Optoelectronics with White LEDs for Backlighting Considerations ON Semiconductor BernieWeir笮 In the past few years, small color screens have changed from rare items to mainstream features in portable consumer electronics such as cell phones, PDAs and digital cameras. One of the challenges of integrating color screens into small portable devices is the need for white light as a backlight. For large displays such as laptops, cold cathode fluorescent lamps (CCFLs) are commonly used, but due to their size limitations and design complexity, they are difficult to use for smaller applications. The white LED has excellent optical properties, making it the solution of choice for small display backlight displays. In addition, white LEDs can also be used as backlights for keyboards. Matching the white LED to the light guide provides a uniform white backlight.
Challenges for White LED Solutions Designers face the challenge of integrating white LEDs in the following ways.
LEDs have unique electrical characteristics, and most portable products are now powered by a single-cell Li-Ion battery. The forward voltage (Vf) of a white LED is a function of the current through the device and its own material properties. But in general, Vf is between 3.23.8V (), while the output voltage of lithium-ion battery is between 3.0 4.2V, so it can meet the electrical characteristics of LED. Therefore, using a LDO that is directly connected to the battery to drive the color LED and a series resistor to regulate the drive current, this simple solution for color LEDs is not suitable for white LEDs. The challenge is, in most cases, Several white LEDs are required. A small display for a mobile phone may require three or four white LEDs, while a larger display for a PDA can use six to eight white LEDs*. The increase in the number of LEDs increases the complexity of the design.
Another problem with backlit displays is that the LEDs need to provide uniform illumination and no bright spots or dark spots on the display. This is common in monochrome LCD displays driven by conventional color LEDs, and because the display is primarily used to display alphanumeric characters, it is an acceptable solution. But for browsing digital images, playing complex games, and browsing the web, this is no longer a useful diode. When the diode is turned on, light is emitted, and the intensity of the light is proportional to the current through the diode. . There are two ways to achieve an approximate match between the illuminance of multiple LEDs. One is to configure the diodes on each branch in parallel with the regulated source, and the other is to arrange all the diodes in series (see) so that the current through each diode is the same. The parallel configuration is biased towards a steady current source rather than a resistor because the forward voltage Vf of the diode can vary by up to 20% even for the same bias conditions, which has a direct effect on the light intensity, making it visible to the user. To. The series configuration eliminates this problem, but requires a high-voltage boost converter (such as the NCP5007). Since 4 to 5 diodes are connected in series, the rated Vf of the series white LED can easily reach 1520V. The specific implementation method is given. Circuit diagram.
In this case, all five diodes are arranged in series, so the current through each device is the same, and the current in the switching regulator can be adjusted by detecting the voltage drop across resistor R1. In addition, by applying a pulse width modulation (PWM) signal to the enabled input, the average current flowing through the diode string can be varied under software control to provide a backlight that is adjustable in brightness.
When the parallel configuration method is adopted, the main characteristic effects of the hot-swappable LD of Table 1 can be used to make the spectrum and the light output characteristics unstable. Therefore, certain reflection measures should be taken for the reflected light, especially the reflected light from the end face of the fiber near the LD, usually the fiber end of the LD side is ground into a fiber end (FiberStub).
The LD is completely inserted into the SFF/SFP transceiver, and the required length is generally 14 coffee. If the plug connection portion of the LD and other portions such as the LD head and the fiber end are removed, the distance from the LD end face to the spot on the fiber edge of the fiber end is 2.9 mm or less. This length is only 1/2 of the fiber-wound coaxial LD ​​module. Generally, the shorter the focal length, the smaller the lens diameter. The lens is preferably a high refractive index material. In one aspect, the smaller the lens diameter, the higher the price of the high refractive index glass material used. Therefore, the performance and cost factors should be considered comprehensively. Among the new products introduced by NEC, the lens used has a lens diameter of 1.8 mm and a refractive index of the lens material of n = 1.78.
Plug-in LD performance The LD module has excellent performance due to careful consideration in device selection, optical path design, and package manufacturing (see Table 1).
Temperature Characteristics Typical IL (Drive Current and Light Output) characteristics of the LD module are shown. Even at a high temperature of 85C, a good temperature characteristic can be obtained with a driving current of 42 mA. Moreover, the tracking error in the -40C85C range can be controlled within *1.0dB.
The plug-in repeatability is high. The LD module can be directly connected to the LC connector. After repeated insertion and removal for 100 times, the characteristic variation is kept within 0.3 dB, indicating that the repeatability is very good.
High reliability As a MAN/LAN optical communication device, the LD module is suitable for various applications such as SONET/SDH and GbpsEthernet, and its reliability is tested by TelcordisGR-468CORE. Through a large number of tests, it can be said that the LD can still work normally after 3000 hours of use, and it performs 1000 temperature cycle tests in the range of -40C85C, and the fiber output change is kept within 10% of the soil. These tests have proved that the optical connection performance of LD is very stable and reliable.
The small-capacity, reliable and reliable hot-swappable LD is well adapted to the application requirements of various network optical communication systems such as MAN and LAN.
(Wang Wei compiled) è±³ (Continued from page 20) Low-voltage inductor boost or charge pump. The main advantage of the parallel configuration is that it can use a low voltage process, but the disadvantage is that each diode requires a separate current source pin, increasing the number of pins in the package and the corresponding PCB pad. In addition, these signals need to be routed to the display module through the connector. Care must also be taken when designing integrated circuits to achieve close matching of each current source.
Summary Many first-generation handset designs use a parallel diode configuration based on a capacitive charge pump scheme, either fractional (1.5X), multiple (2X), or hybrid (1.5/2X) architecture. This is attractive to designers because external capacitors require less space than inductors require. However, the overall system efficiency of this method is low, only between 47% and 80% (depending on the voltage of the lithium-ion battery). As a point, the inductor-based solution can easily achieve 75% 85% efficiency over the same operating voltage range. The effect of efficiency differences is a function of how the product display is used. For example, the PDA display usage is relatively high compared to standard digital phones that only light up when the call is placed and answered. Of course, the feature-rich update phone also includes a camera and gaming features, which also increases the use of the display, thus increasing the importance of a highly efficient backlight solution.
As with all engineering decisions, designers face a range of alternative solutions that must be analyzed and modeled. A number of factors must be considered, such as PCB space, system efficiency, battery capacity, component count, mechanical considerations, customer acceptance, and the cost of using the best solution. Advanced digital consumer products include new features that will further increase the adoption of color LCD displays and display
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