LTC3331_15 Datasheet, PDF (22/34 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

LTC3331
APPLICATIONS INFORMATION
PIEZO MODEL
LTC3331
AC1
V_IN
I_P sin(ωt)
C_P
C_IN
GND
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Figure 4. LTC3331 Voltage Doubler Configuration
A second piezo may be connected from AC2 to ground. This
may be of use if the second piezo is mechanically tuned
to a different resonant frequency present in the system
than the first piezo. To achieve maximum power transfer
from the piezo with the doubler the UVLO window should
be set to the open circuit voltage of the piezo.
Piezoelectric elements are available from the manufactur-
ers listed in Table 5.
Table 5. Piezoelectric Element Manufacturers
Advanced Cerametrics
www.advancedcerametrics.com
Piezo Systems
www.piezo.com
Measurement Specialties
www.meas-spec.com
PI (Physik Instrumente)
www.pi-usa.us
MIDE Technology Corporation www.mide.com
Morgan Technical Ceramics www.morganelectroceramics.com
Electromagnetic Energy Harvesting
Another alternative AC source is an electromagnetic vibra-
tion harvester in which a magnet vibrating inside a coil
induces an AC voltage and current in the coil that can then
be rectified and harvested by the LTC3331. The vibration
could be ambient to the system or it could be caused by
an impulse as in a spring loaded switch.
Solar Energy Harvesting
The LTC3331 can harvest solar energy as the bridge recti-
fier can be used to provide reverse protection for a solar
panel. A solar cell produces current in proportion to the
amount of light falling on it. Figure 5 shows the relationship
between current and voltage for a solar panel illuminated
with several levels of light. The maximum power output
occurs near the knee of each curve where the cell transi-
tions from a constant current device to a constant voltage
500
1500
PANEL CURRENT SANYO 1815 SOLAR PANEL
PANEL POWER
400 ^{1800 LUX}
1200
300
900
1000 LUX
200
600
500 LUX
100
300
200 LUX
0
0
0
1
2
3
4
5
6
V_PANEL (V)
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Figure 5. Typical Solar Panel Characteristics
device. Fortunately, the peak power point doesn’t change
much with illumination and an appropriate UVLO window
can be selected so that the panel operates near the peak
power point for a majority of light conditions.
Two solar panels can be connected to the LTC3331, one
from AC1 to ground and another from AC2 to ground. Each
panel could be aimed in a different direction to capture
light from different angles or at different times of the day
as the sun moves. The panels should be similar so that
the selected UVLO window is optimal for both panels.
BB_IN/BAT_OUT, BAT_IN, V_IN, and V_OUT Capacitors
The input to the buck-boost, BB_IN, must be connected to
BAT_OUT for proper operation. BAT_OUT is the output side
of the low battery disconnect switch. The series resistance
of this switch must be considered when selecting a bypass
capacitor for the common BAT_OUT = BB_IN node. At least
4.7μF to GND or greater should be used. For the higher
I_{PEAK_BB} settings the capacitor may need to be larger to
smooth the voltage at the common BAT_OUT = BB_IN
node. The goal is to average the input current to the buck
boost so that the voltage droop at the common BAT_OUT
= BB_IN node is minimized.
A bypass capacitor of 1µF or greater can also be placed
at the BAT_IN pin. In cases where the series resistance of
the battery is high, a larger capacitor may be desired to
handle transients.
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For more information www.linear.com/LTC3331
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