LTC3412
U
W
U U
APPLICATIO S I FOR ATIO
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, orchooseacapacitorratedatahighertempera-
ture than required. Several capacitors may also be paral-
leled to meet size or height requirements in the design.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, mollypermalloy,
or Kool Mμ® cores. Actual core loss is independent of core
size for a fixed inductor value but it is very dependent on
theinductanceselected.Astheinductanceincreases,core
losses decrease. Unfortunately, increased inductance re-
quires more turns of wire and therefore copper losses will
increase.
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 capacitance 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, ΔVOUT, is determined by:
Ferritedesignshaveverylowcorelossesandarepreferred
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!
⎛
1
⎞
ΔVOUT ≤ ΔI ESR +
⎜
⎟
L
⎝
8fCOUT
⎠
The output ripple is highest at maximum input voltage
since ΔIL increases with input voltage. Multiple capacitors
placedinparallelmaybeneededtomeettheESRandRMS
currenthandlingrequirements.Drytantalum,specialpoly-
mer, aluminum electrolytic and ceramic capacitors are all
available in surface mount packages. Special polymer
capacitors offer very low ESR but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density 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 provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capaci-
tors have excellent low ESR characteristics 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.
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
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the trapezoi-
dal current at the source of the top MOSFET. To prevent
largeripplevoltage, alowESRinputcapacitorsizedforthe
maximum RMS current should be used. RMS current is
given by:
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
VOUT
V
IN
V
IN
VOUT
IRMS = IOUT(MAX)
− 1
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst-case condition is commonly
usedfordesignbecauseevensignificantdeviationsdonot
3412fb
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Linear [ Linear ]
