Micrel, Inc. MIC2593
September 2008
25
M9999-092208
–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). Although no particular sequencing of
the –12V supply relative to the other MIC2593 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. 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 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).
Gate Resistor Guidelines
The MIC2593 controls four external power MOSFETs,
that handle the high currents for each of the two 3.3V and
5V outputs. A capacitor is connected in the application
circuit from each GATE pin of the MIC2593 to ground.
However, an external capacitor, C
GATE
, is not required for
operation of the MIC2593. 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 MIC2593 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 as shown in Figure 15.
These resistances prevent high-frequency MOSFET
source-follower oscillations from occurring. The exact
value of the resistors used is not critical; 10 to 33 is
usually a sufficient choice. Each R
GATE
should be
physically located directly adjacent to the MOSFET gate
lead to which it connects.
MIC2593
GATE
R
GATE
C
GATE
10nF
External
MOSFET
Figure 15. C
GATE
and R
GATE
Connection
