Micrel, Inc.
MIC2085/2086
May 2006
25
M9999-050406
(408) 955-1690
has an R
ON
of (0.0335/2) = 17m& at 25癈.
Assume it has been carrying just about 2.5A for some
time.
When performing this calculation, be sure to use the
highest anticipated ambient temperature (T
A(MAX)
) in
which the MOSFET will be operating as the starting
temperature, and find the operating junction temperature
increase (T
J
) from that point. Then, as shown next, the
final junction temperature is found by adding T
A(MAX
) and
T
J
. Since this is not a closed-form equation, getting a
close approximation may take one or two iterations, but
its not a hard calculation to perform, and tends to
converge quickly.
Then the starting (steady-state) T
J
is:
T
J
E T
A(MAX)
+ TJ
E T
A(MAX)
+ [R
ON
+ (T
A(MAX)
T
A
)(0.005/癈)
(R
ON
)] x I
2
x R
?J-A)
T
J
E 55癈 + [17m& + (55癈-25癈)(0.005)
(17m&)] x (2.5A)
2
x (50癈/W)
T
J
E (55癈 + (0.122W)(50癈/W)
E 61.1癈
Iterate the calculation once to see if this value is within a
few percent of the expected final value. For this iteration
we will start with T
J
equal to the already calculated value
of 61.1癈:
T
J
E T
A
+ [17m& + (61.1癈-25癈)(0.005)(17m&)]
x (2.5A)
2
x (50癈/W)
T
J
E ( 55癈 + (0.125W)(50癈/W)
E 61.27癈
So our original approximation of 61.1癈 was very close
to the correct value. We will use TJ = 61癈.
Finally, add (11.25W)(50癈/W)(0.08) = 45癈 to the
steady-state T
J
to get T
J(TRANSIENT MAX.)
= 106癈. This is
an acceptable maximum junction temperature for this
part.
Figure 12. Transient Thermal Impedance
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