LX1692

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 1

Copyright

© 2004

Rev. 1.2, 12/20/2006

Full Bridge Resonant CCFL Controller

TM

®

FUNCTIO N AL DESCRIPTION

Two operating modes, Strike and Run, are employed by

the LX1692. Upon power up or ENABLE going true, strike

mode is entered.

After a successful strike, e.g., lamp is

ignited, run mode is entered. If ignition is unsuccessful, or

if the lamp extinguishes while running, a fault is declared

and the controller automatically shuts down.

The main oscillator in the LX1692 has two frequency

control loops; a resonant tracking loop and a fixed

frequency loop. The fixed frequency loop is user set via the

I_R resistor and the C_R capacitor value.

The resonant

tracking loop follows the natural resonant frequency of the

load.

Lamp ignition is determined by monitoring the lamp

current feedback voltage at the ISNS pin. If less than 1.0V

during the strike period, the lamp is considered not ignited

and Strike mode continues until ignition is detected or

strike time out (approximately 1 - 2 seconds) is reached. If

greater than 1.0V, strike is declared and a latch is set. The

IC is now in “run” mode. And threshold voltage for strike

detect is reduced to 0.3V to permit a minimum 3:1 analog

dimming ratio to be achieved.

During strike, lamp operating frequency is always

controlled by the resonant frequency tracking loop. At

power up the lamp is not ignited and the loads’ natural

resonant frequency will be typically1.3 to 1.5 times higher

than after the lamp has ignited.

The tracking oscillator frequency will slew to near the

natural resonant frequency of the load.

At open circuit

resonance load Q is high and produces a large rise of

voltage across the lamp, eliminating the need to use a high

transformer turns ratio.

Additionally, since frequency is high, the volt-seconds

applied to the transformer primary is minimized.

This

permits the use of smaller transformers that have reduced

core cross sectional area. During striking operation, ICOMP

is limited to 2.5V until ignition latch is set.

When it starts the striking operation, it starts the striking

frequency as user programmed operating frequency.

And when OVSNS is detected zero cross point , then

tracking oscillator will start to sweep the frequency up to

near to the circuit resonant frequency and it will track the

frequency to reach user programmed open lamp voltage.

At the moment of lamp ignition, operating frequency

immediately switches to the programmed value of the fixed

frequency oscillator. Lamp current flow is sensed by the

strike detection comparator, which decides to return PWM

timing control to the fixed frequency oscillator

The LX1692 has shut down protection for all common

lamp fault conditions. These include the following:

a.

Open or broken lamp

b.

High Voltage Arcing on transformer secondary side

c.

Short across lamp terminals

d.

Short from high side of lamp to ground

e.

Short from low side of lamp to ground ( current

sense resistor shorted)

Three inputs from the lamp are monitored to detect these

conditions,

ISNS,

OV_SNS,

and

OC_SNS.

Fault

protection is designed to prevent fire or smoke from being

generated by terminating inverter operation in the event of

failures in the high voltage components and the power

FET’s. All fault shut down events can only be reset by

ENABLE or VDDP cycling.

When the IC is first powered on or enabled, the inverter

output voltage must be made higher than the normal

operating voltage of the lamp to cause ignition.

The

LX1692 generates this higher “strike” voltage by operating

at the open circuit resonant frequency of the load

inductance and capacitance. Because of its high unloaded

Q, a large resonant rise of voltage occurs across the lamp,

and produces ignition. Both resonant frequency and Q of

the lamp circuit are higher when the lamp is off than when

on. The lamp may not ignite immediately when specified

strike voltage is applied. It is customary to apply strike

voltage for from 0.3 to 3 seconds to insure ignition of cold,

dark, or aged lamps. The LX1692 has a programmable

time out for this purpose. During strike time out, open

lamp voltage is regulated to a value programmed by a

voltage divider across the lamp and sensed at the OV_SNS

pin.