Patent Application
20110080533
The present invention relates generally to driver circuits, and more specifically to driver circuits suitable to drive laser light sources.
Direct modulation of laser diodes for video applications is typically performed using Class A amplifiers in which a series pass transistor varies the current supplied to the laser diode. Class A amplifiers are typically not very power efficient because much of the system power is dissipated by the series transistor. This inefficiency results in wasted power consumption which increases heat and reduces battery life.
In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views.
In some embodiments, light source 140 is a laser light source. For example, in some embodiments light source 140 is a laser diode that produces red, green, or blue laser light. Light source 140 is not limited to laser embodiments. For example, other light sources, such as color filters or light emitting diodes (LEDs) or edge-emitting LEDs, could easily be substituted.
In operation, DAC 110 receives a commanded drive value on node 102 in the form of a digital word. DAC 110 also receives a pixel clock on node 104. At times specified by the pixel clock, DAC 110 converts the commanded drive value on node 102 to an analog laser drive value on node 111. Ramp generator circuits 112 and 116 generate voltage ramp signals that are provided to comparators 150 and 160. In response to the analog laser drive value and the ramp signals, comparators 150 and 160 produce pulse width modulated (PWM) output signals to drive transistors 152 and 162. By modulating the pulse width (time duration to turn on selected transistor), a variety of laser light output levels (referred to herein as “gray levels” or “grayscale”) can be produced using highly efficient switching transistors, thereby saving system power.
The commanded drive value represents a desired output luminance for a particular amount of time (e.g., one pixel). When the commanded drive value represents one pixel of a video display, the commanded value changes at the pixel rate as determined by the pixel clock. In some embodiments, the pixel clock has a fixed period. In other embodiments, the pixel clock has a variable period.
As shown in
Programmable switching power supply 122 may be any type of switching power supply. For example, programmable switching power supply 122 may be a pulse width modulating (PWM) power supply switching at any frequency. Switching power supplies are generally known in the art, and the various embodiments of the present invention are not limited by the implementation details of programmable switching power supply 122.
Transistors 150 and 160 are shown as bipolar junction transistor (BJT), although this is not a limitation of the present invention. Any switching device suitable to provide current to a laser light source may be substituted therefor and is considered equivalent. For example, a field effect transistor (FET) such as a junction FET (JFET) or metal oxide semiconductor FET (MOSFET) may be utilized for transistors 150 and 160 without departing from the scope of the present invention.
Although
In operation, the enabled comparator asserts its output at the beginning of the pixel and de-asserts its output when the corresponding ramp signal reaches the level of the output signal of DAC 110. Because transistors 150 and 160 are coupled in parallel, the comparator PWM output signals are combined to form a composite laser drive signal as shown in
The effective resolution of this scheme depends on the accuracy of the ramp generators, and on the equivalent noise performance of the comparators. In some embodiments, some aspects of non-ideal behavior, such as non-linearity of the ramp waveforms, are calibrated out during calibration of the data path that provides the commanded drive value. The comparator output signal rise and fall times can be slow as long as the pulse width jitter is managed appropriately.
The power supply voltage value needed to produce specific currents (and light levels) may be learned by asserting values within the desired grayscale range, and slowly adjusting the power supply to maintain a suitable power supply voltage. In some embodiments, these calibration pulses are sent at times that the light source would otherwise be inactive. For example, in video applications, a calibration pulse may be sent at the top or bottom of the video frame. Calibration feedback circuit 320 may include any suitable loop filter for feedback. For example, calibration feedback circuit 320 may include a proportional-integral-derivative (PID) controller that is updated when the calibration pulses are issued.
Because the calibration feedback loop operates relatively slowly, in some embodiments, a microprocessor may be in the loop. For example, calibration feedback circuit 320 may include a processor or controller that executes instructions to adjust the power supplies in response to the measured light output.
The commanded drive value on node 402 is a digital word having M bits, and the digital word on node 102 has N bits, where M is greater than N. In operation, dither circuit 410 receives an M bit input word and produces an N bit output word by truncating the input. The truncated bits are added to the next input value and the process repeats. When each input value is truncated, a lesser value output (corresponding to a dimmer light output) results, but the truncated bits increase the value of a later output.
As an example, consider the case in which M=10 and N=4. The commanded drive value can range from zero to 1023, however there are only 16 possible output values: 0, 64, 128, 192, 256, 320, 384, 448, 512, 576, 640, 704, 768, 832, 896, and 960; corresponding to zero through 15 on the N-bit output. Input values from zero to 63 are truncated down to an output value of zero. Similarly, input values from 64 to 127 are truncated down to an output value of 64.
The difference between the input values and the truncated output values is referred to herein as the “residual.” The residual is added to the next input value. For example, if the input value is 522, the truncated output value is 512 and the residual of 10 is added to the next input value.
As another example, consider a consecutive string of input values of 48. The corresponding string of output values will be 0, 64, 64, 64, 0, 64, 64, 64, etc. The average output value is 48.
Because of the truncation, the output value cannot accommodate values over 960. In some embodiments, dither circuit 410 scales the input value down by 960/1023 so that the full range of input values is represented by the full range of possible output values. In other embodiments, dither circuit 410 includes limiting logic to limit the input value to the maximum truncated value, in this example, 960.
In some embodiments, dither circuit 410 employs a “look-ahead” feature that looks at future commanded drive values when producing the current output value. For example, a moving average filter may be employed prior to truncation. In other embodiments, a random number generator weighted by the residual is used to determine which of two adjacent output values will be generated. In still further embodiments, temporal dithering is employed. Any type of dithering may be used to reduce the M-bit commanded drive value to an N-bit drive value without departing from the scope of the present invention.
In the example dithering circuit of
Each tick on the horizontal axis represent one pixel time. When the input value is zero, the output value is also zero. When the input value is one, the output alternates between zero and one as the accumulator overflows. For this example where M=3 and N=2, odd input values cause the output to dither between two output values. In the simplified example of
In operation, image processing component 802 receives video data on node 801, receives a pixel clock from digital control component 890, and produces commanded drive values to drive the laser light sources when pixels are to be displayed. Image processing component 802 may include any suitable hardware and/or software useful to produce commanded drive values from video data. For example, image processing component 802 may include application specific integrated circuits (ASICs), one or more processors, or the like.
Laser light sources 810, 820, and 830 receive commanded drive values and produce light. Laser light sources 810, 820, and 830 may include any of the laser drive apparatus described herein. For example, laser light sources 810, 820, and 830 may include any of apparatus 100 (
Each light source produces a narrow beam of light which is directed to the MEMS mirror via guiding optics. For example, blue laser light source 830 produces blue light which is reflected off mirror 803 and is passed through mirrors 805 and 807; green laser light source 820 produces green light which is reflected off mirror 805 and is passed through mirror 807; and red laser light source 810 produces red light which is reflected off mirror 807. At 809, the red, green, and blue light are combined. The combined laser light is reflected off mirror 850 on its way to MEMS mirror 862. The MEMS mirror rotates on two axes in response to electrical stimuli received on node 893 from MEMS driver 892. After reflecting off MEMS mirror 862, the laser light bypasses mirror 850 to create an image at 880.
The image at 880 may include image artifacts that result from dithering within laser light sources 810, 820, and 830. For example, a faint denim-like pattern may appear when residuals occur with some spatial frequency. These artifacts are most likely to be visible within homogeneous regions of static video. In some embodiments the patterns are kept from moving by clearing the residual value at the end of every frame. This ensures that consecutive static frames are rendered identically. In some embodiments, the pattern are allowed to move from frame to frame. For example, in some embodiments, the residual is retained at the end of every frame. In further embodiments, the residual is randomized at the end of every frame. These artifacts can also be reduced by reducing the dither magnitude (e.g., dither to eight bits rather than six bits).
The MEMS based projector is described as an example application, and the various embodiments of the invention are not so limited. For example, the laser drive apparatus described herein may be used with other optical systems without departing from the scope of the present invention.
Scanning projector 800 may receive image data from any image source. For example, in some embodiments, scanning projector 800 includes memory that holds still images. In other embodiments, scanning projector 800 includes memory that includes video images. In still further embodiments, scanning projector 800 displays imagery received from external sources such as connectors, wireless interface 910, or the like.
Wireless interface 910 may include any wireless transmission and/or reception capabilities. For example, in some embodiments, wireless interface 910 includes a network interface card (NIC) capable of communicating over a wireless network. Also for example, in some embodiments, wireless interface 910 may include cellular telephone capabilities. In still further embodiments, wireless interface 910 may include a global positioning system (GPS) receiver. One skilled in the art will understand that wireless interface 910 may include any type of wireless communications capability without departing from the scope of the present invention.
Processor 920 may be any type of processor capable of communicating with the various components in mobile device 900. For example, processor 920 may be an embedded processor available from application specific integrated circuit (ASIC) vendors, or may be a commercially available microprocessor. In some embodiments, processor 920 provides image or video data to scanning projector 800. The image or video data may be retrieved from wireless interface 910 or may be derived from data retrieved from wireless interface 910. For example, through processor 920, scanning projector 800 may display images or video received directly from wireless interface 910. Also for example, processor 920 may provide overlays to add to images and/or video received from wireless interface 910, or may alter stored imagery based on data received from wireless interface 910 (e.g., modifying a map display in GPS embodiments in which wireless interface 910 provides location coordinates).
Mobile device 1000 includes scanning projector 800 to create an image with light at 880. Mobile device 1000 also includes many other types of circuitry; however, they are intentionally omitted from
Mobile device 1000 includes display 1010, keypad 1020, audio port 1002, control buttons 1004, card slot 1006, and audio/video (A/V) port 1008. None of these elements are essential. For example, mobile device 1000 may only include scanning projector 800 without any of display 1010, keypad 1020, audio port 1002, control buttons 1004, card slot 1006, or A/V port 1008. Some embodiments include a subset of these elements. For example, an accessory projector product may include scanning projector 800, control buttons 1004 and A/V port 1008.
Display 1010 may be any type of display. For example, in some embodiments, display 1010 includes a liquid crystal display (LCD) screen. Display 1010 may always display the same content projected at 880 or different content. For example, an accessory projector product may always display the same content, whereas a mobile phone embodiment may project one type of content at 880 while display different content on display 1010. Keypad 1020 may be a phone keypad or any other type of keypad.
A/V port 1008 accepts and/or transmits video and/or audio signals. For example, A/V port 1008 may be a digital port that accepts a cable suitable to carry digital audio and video data. Further, A/V port 1008 may include RCA jacks to accept composite inputs. Still further, A/V port 1008 may include a VGA connector to accept analog video signals. In some embodiments, mobile device 1000 may be tethered to an external signal source through A/V port 1008, and mobile device 1000 may project content accepted through A/V port 1008. In other embodiments, mobile device 1000 may be an originator of content, and A/V port 1008 is used to transmit content to a different device.
Audio port 1002 provides audio signals. For example, in some embodiments, mobile device 1000 is a media player that can store and play audio and video. In these embodiments, the video may be projected at 880 and the audio may be output at audio port 1002. In other embodiments, mobile device 1000 may be an accessory projector that receives audio and video at A/V port 1008. In these embodiments, mobile device 1000 may project the video content at 880, and output the audio content at audio port 1002.
Mobile device 1000 also includes card slot 1006. In some embodiments, a memory card inserted in card slot 1006 may provide a source for audio to be output at audio port 1002 and/or video data to be projected at 880. Card slot 1006 may receive any type of solid state memory device, including for example, Multimedia Memory Cards (MMCs), Memory Stick DUOS, secure digital (SD) memory cards, and Smart Media cards. The foregoing list is meant to be exemplary, and not exhaustive.
Method 1100 is shown beginning with block 1110 in which a digital value is dithered to represent a digital word having more bits. For example, a six bit output value may be dithered to represent a ten bit input value. Any of the dithering embodiments described above may be used to dither the digital value.
At 1120, a digital value is converted to an analog laser drive value. This corresponds to the operation of DAC 110 (
At 1130, a plurality of ramp signals are generated. For example, in some embodiments, two ramp signals are generated as shown in
At 1140, a plurality of comparators responsive to the analog laser drive signal and the plurality of ramp signals are alternately enabled. For example, referring now back to
At 1150, a laser light source is driven by a plurality of transistors responsive to the plurality of comparators. When the plurality of comparators are alternately enabled, then the plurality of transistors alternately drive the laser light source. This is shown for two comparators in
At 1160, a calibration pulse is sent. In some embodiments, calibration pulses are sent during inactive video periods in a scanning laser projector. For example, calibration pulses may be sent at the end of video frames. Calibration pulses may always have the same drive values, or may have varying drive values.
At 1170, light resulting from the calibration pulse is measured. Referring back to
Although the present invention has been described in conjunction with certain embodiments, it is to be understood that modifications and variations may be resorted to without departing from the scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims.
