Datasheet

LTC3838
26
3838fa
APPLICATIONS INFORMATION
more attractive since it can provide a larger capacitance
for more damping. An aluminum-electrolytic capacitor
with a ripple current rating that is high enough to handle
all of the ripple current by itself will be very large. But
when in parallel with ceramics, an aluminum-electrolytic
capacitor will take a much smaller portion of the RMS
ripple current due to its high ESR. However, it is crucial
that the ripple current through the aluminum-electrolytic
capacitor should not exceed its rating since this will
produce significant heat, which will cause the electrolyte
inside the capacitor to dry over time and its capacitance
to go down and ESR to go up.
The benefit of PolyPhase operation is reduced RMS cur-
rents and therefore less power loss on the input capaci-
tors. Also, the input protection fuse resistance, battery
resistance, and PC board trace resistance losses are also
reduced due to the reduced peak currents in a PolyPhase
system. The details of a close form equation can be found
in Application Note 77 “High Efficiency, High Density,
PolyPhase Converters for High Current Applications”.
Figure 7 shows the input capacitor RMS ripple currents
normalized against the DC output currents with respect
to the duty cycle. This graph can be used to estimate the
maximum RMS capacitor current for a multiple-phase
application, assuming the channels are identical and their
phases are fully interleaved.
Figure 7 shows that the use of more phases will reduce
the ripple current through the input capacitors due to
ripple current cancellation. However, since LTC3838 is
only truly phase-interleaved at steady state, transient RMS
currents could be higher than the curves for the designated
number of phase. Therefore, it is advisable to choose
capacitors by taking account the specific load situations
of the applications. It is always the safest to choose input
capacitors’ RMS current rating closer to the worst case of
a single-phase application discussed above, calculated by
assuming the loss that would have resulted if controller
channels switched on at the same time.
However, it is generally not needed to size the input capaci-
tor for such worst-case conditions where on-times of the
phases coincide all the time. During a load step event, the
overlap of on-time will only occur for a small percentage
of time, especially when duty cycles are low. A transient
event where the switch nodes align for several cycles at
a time should not damage the capacitor. In most applica-
tions, sizing the input capacitors for 100% steady-state
load should be adequate. For example, a microprocessor
load may cause frequent overlap of the on-times, which
makes the ripple current higher, but the load current may
rarely be at 100% of I
OUT(MAX)
. Using the worst-case load
current should already have margin built in for transient
conditions.
The V
IN
sources of the top MOSFETs should be placed
close to each other and share common C
IN
(s). Separating
the sources and C
IN
may produce undesirable voltage and
current resonances at V
IN
.
A small (0.1µF to 1µF) bypass capacitor between the IC’s
V
IN
pin and ground, placed close to the IC, is suggested.
A 2.2 to 10 resistor placed between C
IN
and the V
IN
pin is also recommended as it provides further isolation
from switching noise of the two channels.
Figure 7. Normalized RMS Input Ripple Current
DUTY FACTOR (V
O
/V
IN
)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.9
0.6
0.5
0.4
0.3
0.2
0.1
0
3838 F07
RMS INPUT RIPPLE CURRNET
DC LOAD CURRENT
6-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE