LTC3331_15 Datasheet, PDF (19/34 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

LTC3331
OPERATION
this point the current in the inductor is equal to I_{PEAK_BB}
and the I_{PEAK_BB} comparator will trip turning off M1 and
turning on M2 causing the inductor current to ramp down
to I_ZERO, completing the transition from buck-boost mode
to buck mode.
V_OUT Power Good
A power good comparator is provided for the V_OUT output.
It transitions high the first time the LTC3331 goes to sleep,
indicating that V_OUT has reached regulation. It transitions
low when V_OUT falls to 92% (typical) of its regulation value.
The PGVOUT output is referenced to an internal rail that
is generated to be the highest of V_IN2, BB_IN, and V_OUT
less a Schottky diode drop.
Shunt Battery Charger
The LTC3331 provides a reliable low quiescent current
shunt battery charger to facilitate charging a battery
with harvested energy. A low battery disconnect feature
provides protection to the battery from overdischarge by
disconnecting the battery from the buck-boost input at a
programmable level.
To use the charger connect the battery to the BAT_IN
pin. An internal low battery disconnect PMOS switch is
connected between the BAT_IN pin and the BAT_OUT
pin. The BAT_OUT pin must be connected to BB_IN for
proper operation. A charging resistor connected from V_IN
to BAT_OUT or from CHARGE to BAT_OUT will charge the
battery through the body diode of the disconnect PMOS
until the battery voltage rises above the low-battery connect
threshold. Depending on the capacity of the battery and
the input decoupling capacitor, the common BAT_OUT
= BB_IN node voltage generally rises or falls to V_{BAT_IN}
when the PMOS turns on. Once the PMOS is on the charge
current is determined by the charging resistor, the battery
voltage, and the voltage of the charging source.
As the battery voltage approaches the float voltage, the
LTC3331 shunts current away from the battery thereby
reducing the charging current. The LTC3331 can shunt
up to 10mA. Float voltages of 3.45V, 4.0V, 4.1V, and 4.2V
are programmable via the FLOAT[1:0] pins (see Table 2).
Charging can occur through a resistor connected to V_IN
or the CHARGE pin. An internal set of back to back PMOS
switches are connected between CHARGE and V_IN2 and are
turned on only when the energy harvesting buck converter
is sleeping. In this way charging of the battery only hap-
pens when there is excess harvested energy available and
the V_OUT output is prioritized over charging of the battery.
The charge current available from this pin is limited by the
strength of the V_IN2 LDO and an appropriate charging resis-
tor must be selected to limit this current. The on resistance
of the internal charge switches combined is approximately
60Ω. To charge with higher currents connect a resistor
directly to V_IN. Note that when charging from V_IN the bat-
tery is always being charged. Care must be taken to ensure
that enough power is available to bring up the V_OUT output.
Low Battery Disconnect/Connect: LBD/LBC
The low battery disconnect (V_LBD) and connect (V_LBC)
voltage levels are programmed by the LBSEL and
FLOAT[1:0] pins (see Table 2). As shown in the Block
Diagram the battery disconnects from the common
BAT_OUT = BB_IN node by shutting off the PMOS switch
when the BAT_IN voltage falls below V_LBD. This disconnect
function protects Li-Ion batteries from permanent damage
due to deep discharge. Disconnecting the battery from
the common BAT_OUT = BB_IN node prevents the load
as well as the LTC3331 quiescent current from further
discharging the battery.
Once disconnected the common BAT_OUT = BB_IN node
voltage collapses towards ground. When an input supply is
reconnected the battery charges through the internal body
diode of the disconnect PMOS. The input supply voltage
should be larger than V_{LBC_BAT_OUT} to ensure that the PMOS
is turned on. When the voltage reaches V_{LBC_BAT_OUT}, the
PMOS turns on and connects the common BAT_OUT =
BB_IN node to BAT_IN. While disconnected, the BAT_IN
pin voltage is indirectly sensed through the PMOS body
diode. Therefore V_{LBC_BAT_IN} varies with charge current
and junction temperature. See the Typical Performance
Characteristics section for more information.
Low Battery Select
The low battery disconnect voltage level is programmed by
the LBSEL pin for each float setting. The LBSEL pin allows
the user to trade-off battery run time and maximum shelf
life. A lower battery disconnect threshold maximizes run
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For more information www.linear.com/LTC3331
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