SC8863 Ver la hoja de datos (PDF) - Semtech Corporation

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Fabricante

SC8863

150mA Ultra Low Dropout, Low Noise Regulator

Semtech Corporation 

SC8863

POWER MANAGEMENT

Applications Information (Cont.)

1% tolerance resistors are recommended. The values of

R1 and R2 should be selected such that the current flow

through them is ≥ 10µA (thus R2 ≤ 120kΩ). At high input

voltages and/or high output currents, stability may be

improved by increasing C2 to 2.2µF and reducing R2 to

10kΩ. See “Component Selection - General” for input

capacitor requirements.

Thermal Considerations

The worst-case power dissipation for this part is given

by:

( ) PD(MAX) =

V − V IN(MAX)

OUT (MIN )

• IOUT(MAX) + VIN(MAX) • IQ(MAX)

For all practical purposes, it can be reduced to:

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

SC8863 is 256°C/W. Thus with no additional heatsinking,

T = 135°C.

J(MAX)

The junction temperature can be reduced further by the

use of larger trace widths, and connecting pcb copper

area to the GND pin (pin 2), which connectes directly to

the device substrate. Adding approximately one square

inch of pcb copper to pin 2 will reduce θTH(J-A) to

approximately 130°C/W and TJ(MAX) to approximately

110°C, for example. Lower junction temperatures

improve overall output voltage accuracy. A sample pcb

layout for the Internally Preset Output Voltage circuit on

page 1 is shown in Figure 2 on page 9.

( ) PD(MAX) =

V − V IN(MAX)

OUT (MIN )

• IOUT(MAX)

Looking at a typical application:

VIN(MAX) = 4.2V

VOUT = (3 - 3.5%) = 2.895V worst-case

IOUT = 150mA

TA = 85°C

This gives us:

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. See Figure 2 on page 9 for

a sample layout.

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.

PD(MAX) = (4.2 − 2.895)• 0.150 = 196mW

Using this figure, we can calculate the maximum thermal

impedance allowable to maintain TJ ≤ 150°C:

( ) ( ) θ = (J−A)(MAX)

TJ(MAX) − TA(MAX)

PD(MAX)

= 150 − 85

0.196

= 332°C / W

2) Place the input and output capacitors close to the

device for optimal transient response and device

behavior.

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.

8

www.semtech.com

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