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LX1692B

Full Bridge Resonant CCFL Controller

Microsemi Corporation 

LX1692B

TM

®

Full Bridge Resonant CCFL Controller

PRODUCTION DATA SHEET

FUNCTIONAL DESCRIPTION

OPERATING MODES

OPEN LAMP

Two operating modes, Strike and Run, are employed by the

LX1692B. 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.

OSCILLATOR CHARACTERISTICS

The main oscillator in the LX1692B has a fixed frequency

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

resistor and the C_R capacitor value.

STRIKING THE LAMP

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.

FAULT PROTECTION

The LX1692B 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 from high side of lamp to ground

d. 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 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 LX1692B 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.

Strike time out is programmed by selecting the capacitor

value at the C_TO pin. If the lamp has not ignited before

the end of strike time out, a fault is declared and the IC

outputs are latched off.

HIGH VOLTAGE ARC OR OVER PROGRAMMED VOLTAGE

If a high voltage arc occurs due to intermittent lamp

contacts or component failure, if the over voltage

feedback divider is improperly designed, or if the open

lamp voltage regulation circuitry fails, the peak voltage

on the OV_SNS pin will rise above + 3.2 VDC. This

creates a pulse that increments a 4 bit accumulating

counter. After 16 events are counted, an open lamp fault

is declared and the IC outputs are latched off. This fault

is enabled at all times, including during lamp striking.

The 4 bit counter is reset by signal C_BST which

typically operates at 100 to 300 Hz. Also, OVSNS pin

voltage is greater than 3.2V, then ICOMP pin will be

forced to discharge to 0V about 600ns.

OPEN LAMP VOLTAGE REGULATION

The open lamp voltage regulator regulates the peak voltage

on the OV_SNS pin to +/- 1.97 volts, + the 0.2 volt offset,

with a maximum tolerance +/-8% (+/- 158 mV).

Assuming an additional +/- 5% tolerance for each of the

two capacitors or resistors in the high voltage divider,

maximum open lamp voltage tolerance at the system level

is +/- 18%. At the high side of tolerance, OV_SNS peak

voltage is +2.42V, on the low side of tolerance, OV_SNS

input voltage will be regulated at +1.914 Vpk. If tighter

total voltage regulation is needed in a given application,

the feedback divider can be made with 1% resistors.

Copyright © 2005

Rev. 1.1, 12/20/2006

Microsemi

Integrated Products Division

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

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