MAX8729EEI+T Maxim Integrated Products, MAX8729EEI+T Datasheet - Page 17

• 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

Upon power-up, V
cycle of the high-side MOSFET switches and providing a
measure of soft-start. In addition, the MAX8729 charges
V
after the device is enabled. The DC voltage on V
gradually discharged through an internal resistor during
startup. This feature is equivalent to slowly raising the
overvoltage threshold during startup, so it further
improves the soft-start behavior. The MAX8729 automati-
cally switches over to the constant-frequency operation
after the lamp current reaches regulation.
The MAX8729 is designed to maintain tight control of
the lamp current when a line transient occurs. The
feed-forward control instantaneously adjusts the on-
time for changes in input voltage (V
vides immunity to input-voltage variations and simplifies
loop compensation over wide-input voltage ranges. The
feed-forward control also improves the line regulation
for short DPWM on-times and makes startup transients
less dependent on the input voltage.
Feed-forward control is implemented by increasing the
internal voltage ramp rate for higher V
effect of varying t
while maintaining about the same signal levels at V
Since the required voltage change across the compen-
sation capacitor is minimal, the controller’s response to
input voltage changes is essentially instantaneous.
The MAX8729 controls the brightness of the CCFL by
“chopping” the lamp current on and off using a low-fre-
quency (between 100Hz and 350Hz) digital PWM signal
Figure 5. Theoretical Brightness vs. Control Voltage
FB
to the overvoltage threshold (2.3V, typ) immediately
Feed-Forward Control and Dropout
100
90
80
70
60
50
40
30
20
10
0
0
Digital PWM Dimming Control
COMP
ON
______________________________________________________________________________________
400
as a function of the input voltage
Constant-Frequency, Half-Bridge CCFL
CNTL VOLTAGE (mV)
slowly rises, increasing the duty
800
1200
IN
). This feature pro-
1600
IN
. This has the
Operation
2000
COMP
FB
is
.
either from the internal oscillator or from an external sig-
nal source. In digital PWM operation, COMP controls the
dynamics of the lamp-current envelope. At the beginning
of the digital PWM ON cycle, the average value of the
lamp-current feedback signal is below the regulation
point, so the transconductance error amplifier sources
current into the COMP capacitor. The switch on-time
(t
vides soft-start. At the end of the digital PWM ON cycle,
the MAX8729 turns on a 100µA internal current source.
The current source linearly discharges the COMP capac-
itor, gradually decreasing t
When the SEL pin is connected to ground, the
MAX8729 uses the internal oscillator to generate the
DPWM signal. 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 100Hz and 300Hz (R
103kΩ).
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 2V, which is digitized
to select one of 128 brightness levels. As shown in
Figure 5, the MAX8729 ignores the first 13 steps, so the
first 13 steps all represent the same brightness. When
V
always 10%. When V
change on CNTL results in a 0.78% change in the
DPWM duty cycle. When V
the DPWM duty cycle is always 100%.
To use the external digital PWM signal to control the
brightness, connect SEL to V
the external signal source. The frequency range of the
external signal is between 100Hz and 300Hz with R
150kΩ. In this mode, the brightness control input CNTL
is disabled, and the brightness is proportional 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 is adjusted
accordingly.
ON
CNTL
) gradually increases as V
is between 0 and 203mV, the DPWM duty cycle is
Inverter Controller
f
DPWM
Using the External Digital PWM Signal
CNTL
=
207
Using the Internal Oscillator
ON
is above 203mV, a 15.625mV
CNTL
Hz x
, and providing soft-stop.
CC
R
LF
LF
COMP
is equal to or above 2V,
and connect LSYNC to
is between 217kΩ and
150
k
Ω
rises, which pro-
LF
17
=