Rainbow Electronics MAX8759 - Manual (Page 16)

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