LED drive circuit: close to the pursuit of efficient application

Driving circuit is an important part of LED (light-emitting diode) products, and its technical maturity is gradually increasing with the expansion of the LED market. In the field of illumination, backlight and display panel, the choice of the technical framework of the driving circuit should be matched with the specific application.

As LCD (liquid crystal display) backlight, unshakable status of LED in portable devices, even in large size LCD backlight, LED also began to challenge the CCFL (cold cathode fluorescent lamp) of the mainstream status; and in the field of lighting, LED semiconductor lighting as the most important components, it is by the market because the top pursuit of energy saving and environmental protection, long life, maintenance free and many halo. However, people have been concerned, in the glory of the "Star" behind, how many "supporting role" in silent dedication? LED driver circuit is such a hero behind the scenes".

Drive technology gradually upgrade

The luminescence mechanism of LED is that the forward voltage is applied at both ends, so that the minority carriers and the majority carriers in the semiconductor are complex, and the excess energy is released. The main function of the LED drive circuit is to convert the AC voltage to the constant current power supply, and at the same time, according to the requirements of the LED device, the voltage and current of the LED are matched. White LED, for example, white LED requires about 3.6 volts supply voltage in order to achieve the appropriate brightness control. However, most of the portable electronic products are used in lithium ion batteries for power supply, they are in full power after approximately 4.2 volts, the safety after the power is about 2.8 volts, apparently not by direct drive white LED battery, therefore need to use circuit in the entire battery life cycle uninterrupted LED stable power supply.

In addition, many product designers hope that the brightness of the LED can be changed in different applications, the task will also be given to the driver circuit to complete. PWM (pulse width modulation) technology is a kind of traditional dimming mode, it uses a simple digital pulse, repeatedly switch LED drive system only needs to provide the wide and narrow different digital pulse, easily realizing the change of output current, to adjust the LED brightness. The advantage of this technology is that it can provide high quality white light, as well as the application is simple, high efficiency, but a fatal drawback is easy to produce electromagnetic interference, and sometimes even produce people can hear the noise. Beijing Wang Wang Electronics Technology Co., Ltd. CEO Pei Shiyan briefed reporters on the digital dimming technology, the use of the technology can be real-time control of LED brightness, and to avoid electromagnetic interference.

In the completion of the work of the same time, some manufacturers of LED drive circuit is also integrated with a number of additional features, such as audio amplifier. Huarun powtech (Shanghai) Limited company general manager Ling Hui "China electronic newspaper" in an interview with reporters, there are many companies engaged in PMU (power management unit) design, which is as much as possible to simulate function integration on a single chip, this design can save PCB (the printed circuit board) space, improve reliability and reduce the cost of making some contributions and so on, but whether you need to take this way, ultimately depends on the needs of product application.

System requirements determine the design

As mentioned earlier, the boost is an important task of the LED drive circuit, and the inductor boost and charge pump boost are two different topologies. "Because LED is driven by the current, and the inductance in the current conversion efficiency is highest, it is the biggest advantages of inductive boost mode with high efficiency, if properly designed can be more than 90%; but its disadvantages are also obvious, is a very strong electromagnetic interference, the system requirements for mobile phone and other communication products is very high. With the advent of the charge pump, the majority of mobile phones are no longer using inductance boost mode. Of course, the use of charge pump boost efficiency will be lower than the inductor boost. Ling Hui briefly to reporters cited the advantages and disadvantages of the two ways to boost.

The charge pump and the inductor based converter operate in the switching mode, resulting in voltage and current ripple at the input and output terminals.

"Sometimes, ripple and noise will be coupled to other sensitive circuits (such as mobile phone RF transceivers) that affect product performance. Effect of input ripple is relatively more significant, because the battery power line in the system is public, when the input capacitance of the same switching frequency, driving the same output load, using the same, charge pump architecture and inductance circuit input ripple in the same order, increasing the input capacitance helps reduce input ripple. Maxim portable communications equipment power line director TonyLai explained to the reporter, the output ripple is also a consideration, especially when the lead is longer. As long as an antenna, it is possible to introduce more coupling noise to the circuit, which can be more suitable for the application of the inductor boost circuit with only two leads. When the drive circuit is very close to the placement of the LED, the charge pump is recommended. In addition, the inductance boost circuit switching waveform higher at the junction and Schottky diode inductance, if coupled with a small capacitor in the switch node, rise / slow switch signal down, will help reduce electromagnetic interference, but also affect the efficiency. It can be seen that the final way to boost the design and selection of the system is often based on the needs of the system, in consideration of a variety of technical indicators after the compromise.

Improving efficiency is the eternal theme

It is a problem that the designers must face to improve the conversion efficiency of the drive circuit in the field of lighting applications and backlight applications. increase