Rainbow Electronics MAX8709 - Manual (Page 11)

MAX8709
High-Efficiency CCFL Backlight
Controller with SMBus Interface
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Typical Operating Circuit
The Typical Operating Circuit of the MAX8709 (Figure
1) is a complete CCFL backlight inverter for notebook
TFT LCD panels. The circuit works over an input volt-
age range of 7V to 24V with an RMS lamp current of
6mA. The circuit’s maximum RMS open-lamp voltage is
limited to 1600V. Table 1 lists recommended compo-
nent options, and Table 2 lists the component suppli-
ers’ contact information.
Detailed Description
The MAX8709 controls a full-bridge resonant inverter to
convert an unregulated DC input into a near-sinusoidal
AC output for powering CCFLs. The lamp brightness is
adjusted by turning the lamp on and off with an internal
DPWM signal. The duty cycle of the DPWM signal is
set through an SMBus-compatible 2-wire serial inter-
face. Figure 2 shows the functional diagram of the
MAX8709.
Resonant Operation
The MAX8709 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 cap 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 volt-
age 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 thresh-
old (6mV / R
DS(ON) ), the controller turns off NL2. The
remaining energy in T1 charges up the LX2 node until the
Table 1. Component List
DESIGNATION DESCRIPTION
C1
4.7µF ±20%, 25V X5R
ceramic capacitor (1210)
Murata GRM32RR61E475K
Taiyo Yuden TMK325BJ475MN
TDK C3225X7R1E475M
C2
1µF ±10%, 25V X7R
ceramic capacitor (1206)
Murata GRM31MR71E105K
Taiyo Yuden TMK316BJ105KL
TDK C3216X7R1E105K
C3
15pF ±1pF, 3kVhigh-voltage
ceramic capacitor (1808)
Murata GRM42D1X3F150J
TDK C4520C0G3F150F
C4
0.022µF ±10%, 16V X7R
ceramic capacitor (0402)
Murata GRP155R71C223K
Taiyo Yuden EMK105BJ223KV
TDK C1005X7R1C223K
C5, C6, C8, C9
0.1µF ±10%, 25V X7R
ceramic capacitors (0603)
Murata GRM188R71E104K
Taiyo Yuden TMK107BJ104KA
TDK C1608X7R1E104K
DESIGNATION DESCRIPTION
C7
0.47µF ±10%, 10V X5R
ceramic capacitor (0603)
Taiyo Yuden LMK107BJ474KA
TDK C1608X5R1A474K
D1
Dual silicon switching diode,
common anode (SOT-323)
Central Semiconductor CMSD2836
Diodes Incorporated BAW56W
NH1/2, NL1/2
30V, 0.095 dual N-channel MOSFETs
(6-pin SOT23)
Fairchild FDC6561AN
R1 150
Ω
±1% resistor (0603)
R2 2k
Ω
±5% resistor (0603)
R3 39 Ω ±1% (resistor (0603)
R4, R5 100k Ω ±5% resistors (0603)
T1
CCFL transformer, 1:93 turns ratio
Sumida 5371-400-W1423
TOKO T912MG-1018
Table 2. Component Suppliers
SUPPLIER WEBSITE
Central Semiconductor www.centralsemi.com
Fairchild Semiconductor www.fairchildsemi.com
Murata www.murata.com
Sumida www.sumida.com
Taiyo Yuden www.t-yuden.com
TDK www.components.tdk.com