LTC3839 Datasheet, PDF (24/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
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 LTC3839 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 2.2Ω to 10Ω 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.
C_OUT Selection
The selection of output capacitance C_OUT is primarily
determined by the effective series resistance, ESR, to
minimize voltage ripple. The output voltage ripple ∆V_OUT,
in continuous mode is determined by:
ΔV_OUT
≤
ΔI_L
⎛
⎝⎜ ^RESR
+
8
•
f
1
• C_OUT
⎞
⎠⎟
24
where f is operating frequency, and ∆I_L is ripple content in
the sum of inductor currents of all phases. The output ripple
is highest at maximum input voltage since ∆I_L increases
with input voltage. Typically, once the ESR requirement
for C_OUT has been met, the RMS current rating generally
far exceeds that required from ripple current.
For LTC3839’s multiphase operation, it is advisable to
consider ripple requirements at specific load conditions
when determining the ∆I_L for the output capacitor selec-
tion. At steady state, the LTC3839’s individual phases are
interleaved, and their ripples cancel each other at the output,
so ripple on C_OUT is reduced. During transient, when the
phases are not fully interleaved, the ripple cancellation
may not be as effective. For example, a large step of load
current increase may cause the phases to align for several
cycles, quickly ramping up the total inductor current and
pulling the V_OUT from the droop. While the worst-case ∆I_L
is the sum of the ∆I_L’s of individual phases aligned during
a fast transient, such larger ripple lasts for only a short
time before the phase interleaving is restored.
The choice of using smaller output capacitance increases
the ripple voltage due to the discharging term but can be
compensated for by using capacitors of very low ESR to
maintain the ripple voltage.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount pack-
ages. Special polymer capacitors offer very low ESR but
have lower capacitance density than other types. Tantalum
capacitors have the highest capacitance density but it is
important to only use types that have been surge tested
for use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can be used
in cost-sensitive applications provided that consideration
is given to ripple current ratings and long-term reliability.
Ceramic capacitors have excellent low ESR characteristics
but can have a high voltage coefficient and audible piezo-
electric effects. The high Q of ceramic capacitors with trace
inductance can also lead to significant ringing. When used
as input capacitors, care must be taken to ensure that ring-
ing from inrush currents and switching does not pose an
overvoltage hazard to the power switches and controller.
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