LTC3861-1 Datasheet, PDF (19/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Applications Information
(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 cur-
rent is given by:
( ) _{IRMS ≈IOUT(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 significant 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 coefficients 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 ∆V_OUT is approximately bounded by:
ΔV_OUT
≤
ΔI_L
⎛
⎝⎜ ^ESR
+
8
•
1
f_SW • C_OUT
⎞
⎠⎟
where ∆I_L is the inductor ripple current.
∆I_L may be calculated using the equation:
ΔI_L
=
V_OUT
L • f_SW
⎛
⎝⎜
1–
V_OUT
V_IN
⎞
⎠⎟
Since ∆I_L increases with input voltage, the output ripple
voltage is highest at maximum input voltage. Typically,
once the ESR requirement is satisfied, the capacitance is
adequate for filtering and has the necessary RMS current
rating.
Manufacturers such as Sanyo, Panasonic and Cornell Du-
bilier should be considered for high performance through-
hole capacitors. The OS-CON semiconductor electrolyte
capacitor available from Sanyo has a good (ESR)(size)
product. An additional ceramic capacitor in parallel with
OS-CON capacitors is recommended to offset the effect
of lead inductance.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or transient current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. New special polymer surface
mount capacitors offer very low ESR also but have much
lower capacitive density per unit volume. In the case of
tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. Several excellent
output capacitor choices include the Sanyo POSCAP TPD,
38611f
19