Data Sheet
As an example, for the TO-52 package, θ
JC
is the thermal
resistance between the chip and the case, about 26°C/W. θ
CA
is
the thermal resistance between the case and the surroundings
and is determined by the characteristics of the thermal
connection. Power source P represents the power dissipated
on the chip. The rise of the junction temperature, T
J
, above the
ambient temperature, T
A
, is
T
J
−
T
A
=
P(θ
JC
+ θ
CA
)
(1)
JC
and θ
CA
for several common
thermal media for both the H and F packages. The heat sink
used was a common clip-on. Using Equation 1, the temperature
rise of an AD590 H package in a stirred bath at 25°C, when
driven with a 5 V supply, is 0.06°C. However, for the same
conditions in still air, the temperature rise is 0.72°C. For a given
supply voltage, the temperature rise varies with the current and
is PTAT. Therefore, if an application circuit is trimmed with the
sensor in the same thermal environment in which it is used, the
scale factor trim compensates for this effect over the entire
temperature range.
Table 4. Thermal Resistance
θ
JC
+ θ
CA
(°C/Watt)
H
F
30
10
42
60
45
115
191
480
–
190
–
650
T
INITIAL
AD590
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 16 is usually sufficient to describe the time
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/
)
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
τ (sec)
1
H
F
0.6
0.1
1.4
0.6
5.0
13.5
108
60
–
10.0
–
30
4
TIME
Figure 16. Time Response Curve
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.
Rev. G | Page 9 of 16
00533-013
AD [ ANALOG DEVICES ]
