SC4205EVB 查看數據表（PDF） - Semtech Corporation

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SC4205EVB

Very Low Dropout 1 Amp Regulator With Enable

Semtech Corporation 

SC4205

POWER MANAGEMENT

Applications Information

Introduction

Thermal Considerations

The SC4205 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 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.

The power dissipation in the SC4205 is approximately

equal to the product of the output current and the input

to output voltage differential:

PD ≈ (VIN −VOUT )• IO

The absolute worst-case dissipation is given by:

Component Selection

Input capacitor: a 4.7µF 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 2.2µF,

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 SC4205 is extremely tolerant of

output capacitor ESR values and thus will also work

comfortably with tantalum output capacitors. For refer-

ence, the phase-margin contour of Figure 1 can be used

to choose an appropriate output capacitor for a given

stability requirement.

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.

( ) PD(MAX ) = VIN(MAX ) −VOUT(MIN ) • IO(MAX ) + VIN(MAX ) • IQ(MAX )

For a typical scenario, VIN = 3.3V ± 5%, VOUT = 2.8V and

IO = 1A, therefore:

VIN(MAX) = 3.465V, VOUT(MIN) = 2.744V and IQ(MAX) = 1.75mA,

Thus PD(MAX) = 727mW.

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

calculate the maximum thermal impedance allowable to

maintain TJ ≤ 150°C:

( ) ( ) R = T P − T TH (J −A)(MAX )

J ( MAX )

A( MAX )

D(MAX )

=

150 − 70

.727

= 110°C /W

This should be achievable for the SOIC-8EDP package

using pcb copper area to aid in conducting the heat away,

such as one square inch of copper connected to the pins

of the device. Internal ground/power planes and air flow

will also assist in removing heat. For higher ambient tem-

peratures it may be necessary to use additional copper

area.

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).

 2004 Semtech Corp.

6

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