LED backlight strategy for large and medium size LCD screen

In recent years, LED has made great progress in backlight applications, not only gained popularity in the application of small size backlight LCD display, and began to enter in need of large size LCD display backlighting applications with higher performance and longer working hours, such as GPS, P-DVD, digital photo frame (DPF), and the Internet and other devices.

LED backlight technology has many advantages, including longer life, lower power consumption. More importantly, LED fully comply with environmental requirements, will not cause environmental pollution and other issues.

LED is a current mode device, the need for dedicated driver IC, its brightness is proportional to the current flowing. IC driver with the current control LED constant voltage source and constant current source in two ways. The former can not accurately control the brightness of the LED, while the latter can well avoid the impact of LED discreteness, so as to accurately control the brightness of LED. Constant current source control mode is the main mode of LED driver. The following combination of Wuxi core Microelectronics Co., Ltd. LED backlight driver circuit to do some introduction.

Linear constant current drive, also known as LDO current drive, this method is the lowest cost, and there is no EMI problem, but the main problem is that the battery voltage drops below a certain value, the brightness will change significantly. Therefore, this drive IC method is generally used in low-cost portable products. Core microelectronics AP3600, AP3601, AP3602 and so is the driver IC.

The hybrid mode of charge pump solves the problem of low output current when the battery voltage is low. Depending on the input voltage, the chip operates in a linear, 1.5X or 2X mode. This method has a wide range of input voltage, without inductance, small size and other advantages, widely used in mobile phones and other small LCD screen. These drivers include IC, AP3324, AP3325, etc..

Compared with the charge pump mode, PWM constant current drive mode can be higher than the input voltage, and the stronger the drive can support a greater number of LED, can be applied to larger products (such as GPS, P-DVD, DPF, and the Internet, etc.) LCD backlight module. Core microelectronics AP3127, AP3293 and other PWM constant current drive IC.

AP3127, AP3293 is the core of the micro - Peng for the design of the size of the screen LCD boost backlight LED driver circuit, with up to 1MHz working frequency, allowing the use of small external inductance and capacitive components. The LED is connected in series in order to ensure that the current flowing through each LED is the same, so as to achieve the same brightness. The feedback voltage of the 250mV makes it possible to minimize the power consumption on the current setting resistors and optimize the efficiency of the LED driver applications as much as possible.

AP3127 peak current can be as high as 500mA, and can withstand voltages up to 22V, can drive a string of 6 LED and three series up to 21 LED, using SOT23-6L packaging.

The peak current of the AP3293 can be as high as 1000mA, can withstand up to 40V voltage, drive a string of 10 LED and three string up to 24 LED, using SOT23-6L or WDFN8 package.

Since the feedback voltage of the AP3127/3293 is only 250mV, the power consumption on the resistor is very small, the efficiency is greatly improved, and the service time of the battery is prolonged.

Due to the direct control of the output current rather than voltage, when the load resistance becomes large, the output voltage must be increased to maintain a constant current. Once the load is switched on, the output voltage will rise up to the maximum allowable voltage of the chip. Conventional driver chips generally require a zener diode in parallel with the LED to limit the output voltage. In the design of AP3127/3293, the problem of open circuit protection is considered, and an overvoltage protection circuit is added inside.

In addition, the chip supports PWM dimming, the use of human visual pause principle, with a certain frequency and duty cycle, periodically control white LED conduction current switch back and forth between zero current to the rated current, so as to adjust the brightness. Compared with the dimming method for changing the driving current, the advantage of the dimming method is that the color shift is not generated. In the application, in order to ensure that the human eye can not see the LED flicker, PWM dimming frequency is generally greater than 100Hz.AP3127/3293 support 20Hz~100kHz dimming frequency range, greatly facilitate the system design.

For the 10 inch to 12 inch netbook, chipown microelectronics is developing its AP3295. input voltage at 4.5V~24V, which makes the LED backlight module is compatible with the original input voltage CCFL, can also be powered directly from the battery or adapter, thereby reducing the power conversion part, provided the overall high efficiency system. The scheme also supports PWM dimming. Because of the compatibility of the interface, the user only needs to change the backlight from the CCFL lamp to the aluminum based LED light bar, and the high voltage AC driver is replaced by the low voltage DC driver. Its integrated cost is very close to the traditional CCFL backlight module.

AP3127/3293/3295 has the function of soft start, over voltage, under voltage and over temperature.