LX1691
TM
®
Enhanced Multi-Mode CCFL Controller
P
RODUCTION
D
ATA
S
HEET
DETAILED DESCRIPTION
FEATURE
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
is no longer guaranteed, 10K pull down resistors on the A
OUT
and
B
OUT
pins insure the external power FETs cannot be biased on.
Power On Delay
A power up reset that delays A
OUT
and B
OUT
turn on for
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
DD_CLK input to V
DDA
or ground (see Dimming Table). 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.
REVIEW
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
I
SNS
to GND.
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.
WWW .
Microsemi
.C
OM
D
ESCRIPTION
D
ESCRIPTION
Copyright
©
2003
Rev. 1.0, 7/16/2004
Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 10
MICROSEMI [ MICROSEMI CORPORATION ]
