JP2021110659A - Optical sensor - Google Patents

Abstract

To measure ambient light on the back side of a display panel having translucency.SOLUTION: For example, an optical sensor includes: a light receiving element which generates a light receiving signal IPD corresponding to both of output light of the light emitting element and ambient light; a detection circuit which sequentially generates an integral value DET of the light receiving signal IPD for every measurement period Tm shorter than a light emission period T of the light emitting element; and a processing circuit which sequentially determines a minimum value MIN from a plurality of integrated values DET for every first period Tx equal to or longer than a light emission period T and sets the minimum value MIN or a value corresponding thereto (for example, average value AVE) as a measurement value REG of the ambient light.SELECTED DRAWING: Figure 15

Description

The invention disclosed herein relates to an optical sensor (eg, an illuminance sensor for a smartphone or a proximity sensor).

Optical sensors that detect light are found in a variety of applications.

As an example of the prior art related to the above, Patent Document 1 can be mentioned.

International Publication No. 2018/066143

However, with a conventional optical sensor (particularly, a detection circuit used for this), it is difficult to achieve both high-speed operation and improvement of the SN ratio.

Therefore, one of the inventions disclosed in the present specification is an object of providing an optical sensor capable of achieving both high-speed operation and improvement of an SN ratio, and a detection circuit used therein.

Further, with a conventional optical sensor, it is difficult to measure ambient light on the back surface side of a display panel having translucency (for example, an OLED [organic light emitting diode] panel).

Therefore, one of the inventions disclosed in the present specification is to provide an optical sensor capable of measuring ambient light on the back surface side of a translucent display panel.

For example, the detection circuit disclosed in the present specification includes a first integrator to which an input signal is input, a second integrator to output an output signal, an output end of the first integrator, and the above. Control to monitor the integrated capacitance connected to the input end of the second integrator, the discharge unit connected to the input end of the second integrator, and the second output signal to control the discharge unit. It has a part and.

Further, for example, the optical sensor disclosed in the present specification includes a light receiving element that generates a light receiving signal corresponding to both output light and ambient light of the light emitting element, and a measurement period shorter than the light emitting cycle of the light emitting element. A detection circuit that sequentially generates an integrated value of the received light signal for each, and a detection circuit that sequentially determines a minimum value from a plurality of the integrated values for each first period equal to or longer than the light emission cycle, and determines the minimum value or a value corresponding thereto. It has a processing circuit for measuring ambient light.

The other features, elements, steps, advantages, and properties of the present invention will be further clarified by the detailed description of the embodiments that follow and the accompanying drawings.

According to one of the inventions disclosed in the present specification, it is possible to provide an optical sensor capable of achieving both high-speed operation and improvement of the SN ratio, and a detection circuit used for the optical sensor.

Further, according to one of the inventions disclosed in the present specification, it is possible to provide an optical sensor capable of measuring ambient light on the back surface side of a translucent display panel.

The figure which shows the 1st comparative example of an optical sensor The figure which shows an example of the light detection operation in the 1st comparative example. The figure which shows the 2nd comparative example of the optical sensor The figure which shows an example of the light detection operation in the 2nd comparative example. The figure which shows the relationship between the light-receiving area / integrated capacity ratio, and the SN ratio. Diagram showing the actual waveform and ideal waveform of the analog output signal The figure which shows the relationship between a frequency and a noise amount The figure which shows the 1st Embodiment of an optical sensor The figure which shows an example of the light detection operation in 1st Embodiment The figure which shows the front main part of an electronic device The figure which shows the α1-α2 cross section of an electronic device The figure which shows the 2nd Embodiment of an optical sensor The figure which shows the relationship between the OLED off period and the ambient light measurement period. The figure which shows the relationship between the emission brightness of OLED and the off period (emission brightness 25%). The figure which shows the relationship between the emission brightness of OLED and the off period (emission brightness 50%) The figure which shows the relationship between the emission brightness of OLED and the off period (emission brightness 75%). The figure which shows the relationship between the emission brightness of OLED and the off period (emission brightness 96%) The figure which shows an example of the light detection operation in 2nd Embodiment

<Optical sensor (first comparative example)>
First, prior to the description of the new embodiment of the optical sensor, a comparative example to be compared with the new embodiment will be briefly described. FIG. 1 is a diagram showing a first comparative example of an optical sensor. The optical sensor 10 of this comparative example is a semiconductor integrated circuit device (illuminance sensor IC or the like) that detects light and converts it into an electric signal, and has a light receiving element 11 and a detection circuit 12.

The light receiving element 11 is a photoelectric conversion element that generates a light receiving signal IPD (= current signal) according to the incident light. The received light signal IPD becomes larger as the incident light is stronger, and becomes smaller as the incident light is weaker. As the light receiving element 11, a photodiode or a phototransistor can be preferably used. The light receiving element 11 is generally accompanied by a parasitic capacitor 13 (capacity value Cp).

The detection circuit 12 is a circuit unit that detects the received light signal IPD and generates an analog output signal AOUT, and includes an operational amplifier 121, a capacitor 122, and switches 123 to 125.

The inverting input end (−) of the operational amplifier 121 is connected to the application end of the analog input signal AIN. The non-inverting input end (+) of the operational amplifier 121 is connected to the application end of the bias voltage VB (for example, VB = 0.5V). The output end of the operational amplifier 121 is connected to the application end of the analog output signal AOUT. The analog output signal AOUT is subjected to amplification processing, A / D [analog-to-digital] conversion processing, and the like in a subsequent circuit (not shown).

The capacitor 122 (capacitance value C1) is connected between the inverting input end (−) and the output end of the operational amplifier 121.

The switch 123 is connected in parallel to the capacitor 122, and is turned on / off according to the switching signal SW1. For example, the switch 123 turns on when SW1 = H and turns off when SW1 = L.

The switch 124 is connected between the light receiving element 11 (for example, the cathode of the photodiode) and the inverting input end (−) of the operational amplifier 121, and is turned on / off according to the switching signal SW2. For example, the switch 124 turns on when SW2 = H and turns off when SW2 = L.

The switch 125 is connected between the light receiving element 11 (for example, the cathode of the photodiode) and the application end of the bias voltage VB, and is turned on / off according to the inverting switching signal SW2B (= logical inverting signal of the switching signal SW2). Will be done. For example, the switch 125 turns on when SW2B = H and turns off when SW2B = L.

FIG. 2 is a diagram showing an example of the photodetection operation in the first comparative example, in order from the top, the operating state (START) of the optical sensor 10, the switching signals SW1 and SW2, the inverting switching signal SW2B, and the analog output signal. AOUT is depicted.

Before time t11, it corresponds to the standby period of the optical sensor 10. At this time, SW1 = SW2 = H and SW2B = L. That is, switches 123 and 124 are turned on and switches 125 are turned off. As a result, the detection circuit 12 is in a state where the light receiving signal IPD (and thus the analog input signal AIN) is not integrated, so that AOUT = VB.

Times t11 to t12 correspond to the integration period of the optical sensor 10. At this time, SW1 = SW2B = L and SW2 = H. That is, the switches 123 and 125 are turned off and the switch 124 is turned on. As a result, the detection circuit 12 is in a state of performing the integration operation of the received light signal IPD (and thus the analog input signal AIN), so that the analog output signal AOUT rises from the bias voltage VB.

After time t12, it corresponds to the measurement period of the optical sensor 10. At this time, SW1 = SW2 = L and SW2B = H. That is, switches 123 and 124 are turned off and switches 125 are turned on. As a result, the analog output signal AOUT is held at the signal value immediately before the time t12. This analog output signal AOUT has a voltage value proportional to the magnitude of the received light signal IPD (and thus the intensity of the incident light), and is used as a measured value of the incident light.

By the way, one of the most important characteristics of the optical sensor 10 is the detection sensitivity. As a method of increasing the detection sensitivity, it is conceivable to lengthen the integration period (= time t11 to t12) because the analog output signal AOUT only needs to be large with respect to the same intensity of the incident light.

However, the analog output signal AOUT has an upper limit value (output dynamic range) depending on the power supply voltage of the optical sensor 10 and the circuit method, and when the analog output signal AOUT reaches the upper limit value, the correct integration operation cannot be performed.

For example, when the power supply voltage of the optical sensor 10 is 3V, it is not possible to obtain an analog output signal AOUT of 3V or more regardless of the circuit configuration of the detection circuit 12. Further, since it is necessary to take a voltage margin so that the transistors forming the output stage of the operational amplifier 121 are not saturated, a voltage lower than 3V (for example, 2.8V) is actually the upper limit value of the analog output signal AOUT.

In the following, a second comparative example in which the circuit configuration has been devised so that the analog output signal AOUT does not reach the upper limit value will be described.

<Optical sensor (second comparative example)>
FIG. 3 is a diagram showing a second comparative example of the optical sensor. The optical sensor 10 of this comparative example is based on the first comparative example (FIG. 1), and further includes a discharge unit 126 and a control unit 127.

The discharge unit 126 is connected to the inverting input end (−) of the operational amplifier 121, and discharges the electric charge stored in the capacitor 122 in response to the switching signal SW3 input from the control unit 127. Specifically, the discharge unit 126 discharges the capacitor 122 when, for example, SW3 = H, and stops the discharge operation of the capacitor 122 when SW3 = L.

The control unit 127 compares the analog output signal AOUT with the upper limit value VH and the lower limit value VL (where VL <VB <VH), respectively, and generates a switching signal SW3 for controlling the discharge unit 126. Further, the control unit 127 also has a function of generating integrated value data DATA of the received light signal IPD based on the number of times the capacitor 122 is discharged (= the number of times the switching signal SW3 is raised to a high level).

Further, the capacitance value C1 of the capacitor 122 is not a fixed value but a variable value according to the inverting switching signal S2B. More specifically, when S2B = L, C1 = C1a, and when S2B = H, C1 = C1b (= m × C1a, where m> 1) (for example, m = 32, C1a). = 0.5pF, C1b = 16pF).

In the optical sensor 10 of this comparative example, the stronger the incident light, the more frequently the discharge operation of the capacitor 122 occurs. Therefore, if the digital integrated value data DATA is incremented each time the capacitor 122 is discharged, the incident light can be measured correctly while keeping the analog output signal AOUT within the output dynamic range.

FIG. 4 is a diagram showing an example of the photodetection operation in the second comparative example, in order from the top, the operating state (START) of the optical sensor 10, the switching signals SW1 and SW2, the inverting switching signal SW2B, the switching signal SW, and the analog. The output signal AOUT and the integrated value data DATA are depicted.

Before time t21, it corresponds to the standby period of the optical sensor 10. At this time, SW1 = SW2 = H and SW2B = L. That is, the switches 123 and 124 are turned on and the switch 125 is turned off. As a result, the detection circuit 12 is in a state where the light receiving signal IPD (and thus the analog input signal AIN) is not integrated, so that AOUT = VB.

Since SW3 = L is maintained during the above standby period, the capacitor 122 is not discharged. Further, the integrated value data DATA is set to an initial value (= 0).

Times t21 to t22 correspond to the integration period of the optical sensor 10. At this time, SW1 = SW2B = L and SW2 = H. That is, the switches 123 and 125 are turned off and the switch 124 is turned on. As a result, the detection circuit 12 is in a state of performing the integration operation of the received light signal IPD (and thus the analog input signal AIN), so that the analog output signal AOUT rises from the bias voltage VB.

Further, during the above integration period, every time the analog output signal AOUT reaches the upper limit value VH, the switching signal SW3 is raised to a high level, and the batch discharge operation of the capacitor 122 is performed. As a result, the analog output signal AOUT drops from the upper limit value VH to the bias voltage VB each time the above-mentioned batch discharge operation is performed. That is, the analog output signal AOUT is reduced by the discharge amount V1 (= VH-VB) by one batch discharge operation (for example, VH = 1.1V, VB = 0.5V, V1 = 0.6V).

The integrated value data DATA is incremented by one each time the above-mentioned batch discharge operation is performed. According to this figure, since the batch discharge operation is performed three times during the above integration period, DATA = 3 at time t22.

Times t22 to t23 correspond to the stepwise discharge period of the optical sensor 10. At this time, SW1 = SW2 = L and SW2B = H. That is, the switches 123 and 124 are turned off and the switch 125 is turned on.

Further, during the stepwise discharge period, the capacitance value C1 of the capacitor 122 is switched from the capacitance value C1a (for example, 0.5 pF) in the integration period to a larger capacitance value C1b (for example, 16 pF), and then the stage of the capacitor 122. The discharge operation is repeated. As a result, the analog output signal AOUT decreases by a discharge amount V2 (= V1 / m) smaller than the above-mentioned discharge amount V1 each time the above-mentioned stepwise discharge operation is performed (for example, m = 32, V1 = 0.6V, V2 = 18.8mV). Such a stepwise discharge operation is continued until the time t23 when the analog output signal AOUT falls below the lower limit value VL.

The integrated value data DATA is incremented by 1 / m each time the above-mentioned stepwise discharge operation is performed. According to this figure, since the stepwise discharge operation is performed n times during the stepwise discharge period, DATA = 3+ (n / m) at time t23. As described above, according to the stepwise discharge operation, the decimal point of the integrated value data DATA can be measured, so that the resolution of the integrated value data DATA can be improved.

As a result, in the optical sensor 10 of this comparative example, the integrated value data is the division value obtained by dividing the "voltage proportional to the intensity of the received light signal IPD (and thus the incident light)" by the discharge amount V2 (for example, 18.8 mV). Obtained as DATA.

If this method is adopted, the longer the integration period is set, the higher the detection sensitivity of the optical sensor 10 can be. However, in reality, the integration period cannot be extended indefinitely due to application restrictions. For example, in a proximity sensor for smartphones, it is necessary to complete the integration operation in about 10 to 100 μs.

FIG. 5 is a diagram showing the relationship between the light receiving area / integrated capacity ratio and the SN ratio. The solid line in this figure shows the actual behavior, and the broken line shows the ideal behavior.

If the detection sensitivity is still insufficient even if the above-mentioned integration period is extended to the upper limit value allowed in the application, in order to further increase the detection sensitivity, the area of the light receiving element 11 is increased or the capacitance value C1 of the capacitor 122 is increased. It is necessary to increase the light receiving area / integrated capacitance ratio by reducing the number of light receiving areas.

However, when the area of the light receiving element 11 is increased, the capacitance value Cp of the parasitic capacitor 13 attached to the light receiving element 11 also increases, so that the feedback rate determined by C1 / Cp becomes small. As a result, the closed loop gain of the operational amplifier 121 becomes high, and the noise level of the analog output signal AOUT becomes large, so that the SN ratio cannot be improved as desired (see the comparison between the solid line and the broken line in this figure). Further, the same applies to the case of reducing the capacitance value C1 of the capacitor 122.

FIG. 6 is a diagram showing an actual waveform and an ideal waveform of the analog output signal AOUT, and the operating state START of the optical sensor 10, the switching signal SW1, and the analog output signal AOUT are depicted in order from the top. Regarding the analog output signal AOUT, the solid line shows the actual behavior, and the broken line shows the ideal behavior.

The noise generated in the analog output signal AOUT includes two noise components n1 and n2. The first noise component n1 is the moment when the integration operation is started at time t31, that is, the moment when the switching signal SW1 is switched from the high level to the low level (and the moment when the switch 123 is switched from on to off). It is the voltage fluctuation that occurs. The second noise component n2 is a voltage fluctuation generated during the integration operation.

FIG. 7 is a diagram showing the relationship between the frequency and the amount of noise in the operational amplifier 121. As shown in this figure, the noise generated by the operational amplifier 121 includes flicker noise n11 in the low frequency band and thermal noise n12 in the middle / high frequency band.

In particular, when a format in which the analog input signal AIN is integrated using the operational amplifier 121 is adopted, the thermal noise n12 in the high frequency band mainly affects. Therefore, even if the element size constituting the operational amplifier 121 is increased to suppress the flicker noise n11, there is almost no improvement effect.

Therefore, as the simplest noise countermeasure, it is conceivable to narrow the closed loop bandwidth of the operational amplifier 121 (that is, slow down the operational amplifier 121). According to such noise countermeasures, the thermal noise n12 in the high frequency band can be cut, so that a large noise suppression effect can be obtained.

However, if the operational amplifier 121 is slowed down, there is a trade-off that the settling time of the analog output signal AOUT becomes long (= the discharge speed decreases) during the discharge operation described above.

If the current consumption of the operational amplifier 121 is increased, the thermal noise n12 can be suppressed without slowing down the operational amplifier 121. However, for that purpose, it is necessary to considerably increase the current consumption of the operational amplifier 121, so it cannot be said that it is an effective noise countermeasure.

In the following, in view of the above considerations, we propose a new embodiment that can solve the trade-off between noise suppression and discharge speed reduction, and can achieve both high-speed operation and improvement of the SN ratio.

<Optical sensor (first embodiment)>
FIG. 8 is a diagram showing a first embodiment of the optical sensor. The optical sensor 10 of the present embodiment has an operational amplifier 128, a capacitor 129, a capacitor 12A, a switch 12B, and a delay portion 12C, based on the second comparative example (FIG. 3) described above.

Therefore, unless there is a special need, duplicate explanations will be omitted for the existing components, and emphasis will be given on the new components.

The inverting input end (−) of the operational amplifier 128 is connected to the application end (= one end of the switch 124) of the analog input signal AIN1. The non-inverting input end (+) of the operational amplifier 128 is connected to the application end of the bias voltage VB (for example, VB = 0.5V). The output end of the operational amplifier 128 is connected to the application end of the analog output signal AOUT1.

The capacitor 129 (capacity value C2) is connected between the output end of the operational amplifier 128 and the inverting input end (−) of the operational amplifier 121.

The capacitor 12A (capacitance value C3, where C3 <C2) is connected between the inverting input end (−) and the output end of the operational amplifier 128.

The switch 12B is connected in parallel to the capacitor 12A and is turned on / off according to the switching signal SW1. For example, the switch 12B is turned on when SW1 = H and turned off when SW1 = L.

The delay unit 12C gives a delay to the falling timing of the switching signal SW1 to generate a delay switching signal SW1d, and outputs the delay switching signal SW1d to the switch 123. That is, the switch 123 is turned on / off according to the switching signal SW1d instead of the switching signal SW1. For example, the switch 123 turns on when SW1d = H and turns off when SW1d = L.

In the optical sensor 10 of the present embodiment, the operational amplifier 128, the capacitor 12A, and the switch 12B can be understood as a first integrator 12X that integrates the analog input signal AIN1 to generate the analog output signal AOUT1.

On the other hand, the operational amplifier 121, the capacitor 122, and the switch 123 described above integrate the analog input signal AIN2 (= replace the analog input signal AIN mentioned above) to obtain the analog output signal AOUT2 (= the analog output signal AOUT mentioned above). It can be understood as a second integrator 12Y that generates (replacement).

As described above, the optical sensor 10 of the present embodiment has a capacitor 129 (= integrated capacitance) between the operational amplifier 128 (= corresponding to the first amplifier) in the front stage and the operational amplifier 121 (= corresponding to the second amplifier) in the subsequent stage. It has a cascade structure with (equivalent) inserted.

The discharge unit 126 is connected only to the operational amplifier 121 (particularly the inverting input terminal (−)) in the subsequent stage.

Further, the operational amplifier 128 in the front stage has a narrower closed loop gain bandwidth than the operational amplifier 121 in the latter stage. That is, the operational amplifier 128 is slower than the operational amplifier 121.

Further, the switch 123 is turned off when a predetermined delay time Td has elapsed from the off timing of the switch 12B. That is, the integration start timing of the second integrator 12Y is slightly delayed from the integration start timing of the first integrator 12X.

FIG. 9 is a diagram showing an example of the photodetection operation in the first embodiment, in order from the top, the operating state (START) of the optical sensor 10, the switching signal SW1, the delay switching signal SW1d, and the analog output signal AOUT1 and AOUT2 is depicted.

Regarding the analog output signals AOUT1 and AOUT2, the solid line shows the actual behavior, and the broken line shows the ideal behavior.

Before time t41, it corresponds to the standby period of the optical sensor 10. At this time, SW1 = SW1d = H. That is, both switches 12B and 123 are turned on. As a result, neither the first integrator 12X nor the second integrator 12Y is in a state of performing the integration operation, so that AOUT1 = AOUT2 = VB.

After time t41, it corresponds to the integration period of the optical sensor 10. However, at time t41, only the switching signal SW1 becomes low level, and the delay switching signal SW1d is maintained at high level. That is, only the switch 12B is turned off and the switch 123 remains on. As a result, only the first integrator 12X is in a state of performing the integrating operation, so that only the analog output signal AOUT1 rises from the bias voltage VB.

After that, at time t42, when the delay switching signal SW1d drops to a low level, the switch 123 is turned off. Therefore, since the second integrator 12Y is also in a state of performing the integrative operation, the analog output signal AOUT2 drops from the bias voltage VB.

As described above, in the optical sensor 10 of the present embodiment, the off timings of the switches 12B and 123 are shifted. With such a configuration, the noise component generated by the operational amplifier 128 in the previous stage at the moment when the switch 12B is turned off is absorbed by the operational amplifier 121 in the subsequent stage which has not yet started the integration operation, so that the final analog output signal AOUT2 Has no effect on.

On the other hand, the noise component generated in the operational amplifier 128 in the previous stage during the integration operation is transmitted to the operational amplifier 121 in the subsequent stage through the capacitor 129. However, by making the operational amplifier 128 slower than the operational amplifier 121, the noise component generated in the operational amplifier 128 during the integration operation can be reduced, so that the influence of the noise component can be suppressed. In the first integrator 12X, since the discharge operation of the capacitor 12A is not performed, there is no problem even if the operational amplifier 128 is slowed down.

From the above, the noise component generated by the operational amplifier 128 in the front stage has almost no effect on the noise characteristics of the entire optical sensor 10, and the noise characteristics of the entire optical sensor 10 are determined according to the noise component generated by the operational amplifier 121 in the rear stage. become.

Here, when the capacitance value C1 of the capacitor 122 is the same value as that of the above-mentioned comparative example (FIG. 1 or 3), the gain G of the entire optical sensor 10 is (C2 / C3) times that of the comparative example. Therefore, for example, if the capacitance value C2 of the capacitor 129 is increased, the gain G can be increased without increasing the noise, so that the SN ratio can be improved.

That is, in the optical sensor 10 of the present embodiment, the detection sensitivity is increased (= the inclination of the analog output signal AOUT2 is increased) while keeping the noise level of the analog output signal AOUT2 equal to that of the above-mentioned comparative example (AOUT). ) Is possible.

As described above, in the first integrator 12X, the discharge operation of the capacitor 12A is not performed, so it is necessary to take care so that the analog output signal AOUT1 falls within the output dynamic range. Regarding this, for example, by increasing the capacitance value C3 of the capacitor 12A, the inclination of the analog output signal AOUT1 is suppressed, and the analog output signal AOUT1 does not reach the upper limit value of the output dynamic range during the integration period of the first integrator 12X. It should be designed as follows.

Of course, if only the capacitance value C3 of the capacitor 12A is increased, the gain G (= C2 / C3) will drop from the desired value. Therefore, the capacitance value C2 of the capacitor 129 and the capacitance value C3 of the capacitor 12A may be increased so that the gain G is maintained at a desired value.

From the above, the optical sensor 10 of the present embodiment can solve the trade-off between noise suppression and reduction of the discharge speed, and can achieve both high-speed operation and improvement of the SN ratio.

<Installation in electronic devices>
FIG. 10 is a diagram showing a front main part of an electronic device on which an optical sensor is mounted. In the electronic device (for example, smartphone) X of the present figure, most of the front surface of the housing is occupied by the display panel X1. Therefore, for example, when the display panel X1 is a liquid crystal panel, the optical sensor (for example, the illuminance sensor or the proximity sensor) mounted on the electronic device X must be arranged in the bezel region X2 (for example, the position P0) surrounding the display panel X1. I don't get it. The reason is that since the display panel X1 (= liquid crystal panel) does not transmit light, the optical sensor cannot be arranged on the back surface side of the display panel X1, and only the bezel area X2 has an arrangement space.

However, in recent years, there has been a strong demand for a full display in the electronic device X, and the bezel region X2 has been narrowed, so that it has become difficult to arrange the optical sensor in the bezel region X2.

By the way, as the display panel X1, an OLED panel has been put into practical use in addition to the liquid crystal panel. The OLED is a translucent light emitting element. Therefore, when the OLED panel is used as the display panel X1, the optical sensor can be arranged on the back surface side (for example, the position P1) of the display panel X1. In fact, in the trend of full display and OLED panel, there is a great demand for arranging an optical sensor on the back side of the display panel X1.

FIG. 11 is a diagram showing an α1-α2 cross section of the electronic device X in FIG. As shown in this figure, the display panel X1 (= OLED panel) incorporated on the front side of the housing X11 is formed by superimposing a glass plate X12, an OLED layer X13, and a protective layer X14.

In the OLED layer X13, a large number of a plurality of OLEDs are two-dimensionally arranged as pixels for outputting an arbitrary character or video. As described above, the OLED is a translucent light emitting element. Therefore, both the glass plate X12 and the OLED layer X13 have translucency. On the other hand, the protective layer X14 for protecting the back surface of the OLED layer X13 is often formed of a light-shielding material.

Therefore, when the optical sensor 20 is arranged on the back surface side of the display panel X1 (= OLED panel), the protective layer X14 includes the optical sensor 20 mounted on the substrate X15 (particularly, the light receiving element 21 formed on the surface thereof). In the facing portion, it is preferable to provide an opening 14a for transmitting the ambient light L1 incident from the front surface side of the display panel X1 to the back surface side of the display panel X1.

With such a configuration, the ambient light L1 transmitted through the display panel X1 can be measured by using the optical sensor 20 provided on the back surface side of the display panel X1, so that the electronic device X can be used as an old display. It becomes possible to correspond.

However, if the optical sensor 20 is arranged on the back surface side of the display panel X1, not only the ambient light L1 which is the original measurement target but also the output light L2 of the OLED panel X1 is incident on the optical sensor 20, which causes a measurement error. It ends up.

In view of the above considerations, the following proposes a novel embodiment capable of accurately measuring ambient light on the back surface side of a translucent display panel.

<Optical sensor (second embodiment)>
FIG. 12 is a diagram showing a second embodiment of the optical sensor. The optical sensor 20 of the second embodiment includes a light receiving element 21, a detection circuit 22, a processing circuit 23, a register 24, and an interface circuit 25.

The light receiving element 21 is a photoelectric conversion element that generates a light receiving signal IPD (= current signal) according to the incident light. The received light signal IPD becomes larger as the incident light is stronger, and becomes smaller as the incident light is weaker. As the light receiving element 11, a photodiode or a phototransistor can be preferably used. As can be seen from FIG. 11 above, in addition to the ambient light L1 transmitted through the display panel X1, the output light L2 of the display panel X1 (particularly the OLED that is the light emitting element) is included in the incident light of the light receiving element 21. include.

The detection circuit 22 sets the integrated value S1 of the received signal IPD for each measurement period Tm (for example, Tm ≦ 1 ms) shorter than the light emission cycle T (= 1 / f, for example, if f = 240 Hz, T≈4 ms) of the OLED. Generate sequentially. In order to realize the high-speed integration operation as described above, it is desirable to adopt, for example, the above-mentioned first embodiment (FIG. 8) as the detection circuit 22.

The processing circuit 23 is a functional block that generates a measured value S3 of ambient light L1 by performing predetermined signal processing on the integrated value S1 input from the detection circuit 22, and is a minimum value determination unit 231 and an average value calculation unit 232. including.

The minimum value determination unit 231 sequentially determines the minimum value S2 from a plurality of integral values S1 for each first period Tx (Tx ≧ T, for example, Tx = 5 ms).

The average value calculation unit 232 sequentially calculates an average value from a plurality of minimum values S2 for each second period Ty (Ty> Tx, for example, Ty = 100 ms), and uses the average value as the measured value S3 of the ambient light L1. Output.

The operation of the processing circuit 23 (particularly, the minimum value determination unit 231 and the average value calculation unit 232) and its technical significance will be described in detail later.

The register 24 stores the measured value S3 input from the processing circuit 23.

The interface circuit 25 reads out the measured value S3 stored in the register 24 periodically or in response to an external request, and outputs this to the outside as ambient light measurement data ALSDATA. Incidentally, in the communication method of the interface circuit 25, a serial communication system (e.g., ^{I 2 C [inter-integrated circuit} ] communication method) is preferable.

FIG. 13 is a diagram showing the relationship between the OLED off period Toff and the measurement period Tm of the ambient light L1. The display panel X1 (particularly the OLED forming the OLED) appears to be constantly lit to the naked eye, but is actually PWM-driven so as to repeat the on-period Ton and the off-period Tof in a predetermined light emission cycle T. .. By performing such PWM drive, it is possible to adjust the emission brightness of the display panel X1 by switching the on-duty Don (= the ratio of the on-period Ton to the emission cycle T, Don = Ton / T). It becomes.

Since the output light L2 of the OLED becomes zero (or substantially zero) during the OFF period of the OLED, only the ambient light L1 is incident on the light receiving element 21. Therefore, the speed of the optical sensor 20 is increased so that the detected value S1 in the OLED off period Tof can be obtained. Generally, the typical measurement period Tm of the illuminance sensor is about 100 ms, but for example, this measurement period Tm is set to 1 ms or less.

14A to 14D are diagrams showing the relationship between the emission brightness of the OLED and the off period Toff (emission brightness 25%, 50%, 75%, and 96%), respectively.

For example, when the emission brightness is 25% (FIG. 14A), Toff = Toff1 (for example, 2.8 ms). Further, for example, when the emission brightness is 50% (FIG. 14B), Toff = Toff2 (<Toff1, for example, 2.2 ms), and when the emission brightness is 75% (FIG. 14C), Toff = Toff3 (<Toff2, for example 1, 1). .5ms). Then, at the emission brightness of 96% (FIG. 14D), the off period Toff finally disappears.

As can be seen from each figure, the OLED off period Toff has a length of 1 ms or more unless the emission brightness is close to 100%. Therefore, if the measurement period Tm of the optical sensor 20 (particularly the detection circuit 22) is set to 1 ms or less, the detection value S1 in the OLED off period Toff can be obtained, so that the detection value S1 is not affected by the unnecessary output light L2. , It is possible to measure only the ambient light L1.

When the emission brightness is close to 100%, the OLED off period Toff disappears, so that an error occurs in the measurement result of the ambient light L1. However, the fact that the emission brightness is close to 100% is generally considered to be when the ambient light L1 is very strong. Therefore, the influence of the unnecessary output light L2 is relatively small, and it can be said that it can be sufficiently dealt with by correction by software, for example.

On the contrary, when the emission brightness is close to 0%, it is generally considered that the ambient light L1 is extremely weak (the surroundings are pitch black), so that the influence of the unnecessary output light L2 becomes relatively large. However, in such a case, since the OLED off period Toff is sufficiently longer than the measurement period Tm of the optical sensor 20, the environment is set in the OLED off period Toff where the output light L2 is zero (or almost zero). Only the light L1 can be measured correctly.

FIG. 15 is a diagram showing an example of the light detection operation in the second embodiment, in order from the top, the received light signal IPD, the integrated value S1 (the measurement period Tm), the minimum value S2, the average value S3, and the ambient light measurement data. External requirements for ALSDATA and the interface circuit 25 are depicted.

As described above, the detection circuit 22 sequentially generates the integrated value S1 of the received light signal IPD every measurement period Tm (for example, Tm ≦ 1 ms) shorter than the light emission cycle T of the OLED. However, even if the received light signal IPD is measured at high speed, it is not possible to determine which integrated value S1 is the data when the OLED is emitted and which integrated value S1 is the data when the OLED is not emitted by a single data read. It is impossible. Therefore, in order to acquire the ambient light measurement data ALSDATA when the OLED does not emit light, it is necessary to continue to acquire the integrated value S1 over a certain interval (at least the emission cycle T or more of the OLED).

However, if the integrated value S1 itself is to be processed by an external master (such as a microcomputer (not shown)), it is necessary to perform a data read operation at an extremely high frequency (every measurement period Tm). Therefore, it occupies the communication bus of the master, which may hinder communication with other ICs or increase the current consumption.

As a solution to the above problem, for example, a method of storing the integrated value S1 within a certain interval in the FIFO [first in, first out] memory and reading it collectively later can be considered. However, since the FIFO memory generally has a large logic scale, it leads to an increase in cost and size of the optical sensor.

On the other hand, in the case of the optical sensor 20 of the present embodiment, by introducing the minimum value search algorithm, it is possible to acquire the ambient light measurement data ALSDATA when the OLED does not emit light by performing the data read operation only once without requiring the FIFO memory. Is possible.

The above minimum value search algorithm will be specifically described in detail. First, the minimum value determination unit 231 sequentially determines the minimum value S2 (min1, min2, ..., MinN) from a plurality of integral values S1 for each first period Tx (for example, Tx = 5 ms). As a method for determining the minimum value S2, for example, the newly acquired integral value S1 is compared with the minimum value S2 held up to that point, and if S1 <S2, the minimum value S2 is updated, and if S1> S2. For example, the integrated value S1 may be discarded.

Next, the average value calculation unit 232 sequentially calculates the average value ave (min1: minN) from the plurality of minimum values S2 for each second period Ty (for example, Ty = 100 ms), and the calculation result is the measured value of the ambient light L1. It is stored in the register 24 as S3 (for example, 0h → XXXXh).

The measured value S3 stored in the register 24 is, for example, read out in response to an external request and output externally from the interface circuit 25 as ambient light measurement data ALSDATA.

In the above description, an example of sequentially determining the minimum value S2 from a plurality of integrated values S1 for each first period Tx has been given. For example, not only the minimum value S2 (= min1 to minN) but also the maximum value S4 (= Max1 to maxN) may also be sequentially determined, and the difference value (= S4-S2) between the maximum value S4 and the minimum value S2 may be acquired as the measured value of the output light L2.

Further, in the above description, an example is given in which the average value ave (min1: minN) is obtained from a plurality of minimum values S2 and used as the ambient light measurement data ALSDA. However, for example, the averaging process is omitted and the minimum value is omitted. S2 itself may be treated as ambient light measurement data ALSDATA. In that case, it is advisable to set the first period Tx longer in order to suppress the communication frequency.

<Summary>
Hereinafter, various embodiments disclosed herein will be described in a comprehensive manner.

The detection circuits disclosed in the present specification include a first integrator to which an input signal is input, a second integrator to output an output signal, an output end of the first integrator, and the second integrator. A configuration including an integrator connected between the input end of the integrator, a discharge unit connected to the input end of the second integrator, and a control unit that monitors the output signal and controls the discharge unit. (First configuration).

In the detection circuit having the first configuration, the first integrator may be configured to be slower than the second integrator (second configuration).

Further, the detection circuit having the first or second configuration further has a delay portion that delays the integration start timing of the second integrator with respect to the integration start timing of the first integrator (third configuration). ) May be used.

Further, in the detection circuit having any of the first to third configurations, the control unit sends the second integrator to the second integrator when the output signal exceeds the upper limit value in the integration period of the second integrator. The stored charge may be collectively discharged by the first discharge amount (fourth configuration).

Further, in the detection circuit having the fourth configuration, the control unit uses the electric charge stored in the second integrator until the output signal falls below the lower limit after the expiration of the integration period of the second integrator. A configuration (fifth configuration) may be adopted in which a second discharge amount smaller than the first discharge amount is gradually discharged.

Further, in the detection circuit having any of the first to fifth configurations, the control unit is configured to generate integrated value data of the input signal based on the number of discharges of the discharge unit (sixth configuration). You may.

Further, in the detection circuit having any of the first to sixth configurations, in the first integrator, the inverting input end is connected to the application end of the input signal and the output end is the first end of the integrator. A configuration including a connected first amplifier, a first integrator connected between the inverting input end and an output end of the first amplifier, and a first switch connected in parallel to the first integrator. (7th configuration) may be used.

Further, in the detection circuit having the seventh configuration, in the second integrator, the inverting input end is connected to the second end of the integrator and the output end is connected to the application end of the output signal. A configuration (eighth configuration) including an amplifier, a second integrated capacitance connected between the inverting input end and the output end of the second amplifier, and a second switch connected in parallel to the second integrated capacitance. ) May be used.

Further, in the detection circuit having the eighth configuration, the non-inverting input terminals of the first amplifier and the second amplifier are both connected to the application end of the bias voltage (nineth configuration). You may.

Further, the optical sensor disclosed in the present specification includes a light receiving element that generates a light receiving signal and a detection circuit that has the above-mentioned first to ninth configurations and detects the received light signal ( It is said to be the tenth configuration).

Further, for example, the optical sensor disclosed in the present specification includes a light receiving element that generates a light receiving signal corresponding to both output light and ambient light of the light emitting element, and a measurement period shorter than the light emitting cycle of the light emitting element. A detection circuit that sequentially generates an integrated value of the received light signal for each, and a detection circuit that sequentially determines a minimum value from a plurality of the integrated values for each first period equal to or longer than the light emission cycle, and determines the minimum value or a value corresponding thereto. It has a configuration (11th configuration) including a processing circuit for measuring ambient light.

In the optical sensor having the eleventh configuration, the processing circuit sequentially calculates an average value from a plurality of the minimum values for each second period longer than the first period, and uses the average value as the ambient light. The configuration (12th configuration) may be used as the measured value of.

Further, the optical sensor having the eleventh or twelfth configuration may have a configuration (thirteenth configuration) further having a register for storing the measured value.

Further, the optical sensor having any of the above 11th to 13th configurations may have a configuration (14th configuration) further including an interface circuit for externally outputting the measured value.

Further, in the optical sensor having the configuration of any one of the 11th to 14th, the processing circuit sequentially determines a maximum value from a plurality of the integrated values for each first period, and obtains the maximum value and the minimum value. The difference value of the above may be used as the measured value of the output light (15th configuration).

Further, in the optical sensor having any of the first to fifteenth configurations, the measurement period may be 1 ms or less (the sixteenth configuration).

Further, the electronic device disclosed in the present specification comprises a display panel capable of transmitting ambient light incident from the front surface side to the back surface side, and the display panel having any of the above 11th to 16th configurations. It is configured to have an optical sensor for measuring the ambient light on the back surface side of the above (17th configuration).

In the electronic device having the 17th configuration, the display panel may have a configuration including a translucent light emitting element (18th configuration).

Further, in the electronic device having the eighteenth configuration, the light emitting element may be configured to be PWM [pulse width modulation] driven by PWM [pulse width modulation] according to the emission brightness of the display panel (19th configuration). ..

Further, in the electronic device having the 19th configuration, the light emitting element may have an OLED configuration (20th configuration).

<Other variants>
In addition to the above-described embodiment, the various technical features disclosed in the present specification can be modified in various ways without departing from the spirit of the technical creation. That is, it should be considered that the above-described embodiment is exemplary in all respects and is not restrictive, and the technical scope of the present invention is not limited to the above-described embodiment, and claims for patent. It should be understood that the meaning equal to the scope and all changes belonging to the scope are included.

The invention disclosed in the present specification can be used, for example, in an optical sensor mounted on an electronic device such as a smartphone to measure ambient light.

10, 20 Optical sensor 11, 21 Light receiving element (photodiode)
12, 22 Detection circuit 121 Operational amplifier (second amplifier)
122 Capacitor (2nd integrated capacitance)
123 switch (second switch)
124 switch 125 switch 126 Discharge unit 127 Control unit 128 Operational amplifier (1st amplifier)
129 Capacitor (integral capacity)
12A capacitor (first integrated capacitance)
12B switch (1st switch)
12C Delay part 12X 1st integrator 12Y 2nd integrator 13 Parasitic capacitor 23 Processing circuit 231 Minimum value judgment part 232 Average value calculation part 24 Register 25 Interface circuit X Electronic device (smartphone)
X1 Display panel X2 Bezel area X11 Housing X12 Glass plate X13 OLED layer X14 Protective layer X14a Opening X15 Substrate

Claims (10)

A light receiving element that generates a light receiving signal corresponding to both the output light and the ambient light of the light emitting element,
A detection circuit that sequentially generates an integrated value of the received light signal for each measurement period shorter than the light emitting cycle of the light emitting element, and a detection circuit.
A processing circuit that sequentially determines the minimum value from a plurality of the integrated values for each first period equal to or longer than the light emission cycle, and sets the minimum value or a value corresponding thereto as the measured value of the ambient light.
Has an optical sensor. The optics according to claim 1, wherein the processing circuit sequentially calculates an average value from a plurality of the minimum values for each second period longer than the first period, and uses the average value as a measurement value of the ambient light. Sensor. The optical sensor according to claim 1 or 2, further comprising a register for storing the measured value. The optical sensor according to any one of claims 1 to 3, further comprising an interface circuit for externally outputting the measured value. Claims 1 to 4, wherein the processing circuit sequentially determines a maximum value from a plurality of the integrated values for each first period, and uses a difference value between the maximum value and the minimum value as a measured value of the output light. The optical sensor according to any one of the above. The optical sensor according to any one of claims 1 to 5, wherein the measurement period is 1 ms or less. A display panel that can transmit ambient light incident from the front side to the back side,
The optical sensor according to any one of claims 1 to 6, which measures the ambient light on the back surface side of the display panel.
Electronic equipment with. The electronic device according to claim 7, wherein the display panel includes a translucent light emitting element. The electronic device according to claim 8, wherein the light emitting element is driven by PWM [pulse width modulation] on duty according to the light emission brightness of the display panel. The electronic device according to claim 9, wherein the light emitting element is an OLED [organic light emitting diode].

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