8

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC8863

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

10k

Ω. See “Component Selection - General” for input

capacitor requirements.

Thermal Considerations

The worst-case power dissipation for this part is given

by:

(

)

)

MAX

(

Q

)

MAX

(

IN

)

MAX

(

OUT

)

MIN

(

OUT

)

MAX

(

IN

)

MAX

(

D

I

V

I

V

V

P

•

+

•

−

=

For all practical purposes, it can be reduced to:

(

)

)

MAX

(

OUT

)

MIN

(

OUT

)

MAX

(

IN

)

MAX

(

D

I

V

V

P

•

−

=

Looking at a typical application:

V

V

I

T

This gives us:

()

mW

196

150

.

0

895

.

2

2

.

4

P

)

MAX

(

D

=

•

−

=

Using this figure, we can calculate the maximum thermal

impedance allowable to maintain T

(

)

W

/

C

332

196

.

0

85

150

P

T

T

)

MAX

(

D

)

MAX

(

A

)

MAX

(

J

)

MAX

)(

A

J

(

°

=

−

=

−

=

θ

−

Applications Information (Cont.)

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

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

θ

approximately 130°C/W and T

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.

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.

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.