LTC3109
APPLICATIONS INFORMATION
IꢁTRODUCTIOꢁ
ripple caused by the source’s ESR and the peak primary
switchingcurrent(whichcanreachhundredsofmilliamps).
Since the input voltage may be of either polarity, a ceramic
capacitor is recommended.
The LTC3109 is designed to gather energy from very low
input voltage sources and convert it to usable output
voltages to power microprocessors, wireless transmit-
ters and analog sensors. Its architecture is specifically
tailored to applications where the input voltage polarity is
unknown, or can change. This “auto-polarity” capability
makes it ideally suited to energy harvesting applications
using a TEG whose temperature differential may be of
either polarity.
PELTIER CELL (TꢄERMOELECTRIC GEꢁERꢃTOR)
A Peltier cell is made up of a large number of series-con-
nected P-N junctions, sandwiched between two parallel
ceramic plates. Although Peltier cells are often used as
coolers by applying a DC voltage to their inputs, they will
alsogenerateaDCoutputvoltage,usingtheSeebeckeffect,
when the two plates are at different temperatures.
Applications such as wireless sensors typically require
much more peak power, and at higher voltages, than
the input voltage source can produce. The LTC3109 is
designed to accumulate and manage energy over a long
period of time to enable short power pulses for acquiring
and transmitting data. The pulses must occur at a low
enough duty cycle that the total output energy during the
pulsedoesnotexceedtheaveragesourcepowerintegrated
over the accumulation time between pulses. For many
applications, this time between pulses could be seconds,
minutes or hours.
When used in this manner, they are referred to as thermo-
electricgenerators(TEGs).Thepolarityoftheoutputvoltage
will depend on the polarity of the temperature differential
between the TEG plates. The magnitude of the output volt-
age is proportional to the magnitude of the temperature
differential between the plates.
The low voltage capability of the LTC3109 design allows it
tooperatefromatypicalTEGwithtemperaturedifferentials
as low as 1°C of either polarity, making it ideal for harvest-
ing energy in applications where a temperature difference
exists between two surfaces or between a surface and
the ambient temperature. The internal resistance (ESR)
of most TEGs is in the range of 1Ω to 5Ω, allowing for
reasonablepowertransfer.ThecurvesinFigure2showthe
open-circuit output voltage and maximum power transfer
for a typical TEG with an ESR of 2Ω, over a 20°C range of
temperature differential (of either polarity).
The PGOOD signal can be used to enable a sleeping
microprocessor or other circuitry when V
reaches
OUT
regulation, indicating that enough energy is available for
a transmit pulse.
IꢁPUT VOLTꢃGE SOURCES
The LTC3109 can operate from a number of low input
voltage sources, such as Peltier cells (thermoelectric
generators), or low level AC sources. The minimum input
voltage required for a given application will depend on the
transformer turns ratios, the load power required, and the
internal DC resistance (ESR) of the voltage source. Lower
ESRsourceswillallowoperationfromlowerinputvoltages,
and provide higher output power capability.
1000
100
10
100
10
1
TEG: 30mm SQUARE
127 COUPLES
R = 2Ω
V
MAX P
OUT
(IDEAL)
OC
For a given transformer turns ratio, there is a maximum
recommended input voltage to avoid excessively high
secondary voltages and power dissipation in the shunt
regulator. It is recommended that the maximum input
voltage times the turns ratio be less than 50.
1
0.1
1
10
dT (°C)
100
3109 F02
Note that a low ESR decoupling capacitor may be required
acrossaDCinputsourcetopreventlargevoltagedroopand
Figure 2. TypiAal PerformanAe of a Peltier Cell
ꢃAting as a Power Generator (TEG)
3109fa
12
Linear Systems [ Linear Systems ]
