Low-Cost CCFL Controller

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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:

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

Conditions table.

Table 1. Transformer Specifications (as used in the Typical Operating Circuits)

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Turns Ratio (Secondary/Primary) Push-Pull Type

(Notes 1, 2, 3)

40

Turns Ratio (Secondary/Primary) Half-Bridge Type (Note 3)

80

Frequency

40

80

kHz

Output Power

6

W

Output Current

5

8

mA

Primary DCR

Center tap to one end

200

m

Secondary DCR

500

Primary Leakage

12

μH

Secondary Leakage

185

mH

Primary Inductance

70

μH

Secondary Inductance

500

mH

1000ms (min)

2000

Secondary Output Voltage

Continuous

1000

Note 1: Primary should be bifilar wound with center-tap connection.

Note 2: Turns ratio is defined as secondary winding divided by the sum of both primary windings.

Note 3: This is the nominal turns ratio for driving a 438mm x 2.2mm lamp with a 12V supply. Refer to Application Note 3375 for more

information on push-pull type applications.