LTC3728LCGN-PBF Datasheet, PDF (14/38 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Regulators

LTC3728L/LTC3728LX
Operation (Refer to Functional Diagram)
Theory and Benefits of 2-Phase Operation
The LTC1628 and the LTC3728L family of dual high effi-
ciency DC/DC controllers brings the considerable benefits
of 2-phase operation to portable applications for the first
time. Notebook computers, PDAs, handheld terminals
and automotive electronics will all benefit from the lower
input filtering requirement, reduced electromagnetic in-
terference (EMI) and increased efficiency associated with
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 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 LTC1628
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
I_IN(MEAS) = 2.53A_RMS
(a)
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
DC236 F03a
I_IN(MEAS) = 2.53A_RMS
(b)
DC236 F03b
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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