MAX8752ETA+T Maxim Integrated Products, MAX8752ETA+T Datasheet - Page 7

• Manufacturer Part Number: MAX8752ETA+T
• Description: IC DC-DC CONV TFT LCD 8-TDFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8752ETA+T

• Applications: Converter, TFT, LCD
• Voltage - Input: 1.8 ~ 5.5 V
• Number Of Outputs: 1
• Voltage - Output: 1.8 ~ 13 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-TDFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The MAX8752 is a highly efficient, step-up power sup-
ply designed for TFT-LCD panels. The typical circuit
shown in Figure 1 operates from an input voltage as
low as 1.8V, and produces a MAIN output of 10V at
220mA from 2.5V input while supporting discrete
diode-capacitor charge pumps that produce
-9V at 20mA and +28V at 10mA. If the charge-pump
outputs are not required, the diodes and capacitors
associated with them may be eliminated and the main
output increased to 270mA.
The MAX8752 employs a current-mode, fixed-frequen-
cy, pulse-width modulation (PWM) architecture for fast
transient response and low-noise operation. The high
switching frequency (1.2MHz) 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 function reduce the number of external com-
ponents required. The output voltage can be set from
V
The MAX8752 regulates the output voltage through a
combination of an error amplifier, two comparators, and
several signal generators (Figure 2). The error amplifier
compares the signal at FB to 1.24V and varies the
COMP output. The voltage at COMP determines the
Figure 2. MAX8752 Functional Diagram
IN
SHDN
SUP
LDO
IN
to 13V with an external resistive voltage-divider.
OSCILLATOR
REGULATOR
BOOTSTRAP
CLOCK
LINEAR
STARTUP
AND
OSC
COMPARATOR
SLOPE COMP
_______________________________________________________________________________________
PWM
MAX8752
Detailed Description
CURRENT LIMIT
AND DRIVER
LOGIC
START
SOFT-
∑
TFT LCD Step-Up DC-DC Converter
I
LIMIT
CURRENT
SENSE
ERROR AMP
1.24V
LX
GND
FB
COMP
current trip point each time the internal MOSFET turns
on. As the load changes, the error amplifier sources or
sinks current to the COMP output to set the inductor
peak current necessary to service the load. To maintain
stability at high duty cycles, a slope-compensation sig-
nal 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 cur-
rent through the inductor ramps up linearly, storing
energy in its magnetic field. Once the sum of the cur-
rent-feedback signal and the slope compensation
exceed the COMP voltage, the controller resets the flip-
flop and turns off the MOSFET. Since the inductor cur-
rent is continuous, a transverse potential develops
across the inductor that turns on the diode (D1). The
voltage across the inductor then becomes the differ-
ence between the output voltage and the input voltage.
This discharge condition forces the current through the
inductor to ramp back down, transferring the energy
stored in the magnetic field to the output capacitor and
the load. The MOSFET remains off for the rest of the
clock cycle.
At light loads, this architecture allows the MAX8752 to
“skip” cycles to prevent overcharging the output
capacitor voltage.
In this region of operation, the inductor ramps up to a
peak value of approximately 250mA, discharges to the
output, and waits until another pulse is needed.
The output-current capability of the MAX8752 is a func-
tion of current limit, input voltage, operating frequency,
and inductor value. Because of the slope compensa-
tion used to stabilize the feedback loop, the inductor
current limit depends on the duty cycle. The current
limit is determined by the following equation:
where I
cycle (see the Electrical Characteristics) and D is the
duty cycle.
The output current capability depends on the current-
limit value and is governed by the following equation:
I
OUT MAX
LIM
(
I
_
LIM
EC
= (1.162 - 0.361 x D) x I
)
is the current limit specified at 65% duty
=
⎡
⎢
⎣
I
LIM
Output-Current Capability
−
0 5
f
.
OSC
x D V
x L
IN
⎤
⎥
⎦
x
LIM_EC
V
V
OUT
IN
x
η
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