LM2641MTC-ADJ National Semiconductor, LM2641MTC-ADJ Datasheet - Page 15

• Manufacturer Part Number: LM2641MTC-ADJ
• Description: Power Supply IC
• Manufacturer: National Semiconductor

Specifications of LM2641MTC-ADJ

• Power Dissipation Pd: 883mW
• No. Of Pins: 28
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Design Procedure
The data sheet for the inductor must be reviewed carefully to
verify that the selected component will have the desired in-
ductance at the frequency and current for the application.
Current Rating
This specification may be the most confusing of all when
picking an inductor, as manufacturers use different methods
for specifying an inductor’s current rating.
The current rating specified for an inductor is typically given
in RMS current, although in some cases a peak current rat-
ing will also be given (usually as a multiple of the RMS rat-
ing) which gives the user some indication of how well the in-
ductance operates in the saturation region.
Other things being equal, a higher peak current rating is pre-
ferred, as this allows the inductor to tolerate high values of
ripple current without significant loss of inductance.
In the some cases where the inductance vs. current curve is
relatively flat, the given current rating is the point where the
inductance drops 10% below the nominal value. If the induc-
tance varies a lot with current, the current rating listed by the
manufacturer may be the “center point” of the curve. This
means if that value of current is used in your application, the
amount of inductance will be less than the specified value.
(Continued)
15
DC Resistance
The DC resistance of the wire used in an inductor dissipates
power which reduces overall efficiency. Thicker wire de-
creases resistance, but increases size, weight, and cost. A
good tradeoff is achieved when the inductor’s copper wire
losses are about 2% of the maximum output power.
Selecting An Inductor
Determining the amount of inductance required for an appli-
cation can be done using the formula:
Where:
V
V
F is the switching frequency, F
I
for this is about 30% of the DC output current.
It can be seen from the above equation, that increasing the
switching frequency reduces the amount of required induc-
tance proportionally. Of course, higher frequency operation
is typically less efficient because switching losses become
more predominant as a percentage of total power losses.
It should also be noted that reducing the inductance will in-
crease inductor ripple current (other terms held constant).
This is a good point to remember when selecting an inductor:
increased ripple current increases the FET conduction
losses, inductor core losses, and requires a larger output ca-
pacitor to maintain a given amount of output ripple voltage.
This means that a cheaper inductor (with less inductance at
the operating current of the application) will cost money in
other places.
INPUT CAPACITORS
The switching action of the high-side FET requires that high
peak currents be available to the switch or large voltage tran-
sients will appear on the V
rents, a low ESR capacitor must be connected between the
drain of the high-side FET and ground. The capacitor must
be located as close as possible to the FET (maximum dis-
tance = 0.5 cm).
A solid Tantalum or low ESR aluminum electrolytic can be
used for this capacitor. If a Tantalum is used, it must be able
to withstand the turn-ON surge current when the input power
is applied. To assure this, the capacitor must be surge tested
by the manufacturer and guaranteed to work in such applica-
tions.
Caution: If a typical off-the-shelf Tantalum is used that has
not been surge tested, it can be blown during power-up and
will then be a dead short. This can cause the capacitor to
catch fire if the input source continues to supply current.
Voltage Rating
For an aluminum electrolytic, the voltage rating must be at
least 25% higher than the maximum input voltage for the ap-
plication.
Tantalum capacitors require more derating, so it is recom-
mended that the selected capacitor be rated to work at a
voltage that is about twice the maximum input voltage.
Current Rating
Capacitors are specified with an RMS current rating. To de-
termine the requirement for an application, the following for-
mula can be used:
RIPPLE
IN
OUT
is the maximum input voltage.
is the output voltage.
is the inductor ripple current. In general, a good value
IN
line. To supply these peak cur-
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