LTC3729_15 Datasheet, PDF (25/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
APPLICATIONS INFORMATION
Simplified Visual Explanation of How a 2-Phase
Controller Reduces Both Input and Output RMS Ripple
Current
A multiphase power supply significantly reduces the amount
of ripple current in both the input and output capacitors.
The RMS input ripple current is divided by, and the effective
ripple frequency is multiplied up by the number of phases
used (assuming that the input voltage is greater than the
number of phases used times the output voltage). The output
ripple amplitude is also reduced by, and the effective ripple
frequency is increased by the number of phases used. Figure
10 graphically illustrates the principle.
SW V
SINGLE PHASE
I_CIN
I_COUT
SW1 V
SW2 V
I_L1
DUAL PHASE
I_L2
I_CIN
I_COUT
RIPPLE 3729 F10
Figure 10. Single and PolyPhase Current Waveforms
The worst-case RMS ripple current for a single stage design
peaks at twice the value of the output voltage . The worst-
case RMS ripple current for a two stage design results in
peaks at 1/4 and 3/4 of input voltage. When the RMS cur‑
rent is calculated, higher effective duty factor results and
the peak current levels are divided as long as the currents
in each stage are balanced. Refer to Application Note 19 for
a detailed description of how to calculate RMS current for
the single stage switching regulator. Figures 3 and 4 help to
illustrate how the input and output currents are reduced by
using an additional phase. The input current peaks drop in
half and the frequency is doubled for a 2-phase converter.
The input capacity requirement is reduced theoretically by a
factor of four! A ceramic input capacitor with its unbeatably
low ESR characteristic can be used.
Figure 4 illustrates the RMS input current drawn from the
input capacitance versus the duty cycle as determined
by the ratio of input and output voltage. The peak input
RMS current level of the single phase system is reduced
by 50% in a 2-phase solution due to the current splitting
between the two stages.
An interesting result of the multi-phase solution is that the
V_IN which produces worst-case ripple current for the input
capacitor, V_OUT = V_IN/2, in the single phase design produces
zero input current ripple in the 2-phase design.
The output ripple current is reduced significantly when
compared to the single phase solution using the same
inductance value because the V_OUT/L discharge current
term from the stage(s) that has its bottom MOSFET on
subtracts current from the (V_IN - V_OUT)/L charging current
resulting from the stage which has its top MOSFET on.
The output ripple current is:
I_RIPPLE
=
2V_OUT
fL



1− 2D (1−D) 
1− 2D + 1


where D is duty factor.
The input and output ripple frequency is increased by
the number of stages used, reducing the output capacity
requirements. When V_IN is approximately equal to NV_OUT
as illustrated in Figures 3 and 4, very low input and output
ripple currents result.
Again, the interesting result of 2-phase operation results
in no output ripple at V_OUT = V_IN/2. The addition of more
phases by phase locking additional controllers always
results in no net input or output ripple at V_OUT/V_IN ratios
equal to the number of stages implemented. Designing a
system with a multiple of stages close to the V_OUT/V_IN ratio
will significantly reduce the ripple voltage at the input and
outputs and thereby improve efficiency, physical size, and
heat generation of the overall switching power supply.
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