LTC3331_15 Datasheet, PDF (20/34 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

LTC3331
OPERATION
time by allowing the battery to fully discharge before the
disconnect event. Conversely, by increasing the low battery
disconnect threshold more capacity remains following the
disconnect event which extends the shelf life of the battery.
For maximum run time, tie LBSEL to GND. For extended
shelf life, tie LBSEL to the common BAT_OUT = BB_IN
node. If a high peak current event is expected, users may
temporarily select the lower disconnect threshold. This
avoids disconnecting the battery too early when the load
works against the battery series resistance and temporarily
reduces the common BAT_OUT = BB_IN node.
Ship Mode
A ship mode is provided which manually disconnects the
battery. This may be useful to prevent discharge of the
battery in situations when no harvestable energy is expected
for a long period of time such as during shipping. Bring
the SHIP pin high to engage ship mode. The low battery
disconnect PMOS will turn off, disconnecting the battery
at BAT_IN from the common BAT_OUT = BB_IN node. If
no harvestable energy is present to hold up the common
BAT_OUT = BB_IN node that voltage will collapse. Typically
an additional 1µA of quiescent current will appear on BB_IN
while SHIP mode is engaged.
To exit SHIP mode first bring the SHIP pin low. If the
BB_IN voltage had collapsed while in SHIP mode it must
now be brought above the LBC threshold to reconnect
the battery. This can be done manually or from an energy
harvesting charging source. If harvestable energy had
been propping up the common BAT_OUT = BB_IN node
voltage above the LBC threshold then the battery will be
connected immediately.
Prioritizer
The input prioritizer on the LTC3331 decides whether to use
the energy harvesting input or the battery input to power
V_OUT. If a battery is powering the buck-boost converter
and harvested energy causes a UVLO rising transition on
V_IN, the prioritizer will shut off the buck-boost and turn on
the buck, orchestrating a smooth transition that maintains
regulation of V_OUT.
When harvestable energy disappears, the prioritizer will
first poll the BB_IN voltage. If the BB_IN voltage is above
1.8V the prioritizer will switch back to the buck-boost while
maintaining regulation. If the BB_IN voltage is below 1.8V
the buck-boost is not enabled and V_OUT cannot be supported
until harvestable energy is again available. If the battery
is connected then the BB_IN voltage will be above 1.8V
for every float and LBSEL combination. If the battery is
disconnected the BB_IN voltage will have collapsed below
1.8V and the prioritizer will not switch to the buck-boost
when harvestable energy goes away. In the event that the
battery is depleted and is disconnected while powering
the buck-boost the prioritizer will not switch back to V_IN
until harvested energy is again available.
If either BB_IN or V_IN is grounded, the prioritizer allows
the other input to run if its input is high enough for op-
eration. The specified quiescent current in UVLO is valid
upon start-up of the V_IN input and when the battery has
taken over regulation of the output. If the battery is less
than 1.8V when UVLO is entered and the prioritizer does
not enable the buck-boost several hundred nanoamperes
of additional quiescent current will appear on V_IN.
When the prioritizer selects the V_IN input the current on the
BB_IN input drops to 200nA. However, if the voltage on
BB_IN is higher than V_IN2, a fraction of the V_IN quiescent
current will appear on BB_IN due to internal level shifting.
This only affects a small range of battery voltages and
UVLO settings.
A digital output, EH_ON, is low when the prioritizer has
selected the BB_IN input and is high when the prioritizer
has selected the V_IN input. The EH_ON output is referenced
to V_IN3.
Supercapacitor Balancer
An integrated supercapacitor balancer with 150nA of
quiescent current is available to balance a stack of two
supercapacitors. Typically the input, SCAP, will tie to
V_OUT to allow for increased energy storage at V_OUT with
supercapacitors. The BAL pin is tied to the middle of the
stack and can source and sink 10mA to regulate the BAL
pin’s voltage to half that of the SCAP pin’s voltage. To
disable the balancer and its associated quiescent current
the SCAP and BAL pins can be tied to ground.
3331fb
20
For more information www.linear.com/LTC3331