LTC3707-SYNC
11
3707sfa
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.
Foldback Current, Short-Circuit Detection
and Short-Circuit Latchoff
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 condition 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 prevents net dis-
charge of the RUN/SS capacitor(s) during an overcurrent
and/or short-circuit condition. Foldback current 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 effi cient nature of
the current mode switching regulator.
Theory and Benefi ts of 2-Phase Operation
The LTC1628 and the LTC3707-SYNC dual high effi ciency
DC/DC controllers bring the considerable benefi ts of
2-phase operation to portable applications for the fi rst
time. 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.
Why the need for 2-phase operation? Up until the LTC1628
family of parts, 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 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
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 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.
OPERATION
(Refer to Functional Diagram)
