16
LTC3808
3808f
The corresponding average current depends on the amount
of ripple current. Lower inductor values (higher I
RIPPLE
)
will reduce the load current at which Burst Mode operation
begins.
The ripple current is normally set so that the inductor
current is continuous during the burst periods. Therefore,
I
RIPPLE
≤ I
BURST(PEAK)
This implies a minimum inductance of:
L
VV
fI
V
V
MIN
IN OUT
OSC BURST PEAK
OUT
IN
≤
–
•
•
()
A smaller value than L
MIN
could be used in the circuit,
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
efficiency. In general, though, it is a good idea to keep
I
RIPPLE
comparable to I
BURST(PEAK)
.
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. High efficiency converters generally cannot afford
the core loss found in low cost powdered iron cores, forc-
ing the use of more expensive ferrite, molypermalloy or Kool
Mµ
®
cores. Actual core loss is independent of core size for
a fixed inductor value, but is very dependent on the induc-
tance 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 losses 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. Core saturation results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool Mµ. Toroids are very space efficient,
especially when several layers of wire can be used, while
inductors wound on bobbins are generally easier to sur-
face mount. However, designs for surface mount that do
not increase the height significantly are available from
Coiltronics, Coilcraft, Dale and Sumida.
Schottky Diode Selection (Optional)
The schottky diode D in Figure 12 conducts current during
the dead time between the conduction of the power
MOSFETs. This prevents the body diode of the bottom
N-channel MOSFET from turning on and storing charge
during the dead time, which could cost as much as 1% in
efficiency. A 1A Schottky diode is generally a good size for
most LTC3808 applications, since it conducts a relatively
small average current. Larger diode results in additional
transition losses due to its larger junction capacitance.
This diode may be omitted if the efficiency loss can be
tolerated.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (V
OUT
/V
IN
). To
prevent large voltage transients, a low ESR input capacitor
sized for the maximum RMS current must be used. The
maximum RMS capacitor current is given by:
C
IN
Re •
•–
/
quiredI I
VVV
V
RMS MAX
OUT IN OUT
IN
≈
()
12
This formula has a maximum value at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC3808, ceramic capacitors
can also be used for C
IN
. Always consult the manufacturer
if there is any question.
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement is
APPLICATIO S I FOR ATIO
WUUU
Kool Mµ is a registered trademark of Magnetics, Inc.
