MAX8759 Maxim Integrated Products, MAX8759 Datasheet - Page 16

• Manufacturer Part Number: MAX8759
• Description: CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

Figure 4. Resonant Operation
The second peak f
mined by the secondary leakage inductance (L), the
parallel capacitor (C
ed 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 reso-
nant peak due to the lamp’s infinite impedance. The cir-
cuit displays the characteristics of a parallel-loaded
Low-Cost, SMBus, CCFL Backlight Controller
16
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NH1
NH1
(BODY DIODE TURNS ON FIRST)
NL1
OFF
OFF
NL1
ON
ON
LX1
LX1
f
P
=
P
2π
P
is the parallel resonant peak deter-
), and the series capacitor reflect-
S
L
):
C
C2
C2
1
C C
S
′ +
′
S P
C
V
V
BATT
(a)
BATT
(b)
P
T1
T1
LX2
LX2
NH2
OFF
NL2
ON
NH2
OFF
NL2
ON
NH1
NH1
OFF
NL1
OFF
NL1
ON
ON
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 MAX8759 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
LX1
LX1
C2
C2
V
V
BATT
(c)
BATT
(d)
Lamp-Current Regulation
T1
T1
(BODY DIODE TURNS ON FIRST)
LX2
LX2
NH2
ON
NL2
OFF
NH2
OFF
NL2
ON