LTC3109
12
3109fb
For more information www.linear.com/LTC3109
applicaTions inForMaTion
INTRODUCTION
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.
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
pulse does not exceed the average source power integrated
over the accumulation time between pulses. For many
applications, this time between pulses could be seconds,
minutes or hours.
The PGOOD signal can be used to enable a sleeping
microprocessor or other circuitry when V
OUT
reaches
regulation, indicating that enough energy is available for
a transmit pulse.
INPUT VOLTAGE 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
ESR sources will allow operation from lower input voltages,
and provide 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 decoupling capacitor may be required
across a DC 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).
Since the input voltage may be of either polarity, a ceramic
capacitor is recommended.
PELTIER CELL (THERMOELECTRIC GENERATOR)
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
also generate a DC output voltage, using the Seebeck effect,
when the two plates are at different temperatures.
When used in this manner, they are referred to as thermo-
electric generators (TEGs). The polarity of the output voltage
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
to operate from a typical TEG with temperature differentials
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
reasonable power transfer. The curves in Figure 2 show the
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).
dT (°C)
1
1
TEG V
OPEN-CIRCUIT
(mV)
OUT
10
100
0.1
1
10
10 100
3109 F02
TEG: 30mm SQUARE
127 COUPLES
R = 2Ω
V
OC
MAX P
OUT
(IDEAL)
Figure 2. Typical Performance of a Peltier Cell
Acting as a Power Generator (TEG)
