MIC2590B-2YTQ datasheet(21/23 Pages) MICREL | Dual-Slot PCI Hot Plug Controller

21 / 23 page

Micrel, Inc.

MIC2590B

September 2008

M9999-091808

Using this graph is not nearly as daunting as it may at

first appear. Taking the simplest case first, we’ll assume

that once a fault event such as the one in question

occurs, it will be along time, 10 minutes or more, before

the fault is isolated and the slot 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 0.1sec (=100ms), we see that the

effective thermal impedance of this MOSFET to a single

pulse event of this duration is only 6% 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. So, some

further math, just to get things ready for the finale:

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

the drain leads, no airflow.

Assume the MOSFET has been carrying just about 5A

for some time.

Then the starting (steady-state)TJ is:

TJ ≅ 55°C + (7.3mΩ)(5A)

2(30°C/W)

TJ ≅ 60.5°C

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 67°C:

RON at TJ = 60.5°C =[1+(60.5°C–25°)(0.5%/°C)]×6.35mΩ

RON at TJ = 60.5°C ≅ 7.48mΩ

TJ ≅ 55°C + (7.3mΩ)(5A)

2(30°C/W)

TJ ≅ 60.6°C

At this point, the simplest thing to do is to approximate TJ

as 61°C, which will be close enough for all practical

purposes.

Finally, add (10W)(67°C/W)(0.03) = 21°C to the steady-

state TJ to get TJ(TRANSIENT MAX) = 82°C. The Si4430DY

can easily handle this value of TJ(MAX).

A second illustration of the use of the transient thermal

impedance curves: assume that the system will attempt

multiple retries on a slot showing a fault, with a one

second interval between retry attempts. This frequency

of restarts will significantly increase the dissipation in the

Si4430DY MOSFET. Will the MOSFET be able to handle

the increased dissipation? We get the following:

The same part is operating into a persistent fault, so it is

cycling in a square-wave fashion (no steady-state load)

with a duty cycle of (50msec/second = 0.05).

On the Transient Thermal Impedance Curves, read up

from the X-axis to the line showing Duty Cycle equaling

0.05. The effective Rθ(JA) = (0.7 x 67°C/W) = 4.7°C/W.

Calculating the peak junction temperature:

TJ(PEAK MAX) = [(10W)(4.7°C/W) + 55°C] = 102°C

And finally, checking the RMS power dissipation just to

be complete:

( ) (

)

0.042W

0.05

7.47mΩ

RMS

which will result in a negligible temperature rise.

The Si4430DY is electrically and thermally suitable for

this application.

MOSFET and Sense Resistor Selection Guide

Listed below, by Manufacturer and Type Number, are

some of the more popular MOSFET and resistor types

used in PCI hot plug applications. Although far from

comprehensive, this information will constitute a good

starting point for most designs.

MOSFET Vendors

Key MOSFET Type(s)

Web Address

Vishay (Siliconix)

Si4430DY (“LittleFoot” Series)

Si4420DY (“LittleFoot” Series)

www.siliconix.com

International Rectifier

IRF7413A (SO-8 package part)

Si4420DY (second source to Vishay)

www.irf.com

Fairchild Semiconductor

FDS6644 (SO-8 package part)

FDS6670A (SO-8 package part)

FDS6688 (SO-8 package part)

www.fairchildsemi.com

Resistor Vendors

Sense Resistors

Web Address

Vishay (Dale)

“WSL” Series

www.vishay.com/docs/wsl_30100.pdf

IRC

“OARS” Series

“LR” Series

(second source to “WSL”)

irctt.com/pdf_files/OARS.pdf

irctt.com/pdf_files/LRC.pdf

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