日期：2012年8月24日

red led flood light and led flash

On the market each year must be hundreds of new mobile phones, these phones are the same basic functions, that is communication. Around the phone’s design is to increase the phone’s additional features, the main way to increase mobile phone selling point as well as a new profit point. Different phone difference between live mainly in the peripheral functions, such as appearance, screen colors, brightness, multimedia features, Bluetooth, a camera function. Phone integrated camera feature is the inevitable trend of mobile phone design cell phone camera pixels are increasingly high. To get a better image effect, the mobile phone camera flash function is also becoming increasingly important.
The current mobile phone camera with LED flash. The flash of the LED only requires 3.5 to 4.5V DC voltage 120 to 250mA current can be issued 2000 ~ 7500mcd high brightness light. LED low-voltage flash circuit is simple, efficient, energy-saving, low cost, PCB area, especially for mobile phones, digital cameras and handheld devices, and popular with the favor of hand-held imaging products market.

LED flash drivers to control the forward current program

red led flood light and led flash

According to the output characteristics of the drive circuit, the LED flash driver circuit can be divided into the constant voltage type and the constant current type; circuit works can be divided into a step-up circuit and charge pump circuit.

LED is a current-driven devices, and their brightness is proportional to the current relationship. In the constant voltage drive circuit, often a resistor in series with the LED is used to determine the cause of the forward current is expected to the required voltage to the LED lights. In this way there is a drawback, any change of the LED forward voltage will lead to changes in the LED current, which can not be guaranteed to flow through the LED current is equal to the default, will not be able to ensure that the LED’s constant brightness.

, By detecting the series resistance of the voltage on the LED constant current drive circuit to ensure constant flow through the LED current. This way you can eliminate the forward voltage changes caused by current changes, it can be fixed LED brightness.

Since the scope of work of the cell phone battery voltage is typically 3.6V ~ 4.2V, LED forward voltage is 3V to 4V, so keep low voltage input, the output of high voltage, you must boost circuit voltage will be increased to drive LED. Flash-driven commonly used are two ways to boost, a boost circuit inductor for energy storage component, and the other is the charge pump capacitors for energy storage component.

The boost circuit using inductors as energy storage elements, the advantage of relatively high efficiency.

red led lamps and driver design

To customize the four 7-segment LED display driver chip, for example, the design of the LED display to drive the principle shown in Figure 7. It uses the 89C52 microcontroller’s general-purpose I / O port the P1.0, P1.1, as a simulation of the I2C bus; LED display for four common cathode LED pin is connected to the power of the A / K; display driver chip P87LPC762 for custom, named for the LED lights -762. I2C bus external address of the first chip 000, A0, A1, A2 pin to ground to achieve the rest of the chip address set in turn, can connect up to 8 external chip (not shown). From the schematic point of view, LED-762 can not add any external components can be used as LED drivers, I2C bus connections, take up system resources, at least, the circuit is relatively simple. Connected on the I2C bus, eight LED-762 LED extended median of 32. LED display driver chip applications, can use the same connection for the “m” field. The “m” field LED on the same I2C bus, the maximum extension of 48, enough to meet the general requirements.
In order to improve the driving capability of the I2C bus, in the realization of multi-chip connections, SCL, the SDA should be connected bus matching pull-up resistor.
Software programming of customized LED panel display driver chip
Was controller within the LPC series chips with hardware support for I2C bus interface, the user can direct it as the master of the I2C bus or the I2C bus. Controller via the I2C hardware interrupt handler can receive or send data from the bus; master operation, the I2C bus to the start timing, data timing, response timing, stop timing to detect the I2C bus controller, and corresponding data transfer. On the I2C bus controller based on the I2C bus address to distinguish. The unity of the I2C bus addresses allocated by the I2C bus Commission, a total of seven chip address (it occupies D7 ~ D1-bit), 4 (D7 ~ D4) decided to chip types, users can also customize the chip types, 3 (low D3 ~ D1) chip A0, A1, A2 pin set.

red led lamps and driver design
When used with the LPC series of I2C bus interface chip custom LED display driver chip, customized LED display driver chip is set to be the controller, you want to send data the CPU is set to the I2C bus master. Customized LED display driver chip to receive data through the I2C interrupt the flow chart shown in Figure 8. When the first data received from the I2C bus, the judgment of the chip address, such as the same and write to display data, then send the response timing receiver 4 displays the data, then the I2C interface to return to the idle state. LED dynamic display, LED display drive-based display program according to the LED polarity input, respectively, sent to display segments and bits, and the LED tube will be able to display properly.

 

According to the I2C bus protocol requirements, response time requirements for the master to send data. The shortest period of time determined by the multiplier of the RC oscillator frequency and interrupt response time to the maximum rate can reach 400kbs / s. The minimum rate may be within the LPC series dedicated I2C timer to control, in order to adapt to the low rate of non-standard I2C bus operation, off timer.
6-chip testing and key performance indicators
Requirements in accordance with custom, the complete LED curing of the driver and chip set the parameters by the programmer to the production of chip testing the connection diagram shown in Figure 7. The 89C52 P1.0 and P1.1 as analog I2C bus control line, write simulation I2C driver. A normal data reception, 89C52 host re-reset the I2C bus communication disconnection, and other circumstances can not affect the display driver. If you turn off the timer in custom LED driver I analog I2C program pause, single step debugging custom LED driver can drive display. By Philips LPC series chip port output current capability, 0.5 in. drive common cathode and common anode LEDs, LED brightness are to meet the requirements. In the long-term full-bright display, custom LED driver chip temperature normal, long-term continuous work. In actual use, control, Port A / K and A0, A1, A2 set the input mode when the programming, so the vacant when the input level is undefined, and random variation, and may not display properly, should be based on the address set requirements that force is connected to VCC or GND.

remote controlled spotlights and led advantage

As an emerging technology, semiconductor lighting industry is still in a stage of rapid development, scientific and technological progress we are delighted, but we also realize that both the technical aspects or the rules and regulations of the industry, compared with conventional light sources are not mature, not a sound. To really achieve with LED instead of traditional light sources, there is still a long way to go, there are many technical problems need to be addressed.

remote controlled spotlights and led advantage

3, LED technical problems

In theory LED luminous efficacy of up to 200lm / W, while the existing white LED only 70lm / W or so, and energy-saving fluorescent lamps still lags far behind; and its price compared with conventional light sources, there are big disadvantages (see right).

And therefore how quickly the advantages of the LED lights really play out has become practitioners must face technical problems. To really carve out a new semiconductor lighting era, we need to efforts to overcome technical problems as well as from the following aspects to further standardize the semiconductor lighting market.

3.1, the LED chip

The chip is the core of the LED, the internal quantum efficiency of direct influence on the luminous efficacy of LED, non-radiative recombination rate determines the size of the chip heat production. It can be said that the only manufacture LED chips with good quality, it may have a superior performance of the LED light source.

In order to maximize the internal quantum efficiency, and reduce non-radiative recombination rate, mainly from two aspects to improve the quality of silicon, is to choose the chip structure and defect density of the control chip.

LED chip structure of a single heterostructure, double heterostructure and quantum well structure, its luminous efficiency has great impact. Currently the most widely used and most efficient chip architecture for multi-quantum well structure (Multi-Quantum Well Structure,).

For the LED, the wafer and the substrate lattice constant and thermal expansion coefficient matching, wafer preparation process will directly affect the density of lattice defects. These defects may, under certain conditions, especially for Ⅲ – nitrogen light may be beneficial, but in most cases, because of the presence of these defects, will shorten the continuous working life of the chip, reducing the carrier density and thus reduce the light-emitting output, and may become non-radiative recombination centers.

How to choose a reasonable chip structure, understanding of lattice defects in the light-emitting LED chip mechanism to better control the generation of adverse defects.

replace led bulb and current drivers

White LED usually has four different driver circuit: (a) the voltage source and ballast resistors, (b) current source and ballast resistors, (c) multiple current sources, (d) a current source driver series led lights.

replace led bulb and current drivers
Each white LED forward voltage (Vf) the impact of the current regulation accuracy, depending on the structure of regulation circuit: (a) voltage source and ballast resistors, (b) current source and ballast resistors, (c) multiple current source or a current source connected in series LED. 6 LED Vf curve (three from vendor A and B) are shown in the figure, the regulator output load curve and the LED’s Vf curve of intersection is the regulation of the operating point.

Circuits use a voltage regulator ballast resistors to control the LED current, the advantage of this structure there is considerable room to select the voltage source between the regulator and LED only need one connection endpoint; drawback is low efficiency, which is the main town resistor loss caused, in addition, its control of the LED forward current is not very accurate. Figure 5a test curve can be seen six different LED current range: 14.2mA to 18.4mA average luminance than the manufacturers provided by vendor A LED B is higher, the work of currents higher than 2mA.

The MAX1910 is this structure, this circuit can drive the same vendor products in the same batch to obtain good results. In the above circuit to provide the same current conditions, you can reduce the ballast resistors to reduce power consumption by half.Figure 5b shows the range of six different LED drive current: 15.4mA to 19.6mA, the LED current changes provided by the manufacturer A smaller average from vendor A and vendor B LED control current is the same: 17.5mA. The defects of this structure is the ballast resistor power consumption is still large, and the LED current matching is not very good. However, this circuit a good compromise between performance and ease of circuit.

The current regulation accuracy and matching of the individual current regulator. The MAX1570 uses a current source structure, the current accuracy of 2%, matching up to 0.3%. Low dropout current regulator, you can get higher efficiency. Figure 5c shows that all tested six white LED flood lights current is maintained at a stable 17.5mA, eliminating the need for ballast resistors, which can effectively save board space, but four terminals are required between the regulator and LED. This circuit can provide a high level of performance is based on the competition program of the inductor structure.

Lower feedback threshold to further reduce the sense resistor’s power consumption, because the LEDs are arranged in series connection, and under any working conditions are able to make the LED brightness is consistent. The current accuracy depends on the accuracy of the regulator’s feedback threshold from the LED’s forward voltage variations.The MAX1848 and MAX1561 are two typical examples of this current regulator circuit, conversion efficiency (PLED / PIN) can reach 87% (3 series LED) or 84% (6 LEDs in series). Another advantage of this circuit is between the regulator and LED only requires two connection endpoints, provide some flexibility for the user’s design. However, because the circuit uses inductance, a larger size, higher cost compared with the above-mentioned program, EMI radiation is also larger.

 

replacement led car bulbs and testing method

LED lighting technology standards focus on solving the problem of product characteristics and test methods, the LED is the most important performance indicators is the optical properties and electrical characteristics. The optical properties of luminous flux, light distribution, brightness, spectral distribution, chromaticity coordinates, color rendering index.

LEDs used in lighting fixtures, both for the general in the optical light source similar to the common particularity and its products: LED is a directional light emitting devices; the luminous body is very small, can be approximated point light source; light-emitting affected by environmental and processing process impact.

replacement led car bulbs and testing method

LED lights decay characteristics of the measurement cycle is very long (tens of thousands or even tens of hours). LED illumination in addition to the optical properties are involved, as well as its electrical attachment properties (including energy-saving electrical attachment), mechanical properties, temperature characteristics, and utensils, and security features.

Therefore consider the problem of testing methods in the standard setting process to focus on

The first is to choose the adoption and development of LED characteristics of test equipment, to create a test system. Including: LED optical test systems, temperature monitoring and test systems, electrical testing, environmental testing, durability, reliability test system. The main test system should adopt international advanced tools, test methods should be able to approach international advanced level.

Through cooperation and exchange with foreign laboratories, our LED detection technology and international developments, so that the standard can objectively reflect the technical level of China’s LED lighting compatible with the rapid development of China’s LED lighting, but also consistent with international standards
Further promote the development of LED panel technology.

LED lighting standards for a series of standards

The series of standards in the basis of the standard (or standards) single-tube LED lighting standard and test method standards in LED lighting standards on the basis of this system. LED standards must first be resolved to the characteristics of the LED light source in the lighting requirements, determine the technical requirements of the LED light source, the main technical characteristics and technical parameters of the LED and the LED detection to find, to determine the technical level of China’s LED lighting and related technical specifications and technical requirements.

Quickly due to the development of LED lighting technology, LED lighting development of standards with the development of technology and market needs to adapt. Standardization bodies, inspection bodies, lighting LED research department, production enterprises and application units should complement each other and work together to establish a set to meet China’s national conditions, advanced technology and LED lighting of the national standards.

replacement led flashlight bulbs and power solution

As large numbers of solid state light source, light-emitting diode (LED lights), incandescent increasingly lonely. In the past few years, LED technology has been tremendous progress, making LED cooling, packaging and process technology advances have higher brightness, higher efficiency, longer life and lower cost.

However, the efficiency of energy conversion, thermal management, and production costs, LED still has further room for improvement. For example, LED efficiency has been greatly improved. Many improvements in the LED from the better chip to produce light, and there are better ways to get light from the chip and its package. Similarly, 20mA white LED prices have significantly reduced this LED has just launched, lots of share offer for $ 1. At present, 20mA white LED may be approved purchase price of less than 30 cents to buy.

replacement led flashlight bulbs and power solution

LED lighting applications needs analysis

Of course, all these advances not only promoted the LED as the lighting of use in different applications, but also stimulated the demand for LED driver IC. To understand the design and manufacture of LED driver IC to face obstacles, it is necessary to understand the white LED light-emitting What are the conditions. White LED with a constant current source driver, so that the white points of light will not change (that is, the brightness must be consistent). In addition, since the white LED is a diode, then it must overcome its internal forward voltage drop (Vf). Vf with the white LED current value will change with changes in temperature. Under normal circumstances, within the entire operating temperature range, 20mA white LED Vf in the range from 2.5V to 3.9V. Most applications, multiple white LEDs, these LED may also be configured in parallel, in series or series-parallel hybrid forms. This means that the white LED driver IC must be able to provide for the specific configuration of multiple LED current and voltage and the voltage converter topology used in this IC must also meet the input voltage range and output voltage and current requirements.

The biggest driving force of the LED common applications include automotive and aircraft instrument panel display and instructions, traffic lights, cell phones, flat panel display backlight, miners lamps, buildings and outdoor large-scale stadium lighting, and the current and future years, the LED market growth from the flat panel display backlighting requirements. These displays use LCD screen for TV sets, navigation systems, portable media players, digital billboards, and computer monitors. However, the LED facing the challenges of thermal management technology. Although LED Unlike other light sources, as so much heat radiation, but depending on the output power, some LED may require a suitable heat sink, so that light output will not reduce the life expectancy is not shortened. For example, the power light output for the the 25lm high brightness LED is generally consumed more than 1W. This means that the white LED driver IC with high efficiency must be converted so that it can not become the main cause of heat problems. In addition, in many cases there are space-constrained problem, the LED driver solutions to small footprint, and small size.

Mobile phone, for example, most cell phones today have built-in digital camera that can shoot high-resolution photos and videos. Camera improved performance has also led to the demand for high-power white light, the camera indoors or in dim environments need to use this light, the white LED main light source for a camera phone. Because the white LED has a wide range of modern cell phone designers want the features: small size, high light output, can provide the “Flash” and “video” object of continuous illumination. Has been specifically developed for the integrated camera lighting the high output power LED.

Although the use of high-power LED visible light is very simple, but if the wrong has been designed to improve, to form a high-performance power supply and current control solution is very difficult. Linear Technology’s LTC3454 is a new product designed specifically for high-current camera lighting applications to improve power conversion efficiency and accuracy, and optimize the control of LED current.

replacement led lamps and life span

LED industry in the past in order to profit fully white LED beam, once the development of large-size LED chips trying to take this way to achieve the desired goals, but actually exert power white LED lights continuous beam but will decline more than 1W above, the luminous efficiency is relatively lower 2030 %, in other words white LED brightness several times larger than traditional LED power consumption characteristics beyond the fluorescent lights, you must first overcome the following four topics, including inhibition of the temperature rise to ensure that the service life, improve light-emitting efficiency, as well as luminescent properties equalization.

replacement led lamps and life span
Is to reduce the package thermal impedance on temperature rise problems; maintain LED life is to improve the chip shape, a small chip; specific methods to improve the chip architecture, small chip to improve the LED luminous efficiency; As for the light-emitting characteristics of uniform of specific methods of improvement of the LED package, and these methods have gradually been developed.

The package thermal problem is the fundamental method

It will cause the package thermal impedance is drastically reduced to below 10K / W due to the increased power, foreign companies have developed high-temperature white LED an attempt to improve the problem, However, in practice the high-power LED heat than low-power LED high number of more than ten times the luminous efficiency and temperature rise will be declined substantially, even if the packaging technology allows high-calorie, but the LED chip junction temperature there is likely to exceed the allowable value
And finally the industry has finally realized that the heat dissipation problem is the fundamental method to solve the package.

LED life, for example, to switch to the silicon packaging materials and ceramic packaging materials, make the life of the LED’s to improve a number of white LED light spectrum contains wavelengths below 450nm short wave length light, traditional epoxy encapsulated materials can easily be short-wavelength light damage, high-power white LED light to accelerate the degradation of packaging materials, continuous lighting of less than one million hours, according to the test results of the industry, high-power white LED brightness has been reduced by more than half simply can not meet the basic requirements for lighting long life.

Remove, not easy from the large chip, can achieve the same level with the low-power white LED chip package structure for LED luminous efficiency improvement, mainly due to the current density increased by more than 2 times, not only light, a result, it will cause the luminous efficiency the dilemma of not as good as low-power white LED, improving the electrode structure of the chip, in theory, the light extraction issues can be resolved.

try to reduce the thermal resistance to improve the heat dissipation problem

For light-emitting characteristics of uniformity, is generally believed that as long as the production technology to improve the uniformity and phosphor white LED phosphor materials concentration should be able to overcome this trouble. As described above to improve exert power at the same time, it is necessary to reduce the thermal impedance to improve the heat dissipation problem, the specific contents are: to reduce the chip to the package thermal impedance, inhibiting the thermal impedance of the package to the printed circuit board, improve the smoothness of chip cooling.

In order to reduce the thermal resistance, many foreign LED manufacturers LED chip set heat sink made of copper and ceramic materials (heat sink) surface, and then using the welding heat to the printed circuit board with a wire connected to the use of the cooling fan forced air cooling fins, according to German OSRAM Opto Semiconductors Gmb experimental results confirmed the above-mentioned structure of the LED chip to the thermal impedance of the welding point can reduce the 9K / W is about 1/6 of the traditional LED, the LED package to impose 2W of power, the LED chip junction temperature than welding point high 18K, even if the printed circuit board temperature rises to 500C junction temperature at most only about 700C; previous thermal impedance contrast lower, then the LED chip junction temperature will The temperature of printed circuit board, this way necessary to try to reduce the temperature of the LED chip, in other words reduce the thermal impedance of the LED chip to the welding point, can effectively reduce the burden of the LED chip cooling operation. On the other hand the white LED is able to inhibit the thermal impedance of the structure, if the heat conduction from the package to the printed circuit board, LED temperature rise results luminous efficiency will be dramatically decreased by Matsushita Electric Works to develop printed circuit board and package integration technologies, the 1mm square blue LED flip chip packaged in a ceramic substrate, and then paste then the ceramic substrate in the surface of copper printed circuit board, according to Matsushita said contains a printed circuit board module as a whole, including the thermal impedance is about 15K / W or so.

replacement led light bulbs and power switch

High-power LED lights are increasingly being used in a variety of areas, including automotive lighting, camera phone flash, stage lighting, workplace lighting, landscape lighting, flash, bicycle lights and cars running lights. This article will discuss the various technical problems and distribution switches in LED applications.

At present, the limiting distribution switch is one of the best choice for designers used to drive the LED. This is because the distribution switch operation, and better able to control the distribution system a variety of load. Its function is simple: in the state of the switch is turned on, allowing current to flow to the load, the switch turned off, preventing current flow to the load.

replacement led light bulbs and power switch

This switch also has a limiting function, that is, when the switch is turned on, the maximum current load is equal to the current limit set by the switch. This limit function is necessary to design a constant current LED driver.

Why designer so need to constant-current LED drive? LED light intensity is constant because the flow through the LED current is constant, even in the case of flow through the LED voltage change will not change. This is necessary for the application of the requirements of a stable light intensity.

When the switch is open, the LED will try to absorb as current as possible, but due to limitations of the switch, only 350mA. Therefore, as long as the switch is open, and switch the input voltage (Vin) within the range of 2.5V to 5.5V, the flow through the LED current can be always maintained in the 350mA.

With current-limiting switch, the LED current limit will be able to be adjusted between 200 mA to 2 A by changing the resistor values ​​in Rset. A limiting value range so wide, you can apply to a variety of high-power LED, and can be used for a variety of applications.

This IC with Cslew pin capacitor between Cslew and Vin to connect through it. Cslew capacitors can be used to extend the opening time of the LED. At the same time, it comes with a built-load discharge transistor (P-channel FET). When the switch is off, the FET will release the output current.

In some applications, the need to gradually open and close the LEDs, so the audience will not be sudden change in light conditions and transient blindness. Figure 3 shows one can arbitrarily set the opening time and closing time of the program.

Here, open the input and Vin are related (Figure 3 display Cslew pin) pin Ilimit issued a 5 V signal to open and close the control.

To calculate the limiting value of the circuit in Figure 3, the required sum of 0.5 kΩ and 0.2 kΩ (3.33 kΩ and 0.212 kΩ resistor in the equivalent resistance), obtained 0.7 kΩ. In this case, the limiting value is 245/0.7 kΩ = 350 mA. In addition, through a 680 μF capacitor values, opening and closing time is set at 300 ms. This time is calculated: 2.2 x R x C = 2.2 x 0.2 kΩ x 680 μF = 300 ms.

MIC2007 can switch to turn on and adjust the current limit after the LED is lit. By changing the current limit, it can dynamically adjust the LED light intensity. Shown in Figure 5, there are four resistors are connected to Ilimit pin.

If 4 of the resistor value, the LED light intensity can have four different levels. Several resistors connected to Ilimit pin, you can get several levels of the LED light intensity.

If the resistor’s value, by using different combinations of resistors to adjust the intensity of LED light. For example, if the resistance value of R, R / 2, the R / 4, the R / 8 and R/16 5 resistors have been used, the combination of R, the R / 2 and R / 4 three resistors LED The light intensity is even lower, while the combination of R / R / 8 R/16 three resistors, LED’s light intensity is higher.

Micrel’s MIC2000 series limiting distribution switch allows a maximum of 2A current flows to load such a large current is sufficient for most of the load in the system. Limiting distribution switches to many designers are using the high power LED load to meet the growing demands of a variety of lighting applications in the market today.