LM2641MTC-ADJ Datasheet, PDF (21/31 Pages) Texas Instruments – LM2641 Dual Adjustable Step-Down Switching Power Supply Controller

LM2641
www.ti.com
SNVS040B – JANUARY 2000 – REVISED APRIL 2013
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 inductance 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
decreases 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 application can be done using the formula:
(5)
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
inductance 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 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 transients will appear on the V_IN 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 specified to work in such applications.
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 capacitors require more derating, so it is recommended that the selected capacitor be rated to work at
a voltage that is about twice the maximum input voltage.
Current Rating
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