Abstract:
A method and apparatus for high-side control of an optical transducer provides improved current control and temperature compensation and uses stochastic modulation for improved spectral characteristics.

Description:
This application claims priority to U.S. Provisional Patent Application No. 60/858,821, filed Nov. 13, 2006, the entire contents of which are hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the control of optical transducers and, more particularly, to current control and sensing in optical transducers. 
     BACKGROUND 
     Light-emitting diode (LED) technology has advanced to the point where LEDs can be used as energy efficient replacements for conventional incandescent and fluorescent light sources. One application where LEDs have been employed is in ambient lighting systems using white and color (e.g., red, green and blue) LEDs. Like incandescent and fluorescent light sources, the average luminous flux of an LED&#39;s output is controlled by the average current through the device. Unlike incandescent and fluorescent light sources, however, LEDs can be switched on and off almost instantaneously. As a result, their luminous flux can be controlled by switching circuits that switch the device current between two current states to achieve a desired average current corresponding to a desired luminous flux. This approach can also be used to control the relative intensities of red, green and blue (RGB) LED sources (or any other set of colored LED sources) in ambient lighting systems that mix colored LEDs in different ratios to achieve a desired color. 
       FIG. 1A  illustrates a conventional LED light source  100 , which includes a pulsewidth modulator (PWM)  101 , a switched current source  102  referenced to ground, and an LED  103  floating between a supply voltage Vp and the high impedance side of the switched current source. The PWM  101  uses an n-bit linear counter  104  to count repetitively from 0 to 2 n −1 over a period T=2 n /f clock . A pulsewidth register  105  holds a value between 0 and 2 n −1, representative of a desired duty cycle of the switched current source  102 . A comparator  106  compares the value of the linear counter  104  to the value in the pulsewidth register. When the output of the counter is below the value in the pulsewidth register, the output of the comparator is low. When the output of the counter is at or above the value in the pulsewidth register, the output of the comparator is high. As a result, the duty cycle of the current source, and the average intensity of the LED, can be controlled by changing the value in the pulsewidth register. 
       FIG. 1B  illustrates an array of LED light sources, which may include different color LEDs (e.g., red, green and blue) in different intensity proportions to generate different colors in combination. 
     In LED lighting, the luminous flux output (intensity) of each LED at a given operating current decreases as the junction temperature of the LED increases. LED junction temperature can increase due to power dissipation in the LED and increases in ambient temperature. This effect, illustrated in the curves of  FIG. 1C  for three selected LEDs, can create both luminous flux errors and errors in color mixing because the magnitude of the effect is different for LEDs of different colors. 
     Another temperature effect in LEDs is a shift of the dominant wavelength of an LED as the junction temperature of the LED changes. Typically, the dominant wavelength increases as junction temperature increases, causing a red shift. This effect can cause additional color distortion independent of the luminous flux effects. 
     At any given operating current, the forward bias voltage of an LED is a function of the junction temperature of the LED. If the forward voltages of the LEDs in an illumination array are known, then the junction temperatures can be determined and the overall spectral output of the array (i.e., color and intensity) can be controlled and corrected for changes in the junction temperatures of the LEDs. However, measuring the forward voltage of the LEDs in the conventional configuration is difficult because the LEDs are floating above ground and have a high common-mode voltage. In the conventional configuration, the LED forward voltages are measured as floating differential voltages and have to be measured through level-shifting voltage dividers and differential amplifiers that add complexity and measurement error. Additionally, the voltage dividers can leak current from the LEDs to ground, reducing LED intensity at a given drive level or increasing current consumption at a given intensity level. 
     In conventional LED arrays, the PWM output frequency is fixed, and therefore the spectral content of the control signal is concentrated in the PWM fundamental frequency and its harmonics. This may cause electromagnetic radiation that is concentrated in a narrow frequency range that may interfere with the operation of other circuitry in the illumination system or the local electronic environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which: 
         FIG. 1A  illustrates conventional PWM brightness control of an optical transducer; 
         FIG. 1B  illustrates conventional PWM brightness control of an array of optical transducers; 
         FIG. 1C  illustrates the change in the luminous flux of LEDs as a function of junction temperature; 
         FIG. 2A  illustrates high-side SSDM control of an optical transducer in one embodiment; 
         FIG. 2B  illustrates high-side SSDM control of an optical transducer network in one embodiment; 
         FIG. 2C  illustrates high-side peak and average SSDM current control of an optical transducer in one embodiment; 
         FIGS. 3A-3E  illustrate waveforms for high-side peak and average SSDM current control of an optical transducer in one embodiment. 
         FIG. 4  illustrates a system for high-side stochastic control of an array of optical transducers in one embodiment; and 
         FIG. 5  is a flowchart illustrating a method for high-side stochastic control of an optical transducer in one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Methods and apparatus for controlling optical transducers are described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description. 
     Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. 
     In one embodiment, a method includes controlling the intensity of an optical transducer with a high-side modulator, wherein the optical transducer is referenced to a ground potential, and independently controlling the peak intensity and average intensity of the optical transducer with stochastic signal density modulators. 
     In one embodiment of a high-side SSDM control circuit for optical transducers, as illustrated in  FIG. 2A , a stochastic LED light source  200  includes a stochastic signal density modulator (SSDM)  201  and a controllable current source  202  located on the high-potential side of an LED  203  so that the LED is referenced to ground. As described in greater detail below, this configuration allows the forward voltage of the LED  203  to be measured directly with an instrumentation amplifier without level shifting circuitry that would be required in the conventional floating LED configuration.  FIG. 2A  illustrates a configuration where the cathode of LED  203  is connected to ground, the anode of LED  203  is connected to one terminal of controllable current source  202  and the other terminal of controllable current source  202  is connected to a supply voltage V P . In the embodiment illustrated in  FIG. 2A , V P  would be a positive voltage and controllable current source  202  would source positive bias current to LED  203 . In other embodiments, the orientation of LED  203  may be reversed, in which case V P  would be a negative voltage and controllable current source  202  would source negative bias current to LED  203 . 
     In one embodiment, stochastic signal density modulator  201  includes an n-bit stochastic counter  204 , clocked by a signal f clock , which generates a pseudorandom number sequence of numbers between 0 and 2 n −1 every 2 n  clock cycles, a signal density register  205  that stores a signal density value between 0 and 2 n −1 and a comparator  206  to compare the output of the stochastic counter  204  with the signal density value in the signal density register  206 . When the signal density value in signal density register  205  is greater than the output value of the stochastic counter  204 , the output of comparator  206  is high. When the signal density value is less than or equal to the output value of the stochastic counter, the output of comparator  206  is low. As a result, the output signal (SSDM OUT ) from comparator  206  will have a pseudorandom distribution over the period of the stochastic counter  204 , with an average value determined by the value in the signal density register  205 , and with a spread spectrum (i.e., non-fixed) frequency response due to a non-constant output frequency. The configuration and operation of stochastic signal density modulators is described in detail in copending U.S. patent application Ser. No. 11/598,981 which is incorporated herein in its entirety by reference. 
     In one embodiment, as illustrated in  FIG. 2C , a high-side LED control circuit  300  includes a pair of SSDM control blocks to independently control the peak and average current of an LED. Circuit  300  includes an average current SSDM control block  201 A driven by a low frequency clock signal f clockL  and a peak current SSDM control block  201 B driven by a high frequency clock signal f clockH . Clock signals f clockL  and f clockH  are defined with respect to a cutoff frequency f C  of a lowpass filter  208  as described below. 
     The output of average current SSDM control block  201 A is a low frequency SSDM signal  211 A as illustrated in  FIG. 3A , with a signal density corresponding to the signal density value stored in its signal density register. The output of peak current SSDM control block  201 B is a high frequency SSDM signal as illustrated in  FIG. 3B , with a signal density corresponding to the signal density value stored in its signal density register. The high frequency and low frequency SSDM signals are combined at AND gate  207  to produce a combined SSDM signal  217  as illustrated in  FIG. 3C . The combined SSDM signal  217  is applied to lowpass filter  208 , which has a cutoff frequency f c . Cutoff frequency f c  is selected such that f c  is greater than f clockL  and less than f clockH . In one embodiment, for example, f clockL  may be approximately 5 kilohertz, f clockH  may be approximately 1 megahertz and f c  may be approximately 70 kilohertz. 
     The output of lowpass filter  208  is a control signal  218 , as illustrated in  FIG. 3D , with a DC (direct current) level V DC  determined by the DC component of high frequency SSDM signal  211 B. Control signal  218  varies between V DC  and a peak level V PEAK  with a timing that follows low frequency SSDM signal  211 A. Control signal  218  is applied to a controllable current source  202  that generates an LED current (I LED )  213  through LED  203 , as illustrated in  FIG. 3E , that is proportional to control signal  218 . LED current  213  has a minimum value I MIN  that is proportional to V DC  and a peak value I PEAK  that is proportional to V PEAK . 
     The peak value I PEAK  of LED current  213  may be detected by a sense resistor R SENSE    209 , connected between LED  203  and ground, which develops a voltage V SENSE    219  that is proportional to L ED    213 . R SENSE  may be a small value resistor (e.g., less than 1 Ohm) such that the voltage V SENSE  is much less than the forward voltage across LED  203 , which is typically in the range of 0.7 volts to 1.0 volts for silicon based LEDs. In one embodiment, for example, R SENSE may be approximately 0.1 Ohm and the peak value of I LED  may be approximately 1 Ampere, such that the peak value of V SENSE  is approximately 0.1 volt. 
     As illustrated in  FIG. 2C , the analog sense voltage V SENSE  may be converted to an n-bit digital value by an analog-to-digital converter (ADC)  210 . The n-bit digital value may be compared to an n-bit signal density value in the signal density register (e.g., a signal density register such as signal density register  205 ) in peak current SSDM  201 B. Methods for comparing digital values are known in the art and, accordingly, are not described in detail. If the n-bit digital value from ADC  210  differs by more than a specified amount from the n-bit signal density value in SSDM  201 B, then the signal density value may be adjusted accordingly, up or down, to achieve a desired value of peak current in LED  203 . 
       FIG. 4  illustrates one embodiment of a system  400  for controlling an array of LEDSs. System  400  includes a stochastic controller block  401 , which may include peak and average current SSDMs (such as SSDMs  201 B and  201 A) and an AND gate (such as AND gate  207 ) for each color channel. In one embodiment, as illustrated in  FIG. 4 , the array of LEDs may include a set of primary color LEDs such as a red LED (D R ), a green LED (D G ) and a blue LED (D B ). In other embodiments, the array of LEDs may include other primary or complementary sets of LEDs as well as one or more WHITE LEDs to control color saturation as is known in the art. 
     System  400  may also include a lowpass filter  414  for each color channel (i.e.,  414 R,  414 G,  414 B). The outputs of the lowpass filters drive controllable current sources  410 , which includes a controllable current source for each color channel. In one embodiment, as illustrated in  FIG. 4 , each controllable current source may include a buffer transistor (Q R , QG, QB), a voltage divider (R 1 , R 2 ) RGB  and a MOSFET driver transistor (M R , M G , M B ). The DC component (V DC ) of the filtered control signal in each channel (associated with the corresponding peak current SSDM for that channel) may be selected to drive a corresponding MOSFET in its “variable resistance region” to set a peak current level for that channel. The variable component of the filtered control signal in each channel (associated with the corresponding average current SSDM for that channel) may be selected to set the average current level for that channel by switching the MOSFET on and off in sequence with its SSDM waveform. Therefore, by adjustment of the values of the signal density registers in the SSDM blocks, the relative and absolute intensity of each LED can be controlled to achieve a desired color mix at a desired intensity level. The principles of color mixing and signal density control are described in copending U.S. patent application Ser. No. 11/811,108, which is incorporated herein in its entirety by reference. Each of the color channels may also include a sense resistor (R 3R , R 3G , R 3B ) as described above, which may be used to sense the peak current in each of the LEDs. The use of low value resistors, as described above with respect to  FIG. 2C , reduces the total voltage on the current source side of the LEDs and, in turn, increases the dynamic range and controllability of the controllable current sources. 
     System  400  may also include analog multiplexers (MUXs) and amplifiers to sample and process signals from each of the color channels. In one embodiment, a multiplexer  403  may be used to sample the current sense voltages V SENSE R, V SENSE G and V SENSE B. Another multiplexer  402  may be used to sample the LED voltages V DR , V DG  and V DB . The selected signals from MUX  402  and MUX  403  may be buffered by amplifiers  404  and  405  respectively. MUX  406  and MUX  408  may be configured to measure the sense voltage in each color channel to determine the respective peak LED currents in each channel. MUX  406  and MUX  408  may also be configured to measure the forward voltage across each LED to determine the junction temperature of each LED as described above. 
     To measure the sense voltage of a channel selected by MUX  403 , MUX  406  directs the output of buffer amplifier  405  to MUX  408 . In turn, MUX  408  directs the sense voltage to ADC  409 , which converts the analog sense voltage to a digital value as described above. To measure the forward voltage across one of the LEDs, MUX  402  and MUX  403  select the same channel and direct the respective sense and LED voltages to buffer amplifiers  404  and  405 . The output of buffer amplifier  404  is routed to one input of buffer amplifier  407 . The output of buffer amplifier  405  is directed to the other input of buffer amplifier  407  by MUX  406 . The output of buffer amplifier  407  is proportional to the voltage across the LED in the selected channel, which is the difference between the LED voltage and the sense voltage. MUX  408  directs the output of buffer amplifier  407  to ADC  409 , where it is converted into another digital value that maybe used to adjust signal density values in an associated SSDM module in stochastic controller  401 . System  400  may also include a processor  412  to control stochastic controller  401  as well as multiplexers  402 ,  403 ,  406  and  408  (connections not shown). Processor  412  may be, for example, any suitable type of device known by those of ordinary skill in the art, such as a microprocessor or central processing unit, a controller, special-purpose processor, digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. 
     System  400  may also include a memory  413 , which may be any suitable type of machine-readable storage medium, to store program instructions for processor  412 , calibration data for the LEDs and buffer amplifiers, lookup tables for LED output versus current and junction temperature and the like. A machine-readable medium includes any mechanism for storing or transmitting information in a form (e.g., software, processing application) readable by a machine (e.g., a computer, processor, etc.). The machine-readable medium may include, but is not limited to, magnetic storage media, optical storage media, magneto-optical storage media, read-only memory (ROM), random-access memory (RAM), erasable programmable memory (e.g., EPROM and EEPROM), flash memory or another type of medium suitable for storing electronic instructions. 
     In one embodiment, stochastic controller  401 , processor  412 , memory  413 , ADC  409 , multiplexers  402 ,  403 ,  406  and  408 , and buffer amplifiers  404 ,  405  and  407  may be implemented in a programmable mixed signal device  411  such as a programmable system on a chip (PsoC®) available from Cypress Semiconductor Corporation of San Jose, Calif. 
     System  400  may reside on a common carrier substrate such as, for example, an integrated circuit (IC) die substrate, a multi-chip module substrate, or the like. Alternatively, the components of system  400  may be one or more separate integrated circuits and/or discrete components. 
       FIG. 5  is a flowchart  500  illustrating a method according to one embodiment of the present invention. In operation  501 , an optical transducer is referenced to a ground potential. In operation  502 , the peak and average currents in the optical transducer are independently controlled with stochastic control signals generated by a high-side stochastic modulator. In operation  503 , the peak current in the optical transducer is sensed and fed back to a stochastic signal density modulator to control the intensity of the optical transducer. In operation  504 , the forward voltage across the optical transducer is sensed and fed back to a stochastic signal density modulator to correct for luminous flux and dominant wavelength shifts in the optical transducer due to junction temperature effects. 
     Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.