LTC3115-1
16
Rev. C
For more information
OPERATION
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 inter-
nal control circuitry and power device gate drivers of the
LTC3115-1. 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
regula-
tor includes 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 improve-
ment, 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 LTC3115-1 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 LTC3115-1. The dominant factor depends on the volt-
age drop between V
IN
and V
CC
which is determined by
the dropout voltage of the V
CC
regulator and is propor-
tional to the total load current drawn from V
CC
. The load
current 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 applications
where V
CC
is bootstrapped (powered by V
OUT
or by an
auxiliary supply rail through a Schottky diode) the mini-
mum 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
threshold. Therefore, in order to place the part in shut-
down and reduce the input current to its minimum level
(3µA typical) it is necessary to ensure that the RUN pin
is brought below the worst-case logic threshold (0.3V).
The RUN pin is a high voltage input and can be connected
directly to V
IN
to continuously enable the part when the
input supply is present. If the RUN pin is forced above
approximately 5V it will sink a small current as given by
the following equation:
I
RUN
≅
V
RUN
– 5V
5MΩ
With the addition of an external resistor divider as shown
