AME
AME5258
Inductor Core Selection
Once the value for 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, forcing the use of more expensive ferrite or
mollypermalloy cores. Actual core loss is independent of
core size for a fixed inductor value but it is very depen-
dent on the inductance selected. As the inductance in-
creases, core losses decrease. Unfortunately, increased
inductance requires more turns of wire and therefore cop-
per losses will increase. Ferrite designs have very low
core losses and are preferred at high switching frequen-
cies, so design goals can concentrate on copper loss
and preventing saturation. Ferrite core material saturates
"hard", which means that inductance collapses abruptly
when the peak design current is exceeded. This result in
an abrupt increase in inductor ripple current and conse-
quent output voltage ripple. Do not allow the core to satu-
rate! Different core materials and shapes will change the
size/current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don't radiate energy but generally cost
more than powdered iron core inductors with similar char-
acteristics. The choice of which style inductor to use
mainly depends on the price vs. size requirements and
any radiated field/EMI requirements.
C
IN
and C
OUT
Selection
The input capacitance, CIN, is needed to filter the trap-
ezoidal current at the source of the top MOSFET. To pre-
vent large ripple voltage, a low ESR input capacitor sized
for the maximum RMS current should be used.RMS cur-
rent is given by :
1.5MHz, 600mA
Synchronous Buck Converter
Several capacitors may also be paralleled to meet size
or height requirements in the design. The selection of
COUT is determined by the effective series resistance
(ESR) that is required to minimize voltage ripple and load
step transients, as well as the amount of bulk capaci-
tance that is necessary to ensure that the control loop is
stable. Loop stability can be checked by viewing the load
transient response as described in a later section.
The output ripple, V
OUT
, is determined by :
∆
V
OUT
1
≤ ∆
I
L
ESR
+
8
f
⋅
C
OUT
The output ripple is highest at maximum input voltage
since IL increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and
RMS current handling requirements. Dry tantalum, spe-
cial polymer, aluminum electrolytic and ceramic capaci-
tors are all available in surface mount packages. Special
polymer capacitors offer very low ESR but have lower
capacitance density than other types. Tantalum capaci-
tors have the highest capacitance density but it is impor-
tant 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 provided that consideration
is given to ripple current ratings and long term reliability.
Ceramic capacitors have excellent low ESR characteris-
tics but can have a high voltage coefficient and audible
piezoelectric effects. The high Q of ceramic capacitors
with trace inductance can also lead to significant ringing
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, V
IN
. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at V
IN
large enough to damage the
part.
9
I
RMS
=
I
OUT
(
max
)
⋅
V
OUT
⋅
V
IN
V
IN
V
OUT
−
1
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
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 further derate the
capacitor, or choose a capacitor rated at a higher tem-
perature than required.
Rev.A.05
AMETHERM [ AMETHERM CIRCUIT PROTECTION THERMISTORS ]
