MAX8722 Maxim Integrated Products, MAX8722 Datasheet - Page 14

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

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Figure 5 shows the frequency response of the resonant
tank’s voltage gain under different load conditions.
The primary series capacitor is 1µF, the secondary par-
allel capacitor is 15pF, the transformer turns ratio is
1:93, 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 inverter is designed to operate between these two
resonant peaks. When the lamp is off, the operating
point of the resonant tank is close to the parallel resonant
peak due to the lamp’s infinite impedance. The circuit
Low-Cost CCFL Backlight Controller
Figure 4. Equivalent Resonant Tank Circuit
14
______________________________________________________________________________________
SOURCE
SOURCE
AC
AC
S
):
C'
f
P
S
C
=
S
C
N
f
S
=
S
2
P
, is the parallel resonant peak
1:N
2π L
=
P
2π
), and the series capacitor
(a)
(b)
C'
S
C'
1
1
LC
):
S
L
L
S
+ C
'
S
C
S
S
P
, is the series reso-
P
and f
C
C
P
P
P
, in the fre-
CCFL
R
L
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 will increase until the
lamp is ionized or until it reaches the IC’s secondary volt-
age limit, without regard to the transformer turns ratio or
the input voltage level. 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 MAX8722 uses a lamp-current control loop to regu-
late the current delivered to the CCFL. The heart of the
control loop is a transconductance error amplifier. The
AC lamp current is sensed with a resistor connected in
series with the low-voltage terminal of the lamp. The
voltage across this resistor is fed to the IFB input and is
internally full-wave rectified. The transconductance
error amplifier compares the rectified IFB voltage with a
790mV (typ) internal threshold to generate an error cur-
rent. The error current charges and discharges a
capacitor connected between COMP and ground to
create an error voltage (V
pared with an internal ramp signal to set the high-side
MOSFET switch on-time (t
Figure 5. Frequency Response of the Resonant Tank
4
3
2
1
0
0
20
R
f
S
L
Lamp-Current Regulation
FREQUENCY (kHz)
INCREASING
ON
40
COMP
).
60
). V
COMP
80
f
P
is then com-
100