DC354A-High Power Dual CCFL Demo Board DESCRIPTION Demonstration Board DC354 featuring the LT1768 is a complete solution for regulating current in high power dual CCFL displays. A single input source, either DC or PWM, controls CCFL current via a unique Multimode dimming scheme. Multimode dimming combines both linear and PWM control functions to maximize lamp life, efficiency and dimming range. Accurate maximum and minimum lamp currents can be easily set. The DC354 can detect and protect against lamp failures, which can shorten lamp life or produce hazardous voltages. CONNECTIONS Connect the input supply to the VIN (E1) and GND (E2) terminals. Connect the high side (the high side of the lamp is the shortest wire to the lamp and is usually pink in color) of each CCFL lamp to terminals E6 and E7 respectively. Connect the low side of each CCFL lamp to terminals E4 and E5 respectively. Connect a 0V to 5V DC or PWM voltage source to the PROG (E3) terminal. The SD (E8) terminal (Shutdown pin) can be left open, tied to VIN, or connected to an active low source. The FAULT(E9) terminal (Fault pin) is an open collector output that is active low when the LT1768 detects a fault condition. CURRENT PROGRAMMING Demonstration board DC354 operates from an input supply voltage of 8V to 24V. Below 8V the LT1768 is in undervoltage lockout preventing lamp operation. The DC354 can be placed in shutdown mode by taking the SD (E8) terminal below 1.25V, which results in a typical quiescent current of 65uA. Leaving the SD terminal open or tying it to the VIN (E1) terminal will result in normal operation. Lamp current is controlled by the voltage source connected to the PROG (E3) terminal. The DC 354 takes a 0V to 5V DC or 1kHz PWM signal. The input is filtered by R1 and C4 and then sent to the PROG pin of the LT1768. Referring to Figure 1, the lamp current has five modes of operation depending on the value of the voltage on the LT1768 PROG pin. When the LT1768 PROG pin voltage is less than 0.5V, DC354 will be in off mode and the lamp current will be zero. When the LT1768 PROG pin voltage is between 0.5V and 1.0V, DC354 will be in minimum lamp current mode. The minimum lamp current is set to approximately 10 times the current flowing out 1.26V referenced RMIN pin on the LT1768. With jumper JP1 in the 1-2 position, R8 is connected to the 5V VREF pin and current flows into the RMIN pin, which results in zero lamp current. With jumper JP1 in the 2-3 position, R8 is connected to ground and 12.6uA flows out of the RMIN pin, which results in approximately 126uA of lamp current. The minimum current determines the dimming ratio of the display. Setting the minimum current to zero by connecting R8 to the VREF pin will produce the greatest dimming ratio but may violate lamp specifications Minimum lamp current can be set by putting the LT1768 in minimum current mode (VPROG=0.75V), and then adjusting R8 (jumper in 2-3 position) for the desired current. When the LT1768 PROG pin voltage is between 1.0V and the LT1768 PWM pin voltage (3.0V set by the LT1768 5V VREF pin, and resistor divider R2 & R3) DC354 will be in PWM lamp current mode. In PWM mode, the lamp current will be pulse width modulated between the minimum lamp current and (VPROG- 1V)/3V times the maximum lamp current. The PWM frequency is set to 220hz by the capacitor C3. The PWM frequency is defined by 22Hz/C3uF, with its duty cycle equal to 1-(VPWM-VPROG)/(VPROG- 1V) *100%. Linear Max Current 9 mA Min Current PWM (Freq =220Hz) I (mA) 0% 100% CCFL Off 0.5 1.0 3V (V ) 4.0 5.0 PWM V (V) PROG Figure 1. CCFL Current per Lamp vs PROG Voltage for the DC354 When the LT1768 PROG pin voltage is between the LT1768 PWM pin voltage and 4.0V, DC354 will be in linear lamp current mode. In linear mode, the lamp current increases linearly with the voltage on the LT1768 PROG pin and follows the equation, ILAMP = (VPROG-1V)/3V*(maximum lamp current). For optimum performance, the voltage on the LT1768 PWM pin should be set such that PWM mode starts just below the nominal lamp operating current. When the LT1768 PROG pin voltage is between 4.0V and 5.0V, DC354 will be in maximum lamp current mode. The maximum lamp current is set to approximately 100 times the current flowing out 1.25V referenced RMAX pin on the LT1768. Maximum lamp current can be set by putting the LT1768 in maximum current mode (VPROG=4.5V), and then adjusting R4 for the desired current.