Datasheet

LTC3114-1
17
Rev. C
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OPERATION
is disabled until LDO rises above 2.5V (typical) as long as
V
IN
is above its nominal UVLO threshold level.
Depending on the particular application, either of these
UVLO thresholds could be the limiting factor affecting
the minimum input voltage required for operation. The
LTC3114-1 LDO regulator uses V
IN
for its power input. If
LDO is not bootstrapped, then there exists a voltage drop or
dropout voltage between V
IN
and LDO. The dropout voltage
is proportional to the loading on LDO, which is primarily
due to the gate charge and capacitive charging currents
inherent to the internal power switches. The loading on LDO
and the LDO dropout voltage, therefore, are proportional
to V
IN
and V
OUT
. For this reason, the minimum input volt-
age required for operation is limited by the LDO minimum
voltage as input voltage (V
IN
) will always be higher than
LDO in the normal (non-bootstrapped) configuration. The
Typical Performance Characteristics section of this data
sheet provides guidance on the dropout voltage between
V
IN
and LDO over the range of V
IN
and V
OUT
.
In applications where LDO is bootstrapped (powered by
V
OUT
through a Schottky diode or auxiliary power rail), the
minimum input voltage for operation (after start-up) will
be limited only by the V
IN
UVLO threshold (2.1V typical).
Please note that if the bootstrap voltage is derived from the
LTC3114-1 V
OUT
and not an independent power rail, then
the minimum input voltage required for initial start-up is
still limited by the minimum LDO voltage (2.6V typical).
RUN PIN COMPARATOR
In addition to serving as a logic-level input to enable cer
-
tain functions of the IC, the RUN pin includes an accurate
internal comparator that allows it to be used to set custom
rising and falling on/off thresholds with the addition of an
external resistor divider
. When RUN is driven above its
logic threshold (0.7V typical), the LDO regulator is enabled,
which provides power to the internal control circuitry of
the IC. If the voltage on RUN is increased further so that
it exceeds the RUN comparator accurate analog threshold
(1.2V nominal), all functions of the buck-boost converter
will be enabled and a startup sequence will ensue.
If RUN is brought below the accurate comparator threshold,
the buck-boost converter will inhibit switching, but the LDO
regulator and control circuitry will remain powered unless
RUN is brought below its logic threshold. Therefore, in
order to completely shut down the IC and reduce the V
IN
current to 3µA (typical), it is necessary to ensure that RUN
is brought below its worst-case low-logic threshold of
0.3V. RUN is a high voltage input and can be tied directly
to V
IN
to continuously enable the IC when the input supply
is present. Also note that RUN can be driven above V
IN
or
V
OUT
as long as it stays within the operating range of the
IC, that is, less than 40V. If RUN is forced above 5V, it will
sink a small current as given by the following equation:
I
RUN
≈
V
RUN
– 5V
5MΩ
With the addition of an optional resistor divider as shown
in Figure3, the RUN pin can be used to establish a user-
programmable turn on and turn off threshold.
The buck-boost converter is enabled when the voltage on
RUN reaches 1.205V (nominal). Therefore, the turn-on
voltage threshold on V
IN
is given by:
V
TURNON
= 1.205V 1+
R1
R2
⎛
⎝
⎜
⎞
⎠
⎟
Once the converter is enabled, the RUN comparator in-
cludes a built-in hysteresis of approximately 140mV, so
that the turn-off threshold will be approximately 8.33%
lower than the turn-on threshold. Put another way
, the
internal threshold level for the RUN comparator looks like
1.1V after the IC is enabled.
Figure3. Accurate RUN Pin Comparator
+
–
+
–
ENABLE SWITCHING
ENABLE LDO AND
CONTROL CIRCUITS
1.2V
ACCURATE THRESHOLD
LTC3114-1
LOGIC THRESHOLD
0.7V
31141 F03
RUN
R1
R2
V
IN
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