MIC2582_09 Datasheet, PDF (21/25 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
Thus:
Assume T_A = 55°C maximum, 1 square inch of copper at
the drain leads, no airflow.
Recalling from our previous approximation hint, the part
has an R_ON of (0.0335/2) = 17mΩ at 25°C.
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, and
the calculation tends to converge quickly.
Then the starting (steady-state) T_J is:
T_J ≅ T_A(MAX) + ∆T_J
≅ T_A(MAX) + [R_ON + T_A(MAX) – T_A)(0.005/ºC)(R_ON)]
x I² x R_θ(JA)
T_J ≅ 55ºC + [17mΩ + (55ºC-25ºC)(0.005)(17mΩ)]
MIC2582/MIC2583
x (2.5A)² x (50ºC/W)
T_J ≅ (55ºC + (0.122W)(50ºC/W)
≅ 61.1ºC
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°C:
T_J ≅ T_A + [17mΩ + (61.1ºC-25ºC)(0.005)(17mΩ)]
x (2.5A)² x (50ºC/W)
T_J ≅ (55ºC + (0.125W)(50ºC/W) ≅ 61.27ºC
So our original approximation of 61.1ºC was very close
to the correct value. We will use T_J = 61ºC.
Finally, add the temperature increase due to the
maximum power dissipation calculated from a “single
event”, (11.25W)(50ºC/W)(0.08) = 45ºC to the steady-
state T_J to get T_{J(TRANSIENT MAX.)} = 106ºC. This is an
acceptable maximum junction temperature for this part.
Figure 10. Transient Thermal Impedance
April 2009
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M9999-043009-C