MIC2582 datasheet(21/32 Pages) MICROCHIP

21 / 32 page



2021 Microchip Technology Inc.

DS20006573A-page 21

MIC2582/3

Taking the simplest case first, we’ll assume that once a

fault event such as the one in question occurs, it will be

a long time—ten minutes or more—before the fault is

isolated and the channel is reset. In such a case, we

can approximate this as a “single pulse” event, that is

to say, there’s no significant duty cycle. Then, reading

up from the X-axis at the point where “Square Wave

Pulse Duration” is equal to 0.1 sec (100 ms), we see

that the Zθ(JA) of this MOSFET to a highly infrequent

event of this duration is only 8% of its continuous

Rθ(JA).

This particular part is specified as having an Rθ(JA) of

50°C/W for intervals of 10 seconds or less.

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) = 17 mΩ 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. Because 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:

EQUATION 5-11:

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:

EQUATION 5-12:

So our original approximation of 61.1°C was very close

to the correct value. We will use TJ = 61°C.

Finally, add the temperature increase due to the

maximum power dissipation calculated from a “single

event”, (11.25W)(50°C/W)(0.08) = 45°C to the

steady-state TJ to get TJ(TRANSIENT MAX) = 106°C. This

is an acceptable maximum junction temperature for this

part.

FIGURE 5-5:

Transient Thermal Impedance.

TJ TA MAX









TJ TA MAX





RON TA MAX



 T

–





0.005/C





RON













RJA



TJ 55C 17m 55C 25C

–





0.005





17m











2.5A2



50C/W



TJ 55C 0.122W 50C/W







61.1C



TJ TA 17m 61.1C 25C

–





0.005





17m











2.5A2



50C/W



TJ 55C 0.125W 50C/W







61.27C



Normalized Thermal Transient Impedance, Junction-to-Ambient

Square Wave Pulse Duration (sec)

0.1

0.01

10–4

10–3

10–2

10–1

110

0.2

0.1

0.05

0.02

Single Pulse

Duty Cycle = 0.5

1. Duty Cycle, D =

2. Per Unit Base = R thJA = 50

°C/W

3. TJM – T A = P DMZthJA (t)

Notes:

4. Surface Mounted

P DM

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