MAX8795A Maxim Integrated Products, MAX8795A Datasheet - Page 16

• Manufacturer Part Number: MAX8795A
• Description: TFT-LCD DC-DC Converter
• Manufacturer: Maxim Integrated Products

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TFT-LCD DC-DC Converter with
Operational Amplifiers
Table 2. Component Suppliers
Figure 2. MAX8795A Functional Diagram
16
V
Fairchild
Sumida
TDK
Toshiba
IN
______________________________________________________________________________________
SUPPLIER
IN
COMP
SRC
COM
DRN
SUP
NEG1
OUT1
POS1
NEG2
OUT2
POS2
OUT3
POS3
OP1
OP2
V
OP3
CN
CONTROLLER
CONTROLLER
GATE-ON
STEP-UP
CONTROL
MAX8795A
SWITCH
CONTROLLER
GATE-OFF
EP
BGND
REF
OP4
OP5
408-822-2000
847-545-6700
847-803-6100
949-455-2000
LX
PHONE
PGND
AGND
DRVN
DRVP
NEG4
OUT4
POS4
NEG5
OUT5
POS5
FBP
DEL
CTL
FBN
REF
FB
V
CP
V
MAIN
V
V
V
V
CP
GON
CN
GOFF
408-822-2102
847-545-6720
847-390-4405
949-859-3963
FAX
The MAX8795A contains a high-performance step-up
switching regulator, two low-cost linear-regulator con-
trollers, multiple high-current operational amplifiers,
and startup timing and level-shifting functionality useful
for active-matrix TFT LCDs. Figure 2 shows the
MAX8795A functional diagram.
The main step-up regulator employs a current-mode,
fixed-frequency PWM architecture to maximize loop
bandwidth and provide fast transient response to
pulsed loads typical of TFT-LCD panel source drivers.
The 1.2MHz switching frequency allows the use of low-
profile inductors and ceramic capacitors to minimize
the thickness of LCD panel designs. The integrated
high-efficiency MOSFET and the IC’s built-in digital
soft-start functions reduce the number of external com-
ponents required while controlling inrush currents. The
output voltage can be set from V
nal resistive voltage-divider.
The regulator controls the output voltage and the power
delivered to the output by modulating the duty cycle (D)
of the internal power MOSFET in each switching cycle.
The duty cycle of the MOSFET is approximated by:
Figure 3 shows the functional diagram of the step-up
regulator. An error amplifier compares the signal at FB
to 1.233V and changes the COMP output. The voltage
at COMP sets the peak inductor current. As the load
varies, the error amplifier sources or sinks current to the
COMP output accordingly to produce the inductor peak
current necessary to service the load. To maintain sta-
bility at high duty cycles, a slope-compensation signal
is summed with the current-sense signal.
On the rising edge of the internal clock, the controller sets
a flip-flop, turning on the n-channel MOSFET and applying
the input voltage across the inductor. The current through
the inductor ramps up linearly, storing energy in its
magnetic field. Once the sum of the current-feedback
signal and the slope compensation exceeds the COMP
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D
≈
Detailed Description
V
Main Step-Up Regulator
MAIN
V
MAIN
WEBSITE
−
V
IN
IN
to 18V with an exter-