1. Field of the Invention
The invention relates to line driver circuits and, more particularly, to imaging devices having a fast-settling line driver for efficient high-resolution operation.
2. Description of the Related Art
Imaging systems, especially those in the visible and infrared bands, have many modern applications in various fields including personal digital photography, astronomy, medical imagery, surveillance, security and military target acquisition. Such systems are based on image sensors that convert light into an electrical signal. Light sensing visible imager sensors have become increasingly popular in recent years, especially in digital still cameras and video camcorders. Their popularity has been fueled by the development and availability of new cost-effective image sensor technologies.
Charge-coupled devices (CCD) and complimentary metal-oxide semiconductor (CMOS) are two widely used technologies for fabricating image sensors.
CCDs are an integrated circuit with an array of light-sensitive capacitors that are linked or coupled together. CCD signals require special off-chip processing, which increases the cost of a CCD imaging system.
CMOS image sensors are devices that use complimentary and symmetrical pairs of n-type and p-type field-effect transistors to perform basic logic functions. CMOS technology is used to create microprocessors, microcontrollers, static memory, data converters, amplifiers and other digital and analog circuits. One type of image sensor that can be made using normal CMOS processes is an active pixel sensor (APS). APS imagers include an array of pixels each of which comprise a photodiode to collect the input signal and multiple transistors to buffer and amplify the signal for output.
Although CCD devices currently offer a superior dynamic range, CMOS devices are more cost-effective because they use standard semiconductor processes and offer higher levels of integration on a single chip. Due to advantages of CMOS image sensors, they are more desirable for consumer products.
As the market demand for high resolution imaging products continues to increase, so do efforts to maximize the number of pixels that can be built onto a small chip area. The pixels require some circuitry for transducing and transmitting a signal to other components for processing. This circuitry includes transistors for amplifying and switching signals for line transmission. Typically the signals travel along a wire called a bus to the signal processing components.
One component that is of particular interest to circuit designers is the line driver. A line driver is an amplifier circuit that is used to improve the reliability of a transmission line by driving the input from the pixels onto the line, often called a bus, where they are then sent to other components for processing. Substantial effort has been devoted to improving the quality of line driver circuits, especially in the field of video imaging. As the demand for high definition video increases, video imagers require faster line driver circuits to accommodate large arrays of pixels (e.g., 1920×1080) with a typical 30 Hz or 60 Hz frame rate. Faster line drivers tend to dissipate large amounts of power and can drain portable power sources such as batteries quickly.
A known line driver circuit 100 is shown in FIG. 1. The input signal Vin is connected to the positive terminal of amplifier 102. The output of the amplifier is fed back into the negative input terminal of the amplifier, creating a negative feedback loop that stabilizes the amplifier 102. The output of the amplifier 102 is connected to a bus line 104 through switch 106. The operation of line drivers is discussed in more detail below.
One approach that has been taken to reduce power consumption is to use multiple buses to send data from the pixels to the processing circuitry. A multiple bus architecture allows the pixel signals to be transmitted in parallel, reducing the operating speed of the line drivers. For example, if a dual-bus configuration is used, each of the line drivers along the buses can operate at one half the speed that would be required in a single bus configuration. Although using multiple buses has been effective, it is not practical to simply add more buses to an imager. Buses take up valuable space on a chip, adding significant production costs. They also add complexity to the design thus increasing design verification cost.
As the number of pixels increases, the length of the buses and the number of switches per bus must also increase. One byproduct of longer buses and more switches is an increase in the settling time of the individual line drivers, meaning that the signal at the line driver takes longer to approach the target input voltage. This can be problematic if the line drivers must accommodate a high bit resolution such as 12 bits where the difference between the value the line drive outputs in the given time period and the value it would eventually settle to if a much larger amount of time was available (i.e., the target voltage) must be small when the signal is transmitted onto the bus.
FIG. 2 depicts a model circuit 200 that is electrically analogous to the line driver circuit 100. In circuit 200, gm is the transconductance of the amplifier 102, RSW is the resistance of the switch 106, RLINE is the series resistance of the bus line 104 and CLINE is the capacitance of the bus line 104 (which includes the capacitance of the switches connected to the line). VOUT is the output voltage that is delivered to the bus line 104. The settling time of a circuit is proportional to the RC time constant (τ), a well-known parameter of resistive and capacitive circuits. For 10-bit settling, the settling time is 6.9×τ. While for 12-bit settling the settling time is 8.4×τ. Referring to circuit 200, the RC time constant may be calculated according to the following equation:
  τ  =      RC    ≈                  (                              1                          g              m                                +                      R            SW                    +                      R            LINE                          )            ⁢              C        LINE            Given the configuration of line driver circuit 100, the resistance of the switch RSW usually dominates the time constant, and thus the settling time. One reason for Rsw limiting τ is that, unlike other parameters, Rsw cannot be independently reduced. To reduce Rsw the switch needs to be made wider which increases CLINE.
Several solutions have been used to reduce the settling time of line drivers in high resolution imagers. As mentioned above, the settling time of typical line drivers in these large arrays, is determined mostly by the resistance of the switch 106 (FIG. 1) connecting the amplifier to the bus. The settling time may be decreased by using a larger switch that has less resistance; however, the larger switch also increases the load and the capacitance on the bus. Another solution is to increase the power of the line driver. This method is also insufficient because the adjustment only has a sub-linear effect (i.e., it only affects that gm term) as the switch resistance eventually dominates, keeping the settling time large.
Thus, there is a need for a fast-settling line driver that is capable of operating in high resolution imaging systems.