MAX8722 Maxim Integrated Products, MAX8722 Datasheet - Page 12

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

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The typical operating circuit of the MAX8722 (Figure 1)
is a complete CCFL backlight inverter for TFT-LCD pan-
els. 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.
The MAX8722 controls a full-bridge resonant inverter to
convert an unregulated DC input into a near-sinusoidal,
high-frequency AC output for powering CCFLs. The
lamp brightness is adjusted by turning the lamp on and
off with a digital pulse-width-modulation (DPWM) sig-
nal. The brightness of the lamp is proportional to the
Low-Cost CCFL Backlight Controller
Table 1. List of Important Components
Table 2. Component Suppliers
12
DESIGNATION
Central Semiconductor
Diodes Inc.
Fairchild Semiconductor
Murata
Sumida
Taiyo Yuden
TDK
TOKO
NH1/2, NL1/2
______________________________________________________________________________________
C1
C2
C3
D1
T1
SUPPLIER
Typical Operating Circuit
4.7µF 20%, 25V X5R ceramic capacitor
Murata GRM32RR61E475K
Taiyo Yuden TMK325BJ475MN
TDK C3225X7R1E475M
1µF 10%, 25V X7R ceramic capacitor
18pF 1pF, 3kV, high-voltage ceramic
capacitor
Murata GRM42D1X3F180J
TDK C4520C0G3F180F
Dual silicon switching diode, common
anode, SOT-323
Central Semiconductor CMSD2836
Diodes, Inc. BAW56W
Dual n-channel MOSFETs, 30V, 0.095,
SOT23-6
Fairchild FDC6561AN
CCFL transformer, 1:93 turns ratio
TOKO T912MG-1018
Detailed Description
www.centralsemi.com
www.diodes.com
www.fairchildsemi.com
www.murata.com
www.sumida.com
www.t-yuden.com
www.components.tdk.com
www.tokoam.com
DESCRIPTION
WEBSITE
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 MAX8722.
The MAX8722 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
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
capacitor, C’
secondary, C
the transformer turns ratio, L is the transformer sec-
ondary leakage inductance, and R
resistance that models the CCFL in normal operation.
S
P
is the series capacitance reflected to the
is the secondary parallel capacitor, N is
DS(ON)
), the controller turns off NL2.
Resonant Operation
S
is the primary series
L
is an idealized