MAX8729EEI+T Maxim Integrated Products, MAX8729EEI+T Datasheet - Page 18

• 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

For safety, the MAX8729 monitors the lamp-current
feedback (IFB) to detect faulty or open CCFL lamps
and shorted IFB sense resistor. As described in the
Lamp-Current Regulation section, the voltage on IFB is
internally full-wave rectified. If the rectified IFB voltage
is below 790mV, the MAX8729 charges the TFLT
capacitor with 1µA. The MAX8729 latches off if the volt-
age on TFLT exceeds 4V. Unlike the normal shutdown
mode, the linear regulator output (V
Toggling SHDN or cycling the input power reactivates
the device.
Constant-Frequency, Half-Bridge CCFL
Inverter Controller
Figure 6. Daisy-Chain Operation of Five Controllers
18
DIMMING
ON/OFF
______________________________________________________________________________________
V
CC
CNTL
SHDN
HFCK
LFCK
CNTL
SHDN
HFCK
LFCK
CNTL
SHDN
HFCK
LFCK
CNTL
SHDN
HFCK
LFCK
V
V
V
V
CC
CC
CC
CC
Lamp-Out Protection
PS1
PS1
PS1
PS1
MASTER
SLAVE 1
SLAVE 2
SLAVE 4
CC
PS2
PS2
PS2
PS2
) remains at 5.3V.
DPWM
DPWM
DPWM
DPWM
PSCK
PSCK
PSCK
PSCK
HF
HF
HF
HF
LF
LF
LF
LF
During the delay period, the current-control loop tries to
maintain the lamp-current regulation by increasing the
high-side MOSFET on-time. Because the lamp imped-
ance is very high when it is open, the transformer sec-
ondary rises as a result of the high-Q factor of the
resonant tank. Once the secondary voltage exceeds
the overvoltage threshold, the MAX8729 turns on a
1.2mA 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
during the lamp-out delay period.
The MAX8729 provides cycle-by-cycle primary over-
current protection. A current-sense amplifier monitors
the drain-to-source voltages of both the high-side and
low-side switches when the switches are conducting. If
the voltage exceeds the internal current-limit threshold
(400mV typ), the regulator turns off the high-side switch
at the opposite side of the primary to prevent the trans-
former primary current from increasing further.
The secondary current limit provides fail-safe protection
in case of short-circuit or leakage from the transformer’s
high-voltage terminal to ground. ISEC monitors the volt-
age across a sense resistor placed between the trans-
former’s low-voltage secondary terminal and ground.
The ISEC voltage is internally half-wave rectified and
continuously compared to the ISEC regulation threshold
(1.25V, typ). Any time the ISEC voltage exceeds the
threshold, a controlled current is drawn from COMP to
reduce the on-time of the bridge’s high-side switches. At
the same time, the MAX8729 charges the TFLT capacitor
with a 126µA current. The MAX8729 latches off when the
voltage on TFLT exceeds 4V. Unlike the normal shut-
down mode, the linear regulator output (V
5.3V. Toggling SHDN or cycling the input power reacti-
vates the device.
The MAX8729 supports daisy-chain operation to allow
synchronization and phase shift of multiple MAX8729s.
Up to five MAX8729s can be connected in a daisy-
chain configuration shown in Figure 6. Connecting
CNTL to V
puts the IC in slave mode.
To synchronize the switching frequency, connect the
HFCK pins of the slave IC and master IC together, and
connect the PSCK pin of the master IC to the HF pin of
Synchronization in Daisy-Chain Operation
CC
Primary Overcurrent Protection
enables the daisy-chain operation and
Secondary Current Limit (ISEC)
(HFCK, LFCK, PSCK, DPWM)
CC
) remains at