LTC3707 Linear Technology, LTC3707 Datasheet - Page 11

• Manufacturer Part Number: LTC3707
• Description: High Efficiency / 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
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 potentially boost-
ing 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
external AC or DC signal source to control the instanta-
neous frequency of the oscillator. Voltage should not be
applied to the FREQSET pin prior to the application of
voltage to the V
INTV
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
When the EXTV
dropout linear regulator supplies INTV
is taken above 4.7V, the 5V regulator is turned off and an
internal switch is turned on connecting EXTV
This allows the INTV
efficiency external source such as the output of the regu-
lator itself or a secondary winding, as described in the
Applications Information.
Standby Mode Pin
The STBYMD pin is a three-state input that controls
common 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 con-
troller and activating necessary common internal biasing.
When the STBYMD pin is taken above 2V, both internal
CC
/EXTV
CC
IN
Power
CC
U
pin.
pin is left open, an internal 5V low
CC
(Refer to Functional Diagram)
power to be derived from a high
CC
power. If EXTV
CC
to INTV
CC
pin.
CC
CC
.
linear regulators are turned on independent of the state on
the RUN/SS pins of the two switching regulator control-
lers, providing an output power source for “wake-up”
circuitry. Decouple the pin with a small capacitor (0.01 F)
to ground if the pin is not connected to a DC potential.
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
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% require-
ment, the MOSFET is turned off within 10 s 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 >5 A 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
LTC3707
11
3707f