MIC2085 Datasheet, PDF (23/29 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
MOSFET Voltage Requirements
The first voltage requirement for the MOSFET is that the
drain-source breakdown voltage of the MOSFET must
be greater than V_IN(MAX). For instance, a 16V input may
reasonably be expected to see high-frequency transients
as high as 24V.Therefore, the drain-source breakdown
voltage of the MOSFET must be at least 25V. For ample
safety margin and standard availability, the closest
minimum value should be 30V.
The second breakdown voltage criterion that must be
met is a bit subtler than simple drain-source breakdown
voltage. In MIC2085/86 applications, the gate of the
external MOSFET is driven up to a maximum of 21V by
the internal output MOSFET. 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
(21V – 0V) = 21V. Since most power MOSFETs
generally have a maximum gate-source breakdown of
20V or less, the use of a Zener clamp is recommended
in applications with V_CC ≥ 8V. A Zener diode with 10V to
12V rating is recommended as shown in Figure11. At the
present time, most power MOSFETs with a 20V gate-
source voltage rating have a 30V drain-source break-
down rating or higher. As a general tip, choose surface-
mount devices with a drain-source rating of 30V or more
as a starting point.
Finally, the external gate drive of the MIC2085/86
requires a low-voltage logic level MOSFET when
operating at voltage slower than 3V. There are 2.5V
logic-level MOSFETs avail-able. Please see Table 4,
“MOSFET and Sense Resistor Vendors” for suggested
manufacturers.
MOSFET Steady-State Thermal Issues
The selection of a MOSFET to meet the maximum
continuous current is a fairly straightforward exercise.
First, arm yourself with the following data:
• The value of I_{LOAD(CONT, MAX.)} for the output in
question (see “Sense Resistor Selection”).
• The manufacturer’s data sheet for the candidate
MOSFET.
MIC2085/2086
• The maximum ambient temperature in which the
device will be required to operate.
• Any knowledge you can get about the heat
sinking available to the device (e.g., can heat be
dissipated into the ground plane or power plane,
if using a surface-mount part? Is any airflow
available?).
The data sheet will almost always give a value of on
resistance given for the MOSFET at a gate-source
voltage of 4.5V, and another value at a gate-source
voltage of 10V. As a first approximation, add the two
values together and divide by two to get the on-
resistance of the part with 8V of enhancement. Call this
value R_ON. Since a heavily enhanced MOSFET acts as
an ohmic (resistive) device, almost all that’s required to
determine steady-state power dissipation is to calculate
I2R.The one addendum to this is that MOSFETs have a
slight increase in R_ON with increasing die temperature. A
good approximation for this value is 0.5% increase in
R_ON per °C rise in junction temperature above the point
at which R_ON was initially specified by the manufacturer.
For instance, if the selected MOSFET has a calculated
R_ON of 10mΩ at a T_J = 25°C, and the actual junction
temperature ends up at 110°C, a good first cut at the
operating value for R_ON would be:
R_ON ≅ 10mΩ[1 + (110 - 25)(0.005)] ≅14.3mΩ
The final step is to make sure that the heat sinking
available to the MOSFET is capable of dissipating at
least as much power (rated in °C/W) as that with which
the MOSFET’s performance was specified by the
manufacturer. Here are a few practical tips:
1. The heat from a surface-mount device such a
san SO-8 MOSFET flows almost entirely out of
the drain leads. If the drain leads can be
soldered down to one square inch or more, the
copper will act as the heat sink for the part. This
copper must be on the same layer of the board
as the MOSFET drain.
May 2006
23
M9999-050406
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