Datasheet

LTC3411
11
3411fb
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
ΔI
L
/2.
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst case is commonly used
to design because even signifi cant deviations do not offer
much relief. Note that capacitor manufacturers ripple cur-
rent ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet the size or height requirements of the design. An
additional 0.1μF to 1μF ceramic capacitor is also recom-
mended on V
IN
for high frequency decoupling, when not
using an all ceramic capacitor solution.
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfi ed, the capacitance
is adequate for fi ltering. The output ripple (ΔV
OUT
) is
determined by:
Δ≈Δ +
V I ESR
fC
OUT L
O OUT
1
8
where f = operating frequency, C
OUT
= output capacitance
and ΔI
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI
L
increases
with input voltage. With ΔI
L
= 0.3 • I
LIM
the output ripple
will be less than 100mV at maximum V
IN
and f
O
= 1MHz
with:
ESRC
OUT
< 150mΩ
Once the ESR requirements for C
OUT
have been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantulum
capacitors are all available in surface mount packages.
The OS-CON semiconductor dielectric capacitor avail-
able from Sanyo has the lowest ESR(size) product of any
aluminum electrolytic at a somewhat higher price. Special
polymer capacitors, such as Sanyo POSCAP, offer very
low ESR, but have a lower capacitance density than other
types. Tantalum capacitors have the highest capacitance
density, but it has a larger ESR and it is critical that the
capacitors are surge tested for use in switching power
supplies. An excellent choice is the AVX TPS series of
surface mount tantalums, avalable in case heights ranging
from 2mm to 4mm. Aluminum electrolytic capacitors have
a signifi cantly larger ESR, and is often used in extremely
cost-sensitive applications provided that consideration
is given to ripple current ratings and long term reliability.
Ceramic capacitors have the lowest ESR and cost but also
have the lowest capacitance density, a high voltage and
temperature coeffi cient and exhibit audible piezoelectric
effects. In addition, the high Q of ceramic capacitors along
with trace inductance can lead to signifi cant ringing. Other
capacitor types include the Panasonic specialty polymer
(SP) capacitors.
In most cases, 0.1μF to 1μF of ceramic capacitors should
also be placed close to the LTC3411 in parallel with the
main capacitors for high frequency decoupling.
APPLICATIONS INFORMATION