Rainbow Electronics MAX8759 - Manual (Page 27)

using the following equation to set the desired maxi-
mum RMS secondary voltage V
LAMP(RMS) _ MAX :
where the 2.3V is the typical value of the VFB peak volt-
age when the lamp is open. To set the maximum RMS
secondary voltage to 1800V when C4 is 10pF, use
10nF for C5.
Setting the Secondary Current Limit
The MAX8759 limits the secondary current even if the
IFB_ sense resistors are shorted or transformer sec-
ondary current finds its way to ground without passing
through the sense resistors. ISEC monitors the peak volt-
age across the sense network (R2 and C6 in Figure 1)
connected between the low-voltage terminal of the trans-
former secondary winding and ground. Using an RC-
sense network instead of a single-sense resistor makes
the secondary current-limit frequency dependent. The UL
safety standard requires the AC peak current in a limited-
current circuit should not exceed 0.7mA for frequencies
below 1kHz. For frequencies above 1kHz, the limit of
0.7mA is multiplied by the value of the frequency in kilo-
hertz but should not exceed 70mA peak when the fre-
quency is equal to or above 100kHz. To meet the UL
current-limit specifications, determine the value of R2
using the current limit at 1kHz and determine the value of
C6 using the current limit at 100kHz:
where 1.23V is the typical value of the ISEC peak volt-
age when the transformer secondary is shorted. The
circuit of Figure 1 uses 3.9k
Ω for R2 and 68nF for C6.
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. If the maximum RMS lamp voltage in normal oper-
ation is 700V and the minimum DC input voltage is 7.5V,
the turns ratio should be greater than 104. The turns ratio of
the transformer used in the circuit of Figure 1 is 110.
The next step in the design procedure is to determine the
desired operating frequency range. The MAX8759 is syn-
chronized to the natural resonant frequency of the reso-
nant 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 operation,
the resonant frequency range must be within the operat-
ing frequency range specified by the CCFL transformer
manufacturer. As discussed in the Resonant Operation
section, the resonant frequency range is determined by
transformer secondary leakage inductance L, the primary
series DC blocking capacitors (C
S ), and the secondary
parallel resonant capacitor C P . Since it is difficult to con-
trol the transformer leakage inductance, the resonant tank
design should be based on the existing secondary leak-
age inductance of the selected CCFL transformer. The
leakage inductance values can have large tolerance and
significant variations among different batches. It is best to
work directly with transformer vendors on leakage induc-
tance requirements. The MAX8759 works best when the
secondary leakage inductance is between 250mH and
350mH. Series capacitor C
S sets the minimum operating
frequency, which is approximately two times the series
resonant peak frequency. Choose:
where f MIN is the minimum operating frequency range.
In the circuit of Figure 1, the transformer’s turns ratio is
110 and its secondary leakage inductance is approxi-
mately 300mH. To set the minimum operating frequen-
cy to 30kHz, the total series capacitance needs to be
less than 4.5µF. Therefore, two 2.2µF capacitors (C2
and C3) are used in Figure 1.
Parallel capacitor C
P
sets the maximum operating fre-
quency, which is also the parallel resonant peak fre-
quency. Choose:
In the circuit of Figure 1, to set the maximum operating
frequency to 100kHz, C
P needs to be larger than 8.6pF.
A 10pF high-voltage capacitor (C4) is used in Figure 1.
C
C
f L C N
P
S
MAX S
≥
Ч Ч Ч − 4
2 2 2 π
C
N
f L
S
MIN
≤
Ч Ч
2
2 2 π
N
V
V
LAMP RMS
IN MIN
≥
×
( )
( )
. 0 9
C
mA
kHz V
nF 6
70
2 100 1 23
90 <
Ч Ч
=
π
.
R
V
mA
k 2
1 23
0 7
1 75 > =
.
.
. Ω
C
V
V
C
LAMP RMS MAX
5
2
2 3
4
=
Ч
Ч
( ) _
.
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
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