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| 型號： | LM2642 |
| 文件頁數(shù)： | 17/18頁 |
| 文件大小： | 504K |
| 代理商： | LM2642 |
Design Procedure
(Continued)
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
inductance 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
rating will also be given (usually as a multiple of the RMS
rating) which gives the user some indication of how well the
inductance operates in the saturation region.
Other things being equal, a higher peak current rating is
preferred, 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
IN
is the maximum input voltage.
V
OUT
is the output voltage.
F is the switching frequency, F
OSC
I
RIPPLE
is the inductor ripple current. In general, a good value
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
increase 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
capacitor to maintain a given amount of output ripple volt-
age. This means that a cheaper inductor (with less induc-
tance 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
transients will appear on the V
line. To supply these peak
currents, 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
distance = 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 appli-
cations.
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
application.
Tantalum capacit
L
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17
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