MIC2590B-5BTQ TR【PDF 21/23 页】IC PCI HOT PLUG CTLR DUAL 48TQFP

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Sheet目录45

17页 18页 19页 20页 [21页]22页 23页

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, well 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, theres 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

?JA)

This particular part is specified as having an R

?JA)

50癈/W for intervals of 10 seconds or less. So, some

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

Assume T

= 55癈 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)T

is:

E 55癈 + (7.3m&)(5A)

(30癈/W)

E 60.5癈

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

equal to the already calculated value

of 67癈:

at T

= 60.5癈 =[1+(60.5癈25?(0.5%/癈)]?.35m&

at T

= 60.5癈 E 7.48m&

E 55癈 + (7.3m&)(5A)

(30癈/W)

E 60.6癈

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

as 61癈, which will be close enough for all practical

purposes.

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

state T

to get T

J(TRANSIENT MAX)

= 82癈. The Si4430DY

can easily handle this value of T

J(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 R

?JA)

= (0.7 x 67癈/W) = 4.7癈/W.

Calculating the peak junction temperature:

J(PEAK MAX)

= [(10W)(4.7癈/W) + 55癈] = 102癈

And finally, checking the RMS power dissipation just to

be complete:

)

0.042W

0.05

7.47m&

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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