Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 11
Copyright © 2000
Rev. 1.1, 2006-02-27
WWW.Microsemi .COM
LX1689
Third Generation CCFL Controller
TM
®
DESCRIPTION
FEATURE REVIEW
On-Chip LDO Regulators
Two LDO regulators extend the input voltage range of the IC up
to 28 Volts without using external circuitry as was required with
our previous controllers.
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 delays A
OUT
and B
OUT
turn on for
approximately 16384 x 1/f
O
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)
PWM signal can be used to directly modulate the duty cycle of the
lamp current. In this case the capacitor at BRITE_C is not
installed.
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
frequency can be synchronous to lamp current by selecting internal
clocking, or to an external clock that may be a multiple of the
video vertical frame rate. With an external clock source, three burst
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.
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
duty cycle at the BRITE_IN pin.
Lamp Current Compensation
The BRITE_OUT pin outputs a precision current that is
proportional to the BRITE_IN signal. This current can be applied to
a precision resistor to develop the brightness control voltage at the
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 f
O
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.
Fault Time Out
If the lamp fails to ignite with in approximately 1.6 seconds
(depending on Run Frequency) at maximum strike potential, or if it
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.
The voltage on pin BRITE_C will vary directly with BRITE
input voltage, but does not exceed 1.2V unless a fault condition
occurs.
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.
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.
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