Datasheet
11
LTC3727/LTC3727-1
3727fc
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.
Foldback Current, Short-Circuit Detection
and Short-Circuit Latchoff (LTC3727 Only)
The RUN/SS capacitors are used initially to limit the inrush
current of each switching regulator. After the controller
has been started and been given adequate time to charge
up the output capacitors and provide full load current, the
RUN/SS capacitor is used in a short-circuit time-out
circuit. If the output voltage falls to less than 70% of its
nominal output voltage, the RUN/SS capacitor begins
discharging on the assumption that the output is in an
overcurrent and/or short-circuit condition. If the condi-
tion lasts for a long enough period as determined by the
size of the RUN/SS capacitor, the controller will be shut
down until the RUN/SS pin(s) voltage(s) are recycled.
This built-in latchoff can be overridden by providing a
>5μA pull-up at a compliance of 5V to the RUN/SS pin(s).
This current shortens the soft start period but also pre-
vents net discharge of the RUN/SS capacitor(s) during an
overcurrent and/or short-circuit condition. Foldback cur-
rent limiting is also activated when the output voltage falls
below 70% of its nominal level whether or not the short-
circuit latchoff circuit is enabled. Even if a short is present
and the short-circuit latchoff is not enabled, a safe, low
output current is provided due to internal current foldback
and actual power wasted is low due to the efficient nature
of the current mode switching regulator.
PART NUMBER FUNCTION
LTC3727 With Latchoff Function Available
LTC3727-1 Latchoff Always Disabled
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC3727 dual high efficiency DC/DC controller brings
the considerable benefits of 2-phase operation to portable
applications. Notebook computers, PDAs, handheld ter-
minals and automotive electronics will all benefit from the
lower input filtering requirement, reduced electromag-
netic interference (EMI) and increased efficiency associ-
ated with 2-phase operation.
Why the need for 2-phase operation? Until recently, con-
stant-frequency dual switching regulators 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 capaci-
tor 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
expen
sive 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
LTC3727 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
RMS
2
, 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
(Refer to Functional Diagram)
OPERATIO
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