LTC3878
13
3878fa
applicaTions inForMaTion
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. The largest ripple current
occurs at the highest V
IN
. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
L
V
f I
V
V
OUT
OP IL MAX
OUT
IN MAX
=
•
–
( ) ( )
∆
1
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
tolerate the core loss of low cost powdered iron cores,
forcing the use of more expensive ferrite materials such
as molypermalloy or Kool Mµ cores. A variety of inductors
designed for high current, low voltage applications are
available from manufacturers such as Sumida, Panasonic,
Coiltronics, Coilcraft, Toko, Vishay, Pulse and Wurth.
Inductor Core Selection
Once the inductance value is determined, the type of in
-
ductor must be selected. Core loss is independent of core
size for a fixed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con
-
centrate on
copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc
-
tance collapses
abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
C
IN
and C
OUT
Selection
The input capacitance C
IN
is required to filter the square
wave current at the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
I I
V
V
V
V
RMS OUT MAX
OUT
IN
IN
OUT
≅
( )
• • – 1
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT(MAX)
/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
2000 hours of life, which makes it advisable to de-rate
the capacitor.
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The
∆V
OUT
is approximately bounded by:
∆ ∆V I ESR
f C
OUT L
OP OUT
≤ +
1
8 • •
Since ∆I
L
increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfied, the capacitance is adequate
for filtering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, specialty polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Specialty polymer capacitors offer very low
ESR but have lower specific capacitance than other types.
Tantalum capacitors have the highest specific capacitance
but it is important to only use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR,
but can be used in cost-sensitive applications providing
that consideration is given to ripple current ratings and
long-term reliability. Ceramic capacitors have excellent
low ESR characteristics but can have a high voltage co
-
efficient and
audible piezoelectric effects. The high Q of
ceramic capacitors with trace inductance can also lead to
significant ringing. When used as input capacitors, care
must be taken to ensure that ringing from inrush currents
and switching does not pose an overvoltage hazard to the
power switches and controller. To dampen input voltage
transients, add a small 5µF to 40µF aluminum electrolytic
capacitor with an ESR in the range of 0.5Ω to 2Ω. High
performance though-hole capacitors may also be used,
but an additional ceramic capacitor in parallel is recom
-
mended
to reduce the effect of lead inductance.
