APPLICATION NOTES CONTINUED
HEAT SINKING
To determine if a heat sink is necessary for your applica-
tion and if so, what type, refer to the thermal model and
governing equation below.
Example:
In our example the amplifier application requires each out-
put to drive a 10 volt peak sine wave across a 20 ohm
load for 0.5 amp of output current. For a worst case
analysis we will treat the 0.5 amp peak output current as
a D.C. output current. The power supplies are ±12VDC.
The device is the SOIC version.
1.) Find Driver Power Dissipation
P
D
= [(quiescent current) x (+V
CC
- (-V
CC
))] +
[(V
CC
-V
O
) x I
OUT
]
= (75mA) x (24V) + (2V) x (0.5A) + (2V) x (0.5A)
= 1.8W + 2W
= 3.8W
2.) For conservative design, set T
J
=+150°C.
3.) For this example, worst case T
A
=+25°C
4.) R
θJC
= 3.0°C/W
5.) Rearrange governing equation to solve for R
θSA
R
θSA
= ((T
J
- T
A
)/P
D
) - (R
θJC
) - (R
θCS
)
= ((150°C - 25°C) / 3.8W) - (3°C/W)
≅
29.9°C/W
Thermal Model:
The heat sink in this example must have a thermal resis-
tance of no more than 30°C/W to maintain a junction
temperature of no more than +150°C
.
CASE CONNECTION
The heat sink and lid of the MSK 173 are electrically con-
nected to the negative power supply rail. The user is
urged to keep this in mind when designing the printed
circuit card the MSK 173 will be placed in. There should
be no printed circuit traces making contact with the case
of the device except for -Vcc. The -Vcc plane or ground
for single supply systems, can be used to pull heat away
from the device and must be connected electrically to the
heat sink.
Governing Equation:
T
J
=
P
D
x (R
θJC
+ R
θCS
+ R
θSA
) + T
A
Junction Temperature
Total Power Dissipation
Junction to Case Thermal Resistance
Case to Heat Sink Thermal Resistance
Heat Sink to Ambient Thermal Resistance
Case Temperature
Ambient Temperature
Sink Temperature
Where
T
J
=
P
D
=
R
θJC
=
R
θCS
=
R
θSA
=
T
C
=
T
A
=
T
S
=
UNIVERSAL EVALUATION PC BOARD LAYOUT
DEVICE SOLDERING
The MSK 173 is a highly thermally conductive device and
the thermal path from the package to the internal junc-
tions is very short. Standard surface mount techniques
should be used when soldering the device into a circuit
board. A hole can be cut in the printed circuit board to
allow the heat sink of the package to be thermally bonded
to an external heat sink for high power applications.
4
Rev. H 8/00
MSK [ M.S. KENNEDY CORPORATION ]
