Datasheet

LTC3727A-1
11
3727a1fa
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least effi cient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When sinking cur-
rent 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 effi ciency external source such as the output of the
regulator 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
conditions 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.
Theory and Benefi ts of 2-Phase Operation
The LTC3727A-1 dual high effi ciency DC/DC controller
brings the considerable benefi ts of 2-phase operation
to portable applications. Notebook computers, PDAs,
handheld terminals and automotive 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.
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 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 representative
single-phase dual switching regulator to the LTC3727A-1
2-phase dual switching regulator. An actual measure-
ment 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
OPERATION
(Refer to Functional Diagram)