Detailed Description

The MAX8727 is a highly efficient power supply that

employs a current-mode, fixed-frequency, pulse-width

modulation (PWM) architecture for fast transient

response and low-noise operation. The device regu-

lates 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 cur-

rent necessary to service the load. To maintain stability

at high duty cycles, a slope-compensation signal is

summed with the current-sense signal.

At light loads, this architecture allows the MAX8727 to

“skip” cycles to prevent overcharging the output voltage.

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

peak value of approximately 50mA, discharges to the

output, and waits until another pulse is needed again.

Output Current Capability

The output current capability of the MAX8727 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) 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

TFT-LCD Step-Up DC-DC Converter

_______________________________________________________________________________________

7

GND

LX

IN

FREQ

FB

COMP

4

µA

5

µA

N

ERROR

COMPARATOR

ERROR

AMPLIFIER

SKIP

COMPARATOR

SS

CLOCK

SKIP

BIAS

SHDN

MAX8727

Σ

CURRENT

SENSE

CONTROL

AND DRIVER

LOGIC

SOFT-

START

SLOPE

COMPEN-

SATION

OSCILLATOR

∞

1.24V

Figure 2. MAX8727 Functional Diagram