MAX8751ETJ+ Maxim Integrated Products, MAX8751ETJ+ Datasheet - Page 17

• Manufacturer Part Number: MAX8751ETJ+
• Description: IC CNTRLR CCFL INV 32-TQFN
• Manufacturer: Maxim Integrated Products
• Type: CCFL Controllerr

Specifications of MAX8751ETJ+

• Frequency: 30 ~ 80 kHz
• Current - Supply: 3.2mA
• Voltage - Supply: 6 V ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Lead Free Status / Rohs Status: Details

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
providing soft-start. The dimming control for the CCFL
is provided by changing the duty ratio of the low-fre-
quency DPWM signal.
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:
The adjustable range of the DPWM frequency is between
80Hz and 300Hz (R
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
203mV) all represent 10% brightness. When V
above 203mV, a 15.625mV change on CNTL results in
a 0.78% change in the DPWM duty cycle. When V
is equal to or above 2000mV, the DPWM duty cycle is
always 100%.
Figure 5. Brightness vs. CNTL Voltage
100
90
80
70
60
50
40
30
20
10
0
ON
f
0
DPWM
and bringing lamp current to zero, thus
______________________________________________________________________________________
LF
400
=
is between 390kΩ and 104kΩ).
CNTL VOLTAGE (mV)
208Hz x 150k
Using the Internal Oscillator
800
R
LF
1200
Fixed-Frequency, Full-Bridge CCFL
CNTL
Ω
1600
between 0 and
2000
CNTL
CNTL
is
To use the external DPWM signal to control the bright-
ness, connect SEL to V
external signal source. The frequency range of the exter-
nal signal is within ±40% of the internal oscillator fre-
quency set by R
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%.
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
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.
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.
CC
) remains at 5.35V. Toggling SHDN or cycling the
Inverter Controller
Primary Overcurrent Protection
LF
. In this mode, the brightness control
Using the External DPWM Signal
CC
and connect LSYNC to the
Lamp-Out Protection
17