MAX8722 Maxim Integrated Products, MAX8722 Datasheet - Page 16

• Manufacturer Part Number: MAX8722
• Description: Low-Cost CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

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In digital PWM operation, COMP controls the rise and
fall time of the lamp-current envelope. At the beginning
of the digital PWM on-cycle, V
ually increasing t
end of the digital PWM on-cycle, the COMP capacitor
discharges linearly, gradually decreasing t
viding soft-stop.
There are three ways to set the digital PWM frequency.
1) The digital PWM frequency can be set with an
2) The digital PWM frequency can be clocked by an
Low-Cost CCFL Backlight Controller
Table 3. Digital PWM Frequency Setting
16
Connect FREQ to GND
through an external resistor.
Connect FREQ to V
Connect FREQ to GND
through a 100k
external resistor. Connect SYNC to GND and con-
nect a resistor between FREQ and GND. The digital
PWM frequency is given by the following equation:
The adjustable range of the digital PWM frequency
is between 100Hz and 350Hz (R
353kΩ and 101kΩ). CNTL controls the digital PWM
duty cycle.
external high-frequency signal. Connect FREQ to
V
quency signal. The digital PWM frequency is 1/128
of the frequency of the external signal:
where f
The frequency range of the external signal should
be between 13kHz and 45kHz, resulting in a digital
PWM frequency range between 100Hz and 350Hz.
CNTL controls the DPWM duty cycle.
______________________________________________________________________________________
CC
and connect SYNC to the external high-fre-
f
DPWM
FREQ
EXT
Digital PWM Frequency Setting
is the frequency of the external signal.
resistor.
=
CC
ON
f
209
.
DPWM
, which provides soft-start. At the
Hz
Connect SYNC to GND.
Connect SYNC to an
external high-frequency
signal.
Connect SYNC to V
×
=
169 Ω
COMP
f
128
EXT
k
SYNC
rises linearly, grad-
/
FREQ
R
FREQ
ON
CC
is between
.
and pro-
DPWM is used as the digital PWM
signal output.
DPWM is used as the digital PWM
signal output.
Connect DPWM to an external
low-frequency signal.
3) The digital PWM frequency can be synchronized to
The frequency range of the external signal is between
100Hz and 350Hz. In this mode, the brightness control
input CNTL is disabled, and the brightness is propor-
tional to the duty cycle of the external signal.
Table 3 summarizes the three ways of setting the digital
PWM frequency.
The MAX8722 includes an undervoltage-lockout
(UVLO) circuit. The UVLO circuit monitors the V
age. When V
ables both high-side and low-side MOSFET drivers and
resets the fault latch.
When the MAX8722 is placed in shutdown, all functions
of the IC are turned off except for the 5.4V linear regu-
lator. In shutdown, the linear-regulator output voltage
drops to about 4.5V and the supply current is 6µA (typ).
While in shutdown, the fault latch is reset. The device
can be placed into shutdown by pulling SHDN to its
logic-low level.
For safety, the MAX8722 monitors the lamp-current
feedback (IFB) to detect faulty or open CCFL tubes and
secondary short circuits in the lamp and IFB sense
resistor. As described in the Lamp-Current Regulation
section, the voltage on IFB is internally full-wave recti-
fied. If the rectified IFB voltage is below 600mV, the
MAX8722 charges the TFLT capacitor with 1µA. The
DPWM
an external low-frequency signal. To enable this
mode, connect SYNC to V
GND through a 100kΩ resistor, and connect DPWM
to the external low-frequency signal. The digital
PWM frequency and duty cycle are equal to those
of the external signal.
CC
is below 4.2V (typ), the MAX8722 dis-
The resistor value sets the frequency.
CNTL controls the duty cycle.
The frequency is 1/128 of the
frequency of the external signal. CNTL
controls the duty cycle.
The frequency and duty cycle are
equal to those of the external signal.
FREQUENCY/DUTY CYCLE
Low-Power Shutdown
Lamp-Out Protection
DIGITAL PWM
CC
, connect FREQ to
CC
UVLO
volt-