Super bright LED driving function and application

Abstract : Based on the brief introduction of the characteristics and characteristics of super bright LEDs , the driving methods of LED current limiting, linear regulator and switching regulator are introduced in detail. Based on this, the driving chip of super bright LED is introduced. Features and applications of the MLX10801. And the main features of the current main super bright LED driver chip are given.

Light Emitting Diode (LED) was invented in the 1960s by using electrons and holes in a semiconductor material to combine and release energy, so that the energy band (Energy Gat) is changed to emit light. The energy released. LED has the advantages of small size, long life, low driving voltage, low power consumption, fast reaction rate and good shock resistance. It is widely used in signal indication, digital display and other fields. With the continuous advancement of technology, the development of super bright LED has been successful, especially the successful development of white LED, which makes it more and more used in the field of lantern decoration and even lighting.

1, the characteristics of super bright LED

Compared to traditional lighting, super bright LEDs have the following advantages:

1) Long life, reliable and durable, extremely low maintenance cost LED can be used continuously for 105h, 100 times longer than ordinary incandescent bulbs;

2) High efficiency can now reach 201m/w. It is estimated that by 2005, it will reach 501m/W [1], and the spectrum of LED is almost exclusively concentrated in the visible light band, and its luminous efficiency can reach 80% to 90%, and LEDs can save 1/4 more energy-saving lamps;

3) The color is bright, the light color is purely a 12-inch red traffic light. It uses a low-efficiency 140W incandescent lamp as the light source. After the 2000lm white light is generated by the red filter, the light loss is 90%. The red light of 2001m, and the lamp designed by Lumileds Lighting with 18 red LED light sources, including circuit loss, only consumes 14W, can produce the same light effect;

4) Fast lighting speed The car signal light is an important field of LED light source application. Because the LED response speed is fast (ns level), installing high-position LED brake lights on the car can reduce the occurrence of car rear-end accidents. In recent years, high-brightness LEDs have been used in automotive low-beam lights, such as the Audi A86.O of Audi in Germany, the concept car Yak launched by Fioravanti in Italy at the 2003 Geneva Motor Show, and Ford Motor Company recently at the Detroit Auto Show. The Model U launched at the beginning began to use high-brightness LEDs in the design of headlamps.

Although super bright LEDs have many advantages, they still have the following disadvantages:

1) Low power The single LED power on the market is generally below 5W, and there is no LED with more power. This is the biggest bottleneck for LEDs to become the first choice for lighting.

2) Strict control of temperature LEDs are a kind of semiconductor material. They have the same PN junction as ordinary diodes. Since the power of high-brightness diodes is relatively large, it is the same as power semiconductor devices. The heat dissipation problem needs to be considered. The junction temperature is too high and will directly affect. The life of the LED, and will increase the light decay of the LED, the situation will seriously burn the LED;

3) High price, in addition to low power, the price is difficult to become the main factor of LED. Although LED is currently recognized by most people, it is also favored by most people, but its high price is difficult to be accepted by consumers. Currently, single yellow LED About O. 6 yuan / month, green and blue single LED at 1.8 yuan / month, the price of white LED reached 2.2 ~ 5.5 yuan / month; if you combine dozens of individual LEDs, its cost will be greatly increased For example, if an LED is installed in a lawn lamp, its unit price is equivalent to several times that of a general lawn lamp. If LED is to become the mainstream light source for future lighting, it must be developed in the direction of big lumens, and the cost will be reduced. It is possible to break through.

2, the characteristics of super bright LED

The HPWA-xH00 is a super bright LED from Lumileds Lighting. This article uses it as an example to analyze the characteristics of super bright LEDs. Figure 1 is the relationship between forward voltage drop (VF) and forward current (IF). It can be seen from the curve that when the forward voltage exceeds a certain threshold (about 2V), that is, the so-called turn-on voltage, it can be approximated. , IF is proportional to VF. Table 1 shows the electrical characteristics of the current main super bright LEDs. As can be seen from Table 1, the highest IF of the current super bright LED can reach 1A, and the VF is usually 3 to 4V.

Since the light characteristics of LEDs are generally described as a function of current rather than a function of voltage, the relationship between luminous flux (φV) and IF is shown in Figure 2. Therefore, the use of constant current source driving can better control brightness. In addition, as shown in Table 1, the forward voltage drop of the LED varies widely (up to 1V or more), and the VF-IF curve in Figure 1 shows that a small change in VF causes a large IF change. Causes a large change in brightness. Therefore, the use of constant voltage source drive can not guarantee the consistency of LED brightness, and affect the reliability, life and light decay of LED. Therefore, super bright LEDs are usually driven by a constant current source.

Figure 3 is a plot of temperature versus luminous flux (φV) for HPWA-xH00 LEDs. Figure 3 shows that the luminous flux is inversely proportional to temperature. The luminous flux at 85 °C is half that at 25 °C, while the light output at 25 °C is 25 °C. 1.8 times. The change in temperature also has a certain effect on the wavelength of the LFD. Therefore, good heat dissipation is a guarantee that the LED maintains a constant brightness.

3 Super-bright LED drive circuit is limited by the LED power level, usually need to drive multiple LEDs at the same time to meet the brightness requirements, therefore, a special drive circuit is needed to illuminate the LED. The following is a brief introduction to the main circuit of the LED driver.

3.1 Restriction current limiting circuit
As shown in Figure 4, the resistor current limiting drive circuit is the simplest drive circuit, and the current limiting resistor is calculated according to equation (1).

Where: Vin is the input voltage of the circuit:
VF is the forward current of the IED;
VF is the voltage drop of the LED when the forward current is IF;
VD is the voltage drop of the anti-reverse diode (optional);
y is the number of LEDs per string;
x is the number of strings of parallel LEDs.

3.2 linear regulator

The core of the linear regulator is to use a power transistor or MOSFFET operating in the linear region as a dynamic adjustable resistor to control the load. Linear regulators are available in both parallel and series versions.

Figure 5 (a) shows a parallel linear regulator, also known as a shunt regulator (only one LED is shown in the figure, in fact the load can be multiple LEDs in series, the same below), it is connected in parallel with the LED, when input As the voltage increases or the LED) decreases, the current through the shunt regulator will increase, which will increase the voltage drop across the current limiting resistor to keep the current through the LED constant.

Since the shunt regulator requires a resistor in series, it is not efficient, and it is difficult to make constant adjustments when the input voltage varies widely.

Figure 5(b) shows a series regulator that increases the dynamic resistance as the input voltage increases to keep the voltage (current) on the LED constant.

Since the power transistor or MOSFET has a saturated turn-on voltage, the minimum input voltage must be greater than the sum of the saturation voltage and the load voltage for the circuit to operate properly.

3.3 switching regulator

The above drive technology is not only limited by the input voltage range, but also inefficient. When driving for ordinary LEDs with low power, since the current is only a few mA, the loss is not obvious. When used as a high-brightness LED with a current of several hundred mA or more, the loss of the power circuit becomes a comparison. serious problem. Switching power supply is the most efficient in energy conversion, and can reach more than 90%. Power converters such as Buek, Boost, and Buck-Boost can be used for LED driving, just to meet the constant current driving of LEDs, and use feedback output current instead of detecting output voltage for feedback control.

Fig. 6(a) shows an LED driving circuit using a Buck converter. Unlike a conventional Buek converter, the switching tube S is moved to the rear of the inductor L, so that the S source is grounded, thereby facilitating the driving of the S, and the LED is connected in series with the L. The freewheeling diode D is anti-parallel with the series circuit. The driving circuit is simple and does not require an output filter capacitor, which reduces the cost. However, the Buck converter is a buck converter and is not suitable for applications where the input voltage is low or multiple LEDs are connected in series.


Figure 6(b) shows an LED driver circuit using a Boost converter that pumps the output voltage to a higher desired value than the input voltage through the inductor energy storage to drive the LED at a low input voltage.

Fig. 6(c) shows an LED driving circuit using a Buck-Boost converter. Similar to the Buek circuit, the source of the circuit S can be directly grounded, which facilitates the driving of the S. Boost and Buck-Boosl converters have a larger capacitance than Buck converters, but they all increase the absolute value of the output voltage. Therefore, they are used when the input voltage is low and multiple LEDs need to be driven.

4 super bright LED drive control chip introduction

According to the characteristics of high-brightness LED high-power constant-current driving, many companies have introduced dedicated driver control chips for high-brightness LEDs, such as Melexis, lnfinton, Infineon, Supenex Inc, and Analog. Devices (ADI), etc.

4.1 MLxl0801 chip introduction

The MLXl0801 is an LED driver chip for automotive applications in the Melexi8 public magazine. It can also be used for driving coils such as relays and as an electronic fuse. The block diagram of the MLx10801 chip is shown in Figure 7. It integrates a power MOSFET with a maximum drive current of 350mA. When used as an LED driver, it has the following features:
- The external application circuit is simple;
- internal/external temperature detection protection;
- High efficiency switching power supply drive;
- brightness can be controlled via PWM input;
- The parameters of the LED can be adjusted and stored in the on-chip NV memory.
Although the MLX10801 has only 8 pins, it is powerful, and Table 2 shows the pin functions of the MLXl0801.

Figure 8 shows a typical application circuit of MlJXl0801. The integrated MOSFETs in LED, L, D2, Rsense and MLXl0801 form a typical Buck LED driver circuit. The LEDs in Figure 8 can be connected in series using multiple LEIs depending on the brightness required. The MLXl0801 detects the peak current through the LED by detecting the voltage on the Rsense, compares the current value with the set reference value, and controls the LED current by controlling the pulse width of the DRV0UT. CONTR can be used as an external on/off control or as a PWM signal to control LED blinking. When no control is required, the pin can be connected to the foot VS through the resistor R. DSENSE is used to connect an external thermistor to detect the LED temperature and protect the IED. Although the MLXl0801 has a built-in thermistor inside, the MIX10801 still sets the foot DSENSE to detect the long-distance temperature in order to ensure that the temperature is correctly detected when the chip is far away from the LED. The foot CALIB is used to communicate with the controller for accepting the current of the controller setting LED, allowing the maximum temperature, using internal temperature detection or external temperature detection, anti-shake, soft start time and other parameters.


4.2 Comparison of High-Power LED Driver Chips Table 3 lists the performance comparisons of current major high-power IED driver control chips. When applying high-power IED driver control chips, you can choose according to different applications:

1) When higher power is required, the power device can be selected without a controller integrated in the chip, so that the power device can be separately selected according to actual power requirements;
2) When a higher conversion efficiency is required, such as a portable device, a drive circuit of a switching power supply type can be selected;
3) When applied to equipment with high reliability, a chip with comprehensive control functions such as temperature protection and fault alarm can be selected.

5 Conclusion

With the advancement of technology, the power of ultra-bright LEDs will be further improved, and the cost will be reduced. High-efficiency and energy-saving super bright LEDs will be widely used in the field of lighting. The low-cost, high-reliability drive circuit is the key to ensuring long-lasting brightness of LEDs.