Micrel, Inc.
MIC2582/MIC2583
April 2009
19
M9999-043009-C
MOSFET Selection
Selecting the proper external MOSFET for use with the
MIC2582/83 involves three straightforward tasks:
" Choice of a MOSFET which meets minimum voltage
requirements.
" Selection of a device to handle the maximum
continuous current (steady-state thermal issues).
" Verify the selected parts 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 9. 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.
Figure 9. Zener Clamped MOSFET Gate
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