LTC3115-2_15 Datasheet, PDF (16/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

LTC3115-2
Operation
SOFT-START
To minimize input current transients on power-up, the
LTC3115-2 incorporates an internal soft-start circuit with
a nominal duration of 9ms. The soft-start is implemented
by a linearly increasing ramp of the error amplifier refer-
ence voltage during the soft-start duration. As a result,
the duration of the soft-start period is largely unaffected
by the size of the output capacitor or the output regula-
tion voltage. Given the closed-loop nature of the soft-start
implementation, the converter is able to respond to load
transients that occur during the soft-start interval. The
soft-start period is reset by thermal shutdown and UVLO
events on both V_IN and V_CC.
V_CC REGULATOR
An internal low dropout regulator generates the 4.45V
(nominal) V_CC rail from V_IN. The V_CC rail powers the in-
ternal control circuitry and power device gate drivers of
the LTC3115-2. The V_CC regulator is disabled in shutdown
to reduce quiescent current and is enabled by forcing the
RUN pin above its logic threshold. The V_CC regulator in-
cludes current limit protection to safeguard against short
circuiting of the V_CC rail. For applications where the output
voltage is set to 5V, the V_CC rail can be driven from the
output rail through a Schottky diode. Bootstrapping in this
manner can provide a significant efficiency improvement,
particularly at large voltage step down ratios, and may
also allow operation down to a lower input voltage. The
maximum operating voltage for the V_CC pin is 5.5V. When
forcing V_CC externally, care must be taken to ensure that
this limit is not exceeded.
UNDERVOLTAGE LOCKOUT
To eliminate erratic behavior when the input voltage is
too low to ensure proper operation, the LTC3115-2 incor-
porates internal undervoltage lockout (UVLO) circuitry.
There are two UVLO comparators, one that monitors V_IN
and another that monitors V_CC. The buck-boost converter
is disabled if either V_IN or V_CC falls below its respective
UVLO threshold. The input voltage UVLO comparator has
a falling threshold of 2.4V (typical). If the input voltage
falls below this level all switching is disabled until the input
voltage rises above 2.6V (nominal). The V_CC UVLO has a
falling threshold of 2.4V. If V_CC falls below this threshold
the buck-boost converter is prevented from switching until
V_CC rises above 2.6V.
Depending on the particular application circuit it is pos-
sible that either of these UVLO thresholds could be the
factor limiting the minimum input operating voltage of the
LTC3115-2. The dominant factor depends on the voltage
drop between V_IN and V_CC which is determined by the
dropout voltage of the V_CC regulator and is proportional
to the total load current drawn from V_CC. The load cur-
rent on the V_CC regulator is principally generated by the
gate driver supply currents which are proportional to
operating frequency and generally increase with larger
input and output voltages. As a result, at higher switching
frequencies and higher input and output voltages the V_CC
regulator dropout voltage will increase, making it more
likely that the V_CC UVLO threshold could become the lim-
iting factor. Curves provided in the Typical Performance
Characteristics section of this data sheet show the typical
V_CC current and can be used to estimate the V_CC regulator
dropout voltage in a particular application. In applica-
tions where V_CC is bootstrapped (powered by V_OUT or
by an auxiliary supply rail through a Schottky diode) the
minimum input operating voltage will be limited only by
the input voltage UVLO threshold.
RUN PIN COMPARATOR
In addition to serving as a logic-level input to enable the
IC, the RUN pin features an accurate internal compara-
tor allowing it to be used to set custom rising and falling
input undervoltage lockout thresholds with the addition of
an external resistor divider. When the RUN pin is driven
above its logic threshold (typically 0.8V) the V_CC regulator
is enabled which provides power to the internal control
circuitry of the IC and the accurate RUN pin comparator is
enabled. If the RUN pin voltage is increased further so that
it exceeds the RUN comparator threshold (1.21V nominal),
the buck-boost converter will be enabled.
If the RUN pin is brought below the RUN comparator
threshold, the buck-boost converter will inhibit switching,
but the V_CC regulator and control circuitry will remain
powered unless the RUN pin is brought below its logic
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