In the 21st century, energy consumption has increasingly become the focus of attention in the entire human society. Due to the basic demand for lighting, how to use more energy to generate more lighting has become a huge driving force for exploring new lighting technologies. From the original fuel lighting to incandescent lamps, from fluorescent lamps to various luminescent materials, LED lighting technology has emerged. In today's society, the differentiation of the lighting environment requirements of various media devices further promotes human exploration of how to use various high-brightness LEDs for illumination. The application of LEDs in lighting has attracted wide attention.

LED basic principle and performance characteristics

First, let's introduce the basic principles and performance characteristics of LEDs. The basic structure of the LED is an electroluminescent semiconductor material placed on a leaded shelf and then sealed with epoxy resin to protect the internal core. Therefore, the LED has good seismic performance. The core portion of the light emitting diode is a wafer composed of a p-type semiconductor and an n-type semiconductor, and a transition layer between the p-type semiconductor and the n-type semiconductor is referred to as a PN junction. In some PN junctions of semiconductor materials, the injected minority carriers recombine with the majority carriers to release excess energy in the form of light, thereby directly converting electrical energy into light energy. The PN junction adds a reverse voltage, and minority carriers are difficult to inject, so they do not emit light. When it is in the forward working state (ie, the forward voltage is applied to both ends), when the current flows from the anode of the LED to the cathode, the semiconductor crystal emits light of different colors from ultraviolet to infrared, and the intensity of the light is related to the current.

LED light sources have the following characteristics:

1. Voltage: LED forward conduction working voltage is low, can be driven by low voltage power supply, the power supply voltage varies according to the final product, is a relatively safe lighting equipment, especially suitable for public places.

2. Efficacy: In the case of the same illuminance, the energy consumption is reduced by 80% compared with the incandescent lamp with the same light effect.

3. Applicability: Each unit LED chip is a square of 3-5mm, so it can be prepared into various shapes and is suitable for a variable environment.

4. Stability: usually 100,000 hours, the light decay is the initial 50%.

5. Response time: The response time of the incandescent lamp is millisecond, and the response time of the LED lamp is nanosecond.

6. Environmental pollution: no harmful metal mercury.

7. Color: The LED color and luminous efficiency are related to the materials and processes for making LEDs. Currently, there are three basic colors of red, green and blue.

8. Price: Compared with incandescent lamps, the price of LED is higher. The price of several LEDs can be comparable to the price of an incandescent lamp. However, with the advancement of technology, the price/performance ratio can be gradually improved.

LED drive solution

Since the advent of LEDs, LEDs have received great attention. LED driver devices and drive methods are constantly being updated to drive LEDs more efficiently for human lighting. From the early DC/DC switching power supply to the charge pump, as well as the low-side drive and LDO for cost, people choose different drive lines in different applications, which is a hot spot in the market. The display backlight of portable products is LED backlight application. The main market, the following will be for the backlight display of portable products, introduces several common LED driver solutions.

1. Step-UP DC/DC switching power supply realizes LED driving

The main features of the program are: low input voltage, even as low as 0.7V (single-cell dry battery). From the perspective of technological development, the series-type drive appears earlier, the technology is more mature, and the efficiency is higher. A typical application circuit is shown in Figure 1.

Figure 1: Typical application line for a LED-driven Step-UP DC/DC switching power supply.

For this application mode, the early driver chip mainly adopts the current feedback mode, and the current of the LED is determined according to VFB/Rb. The DC/DC feedback voltage VFB is generally around 1.2V, which limits the further improvement of the effective efficiency, itself DC/ The efficiency of DC is about 80%. In this application, the actual efficiency is reduced more. With the improvement of technology, reducing the VFB voltage to within 0.1V can increase the effective efficiency to over 85%, especially in driving. 2-3 lights in the application state. The advantages and disadvantages of this program are as follows:

Advantages: mature technology, relatively low cost, high brightness uniformity and high safety factor through some new technological innovations, such as the overvoltage protection function shown in Figure 1, or the combination of voltage and current feedback. Especially for backlights with large-size screen displays. Since more LEDs are needed to achieve screen backlighting, brightness uniformity and uniformity are challenges that must be faced.

Disadvantages: The application of its own inductance limits the size and height of the line, and brings problems that engineers are unwilling to face - EMI processing, especially in the vicinity of the RF part, requires special processing for interference, otherwise it will lead to RF reception. The sensitivity is reduced, and even the interference of the audio part, such as the audio output current, interferes with the sound. In addition, in this application case, if an LED fails, the entire string of LEDs will fail, which is the result that people do not want to see.

2. Capacitive charge pump drive mode

This is a relatively new way of driving. Simply put, a capacitive charge pump uses a switch array and an oscillator, logic, and a comparison controller to achieve voltage boosting and capacitors to store energy. Due to its high frequency operation, small ceramic capacitors (1μF) can be used, which minimizes space and costs. The charge pump has two modes of operation, constant voltage mode and constant current mode.

1) Constant voltage mode charge pump. Shengbang Microelectronics' SGM3110 is a charge pump with constant voltage mode. Due to the operating characteristics of the constant voltage mode charge pump, it has a high switching frequency and a large peak transient current. Therefore, engineers need extra attention in the PCB layout. It is required that the peripheral capacitor be as close as possible to the device itself. The peripheral wiring should be as short as possible. The peripheral PCB should be as large as possible. The peripheral capacitor should be directly connected to the ground of the SGM3110. If the wiring is limited, it can pass through the large PCB via. And multiple vias for good grounding.

Efficiency calculation method: η=Iout*Vout/Iin*Vin

Since the device itself has switching losses and capacitor leakage, the actual efficiency is lower than this value, which is related to the manufacturer's process technology. The advantage of using this LED driving mode is that it can simplify peripheral lines and reduce product cost. Especially in the case of small-size screens, 1-2 LEDs are used as screen backlights; or used as flash drives to achieve 250mA current output in 100mS; or a simple DC/DC 5V boost simplified version instead of inductive DC/ DC boost, reducing cost and product EMI issues. The disadvantage is that there is only a simple double boost mode, the efficiency is lower, most of the time efficiency is lower than 70%, the actual utilization efficiency is lower, and a considerable part of it is consumed in the current limiting resistor. When driving multiple LEDs, backlight consistency is completely dependent on the accuracy of the LEDs and current limiting resistors.

2) Constant current mode charge pump. In order to further improve the efficiency of LED driving, Shengbang Microelectronics introduced the constant current mode charge pump SGM3123 with 1X, 1.5X and 2X boost in the second quarter of 2008. For the constant current mode charge pump, through the internal Logic control is used to achieve current sharing for each LED, so that the brightness of the LEDs is kept equal, and at the same time, the driving efficiency of the LED is improved as much as possible.

In this application, the brightness consistency of the LED is maintained by the current mirror control technology, the LED current error of each channel does not exceed 2%, and the 1X, 1.5X boost mode makes the efficiency of driving the LED balanced, and the LED is large. In some working hours, the driving efficiency can be guaranteed to be above 80%. Typical application lines are shown in Figure 2.

Figure 2: Typical application line for the constant current mode charge pump SGM3123.

Use different resistance values ​​to set the LED output current: ILED = Gain * VIset / RIset. Different manufacturers have different Gain coefficients. At the same time as the innovation of charge pump technology, people also found that it uses LED to drive a series of advantages over DC/DC, reduce the cost, reduce the size of the driver board, and avoid EMI interference.

3. For the pursuit of cost and performance, there are also a range of other driving methods such as low-side LED drivers. Compared to the charge pump of the constant current mode, only the charge pump boost is lacking, and the constant current processing portion for the current is the same. The cost versus current mode charge pump ratio is relatively low. The disadvantage is that pure linear processing, when the battery voltage drops to a low, such as 3.6V, in the case of sudden large load: the phone is connected to make a phone call or take a picture, MP3 playing music, MP4 playing video, will cause voltage fluctuations, the voltage will Decrease by 0.1~0.2V, then the system voltage will drop to 3.4V or even lower instantaneously. The normal brightness of the LED cannot be guaranteed, and there is a defect that the screen brightness changes significantly.

Shengbang Microelectronics has been widely used in LED driver chip products.

4. Cost pressures have led to the use of lower cost LDOs for system backlighting. Compared with the above scheme, the LDO is used as the LED driver. In addition to the low cost, no matter whether the voltage mode charge pump SGM3110 drives more than 3 LEDs, there are disadvantages of uneven brightness, and the low-side driving voltage instability causes the brightness to change. .

As a semiconductor company specializing in the development, production and sales of high-performance, high-quality analog/mixed-signal integrated circuits, Shengbang Microelectronics is currently targeting next-generation multimedia display technologies, and will develop more and more market-adapted applications in the near future. Demanded products, such as 7-channel DC/DC, multi-channel PMU, single-channel DC/DC switching power supply, and various LED drivers (DC/DC, charge pump, low-side drive).

At present, Shengbang Microelectronics has more than 200 products in 10 series, which can be widely used in mobile phones, DVDs, digital cameras, notebook computers, automotive electronics, industrial automation, medical instruments, liquid crystal displays and military defense. Shengbang has an advanced quality assurance system and has passed ISO9001 certification in 2004. All products are RoHS compliant.

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