SC1563 データシートの表示（PDF） - Semtech Corporation

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SC1563

Very Low Dropout, Low Quiescent 500mA Regulator

Semtech Corporation 

SC1563

POWER MANAGEMENT

Applications Information

Introduction

Thermal Considerations

The SC1563 is intended for applications such as

graphics cards where high current capability and very low

dropout voltage are required. It provides a very simple,

low cost solution that uses little pcb real estate.

Additional features include a shutdown pin to allow for a

very low power consumption standby mode, and a fully

adjustable output.

The power dissipation in the SC1563 is approximately

equal to the product of the output current and the input

to output voltage differential:

( ) PD ≈ VIN − VOUT • IOUT

The absolute worst-case dissipation is given by:

Component Selection

Input capacitor - a 1µF ceramic capacitor is

recommended. 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 1µF,

along with a 0.1µF ceramic decoupling capacitor is

recommended. Increasing the bulk capacitance will

improve the overall transient response. The use of

multiple lower value ceramic capacitors in parallel to

achieve the desired bulk capacitance will not cause

stability issues. Although designed for use with ceramic

output capacitors, the SC1563 is extremely tolerant of

output capacitor ESR values and thus will also work

comfortably with tantalum output capacitors.

( ) PD(MAX) =

V − V IN(MAX )

OUT (MIN )

• IOUT(MAX ) + VIN(MAX ) • IQ(MAX )

For a typical scenario, VIN = 3.3V ± 5%, VOUT = 2.5V and

IOUT = 500mA, therefore:

VIN(MAX) = 3.465V,

VOUT(MIN) = 2.450V and

IQ(MAX) = 150µA,

Thus PD(MAX) = 0.508W.

Using this figure, and assuming TA(MAX) = 70°C, we can

calculate the maximum thermal impedance allowable to

maintain TJ ≤ 150°C:

( ) ( ) R = TH(J−A)(MAX)

TJ(MAX) − TA(MAX)

PD(MAX )

=

150 − 70

0.508

= 157°C / W

Noise immunity - in very electrically noisy environments,

it is recommended that 0.1µF ceramic capacitors be

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

device pins as possible.

This should be achievable with the SOT-23-5 package

using pcb copper area to aid in conducting the heat away

from the device. Internal ground/power planes and air

flow will also assist in removing heat.

External voltage selection resistors - the use of 1%

resistors, and designing for a current flow ≥ 10µA is

recommended to ensure a well regulated output (thus

R2 ≤ 120kΩ).

 2006 Semtech Corp.

6

www.semtech.com

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