LTC3107
15
3107f
For more information www.linear.com/LTC3107
Introduction
The LTC3107 is designed to gather energy from very low
input voltage sources and use it to extend the life of a
primary battery in applications such as wireless sensors.
The LTC3107 is designed to accumulate and manage
energy over long periods of time to enable short power
bursts for acquiring and transmitting data. These bursts
must occur at a low enough duty cycle such that the total
output energy during the burst does not exceed the aver
-
age source power integrated over the accumulation time
between bursts. In these instances, the battery will not
be used at all, so the battery life may be extended up to
the shelf life of the battery.
Harvesting Input Voltage Sources
The LTC3107 can operate from a number of low input
voltage sources, such as thermoelectric generators, ther
-
mopiles and coil and magnet transducers. The minimum
input voltage required
for a given application will depend
on the transformer turns ratio, the load power required,
and the internal DC resistance (ESR) of the voltage source.
Lower ESR sources (typically less than 10Ω) will allow
the use of lower input voltages, and higher output power
capability.
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.
Note that a low ESR bulk decoupling capacitor may be
required across the input source to prevent large voltage
droop and ripple caused by the source’s ESR and the peak
primary switching current (which can reach hundreds of
milliamps). The time constant of the filter capacitor and
the ESR of the voltage source should be much longer than
the period of the resonant switching frequency.
applicaTions inForMaTion
Peltier Module (Thermoelectric Generator)
A Peltier module (also known as a thermoelectric cooler)
is made up of a number of series-connected P-N junctions,
sandwiched between two parallel ceramic plates. Although
Peltier modules are often used as coolers by applying a
DC voltage to their inputs, they will also generate a DC
output voltage, using the Seebeck effect, when the two
plates are held at different temperatures. The polarity of
the output voltage will depend on the polarity of the tem
-
perature differential between the plates. The magnitude
of the output voltage is proportional to the magnitude of
the temperature differential between
the plates. When
used in this manner, a Peltier module is referred to as a
thermoelectric generator (TEG).
The low voltage capability of the LTC3107 design allows
it to operate from a TEG with temperature differentials
as low as 1°C to 2°C, making it ideal for harvesting
energy in applications where a temperature difference
exists between two surfaces or between a surface and
the ambient temperature. The internal resistance (ACR)
of most TEGs is in the range of 1Ω to 10Ω, allowing for
reasonable power transfer. The curves in Figure 3 show the
open-circuit output voltage and maximum power transfer
for a typical TEG with an ACR of 2Ω over a 20°C range
of temperature differential. It can be seen that an output
power of a few hundred microwatts is easily achievable
with a small temperature differential. This is often more
than enough to satisfy the average power demand of a
low power wireless sensor.
Note that the thermal resistance of most TEGs is typically
quite low (2°K/W to 20°K/W). Therefore, it may be difficult
to sustain a large temperature differential across the TEG.
The temperature differential will depend on the amount of
heat transfer available. In
most applications, this will be
determined by the size of the heat sink used on the TEG,
and the amount
of air flow. For optimal performance, the
thermal resistance of the heat sink should be at least as
low as the thermal resistance of the TEG being used.
