MAX8722 Maxim Integrated Products, MAX8722 Datasheet - Page 18

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

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specified in the Electrical Characteristics table. To set
the RMS lamp current to 6mA, the value of R1 should
be 148Ω. The closest standard 1% resistors are 147Ω
and 150Ω. The precise shape of the lamp-current
waveform, which is dependent on lamp parasitics, influ-
ences the actual RMS lamp current. Use a true RMS
current meter connected between the R1/IFB junction
and the low-voltage side of the lamp to make final
adjustments to R1.
The MAX8722 limits the transformer secondary voltage
during startup and lamp-out faults. The secondary volt-
age is sensed through the capacitive voltage-divider
formed by C3 and C4 (Figure 1). The voltage on VFB is
proportional to the CCFL voltage. The selection of
the parallel resonant capacitor C3 is described in
the Transformer Design and Resonant Component
Selection section. C3 is usually between 10pF and
22pF. After the value of C3 is determined, select C4
using the following equation to set the desired maxi-
mum RMS secondary voltage V
where 2.3V is the typical value of the VFB overvoltage
threshold specified in the Electrical Characteristics table.
To set the maximum RMS secondary voltage to 1600V
using 18pF for C3, use approximately 15nF for C4.
The MAX8722 limits the secondary current even if the
IFB sense resistor (R1) is shorted or transformer sec-
ondary current finds its way to ground without passing
through R1. ISEC monitors the voltage across the
sense resistor R3, connected between the low-voltage
terminal of the transformer secondary winding and
ground. Determine the value of R3 using the following
equation:
where I
transformer secondary current during fault conditions,
and 1.217V is the typical value of the ISEC regulation
point specified in the Electrical Characteristics table. To
set the maximum RMS secondary current in the circuit
of Figure 1 to 22mA, use approximately 40.2Ω for R3.
Low-Cost CCFL Backlight Controller
18
______________________________________________________________________________________
SEC(RMS)
Setting the Secondary Voltage Limit
C
Setting the Secondary Current Limit
4
R
=
3
=
2
_
MAX
×
2
V
×
LAMP RMS MAX
is the desired maximum RMS
2 3
I
SEC RMS MAX
1 217
.
.
V
(
(
LAMP(RMS)
V
)_
)_
×
_
MAX
C
3
:
The transformer is the most important component of the
resonant tank circuit. The first step in designing the
transformer is to determine the turns ratio (N). The ratio
must be high enough to support the CCFL operating
voltage at the minimum supply voltage. N can be cal-
culated as follows:
where V
in normal operation, and V
input voltage. If the maximum RMS lamp voltage in nor-
mal operation is 650V and the minimum DC input volt-
age is 8V, the turns ratio should be greater than 90. The
turns ratio of the transformer used in the circuit of
Figure 1 is 93.
The next step in the design procedure is to determine
the desired operating frequency range. The MAX8722
is synchronized to the natural resonant frequency of the
resonant tank. The resonant frequency changes with
operating conditions, such as the input voltage, lamp
impedance, etc.; therefore, the switching frequency
varies over a certain range. To ensure reliable opera-
tion, the resonant frequency range must be within the
operating frequency range specified by the CCFL
transformer manufacturer. As discussed in the
Resonant Operation section, the resonant frequency
range is determined by the transformer secondary leak-
age inductance L, the primary series DC blocking
capacitor C2, and the secondary parallel resonant
capacitor C3. Since it is difficult to control the trans-
former leakage inductance, the resonant tank design
should be based on the existing secondary leakage
inductance of the selected CCFL transformer. Leakage-
inductance values can have large tolerance and signifi-
cant variations among different batches, so it is best to
work directly with transformer vendors on leakage-
inductance requirements. The MAX8722 works best
when the secondary leakage inductance is between
250mH and 350mH. The series capacitor C2 sets the
minimum operating frequency, which is approximately
two times the series resonant peak frequency. Choose:
LAMP(RMS)
Transformer Design and Resonant
C
2
N
≤
is the maximum RMS lamp voltage
≥
π
0 9
2
.
V
×
LAMP RMS
×
Component Selection
IN(MIN)
f
N
MIN
V
2
(
IN MIN
2
(
×
)
is the minimum DC
)
L