Over the years, light-emitting diodes (Light Emitting Diode, LED) has been widely used in the state of instructions and information display, we are now more common in the red, green and yellow, blue and wide selection of portable applications equipment white products. For example, the white LED is considered the ideal color display backlighting, but in these new LED design power, we must pay attention to its own characteristics. This article will introduce the old and new LED driver, as well as the characteristics of the power they needed performance requirements.
LED lit the simplest way is through the LED series resistance on the voltage source, as long as the working voltage (VB) remains unchanged, will be issued a fixed brightness LED light, but in fact will be bright as the ambient temperature rising weakened, we can control resistance to change luminous intensity.
5 mm diameter standard LED, Figure 1 shows the forward voltage (VF) and forward current (IF) the relationship between, please note that the two ends of the LED forward current pressure will increase with the rise, assuming 10mA forward current of a single green LED in the fixed 5 V to work, then RV is equivalent series resistance (5 V-VF), / 10mA = 300 ohms, the general LED forward voltage of 2 V, shown in Figure 2. Such common light-emitting diodes used arsenic, gallium, phosphorus and other material production portfolio, it is relatively easy to deal with the design, but also for the broad design engineers are familiar with, they have many advantages:
■ luminous color, or wavelength LED forward current, voltage and temperature change is basically to maintain the stability of the standard green LED luminescence wavelength of about 565 nm, the error was only about 25 nm, it will be Parallel to these LED and there will be no problems (Please see Figure 3), the color difference is very small, forward voltage of the normal range of fluctuation variances caused by luminous intensity is not high, we usually can be ignored with the manufacturers, as well as with a number of products any differences between.
■ in the forward voltage at 10 mA forward current is not much difference before, the red LED on the value of change, about 200 mV, 400 mV compared with the other color, please refer to Figure 1.
■ forward current of less than 10 mA circumstances, blue or white LED forward voltage to the low many, it can directly from lithium-ion batteries or three nickel-metal hydride batteries to supply low cost, standard LED control the work of the circuit costs very low. If the voltage is higher than the work of LED maximum forward voltage requirements, then they do not need boost converter or complex and expensive current source, in fact as long as the application can be accepted when the battery is running low when the luminous intensity will be reduced, then LED can also directly from lithium-ion batteries or three nickel-metal hydride batteries.
To the blue LED in a very long period of time have not been developed, design engineers can use the existing red, green and yellow light, early in the components of the blue LED is not true, but a Blue color coating material coated luminous body, a true blue LED, a few years ago using pure silicon carbide (SiC, Silicon Carbon) materials development, but the luminous efficiency is poor, the new generation of components, the introduction of the new GaN substrate material, and meet several times in the first generation version of the luminous efficiency is used in the blue LED material called indium gallium nitride (InGaN, Indium-Gallium-Nitride), luminescence wavelength in the 450 nm to 470 nm the scope of InGaN LED can provide GaN LED five times the luminous intensity.
In fact the real white LED did not realize, such as components in the white LED is not on the spectrum, a very difficult, so we will be white as a mixture of several wavelengths, in the manufacture of white LED must be taken when certain skills, the basement will be blue InGaN materials to be issued by the Blu-ray excitation and the conversion of yellow cover, which has a mixed blue and yellow, will be on the eye for color is white, in Figure 4 shown.
Since white is mixed from several wavelengths, we are unable to define a specific white LED wavelength, white LED colors on the color coordinates by definition, these XY axis to the value of the international Commission on Illumination (CIE, the Commission Internationale De L'Eclairage ) 15.2 document instructions, the white LED performance indicators usually marked increase in a forward current color coordinates of these changes, please refer to Figure 5, unfortunately, the use of InGaN LED technology is not as standard red, green and Yellow types of products that can be dealt with easily.
The main InGaN LED luminescence wavelength, which is current color will be a positive change with the change in Figure 6. For example, white LED material will be different because of conversion, migration to produce colors, as well as with the blue InGaN LED forward voltage caused by changes in the wavelength change, these color changes can be seen in Figure 5, where X and Y axis on the changes represent the color change, as previously mentioned, and there is no specific white LED wavelength. In the forward voltage at 10 mA forward current will be substantial changes, changes in the range of approximately 800 mV, and some LED material change even greater, battery discharge voltage caused by the change will change color, which were attributed to a positive voltage Current changes. At 10 mA forward current circumstances, forward voltage of approximately 3.4 V, a figure determined by manufacturer, the range of approximately from 3.1 V to 4.0 V. Current-voltage characteristics of the LED is also different between the small differences do not exist, directly from the battery voltage LED driver is very difficult, because most of the state after the battery discharge may LED forward voltage below the minimum requirement.
At present the use of portable and battery-powered equipment are used as backlighting white LED lighting, in particular mobile phone, PDA, such as the color display screen needs to white backlight color close to the original color reduction, in most cases, a single white LED and can not meet the demand, the need for LED to a few stars parallel manner.
Figure 7 shows the number of stars random selection of white LED current and voltage curves, will be added to 3.3 V voltage on these LED, please refer to above the dotted line, will have 2 to 5 mA mA the forward current, resulting in changes in the different colors of white, Please refer to Figure 5. In the region of the Y-axis particularly strong changes brought by the lighting display color reproduction distortion, LED itself also has different luminescence intensity, resulting in uneven lighting. Another problem is the minimum required power supply voltage, let LED work must be more than 3 V voltage, lower-potential, and some may be completely LED does not shine.
In the lithium-ion battery is fully charged can provide approximately 4.2 V output voltage, and in a short period of time after the standard will be reduced to about 3.5 V, the battery voltage after discharge will continue to decline further to 3.0 V, so if White LED directly from the battery-powered, there will be the following questions. First, the battery is fully charged, all light-emitting diodes have been able to provide lighting, but there are different luminous intensity and color output when the battery voltage down to the standard of punctuality and luminous intensity will be reduced, while the white color there will be more big difference, design engineers must take into account the battery voltage value, as well as the LED forward voltage resistor in series to calculate the value in the discharge of the battery, some not even LED luminescence.
LED power supply is designed to provide a sufficiently high output voltage, current and the same size available to the parallel connection LED, in Figure 5, if all connected parallel white LED is the current flowing through the same, then will have the same color Coordinate US letter (Maxim) companies against this application provides a control function with the current charge pump products MAX1912.
Figure 8 to the description of the three parallel LED circuit configuration example, a pump will be 1.5 times the input voltage upgrade the type, usually early pump will only redouble its input voltage, but this new technology can provide better efficiency, will be upgraded to input voltage LED just enough work requirements. Connected to the SET pin (pin 10) resistance can ensure that all LED current equal to the internal circuitry SET voltage will be maintained at 200 mV, the current flows through the LED for any ILED = 200mV/10 Ω = 20 mA. If some of the LED current requires less, then we can more than three parallel LED because MAX1912 can provide up to 60 mA current output, reference MAX1912 can be further information and examples of circuit.
If the system can provide diode forward voltage higher than the voltage source, white LED can be simple to work, for example, digital cameras usually include a +5 V power supply, and in this case we do not need to boost function, because includes ownership of the supply voltage of the LED promote adequate Yuyu space. To the circuit in Figure 8, we should choose matching current source, as an example, the MAX1916 can drive up to three parallel LED, please see figure 9.
Working principle is quite simple, RSET control of the resistor connected through the LED current, this way very few of the occupied board space, in a small 6-pin SOT23 packaging chips, and few bypass capacitor, resistor only a plus, LED this and can bring between 0.3% of the outstanding current matching, this configuration provides the same full color position, it will allow each of the same type of LED white.
Portable devices can be part of the environment in accordance with the lighting intensity light output control of the situation, other standby through in a short time by software control to reduce emission intensity, the two work needed LED can be controlled, and the brightness control function should be to the the same forward current manipulate color coordinates so as to avoid the offset, this consistency can be controlled through the use of a current flowing through resistor RSET small digital analog converters to achieve.
6-bit resolution with the converters, such as I2C compatible interface with the MAX5362 SPI compatible interface or with the MAX5365 can bring 32 bands luminous intensity control functions, please refer to Figure 10. Issued by the white LED light will change due to changes in brightness, will be affected because of forward current color coordinates, but this will not be a problem, because the same forward current in the circuit will each diode sent the same light.
One will not affect the brightness of color coordinates known as pulse width modulation control mode PWM (Pulse Width Modulation), can be activated or shut-off function with the most power components to achieve, for example, components for the MAX1916 EN pin pulled low and the potential closure would allow leakage current flowing through the LED to only 1 μ A, it will not luminescence, will be sent EN pin high potential will default forward current flows through the LED, if we veins Add EN width modulated signal to his feet, then we will be able to brightness of the signal is proportional to the effective duty cycle.
Since each LED with the same forward current, and therefore will not color coordinate mobile, but our eyes probably aware that the effective rate cycle changes and that change brightness, higher than the frequency of 25 Hz to avoid the eyes discernible , therefore the PWM brightness control applications, 200-300 Hz switching frequency is a very good choice, a higher frequency may be a problem, because color coordinates will be in LED switch within a short period of time caused the partial color coordinates shift. PWM signal can be output from the microprocessor to the pin or external to provide brightness steps with the changes in the application in accordance with the purpose of this count and width of the buffer.
Apart from the above-mentioned MAX1912 pump components, we can also be used with current control function boost converter, such as the MAX1848 switch-mode voltage converter can generate up to 13 V output voltage, which are sufficient to drive up 3 LED series, please see figure 11, this approach may be the most simple way, the series connected LED necessarily have exactly the same current, and the size of LED current from RSENSE feet and then added to the CTRL voltage decision .
MAX1848 can be according to the above-mentioned one of the ways to achieve brightness control function, the forward current flows through the LED Add CTRL proportional to the voltage pin, as in the Add CTRL MAX1848 voltage less than 100 mV, the Commissioner will enter mode, we can also use it to achieve PWM brightness control function.
LED lit the simplest way is through the LED series resistance on the voltage source, as long as the working voltage (VB) remains unchanged, will be issued a fixed brightness LED light, but in fact will be bright as the ambient temperature rising weakened, we can control resistance to change luminous intensity.
5 mm diameter standard LED, Figure 1 shows the forward voltage (VF) and forward current (IF) the relationship between, please note that the two ends of the LED forward current pressure will increase with the rise, assuming 10mA forward current of a single green LED in the fixed 5 V to work, then RV is equivalent series resistance (5 V-VF), / 10mA = 300 ohms, the general LED forward voltage of 2 V, shown in Figure 2. Such common light-emitting diodes used arsenic, gallium, phosphorus and other material production portfolio, it is relatively easy to deal with the design, but also for the broad design engineers are familiar with, they have many advantages:
■ luminous color, or wavelength LED forward current, voltage and temperature change is basically to maintain the stability of the standard green LED luminescence wavelength of about 565 nm, the error was only about 25 nm, it will be Parallel to these LED and there will be no problems (Please see Figure 3), the color difference is very small, forward voltage of the normal range of fluctuation variances caused by luminous intensity is not high, we usually can be ignored with the manufacturers, as well as with a number of products any differences between.
■ in the forward voltage at 10 mA forward current is not much difference before, the red LED on the value of change, about 200 mV, 400 mV compared with the other color, please refer to Figure 1.
■ forward current of less than 10 mA circumstances, blue or white LED forward voltage to the low many, it can directly from lithium-ion batteries or three nickel-metal hydride batteries to supply low cost, standard LED control the work of the circuit costs very low. If the voltage is higher than the work of LED maximum forward voltage requirements, then they do not need boost converter or complex and expensive current source, in fact as long as the application can be accepted when the battery is running low when the luminous intensity will be reduced, then LED can also directly from lithium-ion batteries or three nickel-metal hydride batteries.
To the blue LED in a very long period of time have not been developed, design engineers can use the existing red, green and yellow light, early in the components of the blue LED is not true, but a Blue color coating material coated luminous body, a true blue LED, a few years ago using pure silicon carbide (SiC, Silicon Carbon) materials development, but the luminous efficiency is poor, the new generation of components, the introduction of the new GaN substrate material, and meet several times in the first generation version of the luminous efficiency is used in the blue LED material called indium gallium nitride (InGaN, Indium-Gallium-Nitride), luminescence wavelength in the 450 nm to 470 nm the scope of InGaN LED can provide GaN LED five times the luminous intensity.
In fact the real white LED did not realize, such as components in the white LED is not on the spectrum, a very difficult, so we will be white as a mixture of several wavelengths, in the manufacture of white LED must be taken when certain skills, the basement will be blue InGaN materials to be issued by the Blu-ray excitation and the conversion of yellow cover, which has a mixed blue and yellow, will be on the eye for color is white, in Figure 4 shown.
Since white is mixed from several wavelengths, we are unable to define a specific white LED wavelength, white LED colors on the color coordinates by definition, these XY axis to the value of the international Commission on Illumination (CIE, the Commission Internationale De L'Eclairage ) 15.2 document instructions, the white LED performance indicators usually marked increase in a forward current color coordinates of these changes, please refer to Figure 5, unfortunately, the use of InGaN LED technology is not as standard red, green and Yellow types of products that can be dealt with easily.
The main InGaN LED luminescence wavelength, which is current color will be a positive change with the change in Figure 6. For example, white LED material will be different because of conversion, migration to produce colors, as well as with the blue InGaN LED forward voltage caused by changes in the wavelength change, these color changes can be seen in Figure 5, where X and Y axis on the changes represent the color change, as previously mentioned, and there is no specific white LED wavelength. In the forward voltage at 10 mA forward current will be substantial changes, changes in the range of approximately 800 mV, and some LED material change even greater, battery discharge voltage caused by the change will change color, which were attributed to a positive voltage Current changes. At 10 mA forward current circumstances, forward voltage of approximately 3.4 V, a figure determined by manufacturer, the range of approximately from 3.1 V to 4.0 V. Current-voltage characteristics of the LED is also different between the small differences do not exist, directly from the battery voltage LED driver is very difficult, because most of the state after the battery discharge may LED forward voltage below the minimum requirement.
At present the use of portable and battery-powered equipment are used as backlighting white LED lighting, in particular mobile phone, PDA, such as the color display screen needs to white backlight color close to the original color reduction, in most cases, a single white LED and can not meet the demand, the need for LED to a few stars parallel manner.
Figure 7 shows the number of stars random selection of white LED current and voltage curves, will be added to 3.3 V voltage on these LED, please refer to above the dotted line, will have 2 to 5 mA mA the forward current, resulting in changes in the different colors of white, Please refer to Figure 5. In the region of the Y-axis particularly strong changes brought by the lighting display color reproduction distortion, LED itself also has different luminescence intensity, resulting in uneven lighting. Another problem is the minimum required power supply voltage, let LED work must be more than 3 V voltage, lower-potential, and some may be completely LED does not shine.
In the lithium-ion battery is fully charged can provide approximately 4.2 V output voltage, and in a short period of time after the standard will be reduced to about 3.5 V, the battery voltage after discharge will continue to decline further to 3.0 V, so if White LED directly from the battery-powered, there will be the following questions. First, the battery is fully charged, all light-emitting diodes have been able to provide lighting, but there are different luminous intensity and color output when the battery voltage down to the standard of punctuality and luminous intensity will be reduced, while the white color there will be more big difference, design engineers must take into account the battery voltage value, as well as the LED forward voltage resistor in series to calculate the value in the discharge of the battery, some not even LED luminescence.
LED power supply is designed to provide a sufficiently high output voltage, current and the same size available to the parallel connection LED, in Figure 5, if all connected parallel white LED is the current flowing through the same, then will have the same color Coordinate US letter (Maxim) companies against this application provides a control function with the current charge pump products MAX1912.
Figure 8 to the description of the three parallel LED circuit configuration example, a pump will be 1.5 times the input voltage upgrade the type, usually early pump will only redouble its input voltage, but this new technology can provide better efficiency, will be upgraded to input voltage LED just enough work requirements. Connected to the SET pin (pin 10) resistance can ensure that all LED current equal to the internal circuitry SET voltage will be maintained at 200 mV, the current flows through the LED for any ILED = 200mV/10 Ω = 20 mA. If some of the LED current requires less, then we can more than three parallel LED because MAX1912 can provide up to 60 mA current output, reference MAX1912 can be further information and examples of circuit.
If the system can provide diode forward voltage higher than the voltage source, white LED can be simple to work, for example, digital cameras usually include a +5 V power supply, and in this case we do not need to boost function, because includes ownership of the supply voltage of the LED promote adequate Yuyu space. To the circuit in Figure 8, we should choose matching current source, as an example, the MAX1916 can drive up to three parallel LED, please see figure 9.
Working principle is quite simple, RSET control of the resistor connected through the LED current, this way very few of the occupied board space, in a small 6-pin SOT23 packaging chips, and few bypass capacitor, resistor only a plus, LED this and can bring between 0.3% of the outstanding current matching, this configuration provides the same full color position, it will allow each of the same type of LED white.
Portable devices can be part of the environment in accordance with the lighting intensity light output control of the situation, other standby through in a short time by software control to reduce emission intensity, the two work needed LED can be controlled, and the brightness control function should be to the the same forward current manipulate color coordinates so as to avoid the offset, this consistency can be controlled through the use of a current flowing through resistor RSET small digital analog converters to achieve.
6-bit resolution with the converters, such as I2C compatible interface with the MAX5362 SPI compatible interface or with the MAX5365 can bring 32 bands luminous intensity control functions, please refer to Figure 10. Issued by the white LED light will change due to changes in brightness, will be affected because of forward current color coordinates, but this will not be a problem, because the same forward current in the circuit will each diode sent the same light.
One will not affect the brightness of color coordinates known as pulse width modulation control mode PWM (Pulse Width Modulation), can be activated or shut-off function with the most power components to achieve, for example, components for the MAX1916 EN pin pulled low and the potential closure would allow leakage current flowing through the LED to only 1 μ A, it will not luminescence, will be sent EN pin high potential will default forward current flows through the LED, if we veins Add EN width modulated signal to his feet, then we will be able to brightness of the signal is proportional to the effective duty cycle.
Since each LED with the same forward current, and therefore will not color coordinate mobile, but our eyes probably aware that the effective rate cycle changes and that change brightness, higher than the frequency of 25 Hz to avoid the eyes discernible , therefore the PWM brightness control applications, 200-300 Hz switching frequency is a very good choice, a higher frequency may be a problem, because color coordinates will be in LED switch within a short period of time caused the partial color coordinates shift. PWM signal can be output from the microprocessor to the pin or external to provide brightness steps with the changes in the application in accordance with the purpose of this count and width of the buffer.
Apart from the above-mentioned MAX1912 pump components, we can also be used with current control function boost converter, such as the MAX1848 switch-mode voltage converter can generate up to 13 V output voltage, which are sufficient to drive up 3 LED series, please see figure 11, this approach may be the most simple way, the series connected LED necessarily have exactly the same current, and the size of LED current from RSENSE feet and then added to the CTRL voltage decision .
MAX1848 can be according to the above-mentioned one of the ways to achieve brightness control function, the forward current flows through the LED Add CTRL proportional to the voltage pin, as in the Add CTRL MAX1848 voltage less than 100 mV, the Commissioner will enter mode, we can also use it to achieve PWM brightness control function.
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