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DS3992 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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DS3992 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 13 page Two-Channel, Push-Pull CCFL Controller _____________________________________________________________________ 9 When both the DS3992 and the DC inverter supplies are at acceptable levels, the DS3992 will attempt to strike the lamps. The DS3992 slowly ramps up the MOSFET gate duty cycle until the lamp strikes. The controller detects that the lamp has struck by detecting current flow in the lamp. If during the strike ramp, the maximum allowable voltage is reached, the controller will stop increasing the MOSFET gate duty cycle to keep from overstressing the system. The DS3992 will go into a fault-handling state if the lamp has not struck after 65,536 lamp cycles. If an overvoltage event is detected during the strike attempt, the DS3992 will dis- able the MOSFET gate drivers and go into the fault- handling state. Once the lamp is struck, the DS3992 moves to the run lamp stage. In the run lamp stage, the DS3992 adjusts the MOSFET gate duty cycle to optimize the lamp cur- rent. The gate duty cycle is always constrained to keep the system from exceeding the maximum allowable lamp voltage. If lamp current ever drops below the lamp out reference point for 65536 lamp cycles, then the lamp is considered extinguished. In this case the MOSFET gate drivers are disabled and the device moves to the fault handling stage. In the case of a lamp overcurrent condition, the DS3992 will instantaneously declare the controller to be in a fault state. If either channel on the DS3992 goes into the fault state, only the faulty channel will be shut down. Once a fault state is entered, the controller will remain in that state until one of the following occurs: •VCC drops below the UVLO threshold. • The SVM input drops below 2.0V. Applications Information Component Selection External component selection has a large impact on the overall system performance and cost. The two most important external components are the transformers and n-channel MOSFETs. The transformer should be able to operate in the 40kHz to 80kHz frequency range of the DS3992, and the turns ratio should be selected so the MOSFET drivers run at 28% to 35% duty cycle during steady-state operation. The transformer must be able to withstand the high open-circuit voltage that will be used to strike the lamp. Additionally, its primary/secondary resistance and inductance characteristics must be considered because they contribute significantly to determining the YES MOSFET GATE DRIVERS ENABLED RUN LAMP STAGE DEVICE AND INVERTER SUPPLIES AT PROPER LEVELS? FAULT STATE [MUST POWER CYCLE THE DS3992 OR TAKE SVM BELOW 2V TO RESET THE CCFL CONTROLLER] LAMP OVERCURRENT [INSTANTANEOUS] STRIKE LAMP [RAMP AND REGULATE TO OVD THRESHOLD] LAMP STRIKE TIMEOUT [65536 LAMP CYCLES] RUN LAMP [REGULATE LAMP CURRENT BOUNDED BY LAMP VOLTAGE] LAMP-OUT TIMEOUT [65,536 LAMP CYCLES] OVERVOLTAGE [64 LAMP CYCLES] IF LINE REGULATION THRESHOLD IS MET Figure 4. Fault-Handling Flowchart |
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