High-Accuracy LED Current Drivers for Display and Lighting Applications

Abstract

Light emitting diodes (LEDs) have been broadly adopted in various applications such as backlight, display, and lighting applications due to their outstanding characteristics including fast response time, wide color gamut, low power consumption, high dynamic range, long lifetime, environmental stability, and wide viewing angle. In high-end LED applications, which demand long lifetime and high brightness or luminance, the LEDs have showed superior performance because inorganic materials in the LED are robust to high current. The luminance or brightness of the LED can generally be controlled by the LED driving circuit and driving method. However, the non-uniform electrical characteristics of the LEDs (forward voltage and forward current) make it difficult to accurately control the luminance or brightness of the LEDs. In addition, the characteristics of the LEDs cannot be fully utilized due to inherent limitations of the conventional LED driving circuits and driving methods. To solve aforementioned issues, this dissertation presents various LED driving circuits and driving methods for backlight, display, and lighting applications. First, a fast switching current regulator is proposed in attempt to realize a high dimming ratio of the LED backlight drivers. The proposed current regulator using a slewing time reduction method improves the dimming ratio by reducing the rising time of the LED forward current. A six-channel LED backlight driver was fabricated in 0.35-μm BCD process technology, which has an n-type MOSFET with a low voltage of 5 V and a high-side lateral double diffused MOSFET (LDMOSFET) with a breakdown voltage of 60 V. The measurement results show that the rising time and minimum pulse width of the LED forward current are reduced to 4.8 ns and 12 ns, respectively, at a current transition of 60 mA. As a result, the dimming ratio of the LED backlight driver using the proposed fast switching current regulator is estimated to be 160,000:1 at a dimming frequency of 500 Hz, which is more than three times higher than that achieved in prior works. Thus, the proposed current regulator is more suitable for high-end displays requiring higher image quality. Second, a driving method of an active matrix micro-pixelated light-emitting diode (AMLED) using an adaptive reference generator is proposed in attempt to achieve highly uniform luminance. The proposed driving method compensates for resistance mismatches between the column lines as well as between the row lines with generating a reference voltage for row lines. To verify the proposed driving method, the AMLED pixel driver ICs and pixel circuit ICs were fabricated in 0.18 μm CMOS process with 1.8 V and 6 V devices. In the measurement, a micro-pixelated LED array with 16×16×RGB was driven using the fabricated pixel driver ICs and circuit ICs, and its pixel currents were measured for luminance uniformity of the AMLED display. The measurement results show that the average deviation of the pixel current was 0.79% for all 768 pixels with an average pixel current of 12.003 μA, indicating that the proposed AMLED display realized highly uniform luminance. Thus, the proposed driving method is suitable for high-end AMLED display applications requiring high image quality. Finally, the average current controller (ACC) is proposed in attempt to accurately regulate the LED forward current in the DC and AC lighting applications. In DC lighting applications, the proposed ACC using a charge sharing method achieves a highly accurate average LED forward current by compensating for an error occurred when the LED forward current is sampled. It was fabricated in 0.35-μm CMOS process technology and occupied a small chip area of 1,028 μm × 728 μm. The experimental results showed that the proposed ACC achieved a deviation in the average LED forward current of less than 0.6% over the entire input voltage range. In addition, the measured maximum power efficiency was 93.6%. Thus, the proposed ACC is suitable for high quality LED lighting systems requiring an accurate LED forward current. In AC lighting applications, the proposed LED lamp driver accurately maintains an average LED forward current by accurately adjusting the duration of the on- and off-times of the LED lamp driver. It also regulates the LED forward current to remove the low-frequency flicker by without being influenced from the low-frequency component of the AC line voltage. In addition, it reduces power consumption by eliminating the necessity of a high-side resistor or high-voltage sensor, which causes high power consumption. The proposed average current controller was fabricated in 0.18 μm BCD process technology. The experimental results of the proposed LED lamp driver showed that the variation in the average LED forward current was less than 0.6%, and moreover the light flicker below 120 Hz was completely removed. In addition, the measured power factor and maximum power efficiency of the proposed LED lamp driver were 0.93 and 89.22%, respectively.