LM2640MTC-ADJ National Semiconductor, LM2640MTC-ADJ Datasheet - Page 16

• Manufacturer Part Number: LM2640MTC-ADJ
• Description: Dual Adjustable Step-Down Switching Power Supply Controller
• Manufacturer: National Semiconductor

www.national.com
Design Procedure
It is important to understand that all inductors are not created
equal, as the method of specifying inductance varies widely.
It must also be noted that the inductance of every inductor
decreases with current. The core material, size, and con-
struction type all contribute the the inductor’s dependence
on current loading. Some inductors exhibit inductance
curves which are relatively flat, while others may vary more
than 2:1 from minimum to maximum current. In the latter
case, the manufacturer’s specified inductance value is usu-
ally the maximum value, which means the actual inductance
in your application will be much less.
An inductor with a flatter inductance curve is preferable,
since the loop characteristics of any switching converter are
affected somewhat by inductance value. An inductor which
has a more constant inductance value will give more consis-
tent loop bandwidth when the load current is varied.
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.
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-
RIPPLE
IN
OUT
is the maximum input voltage.
is the output voltage.
is the inductor ripple current. In general, a good value
OSC
(Continued)
16
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:
It is also recommended that a 0.1 µF ceramic capacitor be
placed from V
cated as close as possible to the V
OUTPUT CAPACITORS
The output capacitor(s) are critical in loop stability (covered
in a previous section) and also output voltage ripple.
The types best suited for use as output capacitors are alumi-
num electrolytics and solid Tantalum.
Aluminum Electrolytics
The primary advantage of aluminum electrolytics is that they
typically give the maximum capacitance-to-size ratio, and
they are reasonably priced. However, it must be noted that
aluminum electrolytics used in high-performance switching
regulator designs must be high frequency, low ESR types
such as Sanyo OSCON or Panasonic HFQ which are spe-
IN
to ground for high frequency bypassing, lo-
IN
line. To supply these peak cur-
IN
pin.