In recent years, the industry began using LED lights to replace CCFL and EL as the LCD backlight (background light for short) compared to CCFL, EL. LED has the following advantages: 1) to make the LCD more realistic colors, LED backlight cho to provide 130% of the NTSC color scale, only 70% of CCFL. Levels expansion of the LCD image chroma saturation, and more realistic; to make the LCD thinner thickness in the 18-inch LCD module, LED backlight thickness is 4mm to 6mm in CCFL 8mm ~ 12nm; 3) long life, up to 50,000 hours; 4) meet the requirements of environmental protection, LED contains no mercury, 5) compared with EL backlight LED backlight does not arise from interference. Therefore, LED backlighting is widely used in PC, TV, car audio, mobile phones, communication devices, personal digital assistant (PDA) and watches, and other fields, it has become the leading products of the LCD backlighting market.

LED market demand in 2002 accounted for about 60% of the total market demand backlight, green, red, blue and white LED LCD backlight, due to price factors, the green LED Habitat mainstream, accounting for 80 of the LED backlight %. Their rated current of 2mA ~ 20mA, the brightness of 600mcd. Due to the higher cost of the white LED is mainly used for color mobile phones and color PDA’s backlighting and automotive instrument lighting.

2 white LED light-emitting mechanism and characteristics

2.1 light-emitting mechanism

Single-chip white LED tube of CaN Containing InGaN active layer light-emitting diodes, it is mainly there are two light-emitting mechanism: one is the combination of blue LED and yellow phosphorus, the emission of yellow light through the blue and phosphorus mixed to produce white light; another is to produce white light through a combination of UV LED and red, blue, green and phosphorus.

2.2 Features

White LED main characteristics are: the forward voltage drop of 3.5 V; luminous efficiency greater than 20lm / W, better than incandescent bulbs, fluorescent second (601m / W ~~ 100lm / W), in 2004, the luminous efficiency can be increased 60lm / w, close to the fluorescent level, which is used extensively for the lighting market; luminous flux for the 231m; small package size. SMD-type white LED Nichia Corporation in 2003, model NSCW215, it is a side view of SMD white LED, height 0.8/1mm current of 20mA, the brightness of 600mcd. Toyoda Gosei introduced SMD-type white LED current of 20mA, the brightness of 100mcd the luminous efficiency of 4.5 lm / W to 5 lmW size of 3.2 mm × 2.8 mm, Model of TG the white. citizen company Nichia white LED die developed by far the world’s smallest white LED, a thickness of 0.55 mm. Nichia company’s non-SMD white LED size is 11.2mm (width) x 7.2mm (length) x 6mm (H), long life, up to 50,000 hours or more. White LED lighting market is attractive, the world LED manufacturers to step up development of high-power white LED, high power of InGaN LEDs such as Nichia Corporation has developed the power of 1 W ~ 2 W, an existing LED 10 times. University of California, solid state lighting and display center program developed in 2007 at the light efficiency of 200lm / W white LED.

3 white LED driver circuit

White LED is mainly used for color mobile phones and color PDA, a uniform color LCD backlight require three to four or more white LEDs, smart phones may require six or more white LED. The increase in the demand of the white LED. A strong impetus to the growth of the white LED driver market. Tony Armstrong, Linear Technology Power Division product marketing manager, estimates: “2003 mobile phone shipments will be over 400 million of which at least 60% to 70% of color mobile phones, in addition there will be a 1000 color screen PDA , the market will demand “hundreds of millions of white LED driver, white LED forward voltage drop of 3.5 V, when a similar single-cell lithium, and therefore need a boost converter to solve the white LED forward voltage problems. , Boost, there are two solutions: First, the charge pump (switched capacitor), with its advantages of small footprint, low efficiency, National Semiconductor has introduced White LED Driver With this switched-capacitor, the company believes, if the boost converter will drive in the off state leakage current; inductor switch boost its advantages of high efficiency, but the total area occupied. At present, most of the white LED driver manufacturers inductor switch boost, such as the Catalyst company CAT32 white LED driver, it works at a fixed frequency of 1.2MHz, it can enhance the low-voltage battery voltage and automatically adjusts the drive current, up to 4 white LEDs connected in series. The company is developing even more advanced than CAT32 white LED drivers, it can control the current through the MPU, and the integration of passive components, thus saving costs. Linear Technology introduced a white LED driver integrated on-chip Schottky diode, drive circuit requires only two external capacitors, a resistor and an inductor. The general white LED driver integrated MOSFETs.

The white LED panel driver circuit consists of a white LED driver and peripheral circuits (including transistors, diodes, inductors, capacitors and resistors, etc.) composition. Drive white LED requires a constant current source, current is 15mA to 20mA. The brightness of the LED depends on its forward current, so more of the white LED used in tandem, to ensure that each white LED current is flowing through the same. Being compiled set of 4 series white LED need 14V voltage, the voltage boost regulator to enhance the single-cell lithium battery (2.7V to 4.2V) it said operating voltage. And drive on and off of Q1. Ql, L1, D1 and C1 form a boost regulator, so that the voltage across C1 increases. When the voltage exceeds the forward voltage drop of the white LEDs in series and the current started. The white LED current sensing resistor R1 connected in series to form a feedback loop is closed. Make the R1 at both ends of the pressure drop is the smallest you can get high efficiency. The voltage across R1, the SP6682 voltage of 0.3V parameters, allows the driver circuit efficiency of 87%. Typically, commercially available integrated boost regulator as a feedback voltage to 1.24V bandgap reference voltage, will allow the Rl both ends to produce 1.24V voltage drop across the So that the conversion efficiency is reduced by 7%. SP6682 0.3V reference voltage is far lower than 1.24V, while reducing the efficiency of the reference voltage is proportional to. MOSFET has a small on-resistance and high switching speed of these parameters is superior to other integrated switch. The breakdown voltage of the MOSFET will limit the maximum output voltage by regulating the voltage to the drive required for a few white LED system. Enable the regulator to shut down and restart in order to accurately control the brightness of white LED with a PWM signal on the SP6682 start pin 6.