The Design Of Linear Regulator Based On White LED Driver

With the colour LCD widely application in cell phones,PDA and digital cameras ,white LED as floodlight is becoming more and more prevalent. In comparison with conventional light, semiconductor lighting, especially GaN-based white LED lighting, is much more superior on power waste and life. Conventional incandescent bulb emits visible light as a result of being heated. It wastes of 90% energy. However, the conversion efficiency of LED is very high. GaN has become the engine of semiconductor lighting industry. The other outstanding merit of GaN-based white LED lighting is protecting environment. It doest have EMI because the DC difference is just only 3V. At the same time, the long life brings little rubbish. But the fluorescent lamp produces much contamination, such as mercury steam. In addition, the lamp-house of GaN-based white LED lighting has smaller size, lighter weight and better orientation. And it can be used in all kinds of abominable conditions, such as in water. All these advantages bring great strike to conventional lighting.Different LEDs have different current and voltage characteristics because of different manufacturers and specifications. Theoretically, the life of LED is over 100 thousand hours. But in fact, the LED is easily destroyed because of the improperness of the design and choosing of the LED driver. Therefore, It is key to choose LED driver properly and design LED driver accurately according to the request of product and environment to be used in. A serial/parallel LED driver with current-regulated, step-up DC/DC converter is designed in this paper, which is used for backlight in cell phones, handheld computers, PDAs, MP3 players and so on. It has simple frame, small size and high efficiency and can be applied in all kinds of conditions flexibly. Connecting with different peripheral devices, the circuit can also have the functions of short circuit protection, soft start and several kinds of dimming control (analog, digital and PWM/PFM). The circuit can drive up to twelve LEDs of two parallel and six series with the input voltage ranging from 2.7v to 5.5v. The output current is adjustable from 0mA to 20mA. The maximal shutdown current is 1uA. Operating frequency is 0.5MHz. It can provide extremely fast transient response and efficiency as high as 85%.At present, LED driving circuit mainly has two kinds of structure: inductor-based boost DC-DC converter and charge pump structure. This paper uses inductor-based boost DC-DC converter to driver LEDs. An internal NMOS switch drives an external inductor, and a Schottky diode delivers the inductor's stored energy to the load to drive LEDs. The chip has a high-powered DC-DC converter, two D/A converters and two LDO circuitries together. Two independent channels drive up to six LEDs per channel. Two internal two-bit D/A converters provide independent programmability of each output channel current. The current on LEDs can be regulated by adjusting the current on resistances to control brightness of LEDs. Linear Regulator stabilizes the output current by stabilizing the output voltage. Then stable brightness is got. Proprietary internal circuitry continuously monitors the current on both strings and automatically adjusts the generated output DC voltage to the lowest minimum value required by the LEDs string with the highest summarized forward voltage. This adaptive nature of the chip ensures operation at the highest possible efficiency. Soft start circuitry prevents excessive current drawn from the supply during power on. Any number of LEDs can be connected in series as long as the summed forward voltages do not exceed the specified operating output voltage range. Although it is not required to have an equal number of LEDs connected in series within each branch, the highest efficiency and best current regulation is always achieved when an equal number of LEDs are serially connected.The paper classifies all kinds of linear regulators and introduces the principle of LDO regulator in detail.The topologies of several sub-blocks are analyzed in detail in the paper, such as Linear Regulator,Power Good,Bias and BG-REF. And ever sub-block has been simulated to ensure meeting the requirement at different supply voltage and temperature.Bias circuitry provides bias current for other sub-blocks in the whole chip, which uses the topology of current mirror. At the same time, it puts out enable signal to make the chip work normally.Band-gap REF circuitry is used for providing other sub-blocks with band-gap reference voltage which is insensitivity to the temperature. In this chip, the band-gap reference voltage is divided into four pieces of output voltage by four resistances. These output voltages are selected by DAC circuitry, and then provide for Linear Regulator to control brightness of LEDs.Linear Regulator stabilizes the output voltage by negative feedback,like LDO. Then, stable brightness is got for stable current on LEDs. At the same time, it gives the high-low voltage to control the digital circuitry.Power Good circuitry can control the boost circuit. When the input voltage is connected to VIN pin, the system is turned on, then, VCH1 and VCH2 become higher and higher gradually. When the value is higher than V1, the output voltage of Power Good circuitry gets down to low to stop boost circuit. When the value is lower than V2(V2