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Volume 74 (Dec 26, 2007)

The laser control core (pulse-width modulator) behind the development of high-speed, high-definition printers

Along with the demand for fast and beautiful printing of business documents and pictures taken with digital cameras, there are mounting needs for high-resolution color laser printers in a wide spectrum of areas ranging from business to personal use. NEC Electronics has developed a laser control core (pulse-width modulator, or PWM) for cell-based IC use that is essential to further increasing the speed and definition of printers and copy machines. Here we will introduce the capabilities of this laser control core as well as the advantages of its implementation at the time of printer development.

How laser printers work and the task of achieving higher definition

Here is a simplified explanation of how a laser printer works. First, a laser beam scans (*1) a positively charged photoconductive drum in accordance with the image data. The electrical charge of the scanned area is then removed and toner is adhered to it. Next, the toner is transferred and set onto the paper, and finally the document is printed (Figure 1). Unlike inkjet printers, which are immensely popular for household use, laser printers are capable of high-speed, high-definition printing.

The speed of a laser printer—its primary feature—is determined by the scanning rate and printing method used. The "tandem method" is on its way to becoming the predominant printing method for high-speed models. This method involves the use of four CMYK (*2) photoconductive drums and four light sources, and makes it possible for all four colors to be scanned and transferred at once. To achieve further improvements in speed, the possibility of increasing the number of beams from four channels (four beams) to eight channels and eventually to 32 channels is currently under consideration.

The key to achieving high definition—another important laser printer feature—is laser beam control. During irradiation of the laser beam onto the photoconductive drum, the laser is turned on and off in accordance with the binary information of the pixel data and microscopic spots are formed on the photoconductive drum. The resolution of each pixel is determined by how finely the width and position of the microscopic spot can be controlled. The ideal is for the laser to be reflected by a polygon mirror that moves at a constant angular velocity and to pass through the fθ lens (*3) so that it can be irradiated onto the photoconductive drum in equally spaced intervals (Figure 2). However, with the tandem method, equally spaced intervals are quite difficult to attain due to the effects of fθ lens characteristics such as dispersion between each of the CMYK units, thus causing subtle color drift. To combat this, use of a technology capable of eliminating subtle color drift by changing the frequency and controlling the on and off timing of the laser beam is effective.

PWM: Enabling authentic reproduction of images through printing

The key to microscopic spot positioning, width control and on and off timing control for laser beams is a PWM technology. With PWM, high-definition printing of images with authentically reproduced shading and edges is made possible by the on and off timing of the laser, in other words, the fine-tuning of width and positioning within each pixel. Moreover, by incorporating a frequency variable function into the PWM, lens and mechanically induced distortion can be corrected, thereby making it possible to come close to achieving ideal conditions (Figures 3-1, 3-2, 3-3).

However, since PWM chips are rather expensive, they have been primarily utilized for intermediate-grade models for business use and had not made much headway in terms of being incorporated into popular models. NEC Electronics has been engaged in the development of low-cost, high-precision PWM since 2002. And by converting PWM to IP as a laser control core that can be used in cell-based ICs, we have succeeded in making substantial cost and performance improvements.

NEC Electronics' PWM core makes 256 gradation and high definition possible through 8-bit pulse-width resolution; it also achieves an increased scanning rate through a pulse output rate of 105 MHz. Add this to the fact that it comes equipped with a frequency variable function effective in correcting lens and mechanical distortion, and what you've got is a PWM core with basic capabilities that is perfect for use in printers and copy machines. As for external synchronizing signals, a synchronizing accuracy to within 1/16 of a pixel is guaranteed (Table 1). This low-price, high-performance and low power consuming PWM core has opened up a myriad of possibilities for equipping intermediate-level and lower models developed for household use with PWM functions.

Successful on-chip transformation of a PWM core without performance degradation

However, the on-chip transformation of the PWM core was by no means an easy undertaking. This is because performance degradation (for example, with operating speed and precision) can occur due to the effects of noise and other things when on-chip transformation of a PWM core is carried out. NEC Electronics successfully developed its own high-speed, high-precision technology to resolve this problem. We were able to achieve this with a PWM core for 150-nanometer (nm) cell-based CB-12 IC use that does not experience performance degradation even when on-chip transformation is carried out. In the future, we will expand this to 90 nm cell-based CB-90 IC and under fine processes. Furthermore, to meet the set configuration needs of customers for high-speed printers using multiple PWM cores, we will strive to offer a broad array of PWM cores for not only cell-based IC use but also for standard product and system-in-package (SIP) product use.

For evaluation purposes, we offer a "laser control core evaluation board" that can be connected to a computer via a USB cable to freely set up all parameters on a GUI base as well as easily confirm the printing of print data. This enables customers to check system compatibility by simply inputting the external synchronizing signal and then receiving the LVDS or LVTTL output. It also allows for smooth introduction of our PWM core at the time of product development.

NEC Electronics' PWM core will continue to be an integral part of the development of printers as they become increasingly sophisticated and gain in popularity.

Notes(*)

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