1. Field of the Invention
The present invention relates to portable personal computers and, more particularly, to portable personal computers having a fluorescent backlit liquid crystal display (LCD) wherein the backlight flicker is reduced by synchronizing the backlight with the display refresh signal.
2. Description of the Prior Art
Various display technologies have been provided on portable personal computers. For example, cathode ray tube (CRT), LCD, gas plasma and electroluminescent display (EL) technologies are known. Early portable personal computers, such as illustrated in U.S. Pat. No. 4,669,053, assigned to the same assignee as the assignee of the present invention, were provided with CRTs. However, the use of a CRT in a portable personal computer results in a relatively large cabinet size as compared to today's briefcase size portable personal computers.
Gas plasma displays in portable personal computers are also known. These displays are formed from a matrix of electrodes separated by a gas such as neon. By applying a voltage to an addressed matrix intersection, the gas is excited and emits an orange-red light. Even though gas plasma d require no backlighting, they consume relatively more power than other display technologies, such as a backlit LCD. Thus, such gas plasma displays place a relatively large drain on the battery driving the portable personal computer.
Another type of display used on portable personal computers is an EL display. Typical EL displays consist of a thin film of an electroluminescent material, such as phosphor, sandwiched between thin films of an insulating dielectric material, disposed adjacent a matrix of electrodes. When an appropriate voltage is applied to an addressed matrix intersection, the phosphors emit light. However, EL displays require alternating current (AC) and consume relatively more power than an LCD. Since EL displays require AC, they are unsuitable for truly portable personal computers, which are not required to be tethered to an external source of AC.
LCD displays are used on various portable personal computers. LCDs offer many advantages, over other display technologies, such as low power consumption and small size. Typical LCD displays are formed from liquid crystals sandwiched between two optical polarizers. Some known personal computers are provided with reflective type LCD displays. These reflective LCD displays do not produce light, but rather depend on ambient light. Consequently, in conditions of low ambient light, these displays provide low contrast and poor readability. In order to resolve this problem, some LCD displays are now backlit to obviate the dependence of the display on ambient light. Different types of light sources are used for backlighting the LCD display, such as high intensity incandescent light bulbs and high intensity fluorescent lights. The fluorescent backlights require the same amount of power as a high intensity incandescent bulb and last twice as long. Moreover, the fluorescent backlights have almost double the intensity of an incandescent light, thus resulting in a 20 to 1 contrast ratio.
Such fluorescent backlit LCD displays are known to be driven by, for example, a square wave developed from the battery supplying power to the computer. One problem with the fluorescent backlit LCD screen is noticeable flicker. The flicker results from the square wave beating with the display refresh signal. In some known backlit LCDs, the display is refreshed at 70 Hz resulting in the display being completely rewritten 70 times a second. In order to eliminate or reduce the flicker, the backlight has to be operated at substantially the same frequency or at harmonic frequencies of the display refresh signal. If the backlight is operated at the same frequency as the display refresh signal, a relatively close tolerance must be maintained on the frequency. For example, there will be no noticeable flicker on a 70 Hz display when the backlight is also operated at 70 Hz. However, a flicker may be noticeable at 71 Hz. Since the flicker is more noticeable at lower frequencies, the frequency of the backlight is generally driven at a harmonic frequency of the display refresh signal, such as 420 Hz.
The signal for the backlight has heretofore been developed by an oscillator, for example, a tank circuit. However, the problem with using such a tank circuit is that the frequency of the oscillator is greatly affected by tolerances in resistors and capacitors used in the oscillator. Thus, it is difficult to rather accurately control the frequency of the backlight signal to eliminate the flicker. Moreover, some known portable personal computers are provided with a backlight signal having an adjustable duty cycle to reduce battery power consumption. The flicker problem becomes much more apparent at such lower duty cycles.