APPLICATION NOTES
HEAT SINKING
To determine if a heat sink is necessary for your application
and if so, what type, refer to the thermal model and governing
equation below.
R
ΘSA
= ((T
J
- T
A
)/P
D
) - (R
ΘJC
) - (R
ΘCS
)
= ((125°C - 80°C) / 0.36W) - 55°C/W - 0.15°C/W
= 125 - 55.15
= 69.9°C/W
This heat sink in this example must have a thermal resistance
of no more than 69.9°C/W to maintain a junction temperature
of no more than +125°C.
Thermal Model:
Typical Applications:
Governing Equation:
T
J=
P
D X
(R
ΘJC
+R
ΘCS
+
R
ΘSA
) +T
A
Where
T
J
=Junction Temperature
PD=Total Power Dissipation
R
ΘJC
=Junction to Case Thermal Resistance
R
ΘCS
=Heat Sink to Ambient Thermal Resistance
T
C
=Case Temperature
T
A
=Ambient Temperature
T
S
=Sink Temperature
Example:
This example demonstrates a worst case analysis for the buffer
output stage. This occurs when the output voltage is 1/2 the
power supply voltage. Under this condition, maximum power
transfer occurs and the output is under maximum stress.
Conditions:
VCC= ±12VDC
Vo= ±6Vp Sine Wave, Freq. = 1KHz
RL= 100Ω
For a worst case analysis we will treat the ±6Vp sine wave as
an 6 VDC output voltage.
1.) Find Driver Power Dissipation
PD= (Vcc-Vo) (Vo/R
L
)
= (12V-6V) (6V/100Ω)
= 360mW
2.) For conservative design, set T
J
=+125°C Max.
3.) For this example, worst case T
A
=+80°C
4.) R
ΘJC
= 55° C/W from MSK 0002RH Data Sheet
5.) R
ΘCS
= 0.15° C/W for most thermal greases
6.) Rearrange governing equation to solve for R
ΘSA
3
Rev. A
4/07
MSK [ M.S. KENNEDY CORPORATION ]
