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Small size LCD display module has long been an important part of handheld digital products. With the increasing demands of consumers on visual aspects, the design of LCD display modules has become more and more important. How to display more information on the 1.8-inch to 2.8-inch LCD screen and improve the display quality to achieve better visual effects has become one of the focuses of many handheld digital product designers. In addition to increasing the resolution of the LCD screen, reducing the latency, and enhancing the software, the LCD screen's backlight design also plays an important role. Traditional design, small size color LCD screen generally uses white LED as the backlight. 1.8-inch to 2.8-inch LCD screen to use more 1 to 4 white LED. In order to achieve a good display, white LED brightness requirements consistent, adjustable brightness; to protect the handheld device requires some LED open circuit protection, and can not affect the normal LED work; to extend the standby time of handheld devices , Requiring LED driver IC quiescent current is very small, high efficiency. In addition, the handheld device integrates many functional modules, the interference between the modules must be reduced to maintain the stability of the entire system, and the white LED driver IC is no exception. BCD introduced the AP3605 is a high performance LED driver IC, is ideal for single-cell lithium battery powered, suitable for most small-size LCD backlight design, cost-effective. The following describes the AP3605's performance, application programs and features. First, AP3605 Introduction AP3605 is a charge pump boost mode with constant current source of white LED driver chip, only through two ceramic capacitors 1uF, AP3605 achieved 1.5 times the boost in a single lithium-powered range can drive 4 The white LED, which is twice as fast as the boost mode, greatly improves efficiency. AP3605 with 4-channel constant current source to ensure that the parallel 4 LED current equal, the same brightness; through an external resistor to set the constant current source current. AP3605 supports PWM dimming, dimming frequency can be up to 50KHz. Up to 1MHz operating frequency makes AP3605 only need 1uF input and output and boost capacitors, and greatly reduce the output voltage ripple. Powered from 2.7V to 5.5V, the AP3605 is designed for single-cell Li-Po design and features a built-in soft-start feature that greatly reduces inrush current during startup, protecting the Li-ion battery and extending battery life. The AP3605 is available in a QFN-3 * 3-16 package. Second, AP3605 backlight typical application circuit Designed by the AP3605 typical white LED driver circuit, powered by a single lithium battery, in addition to AP3605, only by the low-cost 5 peripheral devices. 1, input, output and boost capacitor: Cin, Cout, C1 and C2 are recommended for use with X7R or X5R ceramic capacitors. Due to the operating frequency of the AP3605 up to 1MHz, a 1uF capacitor is sufficient to ensure that the AP3605 outputs 80mA to drive four LEDs. If the desired output current is smaller, use 0.68uF or 0.47uF boost capacitors (C1 and C2). In order to reduce the output ripple, it is recommended that when designing the PCB board, Cout should be given priority to the pins corresponding to the AP3605, followed by the pins C1, C2 and Cin. 2, resistance RISET RISET used to set the LED current, ILED = 36.9 / RISET can be set by the current of each LED. In order to accurately set the current, it is recommended to use 1% or higher precision resistors. 3, white LED AP3605 can drive up to 4 white LED, using a common cathode connection, the customer can choose from 1 to 4 LED screen size as the backlight. When a few LEDs are not needed, just leave their corresponding pins floating. Third, AP3605 white LED driver IC as an important performance 1, the current matching current matching LED driver IC is an important performance. LED is a current-driven device, LED brightness is proportional to the forward current, and the same current, the difference between the positive voltage drop, so in order to ensure uniform brightness, it is best to use constant current drive. As shown in Figure 1, the AP3605 has four constant current sources internally for driving four LEDs. When the forward voltage drop of the LEDs varies from 3V to 4V, the AP3605 guarantees a current deviation of no more than ± 3% , The current matching characteristics are not affected by the forward voltage drop LED, suitable for many types of white LED, a very good LED brightness consistent.