LTC3727LX-1_15 Datasheet, PDF (11/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

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OPERATIO (Refer to Functional Diagram)
Power Good (PGOOD) Pin
The PGOOD pin is connected to an open drain of an
internal MOSFET. The MOSFET turns on and pulls the
pin low when either output is not within ±7.5% of the
nominal output level as determined by the resistive
feedback divider. When both outputs meet the ±7.5%
requirement, the MOSFET is turned off within 10µs and
the pin is allowed to be pulled up by an external resistor
to a source of up to 7V.
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC3727LX-1 dual high efficiency DC/DC controller
brings the considerable benefits of 2-phase operation to
portable applications. Notebook computers, PDAs, hand-
held terminals and automotive electronics will all benefit
from the lower input filtering requirement, reduced elec-
tromagnetic interference (EMI) and increased efficiency
associated with 2-phase operation.
When constant-frequency dual switching regulators oper-
ate both channels in phase (i.e., single-phase operation),
both switches turn 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
LTC3727LX-1
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 reduc-
tion 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 new
LTC3727LX-1 2-phase dual switching regulator. An actual
measurement of the RMS input current under these con-
ditions 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 factor 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. Im-
provements in both conducted and radiated EMI also
directly accrue as a result of the reduced RMS input
current and voltage.
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
I_IN(MEAS) = 2.53A_RMS
(a)
3727LX1 F03a
I_IN(MEAS) = 1.55A_RMS
(b)
3727LX1 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 LTC3727LX-1 2-Phase Regulator Allows Less Expensive Input Capacitors,
Reduces Shielding Requirements for EMI and Improves Efficiency
3727lx1fa
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