Datasheet

LTC3707
12
3707fb
OPERATION
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC1628 and the LTC3707 are the fi rst dual high
effi ciency DC/DC controllers to bring the considerable
benefi ts of 2-phase operation to portable applications.
Notebook computers, PDAs, handheld terminals and au-
tomotive electronics will all benefi t from the lower input
fi ltering requirement, reduced electromagnetic interference
(EMI) and increased effi ciency associated with 2-phase
operation.
Why the need for 2-phase operation? Up until the LTC1628
was introduced, 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 expensive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating effi ciency.
Figure 3 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC3707
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 4 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.
(Refer to Functional Diagram)
I
IN(MEAS)
= 2.53A
RMS
I
IN(MEAS)
= 1.55A
RMS
3707 F03a 3707 F03b
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
Figure 3. 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 LTC1628 2-Phase Regulator Allows Less Expensive Input Capacitors, Reduces
Shielding Requirements for EMI and Improves Effi ciency
(a) (b)