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LTC3860_15 Datasheet, PDF (16/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Current Sharing | |||
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LTC3860
APPLICATIONS INFORMATION
supply can make up the difference. Generally, a capacitor
(particularly a non-ceramic type) that meets the ï¬rst two
parameters will have far more capacitance than is required
to keep capacitance-based droop under control.
The input capacitorâs voltage rating should be at least 1.4
times the maximum input voltage. Power loss due to ESR
occurs not only as I2R dissipation in the capacitor itself,
but also in overall battery efï¬ciency. For mobile applica-
tions, the input capacitors should store adequate charge
to keep the peak battery current within the manufacturerâs
speciï¬cations.
The input capacitor RMS current requirement is simpli-
ï¬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 (VOUT)(IOUT)
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 capacitorâs 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
VOUT/VIN. The maximum RMS capacitor current is given
by:
( ) IRMS â IOUT(MAX)
VOUT VIN â VOUT
VIN
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monly used for design because even signiï¬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 manufacturerâs 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 coefï¬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 electrolyticsâ 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.
COUT Selection
The selection of COUT is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The output ripple ÎVOUT is approximately bounded by:
ÎVOUT
â¤
ÎIL
â
ââ ESR
+
8
â¢
1
fSW â¢
COUT
â
â â
where ÎIL is the inductor ripple current.
ÎIL may be calculated using the equation:
ÎIL
=
VOUT
L ⢠fSW
â
ââ 1â
VOUT
VIN
â
â â
Since ÎIL increases with input voltage, the output ripple
voltage is highest at maximum input voltage. Typically,
once the ESR requirement is satisï¬ed, the capacitance is
adequate for ï¬ltering and has the necessary RMS current
rating.
3860fc
16
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