LTC3719 LINER [Linear Technology], LTC3719 Datasheet - Page 27

• Manufacturer Part Number: LTC3719
• Description: 2-Phase, High Efficiency, Step-Down Controller for AMD Opteron CPUs
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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.
U
U
SW V
I
COUT
I
CIN
W
Figure 10. Single and 2-Phase Current Waveforms
SINGLE PHASE
U
SW1 V
SW2 V
The worst-case RMS ripple current for a single stage
design peaks at an input voltage of twice the output
voltage. 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
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 switch-
ing regulator. Figures 3 and 4 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 this 2-phase converter. The input capacity
requirement is thus reduced theoretically by a factor of
four! Ceramic input capacitors with their unbeatably low
ESR characteristics can be used.
I
COUT
I
CIN
I
I
L1
L2
DUAL PHASE
RIPPLE
3719 F11
LTC3719
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