MAX1996AETI+ Maxim Integrated Products, MAX1996AETI+ Datasheet - Page 10

• Manufacturer Part Number: MAX1996AETI+
• Description: Display Drivers CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX1996AETI+

• Lead Free Status / Rohs Status: Details

High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
Figure 2. Resonant Operation
The MAX1996A drives the four N-channel power
MOSFETs that make up the zero-voltage switching
(ZVS) full-bridge inverter as shown in Figure 1. The LX1
and LX2 switching nodes are AC coupled to the prima-
ry side of the transformer.
Assume that NH1 and NL2 are turned on at the begin-
ning of the cycle as shown in Figure 2(a). The primary
current flows through MOSFET NH1, DC blocking cap
C2, the primary side of transformer T1, and finally MOS-
FET NL2. During this interval, the primary current ramps
up until the controller turns off NH1. When NH1 is off,
the primary current forward biases the body diode of
NL1 and brings the LX1 node down as shown in Figure
2(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 finally NL2. Once the primary current drops
10
______________________________________________________________________________________
(BODY DIODE TURNS ON FIRST)
NH1
NH1
NL1
OFF
OFF
NL1
ON
ON
LX1
LX1
C2
C2
Resonant Operation
V
V
(b)
BATT
(a)
BATT
T1
T1
LX2
LX2
NL2
ON
NL2
ON
NH2
OFF
NH2
OFF
to the minimum current threshold (6mV/R
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 2(c), beginning a
new cycle with the current flowing in the opposite direc-
tion, with NH2 and NL1 on. The primary current ramps
up until the controller turns off NH2. When NH2 is off,
the primary current forward biases the body diode of
NL2, and brings the LX2 node down as shown in Figure
2(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 2(a).
NH1
NH1
OFF
NL1
OFF
NL1
ON
ON
LX1
LX1
C2
C2
V
V
BATT
BATT
(d)
(c)
T1
T1
(BODY DIODE TURNS ON FIRST)
LX2
LX2
NL2
OFF
NL2
ON
NH2
ON
NH2
OFF
DSON
), the