LX8585-xx / LX858A-xx
PRODUCTION DATA SHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 7
Copyright © 1997
Rev. 2.2a, 2005-11-10
WWW.Microsemi .COM
4.6A Low Dropout Positive Regulators
TM
®
APPLICATION NOTE
LOAD REGULATION
(continued)
Even when the circuit is configured optimally, 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 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
IN
(MIN)
during transients.
V
IN (MIN)
= V
OUT
+ V
DROPOUT (MAX)
where: V
IN (MIN)
≡ the lowest allowable instantaneous voltage at
the input pin.
V
OUT
≡ the designed output voltage for the power supply
system.
V
DROPOUT (MAX)
≡ the specified dropout voltage for the
installed regulator.
THERMAL CONSIDERATIONS
The LX8585/85A 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: V
IN
= 5V
V
OUT
= 2.8V, I
OUT
= 5.0A
Ambient Temp. T
A
= 50°C
R
ΘJT
= 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:
T
J
= P
D
(R
ΘJT
+ R
ΘCS
+ R
ΘSA
) + T
A
Where: P
D
≡ Dissipated Power
R
ΘJT
≡ Thermal resistance from the junction to the
mounting tab of the package
R
ΘCS
≡ 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).
R
ΘSA
≡ Thermal resistance from the mounting surface
to ambient (thermal resistance of the heat sink).
T
S
≡ heat sink temperature.
R
JT
R
CS
R
SA
T
J
T
C
T
S
T
A
First, find the maximum allowable thermal resistance of the heat
sink:
()
C/W3.1
C/W)1.0C/W(2.7
5.0A*2.8V)(5.0V
C50C125
R
11.0W
5.0A*2.8V)(5.0VI VVP
)R(R
P
TT
R
ΘSA
OUTOUTIN(MAX)D
ΘCSΘJT
D
AJ
ΘSA
°=
°+°−
−
°−°
=
=
−=−=
+−
−
=
Next, select a suitable heat sink. The selected heat sink must
have R
ΘSA
< 3.1°C/W. Thermalloy heatsink 6296B has R
ΘSA
=
3.0°C/W with 300ft/min airflow.
Finally, verify that junction temperature remains within
specification using the selected heat sink:
C124C50C/W)3.0C/W1.0C/W11W(2.7T
J
°=°+°+°+°=
**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.
A
A
P
P
P
P
L
L
I
I
C
C
A
A
T
T
I
I
O
O
N
N
S
S
