MAX8729EEI+T Maxim Integrated Products, MAX8729EEI+T Datasheet - Page 23

• 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

Figure 8 shows the frequency response of the resonant
tank’s voltage gain under different load conditions. The
primary series capacitor is 1µF, the secondary parallel
capacitor is 15pF, the transformer turns ratio is 1:78,
and the secondary leakage inductance is 260mH.
Notice that there are two peaks, f
quency response. The first peak, f
nant peak determined by the secondary leakage
inductance (L) and the series capacitor reflected to the
secondary (C’
The second peak, f
determined by the secondary leakage inductance (L),
the parallel capacitor (C
reflected to the secondary (C’
The actual resonant frequency is between these two
resonant peaks. When the lamp is off, the operating
point of the resonant tank is close to the parallel reso-
Figure 7. Simplified CCFL Inverter Circuit
SOURCE
SOURCE
AC
AC
C'
S
S
= (C
):
C
S1
S1
f
P
+ C
+ C
______________________________________________________________________________________
=
S2
f
S2
S
P
) / N
2π
, is the parallel resonant peak
=
Constant-Frequency, Half-Bridge CCFL
2
2π
1:N
P
L
(b)
(a)
L
), and the series capacitor
C
C C
1
LC
1
'
S
S
'
):
S
+
'
S
C
L
P
S
S
P
, is the series reso-
and f
C
C
P
P
P
, in the fre-
R
L
nant peak due to the lamp’s infinite impedance. The cir-
cuit displays the characteristics of a parallel-loaded
resonant converter. While in parallel-loaded resonant
operation, the inverter behaves like a voltage source to
generate the necessary striking voltage. Theoretically,
the output voltage of the resonant converter increases
until the lamp is ionized or until it reaches the IC’s sec-
ondary voltage limit. Once the lamp is ionized, the
equivalent-load resistance decreases rapidly and the
operating point moves toward the series-resonant
peak. While in series-resonant operation, the inverter
behaves like a current source.
The leakage inductance of the CCFL transformer is an
important parameter in the resonant tank design. The
leakage inductance values can have large tolerance
and significant variations among different batches. It is
best to work directly with transformer vendors on leak-
age inductance requirements. The MAX8729 works
best when the secondary leakage inductance is
between 200mH and 350mH. The primary capacitive
dividers C1 and C2 set the minimum operating frequen-
cy, which is approximately two times the series-
resonant peak frequency. Choose:
where f
In the circuit of Figure 1, the transformer’s turns ratio is
178 and its secondary leakage inductance is about
300mH. To set the minimum resonant frequency to
40kHz, use a capacitor of 2.2µF or less for C1 and C2.
Figure 8. Frequency Response of the Resonant Tank
MIN
Inverter Controller
is the minimum operating frequency range.
4
3
2
1
C
0
1
=
C
2
20
≤
8
FREQUENCY (kHz)
π
40
R
2
L
INCREASING
x f
N
2
2
60
MIN
x L
80
100
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