Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
2001-05-16
2004-04-20
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Display driving control circuitry
C327S109000, C372S038070, C340S815450
Reexamination Certificate
active
06724376
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-143530, filed May 16, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an LED intensity modulation type driving circuit for controlling emission
on-emission of an LED output in accordance with the high/low level of an input voltage pulse and, more particularly, to an LED driving circuit capable of outputting a high-speed modulated optical signal almost free from an emission pulse waveform distortion that is inevitable in optical data transmission of a high-speed optical data link.
Multimedia data, now widely-used and being increasingly developed, is typically exchanged over a variety of high-speed optical network backbones throughout the world. To be exchanged, optical transmitting and receiving modules play an important role as key components in optical communication systems of long-distance LAN connections, and short to mid-distance LANs supported by fiber-optic channels and Gigabit Ethernet. Purpose-built optical transmitting and receiving modules used in IT (Information Technology) systems currently in use have been developed at a cost, sacrificing versatility at the same time.
Recently, demands have arisen for a wide application range of optical interconnection techniques without limiting them to special applications such as high-performance optical communication and connection between computers, particularly server devices, in order to ensure connectability over long distances and high throughput even for exchange of data between multimedia devices such as home appliances, and to provide even end users with ease of use.
To meet these demands, IEEE 1394a extended from the electrical specification IEEE 1394 standardized in 1995 is further extended to optical applications, promoting standardization of IEEE 1394b which targets an optical data link using a POF (Plastic Optical Fiber) and is applicable to high-speed, low-cost, medium-distance connection. In the future IT field in a broad sense, practical performance is important as an interconnect requirements specification in addition to specifications which define basic transmission performance such as high throughput, regardless of the optical or electrical transmission signal form.
Strong electrical demands have arisen in terms of system mountability so as to realize low power consumption without cooling, have the same electrical interface as that of another IC used in an IT device, and if possible, obtain characteristics which allow operation at the same power supply voltage. Much lower cost than a conventional optical transmitting module is required in terms of cost performance.
The standard draft IEEE 1394b under examination adopts a gradient index plastic optical fiber having a large core system in order to reduce the cost of an optical fiber for use, simplify the internal structure, and reduce the cost of an optical link module itself. An example of this plastic optical fiber is combined with a red light source which falls within the low-loss wavelength region of the fiber.
The light source of the red wavelength region is a light-emitting diode (LED) which emits light in a wavelength region around 650 nm, or an optical semiconductor laser (LD) which oscillates at 650 nm. Of these light sources, the LD must be employed in terms of essential element response characteristics in high-speed S
800
or more which is considered to be the mainstream of the optical data link as the technique to be established in the near future.
On the other hand, low-speed standards S
100
to S
400
will mainly adopt LEDs which can simplify the circuit arrangement and optical coupling system of an optical transmitter that are important factors for reducing the module cost.
In fact, products using LEDs as light sources are commercially available and widely used in an optical data link of several ten Mb/s or less that targets audio and FA systems.
FIG. 2
is a basic block diagram showing a conventional optical transmitting circuit.
As shown in
FIG. 2
, a constant current pulse prepared by ON/OFF-modulating, using a transistor switch, a DC current generated by a constant current source is generated, and the output is applied to a load LED. This method does not pose any technical problems in a low-speed link, up to about 10 Mb/s. In general, however, the switch response characteristic of an optical signal is low due to a large internal capacitance of a device that is a property unique to the LED. The low response characteristic determines the optical data link speed.
As one effective solution for relaxing this constraint, a peaking pulse current in phase with a constant current pulse is superimposed on an ON/OFF-modulated driving constant current pulse in synchronism with level inversion of the driving current switch, thereby accelerating the transient response of the LED.
If the means for compensating for and accelerating low-speed response characteristic is added, the original transmission data rate is as low as several tens of Mb/s, and the essential time constant which determines the device speed unique to the LED is as short as several ns or less. In addition, the signal processing speeds of photoelectric conversion and logic level conversion after optical transmission are substantially negligible with the development of the IC process technique. Optical transmission can be realized in which the signal error rate in data transmission is suppressed to be low so as not to cause any practical problems.
However, if optical transmission in which the transmission bit rate is increased to 100 Mb/s or more is to be realized by an LED along with the recent demands described above, the conventional method cannot be simply extended or applied.
As shown in
FIG. 1
, one of the present inventors has proposed a method capable of transmitting data at around 100 Mb/s by adopting the principle of current peaking driving, devising peaking superimposition, and decreasing the LED driving amplitude.
More specifically, a current bias required for an LED to generate an ON light intensity is always supplied from a DC constant current source to the LED anode. A CMOS buffer converts an external pulse signal input Vp into a rectangular pulse which changes at a low impedance between power supply voltages Vdd and Vss. A bias current input to the LED anode is ON/OFF-modulated by using the pulse. At the same time, a differential current flowing through a capacitance Cp is superimposed and supplied as a peaking current to the LED anode.
A method using an inductor instead of current peaking using a capacitance has also been proposed. The circuit shown in
FIG. 1
that is driven by a conventional constant current switch is used as a main arrangement. A circuit constituted by series-connecting an inductor and resistor is parallel-connected to an LED, and they are driven as the entire load of the current output in place of using only the LED as a load. That is, a peaking current is generated by the inductor in transition and supplied to the LED.
Adding the means for superimposing the peaking current can shorten the transient response time of an output optical signal to some extent. However, effective peaking superimposition inhibits a peaking pulse from completing attenuation within the signal pulse width because of the time constant. A pulse tail or the like is generated, and the bit rate cannot be so increased. Further, the output optical pulse width essentially becomes smaller than the driving pulse width of an electrical signal.
The optical pulse width decreases by 1 ns or more in general, and in some cases by 10 ns, which depends on the LED driving circuit method. These values cannot be ignored when the minimum pulse width of a transmission optical signal is 10 ns or less. The decrease causes great variations in the ON/OFF pulse duty ratio of the transmission signal or additional increase in time jitter, seriously influencing the t
Kaminishi Katsuji
Sakura Shigeyuki
Bell Paul A
Frommer & Lawrence & Haug LLP
Kabushiki Kaisha Toshiba
Saras Steven
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