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UCC1972 Datasheet(PDF) 6 Page - Texas Instruments |
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UCC1972 Datasheet(HTML) 6 Page - Texas Instruments |
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6 / 17 page  6 UCC1972/3 UCC2972/3 UCC3972/3 Circuit Operation A current fed push-pull topology is used to power the CCFL backlight shown in Fig. 1. This topology accommo- dates a wide input voltage and dimming range while re- taining sinusoidal operation of the lamp. The converter consists of a resonant push-pull stage, a high voltage output stage, and a buck pre-stage used to regulate cur- rent in the converter. Referring to Fig. 1, the push-pull stage consists of CRES, Q1, Q2, RB, and T1’s primary and auxiliary windings. The output stage consists of CBALLAST, the lamp, the current sense resistor RS, and T1’s secondary. The reso- nant frequency of the tank is set by the primary induc- tance of T1, along with the resonant capacitor (CRES), and the reflected secondary impedance. The secondary impedance includes the lamp, the ballast capacitor (CBALLAST), the distributed winding capacitance of T1, and the stray capacitance which forms between the lamp, lamp wires, and the backlight reflector. Since the lamp impedance is nonlinear with operating current, the tank resonant frequency will vary slightly with load (typi- cally 1.5:1). The resonant tank consisting of CRES and T1 produces sinusoidal currents (IRES) and voltages and is fed by a controlled DC current (IBUCK) from the buck stage. Note that the BUCK node voltage is ½ the primary tank volt- age, as VBAT is located at the center tap of the trans- former. The high turns ratio transformer (T1) amplifies the sinusoidal tank voltage to produce a sinusoidal sec- ondary voltage that is divided between the lamp and bal- last capacitor. Transistors Q1 and Q2 are driven out of phase at 50 per- cent duty cycle with an auxiliary winding on T1. The winding provides a floating AC voltage source at the res- onant frequency that is used to drive the transistor bases alternately on and off. One leg of the auxiliary winding is tied to the input voltage through base resistor RB, which is sized to provide sufficient base current to the transis- tors. The transistors channel the buck inductor current into opposing ends of the tank at the resonant frequency, supplying energy for the lamp and system losses. The buck power stage consists of inductor LBUCK, MOSFET switch SBUCK, and flyback diode DBUCK.Inor- der to prevent interactions between multiple switching APPLICATION INFORMATION (cont.) C RES VBAT I RES Q2 R B VBAT Q1 T1 AUXILIARY RESONANT PUSH-PULL STAGE T1 PRIMARY C BALLAST FB R S VBAT I LAMP T1 SECONDARY CCFL OUTPUT STAGE I BUCK OUT VBAT D BUCK L BUCK S BUCK BUCK STAGE VBUCK GND VBAT Q1 ON Q2 ON Q1 ON Q2 ON V BUCK Figure 1. Push-pull, output, and buck stages. UDG-98157 |
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