MAX8722 Maxim Integrated Products, MAX8722 Datasheet - Page 15

• Manufacturer Part Number: MAX8722
• Description: Low-Cost CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

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The MAX8722 reduces the voltage stress on the trans-
former’s secondary winding by limiting the secondary
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 MAX8722 turns
on a 400µA 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 voltage-divider.
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 MAX8722’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
increasing the duty cycle of the high-side MOSFET
switches and providing a measure of soft-start.
Second, the MAX8722 charges VFB to the overvoltage
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.
The MAX8722 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
provides immunity to input-voltage variations and sim-
plifies loop compensation over wide input voltage
ranges. The feed-forward control also improves the line
Transformer Secondary Voltage Limiting
______________________________________________________________________________________
Feed-Forward Control and
Low-Cost CCFL Backlight Controller
Dropout Operation
BATT
COMP
Lamp Startup
). This feature
slowly rises,
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
has the effect of varying t
put voltage while maintaining approximately the same
signal levels at V
change across the compensation capacitor is minimal,
the controller’s response to input voltage changes is
essentially instantaneous.
The MAX8722 controls the brightness of the CCFL by
chopping the lamp current on and off using a low-fre-
quency (between 100Hz and 350Hz) digital PWM sig-
nal either from the internal oscillator or from an external
signal source. The CCFL brightness is proportional to
the digital PWM 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 bright-
ness levels. As shown in Figure 6, the MAX8722
ignores the first twelve steps, so the first twelve steps
all represent the same brightness. When V
between 0 and 187.5mV, the digital PWM duty cycle is
always 9.375%. When V
15.625mV change on CNTL results in a 0.78125%
change in the digital PWM duty cycle. When V
equal to or above 2000mV, the digital PWM duty cycle
is always 100%.
Figure 6. Theoretical Brightness vs. Control Voltage
100
90
80
60
70
50
40
30
20
10
0
0
Digital PWM Dimming Control
400
COMP
CONTROL VOLTAGE (mV)
. Since the required voltage
800
CNTL
ON
1200
as a function of the in-
is above 187.5mV, a
1600
2000
BATT
CNTL
CNTL
. This
15
is
is