LTC3731 Linear Technology, LTC3731 Datasheet - Page 25

• Manufacturer Part Number: LTC3731
• Description: Synchronous Buck Switching Regulator Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
regulator. External OPTI-LOOP compensation allows over-
compensation for PC layouts which are not optimized but
this is not the recommended design procedure.
Simplified Visual Explanation of How a 3-Phase
Controller Reduces Both Input and Output RMS
Ripple Current
The effect of multiphase power supply design 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 13 graphically
illustrates the principle.
The worst-case input RMS ripple current for a single stage
design peaks at twice the value of the output voltage. The
worst-case input RMS ripple current for a two stage
design results in peaks at 1/4 and 3/4 of the input voltage,
U
U
W
Figure 13. Single and Polyphase Current Waveforms
I
V
V
V
I
COUT
COUT
V
SW1
SW2
SW3
I
I
SW
CIN
CIN
I
I
I
L1
L2
L3
U
SINGLE PHASE
TRIPLE PHASE
and the worst-case input RMS ripple current for a three
stage design results in peaks at 1/6, 1/2, and 5/6 of the
input voltage. The peaks, however, are at ever decreasing
levels with the addition of more phases. A higher effective
duty factor results because the duty factors “add” as long
as the currents in each stage are balanced. Refer to AN19
for a detailed description of how to calculate RMS current
for the single stage switching regulator.
Figure 6 illustrates the RMS input current drawn from the
input capacitance versus the duty cycle as determined by
the ration of input and output voltage. The peak input RMS
current level of the single phase system is reduced by 2/3
in a 3-phase solution due to the current splitting between
the three stages.
The output ripple current is reduced significantly when
compared to the single phase solution using the same
inductance value because the V
term from the stages that has their bottom MOSFETs on
subtract current from the (V
resulting from the stage which has its top MOSFET on. The
3731 F13
CC
– V
OUT
OUT
/L discharge currents
)/L charging current
LTC3731
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