MAX8727 데이터 시트보기 (PDF) - Maxim Integrated

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MAX8727

TFT-LCD Step-Up DC-DC Converter

Maxim Integrated 

TFT-LCD Step-Up DC-DC Converter

Soft-Start

The MAX8727 can be programmed for soft-start upon

power-up with an external capacitor. When the shutdown

pin is taken high, the soft-start capacitor (CSS) is immedi-

ately charged to 0.4V. Then the capacitor is charged at a

constant current of 4.5µA (typ). During this time, the SS

voltage directly controls the peak inductor current, allow-

ing 0A at VSS = 0.4V to the full current limit at VSS = 1.5V.

The maximum load current is available after the soft-start

is completed. When the SHDN pin is taken low, the soft-

start capacitor is discharged to ground.

Frequency Selection

The MAX8727’s frequency can be user selected to

operate at either 640kHz or 1.2MHz. Connect FREQ to

GND for 640kHz operation. For a 1.2MHz switching fre-

quency, connect FREQ to IN. This allows the use of

small, minimum-height external components while

maintaining low output noise. FREQ has an internal

pulldown, allowing the user the option of leaving FREQ

unconnected for 640kHz operation.

Table 1. Component List

DESIGNATION

DESCRIPTION

10µF ±10%, 6.3V X5R ceramic capacitor

C1

(0805)

Murata GRM21BR60J106K

Taiyo Yuden JMK212BJ106KD

C2, C7, C8

4.7µF±20%, 25V X7R ceramic capacitors

(1206)

Murata GRM31CR71E475M

D1

3A, 30V Schottky diode (M-Flat)

Toshiba CMS02

L1

3.6µH ±30% power inductor

Sumida CDRH6D26-3R6NC

Table 2. Component Suppliers

SUPPLIER

Murata

Sanyo

Sumida

Taiyo Yuden

Toshiba

PHONE

770-436-1300

619-661-4143

847-545-6700

800-348-2496

949-455-2000

Shutdown

The MAX8727 shuts down to reduce the supply current

to 0.1µA when SHDN is low. In this mode, the internal

reference, error amplifier, comparators, and biasing cir-

cuitry turn off, and the n-channel MOSFET is turned off.

The step-up regulator’s output is connected to IN by

the external inductor and rectifier diode.

Applications Information

Step-up regulators using the MAX8727 can be

designed by performing simple calculations for a first

iteration. All designs should be prototyped and tested

prior to production. Table 1 provides a list of power

components for the typical applications circuit. Table 2

lists component suppliers.

External-component-value choice is primarily dictated

by the output voltage and the maximum load current,

as well as maximum and minimum input voltages.

Begin by selecting an inductor value. Once L is known,

choose the diode and capacitors.

Inductor Selection

The minimum inductance value, peak current rating, and

series resistance are factors to consider when selecting

the inductor. These factors influence the converter’s effi-

ciency, maximum output load capability, transient-

response time, and output voltage ripple. Physical size

and cost are also important factors to be considered.

The maximum output current, input voltage, output volt-

age, and switching frequency determine the inductor

value. Very high inductance values minimize the cur-

rent ripple and therefore reduce the peak current,

which decreases core losses in the inductor and I2R

losses in the entire power path. However, large induc-

tor values also require more energy storage and more

turns of wire, which increase physical size and can

increase I2R losses in the inductor. Low inductance val-

ues decrease the physical size but increase the current

ripple and peak current. Finding the best inductor

involves choosing the best compromise between circuit

efficiency, inductor size and cost.

FAX

770-436-3030

619-661-1055

847-545-6720

847-925-0899

949-859-3963

WEBSITE

www.murata.com

www.sanyovideo.com

www.sumida.com

www.t-yuden.com

www.toshiba.com/taec

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