LTC3727A-1 Datasheet, PDF (11/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

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OPERATIO (Refer to Functional Diagram)
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least efficient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When sinking current
while in forced continuous operation, current will be
forced back into the main power supply.
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV_CC pin.
When the EXTV_CC pin is left open, an internal 7.5V low
dropout linear regulator supplies INTV_CC power. If EXTV_CC
is taken above 7.3V, the 7.5V regulator is turned off and an
internal switch is turned on connecting EXTV_CC to INTV_CC.
This allows the INTV_CC power to be derived from a high
efficiency external source such as the output of the regu-
lator itself or a secondary winding, as described in the
Applications Information section.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious condi-
tions that may overvoltage the output. In this case, the top
MOSFET is turned off and the bottom MOSFET is turned on
until the overvoltage condition is cleared.
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.
LTC3727A-1
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC3727A-1 dual high efficiency DC/DC controller
brings the considerable benefits of 2-phase operation to
portable applications. Notebook computers, PDAs,
handheld terminals and automotive electronics will all
benefit from the lower input filtering requirement, reduced
electromagnetic interference (EMI) and increased effi-
ciency associated with 2-phase operation.
Traditionally, 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 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
LTC3727A-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
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