MIC2590B_08 Datasheet, PDF (21/23 Pages) Micrel Semiconductor – Dual-Slot PCI Hot Plug Controller

Micrel, Inc.
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
R_θ(JA).
This particular part is specified as having an R_θ(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 T_A = 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)T_J is:
T_J ≅ 55°C + (7.3mΩ)(5A)²(30°C/W)
T_J ≅ 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 T_J equal to the already calculated value
of 67°C:
R_ON at T_J = 60.5°C =[1+(60.5°C–25°)(0.5%/°C)]×6.35mΩ
R_ON at T_J = 60.5°C ≅ 7.48mΩ
T_J ≅ 55°C + (7.3mΩ)(5A)²(30°C/W)
T_J ≅ 60.6°C
At this point, the simplest thing to do is to approximate T_J
as 61°C, which will be close enough for all practical
purposes.
MIC2590B
Finally, add (10W)(67°C/W)(0.03) = 21°C to the steady-
state T_J to get T_{J(TRANSIENT MAX)} = 82°C. 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_θ(JA) = (0.7 x 67°C/W) = 4.7°C/W.
Calculating the peak junction temperature:
T_{J(PEAK MAX)} = [(10W)(4.7°C/W) + 55°C] = 102°C
And finally, checking the RMS power dissipation just to
be complete:
P_RMS = (5A)² (7.47mΩ) 0.05 = 0.042W
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
Vishay (Siliconix)
International Rectifier
Fairchild Semiconductor
Key MOSFET Type(s)
Si4430DY (“LittleFoot” Series)
Si4420DY (“LittleFoot” Series)
IRF7413A (SO-8 package part)
Si4420DY (second source to Vishay)
FDS6644 (SO-8 package part)
FDS6670A (SO-8 package part)
FDS6688 (SO-8 package part)
Web Address
www.siliconix.com
www.irf.com
www.fairchildsemi.com
Resistor Vendors
Vishay (Dale)
IRC
Sense Resistors
“WSL” Series
“OARS” Series
“LR” Series
(second source to “WSL”)
Web Address
www.vishay.com/docs/wsl_30100.pdf
irctt.com/pdf_files/OARS.pdf
irctt.com/pdf_files/LRC.pdf
September 2008
21
M9999-091808