LTC3108 Linear Technology, LTC3108 Datasheet - Page 12

• Manufacturer Part Number: LTC3108
• Description: Ultralow Voltage Step-Up Converter and Power Manager
• Manufacturer: Linear Technology

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LTC3108
APPLICATIONS INFORMATION
The low voltage capability of the LTC3108 design allows
it to operate from a TEG with temperature differentials
as low as 1°C, making it ideal for harvesting energy in
applications in which a temperature difference exists
between two surfaces or between a surface and the am-
bient temperature. The internal resistance (ESR) of most
cells is in the range of 1Ω to 5Ω, allowing for reasonable
power transfer. The curves in Figure 2 show the open-
circuit output voltage and maximum power transfer for a
typical Peltier cell (with an ESR of 2Ω) over a 20°C range
of temperature differential.
TEG Load Matching
The LTC3108 was designed to present a minimum input
resistance (load) in the range of 2Ω to 10Ω, depending
on input voltage and transformer turns ratio (as shown
in the Typical Performance Characteristics curves). For
a given turns ratio, as the input voltage drops, the input
resistance increases. This feature allows the LTC3108 to
optimize power transfer from sources with a few ohms
of source resistance, such as a typical TEG. Note that a
lower source resistance will always provide more output
current capability by providing a higher input voltage
under load.
Peltier Cell (TEG) Suppliers
Peltier cells are available in a wide range of sizes and power
capabilities, from less than 10mm square to over 50mm
square. They are typically 2mm to 5mm in height. A list
of Peltier cell manufacturers is given in Table 3.
Table 3. Peltier Cell Manufacturers
Fujitaka
www.fujitaka.com/pub/peltier/english/thermoelectric_power.html
FerroTec
www.ferrotec.com/products/thermal/modules
Melcor
www.melcor.com/tec.html
Micropelt
www.micropelt.com
Nextreme
www.nextreme.com
TE Technology
www.tetech.com/Peltier-Thermoelectric-Cooler-Modules.html
Tellurex
www.tellurex.com
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Thermopile Generator
Thermopile generators (also called powerpile generators)
are made up of a number of series-connected thermo-
couples enclosed in a metal tube. They are commonly
used in gas burner applications to generate a DC output
of hundreds of millivolts when exposed to the high tem-
perature of a fl ame. Typical examples are the Honeywell
CQ200 and Q313. These devices have an internal series
resistance of less than 3Ω, and can generate as much as
750mV open-circuit at their highest rated temperature. For
applications in which the temperature rise is too high for
a solid-state thermoelectric device, a thermopile can be
used as an energy source to power the LTC3108. Because
of the higher output voltages possible with a thermopile
generator, a much lower transformer turns ratio can be
used (typically 1:20, depending on the application).
Photovoltaic Cell
The LTC3108 converter can also operate from a single
photovoltaic (solar) cell operating in indoor lighting, where
the available power is orders of magnitude less than in
direct sunlight. This capability allows the LTC3108 to power
circuits at light levels far too low for even very low input
voltage boost converters.
Non-Boost Applications
The LTC3108 can also be used as an energy harvester
and power manager for input sources that do not require
boosting. In these applications the step-up transformer
can be eliminated.
Any source whose peak voltage exceeds 2.5V AC or 5V
DC can be connected to the C1 input through a current-
limiting resistor where it will be rectifi ed/peak detected. In
these applications the C2 and SW pins are not used and
can be grounded or left open.
Examples of such input sources would be piezoelectric
transducers, vibration energy harvesters, low current
generators, a stack of low current solar cells or a 60Hz
AC input.
A series resistance of at least 100Ω/V should be used
to limit the maximum current into the VAUX shunt
regulator.
3108p