MAX8751ETJ+ Maxim Integrated Products, MAX8751ETJ+ Datasheet - Page 16

• Manufacturer Part Number: MAX8751ETJ+
• Description: IC CNTRLR CCFL INV 32-TQFN
• Manufacturer: Maxim Integrated Products
• Type: CCFL Controllerr

Specifications of MAX8751ETJ+

• Frequency: 30 ~ 80 kHz
• Current - Supply: 3.2mA
• Voltage - Supply: 6 V ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Lead Free Status / Rohs Status: Details

mode, at the beginning of the positive half cycle, NH1
and NL2 turn on and the primary current starts ramping
up. The controller turns off NH1 as the primary current
reaches its peak value. The primary current continues to
flow in the same direction, which forward biases the
body diode of NL1, ramping down the primary current.
When the primary current reaches zero, NL2 is turned off
and NH2 is turned on, starting the negative half cycle.
The MAX8751 reduces the voltage stress on the trans-
former’s secondary winding by limiting the secondary
voltage during startup and open-lamp condition. The AC
voltage across the transformer secondary winding is
sensed through a capacitive voltage-divider. The volt-
age across the low-side capacitor of the divider is fed to
the VFB pin of the MAX8751. An overvoltage compara-
tor compares the VFB voltage with 2.25V (typ) internal
threshold. If the sensed voltage exceeds the overvolt-
age threshold, the MAX8751 turns on an internal
1200µA current source, which discharges the COMP
capacitor. The high-side MOSFET’s on-time shortens as
the COMP voltage decreases, hence reducing the
transformer’s secondary peak voltage below the thresh-
old 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. For example, the normal running voltage of
a typical CCFL is approximately 650V
ing voltage can be as high as 1800V
The MAX8751’s unique resonant startup method
ensures reliable striking. Before the lamp is ionized, the
Fixed-Frequency, Full-Bridge CCFL
Inverter Controller
nal of the lamp. The voltage across this resistor is fed to
The MAX8751 uses a lamp-current control loop to regu-
fied IFB voltage with a 790mV (typ) internal reference to
16
late the current delivered to the CCFL. The heart of the
resistor connected in series with the low-voltage termi-
Figure 2. The AC lamp current is sensed with a sense
error amplifier’s output (COMP) and ground to create
transconductance error amplifier compares the recti-
Transformer Secondary Voltage Limiting
the IFB input and is internally full-wave rectified. The
generate an error current. The error current charges
and discharges a capacitor connected between the
control loop is a transconductance error amplifier in
an error voltage (V
with an internal ramp signal to control the high-side
______________________________________________________________________________________
COMP
Lamp-Current Regulation
MOSFET switch on-time (t
). V
COMP
RMS
RMS
is then compared
Lamp Startup
.
, but the strik-
ON
).
lamp impedance is infinite. The transformer secondary
leakage inductance and the high-voltage parallel
capacitor determine the unloaded resonant frequency.
Since the unloaded resonant circuit has a high Q, the
inverter keeps increasing the secondary voltage until
either the lamp is struck or the controller activates the
secondary overvoltage protection.
Upon power-up, V
duty cycle of the high-side MOSFET switches and pro-
viding a measure of soft-start. In addition, the MAX8751
pulls up V
immediately after the device is enabled. The DC volt-
age on V
resistor during startup. This feature is equivalent to
slowly raising the overvoltage threshold during startup,
so it further improves the soft-start behavior. The
MAX8751 automatically switches over to constant-fre-
quency operation after the IFB voltage rises above
open-lamp threshold.
The MAX8751 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
vides immunity to input-voltage variations and simplifies
loop compensation over wide-input voltage ranges. The
feed-forward control also improves the line regulation
for short DPWM on-times and makes startup transients
less dependent on the input voltage.
Feed-forward control is implemented by increasing the
internal voltage ramp rate for higher V
effect of varying t
while maintaining almost the same signal levels at
V
compensation capacitor is minimal, the controller’s
response to input voltage change is essentially
instantaneous.
The MAX8751 controls the brightness of the CCFL by
“chopping” the lamp current on and off using a low-fre-
quency (between 80Hz and 300Hz) DPWM signal
either from the internal oscillator or from an external sig-
nal source. In the DPWM operation, COMP controls the
rise and fall of the lamp current. At the beginning of the
DPWM on-cycle, the lamp current is zero; V
ly rises due to charging from transconductance error
amplifier, and t
lamp current slowly, providing soft-start. The lamp cur-
rent stabilizes after it reaches the regulation point. At
COMP
. Since the required voltage change across the
FB
FB
Feed-Forward Control and Dropout
is gradually discharged through an internal
to the overvoltage threshold (2.25V, typ)
ON
ON
increases gradually, increasing the
COMP
as a function of the input voltage
DPWM Dimming Control
slowly rises, increasing the
IN
). This feature pro-
IN
. This has the
Operation
COMP
linear-