MIC2593 Micrel Semiconductor, MIC2593 Datasheet - Page 22

• Manufacturer Part Number: MIC2593
• Description: Dual-Slot PCI Hot Plug Controller
• Manufacturer: Micrel Semiconductor

MIC2593
must be greater than V
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.
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
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
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 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.
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 data sheet will almost always give a value of on resis-
tance 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 7V to 8V of enhance-
ment (11.5V nominal V
threshold of the MOSFET). Call this value R
heavily enhanced MOSFET acts as an ohmic (resistive)
device, almost all that’s required to determine steady-state
power dissipation is to calculate I
this is that MOSFETs have a slight increase in R
increasing die temperature. A good approximation for this
value is 0.5% increase in R
ture above the point at which R
manufacturer. For instance, if the selected MOSFET has a
M9999-042204
• The value of I
• The manufacturer’s data sheet for the candidate
• The maximum ambient temperature in which the
• Any knowledge you can get about the heat sinking
question (see “Sense Resistor Selection”).
MOSFET.
device will be required to operate.
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?).
LOAD(CONT, MAX)
IN(MAX)
GATE
ON
per C rise in junction tempera-
ON
minus the 3.5V to 4.5V gate
for the slot in question. For
was initially specified by the
2
R. The one addendum to
for the output in
ON
. Since a
ON
with
22
calculated R
temperature ends up at 110 C, a good first cut at the operat-
ing value for R
Next, approximate the steady-state power dissipation (I
using I
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:
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will, a) withstand both the
applied voltage stresses, and b) handle the worst-case continu-
ous I
MOSFET’s ability to handle short-term overload power dissipa-
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.
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
where t
I
nominal, and the fast-trip threshold is 100mV. If the output is
connected to a 0.60
MOSFET for the slot in question will be regulated to 5.0A for
LOAD(CONT, MAX)
1. The heat from a surface-mount device such as an
2. Airflow works. Even a few LFM (linear feet per
3. The best test of a surface-mount MOSFET for an
2
R
P
R power dissipation that it will endure; verifying the
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
trace must be on the same layer of the board as the
MOSFET drain.
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.
application (assuming the above tips show it to be
a likely fit) is an empirical one. Check the MOSFET'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.
LOAD(CONT,max)
D
ON
FLT
(8.93A)
[I
LOAD(CONT, MAX)
ON
10m [1 + (110 – 25)(0.005)]
for the 5V supply has been set to 50ms,
ON
of 10m at T
2
would be:
is 5.0A, the slow-trip threshold is 50mV
14.3m
and the approximated R
load, the output current from the
J
]
2
= 25 C and the actual junction
1.14W
R
ON
14.3m
ON
April 2004
.
Micrel
2
R)