US1075
Thermal Design
The US1075 incorporates an internal thermal shutdown
that protects the device when the junction temperature
exceeds the maximum allowable junction temperature.
Although this device can operate with junction tempera-
tures in the range of 150
°
C ,it is recommended that the
selected heat sink be chosen such that during maxi-
mum continuous load operation the junction tempera-
ture is kept below this number. The example below
shows the steps in selecting the proper Regulator heat
sink for the worst case current consumption using Intel
200MHz microprocessor as the load .
Assuming the following specifications :
V
IN
=
5 V
V
O
=
3.5 V
I
OUT
MAX
=
4.6 A
T
A
=
35
°
C
The steps for selecting a proper heat sink to keep the
junction temperature below 135
°
C is given as :
1) Calculate the maximum power dissipation using :
P
D
=
4.6
×
(
5
−
3.5
)
=
6.9 W
2) Select a package from the Regulator data sheet
and record its junction to case (or Tab) thermal
resistance.
Selecting TO220 package gives us :
θ
JC
=
2.7
°
C / W
3) Assuming that the heat sink is Black Anodized, cal-
culate the maximum Heat sink temperature allowed :
Assume ,
θ
cs=0.05
°
C/W (Heat sink to Case thermal
resistance for Black Anodized)
T
S
=
T
J
−
P
D
×
(
θ
JC
+ θ
CS
)
T
S
=
135
−
6.9
×
(
2.7
+
0.05
)
=
116
°
C
4) With the maximum heat sink temperature calcu-
lated in the previous step, the Heat Sink to Air thermal
resistance (
θ
sa) is calculated by first calculating the
temperature rise above the ambient as follows :
∆
T
=
T
S
−
T
A
=
116
−
35
=
81
°
C
P
D
=
I
O UT
×
(
V
IN
−
V
O UT
)
∆
T
=Temperature Rise Above Ambient
∆
T
P
D
81
.
θ
SA
=
=
117
°
C / W
6.9
θ
SA
=
5) Next , a heat sink with lower
θ
sa than the one calcu-
lated in step 4 must be selected. One way to do this is
to simply look at the graphs of the “Heat Sink Temp
Rise Above the Ambient” vs. the “Power Dissipation” and
select a heat sink that results in lower temperature rise
than the one calculated in previous step. The following
heat sinks from AAVID and Thermaloy meet this crite-
ria.
Thermalloy
AAVID
Air Flow (LFM)
0
100
200
300
6021PB 6021PB 6073PB 6109PB
534202B 534202B 507302
575002
400
7141D
576802B
Rev. 1.0
9/14/99
2-37
UNISEM [ UNISEM ]
