MIC2593 datasheet(24/26 Pages) STMICROELECTRONICS

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MIC2593

Micrel

M9999-042204

April 2004

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)

www.fairchildsemi.com

FDS6670A (SO-8 package part)

FDS6688 (SO-8 package part)

Resistor Vendors

Sense Resistors

Web Address

Vishay (Dale)

WSL Series

WSL 3637 Series

www.vishay.com/docs/wsl_30100.pdf

IRC

OARS Series

irctt.com/pdf_files/OARS.pdf

LR Series

irctt.com/pdf_files/LRC.pdf

(second source to WSL)

Table 5. Power MOSFET and Resistor Values

MOSFET and Sense Resistor Selection Guide

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.

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. 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 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.

Less obviously, but equally important, is the fact that the

internal charge pump for the 3.3V

AUX 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 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,

followed by a "composite capacitor" from the affected MIC2593

input pin to ground, will suffice for almost any situation. A

good composite capacitor for this purpose is the parallel

combination of a 47

µF tantalum bulk decoupling capacitor,

and one 1

µF 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 0.27

µH air-core coil can also be used.

Noisy VIN

To MIC2593

SMT Ferrite Bead

Fair-Rite Products

Type 2743019447

µF

Tanalum

µF

Ceramic

10nF

Ceramic

Figure 14. Filter Circuit for Noisy Supplies

(+3.3V and/or –12V)

It is theoretically possible that high-amplitude, HF noise

reflected back into one or both of the MIC2593’s –12V outputs

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.

–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, this substrate connection does mean that the –12V input

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.

addition, power supply output ringing or L(di/dt) effects in the

wiring and on the PCB itself will cause brief transient voltages

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

SMT part for this application is a type MBRS140T3 (1A, 40V).

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