MIC2582_14 Datasheet, PDF (20/27 Pages) Micrel Semiconductor – Single-Channel Hot Swap Controllers

Micrel, Inc.
MIC2582/MIC5283
MOSFET Selection
Selecting the proper external MOSFET for use with the
MIC2582/83 involves three straightforward tasks.
 The choice of a MOSFET that meets minimum voltage
requirements.
 The selection of a device to handle the maximum
continuous current (steady-state thermal issues).
 Verification of the selected part’s ability to withstand
any peak currents (transient thermal issues).
MOSFET Voltage Requirements
The first voltage requirement for the MOSFET is easily
stated: the drain-source breakdown voltage of the
MOSFET must be greater than V_IN(MAX). For instance, a
12V input may reasonably be expected to see high-
frequency transients as high as 18V. Therefore, the drain-
source breakdown voltage of the MOSFET must be at
least 19V. For ample safety margin and standard
availability, the closest value will be 20V.
The second breakdown voltage criterion that must be met
is a bit subtler than simple drain-source breakdown
voltage, but is not hard to meet. In MIC2582/83
applications, the gate of the external MOSFET is driven up
to approximately 19.5V by the internal output MOSFET
(again, assuming 12V operation).
At the same time, if the output of the external MOSFET (its
source) is suddenly subjected to a short, the gate-source
voltage will go to (19.5V – 0V) = 19.5V. This means that
the external MOSFET must be chosen to have a gate-
source breakdown voltage of 20V or more, which is an
available standard maximum value. However, if operation
is at or above 13V, the 20V gate-source maximum will
likely be exceeded. As a result, an external Zener diode
clamp should be used to prevent breakdown of the
external MOSFET when operating at voltages above 8V. A
Zener diode with 10V rating is recommended as shown in
Figure 8. At the present time, most power MOSFETs with
a 20V gate-source voltage rating have a 30V drain-source
breakdown rating or higher.
As a general tip, choose surface-mount devices with a
drain-source rating of 30V as a starting point.
Finally, the external gate drive of the MIC2582/83 requires
a low-voltage logic level MOSFET when operating at
voltages lower than 3V. There are 2.5V logic level
MOSFETs available. Please see Table 4 “MOSFET and
Sense Resistor Vendors” for suggested manufacturers.
May 23, 2014
Figure 8. Zener-Clamped MOSFET Gate
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Revision 5.0