MIC2590B(2002) データシートの表示（PDF） - Micrel
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MIC2590B Datasheet PDF : 24 Pages
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MIC2590B
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 maxi-
mum. 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 data
sheet 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 RON. 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 RON.
The one addendum to this is that MOSFETs have a slight
increase in RON with increasing die temperature. A good
approximation for this value is 0.5% increase in RON per °C
rise in junction temperature above the point at which RON 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 RDS(ON) of 8.0mΩ
max. at VG-S = 4.5V, and RDS(ON) of 4.7mΩ max. at VG-S =
10V. Then RON is calculated as:
RON
=
(4.7mΩ
+
2
8.0mΩ)
=
6.35mΩ
at 25°C TJ. If the actual junction temperature is estimated to
be 110°C, a reasonable approximation of RON for the
Si4430DY at temperature is:
6.35mΩ1+
(110°
–
25°)
0.5%
°C
=
6.35mΩ1+
(85°)
0.°5C%
≅
9.05mΩ
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 IDRAIN of
7.6A, the voltage drop across the part will be approximately
(7.6A)(9.05mΩ) = 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:
Micrel
• The value of ILOAD(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 I2R. As noted in “MOSFET Maximum On-State
Resistance,” above, the one further concern is the MOSFET’s
increase in RON 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 RON at
temperature is again approximately 9.05mΩ. Again, allow a
maximum IDRAIN of 7.6A:
Power dissipation ≅ IDRAIN2 × RON = (7.6A)2 × 9.05mΩ ≅ 0.523W
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 Rθ(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, t ≤10seconds,”
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 sol-
dered 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 the MOSFET drain.
2. Since the rating for the part is given as “for 10
seconds,” derate the maximum junction tem-
perature by 35°C. This is the standard good
practice derating of 25°C, plus another 10°C to
allow for the time element of the specification.
3. Airflow, if available, works wonders. This is not
the place for a dissertation on how to perform
airflow calculations, but even a few LFM (linear
feet per minute) of air will cool a MOSFET down
August 2002
19
MIC2590B
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