Datasheet

LTC3115-2
15
31152f
For more information www.linear.com/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