LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 14

• Manufacturer Part Number: LTC3858-1
• Description: Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology Corporation

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OPERATION
LTC3858-1
pulses increased the total RMS current fl owing from the
input capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
With 2-phase operation, the two channels of the dual
switching regulator are operated 180 degrees out of phase.
This effectively interleaves the current pulses drawn by the
switches, greatly reducing the overlap time where they add
together. The result is a signifi cant reduction in total RMS
input current, which in turn allows less expensive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating effi ciency.
Figure 2 compares the input waveforms for a representative
single phase dual switching regulator to the LTC3858-1
2-phase dual switching regulator. An actual measure-
ment of the RMS input current under these conditions
shows that 2-phase operation dropped the input current
from 2.53A
reduction in itself, remember that the power losses are
proportional to I
is reduced by a factor of 2.66. The reduced input ripple
voltage also means less power is lost in the input power
path, which could include batteries, switches, trace/con-
nector 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.
14
Figure 2. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators
Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows
Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Effi ciency
RMS
RMS
to 1.55A
2
, meaning that the actual power wasted
(Refer to the Functional Diagram)
RMS
I
IN(MEAS)
. While this is an impressive
= 2.53A
RMS
INPUT CURRENT
INPUT VOLTAGE
3.3V SWITCH
5V SWITCH
500mV/DIV
20V/DIV
20V/DIV
5A/DIV
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 V
the RMS input current varies for single-phase and 2-phase
operation for 3.3V and 5V regulators over a wide input
voltage range.
It can readily be seen that the advantages of 2-phase op-
eration are not just limited to a narrow operating range,
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.
IN
Figure 3. RMS Input Current Comparison
3.0
2.5
2.0
1.5
1.0
0.5
(Duty Cycle = V
0
0
I
IN(MEAS)
V
V
O1
O2
= 5V/3A
= 3.3V/3A
= 1.55A
10
INPUT VOLTAGE (V)
DUAL CONTROLLER
RMS
2-PHASE
SINGLE PHASE
DUAL CONTROLLER
OUT
20
/V
IN
38581 F01
). Figure 3 shows how
30
3858 F03
40
38581f