As technology advances, LEDs are becoming smaller, lighter and higher efficacy. Order to allow the LED to produce the desired brightness, constant current LED driver circuit. Most of today’s cellular phones, miniature white LED and a small LED drive, full color LCD display, accurate and uniform brightness. The battery life is determined mainly by the brightness of the LCD when the phone is in call state, the LCD power consumption is about 1/3 of the total power budget. The LED itself and adjust the current driver circuit consumes power. Miniature LED driver applications, in addition to battery life, board space is also an important consideration, which makes the choice of the LED driver topology becomes more important than ever.
LED driver topology: spatial factors
The most popular handheld devices battery is a lithium-ion battery, the battery voltage range of 4.2V when fully charged down to the discharge state is about 3.3V. In backlighting applications, the white LED panel current of 20mA typically demonstrate about 3.4V forward voltage (VF), but the voltage varies with the type and temperature. In order to avoid during low battery voltage LED flicker, it is necessary to introduce the boost circuit. The two popular boost driver architecture is a switched capacitor and inductor step-up drive.
The switched capacitor charge pump in parallel to drive the LED, the circuit usually requires four 0403-type packages 1μF ceramic capacitor, it offers the most compact solution, and does not require inductors. Figure 1 shows a 2.5 x 2.5mm QFN package, three-channel charge pump drives the example, use an external resistor to set LED current. In addition to the three LED, all the necessary components can be installed in less than one square centimeters of board space.
In comparison, the inductive boost LED driver uses an inductor to increase the voltage and tandem drive LED. The main advantage of this type of drive current flowing through the LED lights, resulting in the desired brightness matching; disadvantage is that the shape of a large wound inductor, the specific size depends on the inductance value and the rated current. For example, in the backlighting applications, the rated current of 200mA of 22μH inductor height 1 ~ 2mm, for 2 × 2mm ~~ 4 × 4mm footprint. The lower the height of the inductor, the greater the surface area. Depending on the type of LED driver, and may also require an external Schottky diode. Suppose a package SOT23 drive, an external Schottky diode, In addition to the three LED, the total board area takes about 1.5mm2.