LX8384-xx/8384A-xx/8384B-xx

PRODUCT DA T ABOOK 1996/1997

7

Copyright © 1997

Rev. 1.9

12/97

APPLICA TION NOTES

Example

Given: V

V

Ambient Temp., T

300 ft/min airflow available

Find: Proper Heat Sink to keep IC's junction

temperature below 125°C.**

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. (1.0°C/W at 6 in-lbs

mounting screw torque.)

to ambient (thermal resistance of the heat sink).

T

S

≡ Heat sink temperature.

First, find the maximum allowable thermal resistance of the

heat sink:

P

D

=(V

= (5.0V-2.8V) * 5.0A

= 11.0W

- (2.7°C/W + 1.0°C/W)

= 3.1°C/W

Next, select a suitable heat sink. The selected heat sink must have

300ft/min air flow.

Finally, verify that junction temperature remains within speci-

fication using the selected heat sink:

T

T

J

T

C

T

S

T

A

T

P

D

125°C - 50°C

(5.0V-2.8V) * 5.0A

mended for long term reliability to keep the junction temperature

below 125°C whenever possible.

LOAD REGULATION (continued)

Even when the circuit is configured optimally, parasitic resistance

can be a significant source of error. A 100 mil 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

limiting circuitry designed to protect each device under overload

conditions.

For continuous normal load conditions, however,

maximum junction 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.

Junction-to-case thermal resistance is specified from the IC

junction to the back surface of the case directly opposite the die.

This is the lowest resistance path for heat flow. Proper mounting

is required to ensure the best possible thermal flow from this area

of the package to the heat sink. Thermal compound at the case-to-

heat-sink interface is strongly recommended. If the case of the

device must be electrically isolated, a thermally conductive spacer

can be used, as long as its added contribution to thermal resistance

is considered. Note that the case of all devices in this series is

electrically connected to the output.