Rainbow Electronics MAX8759 - Manual (Page 13)

MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
______________________________________________________________________________________ 13
Typical Operating Circuit
The MAX8759 typical operating circuit (Figure 1) is a
single-lamp CCFL backlight inverter for notebook com-
puter TFT LCD panels. The input voltage range of the
circuit is from 7.5V to 24V. The maximum RMS lamp
current is set to 6mA and the maximum RMS striking
voltage is set to 1800V. Table 1 lists some important
components and Table 2 lists the component suppliers’
contact information.
Detailed Description
The MAX8759 controls a full-bridge resonant inverter to
convert an unregulated DC input into a high-frequency
AC output for powering CCFLs. The resonant operation
maximizes striking capability and provides near-sinu-
soidal waveforms over the entire input range to improve
CCFL lifetime. The lamp brightness is adjusted by turn-
ing the lamp on and off with a DPWM signal. The
DPWM frequency can be accurately adjusted with a
resistor. The brightness of the lamp is proportional to
the duty cycle of the DPWM signal, which is controlled
either with a 2-wire SMBus-compatible interface, with
an external ALS, or with an external PWM signal. The
device also includes safety features that effectively pro-
tect against single-point fault conditions such as lamp-
out and secondary short-circuit faults. An internal 5.35V
linear regulator powers the MOSFET drivers and most
of the internal circuitry. Figure 2 is the functional dia-
gram of the MAX8759 and Figure 3 is the detailed dia-
gram of the SMBus and ALS input block.
Resonant Operation
The MAX8759 drives four n-channel power MOSFETs
that make up the zero-voltage-switching (ZVS) full-
bridge inverter as shown in Figure 4. Assume that NH1
and NL2 are on at the beginning of a switching cycle
as shown in Figure 4(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 is turned off, the
primary current forward biases the body diode of NL1,
which clamps the LX1 voltage just below ground as
shown in Figure 4(b). When the controller turns on NL1,
its drain-to-source voltage is near zero because its for-
ward-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 cur-
rent 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 volt-
age. The primary current reverses polarity as shown in
Figure 4(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 is turned off, the primary current
forward biases the body diode of NL2, which clamps
the LX2 voltage just below ground as shown in Figure
4(d). After the LX2 node goes low, the controller loss-
lessly 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 4(a). Note that switching transitions on
all four power MOSFETs occur under ZVS conditions,
which reduces transient power losses and EMI.
DESIGNATION DESCRIPTION
C1
10µF ±20%, 25V X5R ceramic capacitor
(1210)
Murata GRM32DR61E106M
TDK C3225X5R1E106M
C2, C3
2.2µF ±10%, 25V X5R ceramic capacitors
(0805)
Murata GRM21BR61E225K
TDK C2012X5R1E225K
C4
10pF ±10%, 3kV HV ceramic capacitor
(1808)
Kemet C1808C100KHGAC
TDK C4520C0G3F100F
NH1/2, NL1/2
Dual n-channel MOSFETs, 30V, 0.095 Ω,
6-pin SOT23
Fairchild FDC6561AN
T1
CCFL transformer, 1:110 turns ratio
TMP UI9.8L type
Table 1. List of Important Components
SUPPLIER WEBSITE
Fairchild
Semiconductor
www.fairchildsemi.com
Kemet www.kemet.com
Murata www.murata.com
TDK www.components.tdk.com
TMP www.tmp.com
Table 2. Component Suppliers