POWER DISSIPATION
The REG102 is available in three different package configu-
rations. The ability to remove heat from the die is different for
each package type and, therefore, presents different consid-
erations in the printed circuit board (PCB) layout. The PCB
area around the device that is free of other components
moves the heat from the device to the ambient air. Although
it is difficult to impossible to quantify all of the variables in a
thermal design of this type, performance data for several
simplified configurations are shown in Figure 7. In all cases,
the PCB copper area is bare copper (free of solder resist
mask), not solder plated, and are for 1-ounce copper. Using
heavier copper will increase the effectiveness in moving the
heat from the device. In those examples where there is
copper on both sides of the PCB, no connection has been
provided between the two sides. The addition of plated
through holes will improve the heat sink effectiveness.
Power dissipation depends on input voltage, load conditions,
and duty cycle and is equal to the product of the average
output current times the voltage across the output element,
V
IN
to V
OUT
voltage drop.
P
D
=
(V
IN
– V
OUT
)
•
I
OUT
(3)
Power dissipation can be minimized by using the lowest
possible input voltage necessary to assure the required
output voltage.
REGULATOR MOUNTING
The tab of the SOT-223 package is electrically connected to
ground. For best thermal performance, this tab must be
soldered directly to a circuit-board copper area. Increasing
the copper area improves heat dissipation, as shown in
Figure 8.
Although the tab of the SOT-223 is electrical ground, it is not
intended to carry current. The copper pad that acts as a heat
sink should be isolated from the rest of the circuit to prevent
current flow through the device from the tab to the ground
pin. Solder pad footprint recommendations for the various
REG102 devices are presented in Application Bulletin
Solder
Pad Recommendations for Surface-Mount Devices
(SBFA015), available from the Texas Instruments web site
(www.ti.com).
DEVICE DISSIPATION vs TEMPERATURE
2.5
CONDITIONS
#1
#2
#3
#4
Power Dissipation (Watts)
2
CONDITION
1
2
3
4
PACKAGE
SOT-223
SOT-223
SO-8
SOT-23
PCB AREA
4in2 Top Side Only
0.5in2 Top Side Only
—
—
θ
JA
53°C/W
110°C/W
150°C/W
200°C/W
1.5
1
0.5
0
0
25
50
75
100
125
Ambient Temperature (°C)
FIGURE 7. Maximum Power Dissipation versus Ambient Temperature for the Various Packages and PCB Heat Sink Configurations.
THERMAL RESISTANCE vs PCB COPPER AREA
180
Circuit-Board Copper Area
REG102
Surface-Mount Package
1 oz. copper
Thermal Resistance,
θ
JA
(°C/W)
160
140
120
100
80
60
40
20
0
0
1
2
REG102
SOT-223 Surface-Mount Package
3
(inches
2
)
4
5
Copper Area
FIGURE 8. Thermal Resistance versus PCB Area for the Five-Lead SOT-223.
12
REG102
www.ti.com
SBVS024F
BURR-BROWN [ BURR-BROWN CORPORATION ]
