LTC3838-1_15 Datasheet, PDF (25/52 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with Dual Differential Output Sensing

LTC3838-1
APPLICATIONS INFORMATION
L_IN
1µH
ESR_(BULK)
+– _VIN
ESL_(BULK)
+
C_IN(BULK)
ESR_(CERAMIC)
ESL_(CERAMIC)
C_IN(CERAMIC)
I_{PULSE(PHASE1)}
I_{PULSE(PHASE2)}
38381 F06
Figure 6. Circuit Model for Input Capacitor
Ripple Current Simulation
For simulations with this model, look at the ripple current
during steady-state for the case where one phase is fully
loaded and the other was not loaded. This will in general
be the worst case for ripple current since the ripple cur-
rent from one phase will not be cancelled by ripple current
from the other phase.
Note that the bulk capacitor also has to be chosen for
RMS rating with ample margin beyond its RMS current
per simulation with the circuit model provided. For a lower
V_IN range, a conductive-polymer type (such as Sanyo
OS‑CON) can be used for its higher ripple current rating
and lower ESR. For a wide V_IN range that also require
higher voltage rating, aluminum-electrolytic capacitors are
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, Poly-
Phase 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 LTC3838-1 is only
truly phase-interleaved at steady state, transient RMS cur-
rents 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.
0.6
0.5
0.4
1-PHASE
2-PHASE
3-PHASE
0.3
4-PHASE
6-PHASE
0.2
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY FACTOR (V_O/V_IN)
38381 F07
Figure 7. Normalized RMS Input Ripple Current
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
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38381f
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