Transformer Secondary Voltage Limiting
The MAX8722A reduces the voltage stress on the
transformer’s secondary winding by limiting the sec-
ondary voltage during startup and open-lamp faults.
The AC voltage across the transformer secondary
winding is sensed through a capacitive voltage-divider.
The small voltage across the larger capacitor of the
divider is fed to the VFB input and is internally half-
wave rectified. An overvoltage comparator compares
the VFB voltage with a 2.3V (typ) internal threshold.
Once the sense voltage exceeds the overvoltage
threshold, the MAX8722A turns on a 1200µA current
source that discharges the COMP capacitor. The high-
side MOSFET on-time shortens as the COMP voltage
decreases, reducing the transformer secondary’s peak
voltage below the threshold set by the capacitive volt-
age-divider.
Lamp Startup
A CCFL is a gas discharge lamp that is normally driven
in the avalanche mode. To start ionization in a nonion-
ized lamp, the applied voltage (striking voltage) must
be increased to the level required for the start of
avalanche. At low temperatures, the striking voltage
can be several times the typical operating voltage.
Because of the MAX8722A’s resonant topology, the strik-
ing voltage is guaranteed. Before the lamp is ionized, the
lamp impedance is infinite. The transformer secondary
leakage inductance and the high-voltage parallel capac-
itor determine the unloaded resonant frequency. Since
the unloaded resonant circuit has a high Q, it can gener-
ate very high voltages across the lamp.
Upon power-up, two soft-start features acting together
smooth the startup behavior. First, V
COMP
slowly rises,
increasing the duty cycle of the high-side MOSFET
switches and providing a measure of soft-start.
Second, the MAX8722A charges VFB to the overvolt-
age threshold (2.3V typ) immediately after the device is
enabled. The DC voltage on VFB is gradually dis-
charged through an internal 300kΩ (typ) resistor during
startup. This feature is equivalent to slowly raising the
overvoltage threshold during startup, so it further
improves the soft-start behavior.
Feed-Forward Control and
Dropout Operation
The MAX8722A is designed to maintain tight control of
the lamp current under all transient conditions. The
feed-forward control instantaneously adjusts the on-
time for changes in input voltage (V
BATT
). This feature
provides immunity to input-voltage variations and sim-
plifies loop compensation over wide input voltage
ranges. The feed-forward control also improves the line
regulation for short on-times and makes startup tran-
sients less dependent on the input voltage.
Feed-forward control is implemented by increasing
the internal voltage ramp rate for higher V
BATT
. This
has the effect of varying t
ON
as a function of the in-
put voltage while maintaining approximately the same
signal levels at V
COMP
. Since the required voltage
change across the compensation capacitor is minimal,
the controller’s response to input voltage changes is
essentially instantaneous.
DPWM Dimming Control
The MAX8722A controls the brightness of the CCFL by
chopping the lamp current on and off using a low-fre-
quency (between 100Hz and 350Hz) DPWM signal
either from the internal oscillator or from an external sig-
nal source. The CCFL brightness is proportional to the
DPWM duty cycle, which can be adjusted from 9.375%
to 100% by the CNTL pin. CNTL is an analog input with
a usable input voltage range between 0 and 2000mV,
which is digitized to select one of 128 brightness levels.
As shown in Figure 6, the MAX8722A ignores the first
12 steps, so the first 12 steps all represent the same
brightness. When V
CNTL
is between 0 and 187.5mV,
the DPWM duty cycle is always 9.375%. When V
CNTL
is
above 187.5mV, a 15.625mV change on CNTL results
in a 0.78125% change in the DPWM duty cycle. When
V
CNTL
is equal to or above 2000mV, the DPWM duty
cycle is always 100%.
MAX8722A
Low-Cost CCFL Backlight Controller
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