Foreword
At present, mobile phone color screen backlight mainly adopts white light LED (WLED). The function of WLED driver is to provide constant current to WLED, reduce the change of LED brightness caused by battery voltage change and the brightness mismatch between different LEDs. Such WLED drivers have been used in large scale applications in mobile phones over time. From 2003 to 2005, the inductor-boosted WLED driver was popular in mobile phones; the fractional charge pump type WLED driver was used on a large scale from 2005 to 2007; the low-pressure drop constant current that emerged in early 2007 and gradually became popular in 2008 The WLED driver has undergone a series of developments. How to choose the right WLED driver? The following is a step-by-step analysis to find the appropriate architecture for the LED driver as it evolves from the WLED driver.
Series Inductor Boost WLED Driver Architecture
As shown in Figure 1, the series inductor-boost WLED driver is convenient to set the operating current of a string of WLEDs through a resistor. But its biggest problem is that because the inductor is used for boosting, the inductor stores energy by converting electrical energy into magnetic energy. During the working process, electromagnetic conversion is continuously performed, which may generate electromagnetic radiation. And electromagnetic interference, which affects the operation of the RF module in the mobile phone. For example, the acceptance sensitivity index of the mobile phone may be affected and decreased when the backlight module works, so that in the area where the base station signal is not strong, the operation of the backlight module may cause the call quality to decrease, resulting in the user complaining about the quality of the mobile phone.
Figure 1: Typical application diagram of a series inductor boost LED driver
Fractional charge pump type WLED driver architecture
Since 2005, the fractional charge pump WLED driver has gradually become popular in mobile phones. The basic architecture is shown in Figure 2:
Figure 2: Typical application diagram for fractional charge pump W LED driver
Usually, this fractional charge pump type WLED driver is multi-mode, which automatically determines the operating mode required by the driver based on the forward voltage drop (VF) and power supply voltage of the WLED. For example, when the VF of the WLED is 3.3. When V, the battery voltage is above 3.8V. Through the internal mode judgment circuit, the driver can stay in the 1X working mode. When the battery voltage drops below enough to maintain the set current of the WLED, the boost pump can be boosted by the charge pump. the goal of.
The advantages of this fractional charge pump type WLED driver are: elimination of inductance, reduced interference to the RF circuit of the mobile phone, and replacement of the inductor energy storage with inexpensive capacitor energy storage, and the cost of peripheral components is further reduced. The package of this chip is generally a QFN 3mm × 3mm 16L package.
The operation of the fractional charge pump type WLED driver has a characteristic: in an ideal case, when the driving current of 80 mA is output, when operating in the 1X mode, the power consumption current is 80 mA (taking into account the consumption of other circuits inside the chip, the current is generally smaller than 81mA); When working in 1.5X mode, the current consumed by the power supply is abruptly changed to 120mA (the current is generally around 123mA considering the consumption of other circuits inside the chip and the switching loss). When working in 2X mode, the current consumed by the power supply is abrupt. It is 160mA. Therefore, one or two transition points are usually seen on the efficiency curve of a fractional charge pump type WLED driver. Therefore, for the fractional charge pump type WLED driver, how to minimize the power supply voltage corresponding to the mode switching and extend the working time of the driver in the 1X mode is the key to improving the overall efficiency. For example, the fractional charge pump type WLED driver shown in Figure 3 exits the 1X mode when the supply voltage is lower than 3.75V. Therefore, it eventually shows the overall efficiency and a power supply voltage as low as 3.5V before exiting 1X. The overall efficiency of the mode charge pump type WLED driver can vary greatly.
Figure 3: Fractional charge pump type WLED driver efficiency line chart
Figure 4: Fractional charge pump type WLED driver supply current and operating mode relationship
Low-dropout constant current WLED driver architecture
Along with the process improvement of the LED, the forward voltage drop of the WLED is decreasing. Figure 5 shows the IV curve of a Nichia NSSW100CT WLED in 2006. It can be seen from the figure that the WLED VF that provides 20mA current is only about 3.2V. At present, the forward voltage drop of the WLED commonly used in mobile phones is generally between 3.0 and 3.2V.
Figure 5: Nichia WLED IV Curve
As the VFL of the WLED continues to drop, does the WLED need to be boosted? This problem is analyzed in conjunction with the characteristics of the lithium ion battery and the actual application.
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