MIC2593
Micrel
MOSFET and Sense Resistor Selection Guide
Noisy VIN
To MIC2593
Listed in Table 5 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 compre-
hensive, this information will constitute a good starting point
for most designs.
SMT Ferrite Bead
Fair-Rite Products
Type 2743019447
47µF
Tanalum
1µF
Ceramic
10nF
Ceramic
Figure 14. Filter Circuit for Noisy Supplies
(+3.3V and/or –12V)
Power Supply Decoupling
It is theoretically possible that high-amplitude, HF noise
reflectedbackintooneorbothoftheMIC2593’s–12Voutputs
could interfere with proper device operation, although such
noisy loads are unlikely to occur in the real world. If this
becomes an application-specific concern, a pair of filters
similar to that in Figure 14 will provide the required HF
bypassing. The capacitors would be connected to the
MIC2593’s –12V output pins, and the ferrite beads would be
placed between the –12V output pins and the loads.
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. 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,
andisthereforeconnectedtothedevice'ssubstrate. Allofthe
subcircuits integrated onto the silicon chip are hence sub-
jected by capacitive 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.
–12V Input Clamp Diode
The –12V input to the MIC2593 is the most negative potential
on the part and is therefore connected to the chip’s substrate
(as described in “Power Supply Decoupling,” above). Al-
though no particular sequencing of the –12V supply relative
to the other MIC2593 supplies is required for normal opera-
tion, thissubstrateconnectiondoesmeanthatthe–12Vinput
must never exceed the voltage on the GROUND pin of the IC
by more than 0.3 volts. Small amounts of internal leakage
current can cause this to happen when the VSTBY pins are
energized and the 12MVIN pins are not energized. In
addition, power supply output ringing or L(di/dt) effects in the
wiringandonthePCBitselfwillcausebrieftransientvoltages
in excess of +0.3V to appear at the –12V input. For this
reason, it is required to clamp the –12V input to ground with
a Schottky diode. A diode rated at 1 amp and 20V to 40V as
shown in our application schematic diagram is suggested.
The diode’s anode should be physically placed directly at the
–12V input to the MIC2593, and its cathode should have as
short a path as possible back to the part’s ground. A good
SMTpartforthisapplicationisatypeMBRS140T3(1A, 40V).
Less obviously, but equally important, is the fact that the
internal charge pump for the 3.3V
supplies is somewhat
AUX
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 ground
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 MIC2593,
followedbya"compositecapacitor"fromtheaffectedMIC2593
input pin to ground, will suffice for almost any situation. A
good composite capacitor for this purpose is the parallel
combination of a 47µF tantalum bulk decoupling capacitor,
and one 1µF and one 0.01µF ceramic capacitor for high-
frequency bypass. A suggested ferrite bead for such use is
Fair-Rite Products Corporation part number 2743019447
(this is a surface-mountable part). Similar parts from other
vendors or a 0.27µH air-core coil can also be used.
MOSFET Vendors
Key MOSFET Type(s)
Web Address
®
Vishay (Siliconix)
Si4430DY (“LITTLE FOOT ” Series)
www.siliconix.com
®
Si4420DY (“LITTLE FOOT ” Series)
International Rectifier
IRF7413A (SO-8 package part)
www.irf.com
Si4420DY (second source to Vishay)
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
WSL 3637 Series
www.vishay.com/docs/wsl_30100.pdf
IRC
OARS Series
LR Series
irctt.com/pdf_files/OARS.pdf
irctt.com/pdf_files/LRC.pdf
(second source to WSL)
Table 5. Power MOSFET and Resistor Values
M9999-042204
24
April 2004
STMICROELECTRONICS [ ST ]
