MAX8785AETI+T Maxim Integrated Products, MAX8785AETI+T Datasheet - Page 13

• Manufacturer Part Number: MAX8785AETI+T
• Description: IC CCFL CTRL 28-TQFN
• Manufacturer: Maxim Integrated Products
• Type: CCFL Controllerr

Specifications of MAX8785AETI+T

• Current - Supply: 1.5mA
• Voltage - Supply: 4.5 V ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 28-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Frequency: -
• Lead Free Status / Rohs Status: Details

the CCFL strike voltage is reached and the lamp impe-
dance begins to decrease. The operating frequency
continues to decrease as the lamp impedance drops
until the correct operating point is reached, somewhere
between Points C and D. Figure 5 shows the Q of reso-
nant tank is very high. If the operating frequency is
close to resonant frequency, then the high Q gives a
very high efficiency for the converter. However, due to
the high Q, the frequency of operation has to be very
close to the resonant frequency, and this reduces the
range of the switching frequency for the inverter. To
ensure optimal performance, careful selection of exter-
nal components is required.
The MAX8785A includes a voltage-controlled oscillator
(VCO) that sets the switching frequency of the
H-bridge. The VCO is controlled by the output of a
transconductance error amplifier that integrates the dif-
ference between the full-wave, rectified lamp-current
feedback signal (IFB) and an internal reference voltage
(800mV typ). As the lamp current-feedback signal
changes with respect to the reference, the error amplifier
sources and sinks current, which appropriately adjusts
COMP, and equivalently, the VCO frequency.
To strike the CCFL, a frequency sweep is initiated by
linearly ramping the COMP capacitor. As the COMP
voltage rises, the VCO sweeps the switching frequency
from the maximum value (set by the R
point where the gain of resonant tank is enough to
strike the lamp. The frequency sweep range is 15% of
the maximum switching frequency.
The MAX8785A has feed-forward control, which main-
tains tight control of the lamp current over the entire
input voltage range. The feed-forward control adjusts
the on-time (t
The current into RATE determines the slope of the inter-
nal ramp. Connect a passive network between V
RATE to change the duty ratio with input voltage. The
RATE pin allows the user to adjust the maximum duty
ratio (D
the PZT used in the application.
MAX
) to achieve the optimum performance of
ON
) by varying the slope of internal ramp.
Variable Frequency Operation
______________________________________________________________________________________
Feed-Forward Control
HF
resistor) to the
IN
Piezoelectric Transformers
and
Full-Bridge Controller for
Figure 6. Brightness vs. V
Connect a resistor between BATT and RATE to set D
where R
frequency and V
operation. The feed-forward network required is depen-
dent upon the PZT used in the application. Further
improvements in the performance at higher input volt-
ages can be achieved by adding a zener diode in
series with the RATE resistor. The MAX8785A has built-
in protection to limit the maximum duty cycle to 45%.
The MAX8785A has both analog and digital inputs to
control brightness.
Connect SEL to GND to enable analog control mode.
The voltage at CNTL controls DPWM duty cycle. The
DPWM frequency is externally set by the resistor at LF
(R
10% to 100%. CNTL has a voltage range of 0 to 2V with
256 brightness levels. Figure 6 shows the response of
DPWM duty cycle to the CNTL voltage. When V
between 0V and 0.23V, the DPWM duty cycle is fixed at
10%. When V
changes linearly with CNTL. When the CNTL voltage is
above 2V, the DPWM duty cycle is fixed at 100%.
LF
). In analog control mode, the adjustment range is
HF
R
100
RATE
is the resistor that sets the maximum switching
80
60
40
20
0
CNTL
0
=
D
MIN
MAX
is 0.23V to 2V, the DPWM duty cycle
0.4
BRIGHTNESS vs. V
is the minimum input voltage of
CNTL
(%)
0.8
V
×
CNTL
R
Analog Dimming Control
HF
(V)
1.2
(
k
Ω ×
Dimming Control
CNTL
)
1.6
V
MIN
( )
V
2.0
CNTL
MAX
13
is
: