Forward voltage drop (VF) and forward current (IF) curve, the curve indicated that, when the forward voltage exceeds a certain threshold value (about 2V), known as the turn-on voltage, the can be approximated that, IF and VF is proportional. Table is currently the main ultra-bright LED’s electrical characteristics. Can see from the table, the current maximum IF Ultra Bright LED up to 1A, while the VF is usually 2 ~ 4V.
As the LED’s optical properties are usually described as a function of current, rather than a function of voltage, luminous flux (φV) and the relationship between the IF curve, therefore, constant current source driver can better control the brightness. In addition, LED’s forward voltage drop larger range (up to 1V above), but the picture above the VF-IF curve indicated, VF will cause small changes in the larger, IF change, causing more brightness big change. Therefore, using constant voltage LED light source driver can not guarantee the consistency and affect LED reliability, lifetime and lumen maintenance. Therefore, the super-bright LED is usually driven by constant current source.
LED temperature and flux (φV) curves can see from the next graph is inversely proportional to flux and temperature, 85 ℃ 25 ℃ when the flux is half the time, but time for a 40 ℃ 25 ℃, the output is 1.8 times. Temperature on the wavelength of LFD have some effect, therefore, good heat dissipation is guaranteed LED brightness constant.
1.)General LED drive circuit description
LED power level due to the restrictions, often to simultaneously drive multiple LED brightness to meet the demand, therefore, require specialized driver circuit to illuminate the LED. Introduced the concept of following LED-type drive circuit.
Current limiting driver circuit is a simple drive circuit, the current limiting resistor is calculated as follows.
Vin to the circuit’s input voltage: VF for the IED of forward current; VF as the LED forward current, IF the pressure drop; VD for the anti-anti-diode voltage drop (optional); y the number of LED per string ; x is the number of parallel LED string.
LED linear mathematical model of the formula: Vo for the opening of a single LED voltage drop; Rs for a single LED in the linearity of the equivalent series resistance. Limiting resistor is on the type of calculation can be written as when the resistance is chosen, the current limiting circuit of the IF and VF grounds of the above equation the relationship between the current limiting circuit is simple, but the fluctuations in the input voltage, the current through the LED will follow the change, so adjust poor performance. In addition, because the access resistance R loss of power xRIF, therefore inefficient.
2.)Linear Regulators Description
Linear regulators are using the work in the linear region of the power transistor or MOSFFET as a dynamic variable resistor to control the load. Linear regulator with parallel and series two.
Parallel linear regulator, also known as shunt regulator (in fact the load may be more LED in series, the same below), which in parallel with the LED, LED when the input voltage increases or decreases, the current through the shunt regulator will increases, this will increase the voltage drop across current limiting resistor, so that a constant current through the LED. As the shunt regulator requires a resistor in series, so efficiency is not high, and relatively wide input voltage range of circumstances is very difficult to constant adjustment. Series regulator, when the input voltage increases, the increased dynamic resistance regulation in order to maintain the LED’s voltage (current) constant. As the power transistor or MOSFET conduction tube has a saturation voltage, therefore, the minimum input voltage must be greater than the saturation voltage and load voltage and the circuit to work properly.
The drive technology is not only limited by the input voltage range, and low efficiency. In general for low-power LED driver, because the current only a few mA, so loss is not obvious, when used as a current of several hundred mA or more of the bright LED driver, the power circuit of the loss becomes more serious problem. Switching power supply is the most efficient energy transformation can be more than 90%. Buek, Boost and Buck-Boost power converter such as LED drivers can be used only to meet the constant current LED driver using the detection output current rather than detecting the output voltage feedback control.
Buck Converter with LED drive circuit, and the different traditional Buek converter, switch S moved to the back of inductance L, so S source grounded, thus facilitating S’s drive, LED and L series, while the freewheeling diode D anti-parallel with the series circuit, the drive circuit is not only simple and does not require the output filter capacitor, reducing the cost. However, Buck converter is a buck converter, input voltage does not apply to low-or multiple LED series of occasions.
Boost Converter with LED driver circuit, the output voltage by inductive energy storage pump to a higher expected value than the input voltage to achieve the low input voltage of the LED driver. Advantage is that the driver IC outputs can be used in parallel, effectively improve the single LED power.
Buck-Boost Converter with LED drive circuit. And Buek circuit similar to the source of the circuit S can be directly grounded, thus facilitating the S drive. Boost and Buck-Boosl converter although one more than the Buck converter capacitor, however, they can raise the absolute value of the output voltage, so the input voltage is low, and the need to drive multiple LED when used more often.
3.)PWM dimming knowledge description
Mobile phones and other consumer electronics products, the white LED is increasingly being used as a display backlight. Recently, many product designers hope that the white LED brightness can be in different situations to make corresponding changes. This means that white LED drive should be able to support the LED brightness adjustment function. Currently there are three main dimming technology: PWM dimming, analog dimming and digital dimming. Many drivers on the market which can support one or more dimming technology. This article describes the three characteristics of each dimming technology, the product designer can select the specific requirements of the technology.
PWM Dimming (PWM) dimming mode – This is the number of pulses by means of a simple, white LED driver repeatedly switch dimming technology. Application’s system only need to provide wide and narrow pulses of different digital, you can simply change the output current to achieve, so adjust the brightness of white LED. PWM dimming is its ability to provide high-quality white light, and the application of simple and efficient! For example, in mobile phone systems, using a dedicated interface for simple PWM duty cycle of the pulse produce any signal, the signal through a resistor, connected to the EN drive interfaces. Most drive manufacturers support the PWM dimming. However, PWM dimming has its disadvantages. Mainly reflected in: PWM dimming is very easy to make white LED driver circuits generate the human ear can hear the noise (audible noise, or microphonic noise). This noise is returned? Usually belong to white LED driver switching power supply device (buck, boost, charge pump, etc.), the switching frequency of 1MHz or so in, so the drive does not produce a typical application is the human ear can hear the noise. But when the drive when the PWM dimming, if the PWM signal of 200Hz to 20kHz frequency just falls between the white LED drive around the inductor and output capacitor will produce the human ear can hear the noise. Therefore, to avoid the use of design the following low-frequency 20kHz.
We all know the role of a low frequency switching signal winding inductance in common (wire winding coil), will make the mutual inductance between the coils produce mechanical vibration, the mechanical vibration frequency falls just above the frequency, inductor noise by the human ear can hear. Inductance produced some noise, partly from the output capacitor. More and more mobile phone designer ceramic capacitor as the output capacitor drive. Ceramic capacitor with piezoelectric properties, which means: When a low-frequency voltage ripple signal acting on the output capacitor, capacitor will send creak of beeps. When the PWM signal is low, the white LED driver to stop working, the output capacitor through the white LED and the bottom of the resistance to discharge. Therefore, PWM dimming, the output capacitor inevitably have a huge ripple. In short, in order to avoid the PWM dimming when audible noise, white LED drivers should be able to offer beyond the scope of the human ear can hear the dimming frequency! Relative to the PWM dimming, if we can change the resistance RS is also able to change the current flowing through the white LED, which changes LED brightness. We call this technology as analog dimming.
Analog dimming the biggest advantage is that it avoids the transfer of light as the noise arising. In analog dimming technologies, LED’s voltage drop is the wizard will decrease as the LED current decreases, making white LED power consumption is also decreased. But different from the PWM dimming technology, the analog dimming white LED driver is always in work mode, and drive power conversion efficiency decreases rapidly as the decline in output current. Therefore, the use of analog dimming technology tends to increase the overall system power consumption. Analog dimming technology is still a drawback light quality. Because it directly changes the current white light LED, making the quality of white light LED white light has changed!
In addition to PWM dimming, analog dimming, now some manufacturers support the drive digital dimming. Digital dimming technology with white LED drivers have the appropriate digital interface. The digital interface can be SMB, I2C, or a single-wire digital interface. The system designer as long as the specific communication protocol, to drive a string of digital signals, you can make white LED light to change.