LTC3729L-6 Datasheet, PDF (11/28 Pages) Linear Technology – PolyPhase, Synchronous Step-Down Switching Regulator

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OPERATIO (Refer to Functional Diagram)
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
of the IC circuitry is derived from INTV_CC. When the
EXTV_CC pin is left open, an internal 5V low dropout
regulator supplies INTV_CC power. If the EXTV_CC pin 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 section. An external Schottky
diode can be used to minimize the voltage drop from
EXTV_CC to INTV_CC in applications requiring greater than
the specified INTV_CC current. Voltages up to 7V can be
applied to EXTV_CC for additional gate drive capability.
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both V_OS+ and V_OS– benefits regulation
in high current applications and/or applications having
electrical interconnection losses. The amplifier has an
output slew rate of 5V/µs and is capable of driving capaci-
tive loads with an output RMS current typically up to
25mA. The amplifier is not capable of sinking current and
therefore must be resistively loaded to do so. The differen-
tial amplifier is configured as a unity-gain differencing
amplifier.
LTC3729L-6
Power Good (PGOOD)
The PGOOD pin is connected to the drain of an internal
MOSFET. The MOSFET turns on when the output is not
within ±10% of its nominal output level as determined by
the feedback divider and when the internal 100µs timer
expires. When the output is within ±10% of its nominal
value, the MOSFET is turned off within a few microseconds
and the PGOOD pin should be pulled up by an external
resistor to a source of up to 7V.
Short-Circuit Detection
The RUN/SS capacitor is used initially to limit the inrush
current from the input power source. Once the controllers
have been given time, as determined by the capacitor on
the RUN/SS pin, to charge up the output capacitors and
provide full load current, the RUN/SS capacitor is then
used as a short-circuit timeout circuit. If the output voltage
falls to less than 70% of its nominal output voltage the
RUN/SS capacitor begins discharging assuming that the
output is in a severe 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 voltage
is recycled. This built-in latchoff can be overidden by
providing a >5µA pull-up current at a compliance of 5V to
the RUN/SS pin. This current shortens the soft-start
period but also prevents net discharge of the RUN/SS
capacitor during a severe overcurrent and/or short-circuit
condition. Foldback current limiting is activated when the
output voltage falls below 70% of its nominal level whether
or not the short-circuit latchoff circuit is enabled.
APPLICATIO S I FOR ATIO
The basic LTC3729L-6 application circuit is shown in
Figure␣ 1 on the first page. External component selection is
driven by the load requirement, and begins with the
selection of R_{SENSE1, 2}. Once R_{SENSE1, 2} are known, L1 and
L2 can be chosen. Next, the power MOSFETs and D1 and
D2 are selected. The operating frequency and the inductor
are chosen based mainly on the amount of ripple current.
Finally, C_IN is selected for its ability to handle the input
ripple current (that PolyPhase operation minimizes) and
C_OUT is chosen with low enough ESR to meet the output
ripple voltage and load step specifications (also minimized
with PolyPhase). The circuit shown in Figure␣ 1 can be
configured for operation up to an input voltage of 28V
(limited by the external MOSFETs).
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