MAX8751 Datasheet, PDF (17/27 Pages) Maxim Integrated Products – Fixed-Frequency, Full-Bridge CCFL Inverter Controller

Fixed-Frequency, Full-Bridge CCFL
Inverter Controller
the end of the DPWM on-cycle, the dimming control
logic turns on a 100µA internal current source, thus dis-
charging the COMP capacitor linearly, gradually
decreasing t_ON and bringing lamp current to zero, thus
providing soft-start. The dimming control for the CCFL
is provided by changing the duty ratio of the low-fre-
quency DPWM signal.
Using the Internal Oscillator
When the MAX8751 is not using an external synchro-
nization signal, the DPWM signal is generated using an
internal oscillator. The frequency of the internal DPWM
oscillator is adjustable through a resistor connected
between LF and GND. The DPWM frequency is given
by the following equation:
f_DPWM
=
208Hz x 150kΩ
R_LF
The adjustable range of the DPWM frequency is between
80Hz and 300Hz (R_LF is between 390kΩ and 104kΩ).
The CCFL brightness is proportional to the DPWM duty
cycle, which can be adjusted from 10% to 100%
through the CNTL pin. CNTL is an analog input with a
usable input voltage range between 0 and 2000mV,
which is digitized to select one of 128 brightness levels.
As shown in Figure 5, the MAX8751 ignores the first 13
steps, and the first 13 steps (V_CNTL between 0 and
203mV) all represent 10% brightness. When V_CNTL is
above 203mV, a 15.625mV change on CNTL results in
a 0.78% change in the DPWM duty cycle. When V_CNTL
is equal to or above 2000mV, the DPWM duty cycle is
always 100%.
100
90
80
70
60
50
40
30
20
10
0
0
400 800 1200 1600 2000
CNTL VOLTAGE (mV)
Figure 5. Brightness vs. CNTL Voltage
Using the External DPWM Signal
To use the external DPWM signal to control the bright-
ness, connect SEL to V_CC and connect LSYNC to the
external signal source. The frequency range of the exter-
nal signal is within ±40% of the internal oscillator fre-
quency set by R_LF. In this mode, the brightness control
input CNTL is disabled, and the brightness is proportion-
al to the duty cycle of the external signal. When the duty
cycle of the external signal is 100%, the CCFL reaches
full brightness. If the duty cycle of the external signal is
less than 10%, the CCFL brightness stays at 10%.
Lamp-Out Protection
For safety, the IFB pin on MAX8751 monitors the lamp
current to detect faulty or open CCFL lamps. As
described in the Lamp-Current Regulation section, the
voltage on IFB is internally full-wave rectified. If the rec-
tified IFB voltage is below 790mV, the MAX8751
charges the TFLT capacitor with a 1µA current source.
The fault latch is set if the voltage on TFLT exceeds 4V.
Unlike normal shutdown mode, the linear regulator out
(V_CC) remains at 5.35V. Toggling SHDN or cycling the
input power reactivates the device.
During the fault delay period, the current-control loop
tries to maintain the lamp-current regulation by increas-
ing the on-time of high-side MOSFETs. Because the
lamp impedance is very high when it is open, the trans-
former secondary rises as a result of the high-Q factor
of the resonant tank. Once the secondary voltage
exceeds the overvoltage threshold, the MAX8751 turns
on a 1200µA internal current source that discharges the
COMP capacitor. The on-time of the high-side
MOSFETs 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.
Primary Overcurrent Protection
The MAX8751 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.
______________________________________________________________________________________ 17