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MIC2590B-2YTQ Datasheet(PDF) 21 Page - Micrel Semiconductor

Part No. MIC2590B-2YTQ
Description  Dual-Slot PCI Hot Plug Controller
Download  23 Pages
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Maker  MICREL [Micrel Semiconductor]
Homepage  http://www.micrel.com
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MIC2590B-2YTQ Datasheet(HTML) 21 Page - Micrel Semiconductor

 
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Micrel, Inc.
MIC2590B
September 2008
21
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
5A
P
2
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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