Rainbow Electronics MAX8709 User Manual
Page 20
MAX8709
High-Efficiency CCFL Backlight
Controller with SMBus Interface
20
______________________________________________________________________________________
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 to make final adjustments to R1.
Setting the Secondary Voltage Limit
The MAX8709 limits the transformer secondary voltage
during lamp striking and lamp-out faults. The secondary
voltage 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 to 22pF. After the
value of C3 is determined, select C4 using the following
equation to set the desired maximum RMS secondary
voltage V
LAMP(RMS)
_
MAX
:
where 510mV is the typical value of the VFB regulation
threshold specified in the Electrical Characteristics
table. If C3 is 15pF, C4 needs to be 21.2nF to set the
desired maximum RMS secondary voltage to 1600V.
The closest standard value of C4 is 22nF.
The resistor R2 is used to set the VFB DC bias point to
0V. Choose the value of R2 as follows:
where f
SW
is the nominal resonant operating frequency.
Setting the Secondary Current Limit
The MAX8709 limits the secondary current even if the
IFB sense resistor is shorted or transformer secondary
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
SEC(RMS)
_
MAX
is the desired maximum RMS
transformer secondary current during fault conditions,
and 1.25V is the typical value of the ISEC regulation
point specified in the Electrical Characteristics table.
Transformer Design and Resonant
Component Selection
The transformer is the most important component of the
resonant tank circuit. The first step in designing the
transformer is to determine the transformer turns ratio.
The ratio must be high enough to support the CCFL
operating voltage at the minimum supply voltage. The
transformer turns-ratio N can be calculated as follows:
where V
LAMP(RMS)
is the maximum RMS lamp voltage
in normal operation, and V
IN(MIN)
is the minimum DC
input voltage.
The next step in the design procedure is to determine
the desired operating frequency range. The MAX8709
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 lamp
transformer manufacturers. 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. The leak-
age inductance values usually have large tolerance
and significant variations among different batches. It is
best to work directly with transformer vendors on leak-
age inductance requirements. The MAX8709 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 fre-
quency. Choose:
where f
MIN
is the minimum operating frequency range.
The parallel capacitor C3 sets the maximum operating
frequency, which is also the parallel resonant peak fre-
quency. Choose:
C3 ≥
C2
(4π
2
× f
2
MAX
× L × C2) - N
2
C
N
f
L
MIN
2
2
2
2
≤
Ч
Ч
π
N
V
V
LAMP RMS
IN MIN
=
×
(
)
(
)
.
0 9
R
V
I
SEC RMS
MAX
3
1 25
2
=
×
.
(
) _
R
f
C
SW
2
10
2
4
=
Ч
Ч
π
C
V
mV
C
LAMP RMS
MAX
4
2
510
1
3
=
Ч
Ч
Ч
(
) _
π
-