The Design Of Serial/parallel White LED Driver IC With Current-regulated Based On Boost Converter

With the development of portable electronics devices, such as notebooks, cell phones and MP3, LCD Displays have been widely used. As a result, it has been an important research task to design backlight circuits for displays. Presently, white LEDs are the best choice for backlighting. Considering characteristics of white LEDs and necessity of uniform brightness in portable field, they need to be driven by special power management circuits.Nowadays, there are three kinds of white LED driver, parallel connection, series connection and series-parallel connection. The type of driver in parallel connection always uses Charge-Pump DC-DC converter to generate enough forward voltage for white LEDs. This type of driver has many advantages, such as low driver voltage, low EMI, low noise and independent brightness control for each channel. While the main disadvantage of this type is the low efficiency compared to connection in series. Moreover, it's very important to consider different forward voltage of white LEDs. Another type of driver in series usually makes use of Boost DC-DC converter to provide enough summarized forward voltage for white LEDs. The most prominent advantage of this type is the higher efficiency compared to parallel connection, and it isn't necessary to consider the different forward voltage of white LEDs, because these LEDs'current are same in the channel. Nevertheless, there are still some disadvantages of this type, such as high EMI, high noise and so on. And the type of series-parallel connection has both advantages of the two mentioned above.The purpose of this paper is to design a high efficiency, high reliability, Boost DC-DC converter, serial/parallel LED driver IC with current regulated. Firstly, we analyzed the theory of Boost converter. According to whether the current of the inductor is continuous, there are two modes of the Boost circuit, continuous mode (CCM) and discontinuous mode (DCM). Considering power converter was a close-loop system, we researched and analyzed the whole system, including two modes'steady state analysis, choosing control mode and stability criteria, which instructed the design of power converter. In the thesis, we used Linear Regulator to provide steady current for LEDs connected in series within each channel, so we also analyzed the basic theory of Linear Regulator. Withal, we pointed out how to change the current in LEDs by Linear Regulator. Finally, we designed the driver IC according to these analyses.In the thesis, the Boost converter worked in DCM mode. We designed a novel digital PWM controller, and we could modulate the Duty-Cycle by controlling the Up/down counter in the DPWM controller. We used DPWM and Pulse Skipping Modulation (PSM) to control the Boost circuit. Therefore, this driver IC could adapt to widely load changed.Some of the main functional modules were researched and designed in the thesis, including band-gap reference voltage, digital-ctr and linear regulator. Due to the careful designing of the reference voltage module, we acquired a high reliability, low temperature coefficient reference voltage, which improved the output current precision. On the basis of the system and circuit level design, the simulation performed in the Hspice indicated successful operation. The simulation's results showed that this driver IC could operate in 500 KHz, with supply voltage from 2.7V to 5.5V, which was suitable for single-cell battery supply application. The maxi output voltage was about 20V, the maxi output current was 40mA. When 3 LEDs were connected in series for each channel, the output current precision was±0.5%, with two 4.7uF off-chip capacitors and a 6.8uH off-chip inductor, and the power efficiency was over 88% for 12V output voltage and 40mA load current. The results indicated that it had met the design requirements of high efficiency and high precision. The white LEDs driver IC would be fabricated with a 0.6um 5V/40V CMOS process.