LTC3701 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3701' PDF : 20 Pages View PDF
U
OPERATIO (Refer to Functional Diagram)
Why the need for 2-phase operation? Until recently, con-
stant frequency dual switching regulators operated both
channels in phase (i.e., single phase operation). This
means that both topside MOSFETs are turned on at the
same time, causing current pulses of up to twice the
amplitude of those from a single regulator to be drawn
from the input capacitor. These large amplitude pulses
increase the total RMS current flowing into the input
capacitor, requiring the use of more expensive input
capacitors, and increasing both EMI and losses in the
input capacitor and input power supply.
With 2-phase operation, the two channels of the LTC3701
are operated 180 degrees out of phase. This effectively
interleaves the current pulses coming from the switches,
greatly reducing the overlap time where they add
together. The result is a significant reduction in the total
RMS input current, which in turn allows for use of less
expensive input capacitors, reduces shielding requirements
for EMI and improves real world operating efficiency.
Figure 3 shows example waveforms for a single switching
regulator channel versus a 2-phase LTC3701 system with
SW1 (V)
Single Phase
Dual Controller
2-Phase
Dual Controller
LTC3701
both channels switching. A single phase dual regulator
system with both sides switching would exhibit twice the
single side numbers. In this example, 2-phase operation
reduced the RMS input current from 1.79ARMS to
0.91ARMS. While this is an impressive reduction in itself,
remember that power losses are proportional to IRMS2,
meaning that actual power wasted is reduced by a factor
of 3.86. The reduced input ripple current also means that
less power is lost in the input power path, which could
include batteries, switches, trace/connector resistances,
and protection circuitry. Improvements in both conducted
and radiated EMI also directly accrue as a result of the
reduced RMS input current and voltage.
Of course, the improvement afforded by 2-phase opera-
tion is a function of the dual switching regulator’s relative
duty cycles, which in turn are dependent upon the input
voltage VIN. Figure 4 shows how the RMS input current
varies for 1-phase and 2-phase operation for 2.5V and
1.8V regulators over a wide input voltage range.
It can be readily seen that the advantages of 2-phase
operation are not limited to a narrow operating range, but
in fact extend over a wide region. A good rule of thumb for
most applications is that 2-phase operation will reduce the
input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
SW2 (V)
IL1
IL2
IIN
3701 F03
Figure 3. Example Waveforms for a Single Switching
Regulator Channel vs 2-Phase LTC3701 System with
Both Channels Switching
2.0
1.8
1.6
SINGLE PHASE
DUAL CONTROLER
1.4
1.2
2-PHASE
DUAL CONTROLER
1.0
0.8
0.6
0.4
0.2 VOUT1 = 2.5V/2A
VOUT2 = 1.8V/2A
0
23 4 56
78
9 10
INPUT VOLTAGE (V)
3701 F04
Figure 4. RMS Input Current Comparison
3701fa
9
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