MAX8722A latches off if the voltage on TFLT exceeds
4V. Unlike the normal shutdown mode, the linear-regu-
lator output (V
CC
) remains at 5.4V. Toggling SHDN or
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 MAX8722A
turns on a 1200µA 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.
Primary Overcurrent Protection (ILIM)
The MAX8722A 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
CC
and GND with the midpoint connected to ILIM. The
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
CC
to select the
default current-limit threshold of 0.2V.
Secondary Current Limit (ISEC)
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
MAX8722A charges the TFLT capacitor with a 116µA
current source. The MAX8722A latches off when the
voltage on TFLT exceeds 4V. Unlike the normal shut-
down mode, the linear-regulator output (V
CC
) remains at
5.4V. Toggling SHDN or cycling the input power reacti-
vates the device.
Linear-Regulator Output (V
CC
)
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 MAX8722A
and is also used to power the MOSFET drivers by con-
necting V
CC
to V
DD
. The V
CC
voltage drops to 4.5V in
shutdown.
Applications Information
MOSFETs
The MAX8722A 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
DS(ON)
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
MAX8722A uses the low-side MOSFET R
DS(ON)
for pri-
mary overcurrent protection, the lower the MOSFET
R
DS(ON)
, the higher the current limit. Therefore, the
user should select a dual, logic-level n-channel
MOSFET with low R
DS(ON)
to minimize conduction loss,
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 zero (see the Resonant Operation sec-
tion). ZVS effectively eliminates the instantaneous
turn-on loss of MOSFETs caused by C
OSS
(drain-to-
source capacitance) and parasitic capacitance dis-
charge, and improves efficiency and reduces
switching-related EMI.
Setting the Lamp Current
The MAX8722A senses the lamp current flowing
through resistor R1 (Figure 1) connected between the
low-voltage terminal of the lamp and ground. The volt-
age across R1 is fed to IFB and is internally full-wave
rectified. The MAX8722A controls the desired lamp cur-
rent by regulating the average of the rectified IFB volt-
age. To set the RMS lamp current, determine R1 as
follows:
