US1075
DT=Temperature Rise Above Ambient
Thermal Design
DT
qSA =
PD
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-
81
qSA =
=11.7 °C/W
6.9
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 .
5) Next , a heat sink with lower qsa 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.
Assuming the following specifications :
VIN =5V
Air Flow (LFM)
VO =3.5V
IOUTMAX =4.6A
TA =35°C
0
100
200
300
400
Thermalloy
AAVID
6021PB 6021PB 6073PB 6109PB 7141D
534202B 534202B 507302
575002 576802B
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 :
PD = IO UT´ VIN- VO UT
PD =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 :
qJC =2.7°C/W
3) Assuming that the heat sink is Black Anodized, cal-
culate the maximum Heat sink temperature allowed :
Assume , qcs=0.05°C/W (Heat sink to Case thermal
resistance for Black Anodized)
TS=TJ- PD ´ qJC + qCS
TS =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 (qsa) is calculated by first calculating the
temperature rise above the ambient as follows :
DT = TS - TA =116- 35=81 °C
Rev. 1.0
9/14/99
2-37
UNISEM [ UNISEM ]
