AD590
Table 4. Thermal Resistance
Medium
Aluminum Block
Stirred Oil
2
Moving Air
3
With Heat Sink
Without Heat Sink
Still Air
With Heat Sink
Without Heat Sink
1
2
θ
JC
+ θ
CA
(°C/Watt)
H
F
30
10
42
60
45
115
191
480
–
190
–
650
τ (sec)
1
H
F
0.6
0.1
1.4
0.6
5.0
13.5
108
60
–
10.0
–
30
response, T (t). Table 4 shows the effective time constant,
τ
, for
several media.
T
FINAL
SENSED TEMPERATURE
T(t) = T
INITIAL
+ (T
FINAL
– T
INITIAL
)
×
(1 – e
–t/τ
)
τ is dependent upon velocity of oil; average of several velocities listed above.
Air velocity @ 9 ft/sec.
3
The time constant is defined as the time required to reach 63.2% of an
instantaneous temperature change.
T
INITIAL
The time response of the AD590 to a step change in
temperature is determined by the thermal resistances and the
thermal capacities of the chip, C
CH
, and the case, C
C
. C
CH
is
about 0.04 Ws/°C for the AD590. C
C
varies with the measured
medium, because it includes anything that is in direct thermal
contact with the case. The single time constant exponential
curve of Figure 13 is usually sufficient to describe the time
τ
4τ
TIME
Figure 13. Time Response Curve
Rev. C | Page 9 of 16
00533-C-013
AD [ ANALOG DEVICES ]
