LTC1628-PG_15 Datasheet, PDF (11/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

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OPERATIO (Refer to Functional Diagram)
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.
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 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
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 (0.01µF)
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.
LTC1628/LTC1628-PG
Fault Coupling Pin
The FLTCPL pin (LTC1628 only) controls two functions
that can operate individually (FLTCPL = 0V) or unilaterally
(FLTCPL = INTV_CC) between the two controllers. When the
FLTCPL pin is grounded (internally tied default mode for
the LTC1628-PG), 1) the FCB input forces continuous
operation only on the first controller when the applied
voltage drops below 0.8V and 2) the short-circuit latchoff
function only latches off the controller having the shorted
output. When the FLTCPL pin is tied to INTV_CC, 1) the FCB
input forces continuous operation on both controllers
when the applied voltage drops below 0.8V and 2) the
short-circuit latchoff function latches off both controllers
when either has a shorted output.
Power Good (PGOOD) Pin
The PGOOD pin (LTC1628-PG only) 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% 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 (or both controllers
as determined by the FLTCPL pin, LTC1628 only) 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
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