MIC2590B Datasheet, PDF (22/24 Pages) Micrel Semiconductor – Dual-Slot PCI Hot Plug Controller

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 capacitors
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 will also work well, or a 0.27µH, air-core
coil can be used.
Noisy V_IN
SMT Ferrite Bead
Fair-Rite Products
Type 2743019447
To MIC2590
47µF
1µF
10nF
Tanalum Ceramic Ceramic
Figure 11. 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 MIC2590B’s –12V
outputs could interfere with proper device operation, al-
though 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.
–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 rela-
tive 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 poten-
tial 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
Micrel
with such a transient is to clamp it with a Schottky diode. The
diode’s anode 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_GATE) is connected in the applica-
tion circuit from each GATE pin of the MIC2590B to ground.
Each C_GATE controls the ramp-up rate of its respective power
output (e.g., 5VOUTB). These capacitors, 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) between each C_GATE
and the 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_GATE should
be physically located directly adjacent to the MOSFET gate
lead to which it connects.
MIC2590B
GATE
R_GATE
47Ω
C_GATE
External
MOSFET
Figure 12. Proper Connection
of C_GATE and R_GATE
MIC2590B
22
August 2002