AN61-1 Application Note 61 August 1994 Practical Circuitry for ...

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Application Note 61Circuit efficiency ranges from 80% to 88% at full load,depending on line voltage. Current mode operation combinedwith the Royer’s consistent waveshape vs inputresults in excellent line rejection. The circuit has none ofthe line rejection problems attributable to the hystereticvoltage control loops typically found in low voltage micropowerDC/DC converters. This is an especially desirablecharacteristic for CCFL control, where lamp intensitymust remain constant with shifts in line voltage. Interactionbetween the Royer converter, the lamp and theregulation loop is far more complex than might be supposed,and subject to a variety of considerations. Fordetailed discussion see Reference 3.Low Voltage Powered LCD Contrast SupplyFigure 10, a companion to the CCFL power supply previouslydescribed, is a contrast supply for LCD panels. It wasdesigned by Steve Pietkiewicz of LTC. The circuit isnoteworthy because it operates from a 1.8V to 6V input,significantly lower than most designs. In operation theLT1300/LT1301 switching regulator drives T1 in flybackfashion, causing negative biased step-up at T1’s secondary.D1 provides rectification, and C1 smooths the outputto DC. The resistively divided output is compared to acommand input, which may be DC or PWM, by the IC’s“I LIM ” pin. The IC, forcing the loop to maintain 0V at theI LIM pin, regulates circuit output in proportion to thecommand input.Efficiency ranges from 77% to 83% as supply voltagevaries from 1.8V to 3V. At the same supply limits, availableoutput current increases from 12mA to 25mA.HeNe Laser Power SupplyHelium-Neon lasers, used for a variety of tasks, are difficultloads for a power supply. They typically need almost10kV to start conduction, although they require only about1500V to maintain conduction at their specified operatingcurrents. Powering a laser usually involves some form ofstart-up circuitry to generate the initial breakdown voltageand a separate supply for sustaining conduction. Figure11’s circuit considerably simplifies driving the laser. TheV IN1.8V TO 6VT14CONTRAST OUTPUT VOUT –4V TO –29V17310 82150k+C122µF35V9D11N5819V INSW+NC100µFNCSENSELT1300ORLT1301SELECTSHDNI LIMSHUTDOWNPGNDGND12kT1 = DALE LPE-5047-AO4512k+2.2µFCOMMAND INPUTPWM OR DC0% TO 100%OR 0V TO 5VAN61 F10Figure 10. Liquid Crystal Display Contrast Supply Operates from 1.8V to 6V with –4V to – 29V Output RangeAN61-6
Application Note 61start-up and sustaining functions have been combinedinto a single, closed-loop current source with over 10kV ofcompliance. The circuit is recognizable as a reworkedCCFL power supply with a voltage tripled DC output. 2When power is applied, the laser does not conduct and thevoltage across the 190Ω resistor is zero. The LT1170switching regulator FB pin sees no feedback voltage, andits switch pin (V SW ) provides full duty cycle pulse widthmodulation to L2. Current flows from L1’s center tapthrough Q1 and Q2 into L2 and the LT1170. This currentflow causes Q1 and Q2 to switch, alternately driving L1.The 0.47µF capacitor resonates with L1, providing boostedsine wave drive. L1 provides substantial step-up, causingabout 3500V to appear at its secondary. The capacitorsand diodes associated with L1’s secondary form a voltagetripler, producing over 10kV across the laser. The laserbreaks down and current begins to flow through it. The 47kresistor limits current and isolates the laser’s load characteristic.The current flow causes a voltage to appear acrossthe 190Ω resistor. A filtered version of this voltage appearsat the LT1170 FB pin, closing a control loop. TheLT1170 adjusts pulse width drive to L2 to maintain the FBpin at 1.23V, regardless of changes in operating conditions.In this fashion, the laser sees constant current drive,Note 2: See References 2 and 3 and this text’s Figure 9.0.01µF5kV1800pF10kV47k5W1800pF10kVL111 8 HV DIODES5 4 1 32V IN9V TO 35V+150Ω2.2µF0.47µF+V SW 10k2.2µFQ1Q2L2MUR405145µHV IN FBLT11700.1µFV C GND+10k10µFV IN1N4002(ALL)190Ω1%LASERHV DIODES =SEMTECH-FM-500.47µF =WIMA 3× 0.15µF TYPE MKP-20Q1, Q2 =ZETEX ZTX849L1 =COILTRONICS CTX02-11128-2L2 =PULSE ENGINEERING PE-92105LASER =HUGHES 3121H-PAN61 F11Figure 11. LASER Power Supply Is Essentially A 10,000V Compliance Current SourceAN61-7
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AN61-1 Application Note 61 August 1994 Practical Circuitry for ...

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