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LT1786F Datasheet(PDF) 11 Page - Linear Technology |
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LT1786F Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page 11 LT1786F TI I G DIAGRA S Timing for SMBus Interface tBUF tLOW tHIGH tr tf tHD:STA tHD:DAT SDA SCL tSU:DAT tSU:STA tSU:STO STOP 1786 TD01 START START STOP tHD:STA Operating Sequence SMBus Write Byte Protocol, with SMBus Address = 0101101B, Command Byte = 0XXXXXXXB and Data Byte = 111111XXB IOUT SCL S = START P = STOP * = OPTIONAL SDA S 1 0 2 1 3 0 4 1 5 1 6 0 7 1 XX XX X X X 1 1 1 1 1 1 X X 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 P FULL-SCALE CURRENT VCC GND 1786 TD02 ZERO-SCALE CURRENT * SMBUS ADDRESS COMMAND BYTE DATA BYTE ADR APPLICATIONS INFORMATION Introduction Current generation portable computers and instruments use backlit Liquid Crystal Displays (LCDs). Cold Cathode Fluorescent Lamps (CCFLs) provide the highest available efficiency in back lighting the display. Providing the most light out for the least amount of input power is the most important goal. These lamps require high voltage AC to operate, mandating an efficient high voltage DC/AC con- verter. The lamps operate from DC, but migration effects damage the lamp and shorten its lifetime. Lamp drive should contain zero DC component. In addition to good efficiency, the converter should deliver the lamp drive in the form of a sine wave. This minimizes EMI and RF emissions. Such emissions can interfere with other devices and can also degrade overall operating efficiency. Sinusoidal CCFL drive maximizes current-to-light conver- sion in the lamp. The circuit should also permit lamp intensity control from zero to full brightness with no hysteresis or “pop-on.” The small size and battery-powered operation associated with LCD equipped apparatus dictate low component count and high efficiency for these circuits. Size con- straints place severe limitations on circuit architecture and long battery life is a priority. Laptop and handheld portable computers offer an excellent example. The CCFL and its power supply can be responsible for almost 50% of the |
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