LTC3861-1 Datasheet, PDF (23/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Applications Information
A multiphase power supply significantly reduces the
amount of ripple current in both the input and output ca-
pacitors. The RMS input ripple current is divided by, and
the effective ripple frequency is multiplied by, the number
of phases used (assuming that the input voltage is greater
than the number of phases used times the output volt-
age). The output ripple amplitude is also reduced by the
number of phases used. Figure 8 graphically illustrates
the principle.
The worst-case RMS ripple current for a single stage
design peaks at an input voltage of twice the output volt-
age. The worst case RMS ripple current for a two stage
design results in peak outputs of 1/4 and 3/4 of input
voltage. When the RMS current is calculated, higher ef-
fective duty factor results and the peak current levels are
divided as long as the current in each stage is balanced.
Refer to Application Note 19 for a detailed description of
how to calculate RMS current for the single stage switching
regulator. Figures 9 and 10 illustrate how the input and
output currents are reduced by using an additional phase.
For a 2-phase converter, the input current peaks drop in
half and the frequency is doubled. The input capacitor
requirement is thus reduced theoretically by a factor of
four! Just imagine the possibility of capacitor savings with
even higher number of phases!
SW1 V
I_CIN
SINGLE PHASE
I_COUT
SW1 V
SW2 V
I_L1
I_L2
DUAL PHASE
I_CIN
I_COUT
RIPPLE
38611 F08
Figure 8. Single and 2-Phase Current Waveforms
1.0
0.9
0.8
1 PHASE
0.7
0.6
0.5
0.4
0.3
2 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_OUT/V_IN)
38611 F09
Figure 9. Normalized Output Ripple Current
vs Duty Factor [I_RMS″ 0.3 (DI_C(PP))]
0.6
1 PHASE
0.5
0.4
0.3
2 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_OUT/V_IN)
38611 F10
Figure 10. Normalized RMS Input Ripple Current
vs Duty Factor for 1 and 2 Output Stages
38611f
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