LX8586 Datasheet, PDF (6/9 Pages) Microsemi Corporation – 6A Low Dropout Positive Regulators

LX8586 / LX8586A
TM
®
6A Low Dropout Positive Regulators
PRODUCTION DATA SHEET
APPLICATION NOTE
LOAD REGULATION (continued)
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 VIN
(MIN) during transients.
V = V + V IN (MIN)
OUT
DROPOUT (MAX)
Where:
VIN (MIN) ≡ the lowest allowable instantaneous voltage
at the input pin.
VOUT ≡ the designed output voltage for the power
supply system.
VDROPOUT (MAX) ≡ the specified dropout voltage for the
installed regulator.
THERMAL CONSIDERATIONS
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.
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 ICs 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.
T_J
T_C
T_S
T_A
R JT R CS R SA
First, find the maximum allowable thermal resistance of the
heatsink:
( ) R ΘSA = TJ − TA − R ΘJT + R ΘCS
P_D
P_D = (V_{IN (MAX )} − V_OUT )I_OUT = (5.0V − 2.8V ) * 5.0 A
R_ΘSA
=
125°C − 50°C
(5.0V − 28V ) * 5.0 A
−
(2.7°C
/W
+ 1.0°C
/W)
= 3.1°C / W
Next, select a suitable heat sink. The selected heat sink must
have R_ΘSA < 3.1°C/W. Thermalloy heat sink 6296B ahs R_ΘSA =
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.
Copyright © 1996
Rev. 1.0a, 2005-11-10
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
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