MIC2582_14 Datasheet, PDF (22/27 Pages) Micrel Semiconductor – Single-Channel Hot Swap Controllers

Micrel, Inc.
MIC2582/MIC5283
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_J  T_A(MAX) + [R_ON + T_A(MAX) – T_A)(0.005/ºC)(R_ON)]
x I² x R_(JA)
T_J  55ºC + [17mΩ + (55ºC-25ºC)(0.005)(17mΩ)]
x (2.5A)² x (50ºC/W)
T_J  (55ºC + (0.122W)(50ºC/W)
T_J  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 TJ = 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 9. Transient Thermal Impedance
May 23, 2014
22
Revision 5.0