TFT-LCD Step-Up DC-DC Converter

6

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Detailed Description

The MAX8740 is a highly efficient power supply that

employs a current-mode, fixed-frequency, PWM archi-

tecture for fast transient response and low-noise opera-

tion. The device 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

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 command the

inductor peak current necessary to service the load. To

maintain stability at high duty cycles, a slope-compen-

sation signal is summed with the current-sense signal.

At light loads, this architecture allows the MAX8740 to

“skip” cycles to prevent overcharging the output voltage.

In this region of operation, the inductor ramps up to a

peak value of approximately 150mA, discharges to the

output, and waits until another pulse is needed again.

Output Current Capability

The output current capability of the MAX8740 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:

cycle (see the Electrical Characteristics table) and D is

the duty cycle.

The output current capability depends on the current-

limit value and is governed by the following equation:

η is the

regulator efficiency (85% nominal), and D is the duty

cycle. The duty cycle when operating at the current

limit is:

D

VV

V

VI

R

V

OUT

IN

DIODE

OUT

LIM

ON

DIODE

=

−+

−×

+

II

DV

fL

V

V

OUT MAX

LIM

IN

OSC

IN

OUT

()

.

=−

××

×

















××

05

η

GND

LX

IN

FREQ

FB

COMP

4

µA

5

µA

N

ERROR

COMPARATOR

ERROR

AMPLIFIER

SKIP

COMPARATOR

SS

CLOCK

SKIP

BIAS

SHDN

MAX8740

Σ

CURRENT

SENSE

CONTROL

AND DRIVER

LOGIC

SOFT-

START

SLOPE

COMPEN-

SATION

OSCILLATOR

∞

1.24V

Figure 2. Functional Diagram