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UCC3973PW Datasheet(PDF) 11 Page - Texas Instruments |
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UCC3973PW Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 24 page  11 UCC1972/3 UCC2972/3 UCC3972/3 Referring to Fig. 5, at time t0 the control signal is brought low and the voltage in the resonant tank begins to build. At time t1 there is sufficient voltage for the lamp to strike and the feedback loop controls the lamp at rated current using a fixed current sense resistor. When the LFD signal is brought low at time T2, the COMP output is low and the OUT pin stops switching. The resonant tank voltage decays until the lamp extin- guishes. If the on time were extended to t3 the average lamp intensity would be increased accordingly, the next low frequency cycle begins at time t4. The time relationship between the resonant and gating frequency has been exaggerated so that the sinusoidal waveforms can be depicted. In order to avoid visible lamp flicker, the low frequency gating rate (t0-t4) should be greater than 100Hz. To prevent “beat” frequency in- terference, it may be advantageous to synchronize the gating frequency to a multiple of the monitor scan rate of the LCD display. This can be accomplished by con- trolling the duty cycle with a timer routine within the LCD’s software program. LFD waveforms at 200Hz and 50% duty cycle are shown in Fig. 6a. Fig. 6b show a time expanded photo of the same waveforms. Channel 1 is lamp voltage at 500V /div, Channel 2 is lamp current at 20mA / div, and Channel 3 is the LFD control voltage. Since the photos are from a digi- tal oscilloscope, alias exists in the waveforms. Lamp Current Control Loop The current control loop for the CCFL circuit is discussed in detail in Unitrode Application Note U-148 and is briefly repeated here for completeness. A block diagram for the current control loop is shown in Fig. 7. The PWM modulator small signal gain is inversely propor- tional to the internal saw tooth ramp and proportional to the input voltage (the inductor’s current slope increases as VBAT increases). The resonant tank and buck inductor form a RLC filter at the center point of the push pull trans- former. The effective L of the filter is dominated by buck in- ductor and the effective C is approximately 8 times the resonant capacitor (CRES) value. This occurs because the reflected ballast capacitance is equal to CRES and the equivalent capacitance at the push-pull center point is four times the capacitance across the tank. The equivalent re- sistance at the push-pull center point is equal to ¼ the tank voltage squared divided by the lamp power. The cor- ner frequency and Q of the filter are: F LC CORNER BUCK RES = ·· 1 28 p (13) Q FL R FILTER BUCK FILTER = 2p (14) APPLICATION INFORMATION (cont.) LAMP VOLTAGE LAMP CURRENT LFD CONTROL SIGNAL t0 t1 t2 t3 t4 ON OFF 5V 0V Figure 5. Low frequency dimming timing waveforms. Figure 6a. LFD at 50% duty cycle. Figure 6b. Time expanded showing lamp strike and feedback delay. |
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