LTC3108

11

3108p

Introduction

The LTC3108 is designed to gather energy from very low

input voltage sources and convert it to usable output volt-

ages to power microprocessors, wireless transmitters and

analog sensors. Such applications typically require much

more peak power, and at higher voltages, than the input

voltage source can produce. The LTC3108 is designed to

accumulate and manage energy over a long period of time

to enable short power bursts for acquiring and transmit-

ting data. The bursts must occur at a low enough duty

cycle such that the total output energy during the burst

does not exceed the average source power integrated

over the accumulation time between bursts. For many

applications, this time between bursts could be seconds,

minutes or hours.

The PGD signal can be used to enable a sleeping micro-

indicating that enough energy is available for a burst.

Input Voltage Sources

The LTC3108 can operate from a number of low input

voltage sources, such as Peltier cells, photovoltaic cells or

thermopile generators. 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 will

allow the use of lower input voltages, and provide higher

output power capability.

APPLICATIONS INFORMATION

Figure 2. Typical Performance of a Peltier Cell Acting as a Thermoelectric Generator

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 will usually

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.

Peltier Cell (Thermoelectric Generator)

A Peltier cell (also known as a thermoelectric cooler) is

made up of a large number of series-connected P-N junc-

tions, sandwiched between two parallel ceramic plates.

Although Peltier cells are often used as coolers by apply-

ing 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. 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 cell is referred to as a thermoelectric

generator (TEG).

010

20

25

515

ΔT (°C)

0.1

0.3

0.2

0

0.1

10

1

0.01

3108 F02