MIC2593
Micrel
must be greater than V
for the slot in question. For
calculated R of 10mΩ at T = 25°C and the actual junction
IN(MAX)
ON
J
instance, the 5V input may reasonably be expected to see
high-frequency transients as high as 6.5V. Therefore, the
drain-source breakdown voltage of the MOSFET must be at
least 7V.
temperature ends up at 110°C, a good first cut at the operat-
ing value for R would be:
ON
R
10mΩ[1 + (110 – 25)(0.005)] 14.3mΩ
ON
2
Next, approximate the steady-state power dissipation (I R)
The second breakdown voltage criteria which must be met is
abitsubtlerthansimpledrain-sourcebreakdownvoltage, but
is not hard to meet. Low-voltage MOSFETs generally have
low breakdown voltage ratings from gate to source as well. In
MIC2593 applications, the gates of the external MOSFETs
are driven from the +12V input to the MIC2593 controller.
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
MOSFETsmustbechosentohaveagate-sourcebreakdown
voltage in excess of 13V; after 12V absolute maximum, the
next commonly available voltage class has a 20V maximum
gate-source voltage. 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.
using I
and the approximated R
.
LOAD(CONT,max)
ON
2
P
[I
] × R
D
LOAD(CONT, MAX)
ON
2
(8.93A) × 14.3mΩ 1.14W
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 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, the copper trace
will act as the heat sink for the part. This copper
tracemustbeonthesamelayeroftheboardasthe
MOSFET drain.
2. Airflow works. Even a few LFM (linear feet per
minute) of air will cool a MOSFET down substan-
tially. If you can, position the MOSFET(s) near the
inlet of a power supply’s fan, or the outlet of a
processor’s cooling fan.
MOSFET Steady-State Thermal Issues
TheselectionofaMOSFETtomeetthemaximumcontinuous
current is a fairly straightforward exercise. First, arm yourself
with the following data:
3. The best test of a surface-mount MOSFET for an
application (assuming the above tips show it to be
alikelyfit)isanempiricalone.ChecktheMOSFET's
temperature in the actual layout of the expected
final circuit, at full operating current. The use of a
thermocouple on the drain leads, or infrared py-
rometer on the package, will then give a reason-
able idea of the device’s junction temperature.
• The value of I
for the output in
LOAD(CONT, MAX)
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 dissi-
pated into the ground plane or power plane if using
a surface mount part? Is any airflow available?).
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will, a) withstand both the
appliedvoltagestresses,andb)handletheworst-casecontinu-
2
ous I R power dissipation that it will endure; verifying the
The data sheet will almost always give a value of on resis-
tancegivenfortheMOSFETatagate-sourcevoltageof4.5V,
and another value at a gate-source voltage of 10V. As a first
approximation, addthetwovaluestogetheranddividebytwo
to get the on-resistance of the part with 7V to 8V of enhance-
MOSFET’sabilitytohandleshort-termoverloadpowerdissipa-
tion without overheating is the lone item to be determined. A
MOSFET can handle a much higher pulsed power without
damage than its continuous dissipation ratings would imply.
The reason for this is that thermal devices (silicon die, lead
frames, etc.) have thermal inertia.
ment (11.5V nominal V
minus the 3.5V to 4.5V gate
GATE
threshold of the MOSFET). Call this value R . Since a
ON
In terms related directly to the specification and use of power
MOSFETs, this is known as “transient thermal impedance.”
Almost all power MOSFET data sheets give a Transient
Thermal Impedance Curve. For example, take the case
heavily enhanced MOSFET acts as an ohmic (resistive)
device, almost all that’s required to determine steady-state
2
power dissipation is to calculate I R. The one addendum to
this is that MOSFETs have a slight increase in R
with
ON
where t
I
for the 5V supply has been set to 50ms,
is 5.0A, the slow-trip threshold is 50mV
increasing die temperature. A good approximation for this
FLT
valueis0.5%increaseinR per°Criseinjunctiontempera-
LOAD(CONT, MAX)
ON
nominal, and the fast-trip threshold is 100mV. If the output is
connected to a 0.60Ω load, the output current from the
MOSFET for the slot in question will be regulated to 5.0A for
tureabovethepointatwhichR wasinitiallyspecifiedbythe
ON
manufacturer. For instance, if the selected MOSFET has a
M9999-042204
22
April 2004
STMICROELECTRONICS [ ST ]
