Abstract:
A light monitor includes a single semiconductor substrate. A light to frequency (LTF) converter is on the single semiconductor substrate, a threshold comparator is on the single semiconductor substrate and coupled to an output of the light to frequency converter, and a light intensity calculator is on the single semiconductor substrate and coupled to an output of the threshold comparator.

Description:
FIELD OF THE INVENTION 
   The present invention relates to light monitors, and in particular, to a light monitor on a single chip. 
   BACKGROUND OF THE INVENTION 
   Recent years have seen rapid growth in the demand for inexpensive, lightweight and robust measuring and control devices. This demand has been manifested in the rapid developments in the field of miniaturized devices, such as lab-on-a-chip. 
   Display backlighting significantly contributes to the battery consumption of mobile devices, such as notebook computers, PDAs and mobile phones. Consequently, it is possible to considerably increase the useful lifetime of a battery by controlling the backlights so that they are dimmed in dark conditions and are only increased when there are high ambient light levels. 
   Beyond the above specific example, the provision of inexpensive, miniaturized light monitoring sensors and control systems (e.g., for car headlights, large building lighting networks, street lighting networks, etc.) will clearly provide significant economic and environmental benefits. Traditional light sensors typically produce an analog output signal. One of the main challenges encountered in previous attempts to provide integrated miniaturized light sensing and control systems has been the problem of combining analog signal processing circuitry with digital signal processing circuitry on the same chip. 
   Accordingly, prior art on-chip light sensors have typically possessed limited data processing capabilities. This has created particular problems since it means that such devices have limited, if any, ability to compensate for manufacturing variations between components. 
   For instance, the Microsemi LX1970 and 71 (http://www.microsemi.com/micnotes/1403. pdf) device is an 8-pin dumb-sensor that requires continual monitoring. Similarly, the TDK BCS series requires continual monitoring. The Texas Instruments TAOS (TSL230R/A/B, TSL235R, TSL245) devices possess a narrow dynamic range with no fine control on the limit and no matching or compensation for component errors. 
   Since the present invention relates to imaging sensors, and more particularly, to a light to frequency converter, it is useful at this point to briefly review the properties of CMOS image sensors and the operation of the light to frequency converter circuit. 
   A brief overview of CMOS image sensors will now be discussed. Recent advances in the design and fabrication of complementary metal oxide semiconductor (CMOS) chips have meant that CMOS imaging sensors are adopting a more dominant position in the low-cost imaging market. 
   One of the main advantages of CMOS imaging sensors is that they can be produced using standard fabrication procedures which are already widely used for producing CMOS chips for computer processors, memory chips, etc. Furthermore, the signal processing and control circuitry for a CMOS imaging sensor can be integrated directly onto the CMOS chip. 
   A light to frequency converter circuit will now be discussed. As an overview, a light to frequency (LTF) converter, as described in U.S. Pat. No. 5,850,195 discloses a CMOS imaging sensor with a large dynamic range. The LTF converter architecture possesses several advantages over traditional imaging sensors. These advantages primarily reside in the following features: integration capacitance tolerance, integration capacitance size, and frequency output. These features will be discussed in more detail below. 
   With respect to integration capacitance tolerance in a conventional light sensor, a photodiode&#39;s capacitance is defined by its well capacitance. However, this feature can be hard to control. Consequently, it is difficult to produce an array of photodiodes with matched sensitivity. 
   In contrast, an LTF converter employs a charge amplifier structure, which ensures that the effective capacitance of the LTF converter is determined by an integration capacitance provided by a feedback capacitor. Since capacitors can be manufactured with tighter controls over their capacitance (e.g., poly-poly or metal-metal capacitors), the variability in the capacitance of the individual LTF converters in an LTF converter array is less than that of a similar number of traditional light sensors. 
   With respect to integration capacitance size, increasing the size of a photodiode should in principle increase its ability to collect incident photons. As a result, this increases its sensitivity to incident light. Larger photodiodes also possess an increased parasitic capacitance. This has the effect of negating the ability of the photodiode to collect more photons, and thereby eliminates any sensitivity benefits of the increased photodiode size. 
   In contrast, the LTF converter employs a charge amplifier structure that isolates the capacitance of the LTF converter&#39;s photodiode from the rest of the LTF converter circuitry. This ensures that the effective capacitance of the LTF converter is determined by the capacitance of its feedback capacitor (as described above). Consequently, it is possible to use a large photodiode in an LTF converter while retaining a small overall circuit capacitance, and thereby producing a high sensitivity detector. 
   With respect to frequency output, on-chip signal processing with traditional analog light sensors is relatively sensitive to noise from the other on-chip circuitry. In contrast, the charge amplifier structure of an LTF converter is readily combined with a comparator to produce a digital signal whose frequency is proportional to the light on the LTF converter&#39;s photodiode. 
   The digital signal produced by an LTF converter is both robust and measurable over a large dynamic range (i.e., 140 dB of dynamic range is practical with the charge amplifier architecture). In addition, the LTF converter system is auto-exposing, insofar as no external control loop is required to ensure that an LTF converter&#39;s photodiode pixel does not saturate. 
   The operation of an LTF converter will now be described below with reference to  FIGS. 1-5 . The LTF converter comprises a control circuit  4 , a photodiode  6  and a current to digital signal converter  8 . The current to digital signal converter  8  uses a switched-capacitor charge metering technique to convert a photo-current from the photodiode  6  to a digital signal of a specific frequency. The current to digital signal converter  8  comprises a bias circuit  10  (which controls the maximum operating speed of the digital signal converter  8 ), an amplifier circuit  14 , a switched feedback capacitor  16  in a charge sensing amplifier circuit (not shown), a comparator  18  and a monostable multivibrator circuit  19 . 
   Referring to  FIG. 2 , the charge sensing amplifier circuit  20  effectively isolates the remaining circuitry of the current to digital signal converter (not shown) from the large capacitance of the photodiode  6  (&lt;100 pF). The charge sensing amplifier  20  comprises an operational amplifier  22  configured in a closed loop configuration with its non-inverting input coupled to a reference voltage (V rt ) and its inverting input connected to the feedback capacitor  16 . The reference voltage (V rt ) is set as low as possible to increase voltage swing while maintaining the depletion region of the PN junction of the LTF converter. The reference voltage (V rt ) is usually set to approximately 0.7V. 
   Since the operational amplifier  22  has a large input impedance, virtually no current flows through it. Consequently, the output of the operational amplifier  22  changes to ensure that the inverting and non-inverting inputs of the operational amplifier  22  remain at the same potential (i.e., V rt ) In the process, a current flows through the feedback capacitor  16  which has the same magnitude (but opposite sign) to the photo-current generated by the photodiode  6  (I pd ) 
   Equation (1) below shows the relationship between the output voltage (V out1 ) from the charge sensing amplifier  20  and the photo-current generated by the photodiode  6 . 
   
     
       
         
           
             
               
                 
                   V 
                   out1 
                 
                 = 
                 
                   
                     - 
                     
                       I 
                       pd 
                     
                   
                   ⁢ 
                   
                     
                       T 
                       int 
                     
                     
                       C 
                       fb 
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   From the above expression it can be seen that the output voltage (V out1 ) from the charge sensing amplifier  20  is independent of the photodiode&#39;s  6  capacitance. Referring to  FIG. 3 , the output voltage (V out1 ) from the charge sensing amplifier  20  is accumulated until it reaches a maximum value (V outmax ) at which point it is reset. 
     FIG. 4  shows a system for resetting an integrating amplifier (not shown) in the amplifier circuit  14 . In this system, the output voltage from the amplifier circuit  14  (V out2 ) is transmitted to the comparator  18 . In the comparator  18 , the output voltage (V out2 ) is compared against a reference voltage (V ref ). If the output voltage (V out2 ) exceeds the reference voltage (V ref ), the comparator  18  transmits a control signal (CTRL) to the monostable multivibrator circuit  19 . In response to the control signal (CTRL), the monostable multivibrator circuit  19  emits a pulsed signal (RESET) to discharge the feedback capacitor  16 . 
   Consequently, the frequency of the control signal (CTRL) is also proportional to the photodiode current I pd  (assuming that the integrating amplifier in the amplifier circuit  14  settles completely during the period of the control signal (CTRL)). The control signal (CTRL) is also fed to a divide-by-two circuit  30  to form the overall output signal (F out ) from the LTF converter. By employing a divide by two circuit  30 , a symmetrical output signal is produced, which is more reliably detected since it no longer includes short pulses. 
   Returning to equation (1), since the rate of change (slope) of the output voltage (V out1 ) from the charge sensing amplifier is proportional to the intensity of the incident light, the frequency of the overall output signal (F out ) from the LTF converter is also proportional to the incident light intensity. This proportionality is more clearly expressed in equation (2) below. 
   
     
       
         
           
             
               
                 
                   F 
                   out 
                 
                 = 
                 
                   
                     I 
                     pd 
                   
                   
                     2 
                     ⁢ 
                     
                       
                         C 
                         fb 
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             V 
                             ref 
                           
                           - 
                           
                             V 
                             rt 
                           
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   It can be seen from equation (2) that although the overall output signal (F out ) from the LTF converter is proportional to the photocurrent (I pd ) from the photodiode, it is also dependent on the reference voltages (V ref , V rt ) and the capacitance of the feedback capacitor C fb . While it is possible to use bandgap reference voltages to accurately produce the above reference voltages, the capacitance of the feedback capacitor is less easily controlled since it is typically subjected to manufacturing variations. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, a light monitor comprises an LTF converter, thresholding means and light intensity calculating means. The LTF converter is in communication with the light intensity calculating means, and the light intensity calculating means is in communication with the thresholding means. The light monitor is provided on a single chip. 
   The light intensity calculating means may comprise a counter and clocking signal generating means to provide a clocking signal for the counter. The clocking signal generating means may comprise a constant current source in communication with a charge sensing amplifier circuit comprising a first feedback capacitor. 
   The light monitor may comprise means for scaling a signal generated by the LTF converter in accordance with the clocking signal, and means for accumulating a resulting scaled signal in the counter to calculate a scaled measurement variable. 
   The light intensity calculating means may comprise a counter, reference signal generating means and a crystal oscillator whose output provides a clocking signal for the counter. The counter may comprise means for accumulating a reference signal generated by the reference signal generating means to calculate a reference variable. 
   The counter may further comprise gain adjustment calculating means to calculate a gain adjustment that includes the deviation between the reference variable and an expected value of same. 
   The counter may further comprise means for accumulating a signal from the LTF converter to calculate a measurement variable. The counter may also further comprise means of adjusting the measurement variable in accordance with the gain adjustment to calculate a scaled measurement variable. 
   The thresholding means may comprise at least one register adapted to contain a value of a first limit variable. The thresholding means may further comprise means of comparing a value of the scaled measurement variable from the light intensity calculating means with a value of the first limit variable. 
   Preferably, the thresholding means may comprise transmission means for transmitting to an external system an indicator of whether the value of the scaled measurement variable exceeds the value of the first limit variable. The transmitting means may comprise a single output pin. 
   Optionally, the thresholding means may comprise at least two registers adapted to contain a value of a first limit variable and a second limit variable. The thresholding means may further comprise means of comparing a value of the scaled measurement variable with a value of the first limit variable and the second limit variable. 
   The thresholding means may comprise transmission means for transmitting to an external system an indicator of whether the value of the scaled measurement variable exceeds the value of the first limit variable; is less than the value of the second limit variable; or is between the values of the first limit variable and the second limit variable. 
   The transmitting means may be in communication with the external system through a bi-directional interface. The bi-directional interface may be an I2C interface, an SPI interface or a CAN interface. The bi-directional interface may be a wireless interface such as a Zigbee interface. 
   The thresholding means may transmit the values of the first and second limit variables to the registers through the bi-directional interface. 
   According to a second aspect of the invention, a lighting control system comprises a light source, a light monitor according to the first aspect, and a control device. The light monitor may comprise means of communicating a first signal representing the intensity of ambient light to the control means. The control means may comprise means of transmitting a control signal to the light source in response to the received count signal. The light source may comprise means of altering its output in accordance with the received control signal. 
   According to a third aspect of the invention, a portable computing device back-light control system comprises the lighting control system according to the second aspect. 
   According to a fourth aspect of the invention, a mobile telecommunications device back-light control system comprises the lighting control system according to the second aspect. 
   According to a fifth aspect of the invention, a street lighting control system comprises the lighting control system according to the second aspect. 
   According to the sixth aspect of the invention, an automotive lighting control system comprises the lighting control system according to the second aspect. The automotive lighting system may comprise headlamps, and a dashboard illumination system. 
   The present invention thus combines an LTF converter with signal processing circuitry to provide a light monitor on a chip. The data from such light monitors could then be used in a control strategy as outlined above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described with reference to the accompanying drawings in which: 
       FIG. 1  is a block diagram of an LTF converter according to the prior art; 
       FIG. 2  is a simplified circuit diagram of a charge sensing amplifier used in the LTF converter shown in  FIG. 1 ; 
       FIG. 3  is a graph of the output voltage from the charge sensing amplifier shown in  FIG. 2 , during a period of time in which the LTF converter shown in  FIG. 1  is exposed to light; 
       FIG. 4  is a detailed block diagram of the current to digital signal converter in the LTF converter shown in  FIG. 1 ; 
       FIG. 5  is a timing diagram for the following signals in the LTF converter shown in  FIG. 1 : a reference voltage (V ref ), an output voltage (V out2 ), a pulsed reset signal, and an overall output signal (F out ); 
       FIG. 6  is a block diagram of the light monitor of the present invention in use in a light control system; 
       FIG. 7  is a block diagram showing an overview of a first embodiment of the LTF converter light intensity calculator component of the light monitor shown in  FIG. 6 ; 
       FIG. 8  is a block diagram of the clock component of the LTF converter light intensity calculator shown in  FIG. 7 ; 
       FIG. 9  is a block diagram of the circuit components of the constant current source of the clock component shown in  FIG. 8 ; 
       FIG. 10  is a block diagram showing an overview of a second embodiment of the LTF converter light intensity calculator component of the light monitor shown in  FIG. 6 ; 
       FIG. 11  is a block diagram of a first embodiment of a light intensity comparator component of the light monitor shown in  FIG. 6 , wherein the light intensity comparator is provided with a wired interface; 
       FIG. 12  is a block diagram of a second embodiment of a light intensity comparator component of the light monitor shown in  FIG. 6 , wherein the light intensity comparator is provided with a single pin output and a memory threshold load; and 
       FIG. 13  is a block diagram of a third embodiment of a light intensity comparator component of the light monitor shown in  FIG. 6 , wherein the light intensity comparator is provided with a wireless interface. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   For the sake of clarity and consistency, the thresholding means and the light intensity calculating means will be respectively referred to in the following description as a thresholding comparator and an LTF converter light intensity calculator. Similarly, in the following description of the first embodiment of the invention, the clocking signal generating means will be known as a signal generator. 
   Most control strategies are based upon the comparison of the value of a measured variable with a target value for that variable. Referring to  FIG. 6 , the light monitor  500  comprises an LTF converter  50 , a LTF converter light intensity calculator  60  and a threshold comparator  70 . The output signal from the light monitor may then be transmitted to a distributed control system or other suitable controller  80 . In response to the data from the light monitor  500 , the controller  80  adjusts the power to a light source  90 . 
   A light intensity calculator  50  will initially be discussed. As a broad overview and in reference to  FIG. 7 , in a first embodiment of the LTF converter light intensity calculator  150 , the digital output signal from an LTF converter  100  is fed into a counter  26  together with a reference signal of a fixed pre-defined frequency (F sysclk ). The reference signal is provided by a signal generator  28  and is used to provide a clocking mechanism for the counter  26 . The counter increments a COUNT variable in accordance with the reference signal (F sysclk ) within the period of the pulse received from the LTF converter  100 . Consequently, the value of the COUNT variable accumulated within the period of the pulse from the LTF converter  100  provides a quantized measurement of the intensity of the light detected by the LTF converter&#39;s photodiode. 
   The relationship between the COUNT variable and the frequency of the output signal from the LTF converter  100  is shown in equation (3) below. 
   
     
       
         
           
             
               
                 COUNT 
                 = 
                 
                   
                     
                       F 
                       sysclk 
                     
                     
                       F 
                       out 
                     
                   
                   = 
                   
                     
                       F 
                       sysclk 
                     
                     
                       ( 
                       
                         
                           I 
                           pd 
                         
                         
                           
                             [ 
                             
                               2 
                               × 
                               
                                 ( 
                                 
                                   
                                     V 
                                     ref 
                                   
                                   - 
                                   
                                     V 
                                     rt 
                                   
                                 
                                 ) 
                               
                             
                             ] 
                           
                           × 
                           
                             C 
                             fb 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   In this equation, F out  and F sysclk  respectively represent the frequency of the output signal from the LTF converter  100  and the signal generator  28 . 
   A system clock circuit will now be discussed with reference to  FIG. 8 . The reference signal (F syscik ) from the signal generator  28  is produced using the same charge sensing amplifier circuit  120  (comprising a comparator  122 , a feedback capacitor  124  and a switch  125 ), amplifier  114 , comparator  118 , monostable multivibrator circuit  119  and capacitor resetting system  130  as that previously described for the LTF converter. 
   However, in the case of the signal generator  28 , the input current to the charge sensing amplifier circuit  120  is provided by a constant current source  30  (instead of the photodiode used in an LTF converter). The constant current source  30  is produced using a bandgap reference voltage  32  and voltage controlled current source  34  as shown in  FIG. 9 . 
   A count measurement and capacitance variability compensation will now be discussed. Returning to  FIGS. 7 and 8 , the current from the constant current source  30  thus fixes the frequency of the output signal (F sysclk ) from the signal generator  28  as shown in equation (4) below. 
   
     
       
         
           
             
               
                 
                   F 
                   sysclk 
                 
                 = 
                 
                   
                     I 
                     ref 
                   
                   
                     [ 
                     
                       2 
                       × 
                       
                         ( 
                         
                           
                             V 
                             ref 
                           
                           - 
                           
                             V 
                             rt 
                           
                         
                         ) 
                       
                       × 
                       
                         C 
                         fb2 
                       
                     
                     ] 
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Combining equations (3) and (4) results in the following equation for the COUNT variable from the counter  26 . 
   
     
       
         
           
             
               
                 COUNT 
                 = 
                 
                   
                     
                       C 
                       fb 
                     
                     
                       C 
                       fb2 
                     
                   
                   × 
                   
                     
                       I 
                       pd 
                     
                     
                       I 
                       ref 
                     
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   From the above equation it can be seen that the COUNT variable is effectively a function of the ratio of the capacitances of the feedback capacitors in the LTF converter  100  and the signal generator  28 . 
   The feedback capacitors in the LTF converter  100  and the signal generator  28  are typically constructed in the metal layers of the chip embodying the two circuits. Accordingly, the capacitance of either of the two feedback capacitors (C fb  or C fb2 ) can be generically described by the following equation: 
                 C   =         ∈   ox     ⁢           ⁢   A       t   ox               (   6   )               
where A represents the area of the capacitor and ε ox  and t ox  respectively represent the dielectric constant and thickness of the silicon dioxide in the chip.
 
   Processes such as chemical metal polishing (CMP) can cause variations to occur in the oxide thickness of capacitors, and thereby cause variations in their capacitances. To ensure that the conversion of charge to voltage in both the LTF converter and the clock are equivalent, the feedback capacitors of both systems (C fb  and C fb2 ) are matched. 
   Consequently, referring to equation (5) any part-to-part variations that occur between the two feedback capacitors will be cancelled out in the calculation of the ratio of the two capacitances in the Count measurement. 
   A second embodiment of the LTF converter light intensity calculator will now be discussed with reference to  FIG. 10 . A low power second embodiment of the LTF converter light intensity calculator comprises an LTF converter  250 , a reference signal generator  228  of the same structure as the reference signal generator employed in the first embodiment of the LTF converter light intensity calculator. However, in contrast with the first embodiment of the LTF converter light intensity calculator, in the second embodiment the reference signal is not used to clock the counter  126 . Instead, the counter  126  is clocked by an external crystal oscillator  29 . 
   In the second embodiment of the LTF converter light intensity calculator, the reference signal is periodically transmitted to the counter for calibration purposes. More particularly, since the frequency of the reference signal F sysclk  is known, it is possible to predict the value of the COUNT variable that would be accumulated over a fixed time interval, when the reference signal is input to the counter. Any deviation from the expected value of the COUNT variable (COUNT ref ) can be ascribed to processing or other drift/variations in the LTF converter light intensity calculator. This deviation can be treated as a calibrating scaling factor and used to correct the COUNT variable measured from the LTF converter. 
   Since, the reference signal is not continually required to clock the counter in the second embodiment of the LTF converter light intensity calculator, the reference signal generator does not represent as significant a drain on the power of the light monitor. 
   A light intensity threshold comparator  70  will now be discussed with reference to  FIG. 11 . The first embodiment of the light intensity threshold comparator  170  comprises a logic unit  36  in which the COUNT variable from the light intensity calculator  60  is compared against pre-defined upper and/or lower limits (α MAX  and α MIN ). The light intensity threshold comparator  170  is provided with three logical output lines L 0 , L 1  and L 2  corresponding to three following logical states:
 
L 0 : COUNT&lt;α MIN 
 
L 1 :α MIN &lt;COUNT&lt;α MAX 
 
L 2 : COUNT&gt;α MAX 
 
   Accordingly, the value of the digital signal transmitted on each of these lines provides an indication of the logical status of the COUNT variable compared with the pre-defined upper and lower limits on same. These logical signals can then be transmitted to a simple controller (e.g., bang-bang controller). 
   The upper and lower limits α MAX  and α MIN  can be set through an interface  38  such as I2C, SPI or CAN. The absolute value of the Count measurement can also be transmitted through the interface  38  to a controller for the implementation of more sophisticated control algorithms. 
     FIG. 12  shows a second embodiment of the light intensity threshold comparator  270  in which the three logical output lines from the light intensity threshold comparator  270  are transmitted from a single pin output. This is achieved using a multiplexor  40  that is controlled via the interface  138 . In addition, the values of the upper and lower thresholds α MAX  and α MIN  can be automatically loaded from a memory (not shown) into the intensity threshold comparator  270  registers (not shown) via the interface  138 . 
     FIG. 13  shows a third embodiment of the light intensity threshold comparator  370 , in which the signals from the logical output lines L 0 , L 1  and L 2  (and the analog COUNT variable) are transmitted to a remote controller (not shown) through a wireless interface  238  rather than a specific output pin (as in the first and second embodiments of the light intensity threshold comparator). This could be achieved by a low power, low latency and inexpensive transmitter, such as a Zigbee transmitter. The light monitor can be readily included in an integrated circuit and is applicable to a broad range of devices including lighting control systems. An example lighting control system  125  is shown in  FIG. 6 . More particularly, the lighting control system  125  is applicable to portable computing device backlighting control systems, mobile telecommunications device back-lighting control systems, street lighting control systems and automotive lighting control systems (i.e., headlight controllers and dashboard illumination controllers). It will be appreciated that those skilled in the art may employ standard techniques to implement the invention in these and other ways. Alterations and modifications may be made to the above without departing from the scope of the invention.