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UCC3973PWTRG4 Datasheet(PDF) 7 Page - Texas Instruments |
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UCC3973PWTRG4 Datasheet(HTML) 7 Page - Texas Instruments |
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7 / 24 page  7 UCC1972/3 UCC2972/3 UCC3972/3 frequencies, the UCC3972/3 synchronizes the buck fre- quency to the frequency of the push-pull stage. The tra- ditional buck topology is inverted to take advantage of the lower RDS(on) characteristics of an N-Channel MOSFET switch (SBUCK). With a sinusoidal voltage across the tank, the resulting output of the buck stage (VBUCK) becomes a full-wave rectified voltage referenced to VBAT as shown in Fig. 1. Lamp current is sensed directly with RS and a parallel di- ode on each half cycle. The resulting voltage across the sense resistor RS is kept at a 1.5V average by the error amplifier, which in turn controls the duty cycle of SBUCK. The buck converter typically operates in continuous cur- rent mode but can operate with discontinuous current as the CCFL is dimmed. Design Procedure A notebook computer backlight circuit will be presented here to illustrate a design based on the UCC3972/3 con- troller. The converter will be designed to drive a single cold cathode fluorescent lamp (CCFL) with the following specifications: Input Voltage Range: The notebook computer will be powered by a 4 cell Lith- ium-Ion battery pack with an operational voltage range of 10V to 16.8V. When the pack is being charged, the back light converter is powered from an AC adapter whose DC output voltage can be as high as 22V. Resonant Tank and Output Circuit The selection of components to be used in the resonant tank of the converter is critical in trading off the electrical and optical efficiencies of the system. The value of the output circuit’s ballast capacitor plays a key role in this trade-off. The voltage across the ballast capacitor is a function of the resonant frequency and secondary lamp current: V I CF CB LAMP BALLAST RESONANT = ·· · 2 p (1) A voltage drop across CBALLAST many times the lamp voltage will make the secondary current insensitive to distortions caused by the non-linear behavior of the lamp, providing a high impedance sinusoidal current source with which to drive the CCFL. This approach im- proves the optical efficiency of the system, as capacitive leakage effects are minimized due to reduced harmonic content in the voltage waveforms. Unfortunately, from an electrical efficiency standpoint, an increased tank voltage produces increased flux losses in the transformer and in- creased circulating currents in the tank. In practice, the voltage drop across the ballast capacitor is selected to be approximately twice the lamp voltage (750V in our case) at rated lamp current. Assuming a 50kHz resonant frequency and 5mA operating current, a ballast capaci- tance of 22pF is selected. Since the lamp and ballast ca- pacitor impedance are 90 degrees out of phase, the vector sum of lamp and capacitor voltages determine the secondary voltage on the transformer. () ( ) VV V SEC CB LAMP =+ 22 (2) The resulting secondary voltage at rated lamp current is 820V. Since the capacitor dominates the secondary im- pedance, the lamp current maintains a sinusiodal shape despite the non-linear behavior of the lamp. As the CCFL is dimmed, lamp voltage begins to dominate the second- ary impedance and current becomes less sinusiodal. Transformer secondary voltage is reduced, however, so high frequency capacitive losses are less pronounced. The value of ballast capacitor has no effect on current regulation since the average lamp current is sensed di- rectly by the controller. Once the ballast capacitor is selected, the resonant fre- quency of the push-pull stage can be determined from the transformer’s inductance (L), turns ratio (N), and the selection of resonating capacitor (CRES). () F LC N C RESONANT PRIMARY RES BALLAST = +· æ èç ö ø÷ 1 2 2 p (3) Output distortion is minimized by keeping the independ- ent resonant frequencies of the primary and secondary circuits equal. This is achieved by making the resonant capacitor equal to the ballast capacitance times the turns ratio squared: () CN C pF F RES BALLAST =· = · = 2 2 67 22 0 1 . m (4) The resulting resonant frequency is about 50kHz, this frequency will vary depending upon the lamp load and amount of stray capacitance in the system. Since the UCC3972/3 has an internal oscillator, it is important that APPLICATION INFORMATION (cont.) Lamp Length 250mm (10”) Lamp Diameter 6mm Striking Voltage (20 °C) 1000V (PEAK) Operating Voltage (5mA) 375V (RMS) Full Rated Current 5mA Full Rated Power 1.9W Table 1. Lamp Specifications |
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