MIC2590B Micrel Semiconductor, MIC2590B Datasheet - Page 21

• Manufacturer Part Number: MIC2590B
• Description: Dual-Slot PCI Hot Plug Controller
• Manufacturer: Micrel Semiconductor

Assume the MOSFET has been carrying just about 5A for
some time.
Then the starting (steady-state)T
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
At this point, the simplest thing to do is to approximate T
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 T
to get T
handle this value of T
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
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
August 2002
MIC2590B
T
T
R
R
T
T
MOSFET Vendors
Vishay (Siliconix)
International Rectifier
Fairchild Semiconductor
Resistor Vendors
Vishay (Dale)
IRC
J
J
J
J
ON
ON
J(TRANSIENT MAX)
≅ 55°C + (7.3mΩ)(5A)
≅ 60.5°C
≅ 55°C + (7.3mΩ)(5A)
≅ 60.6°C
at T
at T
J
equal to the already calculated value of 67°C:
J
J
= 60.5°C = [1+(60.5°C–25°)(0.5%/°C)]×6.35mΩ
= 60.5°C ≅ 7.48mΩ
J(MAX)
= 82°C. The Si4430DY can easily
.
2
2
(30°C/W)
(30°C/W)
J
is:
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)
Sense Resistors
“WSL” Series
“OARS” Series
“LR” Series
(second source to “WSL”)
J
as
J
21
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
Calculating the peak junction temperature:
And finally, checking the RMS power dissipation just to be
complete:
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.
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,
T
P
J(PEAK MAX)
RMS
θ(JA)
=
( ) (
5A
= (0.7 x 67°C/W) = 4.7°C/W.
2
Web Address
www.siliconix.com
www.irf.com
www.fairchildsemi.com
Web Address
www.vishay.com/docs/wsl_30100.pdf
irctt.com/pdf_files/OARS.pdf
irctt.com/pdf_files/LRC.pdf
= [(10W)(4.7°C/W) + 55°C] = 102°C
7.47m
Ω
)
0.05
=
0.042W
MIC2590B
Micrel