LED backlight driver for medium size LCD

Medium-sized LCDs ranging from 12 inches to 17 inches are mainly used in portable DVDs, notebook computers and car navigation systems. The LED driving strategy of its backlight application is developed on the basis of the LED backlight driving scheme of the small-size LCD screen, and it is compatible with the current CCFL backlight driver as much as possible in the interface. The user only needs to use the CCFL lamp for the backlight source. The tube is replaced with an aluminum-based LED light-emitting strip, and the high-voltage complex AC driver can be replaced with a low-voltage simple DC driver. The overall cost is very close to that of the traditional CCFL backlight module. Compared with the LED backlight of small-sized LCDs, the current medium-sized LED backlight schemes only increase the number of LEDs, and consider the series and parallel LED groups as a whole load, and control the flow through this The overall current of the LED group adjusts the brightness of the LCD screen. The LD0 driving LED group as shown in Figure 1 is a relatively common driving scheme. It makes full use of the 12V DC input voltage required by the CCFL driver. The total output current value is set by the external sampling resistor R1, and the LDO can maintain it. The total output current is constant and supports PWM dimming. MAX16803 can directly support currents up to 350mA, and the total current can be as high as 2A when an external BJT is connected. Its series of MAX16804 also integrates the PWM generator inside the chip, which can directly support analog dimming signals, further simplifying circuit design. The biggest advantage of LDO driving the LED group is that the circuit is very simple, and there is basically no EMI problem, it is more suitable to be built with the LED light group and placed directly in the LCD screen. The main problem is that the efficiency is relatively low, because the voltage difference between the input voltage and the actual driving voltage required by the LED group is added to the MAX16803 LDO. When the voltage difference is relatively large, not only the overall efficiency is low, but also the MAX16803 The fever is also more. Since there is no current sharing mechanism among the LED strings connected in parallel in the LED group, the only way to reduce the impact of uneven current distribution among the LED strings is to select LEDs with relatively consistent forward voltages. In order to improve the driving efficiency of the entire LED group, replacing LD0 with a switching power supply is the most direct method, as shown in Figure 2. Compared with the LDO drive, the biggest advantage of using a switching power supply is to improve the overall drive efficiency. The input voltage of MAX16819 is 4.5-28V, which makes the LED backlight module compatible with the original CCFL input voltage, and can also be directly powered by a battery or an adapter, which reduces the power conversion link and improves the overall efficiency of the system. This solution also directly supports PWM dimming. Just like LDO driving the LED group, even if the LED group is driven by a switching power supply, it cannot fundamentally solve the effect of uneven current distribution among the LED strings. Controlling the total current of the LED group cannot achieve the even distribution of the current among the LED strings. Even if the LEDs with relatively consistent electrical parameters are selected (which means that the cost of the LED will rise significantly), under the same driving voltage, the total conductance of the LED group The LED string with high turn-on voltage may not be fully turned on. If the total conduction voltage is low, it may be through conduction, and a current exceeding its rated value will flow. As a result, the LED strings will not emit light uniformly, and some LEDs may be damaged in advance. In order to eliminate this hidden danger, the respective characteristics of the switching power supply and LDO can be combined, so that the total power supply of the LED group is realized by the switching power supply to improve the overall efficiency of the system. That is, the LD0 connected in series in each branch is used to detect the current passing through the LED of this branch. The voltage difference between the total output voltage common to the LED group and the actual forward voltage of each LED string is borne by the series LD0, so that The current between the LED strings is the same. As shown in Figure 3, in the loop of each LED branch, the MAX16807 has a built-in LDO, which can automatically adjust the voltage value of the input port according to the passing LED current, that is, the actual load voltage on each string of LED branches. So that the LED current of each branch is consistent with the set value. The system can also control the switching of the LEDs on each branch and the duty cycle of PWM dimming through Pc. Figure 2 Medium-sized LCD backlight-switching power supply driving LED group Figure 3 Medium-sized LCD backlighting-switching power supply + LD0 driving 8 strings of LEDs
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