LTC3107
10
3107f
For more information www.linear.com/LTC3107
VAUX
The active circuits within the LTC3107 are powered from
VAUX, which should be bypassed with a capacitor of 10µF
minimum.
The quiescent current draw on VAUX is typically just 6µA.
If harvested energy is available, this current will come from
the harvesting source. If there is no harvesting energy
available, the VAUX supply current will come from V
BAT
.
A shunt regulator limits the maximum voltage on VAUX
to 4.3V typical. It shunts to ground any excess harvested
current into VAUX when there is no load on the converter
or the input source is generating more power than is
required by the load. If the optional storage capacitor is
connected to VSTORE, then the excess current will be used
to charge the storage capacitor, and current will not be
shunted to ground until the storage capacitor is charged
up to the 4.3V clamp level.
Voltage Reference
The LTC3107 includes a precision, micropower reference,
for accurate regulated output voltages. This reference
becomes active as soon as VAUX exceeds 1.9V.
Low Dropout Linear Regulator (LDO)
The LTC3107 includes a low current LDO to provide a
regulated 2.2V output for powering low power processors.
The LDO is powered by the higher
of VAUX or V
OUT
, and
requires a minimum of 2.2µF ceramic decoupling capacitor.
Larger capacitor values can be used without limitation.
If the LDO is not being used, the VLDO pin should be tied
to VAUX.
V
OUT
The LTC3107 is designed to fit seamlessly into existing
applications that run from a primary battery, while adding
the benefit of energy harvesting to increase the life of the
battery. The main output voltage on V
OUT
is designed to
track the battery voltage on V
BAT
. If no harvested energy
is available, or the energy is insufficient to maintain V
OUT
,
then V
OUT
will be hysteretically regulated 230mV below
V
BAT
by periodically connecting it to V
BAT
.
When enough harvested energy is available to power the
load, V
OUT
will be hysteretically regulated to a voltage
typically 30mV below V
BAT
, and the battery will not be
used. In this condition, the current drain on the battery is
only 80nA typical.
In a typical application, a bulk decoupling capacitor (usually
a few hundred microfarads) is connected to V
OUT
to allow
it to ride-through small, periodic load transients typical of
a wireless sensor application. If the V
OUT
capacitor is sized
appropriately (see the Applications Information section
for
more detail), and the average harvested input power
exceeds the average load power, then battery energy will
never be used.
BAT_OFF
The BAT_OFF output is a digital output with an internal
pull-up to V
OUT
. BAT_OFF is an indicator of when the
battery is being used to help maintain V
OUT
. If BAT_OFF
is high, it indicates that V
OUT
(and VLDO) are being
powered entirely by the harvested input power (including
the VSTORE capacitor), and the battery is not being used.
In this case, the battery current draw is only 80nA typical.
When BAT_OFF goes low, it indicates that the battery is
being used to help maintain V
OUT
and VLDO in regulation.
This indicates that either there is no harvested energy
available, or it is insufficient to power the load entirely.
If the C
OUT
capacitor is not sized properly, the BAT_OFF
indicator may go low during a pulsed load event, to in-
dicate that
current is being drawn from the battery. See
the Applications Information
section of this data sheet for
guidance on sizing the C
OUT
capacitor.
VSTORE
The VSTORE output can be used to charge an optional
storage capacitor, after V
OUT
has reached regulation.
The VSTORE capacitor value can range from hundreds of
micro-farads up to Farads. Once V
OUT
has reached regula-
tion, the VSTORE output will be allowed to charge up to
the maximum V
AUX voltage if excess harvested energy is
available. The storage capacitor on VSTORE can be used
to power the system in the event that the input source is
lost, or is unable to provide the current demanded by the
loads on V
OUT
and VLDO, or simply to supplement the
operaTion
