Patent Publication Number: US-9907135-B2

Title: LED drive apparatus, systems and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 14/030,746 filed Sep. 18, 2013, which claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/704,131 filed Sep. 21, 2012, both of which are hereby fully incorporated herein by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     This relates generally to light-emitting diodes (LEDs), and more particularly to LED luminance control suitable for high dynamic range ambient light environments. 
     BACKGROUND 
     Automotive LED application designs, such as LED illuminated micro-display console systems and heads-up display (HUD) systems, face challenging requirements. These include extended operating temperature range, very wide dimming ratio (ratio of fullest brightness image for full sunlight to lowest brightness image for night darkness), and tight/high quality color point control requirements. 
     Typical solutions use LED current as the primary feedback mechanism. Some solutions perform dimming by decreasing either amplitude or duration of current through one or more LEDs. 
     Texas Instruments DLP® DMD projection technology is a mature technology widely used in numerous display applications, hand held projectors, conference rooms, and digital theaters. 
     SUMMARY 
     In described examples for controlling a level of luminance produced by a color light-emitting diode (LED) array, a method includes: for a predetermined flux bit-slice period, activating a color enable signal to select a primary color LED and to select a predetermined light flux magnitude set-point; selectively charging an energy storage device and discharging the energy storage device through the selected primary color LED to generate a light flux output during the flux bit-slice period; adjusting a rate of selectively charging the energy storage device to maintain a magnitude of the light flux output at the predetermined light flux magnitude set-point during the flux bit-slice period; and adjusting the predetermined light flux magnitude set-point over the life of the selected LED as the selected LED ages as a function of an anode-to-cathode voltage drop across the selected LED for a given magnitude of current flowing through the selected LED. 
     In other examples for controlling a level of luminance produced by a color light-emitting diode (LED) array, a method includes: for a predetermined flux bit-slice period, activating a color enable signal to select a primary color LED and a predetermined light flux pulse magnitude set-point; establishing a number of light flux pulses to be generated during the flux bit-slice period; during the flux bit-slice period and for each light flux pulse, discharging current from an energy storage device into the selected LED; during the flux bit-slice period and for each light flux pulse, bypassing current from the selected LED and recharging the energy storage device when a sensed light flux magnitude reaches the light flux magnitude set-point; and adjusting the predetermined light flux pulse magnitude set-point and/or the number of light flux pulses to be generated during the flux bit-slice period over the life of the selected LED as the selected LED ages as a function of an anode-to-cathode voltage drop across the selected LED for a given magnitude of current flowing through the selected LED. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a prior art diagram illustrating luminance control of a single LED using a three-bit binary word to “bit-weight” a stream of LED drive current pulses. 
         FIG. 2A  is a prior art diagram illustrating a stream of LED drive current pulses created by combining multiple, differently bit-weighted sub-streams of LED drive current pulses, each sub-stream corresponding to a particular primary color. 
         FIG. 2B  is a prior art diagram illustrating various lengths or “bit slices” of LED drive current pulses combined to create an example composite drive current signal to drive a color LED array. 
         FIG. 3  is a prior art diagram illustrating luminance over time of each primary color of a white-balanced output of a color LED array at full brightness. 
         FIG. 4  is a prior art diagram illustrating luminance over time of each primary color of a white-balanced output of a color LED array dimmed to 50% of full brightness by attenuating current flow through the LEDs. 
         FIG. 5  is a diagram illustrating timing of current pulses associated with each primary color of a color LED array dimmed to 10% of full brightness by time-attenuating each bit slice according to example embodiments. 
         FIG. 6  is a diagram illustrating luminance over time of each primary color of a white-balanced output of a color LED array dimmed to 2.5% of full brightness using both current attenuation and time attenuation according to example embodiments. 
         FIG. 7  is a diagram illustrating luminance over a single bit-slice time period of an LED operating in continuous mode according to example embodiments. 
         FIG. 8  is a diagram illustrating luminance pulses from an LED operating in discontinuous mode over a bit-slice period according to example embodiments. 
         FIG. 9  is a diagram illustrating luminous pulses over a bit-slice period of an LED operating in discontinuous mode for example dimming ratios achievable by varying the number and amplitude of pulses according to example embodiments. 
         FIGS. 10A, 10B and 10C  are a flow diagram illustrating a method of controlling a level of luminance produced by a color LED array according to example sequences. 
         FIG. 11  is a timing diagram illustrating timing associated with continuous-mode operation according to the example sequences illustrated by the method of  FIGS. 10A, 10B and 10C . 
         FIGS. 12A, 12B and 12C  are a flow diagram illustrating a method of dimming a color LED array in discontinuous-mode operation according to example sequences. 
         FIG. 13A  is a timing diagram illustrating timing associated with discontinuous-mode operation according to the example sequences illustrated by the method of  FIGS. 12A, 12B and 12C . 
         FIG. 13B  is a timing diagram with an expanded time axis illustrating timing associated with discontinuous-mode operation according to the example sequences illustrated by the method of  FIGS. 12A, 12B and 12C . 
         FIGS. 14A and 14B  are a schematic diagram illustrating an apparatus for controlling levels of luminance produced by a color LED array according to example embodiments. 
         FIG. 15  is a system diagram illustrating an example head-up display system using apparatus for controlling levels of luminance produced by a color LED array according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a prior art diagram illustrating luminance control of a single LED using a three-bit binary word to “bit-weight” a stream of LED drive current pulses. Accordingly, the size of the binary control word determines the average number of current pulses per time applied to the LED. And, the resulting intensity of light emanating from the LED is a function of the average number of current pulses per time. In this description, the term “bit-slice” means a period of time (e.g., the period of time  110 ) during which one or more pulses of current are applied to an LED. 
       FIG. 2A  is a prior art diagram illustrating a stream of LED drive current pulses created by combining multiple, differently bit-weighted sub-streams of LED drive current pulses, each sub-stream corresponding to a particular primary color. 
       FIG. 2B  is a prior art diagram illustrating various lengths or “bit slices” of LED drive current pulses combined to create an example composite drive current signal  212  to drive a color LED array. In this example, current pulses of equal bit-slice length are created for each primary color. Such balancing in the temporal domain may be done to create a net white point in the color domain. However, the bit-slice lengths for a particular group of primary colors vary over time. The latter technique may assist in the visual integration of an image by the human eye to avoid the appearance of flicker, for example. 
       FIG. 3  is a prior art diagram illustrating luminance over time of each primary color of a white-balanced output of a color LED array at full brightness. Such luminous output may result from the current drive signal  212  of  FIG. 2B . The luminous intensity of each primary color is different in this example. Such magnitude differences may be implemented to maintain a given white point and avoid a color cast, given the equivalence of bit-slice periods for each primary color. 
       FIG. 4  is a prior art diagram illustrating luminance over time of each primary color of a white-balanced output of a color LED array dimmed to 50% of full brightness by attenuating current flow through the LEDs. Dimming via current flow attenuation is a traditional means of dimming, but is insufficient for high dimming ratios for at least the reasons mentioned hereinabove. 
       FIG. 5  is a diagram illustrating timing of current pulses associated with each primary color of a color LED array dimmed to 10% of full brightness by time-attenuating each bit slice according to example embodiments. Current is turned on for a selected portion of each bit slice period, resulting in a dimming ratio that is a function of the on-time. 
       FIG. 6  is a diagram illustrating luminance over time of each primary color of a white-balanced output of a color LED array dimmed to 2.5% of full brightness using both current attenuation and time attenuation according to example embodiments. Some embodiments operate in a manner referred to herein as continuous mode (“CM”). CM operation includes controlling both the magnitude of current through a selected LED and the on-time of the LED as a percentage of the bit-slice period. The net dimming ratio is a function of the mathematical product of the current attenuation and the time attenuation. In the example of  FIG. 6 , the attenuation is (0.10)*(0.25) or 2.5%. 
       FIG. 7  is a diagram illustrating luminance over a single bit-slice time period of an LED operating in CM according to example embodiments.  FIG. 7  illustrates a dimming ratio of 32:1 accomplished by limiting light flux magnitude to 25% of maximum available amplitude and limiting light flux pulse width to 12.5% of the bit-slice period, resulting in an attenuation factor of (25%)*(12.5%)=0.03125 or 32:1. Embodiments herein use CM operation in high ambient light situations when lower dimming ratios are appropriate. 
       FIG. 8  is a diagram illustrating luminance pulses from an LED operating in a manner referred to herein as discontinuous mode (“DM”) over a bit-slice period according to example embodiments. During DM operation, multiple light flux pulses of a selected magnitude are generated. 
       FIG. 9  is a diagram illustrating luminous pulses over a bit-slice period of an LED operating in DM for example dimming ratios according to example embodiments. A flux magnitude feedback loop is used to very accurately control flux pulse magnitude such that extremely small pulses may be created, as further described hereinbelow. DM operation is used by embodiments herein in low ambient light conditions when very large dimming ratios are appropriate. The two examples  910  illustrates that the dimming ratio (e.g., in this example 20:1) may be controlled via the number and/or sizes of the multiple light flux pulses. 
       FIGS. 10A, 10B, and 10C  are a flow diagram illustrating a method  1000  of controlling a level of luminance produced by a color LED array according to example sequences.  FIG. 11  is a timing diagram illustrating timing associated with CM operation according to the example sequences illustrated by the method of  FIGS. 10A, 10B and 10C . The following description of the method  1000  references the timing diagram of  FIG. 11  to describe control methods associated with CM operation. 
     The method  1000  includes selectively charging an energy storage device and discharging the energy storage device through the selected primary color LED to generate a light flux output during the flux bit-slice period (e.g., the period  1108  illustrated by the dashed-line waveform  1110  of  FIG. 11 ). The method  1000  also includes adjusting a rate of selectively charging the energy storage device to maintain a magnitude of the light flux output at the predetermined light flux magnitude set-point (e.g., the set-point  1115 ) of  FIG. 11 ) during the flux bit-slice period. In some versions, the method  1000  further includes adjusting the predetermined light flux magnitude set-point over the life of the selected LED as the selected LED ages. The amount of aging and corresponding adjustment in set-point is a function of the anode-to-cathode voltage drop across the selected LED for a given magnitude of current through the selected LED. 
     The method  1000  commences at block  1010  with selecting a primary color for the current bit-slice period and continues at block  1014  with selecting a maximum LED current threshold value. The method  1000  also includes setting a pulse period timer value associated with the flux bit-slice period  1108 , at block  1016 . The method  1000  further includes activating a current drive enable signal (e.g., the signal “D_EN”  1118  of  FIG. 11 ) to begin charging the energy storage device, at block  1020 . 
     The method  1000  continues at block  1023  with activating a color enable signal (e.g., the “G_EN” signal  1124  of  FIG. 11 ) associated with the selected primary color. The method  1000  includes selecting a predetermined light flux magnitude set-point control signal (e.g., the control signal  1110 ) associated with the selected primary color using the color enable signal (e.g., the color enable signal  1124 ), at block  1026 . The method  1000  also includes enabling a pass transistor corresponding to an LED of the selected primary color using the color enable signal, at block  1028 . 
     The method  1000  continues at block  1031  with disabling a current bypass switch used to shunt current away from the LED array. In some versions of the method  1000 , the latter operation may be accomplished by transitioning a “shunt enable signal” (e.g., the “S_EN” signal of  FIG. 11 ) to a low state. The method  1000  includes sensing the magnitude of the light flux output from the selected LED at a flux sensor and generating a corresponding flux level signal (e.g., the signal  1135  of  FIG. 11 ), at block  1034 . The method  1000  also includes comparing the sensed magnitude of the light flux output from the selected LED to the light flux magnitude set-point, at block  1039 . The result of the compare operation is illustrated by example as the “F_CMP_OUT” signal  1140  of  FIG. 11 ). The method  1000  further includes determining if the sensed magnitude of the light flux output is greater than or equal to the light flux magnitude set-point, at block  1041 . If so, the method  1000  includes disabling a current drive source (e.g., via the signal “CNTRL_OUT”  1145 ) to the energy storage device, at block  1044 , until the sensed magnitude of the light flux output is less than the light flux magnitude set-point as determined at block  1041 . 
     If the sensed magnitude of the light flux output is less than the light flux magnitude set-point, the method  1000  continues at block  1047  with sensing a magnitude of current flowing through the selected primary color LED and generating a corresponding current magnitude signal. The method  1000  includes comparing the sensed magnitude of current flowing through the selected primary color LED to a maximum LED current threshold value, at block  1050 . The method  1000  also includes determining whether the sensed magnitude of current flowing through the selected primary color LED is equal to or greater than the maximum LED current threshold value, at block  1053 . If so, the method  1000  includes disabling the current drive source to the LED array to limit the magnitude of current flowing through the selected primary color LED to the maximum LED current threshold value, at block  1044 , and continuing to sense the level of the light flux output, at block  1034 . 
     The method  1000  continues at block  1058  with determining whether the flux bit-slice timer has expired. If not, the method  1000  includes continuing to sense the level of the light flux output, at block  1034 . If the flux bit-slice timer value has expired, the method  1000  includes enabling the current bypass switch to sharply terminate current flow through the selected LED, at block  1061 . Upon flux bit-slice timer expiration, the method  1000  also includes deactivating the current drive enable signal to disable the LED current source, at block  1064 , and deactivating the primary color enable signal, at block  1067 . The latter operation in turn disables the pass transistor associated with the selected LED and de-selects the flux magnitude set-point signal. 
       FIGS. 12A, 12B and 12C  are a flow diagram illustrating a method  1200  of dimming a color LED array in discontinuous-mode (DM) operation according to example sequences.  FIG. 13A  is a timing diagram illustrating timing associated with DM operation according to the example sequences illustrated by the method of  FIGS. 12A, 12B and 12C .  FIG. 13B  is a timing diagram with an expanded time axis illustrating timing associated with DM operation according to the example sequences illustrated by the method of  FIGS. 12A, 12B and 12C . 
     The method  1200  includes discharging current from an energy storage device into a selected LED during a flux bit-slice period (e.g., the period  1308  of  FIG. 13A ) to create each of one or more light flux pulses (e.g., the four red pulses within the dashed-line waveform  1310 ). The method  1200  also includes bypassing current from the selected LED and recharging the energy storage device when a sensed light flux magnitude (e.g., the magnitude  1312  of  FIGS. 13A and 13B ) reaches a light flux magnitude set-point control signal amplitude (e.g., the magnitude set-point amplitude  1315  of  FIGS. 13A and 13B ) to quickly terminate each light flux pulse. 
     In some versions, the method  1200  may also include making adjustments for LED aging over the life of the selected LED to maintain a consistent white point. Such adjustments may include adjusting the predetermined light flux pulse magnitude set-point and/or the number of light flux pulses to be generated during the flux bit-slice period. Aging may be determined by measurements taken of the anode-to-cathode voltage drop across the selected LED for a given magnitude of current flowing through the selected LED. 
     The method  1200  commences at block  1205  with selecting a primary color for the current bit-slice period and continues at block  1208  with initializing a bit-slice pulse counter used to track the number of light flux pulses. The method  1200  includes establishing a number of light flux pulses to be generated during the flux bit-slice period, at block  1211 . The method  1200  also includes establishing a current supply set-point control signal for the current bit-slice, at block  1212 . The method  1200  further includes activating a current drive enable signal (e.g., the “D_EN” signal  1318  of  FIG. 13A ) to begin charging the energy storage device supplying current to the LED array, at block  1213 . 
     The method  1200  also includes activating a color enable signal (e.g., the “G_EN” signal  1325  of  FIG. 13A ) associated with the primary color, at block  1215 . The color enable signal is used to select the primary color LED and the predetermined light flux pulse magnitude set-point for the predetermined flux bit-slice period. 
     The method  1200  continues at block  1221  with selecting the flux pulse magnitude set point control signal (e.g., “PWM_OUT”  1315  of  FIGS. 13A and 13B ) associated with the selected primary color for the bit-slice period. The method  1200  also includes enabling a pass transistor corresponding to the selected LED, at block  1225 . The method  1200  further includes disabling a current bypass switch to enable current from the energy storage device to the selected LED, at block  1228 . In some versions of the method  1200 , a falling edge of a shunt enable signal (e.g., “S_EN”  1330  of  FIGS. 13A and 13B ) may be used to disable the current bypass switch. As bypassed current begins to decrease, forward voltage at the selected LED (e.g., the forward voltage signal  1333  of  FIG. 13B ) begins to increase during the period  1334 . Current begins to flow through the selected LED and light flux begins to be sensed at the point  1335 . 
     The method  1200  also includes maintaining an available current (e.g., the current  1350  of  FIG. 13B ) through an energy storage device such as an inductor used to supply current to the LED array. The method  1200  thus includes sensing a magnitude of current flowing through the energy storage device and generating a corresponding current magnitude signal, at block  1231 . The method  1200  also includes comparing the magnitude of current flowing through the energy storage device to the current supply set-point control signal (e.g., the set-point control signal  1355  of  FIG. 13B ), at block  1233 . The method  1200  further includes determining whether the magnitude of current flowing through the energy storage device is greater than or equal to the magnitude of the current supply set-point control signal, at block  1236 . If so, the method  1200  includes disabling the current drive source at block  1238  to decrease current to the energy storage device until the magnitude of current flowing through the energy storage device is less than the magnitude of the current supply set-point control signal and then re-enabling the current source at block  1241 . 
     The method  1200  continues at block  1244  with sensing the magnitude of the light flux output from the selected LED and generating a corresponding flux level signal (e.g., the flux level signal  1360  of  FIG. 13B ). The method  1200  includes comparing the magnitude of the sensed flux level signal  1360  to the magnitude of the flux level set-point control signal  1315 , at block  1247 . The method  1200  also includes determining whether the sensed magnitude of the light flux output is greater than or equal to the magnitude of the light flux magnitude set-point control signal, at block  1250 . (See, e.g., the “F_CMP_OUT” signal  1370  of  FIG. 13B .) If not, the method  1200  continues with sensing energy storage device current magnitude at block  1231 . 
     If the sensed magnitude of the light flux output is greater than or equal to the magnitude of the light flux magnitude set-point control signal (e.g., at the point  1312  of  FIG. 13B ), the method  1200  includes re-enabling the current bypass switch to shunt current away from the LED array and terminate the light flux output pulse from the selected LED, at block  1254 . (See, e.g., the rising edge  1375  of the current bypass switch enable signal S_EN.) In the latter case, the method  1200  also includes incrementing the bit-slice pulse counter, at block  1258 , and determining whether a count of the bit slice pulse counter is equal to the number of light flux pulses to be generated during the bit-slice period, at block  1261 . If not, the method  1200  continues at block  1228  with generating an additional pulse. 
     If the count of the bit slice pulse counter is equal to the number of light flux pulses to be generated during the bit-slice period, the method  1200  includes deactivating the color enable signal to disable the pass transistor associated with the selected LED and to disable the flux pulse magnitude set-point signal, at block  1264 . The method  1200  then repeats at block  1205  with selecting another primary color for a next bit-slice period. 
       FIGS. 14A and 14B  are a schematic diagram illustrating an apparatus  1400  for controlling luminance levels produced by a color LED array according to example embodiments. The apparatus  1400  operates in two modes, continuous mode (CM) and discontinuous mode (DM). Operating in CM, the apparatus  1400  is capable of performing the method  1000  described hereinabove. Operating in DM, the apparatus  1400  is capable of performing the method  1200  described hereinabove. Accordingly, some components of the apparatus  1400  operate in a certain way in CM and in a different way in DM. Consequently, the apparatus  1400  will be described twice, first in the context of CM operation and then in the context of DM operation. 
     Referring to  FIG. 14B , the apparatus  1400  includes a parallel array of LEDs  1405 . The parallel array of LEDs  1405  includes one or more LEDs  1407 ,  1408 , and  1409  corresponding to each of three primary colors (e.g., red, green and blue). The apparatus  1400  also includes a light flux sensor  1412  flux-coupled to the LED array to sense a magnitude of light flux output from a selected LED. 
     Operating in CM, the apparatus  1400  further includes a pulse width modulation (PWM) selector  1414  communicatively coupled to the LED array  1405 . The PWM selector  1414  selects a predetermined light flux magnitude set-point signal (e.g., PWM 1 , PWM 2 , or PWM 3 ) corresponding to a predetermined primary color. The flux magnitude set-point signal PWM_OUT is selected by a color enable signal (e.g., R_EN, G_EN, or B_EN) for a predetermined flux bit-slice period. 
     The apparatus  1400  also includes a flux comparator  1418  coupled to the PWM selector  1414  and to the light flux sensor  1412 . The flux comparator  1418  compares the sensed magnitude of light flux output  1419  to the light flux magnitude set-point signal PWM_OUT appearing at the input  1420 . 
     The apparatus  1400  further includes a current drive circuit  1424  communicatively coupled to the flux comparator  1418 . The current drive circuit  1424  selectively charges an energy storage device  1426  (e.g., an inductor) coupled between the current drive circuit  1424  and a common anode terminal  1428  of the LED array  1405 . The energy storage device  1426  supplies current to the LED array  1405  when the sensed magnitude of light flux output is less than the light flux magnitude set-point. 
     The apparatus  1400  also includes a primary color pass transistor (e.g., the Red color pass transistor  1431 ) coupled in series with each primary color LED. Each pass transistor is capable of being enabled using a primary color enable signal (e.g., the R_EN signal  1432 ) to select an associated primary color LED (e.g., the Red LED  1407  in this example). 
     The apparatus  1400  further includes a current bypass switch  1435  coupled between an output  1428  of the energy storage device  1426  and a resistor  1437  to ground. The current bypass switch  1435  provides fast LED turn-on and turn-off times by selectively shunting current away from the LED array  1405 . 
     The apparatus  1400  also includes a current control logic module  1440  coupled to the current drive circuit  1424 . The current control logic module  1440  enables the current drive circuit  1424  during the bit-slice period when the sensed magnitude of light flux output (e.g., the signal appearing at the input  1419  of the flux comparator  1418 ) is less than the light flux magnitude set-point (e.g., the signal appearing at the input  1420  of the flux comparator  1418  and no over-current condition exists. 
     The apparatus  1400  further includes a current comparator  1443  coupled to the current control logic module  1440 . A signal created by the voltage drop across the resistor  1437  is representative of the magnitude of current flowing through the selected primary color LED and appears at an input  1445  of the current comparator  1443 . The current comparator  1443  compares the latter signal to a predetermined maximum LED current threshold signal “C_SET” appearing at an input  1446  of the current comparator  1443 . The output “C_CMP” of the current comparator  1443  toggles the current control logic to maintain the magnitude of current flowing through the selected primary color LED at or below the predetermined maximum value of the LED current threshold signal “C_SET.” 
     Referring to  FIG. 14A , the apparatus  1400  also includes a CM logic module  1450  communicatively coupled to the current control logic module  1440  of  FIG. 14B . The CM logic module  1450  provides a drive enable signal “D_EN” for precise turn-on and turn-off of the selected LED (e.g., the Red LED  1407  when R_EN is active). The CM logic module  1450  also provides a shunt enable signal “S_EN” to control the current bypass switch  1435 . The apparatus  1400  further includes a master control logic module  1455  coupled to the CM logic module  1450 . The master control logic module  1455  initiates a sequence of flux bit slices and generates the set of primary color enable signals R_EN, G_EN, and B_EN. The S_EN signal is selected from the CM logic module  1450  by a selector  1452  when a DM signal  1456  from the master control logic module  1455  is inactive. 
     The apparatus  1400  also includes a PWM logic module  1458  coupled to the PWM selector  1414 . The PWM logic module  1458  generates the predetermined light flux magnitude set-point signals associated with the predetermined primary colors for the predetermined flux bit-slice periods. The PWM logic module  1458  also generates the predetermined maximum LED current threshold signal “C_SET.” 
     The apparatus  1400  further includes an LED aging compensation logic module  1462  coupled to the PWM logic module  1458 . The LED aging compensation logic module  1462  monitors the anode-to-cathode voltage drop across the selected LED for a given current flowing through the selected LED to determine how the LED characteristic curve ages over time. The LED aging compensation logic module  1462  then adjusts the predetermined light flux magnitude set-point during the life of the selected LED as the selected LED ages. The apparatus  1400  includes a high-side voltage analog-to-digital converter (ADC)  1464  coupled to the LED aging compensation logic module  1462 . The high-side voltage ADC converts a sensed anode voltage of the selected LED to a digital signal for analysis by the LED aging compensation logic module  1462 . The apparatus  1400  also includes a low-side voltage and current ADC  1466  coupled to the LED aging compensation logic module  1462 . The low-side voltage and current ADC  1466  converts a sensed cathode voltage of the selected LED to a digital signal for analysis by the LED aging compensation logic module  1462 . 
     The apparatus  1400  will now be described with reference to its structure and operation in DM. Operating in DM, the apparatus  1400  includes the parallel array of LEDs  1405  and the light flux sensor  1412  as described hereinabove in the context of CM operation. The apparatus  1400  also includes the PWM selector  1414  communicatively coupled to the LED array  1405 . Operating in DM, the PWM selector  1414  selects a predetermined light flux magnitude set-point signal associated with a predetermined primary color for a predetermined number of light flux pulses to be generated during a bit-slice period. 
     The flux comparator  1418  is coupled to the PWM selector  1414  and to the light flux sensor  1412  to compare the sensed magnitude of light flux output to the light flux magnitude set-point signal. The current bypass switch  1435  is coupled between the output  1428  of the energy storage device  1426  and the resistor  1437  to ground. The current bypass switch  1435  is to be disabled to initiate a ramp-up of LED forward voltage in order to create a leading edge of a light flux pulse and to be enabled to shunt current away from the selected LED in order to terminate the light flux pulse when the sensed magnitude of light flux output is equal to or greater than the light flux magnitude set-point, as described hereinabove with reference to  FIG. 13B . 
     Operating in DM, the apparatus  1400  includes the primary color pass transistors  1407 ,  1408 , and  1409 , each coupled in series with a primary color LED, each pass transistor capable of being enabled using a primary color enable signal to select an associated primary color LED, as described hereinabove with regard to CM operation. 
     The apparatus  1400  also includes the current comparator  1443  communicatively coupled to the LED array  1405 . In DM, the current comparator  1443  compares the magnitude of current flowing through the energy storage device  1426  to a predetermined magnitude of current to be regulated through the energy storage device  1426 . The current drive circuit  1424  is communicatively coupled to the energy storage device  1426  to selectively charge the energy storage device  1426 . The current control logic module  1440  coupled between the current comparator  1443  and the current drive circuit  1424  enables the current drive circuit  1424  when the magnitude of current flowing through the energy storage device is less than the predetermined magnitude of current to be regulated through the energy storage device. 
     The apparatus  1400  further includes a DM logic module  1470  communicatively coupled to the current bypass switch  1435 . The DM logic module  1470  selectively enables and disables the current bypass switch  1435  via the S_EN signal to control the width of each light flux pulse. (See, e.g., the S_EN waveform  1330  of  FIG. 13B .) When the sensed light flux pulse reaches the flux pulse magnitude set-point  1315  at the point  1312 , the waveform  1370  of the flux comparator output F_CMP_OUT goes low. F_CMP_OUT is an input to the DM logic module  1470  and results in S_EN transitioning to a high state. The high state of S_EN turns on the current bypass switch, which abruptly shunts current stored in the energy storage device away from the selected LED and thus abruptly terminates the flux pulse. 
     The master control logic module  1455  is coupled to the DM logic module  1470  to establish DM operation when large dimming ratios are desirable due to low ambient light levels. The master control logic module  1455  initiates a sequence of flux bit slices and sequences a set of primary color enable signals used to select the predetermined light flux magnitude set-point signal associated with the predetermined primary color for the predetermined number of light flux pulses. The master control logic module  1455  also loads a lookup table (LUT) (not shown) located in the DM logic module with the number of pulses to be generated for the current and/or subsequent bit-slices. At run-time, the LUT resets the S_EN signal  1330  to initiate each pulse. 
     The PWM logic module  1458  is coupled to the PWM selector  1414  in DM to generate the predetermined light flux magnitude set-point signal associated with the predetermined primary color for the predetermined number of light flux pulses to be generated during the bit-slice period. The PWM logic module  1458  also generates a signal C_SET representing the predetermined magnitude of current to be regulated through the energy storage device. 
     The LED aging compensation logic module  1462  and associated ADCs  1464  and  1466  are structured and operate in DM as described hereinabove in the context of CM operation. 
       FIG. 15  is a system diagram illustrating an example HUD system  1500  using apparatus for controlling levels of luminance produced by a color LED array according to example embodiments. The HUD system  1500  includes a parallel array of LEDs  1405  consisting of at least one LED corresponding to each of three primary colors. The HUD system  1500  also includes an energy storage device  1426  coupled to the LED array  1405  to supply current to the LED array  1405 . The HUD system  1500  further includes a current bypass switch  1435  coupled to the energy storage device  1426 . The current bypass switch  1435  shunts current away from a selected LED to provide a fast turn-off time. The HUD system  1500  further includes a light flux sensor  1412  flux-coupled to the LED array  1405  to sense a magnitude of a light flux output from a selected LED. The HUD system  1500  also includes a primary color pass transistor (e.g., the pass transistor  1431 ) coupled in series with each primary color LED Each pass transistor is capable of being enabled using a primary color enable signal to select an associated primary color LED. 
     The HUD system  1500  also includes a CM and DM dimming control module  1505 . The control module  1505  receives: a light flux sense signal from light flux sensor  1412 ; and a high side voltage sense signal and a low side voltage sense and current sense signal from the LED array  1405 . The dimming control module  1505  controls current supplied to the energy storage device  1426  and controls the state of the current bypass switch  1435  as described hereinabove in the contexts of CM and DM operation. 
     Accordingly, the dimming control module  1505  includes the pulse width modulation (PWM) selector  1414 , the flux comparator  1418 , the current drive circuit  1424 , the current comparator  1443 , the current control logic module  1440 , the CM logic module  1450 , the DM logic module  1470 , the master control logic module  1455 , the PWM logic module  1458 , the LED aging compensation logic module  1462 , the high-side ADC  1464 , and the low-side voltage and current ADC  1466 , all coupled together to operate as described hereinabove in the contexts of CM and DM operation. 
     The HUD system  1500  also includes a digital micro-mirror device (DMD)  1510  optically coupled to the LED array  1405 . The DMD  1510  includes a two-dimensional array of pixel-sized mirrors. The mirrors form and project an image by selectively aiming light flux output from the selected LED into or away from an optics system  1520  on a pixel-by-pixel basis. The optics system  1520  is also a component of the HUD system  1500  and is optically coupled to the LED array  1405  via the DMD  1510 . The optics system  1520  projects the image formed by the DMD  1510  onto a windshield  1530 . 
     Apparatus, systems and methods described herein may be useful in applications other than dimming light flux from LED arrays in high contrast ratio ambient light conditions. Examples of the methods  1000  and  1200  and apparatus  1400  for controlling levels of luminance produced by an LED array and the HUD system  1500  provide a general understanding of the sequences of various methods and the structures of various embodiments. They do not serve as complete descriptions of all elements and features of methods, apparatus and systems that might use these example sequences and structures. 
     The various embodiments may be incorporated into semiconductor analog and digital circuits for use in receptacle power converters, electronic circuitry used in computers, communication and signal processing circuitry, single-processor or multi-processor modules, single or multiple embedded processors, multi-core processors, data switches, and application-specific modules including multi-layer, multi-chip modules, among others. Such apparatus and systems may further be included as sub-components within a variety of electronic systems, such as televisions, cellular telephones, personal computers (e.g., laptop computers, desktop computers, handheld computers, tablet computers, etc.), workstations, radios, video players, audio players (e.g., MP3 (Motion Picture Experts Group, Audio Layer 3) players), motor vehicles, medical devices (e.g., heart monitor, blood pressure monitor, etc.), set top boxes, and others. 
     Apparatus and methods described herein provide color LED array dimming capabilities applicable to operation in an extremely wide dynamic range of ambient light conditions. Light flux levels sensed from a color LED array are fed back to control both current availability to the LED array and to disable/re-enable a current bypass switch to quickly shunt stored-energy current to or away from a selected LED. 
     In CM operation, a single flux pulse is created for the duration of the bit-slice period. Feedback from the light flux sensor is used to pulse current to an energy storage device used to supply current to the selected LED such as to maintain the output light flux at a predetermined level or set-point during the bit-slice period. A particular dimming level is achieved by establishing both the bit-slice period length and the flux magnitude set-point. 
     In DM operation, one or more short flux pulses are created during the bit-slice period. Both the turn-on and the turn-off time of each such DM flux pulse are controlled by alternately removing and then re-establishing a current shunt from the energy storage device to ground. Flux pulse magnitude is controlled by recognizing when the sensed flux pulse magnitude has reached a predetermined set-point. The resulting flux compare signal is used to re-establish the current shunt and to thus abruptly turn off current to the selected LED. A flux pulse of precise amplitude with a sharply falling edge results. Unexpectedly high dimming ratios on the order of 1:4000 or more are achievable in DM operation. 
     Accordingly, apparatus, systems and methods described herein implement dynamic dimming of a color LED array, such as may be used in various applications operating in high dynamic range ambient light conditions. Such applications may include projection systems of various types including HUD systems, color display panels, outdoor signage, etc. 
     Embodiments herein may operate in one or both of two modes, “continuous mode” (CM) and “discontinuous mode” (DM). Lower dimming ratios are available in CM and higher dimming ratios are available in DM. Consequently, a device or system operating in an extremely wide dynamic range of ambient light may transition back and forth between CM operation during periods of high ambient light and DM operation during periods of low ambient light. However, methods and structures herein do not so require, because each mode of operation is distinctly supported. 
     Both modes of operation use light flux levels sensed from the color LED array as a feedback signal. The light flux feedback signal is used to control both current availability to the LED array and the state of a current bypass switch. The current bypass switch is capable of quickly shunting stored-energy current to or away from a selected LED. In both modes of operation, a target flux level is selected as is a time period referred to herein as a “bit-slice” period. One or more single primary color LEDs are selected from the array for operation during a single bit-slice period. For example, for an array with a single LED per additive primary color, only a single red, green, or blue LED would be selected for operation during a bit-slice period. 
     In CM operation, a single flux pulse is created for the duration of the bit-slice period. Feedback from the light flux sensor is used to pulse current to an energy storage device used to supply current to the selected LED such as to maintain the output light flux at a predetermined level or set-point during the bit-slice period. A particular dimming level is achieved by establishing both the bit-slice period length and the flux magnitude set-point. Dimming ratios on the order of 1:32 are achievable in CM operation, with the limiting factor being unevenness of tracking between LED current and flux output at low levels of LED current. 
     In DM operation, one or more short flux pulses are created during the bit-slice period. Both the turn-on and the turn-off times of each such DM flux pulse are controlled by alternately removing and then re-establishing a current shunt from the energy storage device to ground. Flux pulse magnitude is controlled by recognizing when the sensed flux pulse magnitude has reached a predetermined set-point. A resulting flux compare signal is used to re-establish the current shunt and to thus abruptly turn off current to the selected LED. A flux pulse of precise amplitude with a sharply falling edge results. Very high dimming ratios on the order of 1:4000 or more are achievable in DM operation. 
     An example automotive and/or aircraft HUD system embodiment is also described and claimed. In some embodiments, the example HUD system uses Texas Instruments DLP® DMD projection technology in conjunction with the CM and DM dimming apparatus and methods described in detail herein. 
     Light flux levels sensed from a color LED array control both current availability to the array and disable/re-enable a current bypass switch to shunt stored-energy current to or away from a selected LED. In continuous mode, a single flux pulse is created for the duration of a pre-established period. Feedback from the flux sensor pulses current to an energy storage device to maintain the light flux at a predetermined set-point. A particular dimming level is achieved by establishing both the pulse period and the flux magnitude. In discontinuous mode, one or more short flux pulses are created. Both the turn-on and the turn-off time of each flux pulse is controlled by alternately removing and then re-establishing a current shunt from the energy storage device to ground. Flux pulse magnitude is controlled by recognizing when the flux pulse has reached a predetermined set-point and re-establishing the current shunt to abruptly turn off current to the selected LED. 
     In the drawings, arrows at one or both ends of connecting lines show general directions of electrical current flow, data flow, logic flow, etc., but without limiting such flows to only particular directions (e.g., without precluding such flows in opposite directions). 
     Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.