LTC3729_15 Datasheet, PDF (11/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
OPERATION (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 regulator 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_OUT+ and V_OUT– benefits regulation
in high current applications and/or applications having
electrical interconnection losses.
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 ±7.5% of its nominal output level as determined by
the feedback divider. When the output is within ±7.5% of
its nominal value, the MOSFET is turned off within 10µs
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 volt‑
age 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.
APPLICATIONS INFORMATION
The basic LTC3729 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).
R_SENSE Selection For Output Current
R_{SENSE1, 2} are chosen based on the required output cur‑
rent. The LTC3729 current comparator has a maximum
threshold of 75mV/R_SENSE and an input common mode
range of SGND to 1.1( INTV_CC). The current comparator
threshold sets the peak inductor current, yielding a maxi‑
mum average output current I_MAX equal to the peak value
less half the peak-to-peak ripple current, ∆I_L.
Allowing a margin for variations in the LTC3729 and external
component values yields:
R_SENSE = (50mV/I_MAX)N
where N = number of stages.
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