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LTC3331

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time by allowing the battery to fully discharge before the

disconnectevent.Conversely,byincreasingthelowbattery

disconnect threshold more capacity remains following the

disconnecteventwhichextendstheshelflifeofthebattery.

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

worksagainstthebatteryseriesresistanceandtemporarily

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 bat-

tery 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. Typi-

cally 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

TheinputprioritizerontheLTC3331decideswhethertouse

the energy harvesting input or the battery input to power

VOUT. If a battery is powering the buck-boost converter

and harvested energy causes a UVLO rising transition on

VIN, the prioritizer will shut off the buck-boost and turn on

the buck, orchestrating a smooth transition that maintains

regulation of VOUT.

OPERATION

When harvestable energy disappears, the prioritizer will

first poll the BB_IN voltage. If the BB_IN voltage is above

1.8Vtheprioritizerwillswitchbacktothebuck-boostwhile

maintaining regulation. If the BB_IN voltage is below 1.8V

thebuck-boostisnotenabledandVOUTcannotbesupported

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 VIN

until harvested energy is again available.

If either BB_IN or VIN 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 VIN 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 VIN.

When the prioritizer selects the VINinputthecurrentonthe

BB_IN input drops to 200nA. However, if the voltage on

BB_IN is higher than VIN2, a fraction of the VIN 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

hasselectedtheVINinput.TheEH_ONoutputisreferenced

to VIN3.

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

VOUT to allow for increased energy storage at VOUT 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.

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