MAX8722A
Typical Operating Circuit
The typical operating circuit of the MAX8722A (Figure
1) is a complete CCFL backlight inverter for TFT-LCD
panels. The input voltage range of the circuit is from 8V
to 24V. The maximum RMS lamp current is set to 6mA,
and the maximum RMS striking voltage is set to 1600V.
Table 1 lists some important components, and Table 2
lists the component suppliers’ contact information.
Detailed Description
The MAX8722A controls a full-bridge resonant inverter
to convert an unregulated DC input into a near-sinu-
soidal, high-frequency AC output for powering CCFLs.
The lamp brightness is adjusted by turning the lamp on
and off with a signal. The brightness of the lamp is pro-
portional to the duty cycle of the DPWM signal, which is
set through an analog voltage on the CNTL pin. Figure
2 shows the functional diagram of the MAX8722A.
Resonant Operation
The MAX8722A drives the four n-channel power
MOSFETs that make up the zero-voltage-switching
(ZVS) full-bridge inverter as shown in Figure 3. Assume
that NH1 and NL2 are turned on at the beginning of a
switching cycle as shown in Figure 3(a). The primary
current flows through MOSFET NH1, DC blocking
capacitor C2, the primary side of transformer T1, and
MOSFET NL2. During this interval, the primary current
ramps up until the controller turns off NH1. When NH1
turns off, the primary current forward biases the body
diode of NL1, which clamps the LX1 voltage just below
ground as shown in Figure 3(b). When the controller
turns on NL1, its drain-to-source voltage is near zero
because its forward-biased body diode clamps the
drain. Since NL2 is still on, the primary current flows
through NL1, C2, the primary side of T1, and NL2.
Once the primary current drops to the minimum current
threshold (6mV/R
DS(ON)
), the controller turns off NL2.
The remaining energy in T1 charges up the LX2 node
until the body diode of NH2 is forward biased. When
NH2 turns on, it does so with near-zero drain-to-source
voltage. The primary current reverses polarity as shown
in Figure 3(c), beginning a new cycle with the current
flowing in the opposite direction, with NH2 and NL1 on.
The primary current ramps up until the controller turns
off NH2. When NH2 turns off, the primary current for-
ward biases the body diode of NL2, which clamps the
LX2 voltage just below ground as shown in Figure 3(d).
After the LX2 node goes low, the controller losslessly
turns on NL2. Once the primary current drops to the
minimum current threshold, the controller turns off NL1.
The remaining energy charges up the LX1 node until
the body diode of NH1 is forward biased. Finally, NH1
losslessly turns on, beginning a new cycle as shown in
Figure 3(a). Note that switching transitions on all four
power MOSFETs occur under ZVS condition, which
reduces transient power losses and EMI.
A simplified CCFL inverter circuit is shown in Figure
4(a). The full-bridge power stage is simplified and rep-
resented as a square-wave AC source. The resonant
tank circuit can be further simplified to Figure 4(b) by
removing the transformer. C
S
is the primary series
capacitor, C’
S
is the series capacitance reflected to the
secondary, C
P
is the secondary parallel capacitor, N is
the transformer turns ratio, L is the transformer sec-
ondary leakage inductance, and R
L
is an idealized
resistance that models the CCFL in normal operation.
Figure 5 shows the frequency response of the resonant
tank’s voltage gain under different load conditions.
Low-Cost CCFL Backlight Controller
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