MIC2582 Ver la hoja de datos (PDF) - Micrel

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componentes Descripción

Fabricante

MIC2582

Single Channel Hot Swap Controllers

Micrel 

Micrel, Inc.

Thus:

Assume TA = 55°C maximum, 1 square inch of copper at

the drain leads, no airflow.

Recalling from our previous approximation hint, the part

has an RON 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 (TA(MAX)) in

which the MOSFET will be operating as the starting

temperature, and find the operating junction temperature

increase (∆TJ) from that point. Then, as shown next, the

final junction temperature is found by adding TA(MAX) and

∆TJ. 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) TJ is:

TJ ≅ TA(MAX) + ∆TJ

≅ TA(MAX) + [RON + TA(MAX) – TA)(0.005/ºC)(RON)]

x I2 x Rθ(JA)

TJ ≅ 55ºC + [17mΩ + (55ºC-25ºC)(0.005)(17mΩ)]

MIC2582/MIC2583

x (2.5A)2 x (50ºC/W)

TJ ≅ (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 TJ equal to the already calculated value

of 61.1°C:

TJ ≅ TA + [17mΩ + (61.1ºC-25ºC)(0.005)(17mΩ)]

x (2.5A)2 x (50ºC/W)

TJ ≅ (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 TJ to get TJ(TRANSIENT MAX.) = 106ºC. This is an

acceptable maximum junction temperature for this part.

Figure 10. Transient Thermal Impedance

April 2009

21

M9999-043009-C

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