P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX8384-xx/8384A-xx/8384B-xx
5 A L O W
D
R O P O U T
P
O S I T I V E
R
E G U L A T O R S
P R O D U C T I O N D A T A S H E E T
APPLICATION NOTES
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, a50 millivolt dropwill appearbetween theregulatorand
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.
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: VIN = 5V
VOUT = 2.8V, IOUT = 5.0A
Ambient Temp., TA = 50°C
RθJT = 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.**
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
enoughthatthevoltageattheinputpindoesnotdropbelowVIN(MIN)
during transients.
Solution: The junction temperature is:
TJ = PD (RθJT + RθCS + RθSA) + TA
where: PD ≡ Dissipated power.
RθJT ≡ Thermal resistance from the junction to the
mounting tab of the package.
RθCS ≡ 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.)
RθSA
≡
Thermal resistance from the mounting surface
to ambient (thermal resistance of the heat sink).
TS ≡ Heat sink temperature.
TJ TC TS
TA
VIN (MIN) = VOUT + VDROPOUT (MAX)
RθJT
RθCS
RθSA
where: VIN (MIN)
VOUT
≡ the lowest allowable instantaneous
voltage at the input pin.
≡ the designed output voltage for the
power supply system.
First, find the maximum allowable thermal resistance of the
heat sink:
TJ - TA
RθSA
=
- (RθJT + RθCS)
PD
VDROPOUT (MAX) ≡ the specified dropout voltage
for the installed regulator.
PD = (VIN(MAX) - VOUT) IOUT = (5.0V-2.8V) 5.0A
*
= 11.0W
THERMAL CONSIDERATIONS
125°C - 50°C
RθSA
=
- (2.7°C/W+ 1.0°C/W)
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
(5.0V-2.8V) 5.0A
*
= 3.1°C/W
Next,selectasuitableheatsink. Theselectedheatsinkmusthave
RθSA≤ 3.1°C/W. Thermalloyheatsink6296BhasRθSA =3.0°C/Wwith
300ft/min air flow.
Finally, verify that junction temperature remains within speci-
fication 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 recom-
mended for long term reliability to keep the junction temperature
below 125°C whenever possible.
Copyright © 1997
Rev. 1.9 12/97
7
MICROSEMI [ Microsemi ]
