SC4215

7

Applications Information (continued)

Introduction

The SC425 is intended for applications where high

current capability and very low dropout voltage are

required. It provides a very simple, low cost solution that

uses very little PCB real estate. Additional features include

an enable pin to allow for a very low power consumption

standby mode, and a fully adjustable output.

Component Selection

Input capacitor: A large bulk capacitance ≥ 0µF/A

(output load) should be closely placed to the input supply

pin of the SC425 to ensure that Vin does not sag below

.4V. Also a minimum of 4.7µF ceramic capacitor is recom-

mended to be placed directly next to the Vin pin. This

allows for the device being some distance from any bulk

capacitance on the rail. Additionally, input droop due to

load transients is reduced, improving load transient

response. Additional capacitance may be added if required

by the application.

Output capacitor: A minimum bulk capacitance of ≥

0µF/A (output load), along with a 0.µF ceramic decou-

pling capacitor is recommended. Increasing the bulk

capacitance will improve the overall transient response.

The use of multiple lower value ceramic capacitors in par-

allel to achieve the desired bulk capacitance will not cause

stability issues. Although designed for use with ceramic

output capacitors, the SC425 is extremely tolerant of

output capacitor ESR values and thus will also work com-

fortably with tantalum output capacitors.

Noise immunity: In very electrically noisy environments,

it is recommended that 0.µF ceramic capacitors be placed

from IN to GND and OUT to GND as close to the device

pins as possible.

Internal voltage selection: By connecting the ADJ pin to

GND, an internal resistor divider will regulate the output

voltage to 2.5V.

If the ADJ pin is connected directly to the VO pin, the

output voltage will be regulated to the 0.8V internal

reference.

External voltage selection resistors: The use of %

resistors, and designing for a current flow ≥ 0µA is rec-

ommended to ensure a well regulated output (thus R2

≤50kΩ). A suitable value for R2 can be chosen in the range

of kΩ to 50kΩ. R can then be calculated from.

Enable: Pulling this pin below 0.4V turns the regulator off,

reducing the quiescent current to a fraction of its operat-

ing value. A pull up resistor up to 400kOhms should be

connected from this pin to the VIN pin in applications

where supply voltages of Vin < .9V are required. For

applications with higher voltages than .9V, EN pin could

be left open or connected to VIN.

Thermal Considerations

The power dissipation in the SC425 is given by:

The allowable power dissipation will be dependant on the

thermal impedance achieved in the application. The derat-

ing curve below is valid for the thermal impedance speci-

fied in the Thermal Information section on page .

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