LTC3707 Datasheet, PDF (11/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
OPERATION (Refer to Functional Diagram)
be forced back into the main power supply potentially
boosting the input supply to dangerous voltage levels—
BEWARE!
Frequency Setting
The FREQSET pin provides frequency adjustment of the
internal oscillator from approximately 140kHz to 310kHz.
This input is nominally biased through an internal resistor
to the 1.19V reference, setting the oscillator frequency to
approximately 220kHz. This pin can be driven from an ex-
ternal AC or DC signal source to control the instantaneous
frequency of the oscillator. Voltage should not be applied
to the FREQSET pin prior to the application of voltage to
the V_IN pin.
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 5V low dropout
linear regulator supplies INTV_CC power. If EXTV_CC is taken
above 4.7V, the 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 regulator itself
or a secondary winding, as described in the Applications
Information.
Standby Mode Pin
The STBYMD pin is a three-state input that controls com-
mon circuitry within the IC as follows: When the STBYMD
pin is held at ground, both controller RUN/SS pins are pulled
to ground providing a single control pin to shut down both
controllers. When the pin is left open, the internal RUN/SS
currents are enabled to charge the RUN/SS capacitor(s),
allowing the turn-on of either controller and activating
necessary common internal biasing. When the STBYMD
pin is taken above 2V, both internal linear regulators are
turned on independent of the state on the RUN/SS pins
of the two switching regulator controllers, providing an
output power source for “wake-up” circuitry. Decouple the
pin with a small capacitor (0.01μF) to ground if the pin is
not connected to a DC potential.
Output Overvoltage Protection
An overvoltage comparator, 0V, 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 in-
ternal MOSFET. The MOSFET turns on and pulls the pin
low when both the outputs are not within ± 7.5% of their
nominal output levels as determined by their resistive
feedback dividers. 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 controllers 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 4.2V 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 dissipated is low due to the efficient nature
of the current mode switching regulator.
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