LX8117-xx/8117A-xx/8117B-xx

PRODUCT DA T ABOOK 1996/1997

Copyright © 1999

Rev. 1.4

3/99

8

APPLICA TION NOTES

THERMAL CONSIDERATIONS (continued)

Example

Given: V

I

Find: The size of a square area of 1oz. copper circuit-

board trace-foil that will serve as a heatsink,

adequate to maintain the junction temperature of the

LX8117 in the ST (SOT-223) package within

specified limits.

Solution: The junction temperature is:

T

where: P

D

JT

≡ Thermal resistance from the junction to the

mounting tab of the package.

between the IC and the surface on which

it is mounted.

of the heatsink to ambient.

T

S

≡ Heat sink temperature.

First, find the maximum allowable thermal resistance of the

heat sink:

P

D

=

P

D

= 1.4W

A test was conducted to determine the thermal characteristics of

1 oz. copper circuit-board trace material. The following equation

describes the observed relationship between the area of a square

copper pad, and the thermal resistance from the tab of a SOT-223

package soldered at the center of the pad to ambient.

Area

square pad with area:

Area

will be required to maintain the LX8117 junction temperature

within specified limits.

T

J

T

C

T

S

T

A

T

P

D

LOAD REGULATION (continued)

Even when the circuit is optimally configured, parasitic resistance

can be a significant source of error. A 100 mil (2.54 mm) wide PC

trace built from 1 oz. copper-clad circuit board material has a

parasitic resistance of about 5 milliohms per inch of its length at

room temperature. If a 3-terminal regulator used to supply 2.50 volts

is connected by 2 inches of this trace to a load which draws 5 amps

of current, a 50 millivolt drop will appear between the regulator and

the load. Even when the regulator output voltage is precisely

2.50 volts, the load will only see 2.45 volts, which is a 2% error. It

is important to keep the connection between the regulator output

pin and the load as short as possible, and to use wide traces or

heavy-gauge wire.

The minimum specified output capacitance for the regulator

should be located near the reglator package. If several capacitors

are used in parallel to construct the power system output capaci-

tance, any capacitors beyond the minimum needed to meet the

specified requirements of the regulator should be located near the

sections of the load that require rapidly-changing amounts of

current. Placing capacitors near the sources of load transients will

help ensure that power system transient response is not impaired

by the effects of trace impedance.

To maintain good load regulation, wide traces should be used on

the input side of the regulator, especially between the input

capacitors and the regulator. Input capacitor ESR must be small

enough that the voltage at the input pin does not drop below V

IN (MIN)

during transients.

V

where: V

IN (MIN)

voltage at the input pin.

V

OUT

power supply system.

V

for the installed regulator.

THERMAL CONSIDERATIONS

The LX8117 regulators have internal power and thermal limiting

circuitry designed to protect each device under overload conditions.

For continuous normal load conditions, however, maximum junc-

tion temperature ratings must not be exceeded. It is important to

give careful consideration to all sources of thermal resistance from

junction to ambient. This includes junction to case, case to heat sink

interface, and heat sink thermal resistance itself.

3.1°C/W