MAX8740 데이터 시트보기 (PDF) - Maxim Integrated

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MAX8740

TFT-LCD Step-Up DC-DC Converter

Maxim Integrated 

TFT-LCD Step-Up DC-DC Converter

Soft-Start

The MAX8740 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 MAX8740’s frequency can be user selected to oper-

ate at either 640kHz or 1.2MHz. Connect FREQ to GND

for 640kHz operation. For a 1.2MHz switching frequency,

connect FREQ to IN. This allows the use of small, mini-

mum-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

10µF ±20%, 25V X5R ceramic capacitors

(1210)

TDK C3225X5R1E106M,

Taiyo Yuden TMK325BJ106MM

D1

3A, 40V Schottky diode (SM8)

Central Semiconductor CMSH3-40M

3.3µH ±30%, 4.0A power inductor

L1

Sumida CDRH8D28-3R3, 3.3µH

(alternate : Sumida CDRH103R-3R3, 3.3µH)

Table 2. Component Suppliers

SUPPLIER

Murata

Sumida

Taiyo Yuden

TDK

Toshiba

PHONE

770-436-1300

847-545-6700

800-348-2496

847-803-6100

949-455-2000

Shutdown

The MAX8740 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 MAX8740 can be designed

by performing simple calculations for a first iteration. All

designs should be prototyped and tested prior to pro-

duction. 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

847-545-6720

847-925-0899

847-390-4405

949-859-3963

WEBSITE

www.murata.com

www.sumida.com

www.t-yuden.com

www.component.tdk.com

www.toshiba.com/taec

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