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

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