Microsemi

Integrated Products

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

Page 10

Copyright

© 2003

Rev. 1.0, 7/16/2004

LX1691

Enhanced Multi-Mode CCFL Controller

TM

®

DE TAILED DESCRIP T ION

FEATURE REVIEW

On-Chip LDO Regulator

An LDO regulator keeps critical analog control circuits

operating optimally over the entire input voltage range. Brightness

control voltage and operating frequency are extremely stable.

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

Power On Delay

approximately 100 milliseconds after power is applied. This give

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 pin offers a high impedance input with a linear

control range of 0 to 2.0V The function of the pin depends on

which of six dimming modes the controller is programmed. (see

dimming table page 4) A new feature of the LX1691 allows the

DD_CLK input to control the burst frequency and duty cycle while

the BRITE_IN input controls the output current amplitude (see

additional information below).

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.

When in Digital mode, the dimming burst frequency can be

synchronous to lamp current by selecting internal clocking, or to

an external clock. With an external clock source, burst frequency

will be the clocks frequency divided by 512.

When using the

internal clock source the burst frequency will be the internal

oscillator divided by 1024 which is equivalent to lamp current

frequency divided by 512.

Brightness Polarity and Dual Mode Dimming Control

In Analog dimming mode 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

If

simultaneous amplitude control and digital dimming is desired,

apply a PWM signal to DD_CLK. The lamp current waveform

will exactly follow DD_CLK (DD_CLK is not divided in this

mode); e.g., lamp current flows when DD_CLK is high and stops

when it is low.

In Digital dimming mode, lamp current duty cycle is either

directly or inversely proportional to DC input voltage at the

BRITE_IN pin, depending on the state of DD_CLK. If external

clock is selected, duty cycle will be directly proportional to voltage

at BRITE_IN and the burst period will be DD_CLK period X 512.

Lamp current amplitude can be set and varied by applying a

separate DC voltage at the BRITE_OUT pin.

This voltage can

range from zero to 2.0 VDC. Zero volts will produce zero current

(and therefore a lamp fault).

2.0V will produce the maximum

current as defined by the value of the current sensing resistor from

Strike Voltage Generation

Improved strike voltage generation circuits ramp strike voltage

slowly to programmed maximum potential and hold it there for

approximately 350mSec @65Hz operating frequency. This insures

a worst case lamp will strike at any temperature. Strike potential is

removed immediately when the lamp strikes or if the time limit is

reached.

Strike Detection

The LX1691 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.

Fault Time Out

If the lamp fails to ignite within approximately 350mSec, or if it

extinguishes after ignition, or if the clock signal at the DD_CLK pin

terminates when in external digital dim mode, the output drive is

shut down.

Fault mode will also be invoked if the lamp is short circuited or

left open for more than 350mSec.

On Chip Rectifiers

Integrating full wave rectifiers for each of three lamp inputs

significantly reduces lamp feedback component count.

Current

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

Sense (OV_SNS) signals are 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.

Complete Fault Protection

In addition to the lamp fault time out, maximum output voltage

and current under all fault conditions is regulated. 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.

Familiar Magnetics

The LX1691 can use the same magnetics as all other Direct Drive

controllers. Refer to Application Note 13 for transformer design

and power FET selection criteria.