Micrel, Inc. MIC2590B
September 2008
19
M9999-091808
The second breakdown voltage criteria which must be
met is a bit subtler than simple drain-source breakdown
voltage, but is not hard to meet. Low-voltage MOSFETs
generally have low breakdown voltage ratings from gate
to source as well. In MIC2590B applications, the gates of
the external MOSFETs are driven from the +12V input to
the IC. That supply may well be at 12V + (5% x 12V) =
12.6V. At the same time, if the output of the MOSFET (its
source) is suddenly shorted to ground, the gate-source
voltage will go to (12.6V – 0V) = 12.6V. This means that
the external MOSFETs must be chosen to have a gate-
source breakdown voltage in excess of 13V; after 12V
absolute maximum the next commonly available voltage
class has a permissible gate-source voltage of 20V
maximum. This is a very suitable class of device. 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, look to surface mount
devices with a drain-source rating of 30V as a starting
point.
MOSFET Maximum On-State Resistance
The MOSFETs in the +3.3V and +5V MAIN power paths
will have a finite voltage drop, which must be taken into
account during component selection. A suitable
MOSFET’s datasheet will almost always give a value of
on resistance 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 device with 7
Volts of enhancement (keep this in mind; we’ll use it in the
following Thermal Issues sections). The resulting value is
conservative, but close enough. Call this value R
ON
. Since
a heavily enhanced MOSFET acts as an ohmic (resistive)
device, almost all that is required to calculate the voltage
drop across the MOSFET is to multiply the maximum
current times the MOSFET’s R
ON
. 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, the Vishay
(Siliconix) Si4430DY, which is a commonly used part in
this type of application, has a specified R
DS(ON)
of 8.0m
max. at V
G-S
= 4.5V, and R
DS(ON)
of 4.7m max. at V
G-S
=10V. Then R
ON
is calculated as:
()
6.35m
2
8.0m4.7m
R
ON
=
+
=
at 25°C T
J
. If the actual junction temperature is estimated
to be 110°C, a reasonable approximation of R
ON
for the
Si4430DY at temperature is:
() ()
9.05m
C
0.5%
8516.35m
C
0.5%
2510016.35m ≅
⎥
⎦
⎤
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
°
°+=
⎥
⎦
⎤
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
°
°−°+
Note that this is not a closed-form equation; if more
precision were required, several iterations of the
calculation might be necessary. This is demonstrated in
the section “MOSFET Transient Thermal Issues.”
For the given case, if Si4430DY is operated at an I
DRAIN
of
7.6A, the voltage drop across the part will be
approximately (7.6A)(9.05m) = 69mV.
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.
• 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?).
Now it gets easy: steady-state power dissipation is found
by calculating I
2
R. As noted in “MOSFET Maximum On-
State Resistance,” above, the one further concern is the
MOSFET’s increase in R
ON
with increasing die
temperature. Again, use the Si4430DY MOSFET as an
example, and assume that the actual junction temperature
ends up at 110°C. Then R
ON
at temperature is again
approximately 9.05m. Again, allow a maximum I
DRAIN
of
7.6A:
()
0.523W9.05m7.6ARInDissipatio Power
2
ON
2
DRAIN
≅×=×≅
The next 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. Formally put, the steady-state electrical
model of power dissipated at the MOSFET junction is
analogous to a current source, and anything in the path of
that power being dissipated as heat into the environment
is analogous to a resistor. It’s therefore necessary to
verify that the thermal resistance from the junction to the
ambient is equal to or lower than that value of thermal
resistance (often referred to as R
(JA)
) for which the
operation of the part is guaranteed. As an applications
issue, surface mount MOSFETs are often less than
ideally specified in this regard—it’s become common
practice simply to state that the thermal data for the part is
specified under the conditions “Surface mounted on FR-4
board, t10seconds,” or something equally mystifying. So
here are a few practical tips:
1. The heat from a surface mount device such as an
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 of copper the
copper will act as the heat sink for the part. This
copper must be on the same layer of the board as
