Datasheet

IN OUT
OUT
V V
D
L = 5
2 I s
-
´ ¦
UCD7242
SLUS962B –JANUARY 2010–REVISED AUGUST 2012 www.ti.com
(5)
For example, plotting this result as a function of V
IN
and V
OUT
results in:
Figure 17. Inductance vs. V
IN
and V
OUT
In this graph I
OUT
is 10A, the switching frequency is 750kHz and the inductor ΔI is 4A. If the switching frequency
is cut in half then the resulting inductance would be twice the value shown. Notice that the maximum inductance
occurs at the maximum V
IN
and V
OUT
shown on the plot. In general, this inductance value should be used in
order to keep the inductor ripple current from becoming too large over the range of supported V
IN
and V
OUT
.
INDUCTOR LOSSES AND SATURATION
The current rating of an inductor is based on two things: the current necessary to raise the component
temperature by 40°C and the current level necessary to reduce the inductance to 80% of its initial value
(saturation current
(1)
). The current rating is the lower of these two numbers. Both of these factors are influenced
by the choice of core material. Popular materials currently in use are: ferrite, powdered alloy and powdered iron.
Ferrite is regarded as the highest performance material and as such is the lowest loss and the highest cost. Solid
ferrite all by itself will saturate with a relatively small amount of current. This can be addressed by inserting a gap
into the core. This, in effect, makes the inductor behave in a linear manner over a wide DC current range.
However, once the inductance begins to roll off, these gapped materials exhibit a “sharp” saturation
characteristic. In other words, the inductance value reduces rapidly with increases in current above the saturation
level. This small inductance that results, can produce dangerously high current levels.
(1) Although “saturation current” is standard terminology among many inductor vendors, technically saturation does not occur until the
relative permeability of the core is reduced to approximately 1. This is a value much larger than what is typically seen on data sheets.
20 Submit Documentation Feedback Copyright © 2010–2012, Texas Instruments Incorporated
Product Folder Links: UCD7242