Principle and Application of LCD Backlight Driving Charge Pump

1 Introduction Mobile phones, cordless phones, walkie-talkies and other portable communications products are the use of batteries as a power supply, the rational use of the battery, fully tap its potential to improve efficiency and extend battery life. The LCD screen of the mobile phone needs a high brightness white LED to light up. The white LED needs a stable 5V working voltage. If the working voltage drops, the brightness of the white LED is reduced and the color is not clear. The display effect of the LCD color screen is not satisfactory. White LED can not be directly powered by the battery, because the battery started to use, the voltage decreases, the impact of display. Therefore, the circuit design requires the use of a step-up charge pump to drop the voltage in a long period of time stable at 5V. 2 charge pump classification, working principle and typical application circuit 2.1 Charge Pump Classification Charge Pump can be divided into: - Switching regulator booster pump, as shown in Figure 1 (a) below. - No adjustment Capacitive charge pump, as shown in Figure 1 (b) below. - Adjustable capacitive charge pump, as shown in Figure 1 (c) below 2.2 Working Process Three kinds of charge pump work processes are: first store energy, and then release the energy in a controlled manner to obtain the required output voltage. Switching regulators booster pumps use inductors to store energy, while capacitive charge pumps use capacitors to store energy. Capacitive charge pump through the switch array and oscillator, logic circuit, compare the controller to achieve voltage increase, the use of capacitors to store energy. Due to the higher frequency of operation, the use of small ceramic capacitors (1μF), occupy the smallest space, lower cost. The charge pump can provide ± 2 times the output voltage with only an external capacitor. The losses come mainly from the capacitor's equivalent series resistance (ESR) and the internal switching transistor's RDS (ON). The charge pump converter does not use an inductor, so its radiated EMI can be neglected. Input noise can be filtered with a small capacitor. Its output voltage is factory pre-trimmed and can be adjusted via a back-end on-chip linear regulator so the charge pump can be designed to increase the number of switching stages of the charge pump to provide adequate activity for the back-end regulator space. Charge pumps are ideal for portable applications. The internal structure of the capacitive charge pump shown in Figure 2. It is actually a system of reference, comparison, conversion and control circuits. 2.3 Several different charge pump applications Figure 3 (a), (b), (c) for the three typical application circuit. AAT3110 parallel output power supply output, less external components, alignment is simple, high conversion efficiency. AAT3110 micro-boost power charge pump is the United States Yannuo Logic Technology Co., Ltd. (AATI) developed micro-power switch capacitor voltage boost converter. It provides a stable 5V output voltage with no other inductor necessary for operation of the booster pump and only three small ceramic capacitors on the periphery. It can output 100mA current, can drive 4 ~ 5 white or blue LED, in order to meet the color LCD backlight application. AAT3110 is characterized by small quiescent current, high conversion efficiency, load capacity, is ideal for battery power applications. The AAT3110 operates from a high-frequency boost of 750kHz and uses pulsed-hopping to extract a stable output voltage from a varying input voltage. Input voltage range of 2.7V ~ 5.5V, 5V output voltage stability, the conversion efficiency of more than 90%, output ripple much smaller than similar products, ESD is greater than 2kV, with short circuit protection, over-temperature protection. The AAT3110 consumes little power, at only 13μA, with less than 1μA in shutdown. Compared with similar products of other companies, the AAT3110 has a large output current and small ripple (as shown in Figure 4). The AAT3110 is low in price and can supply power to the parallel-connected LEDs in practical applications (as shown in Figure 5). Low cost of technology, peripheral circuits do not use inductors, no EMI radiation.

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