MAX8729EEI+T Maxim Integrated Products, MAX8729EEI+T Datasheet - Page 16

• Manufacturer Part Number: MAX8729EEI+T
• Description: Display Drivers Constant-Frequency H alf-Bridge CCFL Inve
• Manufacturer: Maxim Integrated Products

Specifications of MAX8729EEI+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

continues below ground to start the negative cycle.
During the negative half cycle, the controller turns off
the low-side switch at t
turns on the high-side switch under ZVS conditions and
a new cycle begins. In both cases, A and B, ZVS oper-
ation reduces the turn-on switching losses of both
power switches, resulting in better efficiency.
The MAX8729 operates in resonant mode during start-
up. In resonant operation, the inverter keeps increasing
the secondary voltage until either the lamp is struck or
the controller activates overvoltage protection. In reso-
nant mode, the switching frequency is synchronized
with the natural resonant frequency of the resonant tank
circuit composed of: transformer leakage inductance,
primary capacitive divider, and secondary resonant
capacitor. The synchronization and phase-shift func-
tions are disabled during startup. Figure 4 demon-
strates the resonant operation, with a timing diagram of
the primary current and gate signals. In the resonant
mode, the high side turns on at the beginning of the
positive half cycle. The primary current ramps up. The
controller turns off the high-side switch at t
the lamp current. The primary current continues to flow
in the same direction, which forward biases the body
diode of the low-side switch after the high-side switch
is off. When the controller turns on the low-side switch,
the voltage drop across the switch is nearly zero. This
ZVS operation results in lower switching losses. With
DL on, the primary current ramps down through zero
until t
After which, the controller turns on the high-side switch
with ZVS condition and a new cycle begins. The ZVS
operation of this architecture reduces the turn-on
switching losses of both power switches, resulting in
better efficiency.
Constant-Frequency, Half-Bridge CCFL
Inverter Controller
Figure 4. Resonant-Operation Timing Diagram
16
PRIMARY CURRENT
______________________________________________________________________________________
2
, when the controller turns off the low-side switch.
DH
DL
t
1
4
. After which, the controller
Resonant Startup
t
2
1
to regulate
The MAX8729 uses a lamp-current control loop to regu-
late the current delivered to the CCFL. The heart of the
control loop is a transconductance error amplifier in
Figure 2. The AC lamp current is sensed with a sense
resistor connected in series with the low-voltage termi-
nal of the lamp. The voltage across this resistor is fed to
the IFB input and is internally full-wave rectified. The
transconductance error amplifier compares the recti-
fied IFB voltage with a 790mV (typ) internal reference to
generate an error current. The error current charges
and discharges a capacitor connected between the
error amplifier’s output (COMP) and ground to create
an error voltage (V
with an internal ramp signal to control the high-side
MOSFET switch on-time (t
The MAX8729 reduces the voltage stress on the trans-
former’s secondary winding by limiting the secondary
voltage during startup and open-lamp fault. 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 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 MAX8729 turns
on a 1.2mA current source that discharges the COMP
capacitor. As the COMP voltage decreases, the high-
side MOSFET’s on-time shortens, which reduces the
transformer secondary peak voltage. The MAX8729
stops discharging the COMP capacitor after the sec-
ondary peak voltage is below the threshold set by the
capacitive voltage-divider. This mechanism effectively
limits the secondary voltage.
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 to start ionization in
the lamp. For example, the normal running voltage of a
typical CCFL is around 650V
age can be as high as 1800V
The MAX8729’s unique resonant startup method
ensures reliable striking. Before the lamp is ionized, the
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.
Transformer Secondary Voltage Limiting
COMP
Lamp-Current Regulation
ON
). V
).
RMS
RMS
COMP
.
, but the striking volt-
is then compared
Lamp Startup