MAX8722 Maxim Integrated Products, MAX8722 Datasheet - Page 17

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

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MAX8722 latches off if the voltage on TFLT exceeds
4V. Unlike the normal shutdown mode, the linear-regu-
lator output (V
cycling the input power reactivates the device.
During the delay period, the current control loop tries to
maintain lamp-current regulation by increasing the
high-side MOSFET on-time. Because the open-circuit
lamp impedance is very high, the transformer sec-
ondary voltage rises as a result of the high Q-factor of
the resonant tank. Once the secondary voltage
exceeds the overvoltage threshold, the MAX8722 turns
on a 400µA current source that discharges the COMP
capacitor. The on-time of the high-side MOSFET is
reduced, lowering the secondary voltage, as the COMP
voltage decreases. Therefore, the peak voltage of the
transformer secondary winding never exceeds the limit
set by a capacitive voltage-divider during the lamp-out
delay period.
The MAX8722 senses transformer primary current in
each switching cycle. When the regulator turns on the
low-side MOSFET, a comparator monitors the voltage
drop from LX_ to GND. If the voltage exceeds the cur-
rent-limit threshold, the regulator turns off the high-side
switch at the opposite side of the primary to prevent
further increasing the transformer primary current.
The current-limit threshold can be adjusted using the
ILIM input. Connect a resistive voltage-divider between
V
current-limit threshold measured between LX_ and
GND is 1/5 of the voltage at ILIM. The ILIM adjustment
range is 0 to 3V. Connect ILIM to V
default current-limit threshold of 0.2V.
The secondary current limit provides fail-safe current
limiting in case a failure, such as a short circuit or leak-
age from the lamp high-voltage terminal to ground, pre-
vents the current control loop from functioning properly.
ISEC monitors the voltage across a sense resistor
placed between the transformer’s low-voltage sec-
ondary terminal and ground. The ISEC voltage is inter-
nally half-wave rectified and continuously compared to
the ISEC regulation threshold (1.25V typ). Any time the
ISEC voltage exceeds the threshold, a controlled cur-
rent is drawn from COMP to reduce the on-time of the
bridge’s high-side switches. At the same time, the
MAX8722 charges the TFLT capacitor with a 116µA cur-
rent source. The MAX8722 latches off when the voltage
on TFLT exceeds 4V. Unlike the normal shutdown
mode, the linear-regulator output (V
CC
and GND with the midpoint connected to ILIM. The
Primary Overcurrent Protection (ILIM)
CC
Secondary Current Limit (ISEC)
) remains at 5.4V. Toggling SHDN or
______________________________________________________________________________________
Low-Cost CCFL Backlight Controller
CC
) remains at 5.3V.
CC
to select the
Toggling SHDN or cycling the input power reactivates
the device.
The internal linear regulator steps down the DC input
voltage to 5.4V (typ). The linear regulator supplies
power to the internal control circuitry of the MAX8722
and is also used to power the MOSFET drivers by con-
necting V
shutdown.
The MAX8722 requires four external n-channel power
MOSFETs NL1, NL2, NH1, and NH2 to form a full-
bridge inverter circuit to drive the transformer primary.
The regulator senses the on-state drain-to-source volt-
age of the two low-side MOSFETs NL1 and NL2 to
detect the transformer primary current, so the R
of NL1 and NL2 should be matched. For instance, if
dual MOSFETs are used to form the full bridge, NL1
and NL2 should be in one package. Since the
MAX8722 uses the low-side MOSFET R
mary overcurrent protection, the lower the MOSFET
R
user should select a dual, logic-level n-channel
MOSFET with low R
and keep the primary current limit at a reasonable level.
The regulator uses zero-voltage switching (ZVS) to soft-
ly turn on each of the four switches in the full bridge.
ZVS occurs when the external power MOSFETs are
turned on when their respective drain-to-source volt-
ages are near 0V (see the Resonant Operation section).
ZVS effectively eliminates the instantaneous turn-on
loss of MOSFETs caused by C
capacitance) and parasitic capacitance discharge, and
improves efficiency and reduces switching-related EMI.
The MAX8722 senses the lamp current flowing through
resistor R1 (Figure 1) connected between the low-volt-
age terminal of the lamp and ground. The voltage
across R1 is fed to IFB and is internally full-wave recti-
fied. The MAX8722 controls the desired lamp current
by regulating the average of the rectified IFB voltage.
To set the RMS lamp current, determine R1 as follows:
where I
790mV is the typical value of the IFB regulation point
DS(ON)
LAMP(RMS)
, the higher the current limit. Therefore, the
CC
to V
Applications Information
R
Linear-Regulator Output (V
1
DD
is the desired RMS lamp current and
=
DS(ON)
. The V
2 2
Setting the Lamp Current
π
×
to minimize conduction loss,
×
CC
I
LAMP RMS
790
voltage drops to 4.5V in
mV
OSS
(
)
(drain-to-source
DS(ON)
MOSFETs
DS(ON)
for pri-
CC
17
)