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LT1186F Datasheet(PDF) 11 Page - Linear Technology |
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LT1186F Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page 11 LT1186F 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 de- vices 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 are responsible for almost 50% of the battery drain. Additionally, all components, including PC board and hardware, usually must fit within the LCD enclosure with a height restriction of 5mm to 10mm. The CCFL regulator drives an inductor that acts as a switched-mode current source for a current-driven Royer- class converter with efficiencies as high as 90%. The control loop forces the CCFL PWM to modulate the aver- age inductor current to maintain constant current in the lamp. The constant current value, and thus lamp intensity, is programmable. This drive technique provides a wide range of intensity control. A unique lamp-current pro- gramming block permits either grounded lamp or floating lamp configurations. Grounded lamp circuits directly sense one-half of average lamp current. Floating lamp circuits directly sense the Royer’s primary-side converter current. Floating-lamp circuits provide symmetric differential drive to the lamp and reduce the parasitic loss from stray lamp- to-frame capacitance, extending illumination range. Block Diagram Operation The LT1186F is a fixed frequency, current mode switching regulator. A fixed frequency, current mode switcher con- trols switch duty cycle directly by switch current rather than by output voltage. Referring to the block diagram for the LT1186F, the switch turns ON at the start of each oscillator cycle. The switch turns OFF when switch current reaches a predetermined level. The control of output lamp current is obtained by using the output of a unique programming block to set current trip level. The current mode switching technique has several advantages. First, it provides excellent rejection of input voltage variations. Second, it reduces the 90 ° phase shift at mid-frequencies in the energy storage inductor. This simplifies closed-loop frequency compensation under widely varying input volt- age or output load conditions. Finally, it allows simple pulse-by-pulse current limiting to provide maximum switch protection under output overload or short-circuit condi- tions. The LT1186F incorporates a low dropout internal regula- tor that provides a 2.4V supply for most of the internal circuitry. This low dropout design allows input voltage to vary from 3V to 6.5V with little change in quiescent current. An active low shutdown pin typically reduces total supply current to 35 µA by shutting off the 2.4V regulator and locks out switching action for standby operation. The IC incorporates undervoltage lockout by sensing regulator dropout and locking out switching below about 2.5V. The regulator also provides thermal shutdown protection that locks out switching in the presence of excessive junction temperatures. A 200kHz oscillator is the basic clock for all internal timing. The oscillator turns on the output switch via its own logic and driver circuitry. Adaptive anti-sat circuitry detects the onset of saturation in the power switch and adjusts base drive current instantaneously to limit switch saturation. This minimizes driver dissipation and provides rapid turn- off of the switch. The CCFL power switch is guaranteed to provide a minimum of 1.25A in the LT1186F. The anti-sat |
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