SC1457ISK-18TRT 查看數據表（PDF） - Semtech Corporation

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SC1457ISK-18TRT

150mA Ultra Low Dropout Regulator with Undervoltage Flag

Semtech Corporation 

SC1457

POWER MANAGEMENT

Applications Information (Cont.)

Thermal Considerations

The worst-case power dissipation for this part is given

by:

( ) (1) PD(MAX) =

V − V IN(MAX)

OUT(MIN)

• IOUT(MAX) + VIN(MAX) • IQ(MAX)

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

to the following expression:

( ) PD(MAX) =

V − V IN(MAX)

OUT(MIN)

• IOUT(MAX)

(2)

Looking at a typical application, 3.3V to 2.8V at 150mA:

VIN(MAX) = 3.3 + 5% = 3.465V

VOUT(MIN) = 2.8V - 2% = 2.744V

IOUT = 150mA

TA = 85°C

Inserting these values into equation (2) gives us:

PD(MAX) = (3.465 − 2.744)• 0.150 = 108mW

With the standard SOT-23-5/TSOT-23-5 Land Pattern

shown at the end of this datasheet, and minimum trace

widths, the thermal impedance junction to ambient for

SC1457ISK is 256°C/W. Thus no additional heatsinking

is required for this example.

The junction temperature can be reduced further (or

higher power dissipation can be allowed) by the use of

larger trace widths and connecting PCB copper to the

GND pin (pin 2), which connects directly to the device

substrate. Adding approximately one square inch of PCB

copper to pin 2 will reduce θJA to approximately

130°C/W and T

for the example above to

J(MAX)

approximately 100°C for the SOT-23-5 package. The use

of multi layer boards with internal ground/power planes

will lower the junction temperature and improve overall

output voltage accuracy.

Layout Considerations

While layout for linear devices is generally not as critical

as for a switching application, careful attention to detail

will ensure reliable operation.

Using this figure, we can calculate the maximum thermal

impedance

allowable

to

maintain

T

J

≤

125°C:

( ) ( ) θJA(MAX) =

TJ(MAX) − TA(MAX)

PD(MAX )

=

125 − 85

0.108

= 370°C / W

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.

 2004 Semtech Corp.

6

www.semtech.com

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