Datasheet
LTC3827
14
3827ff
OPERATION
THEORY AND BENEFITS OF 2-PHASE OPERATION
Why the need for 2-phase operation? Up until the
2-phase family, constant-frequency dual switching regula-
tors operated both channels in phase (i.e., single-phase
operation). This means that both switches turned on at
the same time, causing current pulses of up to twice the
amplitude of those for one regulator to be drawn from the
input capacitor and battery. These large amplitude current
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 expen
sive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating effi ciency.
Figure 1 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC3827
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
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 Effi ciency
I
IN(MEAS)
= 2.53A
RMS
I
IN(MEAS)
= 1.55A
RMS
3827 F01b3827 F01a
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
(a) (b)
(Refer to Functional Diagram)