Datasheet

LTC4081
21
4081f
Accepting larger values of ΔI
L
allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability. A
reasonable starting point for setting ripple current is ΔI
L
=0.3 • I
LIM
, where I
LIM
is the peak switch current limit.
The largest ripple current occurs at the maximum input
voltage. To guarantee that the ripple current stays below a
specifi ed maximum, the inductor value should be chosen
according to the following equation:
L
V
fI
V
V
OUT
L
OUT
IN
MAX
≥−
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
()
0
1
•
•
Δ
For applications with V
OUT
= 1.8V, the above equation
suggests that an inductor of at least 6.8μH should be used
for proper operation.
Many different sizes and shapes of inductors are
available from numerous manufacturers. To maximize
effi ciency, choose an inductor with a low DC resistance.
Keep in mind that most inductors that are very thin or
have a very small volume typically have much higher core
and DCR losses and will not give the best effi ciency. Also
choose an inductor with a DC current rating at least 1.5
times larger than the peak inductor current limit to ensure
that the inductor does not saturate during normal opera-
tion. To minimize radiated noise use a toroid or shielded
pot core inductor in ferrite or permalloy materials. Table
1 shows a list of several inductor manufacturers.
APPLICATIO S I FOR ATIO
WUU
U
Table 1. Recommended Surface Mount Inductor Manufacturers
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Murata www.murata.com
Toko www.tokoam.com
Input and Output Capacitor Selection
Since the input current waveform to a buck converter is a
square wave, it contains very high frequency components.
It is strongly recommended that a low equivalent series
resistance (ESR) multilayer ceramic capacitor be used to
bypass the BAT pin which is the input for the converter.
Tantalum and aluminum capacitors are not recommended
because of their high ESR. The value of the capacitor on
BAT directly controls the amount of input voltage ripple for
a given load current. Increasing the size of this capacitor
will reduce the input ripple.
To prevent large V
OUT
voltage steps during transient
load conditions, it is also recommended that a ceramic
capacitor be used to bypass V
OUT
. A typical value for this
capacitor is 4.7μF.
Multilayer Ceramic Chip Capacitors (MLCC) typically have
exceptional ESR performance. MLCCs combined with a
carefully laid out board with an unbroken ground plane will
yield very good performance and low EMI emissions.