LX8586 / LX8586A

Microsemi

Integrated Products Division

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

Page 6

Copyright

© 1996

Rev. 1.0a, 2005-11-10

6A Low Dropout Positive Regulators

TM

®

APPLICATION NOTE

Even when the circuit is optimally configured, 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

(MIN)

during transients.

Where:

≡ the lowest allowable instantaneous voltage

at the input pin.

≡ the designed output voltage for the power

supply system.

≡ the specified dropout voltage for the

installed regulator.

The LX8586/86A 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.

Given:

300 ft/min airflow available

Find:

Proper Heat Sink to keep ICs junction temperature

below 125°C.**

Solution:

The junction temperature is:

(

)

A

ΘSA

ΘCS

ΘJT

D

J

T

R

R

R

P

T

+

+

+

=

Where:

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.)

surface to ambient (thermal resistance of the heat

sink)

T

J

T

C

T

S

T

A

First, find the maximum allowable thermal resistance of the

heatsink:

()

W

C

W

C

W

C

A

V

V

C

C

R

A

V

V

I

V

V

P

SA

OUT

OUT

MAX

IN

D

/

1

.

3

)

/

0

.

1

/

7

.

2

(

0

.

5

*

)

28

0

.

5

(

50

125

0

.

5

*

)

8

.

2

0

.

5

(

)

(

R

R

P

T

T

R

)

(

ΘCS

ΘJT

D

A

J

ΘSA

°

=

°

+

°

−

−

°

−

°

=

−

=

−

=

+

−

−

=

Θ

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

specification using the selected heat sink.

**TJ = 11W (2.7°C/W + 1.0°C/W + 3.0°C/W) + 50°C = 124°C.

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.