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LTC3109

3109fa

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 VOUT 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)

100

1000

0.1

100

3109 F02

TEG: 30mm SQUARE

127 COUPLES

R = 2Ω

VOC

MAX POUT

(IDEAL)

Figure 2. Typical Performance of a Peltier Cell

Acting as a Power Generator (TEG)

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