In recent years, in LED backlight application has got great development, not only in small size LCD backlight application becomes universal, and began to enter needs higher performance and longer working hours medium size LCD backlight application, such as GPS, P - DVD, digital photo frame (DPF), and netbooks and other equipment.
LED backlight technology many advantages, including longer life, lower power consumption. More importantly, LED completely conform to the requirements of environmental protection, not cause environmental pollution and other issues.
LED is current type device, need special driver IC. Its brightness is proportional to the flow of current. With a LED driver IC control current general persistence voltage source and constant current source in two ways. Unable to accurately control the LED brightness, while the latter can avoid LED the effects of discrete, so as to precisely control the LED brightness. Constant current source control mode is the mainstream of LED drive mode. The following core friends in wuxi microelectronics co., LTD., LED backlight driving circuit to introduce some.
Linear constant current drive is also called "all flow drive, the lowest cost of this method, and there is no EMI problem, but the main problem is the battery voltage drops below a certain value, the brightness will be a significant change. Therefore, the driver IC method commonly used in low-cost portable products. Core friends microelectronics AP3600, AP3601, AP3602 is such a driver IC.
Charge pump mixed mode driver solved under the condition of the battery voltage is low, the obvious drop in output current. According to the input voltage, the chip work in linear, 1.5 X or 2 X model. This approach has a wide range of input voltage, no inductance, the advantages of small volume, it is widely used in mobile phones and other small LCD screen. The driver IC friends including core microelectronics AP3324, AP3325, etc.
Compared to the way of charge pump, PWM constant current drive mode can be higher than the input voltage, and stronger drive ability, can support more number of leds, can be applied to larger size of the product (such as GPS, P - DVD, DPF, and netbooks, etc) of the LCD backlight module. Core friends microelectronics AP3127, AP3293 is PWM constant current driver IC, etc.
AP3127, AP3293 is core size LCD screen in the friends of microelectronics for booster type backlight LED drive circuit design, using up to 1 MHZ operating frequency, allowed the use of small external inductance and capacitance component. Leds in the form of series connection, make sure the current flows through each LED the same, to obtain consistent brightness. The feedback voltage of 250 mv makes it current setting resistance on the consumption of power is very small, as much as possible to optimize the efficiency when it is applied in white LED driver.
AP3127 switch tube can be as high as 500 ma peak current, and can withstand up to 22 v voltage, can drive a series of 6 leds and three string of up to 21 LED, the SOT23-6 l packaging format.
AP3293 switch tube of the peak current can be as high as 1000 mA, can withstand up to 40 v voltage, drive a series of 10 three string of up to 24 LED, LED and the SOT23-6 l or WDFN8 packaging form.
Due to AP3127/3293 feedback voltage is 250 mv, only so little resistance on the power consumption, greatly improve the efficiency, prolong the using time of the battery.
Because it is directly control the output current rather than voltage, when the load resistance becomes larger, must increase in order to maintain a constant current output voltage. Once open load, the output voltage would immediately rise beyond chip, the maximum allowable voltage breakdown chips. Traditional driver chip general requirements on the output side LED is placed in parallel with a zener diode to limit the output voltage. AP3127/3293 considered in the design of the open load protection, internally joined a overvoltage protection circuit, without additional diode protection.
In addition, the chip support PWM dimming, using the human eye visual suspension principle, at a certain frequency and duty ratio, periodic control of white LED to the current flow in the zero current switching back and forth between the rated working current, so as to adjust the brightness. Compared to change the way the size of the drive current dimmer, the advantage of this way of dimming is not produce color migration. In application, to ensure that the human eye can't see the LED flashing, PWM dimming generally greater than 100 hz frequency. AP3127/3293 support 20 hz ~ 100 KHZ frequency range that move light, greatly facilitate the system design.
In 10 inches and 12 inches netbooks, core friends microelectronics are developing AP3295. The input voltage of 4.5 V ~ 24 V, the LED backlight module can be compatible with the original CCFL input voltage, can also directly by the battery or adapter power supply, thereby reducing the power transformation, improve the overall efficiency of the system. The scheme also supports PWM dimming. Due to the interface compatibility, users only need to replace the back light by CCFL tube with aluminum LED bar, at the same time to replace high complex ac drive with low pressure can be simple dc drives. The comprehensive cost with the traditional CCFL backlight module has been very close.
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Source: BVSION Electronic