Datasheet

LTC3861
24
3861fa
For more information www.linear.com/LTC3861
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 9 graphically illustrates
the principle.
The worst-case RMS ripple current for a single stage de
-
sign peaks at an input voltage of twice the output voltage.
The
worst case RMS ripple current for a two stage design
applicaTions inFormaTion
results in peak outputs of 1/4 and 3/4 of input voltage.
When the RMS current is calculated, higher effective 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 10 and 11 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!
3861 F09
SW1 V
I
CIN
I
COUT
SINGLE PHASE
SW1 V
SW2 V
I
CIN
I
L2
I
L1
I
COUT
DUAL PHASE
RIPPLE
Figure 9. Single and 2-Phase Current Waveforms
DUTY FACTOR (V
OUT
/V
IN
)
0.1
DI
C(P-P)
V
O
/L
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.3
0.5
0.6
3861 F10
0.2 0.4
0.7
0.8
0.9
1 PHASE
2 PHASE
Figure 10. Normalized Output Ripple Current
vs Duty Factor [I
RMS
″ 0.3 (DI
C(PP)
)]
0
0.1
0.2
0.3
0.4
3861 F11
0.5
0.6
DUTY FACTOR (V
OUT
/V
IN
)
0.1
RMS INPUT RIPPLE CURRENT
DC LOAD CURRENT
0.3
0.5
0.6
0.2 0.4
0.7
0.8
0.9
1 PHASE
2 PHASE
Figure 11. Normalized RMS Input Ripple Current
vs Duty Factor for 1 and 2 Output Stages