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LT1768IGN Datasheet(PDF) 11 Page - Linear Technology |
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LT1768IGN Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT1768 11 APPLICATIONS INFORMATION per lamp to 1mA for all PROG voltages but limits the dimming ratio to 6:1. Trace B in Figures 3a and 3b shows Figure 1’s CCFL current waveform operating at 1mA in PWM mode. Maximum current mode (VPROG > 4V) sets the VC source current to five times the current out of the 1.25V refer- enced RMAX pin. Setting RRMAX to produce CCFL current equal to the manufacturer’s maximum rating in this mode insures no degradation in the specified lamp lifetime. For example, setting R4 in the circuit in Figure 1 to 16.2k sets the maximum CCFL current to 9mA (9mA is only a typical maximum lamp current used for illustration, consult lamp specifications for the actual value). Trace A in Figure 3a and 3b shows Figure 1’s CCFL current waveform operating at 9mA in maximum current mode. Figure 3a. CCFL Current for Circuit in Figure 1 Figure 3b. CCFL Current for Circuit in Figure 1 In linear mode (VPWM < VPROG < 4V), VC source current is controlled linearly with the voltage on the PROG pin. The equation for the VC source current in linear mode is IVC = (VPROG – 1V)/3V (IRMAX • 5). For the best current to light conversion and highest efficiency, VPWM should be set to make the LT1768 normally operate in the linear mode. For example, in the circuit in Figure 1, linear mode runs from VPROG = 3V to VPROG = 4V with lamp current equal to (3mA)(VPROG–1V)/1V. In PWM Mode (1V < VPROG < VPWM), the VC source current is modulated between the value set by minimum current mode and the value for IVC in linear mode with VPROG = VPWM. The PWM frequency is equal to 22Hz/CT(µF) with its duty cycle set by the voltages on the PROG and PWM pins and follows the equation: DC = [1 – (VPWM – VPROG)/(VPWM – 1V)] • 100% The LT1768’s PWM mode enables wide dimming ratios while reducing the high crest factor found in PWM only dimming solutions. In the example of Figure 1, PWM mode runs from VPROG = 1V to VPROG = 3V with CCFL current modulated between 0mA and 6mA. The PWM modulation frequency is set to 220Hz by capacitor C3. When combined, these five modes of operation allow creation of a DC controlled CCFL current profile that can be tailored to each particular display. With linear mode CCFL current control over the most widely used current range, and PWM mode at the low end, the LT1768 enables wide dimming ratios while maximizing CCFL lifetimes. Lamp Feedback Current In a typical application, the DIO1/2 pins are connected to the low voltage side of the lamps. Each DIO pin is the common connection between the cathode and anode of two internal diodes (see Block Diagram). The remaining terminals of the diodes are connected to PGND. Bidirec- tional lamp current flows into the DIO1/2 pins and their diodes conduct alternately on the half cycles. The diode that conducts on the negative cycle has a percentage of its current diverted into the VC pin. This current nulls against the VC source current specified by the Multimode Dim- ming section. A single capacitor on the VC pin provides both stable loop compensation and an averaging function to the halfwave-rectified lamp current. Therefore, current into the VC pin from the lamp current programming section relates to average lamp current. The overall gain from the resistor current to average lamp current is equal to the gain from the Multimode Dimming block divided by the gain from the DIO pin to the VC pin, TRACE A VPROG = 4.5V ILAMP = 9mARMS TRACE B VPROG = 1.125V ILAMP = 1mARMS 1ms/DIV TRACE A VPROG = 4.5V ILAMP = 9mARMS TRACE B VPROG = 1.125V ILAMP = 1mARMS 100 µs/DIV |
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