MIC2085 Datasheet, PDF (25/29 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
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, but
it’s not a hard calculation to perform, and tends to
converge quickly.
Then the starting (steady-state) T_J is:
T_J ≅ T_A(MAX) + ∆TJ
≅ T_A(MAX) + [R_ON + (T_A(MAX) –T_A)(0.005/°C)
(R_ON)] x I² x R_θ(J-A)
T_J ≅ 55°C + [17mΩ + (55°C-25°C)(0.005)
(17mΩ)] x (2.5A)² x (50°C/W)
T_J ≅ (55°C + (0.122W)(50°C/W)
≅ 61.1°C
MIC2085/2086
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 + (61.1°C-25°C)(0.005)(17mΩ)]
x (2.5A)² x (50°C/W)
T_J ≅ ( 55°C + (0.125W)(50°C/W)
≅ 61.27°C
So our original approximation of 61.1°C was very close
to the correct value. We will use TJ = 61°C.
Finally, add (11.25W)(50°C/W)(0.08) = 45°C to the
steady-state T_J to get T_{J(TRANSIENT MAX.)} = 106°C. This is
an acceptable maximum junction temperature for this
part.
Figure 12. Transient Thermal Impedance
May 2006
25
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