DS3991
Low-Cost CCFL Controller
______________________________________________________________________________________ 11
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 stops increasing the
MOSFET gate duty cycle to keep from overstressing the
system. The DS3991 goes 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 DS3991 disables the MOSFET gate drivers and
goes into the fault handling state.
Once the lamp is struck, the DS3991 moves to the run-
lamp stage. In the run-lamp stage, the DS3991 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 65,536 lamp cycles, the
lamp is considered extinguished. In this case the MOS-
FET gate drivers are disabled and the device moves to
the fault-handling stage.
In the case of a lamp overcurrent, the DS3991 instanta-
neously declares the controller to be in a fault state. If
the DS3991 goes into the fault state, the DS3991 shuts
down. Once a fault state is entered, the controller
remains in that state until one of the following occurs:
• V
CC
drops below the UVLO threshold
• SVML input drops below 2.0V
• SVMH input goes above 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 MOSFETs.
The transformer should be able to operate in the 40kHz
to 80kHz frequency range of the DS3991, 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 is used to strike the lamp.
Additionally, its primary/secondary resistance and
inductance characteristics must be considered
because they contribute significantly to determining the
efficiency and transient response of the system. Table 1
shows a transformer specification that has been utilized
for a 12V inverter supply, 438mm x 2.2mm lamp design.
The MOSFETs must have a threshold voltage that is low
enough to work with logic-level signals, a low on-resis-
tance to maximize efficiency and limit the MOSFET’s
power dissipation, and a breakdown voltage high
enough to handle the transient. For push-pull topolo-
gies, the breakdown voltage of the MOSFETs should be
a minimum of 3x the inverter voltage supply.
Additionally, the total gate charge must be less than
Q
G
, which is specified in the
Recommended Operating
Conditions
table.
Table 1. Transformer Specifications (as used in the
Typical Operating Circuits
)
