Micrel, Inc. MIC2590B
September 2008
22
M9999-091808
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
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
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
To MIC2590B
SMT Ferrite Bead
Fair-Rite Products
Type 2743019447
47µF
Tantalum
1µF
Ceramic
10nF
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,
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.
–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
GATE
) is connected in the
application 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., 5V
OUTB
). 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; 47 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
C
GATE
External
MOSFET
Figure 12. Proper Connection of C
GATE
and R
GATE
