Datasheet

LTC3860
16
3860fc
APPLICATIONS INFORMATION
supply can make up the difference. Generally, a capacitor
(particularly a non-ceramic type) that meets the fi rst two
parameters will have far more capacitance than is required
to keep capacitance-based droop under control.
The input capacitor’s voltage rating should be at least 1.4
times the maximum input voltage. Power loss due to ESR
occurs not only as I
2
R dissipation in the capacitor itself,
but also in overall battery effi ciency. For mobile applica-
tions, the input capacitors should store adequate charge
to keep the peak battery current within the manufacturer’s
specifi cations.
The input capacitor RMS current requirement is simpli-
fi ed by the multiphase architecture and its impact on the
worst-case RMS current drawn through the input network
(battery/fuse/capacitor). It can be shown that the worst-
case RMS current occurs when only one controller is
operating. The controller with the highest (V
OUT
)(I
OUT
)
product needs to be used to determine the maximum
RMS current requirement. Increasing the output current
drawn from the other out-of-phase controller will actually
decrease the input RMS ripple current from this maximum
value. The out-of-phase technique typically reduces the
input capacitor’s RMS ripple current by a factor of 30%
to 70% when compared to a single phase power supply
solution.
In continuous mode, the source current of the top N-channel
MOSFET is approximately a square wave of duty cycle
V
OUT
/V
IN
. The maximum RMS capacitor current is given
by:
I
RMS
≈I
OUT(MAX)
V
OUT
V
IN
–V
OUT
()
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even signifi cant deviations
do not offer much relief. The total RMS current is lower
when both controllers are operating due to the interleav-
ing of current pulses through the input capacitors. This
is why the input capacitance requirement calculated
above for the worst-case controller is adequate for the
dual controller design.
Note that capacitor manufacturer’s ripple current ratings
are often based on only 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 also be paralleled to meet size or
height requirements in the design. Always consult the
manufacturer if there is any question.
Ceramic, tantalum, OS-CON and switcher-rated electrolytic
capacitors can be used as input capacitors, but each has
drawbacks: ceramics have high voltage coeffi cients of
capacitance and may have audible piezoelectric effects;
tantalums need to be surge-rated; OS-CONs suffer from
higher inductance, larger case size and limited surface
mount applicability; and electrolytics’ higher ESR and
dryout possibility require several to be used. Sanyo
OS-CON SVP, SVPD series; Sanyo POSCAP TQC series
or aluminum electrolytic capacitors from Panasonic WA
series or Cornell Dubilier SPV series, in parallel with a
couple of high performance ceramic capacitors, can be
used as an effective means of achieving low ESR and high
bulk capacitance.
C
OUT
Selection
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The output ripple ΔV
OUT
is approximately bounded by:
ΔV
OUT
≤ΔI
L
ESR +
1
8•f
SW
•C
OUT
⎛
⎝
⎜
⎞
⎠
⎟
where ΔI
L
is the inductor ripple current.
ΔI
L
may be calculated using the equation:
ΔI
L
=
V
OUT
L•f
SW
1–
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
Since ΔIL increases with input voltage, the output ripple
voltage is highest at maximum input voltage. Typically,
once the ESR requirement is satisfi ed, the capacitance is
adequate for fi ltering and has the necessary RMS current
rating.