SC1454 查看數據表（PDF） - Semtech Corporation

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SC1454

Dual 150mA Ultra Low Dropout, Low Noise Regulator

Semtech Corporation 

SC1454

POWER MANAGEMENT

Applications Information (Cont.)

R1 and R2 should be selected such that the current

flowing through them is ≥ 10µA (thus R2 ≤ 120kΩ).

Thermal Considerations

The worst-case power dissipation for this part is given

by:

( ) PD(MAX ) =

V − V IN(MAX )

OUTA (MIN )

• IOUTA (MAX )

( ) +

V − V IN(MAX )

OUTB (MIN )

• IOUTB (MAX )

(1)

+ VIN(MAX ) • IQ(MAX )

For all practical purposes, equation (1) can be reduced

to the following expression:

( ) PD(MAX ) =

V − V IN(MAX )

OUTA (MIN )

• IOUTA (MAX )

( ) +

V − V IN(MAX )

OUTB (MIN )

• IOUTB (MAX )

(2)

impedance allowable to maintain TJ ≤ 125°C:

( ) θJA(MAX ) =

TJ(MAX ) − TA(MAX )

PD(MAX )

=

(125 − 85)

0.334

= 120°C / W

This target value can be achieved by using one square

inch of board copper connected to the GND pin (pin 3),

which connects directly to the device substrate.

Increasing this area or the use of multi layer boards will

lower the junction temperature and improve overall

output voltage accuracy.

Layout Considerations

Looking at a typical application:

VIN(MAX) = 4.2V

VOUTA = 3V - 2% (worst case) = 2.94V

VOUTB = 3.3V - 2% (worst case) = 3.234V

IOUTA = IOUTB = 150mA

TA = 85°C

Inserting these values into equation (2) above gives us:

PD(MAX ) = (4.2 − 2.94)• 0.15 + (4.2 − 3.234)• 0.15

= 0.189 + 0.145

= 0.334W

Using this figure, we can calculate the maximum thermal

While layout for linear devices is generally not as critical

as for a switching application, careful attention to detail

will ensure reliable operation.

1) Attaching the part to a larger copper footprint will

enable better heat transfer from the device, especially

on PCBs where there are internal ground and power

planes.

2) Place the input, output and bypass capacitors close

to the device for optimal transient response and device

behaviour.

3) Connect all ground connections directly to the ground

plane. If there is no ground plane, connect to a common

local ground point before connecting to board ground.

 2005 Semtech Corp.

7

www.semtech.com

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