Home >stn display > LED driver circuit the pursuit of high technology upgrade
LED driver circuit the pursuit of high technology upgrade
2011-12-12 by seoer1

Drive circuit is LED (light emitting diodes) products, an important part of the maturity of the technology market is expanding with the LED gradually increased. In terms of lighting, backlighting or the display panel area, drive circuit and the technical architecture of choice should match the specific application.

As LCD supplier (liquid crystal display) backlight, LED's position in the portable product can not be shaken, even in large-size STN display backlight which, LED also began to challenge the CCFL (cold cathode fluorescent lamp) of the mainstream; and in the lighting field, LED lighting as the most critical semiconductor components, but also because braved energy saving, environmental protection, long life, maintenance-free, and many ring and sought after by the market. However, people may have concerns in the dazzling "star" behind, the number of "supporting role" in the quiet dedication? LED driver circuit is such an "unsung heroes."

Escalation of drive technology

LED light-emitting mechanism is that it ends with a positive voltage, the semiconductor in the minority carrier and majority carrier complex occurs, release excess energy, which led to the photon emission. LED driver circuit's main function is to convert AC voltage constant current power supply, and complete in accordance with the requirements of LED devices and LED voltage and current match. The white LED, for example, takes about 3.6 volt white LED supply voltage to achieve the proper brightness control. However, most portable electronic products using lithium-ion batteries for power, they are fully charged after about 4.2 volts, security is discharged after about 2.8 volts, the white LED is clearly not directly driven by the battery, and thus need to use the boost circuit throughout the battery cycle uninterrupted and stable power supply for the LED.

In addition, many product designers want the LED brightness in different applications can make the appropriate changes, this task will be to drive the circuit to complete. PWM (Pulse Width Modulation) technology is a traditional way of dimming, which uses a simple digital pulse, repeated switching LED driver, the system only needs to provide wide and narrow pulses of different digital, you can simply change the output current to achieve, to adjust the LED brightness. The technology is its ability to provide high-quality white light, and the application of simple, high efficiency, but a fatal drawback is prone to electromagnetic interference, and sometimes even have the human ear can hear the noise. Beijing Best Electronic Technology Co., Ltd. Chief Executive Officer Peishi Yan Wang introduced the digital dimming technology, the use of the technology in real-time control of LED brightness, and to avoid electromagnetic interference.

Upon completion of "own work" at the same time, some manufacturers in the LED driver circuit is also integrated some additional features, such as audio amplifier. Resources Powtech (Shanghai) Co., Ltd. Hui Ling, General Manager, said there are already many companies engaged in PMU (Power Management Unit) design, that is as much as possible the number of analog functions in a single chip, such a design can savings in PCB (printed circuit board) space, improve reliability, reduce costs, has made some contributions, but the need to take this approach, ultimately depends on the product applications.

System to decide the design

As mentioned earlier, boost LED driver circuit is an important task, and the inductor boost and boost charge pump topology are two different modes. "As the LED is driven by the current, and the inductor current during the maximum conversion efficiency, so the inductive boost mode of the biggest advantages is the high efficiency, if properly designed, can be more than 90%; but its disadvantages are also obvious, that is, electromagnetic interference strong, mobile phones and other communications products for the system requirements are very high charge pump with the emergence of many cell phones are no longer using inductive boost mode, of course, the step-up mode charge pump efficiency will be lower than the inductor step-up. "Ling Hui briefly enumerate the merits of the two step approach.

Charge pump and inductor-based converters in switch mode, when the input, produce output voltage and current ripple.
"Sometimes, ripple and noise can couple into other sensitive circuit (such as mobile phone RF transceiver), affect product performance. Input ripple effect is relatively more significant, because the battery power cord is common in the system, when the switch the same frequency, drive the same output load, using the same input capacitance, the charge pump circuit architecture and the input inductor ripple in the same order of magnitude, increasing the input capacitance helps to reduce input ripple. "Maxim's portable communication device power TonyLai product line director, explains, "the output ripple is also a factor, especially when the lead is long. like a long wire antenna, able to introduce more noise into the circuit, due to inductive boost circuit requires only two leads, may be more suitable for this application. When the LED drive circuit and the placement of very close, the charge pump is recommended. In addition, inductive boost circuit inductor and Schottky diode junctions have a higher rate of switching waveform, if the switching node to add a small capacitor, slowing the switching signal rising / falling, will help to reduce electromagnetic interference, but will affect the efficiency. "can be seen, the final step-up approach and design choices are often based on system requirements In after comprehensive consideration of various technical indicators to reach a compromise.

Efficiency is the eternal theme

Whether or backlighting applications in the field of lighting applications, to improve the conversion efficiency of the drive circuit is the product designer must face. Improve the conversion efficiency will not only help to extend the standby time of portable products, but also an important problem to solve the LED heat dissipation means. In the lighting field, the use of high-power LED, thereby increasing the conversion efficiency is especially important.

About the results in improving efficiency, Peidan Yan said: "The thinking eMemory charge pump boost mode choose two multiplication factors: 1 and 1.5 times when the voltage between 3.8V and 4.2V, the chip automatically 1 times selected mode, because this voltage is sufficient to drive white LED; and when the voltage drops below 1.5V, the chip will automatically switch to 1.5-fold pattern, in this mode, even if the voltage drops to 2.8V, LED is still able to work so the hybrid model in improving the conversion efficiency, but also possible to extend battery life. "

Hangzhou Silan Microelectronics Co., Ltd. Jian-Xing Wu, deputy director of the Institute said, only from the technical point of view, to improve the conversion efficiency is not complicated, but the device itself to be bound by the external conditions, such as size limits, that is to be limited space as much as possible to improve efficiency, which is the challenge faced by designers. "In lighting applications, LED device itself, to improve the photoelectric conversion efficiency will be more meaningful."

  keywords:stn display