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>> Analysis of LED Backlighting Strategy for Medium and Large Size LCD Panel 28th,Mar,2017
In recent years, LED backlighting applications have been developed by leaps and bounds, not only in the small size of the LCD display backlight applications are popular, and began to enter the need for higher performance and longer working hours of large-size LCD display backlight applications, Such as GPS, P-DVD, digital video box (DPF), and the Internet and other equipment.
LED backlight technology a lot of advantages, including longer life, lower power consumption. More importantly, LED fully meet the environmental requirements, will not cause environmental pollution and other issues.
LED is a current-mode device that requires a dedicated driver IC. Its brightness is proportional to the current flowing. With the driver IC control LED current generally have constant voltage source and constant current source in two ways. The former can not accurately control the LED brightness, while the latter can be a good way to avoid the impact of LED discrete, and thus precisely control the LED brightness. Constant current source control mode is the current mainstream LED driver mode. The following combination of Wuxi core friends Microelectronics Co., Ltd. LED backlight drive circuit to do some introduction.
Linear constant current drive, also known as LDO current drive, this method is the lowest cost, and no EMI problem, but the main problem is the battery voltage dropped to a certain value below, the brightness will be significantly changed. Therefore, this driver IC method is generally used in low-cost portable products. Chip friends microelectronics AP3600, AP3601, AP3602, etc. is such a driver IC.
The charge pump hybrid mode driver solves the problem that the output current drops significantly when the battery voltage is low. Depending on the input voltage, the chip operates in linear, 1.5X or 2X mode. This way has a wide range of input voltage, no inductance, small size, etc., widely used in mobile phones and other small LCD screen. These driver ICs include Micron Microelectronics AP3324, AP3325 and so on.
Compared with the charge pump method, PWM constant current drive mode can be higher than the input voltage, and drive more powerful, can support a greater number of LED, can be applied to larger size products (such as GPS, P-DVD , DPF, and the Internet and other products) LCD backlight module. Chip friends microelectronics AP3127, AP3293, etc. is the PWM constant current driver IC.
AP3127, AP3293 is the core of the microelectronics designed for medium-size LCD screen design step-up backlight LED driver circuit, using up to 1MHz operating frequency, allowing the use of small external inductance and capacitance components. The LEDs are connected in series to ensure that the current flowing through each LED is the same, resulting in consistent brightness. The 250mV feedback voltage makes the power dissipated on the current setting resistor very small, optimizing the efficiency of white LED driver applications as much as possible.
As the AP3127 / 3293 feedback voltage is only 250mV, so the resistance on the power consumption is very small, greatly improve the efficiency and extend the battery life.
Since the output current is directly controlled rather than the voltage, when the load resistance becomes large, the output voltage must be increased to maintain a constant current. And once the load is open, the output voltage will immediately rise until the chip exceeds the maximum allowable voltage, breakdown chip. The traditional driver chip generally requires a zener diode placed in parallel with the LED at the output to limit the output voltage. The AP3127 / 3293 takes into account the problem of open-load protection in the design, and incorporates an overvoltage protection circuit internally without the need for external diode protection.
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