Two-Channel, Push-Pull CCFL Controller

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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:

•

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