subject: Why Linear Led Driver Ic Must Replace Bias Resistor - Electronics Industry - 3d/4d Driving Flight [print this page] Using simple resistor limits the current in the LED string seems the most economic and simple, so why should we think hard and use of linear drive IC?
Fact, the linear drive, a lot of benefits, including:
Avoid using complicated, expensive device (the forward voltage compensation for the changing needs of different types of resistors). By pulse-width modulation (PWM) mode adjust the brightness.
system unstable because the power can be used to reduce the system cost.
reduce the required board space. use more LED, and improve overall efficiency.
ideal LED bias and protection, maximum service life.
In many applications, such as signs, rope lights, billboards, architectural lighting, automotive lighting, aircraft lighting and so on, the linear drive can meet the relevant applications.
Addition to the above advantages, the use of battery-powered and need to boost the power supply voltage for mobile devices, linear actuator also has its limitations.
Below describes the advantages of linear LED drivers, LED drivers for the deployment of advice.
Constant edgeAs with Diode, LED needs a constant current source, rather than a constant voltage source. Use of series resistance allows LED current to maintain constant pressure drop requires a lot of resistors, it will reduce system efficiency. On the other hand, if the system voltage or LED forward voltage changes in a small series resistor voltage drop will result in a larger required LED current bias.
To maintain a constant LED current to prevent because the system voltage or LED forward voltage generated by the over-current LED damage caused by overheating so. Depending on the LED forward voltage adjustment resistors in series, this approach is outdated. LED drivers help to improve the accuracy of the system brightness, light quality at the same time minimize the reduction.
Fig 1 resistor in series vs constant current LED drive LED drive TLE4242
Figure 1 shows the use of the three nominal forward current of 350mA of LED automotive applications. Select series resistor with great difficulties: the low battery voltage conditions, low forward current, LED brightness is insufficient. If there is transient (load dump, dual battery), LED is likely to suffer. Low battery voltage conditions, the constant current source to prevent LED damage, provide better brightness.
Improve system efficiency low pressure drop
LED string depends on the maximum number of LED or LED drive resistor voltage drop. If the use of resistors, in order to obtain the most current needs of the larger constant pressure drop. However, this means to produce heat rather than light. Linear LED driver can lower the pressure drop, providing a constant current, so that you can use in the LED string more LED, to improve the efficiency of the whole system. TLE42xx typical pressure drop for the series of 0.5V, the maximum pressure drop is 0.7V, BCR4xx series of typical voltage drop is 1.2V, the maximum voltage drop is 1.5V.
Not passive filter components And rose to high pressure through the supply chain for the long LED switch mode converter of different, parallel LED string of unique advantages: As the linear drive does not light, so no need to use passive filtering components.
For high-side switch TLE4241 and TLE4242LED drive shutdown mode, quiescent current less than 1 A, making them suitable for high-side switch.
Adjust brightness by PWM Are two ways to adjust the brightness of LED: LED forward current level of adjustment or predefined forward current for PWM regulator. In view of the following two reasons, it is recommended not to use the current method of adjusting forward. First of all, LED range in brightness, and there is no work in the best efficiency point. Second, the forward current is different from the nominal LED current may cause the output of the light color change. PWM dimming signal-regulated by low-frequency PWM LED input, solve the two problems. LED current drive level in a single conduction conditions, the LED brightness can be changed on-the average time to adjust. Depending on the load cycle, the frequency of not less than 200Hz; Typically, 500Hz ~ 1kHz enough to use. PWM control unit integrated to TLE4241, TLE4242 or BCR450 and other single-chip solution. BCR40xLED drive series to allow the completion of an external digital transistor PWM dimming.
LED diagnosticsTo identify faults LED, TLE42xx series of conditions in the status output to indicate open load. It can also be used directly to VCC with pullup resistors connected microprocessors. Figures 2 and 3 Infineon LED drivers for the introduction of different applications.
Protection and security LED usually has a positive temperature coefficient, that is, as the LED forward voltage LED temperature decreases as the temperature increases lead to LED consumes more current. This will potentially lead to loss of control and LED heat damage. Therefore, the need to control the diode current, to keep it constant.
Figure 2 be used in conjunction with an external power stage, low-cost BCR450 Linear LED Driver
TLE4xxx and BCR4xx linear constant current LED driver suitable for harsh environments, such as traffic lighting, building lighting, railways, transportation or automotive applications. The product allows transient voltage up to 45V (the decision by the model), can be as high as the junction temperature of 150 to work, can withstand high heat temperatures. If the system fails, the over-current and over-temperature protection to protect the IC and its application. TLE4xxx series can withstand reverse voltage connection.
Cooling and heat transfer Voltage in the system to changing conditions, in order to minimize the heat, get constant brightness, LED maximum forward voltage should be close (but equal to or less than) minus the LED drive power supply voltage drop. When the LED forward voltage to the minimum value, maximum value when the input voltage, LED drive, the largest loss.
Figure 3 TLE4241 LED driver with PWM control, and open load detection
There are several cooling and temperature gradient appears to prevent the PCB ways:
parallel use of several small and affordable package, subject to the PCB on their separation.
make the drive circuit and power transistor isolation. This principle can also meet the practical requirements of the power transistor diode current.
high performance TAB package (such as small SCT595 or large TO263 package), to achieve good thermal contact with the PCB, the thermal resistance to a minimum.
Summary
Linear constant-current LED drive LED drive is the most cost-effective way. It can increase flexibility and technological superiority
