Over the past few years, the small color LCD display has been integrated into the increasingly broad range of products. Color display was configured as a luxury phone, but now, even in entry-level phone, color has become a standard. Fortunately, the mobile phone industry of the economic scale (global mobile phone shipments of nearly 1 billion) to reduce the cost of the LCD color display, and to integrate them into both the portable medical equipment, universal entertainment remote control, digital photo frame / image Viewer, educational toys, or the latest features with VoIP WiFi cordless phones and other products are attractive.
Color LCD display requires white backlighting, so that users can be in any normal lighting conditions the view. The backlight subsystem includes a high-brightness white light emitting diode (LED) array, a diffuser (diffuser) in order to diffuse light and a backlight power drives will be available as a constant current regulator to drive the LED. A 1 to 1.5 inches of the screen may contain 2 to 4 LED, and a 3.5-inch screen may be easily to contain 6 to 10 LED. For the LED, its light output and current is proportional to, and because the LED has a very steep current - voltage (IV) curves, the current flowing through the LED is very important to closely matched, so as to ensure a balanced backlighting, LED normally distributed because LCD display side. In addition, it requires software control allows users to adjust brightness, as well as to compensate for ambient light conditions. According to the different currents flowing through the LED, LED color points (color point) may be drift. Therefore, the LED current is set to a fixed value and pulse-width modulated LED light output to reduce average very common. Hand-held product design in small color LCD display integrated and thus achieve cost, performance and battery life an appropriate balance, there is a series of factors to be considered.
Battery-powered LED driver circuit needs to be optimized architecture, these structures exist to deal with many challenges, such as space is limited, you need energy efficiency, and battery voltage variations - either may be higher than the LED forward voltage may also be low. There are two commonly used topologies, namely, the charge pump LED configuration using parallel architecture / structure and the LED current source inductor in series configuration step-up structure. The two solutions have to be considered a compromise of factors, such as the boost structure can ensure that all the LED current flows through the same size but requires the use of inductive energy conversion, and charge pump architecture uses a small capacitor for energy conversion, but all the LED in parallel arrangement may become too closely matched, so the current balance of the backlight faced a difficult problem. Figure 1 shows an example of these two architectures.
Should be designed to do the following
1. Assess the use of time about the display
Choose white LED drivers, the need to consider the display frequency of use. If the display backlight will be watching for a long time, with high efficiency converters for battery life is vital. Need more large screen LED, which displays the application will use a long time from the energy efficient benefits of a boost topology. Conversely, if only for a short time the display backlight, it may not be a key to the efficiency of the design parameters.
2. LED choose carefully
LED technology continues to rapidly improve, manufacturers in the use of new materials, manufacturing technology and design to the LED current release of the same size greater light output, so that a few years ago need to be 4 LED backlit display is now may use two LED backlight can achieve the same brightness. Not only that, the last commonly used cold cathode fluorescent lamp (CCFL) for backlighting of 4-7 inches larger displays, is now turning to LED backlighting. In addition, LED's forward voltage is trend lower. Therefore, not only need to consider the drive manufacturer's data sheet drive efficiency curve, but also based on LED on the drive using the selected assessment to be considered. Table 1 lists some of the important specifications several LED shows the forward voltage and the LED brightness in areas such as differences. It should be noted that large changes in the scope forward voltage, which means that the efficiency of the drive LED specification limits should be used for evaluation.
3. Note wiring
Even if each LED with 10 to 20mA to drive a very low current flowing through the converter was also much higher peak current. For purposes of this inductor topology is still the case, because the peak switch current of the average LED current may be 10 to 20 times. Therefore, use of appropriate low-loss cabling technology. Charge pump topology, the capacitor should be arranged in the adjacent location of the drive, so to minimize the loop area in order to avoid switching noise radiation. For the inductive boost converter, the input and output capacitors and inductive drive in nearby locations. In addition, the current setting resistor (Rfb) should be connected directly to the chip ground, because the internal reference and error detection voltage direct impact between the LED current accuracy.
4. In the real test of your product, measured
Consider the display in bright light conditions outside the performance, and ensure that the software dimming control with sufficient dynamic range, resulting in the expected light conditions to fully dimming of the display.
Following issues should be taken to avoid
1. Forget to consider the boundaries and failure mode
Errors will always happen, and if LED open or shorted to ground, the driver should be how to deal with this problem? Inductor boost the drive for, if the LED string open, the output will surge, as a constant current to charge the output capacitor will thus need to over-voltage protection, but this feature may or may not be integrated in the drive. This factory testing may become a problem because the display in some test steps may not be installed. In addition, the assessment of the surge product is open conditions is important, because here a large number of current consumption may be reduced to the minimum operating battery voltage is below the threshold. Soft start and / or software modules in different circuits will be able to sort this problem is minimized.
2. Just stare at peak efficiency
Since the user can adjust the backlight brightness, the display backlight is expected to be considered working hours in most of the drive efficiency. Assess the efficiency of the drive when the LED is expected to consider the working conditions, the battery voltage range and forward voltage. Inductive drive has a better peak efficiency, change the input and output voltage have a greater tolerance.
3. Ignore the external component selection
In all design cases, they should consider a low equivalent series resistance (ESR) X5R or X7R ceramic capacitor to minimize losses. Also in the inductive case, (if external) Schottky rectifier forward voltage drop and the inductor ESR affects efficiency. For example, the use of the inductor with 0.3 ESR to 5 to 20mA current driving the LED in series, the efficiency with 1.3 ESR than 5% higher inductance. Of course, this is not without cost, because the lower the inductance of the ESR to provide the same inductance value corresponding to the larger size. Fortunately, today there are many new small inductor, which allow the LED driver in the display below.