Types of LED driving technologies

For display technology, not only does video signal processing require high definition, but the quality of backlight adjustment also directly affects the video display effect. In order to carry out "high definition" technology to the end, manufacturers have also put a lot of effort into LED backlight driving technology.

Traditionally, LED dimming is accomplished by adjusting the forward current in the LED using a DC signal or filtered PWM. Reducing the LED current will have the effect of adjusting the LED light output intensity, however, changes in forward current will also change the color of the LED because the chromaticity of the LED changes with changes in current. Many applications, such as automotive and LCD TV backlighting, cannot tolerate any color drift in the LED. In these applications, a wide dimming range is required due to the varying light variations in the surrounding environment and the human eye's sensitivity to small changes in light intensity. Controlling the LED brightness by applying a PWM signal allows the LED to be dimmed without changing the color.

True Color PWM dimming, as it is often called, uses a PWM signal to adjust the brightness of the LED. It actually requires turning the LED on and off at full current and PWM frequency. The human eye's resolution limit is 60 frames per second. By increasing the PWM frequency (such as to 80~100Hz), the human eye can perceive the pulsating light source as continuously on. In addition, by adjusting the duty cycle (the total amount of on time), the brightness of the LED can also be controlled. Currently, many LCD TV designers require dimming ratios as high as 3,000:1 to adjust the brightness for a variety of ambient lighting conditions.

系统工程师在挑选背光源时，究竟是该选RGB还是白光LED呢？凌力尔特公司电源产品市场经理Tony Armstrong指出，背光源(RGB或白光LED)的选择可以根据特定最终产品的要求来进行。不过，采用RGB LED来替代白光LED是有正当理由的，因为前者具有更加宽广的色谱，这将显著地改善图像质量，以挑选LCD TV为例，消费者对色彩逼真度的关注程度是最高的。然而，采用RGB LED需要一个更大、更复杂而且更昂贵的解决方案。

Linear Technology's LT3476 four-channel LED driver is an example of an RGB LED driver. Each channel of the LT3476 can drive up to 8 1A LEDs (red, green, blue or white) in series, allowing the LT3476 to drive up to 32 1A LEDs and provide up to 96% efficiency. Each of the four channels can operate independently through its independent True Color PWM signal, enabling its independent dimming with a dimming ratio of up to 1,000:1. The fixed frequency, current mode architecture allows stable operation over a wide range of supply and output voltages. A frequency adjustment pin allows the user to set the switching frequency in the range of 200kHz~2MHz to optimize efficiency and minimize the size of external components. Its thermally enhanced 5×7mm QFN package provides a very compact solution footprint, which is very suitable for 100W LED applications commonly found in large LCD TVs.

In addition to using PWM signals as control signals for brightness adjustment like Linear Technology, are there other technologies that can achieve good dimming effects? Brian Law, a field application engineer at Fairchild Semiconductor, has his own insights. He believes that one of the key requirements that distinguishes LED drivers from standard boost converters is that the LED driver can control the brightness level of the LED. There are three common methods for adjusting brightness: using SET resistors; using PWM technology; and linear adjustment. The simplest method is to use an external SET resistor in the IC. Although this is an effective method, it lacks flexibility and does not allow users to change the light intensity. Using PWM technology and linear adjustment can dynamically control the light intensity. However, both methods have disadvantages. Using PWM to dim LEDs will cause EMI noise and cause the color of white LEDs to shift toward the blue spectrum. It may be a slight shift, but it can be detected in sensitive applications. Linear regulation will reduce efficiency and cause the color of white LEDs to shift toward the yellow spectrum. As for which method is better, the industry has not yet reached a consensus. As a result, Fairchild Semiconductor has developed the latest LED driver that can adjust the brightness using one or a combination of the two methods. The FAN5617 uses a charge pump architecture to drive three parallel LEDs and incorporates a single-wire bus that lets the user set the light intensity either linearly or by applying a PWM signal.

However, Fairchild is not the first company to use this "integrated" dimming technology. According to Pei Shiyan, president of Beijing Siwang Electronic Technology Co., Ltd., the company's recently launched inductorless Charge Pump WLED driver IC SE3362 also uses similar technology. SE3362 has an internal current matching control circuit that can effectively control the LED current so that the current of each WLED has a high matching degree to ensure the consistency of WLED brightness. The WLED current can be set through an external Rset resistor, and its brightness can also be adjusted in a variety of ways:

1. Use a DC voltage to set the current of the RSET pin, thereby changing the WLED current and adjusting the brightness.
2. Use the PWM signal as a control signal to adjust the brightness through the duty cycle.
3. Apply GPIO to the control end to adjust the brightness through the duty cycle.
4. Connect different conversion resistors in parallel at both ends of RSET to change the WLED current to adjust the brightness.

This driver IC can drive 4 WLEDs in parallel at the same time, with a maximum current of 30mA/WLED. It is suitable for driving the backlight LEDs of LCD screens such as mobile phones, PDAs, DSCs and PMPs.

Improving operability and competing in humanization in the flash field

For flash applications, simply improving efficiency is not enough to satisfy device manufacturers. With the popularity of camera phones, people's requirements for camera performance are getting higher and higher. For LED drivers for flash applications, operability is a performance that adds points, and this is also the direction that many LED driver manufacturers are working towards.

The LED driver AAT1270 recently launched by AAT Logic provides designers with highly flexible control of flash and movie mode operations. The product can set the full-scale flash LED current through an independent external resistor. When the movie mode is applied, the current ratio of the default current value is set to 7.3:1. The movie mode current can also be programmed to 15 other ratios through AAT Logic's proprietary single-line simple serial control (S2Cwire) interface. In addition, in order to prevent thermal damage to the flash LED, the AAT1270 is equipped with a user-programmable safety timer that can stop the flash activity within a set length of time, and there is also an independent flash support pin to simultaneously start the flash/flash operation and the safety timer. Finally, the AAT1270 is also equipped with an asynchronous flash inhibition pin that can reduce the flash current in the shooting mode during high battery demand.

同样，为了提升用户的照相体验，恩智浦(NXP)半导体所发布的高效照相手机LED闪光驱动器UBA3001则可在电池供电型手机中提供最大闪光效果输出。增强的闪光性能可提高不良照明条件下的图像质量，由此显著改善用户的视觉体验。恩智浦UBA3001可为单颗高亮度LED提供1安培电流，由于它具备自动升压与降压转换功能，无论电池电量多少都可以获得极好的闪光性能。该解决方案专为高要求的照相手机而设计，并提供一些专门的特性，如发射模式期间降低电流、自动关机、真正LED关机，以及不同应用下电流设置的完全控制，包括闪光灯、手电，以及“相机/视频-开”的指示灯模式。

Compete for the "potential stocks" of lighting applications and overcome efficiency obstacles

As the call for environmental protection grows, energy-saving lighting technology has received more and more attention from the public. Many countries in Europe and the United States have even enacted laws to regulate lighting management. LED lighting technology has become the "vanguard" of energy-saving lighting technology with its advantages of low power consumption and long life.

In addition to traditional lighting markets such as performance lighting, landscape lighting, residential lighting, and fireproof lighting, the automotive headlight lighting market is also a great space for the development of LED lighting technology. Tony Armstrong even believes that the fastest growing LED driver integrated circuit market is automotive headlight lighting, and the annual compound growth rate will exceed 150% from now to 2011. A popular application area is the dashboard backlighting, interior lighting, and brake lights of many cars and trucks. Luxury car manufacturers are increasingly adopting the latest solid-state LED lighting technology to provide interior and exterior lighting with these lighter, smaller, and more durable devices to improve the beauty of future models.

For various lighting applications, how to improve the LED luminous efficiency is the most concerned topic for LED driver manufacturers. In order to promote the rapid growth of the LED lighting market as soon as possible, various manufacturers have also launched a series of excellent LED driver products.

Macroblock Technology recently developed a 1.2A "constant current mode" high-efficiency DC/DC converter MBI6650, which uses the innovative Hysteretic PFM technology. In addition to avoiding interference with audio, it can also have a fast switching frequency under high current conditions and a slow switching frequency under low current conditions, making the system conversion efficiency of MBI6650 up to 85% and 90% under light load and heavy load conditions respectively, greatly improving the efficiency of LED lighting system operation and achieving power saving effects. For high-power LED lighting applications, the chip's input current range has also been expanded to 9~36V. In addition, the MBI6650's built-in all-round protection measures and heat sink can improve the stability and safety of the MBI6650 during operation.

In addition to improving luminous efficiency, increasing integration is also the direction of manufacturers' efforts. Maxim's high-efficiency LED drivers MAX16819 and MAX16820, launched this year, only require an external MOSFET and a small number of passive components to drive LEDs with a current of up to 3A. Each device can drive up to 6 LEDs in series, providing output power from 1W to more than 25W, with an efficiency of up to 94%. The high-side current detection function of this series of drivers makes the device essentially different from the standard buck PWM controller. Since the voltage drop of its external current-sense resistor is only 200mV, the power consumption of MAX16819/MAX16820 is reduced. In addition, high-side current detection and integrated current regulation circuits reduce the number of external components, while the output LED current accuracy can reach ±5%. MAX16819 and MAX16820 do not require the use of external capacitors and have current ripples of 30% and 10% respectively. This series of products is suitable for energy-saving lamp sources and front and rear lighting lamps of automobiles.

Although the LED driver market has a promising future, there are still some technical factors that restrict its development speed. Tony Armstrong pointed out that the main challenges facing LED driver technology include: 1. Powering one or several LED strings with a battery voltage that may be lower than, equal to or higher than the load voltage; 2. Dimming LEDs efficiently with a large dimming ratio while maintaining color characteristics at high and low brightness; 3. High-efficiency operation of LED drivers is a key requirement, especially when driving HB LEDs, because all power that is not output as light is dissipated as heat. Therefore, semiconductor manufacturers still have a lot to do in their pursuit of greater development of LED technology.

Comparison of characteristics of various white light LED drive circuits

After Mr. Nakamura of Nichia Chemical discovered blue LED in 1996, white LED was regarded as the component with the most development potential in lighting source. Therefore, the improvement of white LED performance and commercial application immediately became the focus of research in various countries. At present, white LED has been applied to public place sidewalk lights, automobile lighting, traffic signs, portable electronic products, liquid crystal display and other fields. Since white LED also has the characteristics of rich three primary color temperature and high luminous efficiency, it is generally considered to be very suitable for backlighting source of liquid crystal display. Therefore, various manufacturers have successively launched white LED dedicated driving circuits and related components. In view of this, this article briefly explains the characteristics of LED dedicated driving circuits and future development trends.

1 Reasons for constant current drive

1.1 The luminosity of white LEDs is regulated by forward current

The forward voltage of white light LEDs is usually regulated to a minimum of 3.0V and a maximum of 4.0V at 20mA. That is, if a certain forward voltage is simply applied, the forward current will vary over a wide range.

Figure 1 shows the results of testing the forward voltage and forward current characteristics of three types of white light LED samples randomly selected from the products of LED companies A and B. According to the test results, if the six types of white light LEDs are driven by a 3.4V forward voltage, the forward current will vary greatly within the range of 10~44mA. Table 1 shows the electrical and optical characteristics of white light LEDs.

Since the luminosity and chromaticity of white light LEDs are measured using a constant current method, they are usually driven with a constant current to obtain the expected brightness and chromaticity.

Table 2 shows the ranks of optical coordinates (IF=25mA, Ta=250C).

1.2 Avoid forward current exceeding the allowable current value

To ensure the reliability of white light LEDs, it is basically necessary to try to prevent the forward current from exceeding the absolute maximum design value (rated value) of the white light LED.

In Figure 2, the maximum forward current of a white light LED is 30mA. As the ambient temperature rises, the allowable forward current continues to decrease. If the ambient temperature is 50°C, the forward current cannot exceed 20mA. In addition, it is difficult to control the current value flowing into the LED using a constant voltage drive method, so the reliability of the LED cannot be maintained.

2. Driving method of white light LED

FIG3 shows four commonly used power supply circuits for driving white light LEDs; FIG4 shows the Regulation accuracy characteristics of the above six randomly sampled white light LEDs after stabilization.

The test results in Figure 4 show that the load characteristics of the ReguLator appear at the VF corner of the white light LED, that is, the intersection point in the figure is the stable operating point of each white light LED.

2.1 How to drive the voltage regulator

The circuit in Figure 3 (a) uses a voltage regulator and a BaLLast resistor to control the LED current. The advantage of this circuit is that there are many types of voltage regulators, the designer has more freedom to choose, and there is only one point of contact between the voltage regulator and the LED. The disadvantage is that the power loss caused by the BaLLast will lead to deterioration of efficiency. In addition, the forward current of the LED cannot be precisely controlled.

As can be seen in Figure 4 (a), the forward current of six randomly sampled white light LEDs has a wide distribution range from 14.2mA to 18.4mA, so the (average) forward current of the LED of manufacturer A is as high as 2.0mA. In contrast, although the ReguLator used in the circuit of Figure 4 (b) has the advantages of small size and low cost, its disadvantage is that it may not meet the requirements of performance and reliability, which means that the practicality of this circuit is relatively weak.

2.2 Voltage Regulator drive method using constant current output

Although the circuit in Figure 3 (b) can stabilize all currents flowing into the LEDs, a group of BaLLast resistors are specially set in the circuit to match the electrical characteristics of each LED.

The MAX1910 in Figure 3 (b) is a constant current output type voltage regulator. Although this circuit uses white light LEDs from the same manufacturer and the same lot number (Lot), it has achieved excellent matching. However, when using LEDs from different manufacturers and lots, there will be a large difference in the distribution of characteristics. This current regulator uses a similar method to Figure 3 (a) to control the drive current, but it can reduce the power consumption of the Ballast resistor by about half.

The test results in Figure 4 (b) show that the current flowing into the six randomly sampled white light LEDs varies greatly from 15.4mA to 19.6mA. Therefore, the LEDs of both manufacturers A and B are driven with an average current of 17.5mA. The disadvantages of this circuit are that the power loss caused by the BaLLast resistor may remain, and the matching of the LED current cannot be achieved; however, overall, this circuit has both action characteristics and simplicity, so it has considerable use value.

2.3 Driving method using output type MuLti PuLL current regulator

The circuit of Figure 3 (c) can stabilize the current flowing into the LEDs, so there is no need to use a Ballast resistor, and the accuracy and matching of the current ReguLator are dominated by the respective current ReguLator.

The MAX1570 IC in Figure 3(c) can achieve the above current regulation to achieve the current accuracy of 2% standard and the current matching of 0.3% standard.

The current regulator composed of MAX1570 IC is a low drop-out type, so its operation efficiency is very high. The test results of Figure 4 (c) show that when the driving circuit of Figure 3 (c) is used, the current flowing into the six randomly sampled white light LEDs is stabilized to 17.5mA.

Although four connection terminals are required between the ReguLator and the LED, this circuit does not require a BaLLast resistor, so the packaging area can be effectively suppressed. This makes it very suitable for applications in areas such as small LCD panels where the packaging space is extremely narrow.

2.4 Using boost current regulator drive method

The circuit in Figure 3 (d) uses an inductor that can stabilize the current to form a so-called high-efficiency Step Up Converter. The biggest feature of this circuit is the Feed Back ThreshoLd voltage, which can reduce the power loss of the current detection resistor. In addition, the LED is connected in series, so the current flowing into the white light LED can be fully matched with the LED even under various requirements.

The accuracy of the current is basically determined by the accuracy of the Feed Back ThreshoLd of the Regulator, so it will not be affected by the forward voltage of the LED.

The efficiency (PLED/PIN) of the current regulator composed of MAX1848 and MAX1561 ICs is: three LEDs + MAX1848, 87%; six LEDs + MAX-1561, 84%.

Another advantage of the Step Up Converter is that two connection terminals are required between the Regu-Lator and the LED, and the number of LEDs used will not be affected by the type of Step Up Converter, which means that designers will have more room for choice. Therefore, the Step Up Converter is widely used in LCD panels of various sizes; the disadvantages of the circuit are the height of the inductor, the high cost of the components, and EMI radiation interference.

3 Conclusion

The above introduces the commonly used driving circuits for white light LEDs, and explores the advantages, disadvantages and characteristics of each circuit in actual operation through experiments. Due to the limitations of LED structure, there are problems such as difficulty in controlling wavelength and driving current accuracy. With the increasing demand for white light LED backlight modules, how to improve the above wavelength and current accuracy problems and reduce the manufacturing cost of the driving circuit have become problems that must be overcome.