measurement mode is within a set range in which the illuminance changes with time, the A / D conversion means is shifted to the temperature measurement mode, and the temperature changes with time in the temperature measurement mode. A temperature / illuminance time change rate detection unit that shifts the A / D conversion unit to an illuminance measurement mode may be further provided when the A / D conversion unit is in a certain set range.

  Thus, when the temperature or illuminance changes quickly, the temperature and illuminance can be accurately measured in real time by increasing the frequency of measuring the temperature or illuminance.

  In the illuminance sensor, when the temperature changes in the temperature measurement mode and within a set range, the A / D converted temperature digital data is stored in the temperature register and the illuminance measurement mode is entered. If the illuminance measurement mode is within a set range where the illuminance changes over time, the digital data of the A / D converted temperature is stored in the illuminance register and the temperature measurement mode is entered. And the illuminance measurement mode may be alternately repeated.

  Thus, when the temperature or illuminance changes quickly, the frequency of temperature or illuminance measurement can be increased, and the latest temperature or illuminance digital data can be stored in the temperature register or illuminance register.

  In the illuminance sensor, a low-pass filter circuit may be provided after the switch circuit.

  Thereby, the error at the time of A / D conversion by the switching noise at the time of switching illumination intensity measurement mode and temperature measurement mode can be reduced.

  The illuminance sensor stores PWM (Pulse Width Modulation) dimming data in which a plurality of different duty ratios are set, and outputs one PWM dimming data according to the illuminance level corresponding to the illuminance digital data. A luminance setting register unit that outputs the PWM signal based on the output of the previous luminance setting register unit.

  Thus, the duty ratio of the PWM signal can be varied according to the illuminance level corresponding to the illuminance digital data. Therefore, by turning on the backlight when the PWM signal is at a high level and turning off the backlight when the PWM signal is at a low level, the brightness of the backlight can be controlled to be in proportion to the duty ratio of the PWM signal.

  Since the light receiving module of the present invention uses any one of the above illuminance sensors, electrical measurement can be realized with a small mounting area, and the liquid crystal panel can be made thinner and the device can be made smaller.

  As described above, the illuminance sensor of the present invention has an A / D conversion unit that converts logarithmically compressed voltage and thermal voltage into digital data of illuminance and temperature, and a temperature by the thermal voltage generated by the thermal voltage generation unit. In the temperature measurement mode for measuring, the thermal voltage generation means and the A / D conversion means are connected, and in the illuminance measurement mode for measuring the ambient brightness by the photocurrent generated by the light receiving element, the current-voltage conversion means And a switch circuit for connecting the A / D conversion means.

  Therefore, it is possible to provide an illuminance sensor and a light receiving module that can measure the temperature of the LCD panel and ambient ambient light or the illuminance of the liquid crystal backlight while reducing the mounting area.

  An embodiment of the present invention will be described with reference to FIG.

  FIG. 1 is a block diagram of a liquid crystal display device 1 with automatic light control of a liquid crystal backlight according to the present invention. The liquid crystal display device 1 includes a light receiving element (photodiode PD) 2, an IV conversion circuit 3, a thermal voltage generation circuit 4, a switch SW1, an A / D conversion unit 5, a switch SW2, an illuminance register 6, a temperature register 7, and a register. An R / W serial interface 9, a controller 10, an LCD panel 11, and a backlight 12 are provided. The illuminance sensor IC 13 includes an IV conversion circuit 3, a thermal voltage generation circuit 4, a switch SW1, an A / D conversion unit 5, a switch SW2, an illuminance register 6, a temperature register 7, a register R / W serial interface 9, And terminals 14-16. Signals transmitted from the controller 10 are input to the terminals 14 to 16.

  In FIG. 1, the light receiving element 2 generates a photocurrent Ipd proportional to light incident from the outside and outputs it to the IV conversion circuit 3. The IV conversion circuit 3 converts the photocurrent Ipd output from the light receiving element 2 into a logarithmically compressed voltage. The logarithmically compressed voltage is input to the A / D converter 5 via the switch SW1 and converted into digital data.

The thermal voltage generation circuit 4 generates a thermal voltage Vt proportional to the temperature of the LCD panel 11 and outputs it to the A / D conversion unit 5 via the switch SW1. Here, the thermal voltage Vt is
Vt = k · T / q (1)
It becomes. Here, k is a Boltzmann constant, T is the absolute temperature of the LCD panel 11, and q is an elementary charge.

  The switch SW1 connects the output of the IV conversion circuit 3 and the input of the A / D converter 5 in the illuminance measurement mode (illuminance sensor mode) based on the switch switching signal SW transmitted from the controller 10. In the temperature measurement mode (temperature sensor mode), the output of the thermal voltage generation circuit 4 and the input of the A / D converter 5 are connected.

  Based on the switch switching signal SW transmitted from the controller 10, the switch SW2 connects the output of the A / D converter 5 and the illuminance register 6 in the illuminance measurement mode (illuminance sensor mode), and the temperature measurement mode. In the (temperature sensor mode), the A / D converter 5 and the temperature register 7 are connected.

  Based on the switch switching signal SW transmitted from the controller 10, the A / D conversion unit 5 converts the logarithmically compressed voltage output from the IV conversion circuit 3 into digital data in the illuminance measurement mode. Thus, the illuminance that is an analog amount is converted into a digital amount. Similarly, in the temperature measurement mode, the thermal voltage output from the thermal voltage generation circuit 4 is converted into digital data, thereby converting the analog temperature into a digital quantity.

  Next, the A / D converter 5 stores the converted digital data in the illuminance register 6 in the illuminance measurement mode, and in the temperature register 7 in the temperature measurement mode.

The controller 10 reads the illuminance data and the temperature data stored in the illuminance register 6 and the temperature register 7 through the register R / W serial interface 9, the signal line SCL (Serial CLock), and the signal line SDA (Serial DAta). Based on these values, the brightness of the backlight 12 is controlled so as to be optimized, and an optimum driving voltage is generated so as to optimize the contrast of the liquid crystal and supplied to the LCD panel 11. The signal lines SCL and SDA are general I ² C bus signal lines. I ² C is an abbreviation for Inter Integrated Circuit, and was proposed in 1980 by Philips. The I ² C interface is a serial interface for transmitting information between devices located relatively close to each other by two signal lines SCL and SDA. Examples of the transmission speed include 100 kbps, 400 kbps (fast mode), and 3.4 Mbps (high speed mode).

  In this way, the illuminance sensor and the temperature sensor are integrated on a single chip and mounted on the light receiving module, and the illuminance measurement mode and the temperature measurement mode can be switched arbitrarily, and the A / D converter is shared, which is necessary for the liquid crystal panel It is possible to realize a simple sensor with a smaller mounting area.

[Example 1]
FIG. 2 is a block diagram of the logarithmic compression amplifier of the illuminance sensor, the thermal voltage generation circuit of the temperature sensor, and the A / D conversion unit according to the first embodiment of the present invention. For convenience of explanation, members having the same functions as those shown in the drawings of Embodiment 1 are given the same reference numerals, and descriptions thereof are omitted.

First, the operation in the illuminance measurement mode will be described. The light receiving element (photodiode PD) 2 generates a photocurrent Ipd proportional to the light incident from the outside. The photocurrent Ipd is converted into a logarithmically compressed voltage by a logarithmic compression amplifier 17 including an emitter-base PN junction diode of the PNP transistor QP1 and an amplifier AMP1. The output voltage V1 of the logarithmic compression amplifier 17 is V1 = Vref−Vt · ln (Ipd / Is) (2)
It becomes. Here, Vt is a thermal voltage, Vt = k · T / q, k is a Boltzmann constant, T is an absolute temperature of the LCD panel 11, q is an elementary charge, Is is a reverse saturation current, and Vref is a reference voltage. .

Similarly, a reference current source Iref1 that supplies a reference current Iref whose temperature coefficient is equal to the temperature coefficient of the photodiode current is also a logarithmic compression amplifier including a PN junction diode between the emitter and base of the PNP transistor QP2 and an amplifier AMP2. 18 is converted to a logarithmically compressed voltage. The output voltage V2 of the logarithmic compression amplifier when the reference current Iref1 is inputted is V2 = Vref−Vt · ln (Iref / Is) (3)
It becomes. As a voltage for providing an offset, the voltage V3 is obtained by adding a voltage A · Vt proportional to the thermal voltage Vt to the reference voltage Vref V3 = Vref + Vt · A (4)
Set to.

  The voltage V3 is generated by the resistor R, the current source I, and the buffer B1. The resistor R has one end connected to the power supply 23 that outputs the reference voltage Vref, and the other end connected to the input of the buffer B1. The current source has one end connected to the power supply 24 that outputs the voltage Vcc, and the other end connected to the input of the buffer B1. Further, the output of the buffer B1 is connected to one end of the resistor R3.

The voltages V1, V2, and V3 are calculated by an addition / subtraction circuit 19 including a resistor R1, a resistor R2, a resistor R3, a resistor R4, and an amplifier AMP3. When R1 = R2 = R and R3 = R4 = N · R are set, the output voltage V4 of the addition / subtraction circuit 19 is V4 = N · (−V1 + V2) + V3
= N · (− (Vref−Vt · ln (Ipd / Is)) + (Vref−Vt · ln (Iref / Is))) + (Vref + A · Vt)
= Vref + Vt. (A + N.ln (Ipd / Iref)) (5)
Thus, the term of the reverse saturation current Is having temperature dependence can be canceled.

  The constant N used to set the resistance values of the resistors R3 and R4 is a constant determined by the output voltage Vdac of the 8-bit D / A converter 22 described later and the range of illuminance to be detected. It is determined within the range. The 8-bit D / A converter 22 is included in the A / D converter 5. When the constant N is increased, the amplitude of the output voltage V4 increases and it is difficult to reduce the output voltage Vdac. On the contrary, if the constant N is reduced, the amplitude of the output voltage V4 is reduced, so that the conversion accuracy of the A / D converter 5 is required to be higher. That is, there is a trade-off relationship between lowering the output voltage Vdac generated inside the A / D converter 5 and the conversion accuracy of the A / D converter 5.

  The above equation of V4 is transformed into the format of V4 = Vref + C × log (Ev) (6). Here, Ev is illuminance (lux).

Assuming that the photocurrent flowing through the light receiving element (PD) at an illuminance of 1 lux is Ipd_1lx, the output voltage V4 of the adder / subtractor circuit 19 is V4 = Vref + Vt · (A + ln (Ipd / Ipd_1lx) + N · ln (Ipd — 1lx / Iref))
= Vref + Vt. (A + N.ln (Ev) + N.ln (Ipd_1lx / Iref)) (7)
The logarithm base conversion formula ln (X) = log (X) / log (e) ≈2.3025 × log (X) (8)
Than,
V4 = Vref + Vt. (A-N.ln (Iref / Ipd_1lx) + N.2.32525.log (Ev)) (9)
It becomes.

Here, when the constant A is set so that A−N · ln (Iref / Ipd — 1lx) = 0 (10), V4 = Vref + N · 2.3305 · Vt · log (Ev)
Thus, a voltage corresponding to the logarithm of the illuminance having a logarithm base of 10 can be obtained.

  Thus, by converting the illuminance to a logarithmically compressed voltage, the dynamic range becomes wider than when not converting to a logarithmically compressed voltage, from a low illuminance of 10 lux or less to a high illuminance of 10,000 lux or more. There is an advantage that the resolution at the time of low illuminance can be increased as compared with the case where the output current of the photodiode (light receiving element) is directly A / D converted. An equivalent function can be obtained even if a diode is used in place of the QP1 and QP2 PNP transistors.

  Next, the operation in the temperature measurement mode will be described. The temperature measurement mode is entered when the switch SW1 is connected to the reference current source Iref2 that supplies the reference current Iref.

The value of the output voltage V4 of the addition / subtraction circuit 19 in the illuminance sensor mode is V4 = Vref + Vt · (A + N · ln (Ipd / Iref)) (5)
As described above, in the temperature measurement mode, Ipd in the above equation (5) becomes Iref, and the term N · ln (Ipd / Iref) becomes zero.
V4 = Vref + A · Vt (11)
It is expressed. Since Vt = k · T / q, the output voltage V4 of the adder / subtractor circuit 19 in the temperature sensor mode is the sum of the reference voltage Vref having no temperature dependence and the term of A · Vt proportional to the temperature.

Next, the operation of the A / D conversion unit 5 in the illuminance measurement mode will be described. In the illuminance measurement mode, the logarithmically compressed illuminance analog signal is an 8-bit resolution A / D converter (A / D converter) having a comparator COMP1, an 8-bit up / down counter 21, and an 8-bit D / A converter 22. 5 is input. Here, the output voltage Vdac of the D / A converter 22 is Vdac = Vref + B · Vt (12)
Set to be.

  In the A / D converter 5, the comparator COMP1 compares the output voltage V4 of the adder / subtractor circuit 19 with the output voltage Vdac of the D / A converter 22. When V4> Vdac, the output of the comparator COMP1 is at a high level, and the up / down counter 21 is counted up in synchronization with A / DCLK which is a clock signal of the A / D converter 5. On the contrary, when V4 <Vdac, the output of the comparator COMP1 is at the low level, and the up / down counter 21 is counted down in synchronization with A / DCLK.

The 8-bit digital signal output from the up / down counter 21 is input to the D / A converter 22, and B changes with an accuracy of 256 gradations (8 bits) with respect to the full range. Feedback is applied so that the outputs Vdac and V4 have the same voltage. In this case, V4 = Vdac (13)
Vref + 2.3025 · Vt · log (Ev) = Vref + B · Vt (14)
Than,
B = 2.3025 · log (Ev) (15)
Thus, the illuminance Ev is obtained from the equation (15). By configuring the A / D conversion unit 5 as described above, the thermal voltage Vt in the output voltage V4 of the adder / subtractor circuit 19 is canceled, so that the A / D of the logarithm-compressed illuminance signal with less temperature dependency is obtained. D conversion is possible.

Next, the operation of the A / D conversion unit 5 in the temperature measurement mode will be described. In the temperature measurement mode, the analog voltage obtained by adding the voltage A · Vt proportional to the temperature to the reference voltage Vref is a resolution 8 including the comparator COMP1, the 8-bit up / down counter 21, and the 8-bit D / A converter 22. The data is input to a bit A / D converter (A / D converter) 5. Here, the output voltage Vdac of the D / A converter 22 is Vdac = Vref + B · Vconst (16)
Set to be. Here, Vconst is a voltage source that does not depend on temperature, and feedback is applied so that the output voltages Vdac and V4 of the D / A converter 22 become equal as in the illuminance mode. Therefore, V4 = Vdac (17)
Vref + A · Vt = Vref + B · Vconst (18)
Than,
B = A · Vt / Vconst
= A · (k / q) / Vconst · T (19)
Since B is a value proportional to the temperature T, the temperature T can be measured by the configuration shown in FIG.

[Example 2]
FIG. 3 shows a second embodiment of the present invention. As shown in FIG. 3, the logarithmic compression amplifier of the illuminance sensor, the thermal voltage generation circuit of the temperature sensor, and the A / D shown in FIG. 2 are also provided by installing a switch SW3 for switching between the voltage V3 and the output voltage of the amplifier AMP3. The same function as the conversion unit can be obtained. In the switch SW2 illuminance measurement mode, the output of the amplifier AMP3 and the comparator COMP1 are connected, and in the temperature measurement mode, the output of the buffer B1 and the comparator COMP1 are connected.

Example 3
FIG. 4 shows a specific embodiment of the D / A converter 22. Due to the current mirror having the PMOS transistors MP1 and MP2, the currents flowing through the drains or sources of the PMOS transistors MP1 and MP2 are equalized. Assuming that the current flowing through the drains or sources of the PMOS transistors MP1 and MP2 is I1, the base-emitter voltage Vbe1 of the PNP transistor QP11 is Vbe1 = Vt · ln (I1 / Is) (20)
It is represented by

The PNP transistor QP12 is a PNP transistor having an emitter area four times that of the PNP transistor QP11. The voltage Vbe2 between the base and the emitter of the PNP transistor QP12 is Vbe2 = Vt · ln (I1 / 4Is) (21)
It is represented by

Since the voltage difference between the voltage Vbe1 and the voltage Vbe2 becomes equal to the voltage across the resistor Rref connected to the emitter of the PNP transistor QP12, Vbe1 = Vbe2 + I1 · Rref (22)
I1 = Vt · ln4 / Rref (23)
It becomes.

In the illuminance measurement mode, the selector switch SW4 is connected to the gates of the PMOS transistors MPa0 to MPa7 and the gate of the PMOS transistor MP2. The PMOS transistor MP0, which has the same gate length as the PMOS transistor MP2, and whose gate width is 1, 2, 4, 8, 16, 32, 64, and 128 times that of the PMOS transistor MP2. The gates and drains of MP1, MP2, MP3, MP4, MP5, MP6, and MP7 are connected in common. Further, switches are connected between the sources of the PMOS transistors MPa0 to MPa7 and the power supply 24 that outputs the voltage Vcc, respectively, and A / D conversion data a0 to a7 that are digital outputs of the up / down counter 21 ON / OFF is controlled. When A / D conversion data a0 to a7, which are switch control signals, are at a high level, the switch is turned on, and a current corresponding to the gate width of the PMOS transistors MPa0 to MPa7 flows to the drain. The current Idac flowing through the resistor Rdac connected to the drains of the PMOS transistors MPa0 to MPa7 is Idac = (1 · a0 + 2 · a1 + 4 · a2 + 8 · a3 + 16 · a4 + 32 · a5 + 64 · a6 + 128 · a7) · I1 (24)
Represented by
Vdac = Vref + Idac · Rdac
= Vref + (1 * a0 + 2 * a1 + 4 * a2 + 8 * a3 + 16 * a4 + 32 * a5 + 64 * a6 + 128 * a7) * I1 * Rdac
= Vref + (1 * a0 + 2 * a1 + 4 * a2 + 8 * a3 + 16 * a4 + 32 * a5 + 64 * a6 + 128 * a7) * ln4 * (Rdac / Rref) * Vt (25)
It becomes. By making the resistance Rdac and the resistance Rref the same kind of resistance with the same temperature coefficient,
Vdac = Vref + B · Vt (12)
It becomes possible to make it form.

  Next, the operation of the D / A converter 22 in the temperature measurement mode will be described. When the PMOS transistor MP2 and the PMOS transistor MP3 are MOS transistors having the same size, the drain currents of the PMOS transistor MP2 and the PMOS transistor MP3 are both equal to I1. A resistor Rbg and a diode-connected PNP transistor QP13 are connected in series to the drain of the PMOS transistor MP3. The sum of the emitter-base voltage Vbe3 of the PNP transistor QP13 and the voltage across the resistor Rbg is the voltage Vbg.

I1 = Vt · ln4 / Rref (23)
Thus, the voltage Vbg is expressed by the following equation.

Vbg = Vbe3 + Vt. (Rbg / Rref) .ln4 (26)
When the resistor Rbg and the resistor Rref are configured to be the same type of resistor having the same temperature coefficient, the voltage Vbe3 has a negative temperature coefficient, and Vt · (Rbg / Rref) · ln4 has a positive temperature coefficient. Have. By setting (Rbg / Rref) so that the temperature coefficients of these two voltages are just cancelled, a voltage Vbg with very little temperature dependence can be obtained. Normally, the temperature coefficient of Vbg can be minimized by setting the voltage Vbg = 1.2V.

The voltage Vgb is received by a buffer having AMP1 and an NMOS transistor MN3, and converted to a current I2 by a resistor R1. The current I2 is I2 = Vbg / R1 (27)
The current I2 flows through the drain of the PMOS transistor MP4 represented by In the temperature measurement mode, the changeover switch SW4 is set so that the gate of the PMOS transistor MP4 and the gates of the PMOS transistors MPa0 to MPa7 are connected. As in the illuminance sensor mode, when the A / D conversion data a0 to a7, which are switch control signals, are at a high level, the switch is turned on, and a current corresponding to the gate width of the PMOS transistors MPa0 to MPa7 is supplied to the drain. Flowing. The current Idac flowing in the resistor Rdac connected to the drains of the PMOS transistors MPa0 to MPa7 is Idac = (1 · a0 + 2 · a1 + 4 · a2 + 8 · a3 + 16 · a4 + 32 · a5 + 64 · a6 + 128 · a7) · I2 (28)
Represented by
Vdac = Vref + Idac · Rdac
= Vref + (1 * a0 + 2 * a1 + 4 * a2 + 8 * a3 + 16 * a4 + 32 * a5 + 64 * a6 + 128 * a7) * I2 * Rdac
= Vref + (1 · a0 + 2 · a1 + 4 · a2 + 8 · a3 + 16 · a4 + 32 · a5 + 64 · a6 + 128 · a7) · Vbg · (Rdac / R1) (29)
It becomes. Since Vconst = Vbg · (Rdac / R1),
Vdac = Vref + B · Vconst (16)
It becomes possible to make it form. By making the resistor Rdac and the resistor R1 of the same type having the same temperature coefficient, Vconst can be a voltage source independent of temperature.

  The combination of “1” and “0” of A / D conversion data a0 to a7 makes it possible to control B with 256 gradations (8-bit accuracy), and to convert analog amounts of illuminance and temperature into digital data with high accuracy. Can be converted. Further, by switching the changeover switch SW1 between the illuminance measurement mode and the temperature measurement mode, it is possible to share one A / D converter in the illuminance measurement mode and the temperature measurement mode.

Example 4
In the present embodiment, the resolution of the A / D converter (A / D converter) 5 is described as 8 bits, but the resolution can be further improved by increasing the number of bits of the A / D converter 5. . FIG. 5 shows the output voltage Vdac of the D / A converter 22 that is a digital output signal when the output voltage V4 of the adder / subtractor circuit 19 that is an analog input signal is input to the A / D converter 5.

  When the up / down counter 21 is the initial value “00000000”, if an analog signal V4 as shown in FIG. 5 is input to the A / D converter 5, it is synchronized with the clock signal A / DCLK of the A / D converter 5. Thus, the digital output signal Vdac rises one step at a time, and feedback is applied so that the analog input signal V4 and the digital output signal Vdac match.

  In this way, A / D conversion data a0 to a7 which are digital signals corresponding to the analog signal (illuminance or temperature) input to the A / D conversion unit 5 are output. The digitally converted illuminance data and temperature data are stored in an illuminance register and a temperature register, respectively, and transferred to the controller 10 as liquid crystal control data via the register R / W serial interface 9.

Example 5
FIG. 6 shows a block diagram of a liquid crystal display device 25 with liquid crystal backlight automatic light control according to a fifth embodiment of the present invention. In the first embodiment, the direct illuminance measurement mode and the temperature measurement mode are switched by the control signal from the controller 10, but in this embodiment, the temperature measurement mode and the illuminance are switched by the temperature / illuminance mode switching register 26. It is characterized by switching between measurement modes.

  For example, in order to switch between the temperature measurement mode and the illuminance measurement mode, the temperature measurement mode is set when a certain bit in the temperature / illuminance mode switching register 26 is “1”, and the illuminance measurement mode is set when “0”. The measurement mode can be switched. The temperature measurement mode may be set when a certain bit in the temperature / illuminance mode switching register 26 is “0”, and the illuminance measurement mode may be set when the bit is “1”. The controller 10 can arbitrarily switch between the temperature measurement mode and the illuminance measurement mode by setting the value of the temperature / illuminance mode switching register 26 via the register R / W serial interface 9. Thereby, it is possible to omit the signal line for transmitting the switch switching signal SW from the controller 10 of FIG.

Example 6
FIG. 7 is a block diagram of a liquid crystal display device 27 with automatic liquid crystal backlight dimming according to a sixth embodiment of the present invention. In this embodiment, a temperature / illuminance mode switching time register 28 is provided, and two pieces of time information, a time for switching from the temperature measurement mode to the illuminance measurement mode and a time for switching from the illuminance measurement mode to the temperature measurement mode, are expressed as temperature / illuminance. A timer 29 that can be set by the mode switching time register 28 and generates a switching signal according to the data of the measurement mode switching time stored in the register is provided.

  By setting the switching time via the controller 10, the temperature and illuminance can be measured alternately, and the latest temperature information and illuminance information can always be stored in the temperature register and illuminance register. Therefore, the backlight 12 and the contrast of the LCD panel 11 can be controlled to the optimum state in real time according to the external temperature and illuminance environment.

Example 7
FIG. 8 is a block diagram of a liquid crystal display device 30 with liquid crystal backlight automatic light control according to a seventh embodiment of the present invention. In this embodiment, there is provided a temperature / illuminance time change rate detection circuit 31 for detecting time changes in the values of the illuminance register 6 and the temperature register 7, and the time change of illuminance in the illuminance measurement mode, that is, the range in which the illuminance change rate is present When in, the A / D conversion unit 5 is shifted to the temperature measurement mode. Further, when the temperature changes in the temperature measurement mode, that is, when the temperature change rate is within a certain range, the A / D converter 5 is shifted to the illuminance measurement mode.

  Specifically, the temperature / illuminance time change rate detection circuit 31 first monitors the values of the temperature register 7 and the illuminance register 6 every certain period (time T2−time T1). That is, the temperature Ta1 and the illuminance Ev1 are monitored at time T1, and the temperature Ta2 and the illuminance Ev2 are monitored at time T2. Then, from these temperatures and illuminances, a temperature change rate αTa and an illuminance change rate αEv are calculated using the following equations (30) and (31).

αTa = (Ta2-Ta1) / (T2-T1) (30)
αEv = (Ev2-Ev1) / (T2-T1) (31)
As a result, when the temperature or illuminance changes quickly, the temperature and illuminance can be accurately measured in real time by increasing the measurement frequency of the temperature or illuminance.

Example 8
FIG. 9 is a block diagram of a liquid crystal display device 32 with liquid crystal backlight automatic light control according to an eighth embodiment of the present invention. In this embodiment, an LPF (low-pass filter) 33 is provided between the temperature measurement mode / illuminance measurement mode changeover switch SW1 and the A / D conversion unit 5, thereby enabling A / D due to switching noise when each measurement mode is switched. It is possible to reduce errors during conversion.

Example 9
FIG. 10 is a block diagram of a liquid crystal display device 34 with liquid crystal backlight automatic light control according to a ninth embodiment of the present invention. In the present embodiment, a temperature / illuminance time change rate detection circuit 31 that detects a time change in the values of the illuminance register 6 and the temperature register 7 is provided, and when the illuminance time change is in a certain range in the illuminance measurement mode. Move to temperature measurement mode. Further, when the temperature measurement mode is in a range where the temperature changes with time, the illuminance measurement mode is entered. Further, a PWM register 35 for storing the duty ratio of the output pulse corresponding to the value of the illuminance register, and a PWM controller 36 for outputting a PWM signal at the duty ratio set by the value of the PWM register 35 are provided.

  Here, the PWM signal will be described based on the waveform diagram shown in FIG. The duty ratio of the PWM signal is defined by (time when the signal is high level / PWM period). It can be seen that as the duty ratio increases, the proportion of time that the signal is at a high level increases. When the duty ratio = 0%, the output signal is always at the low level, and when the duty ratio = 100%, the output signal is always at the high level.

  By turning on the backlight 12 when the PWM signal is at a high level and turning off the backlight 12 when the PWM signal is at a low level, the brightness of the backlight 12 can be controlled to a brightness proportional to the duty ratio. Therefore, the output of the PWM controller 36 is directly connected to a backlight driving circuit (not shown) included in the backlight 12 without going through the controller 10, and the backlight 12 can be automatically dimmed. Further, the temperature characteristic of the backlight 12 can be corrected by varying the duty ratio of the PWM output according to the temperature.

  Next, FIG. 12 shows the DUTY ratio of the PWM output signal corresponding to the illuminance level. The illuminance level is set in 16 steps on the horizontal axis so as to correspond to the upper 4 bits of the digital illuminance data output from the 8-bit A / D converter. For each illuminance level, the duty ratio of the PWM signal can be set to 256 gradations (8 bits) in the range of 0 to 100%.

  In such a dimming table, when the DUTY of the PWM signal is proportional to the luminance of the backlight, the luminance of the backlight increases as the illuminance level increases between 0 and 9, and the luminance level ranges from 9 to 12 In this case, the brightness of the backlight is kept almost constant even if the illuminance level changes. Further, in the section where the illuminance level is 12 to 13, the operation of decreasing the luminance of the backlight is performed as the illuminance increases.

[Summary of Embodiment]
In order to solve the above problems, the illuminance sensor of the present invention is a light receiving element 2 that generates a photocurrent according to ambient brightness, and an I− that converts the photocurrent of 2 from the light receiving element into a logarithmically compressed voltage. In an illuminance sensor including a V conversion circuit 3 and a thermal voltage generation circuit 4 that generates a thermal voltage proportional to temperature, an A / A that converts the logarithmically compressed voltage and the thermal voltage into digital data of illuminance and temperature. In the temperature measurement mode in which the temperature is measured by the thermal voltage generated by the D conversion unit 5 and the thermal voltage generation circuit 4, the thermal voltage generation circuit 4 and the A / D conversion unit 5 are connected to generate the light receiving element 2. In the illuminance measurement mode in which the ambient brightness is measured by the photocurrent, the switch SW1 that connects the IV conversion circuit 3 and the A / D conversion unit 5 is provided.

  According to the above-described invention, the switch SW1 outputs data proportional to the illuminance in the illuminance measurement mode, and outputs data proportional to the temperature in the temperature measurement mode. In addition, the connection between the IV conversion circuit 3 and the thermal voltage generation circuit 4 and the A / D conversion unit 5 is switched.

  Therefore, since the A / D converter 5 can be shared between the illuminance measurement mode and the temperature measurement mode, electrical measurement in the illuminance measurement mode and the temperature measurement mode can be realized with a small mounting area, and the liquid crystal panel can be thinned. And downsizing of the equipment becomes possible.

  The illuminance sensor may further include a temperature / illuminance mode switching register 26 that stores a bit for switching between the temperature measurement mode and the illuminance measurement mode.

  Thereby, the signal line which transmits switch switching signal SW which switches the measurement mode of A / D conversion part 5 between temperature measurement mode and illuminance measurement mode can be omitted.

  In the illuminance sensor, the temperature / illuminance mode switching time register 28 for storing time information for switching between the temperature measurement mode and the illuminance measurement mode at regular intervals, and the temperature measurement mode and the illuminance measurement mode based on the time information. You may further provide the timer 29 switched for every fixed time.

  Thereby, the temperature and the illuminance can be measured alternately by setting the switching time via the controller 10. Therefore, the backlight 12 and the contrast of the LCD panel 11 can be controlled to the optimum state in real time in accordance with the external temperature and illuminance environment.

  In the illuminance sensor, when the illuminance measurement mode is within a set range in which the illuminance changes with time, the A / D conversion means is shifted to the temperature measurement mode, and the temperature changes with time in the temperature measurement mode. A temperature / illuminance time change rate detection circuit 31 that shifts the A / D conversion means to the illuminance measurement mode may be further provided when the A / D conversion means is in a certain set range.

  Thus, when the temperature or illuminance changes quickly, the temperature and illuminance can be accurately measured in real time by increasing the frequency of measuring the temperature or illuminance.

  In the illuminance sensor, when the temperature changes in the temperature measurement mode and within a set range, the digital data of the A / D converted temperature is stored in the temperature register 7 and the illuminance measurement mode is entered. In the illuminance measurement mode, when the illuminance is within a set range where the illuminance changes over time, the digital data of the A / D converted temperature is stored in the illuminance register 6 and the temperature measurement mode is entered. The measurement mode and the illuminance measurement mode may be alternately repeated.

  As a result, when the temperature or illuminance changes quickly, the frequency of measuring the temperature or illuminance can be increased, and the latest temperature or illuminance digital data can be stored in the temperature register 7 or the illuminance register 6.

  In the illuminance sensor, an LPF 33 may be provided in the subsequent stage of the switch SW1.

  Thereby, the error at the time of A / D conversion by the switching noise at the time of switching illumination intensity measurement mode and temperature measurement mode can be reduced.

  The illuminance sensor stores PWM (Pulse Width Modulation) dimming data in which a plurality of different duty ratios are set, and outputs one PWM dimming data according to the illuminance level corresponding to the illuminance digital data. And a PWM controller 36 that outputs a PWM signal based on the output of the PWM register 35.

  Thus, the duty ratio of the PWM signal can be varied according to the illuminance level corresponding to the illuminance digital data. Therefore, the brightness of the backlight 12 can be controlled to be in proportion to the duty ratio of the PWM signal by turning on the backlight 12 when the PWM signal is at a high level and turning off the backlight 12 when the PWM signal is at a low level.

  Since the light receiving module of the present invention uses any one of the above illuminance sensors, electrical measurement can be realized with a small mounting area, and the liquid crystal panel can be made thinner and the device can be made smaller.

  The illuminance sensor of the present invention and the light receiving module using the illuminance sensor are suitable for a mobile phone, a liquid crystal television, and the like because the mounting area can be reduced by sharing the A / D conversion means in the illuminance measurement mode and the temperature measurement mode. Can be used.

1 is a block diagram of a liquid crystal display device with liquid crystal backlight automatic light control according to an embodiment of the present invention. It is a block diagram of the logarithmic compression amplifier of the illumination sensor which concerns on 1st Example of this invention, the thermal voltage generation circuit of a temperature sensor, and an A / D conversion part. It is a block diagram of the logarithmic compression amplifier of the illumination intensity sensor which concerns on 2nd Example of this invention, the thermal voltage generation circuit of a temperature sensor, and an A / D conversion part. It is a circuit diagram of a D / A converter concerning an embodiment of the present invention. It is a graph which shows a digital output signal when an analog input signal is input into the A / D converter. It is a block diagram of the liquid crystal display device with a liquid crystal backlight automatic light control which concerns on the other Example of this invention. It is a block diagram of the liquid crystal display device with a liquid crystal backlight automatic light control which concerns on the other Example of this invention. It is a block diagram of the liquid crystal display device with a liquid crystal backlight automatic light control which concerns on the other Example of this invention. It is a block diagram of the liquid crystal display device with a liquid crystal backlight automatic light control which concerns on the other Example of this invention. It is a block diagram of the liquid crystal display device with a liquid crystal backlight automatic light control which concerns on the other Example of this invention. It is a wave form diagram which shows a PWM signal. It is a graph which shows the DUTY ratio of the PWM output signal corresponding to an illumination level. This is a conventional liquid crystal display device with automatic liquid crystal backlight dimming.

Explanation of symbols

1, 25, 27, 30, 32, 34 Liquid crystal display device 2 Light receiving element 3 IV conversion circuit (current voltage conversion means)
4 Thermal voltage generation circuit (thermal voltage generation means)
5 A / D converter (A / D converter)
6 Illuminance register 7 Temperature register 9 Register R / W serial interface 10 Controller 11 LCD panel 12 Backlight 13 Illuminance sensor IC
14 to 16 terminals 17, 18 logarithmic compression amplifier 19 addition / subtraction circuit 21 8-bit up / down counter 22 8-bit D / A converter 23, 24 power supply 26 temperature / illuminance mode switching register 28 temperature / illuminance mode switching time register 29 timer (switching means) )
31 Temperature / illuminance time change rate detection circuit (temperature / illuminance time change rate detection means)
33 LPF (low-pass filter circuit)
35 PWM register (brightness setting register)
36 PWM controller (PWM output signal output unit)
A constant AMP1 to AMP3 amplifier A / DCLK clock signal B1 buffer COMP1 comparator
Ev Illuminance I Current source I2 Current Idac Current Ipd Photocurrent Iref Reference current Iref1, Iref2 Reference current source Is Reverse saturation current MN3 NMOS transistor MP1-MP4 PMOS transistor MPa0-MPa7 PMOS transistor N constants QP1, QP2, QP11-QP13 R, R1 to R4, Rbg, Rdac, Rref Resistance SCL, SDA Signal line SW Switch switching signal SW1 Switch (switch circuit)
SW2 to SW4 switch T absolute temperature V1 to V4 output voltage Vdac digital output signal Vref reference voltage Vt thermal voltage a0 to a7 A / D conversion data

Claims (8)

A light receiving element that generates a photocurrent according to ambient brightness;
Current-voltage conversion means for converting the photocurrent from the light receiving element into a logarithmically compressed voltage;
In an illuminance sensor comprising a thermal voltage generating means for generating a thermal voltage proportional to the temperature,
A / D conversion means for converting the logarithmically compressed voltage and the thermal voltage into digital data of illuminance and temperature;
In the temperature measurement mode in which the temperature is measured by the thermal voltage generated by the thermal voltage generation means, the thermal voltage generation means and the A / D conversion means are connected, and the ambient brightness is increased by the photocurrent generated by the light receiving element. An illuminance sensor comprising a switch circuit for connecting the current / voltage conversion means and the A / D conversion means in the illuminance measurement mode for measuring the thickness.   The illuminance sensor according to claim 1, further comprising a temperature / illuminance mode switching register for storing a bit for switching between the temperature measurement mode and the illuminance measurement mode. A temperature / illuminance mode switching time register for storing time information for switching between the temperature measurement mode and the illuminance measurement mode at regular intervals;
The illuminance sensor according to claim 1, further comprising switching means for switching between the temperature measurement mode and the illuminance measurement mode at regular intervals based on the time information.   When the illuminance measurement mode is within a set range where the illuminance changes over time, the A / D conversion means is shifted to the temperature measurement mode, and the temperature change is set during the temperature measurement mode. 2. The illuminance sensor according to claim 1, further comprising a temperature / illuminance time change rate detection unit that shifts the A / D conversion unit to an illuminance measurement mode when it is within a range. If the time change in temperature is within the specified range during the temperature measurement mode, the A / D converted temperature digital data is stored in the temperature register and the illuminance measurement mode is entered.
If the time change in illuminance is within the specified range in the illuminance measurement mode, the temperature measurement is performed by storing the digital data of the A / D converted temperature in the illuminance register and shifting to the temperature measurement mode. The illuminance sensor according to claim 4, wherein the mode and the illuminance measurement mode are alternately repeated.   The illuminance sensor according to claim 1, further comprising a low-pass filter circuit between the switch circuit and the A / D conversion means. A brightness setting register that stores PWM (Pulse Width Modulation) dimming data in which a plurality of different duty ratios are set, and outputs one PWM dimming data according to the illuminance level corresponding to the illuminance digital data And
The illuminance sensor according to claim 1, further comprising: a PWM output signal output unit that outputs a PWM signal based on an output of the previous brightness setting register unit.   A light receiving module using the illuminance sensor according to any one of claims 1 to 7.

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