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UCC381DP-ADJG4 Datasheet(PDF) 7 Page - Texas Instruments

Part # UCC381DP-ADJG4
Description  Low Dropout 1 Ampere Linear Regulator Family
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

UCC381DP-ADJG4 Datasheet(HTML) 7 Page - Texas Instruments

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7
UCC281-3/-5/-ADJ
UCC381-3/-5/-ADJ
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 • FAX (603) 424-3460
Thermal Design
The Packing Information section of the data book con-
tains reference material for the thermal ratings of various
packages. The section also includes an excellent article
Thermal Characteristics of Surface Mount Packages, that
is the basis of the following discussion.
Thermal design for the UCC381 includes two modes of
operation, normal and pulsed mode. In normal operation,
the linear regulator and heat sink must dissipate power
equal to the maximum forward voltage drop multiplied by
the maximum load current. Assuming a constant current
load, the expected heat rise at the regulator’s junction
can be calculated as follows:
()
T
P
jc
ca
C
RISE
DISS
=•
+
°
θθ
(5)
Where theta is thermal resistance and PDISS is the power
dissipated. The thermal resistance of both the SOIC-8
DP package (junction to case) is 22 degrees Celsius per
Watt. In order to prevent the regulator from going into
thermal shutdown, the case to ambient theta must keep
the junction temperature below 150C. If the LDO is
mounted on a 5 square inch pad of 1 ounce copper, for
example, the thermal resistance from junction to ambient
becomes 40-70 degrees Celsius per Watt. If a lower ther-
mal resistance is required by the application, the device
heat sinking would need to be improved.
When the UCC381 regulator is in pulsed mode, due to
an overload or short circuit in the application, the maxi-
mum
average power dissipation is calculated as follows:
()
()
P
VV
I
T
T
Watts
PULSE avg
IN
OUT
CL
ON
ON
=
−•


33
(6)
As seen in equation 6, the average power during a fault
is reduced dramatically by the duty cycle, allowing the
heat sink to be sized for normal operation. Although the
peak power in the regulator during the TON period can be
significant, the thermal mass of the package will gener-
ally keep the junction temperature from rising unless the
TON period is increased to tens of milliseconds.
Ripple Rejection
Even though the UCC381 linear regulators are not opti-
mized for fast transient applications (Refer to UC182
“Fast LDO Linear Regulator”), they do offer significant
power supply rejection at lower frequencies. Fig 5. de-
picts ripple rejection performance in a typical application.
The performance can be improved with additional filter-
ing.
APPLICATION INFORMATION (cont.)
0
10
20
30
40
50
60
70
80
90
1.0E+02
1.0E+03
1.0E+04
1.0E+05
FREQUENCY
10uF,
IOUT = 100mA
1uF, IOUT = 100mA
1uF, IOUT =1A
10uF, IOUT =1A
Figure 5. Ripple rejection vs. frequency.


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