Microsemi Inc.

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 7

Copyright

© 2000

Rev. 2.1f, 2006-01-20

LX8384x-xx

5A Low Dropout Positive Regulators

TM

®

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 regulator package. If several capacitors

are used in parallel to construct the power system output

capacitance, 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

X)

DROPOUT(MA

OUT

ΙΝ(ΜΙΝ)

V

V

V

+

=

≡ the lowest allowable instantaneous

voltage at the input pin.

≡ the designed output voltage for the

power supply system.

installed regulator.

THERMAL CONSIDERATIONS

The LX8384/84A/84B regulators have internal power and

thermal 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.

Example

Given:

=

5V

2.8V

=

5.0A

=

50°C

=

2.7°C/W for TO-220

300 ft/min airflow available

Find:

Proper Heat Sink to keep IC’s junction temperature

below 125°C.**

Solution: The junction temperature is:

A

θSA

θCS

θJT

J

T

)

R

R

(R

T

+

+

+

=

D

P

where: P

D

≡

Dissipated power.

≡

Thermal resistance from the junction to

the mounting tab of the package.

≡

Thermal resistance through the

interface between the IC and the

surface on which it is mounted.

(1.0°C/W at 6 in-lbs mounting screw

torque).

≡

Thermal resistance from the mounting

surface to ambient (thermal resistance

of the heat sink).

≡

Heat Sink Temperature.

T

J

T

C

T

S

T

A

R

θ JT

R

θCS

R

θSA

First, find the maximum allowable thermal resistance of the

heat sink:

()

CS

θ

θJT

D

A

J

SA

θ

R

R

P

T

T

R

+

−

−

=

11.0W

5.0A

2.8V

5.0V

=

×

−

=

−

=

D

D

P

P

)

(

I

)

V

(V

OUT

OUT

IN(MAX)

C/W

C/W)

1.0

C/W

(2.7

5.0A

*

2.8V)

(5.0V

C

50

C

125

°

=

°

+

°

−

−

°

−

°

=

1

.

3

R

R

θSA

θSA

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

3.0°C/W with 300ft/min air flow.

Finally, verify that junction temperature remains within speci-

fication using the selected heat sink:

C

124

C

50

C/W)

3.0

C/W

1.0

C/W

11W(2.7

°

=

°

+

°

+

°

+

°

=

J

J

T

T

** Although the device can operate up to 150°C junction, it is recommended for long

term reliability to keep the junction temperature below 125°C whenever possible.