Datasheet

LTC3890
14
3890fc
OPERATION
(Refer to the Functional Diagram)
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 significant 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 efficiency.
Figure 1 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC3890
2-phase dual switching regulator. An actual measurement of
the RMS input current under these conditions shows that
2-phase operation dropped the input current from 2.53A
RMS
to 1.55A
RMS
. While this is an impressive reduction in itself,
remember that the power losses are proportional to I
RMS
2
,
meaning that the actual power wasted is reduced by a fac-
tor 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/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 V
IN
(Duty Cycle = V
OUT
/V
IN
). Figure 2 shows how
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 chan-
nel operating at maximum current and 50% duty cycle.
I
IN(MEAS)
= 2.53A
RMS
I
IN(MEAS)
= 1.55A
RMS
3890 F01
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
Figure 1. 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 Efficiency
Figure 2. RMS Input Current Comparison
INPUT VOLTAGE (V)
0
INPUT RMS CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
10 20 30 40
3890 F02
SINGLE PHASE
DUAL CONTROLLER
2-PHASE
DUAL CONTROLLER
V
O1
= 5V/3A
V
O2
= 3.3V/3A
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