MAX8740 データシートの表示（PDF） - Maxim Integrated

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MAX8740

TFT-LCD Step-Up DC-DC Converter

Maxim Integrated 

TFT-LCD Step-Up DC-DC Converter

SHDN

COMP

FB

FREQ

SKIP

COMPARATOR

BIAS

SKIP

IN

4µA

SOFT-

START

SS

1.24V

OSCILLATOR

ERROR

AMPLIFIER

∞

SLOPE

COMPEN-

SATION

ERROR

COMPARATOR

CLOCK

Σ

CONTROL

AND DRIVER

LOGIC

CURRENT

SENSE

LX

N

GND

5µA

MAX8740

Figure 2. Functional Diagram

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:

ILIM = (1.26 - 0.35 x D) x ILIM_EC

where ILIM_EC is the current limit specified at 71% duty

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:

IOUT(MAX)



= ILIM

−

0.5 ×

D

×

VIN





×

fOSC × L 

VIN

VOUT

×

η

where ILIM is the current limit calculated above, η is the

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

cycle. The duty cycle when operating at the current

limit is:

D=

VOUT − VIN + VDIODE

VOUT − ILIM × RON + VDIODE

where VDIODE is the rectifier diode forward voltage and

RON is the on-resistance of the internal MOSFET.

6 _______________________________________________________________________________________

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