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5962-9462201MVA Datasheet(PDF) 4 Page - Texas Instruments |
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5962-9462201MVA Datasheet(HTML) 4 Page - Texas Instruments |
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4 / 14 page  Figure 1 UC1871 UC2871 UC3871 Figure 1 shows a complete application circuit using the UC3871 Resonant Fluorescent lamp and LCD driver. The IC provides all drive, control and housekeeping func- tions to implement CCFL and LCD converters. The buck output voltage (transformer center-tap) provides the zero crossing and synchronization signal. The LCD supply modulator is also synchronized to the resonant tank. The buck modulator drives a P-channel MOSFET di- rectly, and operates over a 0-100% duty-cycle range. The modulation range includes 100%, allowing operation with minimal headroom. The LCD supply modulator also di- rectly drives a P-channel MOSFET, but it’s duty-cycle is limited to 95% to prevent flyback supply foldback. The oscillator and synchronization circuitry are shown in Figure 2. The oscillator is designed to synchronize over a 3:1 frequency range. In an actual application however, the frequency range is only about 1.5:1. A zero detect comparator senses the primary center-tap voltage, gen- erating a synchronization pulse when the resonant wave- form falls to zero. The actual threshold is 0.5 volts, providing a small amount of anticipation to offset propa- gation delay. The synchronization pulse width is the time that the 4mA current sink takes to discharge the timing capacitor to 0.1 volts. This pulse width sets the LCD supply modulator minimum off time, and also limits the minimum linear control range of the buck modulator. The 200 µA current source charges the capacitor to a maximum of 3 volts. A comparator blanks the zero detect signal until the capaci- tor voltage exceeds 1 volt, preventing multiple synchroni- zation pulse generation and setting the maximum frequency. If the capacitor voltage reaches 3 volts (a zero detection has not occurred) an internal clock pulse is generated to limit the minimum frequency. TYPICAL APPLICATION APPLICATION INFORMATION 4 |
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