MAX8751 Maxim Integrated Products, MAX8751 Datasheet - Page 23

• Manufacturer Part Number: MAX8751
• Description: Full-Bridge CCFL Inverter Controller
• Manufacturer: Maxim Integrated Products

where f
In the circuit of Figure 1, the transformer’s turns ratio is
120 and its secondary leakage inductance is 200mH.
To set the minimum resonant frequency to 30kHz, use
2.2µF capacitor for C
The parallel capacitor CP sets the maximum operating
frequency, which is also the parallel resonant peak fre-
quency. The capacitance CP can be chosen as below:
In the circuit of Figure 1, to set the maximum resonant
frequency to 95kHz; use 15pF for C
The transformer core saturation should also be consid-
ered when selecting the operating frequency. The pri-
mary winding should have enough turns to prevent
transformer saturation under all operating conditions.
Use the following expression to calculate the minimum
number of turns N1 of the primary winding:
Figure 7. Simplified CCFL Inverte r Circuit
SOURCE
SOURCE
MIN
AC
AC
C
is the minimum operating frequency range.
P
>
Cs' = Cs'/N
4
C
C
π
______________________________________________________________________________________
S
2
S
≤
S
x f
2
.
4π
2
MAX
1:N
2
(a)
(b)
L
x f
C
N
x L x C
2
S
2
MIN
Fixed-Frequency, Full-Bridge CCFL
L
P
x L
.
S
−
N
C
C
P
2
P
R
L
where D
0.4) of the high-side switch, V
input voltage, B
and S is the minimal cross-section area of the core.
The COMP capacitor sets the speed of the current-reg-
ulation loop that is used during startup, maintaining
lamp-current regulation, and during transients caused
by changing the input voltage. To maintain stable oper-
ation, the COMP capacitor (C
least 3.3nF.
As discussed in the DPWM Dimming Control section,
the COMP capacitor also limits the dynamics of the
lamp-current envelope in DPWM operation. At the end
of the DPWM ON cycle, the MAX8751 turns on a 100µA
internal current source to linearly discharge the COMP
capacitor. Use the following equation to set the fall time:
where t
and V
by resonant tank. At the beginning of the DPWM ON
cycle, the COMP capacitor is charged by transconduc-
tance error amplifier, so the charge current is not con-
stant. Because the average charge current is around
Figure 8. Frequency Response of the Resonant Tank
COMP
FALL
MAX
Inverter Controller
is the dynamic COMP voltage determined
is the fall time of the lamp-current envelope
4
3
2
1
0
is the maximum duty cycle (approximately
C
0
S
N1
COMP
is the saturation flux density of the core,
>
20
D
COMP Capacitor Selection
MAX
=
B x S x f
R
L
S
FREQUENCY (kHz)
INCREASING
100
40
x V
IN(MAX)
µ
V
A x t
COMP
IN(MAX)
COMP
MIN
60
FALL
is the maximum DC
) needs to be at
80
100
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