LX8586 Microsemi Corporation, LX8586 Datasheet - Page 6

• Manufacturer Part Number: LX8586
• Description: Low Drop Out Regulator - Positive Adjustable
• Manufacturer: Microsemi Corporation

Copyright © 1996
Rev. 1.0a, 2005-11-10
L
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
enough that the voltage at the input pin does not drop below V
(MIN)
Where:
T
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
OAD
HERMAL
The LX8586/86A regulators have internal power and thermal
during transients.
R
TM
EGULATION
C
ONSIDERATIONS
V
V
at the input pin.
V
supply system.
V
installed regulator.
IN (MIN)
IN (MIN)
OUT
DROPOUT (MAX)
≡ the designed output voltage for the power
(continued)
≡ the lowest allowable instantaneous voltage
= V
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
OUT
≡ the specified dropout voltage for the
+ V
DROPOUT (MAX)
A P P L I C A T I O N N O T E
Integrated Products Division
®
Microsemi
IN
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:
Find:
Solution:
Where:
heatsink:
have R
3.0°C/W with 300ft/min air flow.
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.
E
R
P
R
=
6A Low Dropout Positive Regulators
XAMPLE
D
Θ
ΘSA
3
First, find the maximum allowable thermal resistance of the
Next, select a suitable heat sink. The selected heat sink must
Finally, verify that junction temperature remains within
SA
1 .
=
°
(
C
=
V
ΘSA
=
IN
/
5 (
T
W
(
< 3.1°C/W. Thermalloy heat sink 6296B ahs R
J
0 .
MAX
125
V
V
Ambient Temp., T
R
300 ft/min airflow available
Proper Heat Sink to keep ICs junction temperature
below 125°C.**
P
R
mounting tab of the package.
R
between the IC and the surface on which it is
mounted. (1.0°C/W at 6 in-lbs mounting screw
torque.)
R
surface to ambient (thermal resistance of the heat
sink)
T
The junction temperature is:
P
T
T
−
V
P
D
D
S
ΘJT
ΘJT
ΘCS
ΘSA
J
IN
OUT
J
T
RODUCTION
≡ Dissipated Power
≡ Heat sink temperature.
−
°
=
)
A
= 5V
C
R
= 2.7°C/W for TO-220
28
−
≡ Thermal resistance from the junction to the
= 2.8V, I
P
≡ Thermal resistance through the interface
−
−
≡ Thermal resistance from the mounting
D
V
V
(
JT
OUT
50
(
R
* )
R
°
T
ΘJT
ΘJT
C
5
C
)
0 .
I
R
OUT
OUT
A
+
+
CS
R
−
D
R
A
= 5.0A
ΘCS
2 (
ΘCS
=
ATA
= 50°C
T
LX8586 / LX8586A
S
7 .
5 (
R
)
°
0 .
+
C
S
V
SA
R
/
HEET
W
−
ΘSA
2
+
T
8 .
A
1
)
V
0 .
+
* )
°
T
C
A
5
/
0 .
W
A
)
ΘSA
=
Page 6