Patent Publication Number: US-7907061-B2

Title: Proximity sensors and methods for sensing proximity

Description:
PRIORITY CLAIM 
     This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/988,047, filed Nov. 14, 2007, which is incorporated herein by reference. 
    
    
     SUMMARY 
     In accordance with an embodiment of the present invention, a proximity sensor includes a driver, a photo-diode (PD) and an analog-to-digital converter (ADC). The proximity sensor can also include a controller to control the driver. The driver selectively drives a light source, e.g., an infrared (IR) light emitting diode (LED). The PD, which produces a current signal indicative of the intensity of light detected by the PD, is capable of detecting both ambient light and light produced by the light source that is reflected off an object. The ADC receives one or more portion of the current signal produced by the PD. The ADC produces one or more digital output that can be used to estimate the proximity of an object to the PD in a manner that compensates for ambient light detected by the PD and transient changes to the detected ambient light. 
     In accordance with an embodiment, a method for use in monitoring the proximity of an object includes detecting an intensity of both ambient light and light produced by a light source that is reflected off the object, during one or more time period. During one or more further time period, the intensity of the ambient light is detected. Based on the intensities detected, an output can be produced that indicative of the intensity of the detected light produced by the light source that is reflected off the object, with the affect of the ambient light and transient changes thereto substantially removed. Such an output can be used to estimate the proximity of the object. 
     Further and alternative embodiments, and the features, aspects, and advantages of the embodiments of invention will become more apparent from the detailed description set forth below, the drawings and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows the spectrum of different types of light. 
         FIG. 1B  shows an exemplary spectral response of an infrared (IR) photo-diode. 
         FIG. 2  shows a monolithic low-cost and low-power IR proximity sensor, according to an embodiment of the present invention, along with a corresponding possible timing diagram. 
         FIG. 3  shows an exemplary spectral response of the photo-diode (PD) of the proximity sensor of  FIG. 2 . 
         FIG. 4  shows an implementation of the analog-to-digital converter (ADC) of the proximity sensor of  FIG. 2 , according to an embodiment of the present invention, along with a corresponding possible timing diagram. 
         FIG. 5  shows a monolithic proximity sensor, according to a further embodiment of the present invention, along with a corresponding possible timing diagram. 
         FIG. 6  shows a monolithic proximity sensor, according to still another embodiment of the present invention, along with a corresponding possible timing diagram. 
         FIG. 7  shows a dual-input single output ADC, which can be used as the ADC in the proximity sensor of  FIG. 6 , in accordance with an embodiment of the present invention. 
         FIG. 8  shows another dual-input single output ADC, which can be used as the ADC in the proximity sensor of  FIG. 6 , in accordance with an embodiment of the present invention. 
         FIG. 9  shows a schematic of the  1 -bit digital-to-analog converter (DAC), in accordance with an embodiment of the present invention, which can be used in the ADC of  FIG. 8 . 
         FIG. 10  is a high level flow diagram that is used to summarize various methods for determining the proximity of an object, in accordance with various embodiments of the present invention. 
         FIG. 11  is a high level block diagram of a system according to an embodiment of the present invention. 
     
    
    
     BACKGROUND 
     Infrared (IR) proximity sensors are becoming popular in cell-phone and handheld-device applications. For example, the sensor can be used to control a touch-screen interface for portable electronics devices. When an object, such as a person&#39;s finger, is approaching, the sensor detects the object. When the object is detected, a touch-screen interface or the like may perform an action such as enabling or disabling a display backlight, displaying a “virtual scroll wheel,” navigation pad or virtual keypad, etc. 
     A conventional analog-output IR proximity sensor typically includes discrete components, including an infrared (IR) light emitting diode (LED), a switch to turn the IR LED on and off, and an IR photo-diode (PD). During normal operation, the switch delivers current to the IR LED. The IR light emitted from the IR LED (or at least a portion of the IR light) will be reflected by an object when there is any, and be received by the PD. The PD converts the reflected light, as well as ambient light, to a current going to a resistor connected in parallel with the photo-diode. The analog output is the voltage across the resistor. The intensity of the reflected IR light received by the photo-diode is decreased at a rate of about 1/(4*X^2), where X is the distance between the object and the PD. However, as just mentioned, the total IR light received by the PD also includes ambient IR light, which may be from sun light, halogen light, incandescent light, fluorescent light, etc.  FIG. 1A  shows the spectrum of these different types of light. 
     In order to improve the signal-to-noise ratio of the sensor, the PD of the convention analog-output proximity sensor is typically made with a relatively large sensor area and with a special package, which has a narrow band-pass filter with the peak at the IR LED&#39;s emitting wavelength. A typical spectral response of such an IR PD is shown in  FIG. 1B . Additionally, to improve the signal-to-noise ratio, a relatively high current is typically used to drive the IR LED in order to emit a stronger IR light signal. The use of the large size sensor area, the special package and the high current make such conventional IR proximity sensors unsuitable, or at least not optimal, for cell-phone and other handheld-device applications. 
     DETAILED DESCRIPTION 
       FIG. 2  shows a monolithic low-cost and low-power proximity sensor  200 , according to an embodiment of the present invention, which includes a monolithic chip including a CMOS-integrated photo-diode  202 , an analog-to-digital converter (ADC)  204 , an IR LED driver  206  and a timing controller  208 . The IR LED driver  206 , which is controlled by the timing controller  208 , selectively drives an external IR LED  210 . In accordance with specific embodiments, the photo-diode (PD)  202  is a regular PN junction diode without any spectrum filter. A typical spectral response of such a PD is shown in  FIG. 3 .  FIG. 4  shows one possible implementation of the ADC  204 , and more specifically, a charge balanced ADC  404 . 
     A benefit of the sensor  200  of  FIG. 2  is that by providing direct conversion of a photo-current to a digital output, with the IR LED driver&#39;s output modulation, relatively small current signals can be processed with low-offset and high resolution. The principle of the operation of the sensor  200 , in accordance with a specific embodiment of the present invention, is as follows:
         During a 1 st  conversion time, the IR LED driver  206  is off (i.e., switch S 0  in  FIG. 2  is open), and thus the external IR LED  210  is off (i.e., not producing any IR light). The output of the ADC  204  (DATA 1 ) is indicative of (e.g., proportional to) the intensity of ambient light; and   During a 2 nd  conversion time, the IR LED driver  206  is on (i.e., switch S 0  is closed), and thus the external IR LED  210  is on (i.e., producing IR light).       

     The output of the ADC  204  (DATA 2 ) is indicative of (e.g., proportional to) the intensity of the ambient light and received IR light from the IR LED  210  reflected toward and detected by the PD  202 . Note that when an object is not in proximity to the sensor  200 , substantially no IR light produced by the IR LED  210  should be reflected back toward the PD, and thus, during this condition, the output of the ADC  204  (DATA 2 ) will again be indicative of (e.g., proportional to) the intensity of the ambient light. Accordingly, the IR LED  210  and the PD  202  are preferably arranged, relative to one another, such that no IR light can travel directly from the IR LED  210  to the PD  202 , but rather, the PD  202  should preferably only detect light from the IR LED  210  that has been reflected off an object  201  in proximity to the sensor  200 . As the term is used herein, ambient light refers to background light, i.e., light already existing in an indoor or outdoor setting that is not caused by light produced by the IR LED  210 . Such ambient light includes radiation over a wide range of wavelengths, including IR wavelengths. 
     The DATA 1  and DATA 2  values can be stored (e.g., in a memory  220 , which may RAM, EPROM, registers, etc), allowing for their subtraction (e.g., by a processor  230  or digital subtraction circuitry). The DATA 1  value is indicative of the intensity of ambient light (which can result from various light sources, such as those shown in  FIG. 1A , and can include both visible and IR light, as can be appreciated from the PD spectrum shown in  FIG. 3 ). The DATA 2  value is indicative of the intensity of both the ambient light and the IR light produced by the IR LED  210  that was reflected off an object and detected by the PD  202 . The subtraction can be a weighted subtraction, e.g., if the 1 st  and 2 nd  conversion times are not of equal duration. If the ambient light does not change during the 1 st  and 2 nd  conversion times, the subtraction of DATA 2 -DATA 1  results in a value that is substantially proportional only to the intensity of the received IR light from the IR LED (i.e., the affect of the ambient light gets subtracted out), which should increase as an object gets closer to the sensor  200 , and more specifically, closer to the PD  202 . Conversely, the value of DATA 2 -DATA 1  should decrease as an object gets farther away from the sensor  200 . Accordingly the value of DATA 2 -DATA 1  can be used to estimate proximity of the object. 
     As shown in the timing diagram of  FIG. 2 , the DATA 1  and DATA 2  outputs from the ADC  204  alternate. A plurality of values (produced by subtracting DATA 1  from DATA 2 ) can be determined, and then added (e.g., integrated or accumulated, using an integrator or accumulator), and the summed value can be used to estimate proximity of an object. Alternatively, a plurality of values (produced by subtracting DATA 1  from DATA 2 ) can be determined, and then averaged, and the averaged value can be used to estimate proximity of an object. In another embodiment, a plurality of DATA 1  values can added (e.g., integrated or accumulated, using an integrator or accumulator) to produce a summed DATA 1 , and a plurality of DATA 2  values can be added to produce an summed DATA 2 , and then the summed DATA 1  can be subtracted from the summed DATA 2 , to produce a value that can be used to estimate proximity of an object. In still another embodiment, a plurality of DATA 1  values can be averaged to produce an averaged DATA 1 , and a plurality of DATA 2  values can be averaged to produce an averaged DATA 2 , and then the averaged DATA 1  can be subtracted from the averaged DATA 2 , to produce a value that can be used to estimate proximity of an object. These are just a few examples, which are not meant to be limiting. 
     As long as the ambient light does not change in the course of the 1 st  and 2 nd  conversion times, the embodiment of  FIG. 2  can provide high sensitivity for proximity sensing, even when there exists a lot of ambient light, because the ADC  204  can provide enough number of bits of data. However, in cell-phone and handheld device applications, environmental changes can occur relatively quickly, resulting in transient changes to the ambient light. 
     It is noted that the conversion times and DATA values can be numbered differently. In other words, it can be that during a 1 st  conversion time the IR LED  210  is on, and during a 2 nd  conversion time the IR LED is off. Using this numbering, the DATA 1  value would be indicative of the intensity of both the ambient light and the IR light produced by the IR LED  210  that was reflected off and detected by the PD  202 , and the DATA 2  value would be indicative of the intensity of the ambient light. Here, the value of DATA 1 -DATA 2  could be used to estimate proximity of the object. 
       FIG. 5  shows a monolithic proximity sensor  500 , according to a further embodiment of the present invention. Here, two ADCs  204   1  and  204   2  are used and the IR LED driver  206  is turned on and off, e.g., with 50% duty-cycle. Each on-time includes M clock periods, where M is an integer (1, 2, 3 . . . ). The output current of the photo-diode  202  is switched between the inputs of the two ADCs  204   1  and  204   2  according to the timing diagram. With this dual-ADCs architecture and this switch-timing scheme, the effect of the ambient light change is substantially removed. In this embodiment, to compensate for gain mismatch between the two ADCs  204   1  and  204   2  that can cause errors in proximity sensing, a trimming circuit can be used to improve the gain matching. The charge balanced ADC  404 , shown in  FIG. 4 , can be used to implement the two ADCs  204   1  and  204   2 . 
       FIG. 6  shows a monolithic proximity sensor  600 , according to another embodiment of the present invention. Here, a dual-input single output ADC  604  is used. In specific embodiments, the dual-input single output ADC  604  can include dual-integrators, which would eliminate any need for a trimming circuit. The output of the ADC  604  is substantially proportional to only the intensity of received IR light emitted from the IR LED  210  (i.e., the effect of the ambient light is substantially removed). 
       FIG. 7  shows one embodiment of a dual-input single output ADC  702 , which can be used for ADC  604  in  FIG. 6 .  FIG. 8  shows another embodiment of a dual-input single output ADC  804 , which can be used for ADC  604  in  FIG. 6 . Alternative embodiments of a dual-input single output ADC besides those shown in  FIGS. 7 and 8  are also possible, and within the scope of the present invention. Ideally, the dual-input single output ADC in current mode will realize the following function:
 
Data=(( I in1− I in2)/ I ref)*2^ N    (1)
 
     Here, Data is the digital output of the ADC, N is the number of bits in the output of the ADC, Iref is the reference current, and Iin 1  and Iin 2  are the dual current inputs. 
     Referring to  FIG. 7 , the dual-input single output ADC  702 , according to an embodiment of the present invention, is implemented with two of conventional single input ADCs (ADC  704   1  and ADC  704   2 ) and a digital subtraction circuit  708 . The output can be expressed as:
 
Data=( I in1/ I ref1− I in2/ I ref2)*2^ N    (2)
 
     Here, Iin 1  and Iin 2  are the input currents, respectively, for the ADC  704   1  and the ADC  704   2 , and Iref 1  and Iref 2  are the ADCs&#39; reference currents. In order to realize the function given by Equation (1) from Equation (2), a trimming circuit should be used to achieve the gain-matching, i.e., to match Iref 1  to Iref 2 . The higher the resolution of the ADC (i.e., the greater the number of output bits), the more difficult it would be to implement such a trimming circuit. 
       FIG. 4 , introduced above, shows some exemplary details of each of the ADC  704   1  and the ADC  704   2 , where each ADC is implemented as a conventional single input ADC  404  which relies on the charge-balancing technique. As shown in  FIG. 4 , each ADC can include an integrator  412 , a comparator  414 , a D flip-flop (dff)  416 , and a counter  418 . For each data (i.e., analog to digital) conversion with N bits, 2^N clock periods are needed. During each conversion time, the number of 1s from the dff  416  are counted, and a charge of Tclock*Iref is delivered to the integrator  412  for each corresponding 1. Here, Tclock is the clock period and Iref is the reference current. According to charge conservation:
 
 I in* T clock*2^ N=I ref* T clock*Data   (3)
 
     Here, Iin is the input current and Data is the counter&#39;s output. The left side of the equation represents the total charge removed from the integrator by the input current, and the right side represents the total charge delivered to the integrator by the reference current. From (3), the digital output can be expressed as:
 
Data=( I in/ I ref)*2^ N    (4)
 
       FIG. 8  shows the architecture of the dual-input single output ADC  804 , implemented with dual integrators, in accordance with a further embodiment of the present invention. As mentioned above, the dual input ADC  804  can be used to implement the dual input ADC  604  in  FIG. 6 . The ADC  804  realizes the function given by (1), and at the same time alleviates any need for a trimming circuit. The ADC  804  is shown as including a pair of integrators 412 A  and  412   B , a pair of comparators  414   A  and  414   B , a pair of D flip-flops  416   A  and  416   B , and an up-down counter  818 . The ADC  404  is also shown as including a 1-bit DAC  820 , and a time delay  822 . The operation of the ADC  404  is described below. Suppose M=1 for the simplicity:
         During SWITCH=H, the output from the comparator  414   A , which compares the output of the integrator  412   A  to a bias voltage (vbias), is enabled. If a 1 from the output of the comparator  414   A  is latched by dff  416   A  on the clock&#39;s falling edge, a charge of 2*Iref*Tclock is delivered to the integrator  412   A ; at the same time, the 1 is counted up by the up/down counter  818 .   During SWITCH=L, the output from comparator  414   B , which compares the output of the integrator  412   B  to the same bias voltage (vbias), is enabled. If a 1 from the output of the comparator  414   B  is latched by dff  416   B  on the clock&#39;s falling edge, a charge of 2*Iref*Tclock is delivered to integrator  412   B ; at the same time, the 1 is counted down by the up/down counter  818 .       

     For each conversion, the numbers of count-up and count-down can be expressed as:
 
 I in A*T clock*2^( N+ 1)=(2* I ref)* T clock*UP   (5)
 
 I in B*T clock*2^( N+ 1)=(2* T ref)* T clock*DOWN   (6)
 
     According to (5) and (6), the output of the up/down counter  818  can be expressed as: 
     
       
         
           
               
             
               
                 
                   
                     
                       
                         
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     In accordance with an embodiment, an accumulator can be used in place of the up-down counter  818 . 
       FIG. 9  shows a schematic of the 1-bit DAC (digital-to-analog converter)  820 , in accordance with an embodiment of the present invention, which produces three output, and which is useful with the ADC architecture with dual integrators. CtrlA is used to steer the reference current between the outputs of vbias and IoutA; CtrlB is used to steer the reference current between the output of vbias and IoutB. When CtrlA=1 and CtrlB=0, then the current of Iref*2 flows to IoutA. When the CtrlA=0 and CtrlB=1,then the current of Iref*2 flows to IoutB. When the CtrlA=0 and CtrlB=0, the current of Iref*2 flows to Vbias. Referring back to  FIG. 8 , there is no case where CtrlA=1 and CtrlB=1, i.e., no case where both switches shown in block  820  are closed. Referring again to  FIG. 9 , the INVERTER-gate and two cross-coupled NOR-gates to CtrlA or CtrlB are used to generate two pairs of over-lapping clocks to prevent the cut-off of the reference current during the steering. The three OR-gates are used to match the time delays of over-lapping clocks for each pair. Alternative configurations of the 1-bit DAC  820  are possible, and within the scope of the present invention. 
     It is within the scope of the present invention to use alternative light sources, i.e., besides an LED. For example, a laser diode can be used to produce light in place of an LED. Alternatively, an incandescent light can be used in place of an LED. These are just a few examples, which are not meant to be limiting. In the above described embodiments, the light source (e.g., LED  210 ) was described as producing IR light. In alternative embodiments, a controlled light source can produce alternative wavelengths of light, such as, but not limited to, light in the visible spectrum (e.g., blue, green or red light). 
     The high level flow diagram of  FIG. 10  is used to describe various methods, according to various embodiments of the present invention, for use in monitoring the proximity of an object. Referring to  FIG. 10 , at step  1000 , a light source is controlled, e.g., using a controller and/or a driver. For example, as described above, the light source can be selectively turned on and off. At step  1002 , an intensity of both ambient light and light produced by the light source that is reflected off the object is detected during one or more time period. At step  1004 , the intensity of the ambient light is detected during one or more further time period, while the light source is not producing light. For example, detection time periods of step  1002  and detection time periods of time  1004  can be interspersed, as was described above. At step  1006 , based on the intensities detected at steps  1002  and  1004 , an output is produced that is indicative of the intensity of the detected light produced by the light source that is reflected off the object, and that compensates for the ambient light including transient changes thereto. Preferably, the affect of the ambient light is substantially removed, so that the ambient light will not affect estimates of the proximity of the object, which can be determined at step  1008 . For example, at step  1008 , the output produced at step  1006  can be compared to one or more threshold to estimate the proximity of the object. In accordance with certain embodiments, the output can be produced at step  1006 , by subtracting the intensity detected at step  1004  from the intensity detected at step  1002 . Such a subtraction may be a weighted subtraction. In accordance with specific embodiments, a dual-input single output analog to digital converter (ADC) is used to produce the output that can be used to estimate proximity of the object, as was described above, e.g., with reference to  FIGS. 6 and 8 . 
     Proximity sensors of embodiments of the present invention can be used in various systems, including, but not limited to, cell-phones and handheld-devices. Referring to the system  1100  of  FIG. 11 , for example, a proximity sensor (e.g.,  200 ,  500  or  600 ) can be used to control whether a subsystem  1106  (e.g., a touch-screen, backlight, virtual scroll wheel, virtual keypad, navigation pad, etc.) is enabled or disabled. For example, the proximity sensor can detect when an object, such as a person&#39;s finger, is approaching, and based on the detection either enable (or disable) a subsystem  1106 . More specifically, one or more output of the proximity sensor (e.g.,  200 ,  500  or  600 ) can be provided to a comparator or processor  1104  which can, e.g., compare the output(s) of the proximity sensor to a threshold, to determine whether the object is within a range where the subsystem  1106  should be enabled (or disabled, depending on what is desired). Multiple thresholds can be used, and more than one possible response can occur based on the detected proximity of an object. For example, a first response can occur if an object is within a first proximity range, and a second response can occur if the object is within a second proximity range. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. 
     The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.