LT1129/LT1129-3.3/LT1129-5
APPLICATI
S I FOR ATIO
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). The
power dissipated by the device will be made up of two
components:
1. Output current multiplied by the input/output voltage
differential: I
OUT
×
(V
IN
– V
OUT
), and
2. Ground pin current multiplied by the input voltage:
I
GND
×
V
IN
.
The ground pin current can be found by examining the
Ground Pin Current curves in the Typical Performance
Characteristics. Power dissipation will be equal to the sum
of the two components listed above.
The LT1129 series regulators have internal thermal limit-
ing designed to protect the device during overload condi-
tions. For continuous normal load conditions the maxi-
mum junction temperature rating of 125°C must not be
exceeded. It is important to give careful consideration to
all sources of thermal resistance from junction to ambient.
Additional heat sources mounted nearby must also be
considered.
For surface mount devices heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Experiments have shown that the
heat spreading copper layer does not need to be electri-
cally connected to the tab of the device. The PC material
can be very effective at transmitting heat between the pad
area, attached to the tab of the device, and a ground or
power plane layer either inside or on the opposite side of
the board. Although the actual thermal resistance of the PC
material is high, the length/area ratio of the thermal
resistor between layers is small. Copper board stiffeners
and plated through holes can also be used to spread the
heat generated by power devices.
The following tables list thermal resistances for each
package. For the TO-220 package, thermal resistance is
given for junction-to-case only since this package is
usually mounted to a heat sink. Measured values of
thermal resistance for several different board sizes and
copper areas are listed for each package. All measure-
ments were taken in still air on 3/32" FR-4 board with 1-oz
copper. This data can be used as a rough guideline in
U
estimating thermal resistance. The thermal resistance for
each application will be affected by thermal interactions
with other components as well as board size and shape.
Some experimentation will be necessary to determine the
actual value.
Table 1. Q Package, 5-Lead DD
COPPER AREA
TOPSIDE*
BACKSIDE
2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm
125 sq. mm
2500 sq. mm
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq. mm
2500 sq. mm
2500 sq. mm
25°C/W
27°C/W
35°C/W
* Tab of device attached to topside copper
W
U
UO
Table 2. ST Package, 3-Lead SOT-223
COPPER AREA
TOPSIDE*
BACKSIDE
2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm
225 sq. mm
100 sq. mm
2500 sq. mm
2500 sq. mm
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq. mm
2500 sq. mm
2500 sq. mm
2500 sq. mm
45°C/W
45°C/W
53°C/W
59°C/W
* Tab of device attached to topside copper
Table 3. S8 Package, 8-Lead Plastic SOIC
COPPER AREA
TOPSIDE*
BACKSIDE
2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm
225 sq. mm
100 sq. mm
2500 sq. mm
2500 sq. mm
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500 sq. mm
2500 sq. mm
2500 sq. mm
2500 sq. mm
55°C/W
55°C/W
63°C/W
69°C/W
* Device attached to topside copper
T Package, 5-Lead TO-220
Thermal Resistance (Junction-to-Case) = 5°C/W
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 4.5V to 5.5V, an output current range of 0mA to
500mA, and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be?
The power dissipated by the device will be equal to:
I
OUT MAX
×
(V
IN MAX
– V
OUT
) + (I
GND
×
V
IN MAX
)
where, I
OUT MAX
= 500mA
V
IN MAX
= 5.5V
I
GND
at (I
OUT
= 500mA, V
IN
= 5.5V) = 25mA
9
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