LTC3707 Datasheet, PDF (12/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
OPERATION (Refer to Functional Diagram)
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC1628 and the LTC3707 are the first dual high
efficiency DC/DC controllers to bring the considerable
benefits of 2-phase operation to portable applications.
Notebook computers, PDAs, handheld terminals and au-
tomotive electronics will all benefit from the lower input
filtering requirement, reduced electromagnetic interference
(EMI) and increased efficiency 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 flowing 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 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 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_RMS2,
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.
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
I_IN(MEAS) = 2.53A_RMS
(a)
3707 F03a
I_IN(MEAS) = 1.55A_RMS
(b)
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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 Efficiency
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