MIC2590B-5BTQ Micrel Inc, MIC2590B-5BTQ Datasheet - Page 22

• Manufacturer Part Number: MIC2590B-5BTQ
• Description: IC PCI HOT PLUG CTLR DUAL 48TQFP
• Manufacturer: Micrel Inc
• Type: Hot-Swap Controllerr

Specifications of MIC2590B-5BTQ

• Applications: PCI, PCI-X
• Internal Switch(s): No
• Voltage - Supply: 3.3V, 5V, ±12V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 48-TQFP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Noisy V
Power Supply Decoupling
In general, prudent system design requires that power
supplies 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 1.2micron 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 subjected by
capacities 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
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
MIC2590B, followed by a "composite capacitor" from the
affected MIC2590B 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 each 1µF and 0.01µF
ceramic
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 will also
work well, or a 0.27µH, air-core coil can be used.
It is theoretically possible that high-amplitude, HF noise
reflected back into one or both of the MIC2590B’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 11 will provide the
required HF bypassing. The capacitors would be
connected to the MIC2590B’s –12V output pins, and the
ferrite beads would be placed between the –12V output
pins and the loads.
Micrel, Inc.
September 2008
IN
Fair-Rite Products
Type 2743019447
SMT Ferrite Bead
Figure 11. Filter Circuit for Noisy Supplies
capacitors
(+3.3V and/or –12V)
for
47µF
Tantalum
high-frequency
1µF
Ceramic
AUX
10nF
Ceramic
supplies is
bypass.
To MIC2590B
A
22
–12V Input Clamp Diode
The –12V input to the MIC2590B is the most negative
potential on the part, and is therefore connected to the
chip’s substrate (as described in “Power Supply
Decoupling,” above). Although no particular sequencing of
the –12V supply relative to the other MIC2590B supplies
is required for normal operation, 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. In some systems, even though the –12V supply will
discharge towards ground potential when it is turned OFF,
the possibility exists that 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. The simplest way to deal with
such a transient is to clamp it with a Schottky diode. The
diode’s a node should be physically placed directly at the
–12V input to the MIC2590B, 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 ON Semiconductor’s
type MBRS140T3 (1A, 40V). Although the 40V rating of
this part is a bit gratuitous, it is an inexpensive industry-
standard part with many second sources. Unless it is
absolutely known in advance that the voltage on the “–
12V” inputs will never exceed 0.0V at the IC’s –12V input
pins, it’s wise to at least leave a position for this diode in
the board layout and then remove it later. This final
determination should be made by observations of the
voltage at the –12V input with a fast storage oscilloscope,
under turn-on and turn-off conditions.
Gate Resistor Guidelines
The MIC2590B controls four external power MOSFETs,
which handle the high currents for each of the two 3.3V
and 5V outputs. A capacitor (C
application circuit from each GATE pin of the MIC2590B
to ground. Each C
respective power output (e.g., 5V
which are typically in the 10nF range, cause the GATE
outputs of the MIC2590B to have very low AC
impedances to ground at any significant frequency. It is
therefore necessary to place a modest value of gate
damping resistance (R
gate of its associated MOSFET. These resistances
prevent high-frequency MOSFET source-follower oscilla-
tions from occurring. The exact value of the resistors used
is not critical; 47Ω is usually a good choice. Each R
should be physically located directly adjacent to the
MOSFET gate lead to which it connects.
Figure 12. Proper Connection of C
MIC2590B
GATE
GATE
GATE
controls the ramp-up rate of its
C
GATE
) between each C
R
GATE
GATE
OUTB
) is connected in the
). These capacitors,
GATE
External
MOSFET
M9999-091808
and R
GATE
MIC2590B
GATE
and the
GATE