MIC2590B(2002) データシートの表示（PDF） - Micrel
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MIC2590B Datasheet PDF : 24 Pages
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MIC2590B
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 imped-
ance 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 signifi-
cantly increase the dissipation in the Si4430DY MOSFET. Will
Micrel
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:
PRMS = (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
Power Supply Decoupling
In general, prudent system design requires that power sup-
plies used for logic functions should have less than 100mV of
noise at frequencies of 100kHz and above. This is especially
true given the speeds of moden logic families, such as the 1.2
micron CMOS used in the MIC2590B. In particular, the –12V
supply should have less than 100mV of peak-to-peak noise
at frequencies of 1MHz or higher. This is because the –12V
supply is the most negative potential applied to the IC, and is
therefore connected to the device's substrate. All of the
subcircuits integrated onto the silicon chip are hence sub-
jected by capacitives coupling to any HF noise on the –12V
supply. While individual capacitances are quite low, the
amount of injected energy required to cause a "glitch" can
also be quite low at the internal nodes of high speed logic
circuits.
Less obviously, but equally important, is the fact that the
internal charge pump for the 3.3VAUX supplies is somewhat
susceptible to noise on the +12V input when that input is at or
near zero volts. The +12V supply should not carry HF noise
in excess of 200mV peak-to-peak with respect to chip gound
when it is in the "off" state.
If either the –12V input, the +12V input, of both supplies do
carry significant HF noise (as can happen when they are
locally derived by a switching converter), the solution is both
small and inexpensive. An LC filter made of a ferrite bead
between the noisy power supply input and the MIC2590B,
August 2002
21
MIC2590B
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