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LX1689

Third Generation CCFL Controller

Microsemi Corporation 

LX1689

TM

®

Third Generation CCFL Controller

PRODUCTION DATA SHEET

DESCRIPTION

FEATURE REVIEW

On-Chip LDO Regulators

Lamp Current Compensation

Two LDO regulators extend the input voltage range of the IC up The BRITE_OUT pin outputs a precision current that is

to 28 Volts without using external circuitry as was required with proportional to the BRITE_IN signal. This current can be applied to

our previous controllers.

a precision resistor to develop the brightness control voltage at the

Under Voltage Lockout

If the battery input voltage is too low for the controller to

function properly, it will turn itself off, preventing spurious

operation. If the battery voltage falls to less than 1V where UVLO

is no longer guaranteed, 10K pull down resistors on the AOUT and

BOUT pins insure the external power FETs cannot be biased on.

Power On Delay

A power up reset delays AOUT and BOUT turn on for

approximately 16384 x 1/fO milliseconds after power is applied.

This gives extra time for the BRITE_IN source voltage to stabilize

so the lamp is not inadvertently powered up at high brightness and

then suddenly lowered, creating an undesirable light flash.

Enhanced BRITE Conditioning Circuitry

The BRITE_IN input is now enhanced to accept either DC

voltage or logic PWM signals. When PWM signals are input, their

levels are clipped at 2V and 0.5V so lamp current will not be

affected by variations in logic signal level. In addition, the

BRITE_C pin permits filtering DC inputs and converting high

frequency PWM inputs to DC voltages with the addition of only a

single external capacitor. A low frequency (less than 500Hz)

error amplifiers non-inverting input. Since the output is constant

current, designers can easily compensate lamp current with respect

to temperature, input voltage, ambient or lamp light output, and

combinations of these conditions by using various temperature or

light sensitive components in combination with resistors. This

capability is very useful in automotive and outdoor applications

where operating temperatures and ambient light vary over wide

ranges. See functional pin description for details.

Strike Voltage Generation

Improved strike voltage generation circuits ramp strike voltage to

5X fO and repeats it’s cycle unless excessive high voltage is sensed

at OV_SNS. If OV_SNS is detected during strike, strike voltage

will not ramp and will hold the current voltage until total strikes

lamp cycles numbers reach 245,760. Strike potential is removed

immediately when the lamp strikes or if the time limit is reached.

Strike Detection

The LX1689 includes a new lamp strike detection scheme that

saves a package pin and three external components. Internal circuits

monitor lamp current pulses at the I_SNS input to determine if the

lamp strikes and if it stays ignited once operational.

PWM signal can be used to directly modulate the duty cycle of the Fault Time Out

lamp current. In this case the capacitor at BRITE_C is not

If the lamp fails to ignite with in approximately 1.6 seconds

installed.

(depending on Run Frequency) at maximum strike potential, or if it

Digital or Analog Dimming Modes

A DIM_MODE input pin selects either Analog or Digital mode.

In Analog mode DC voltage at BRITE_IN controls lamp current

amplitude. In Digital mode it controls digital dimming duty cycle

with amplitude fixed at a value set by the external current scaling

resistor (BRITE_R). When in Digital mode, the dimming burst

extinguishes while enabled, or the external clock frequency at the

DIM_CLK pin terminates, the output drive is shut down and the

BRITE_C pin is driven high. This pin can be monitored with a

CMOS gate to obtain a logical indication that a lamp fault has

occurred. It is especially useful in multiple lamp applications or for

system diagnostic input.

frequency can be synchronous to lamp current by selecting internal The voltage on pin BRITE_C will vary directly with BRITE

clocking, or to an external clock that may be a multiple of the input voltage, but does not exceed 1.2V unless a fault condition

video vertical frame rate. With an external clock source, three burst occurs.

rate selections are available by programming the DIV_248 input to

divide the source clock by 2,4, or 8. This clock source is further

divided by 64 generating the internal burst ramp waveform. Using

the internal clock as source the DIV_248 input changes to divide

by 4, 8, or 16. This feature allows the designer to set a burst

frequency in the range of 100 to 500Hz. The external clock source

must not be interrupted unless the BRITE_IN is set > 2V or the

lamp will extinguish.

On Chip Rectifier

Integrating full wave rectifiers for each of three lamp inputs has

significantly reduced lamp feedback component count. Current

Sense (I_SNS), Over Current Sense (OC_SNS) and Over Voltage

Sense (OV_SNS) signals are now detected using only one external

scaling resistor or capacitor each. Rectification accuracy is

improved with high performance on chip rectifiers to provide better

lamp current and voltage regulation.

Brightness Polarity Control

In Analog dimming mode or internal Digital Dimming, the IC

can be programmed to either increase or decrease lamp current

amplitude as a function of increasing signal at the BRITE_IN pin

by simply connecting the DIM_CLK input to ground or VDD_A

or open(see Dimming Table). If External Digital dimming mode is

used, lamp current amplitude is constant and its duty cycle is

always directly proportional to DC input voltage and / or PWM

Complete Fault Protection

In addition to the faulty lamp time out, lamp open, lamp shorted,

and either lamp terminal shorted to ground are detected. Open

circuit voltage can never go higher than the preset maximum strike

potential and total current from the circuit is safely limited with a

scaling resistor. UL safety specifications can now be easily met in

any application.

duty cycle at the BRITE_IN pin.

Copyright © 2000

Rev. 1.1, 2006-02-27

Microsemi

Integrated Products

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

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