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LTC3108-1

31081fa

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 ﬂame. 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 LTC3108-1. Because

of the higher output voltages possible with a thermopile

generator, a lower transformer turns ratio can be used

(typically 1:20, depending on the application).

Photovoltaic Cell

The LTC3108-1 converter can also operate from a single

photovoltaic cell (also known as a PV or solar cell) at light

levels too low for other low input voltage boost convert-

ers to operate. However, many variables will affect the

performance in these applications. Light levels can vary

over several orders of magnitude and depend on light-

ing conditions (the type of lighting and indoor versus

outdoor). Different types of light (sunlight, incandescent,

ﬂuorescent) also have different color spectra, and will

produce different output power levels depending on which

type of photovoltaic cell is being used (monocrystalline,

polycrystalline or thin-ﬁlm). Therefore, the photovoltaic

cell must be chosen for the type and amount of light avail-

able. Note that the short-circuit output current from the

cell must be at least a few milliamps in order to power the

LTC3108-1 converter

Non-Boost Applications

The LTC3108-1 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 rectiﬁed/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.

COMPONENT SELECTION

Step-Up Transformer

The step-up transformer turns ratio will determine how

low the input voltage can be for the converter to start.

Using a 1:100 ratio can yield start-up voltages as low as

20mV. Other factors that affect performance are the DC

resistance of the transformer windings and the inductance

of the windings. Higher DC resistance will result in lower

efﬁciency. The secondary winding inductance will deter-

mine the resonant frequency of the oscillator, according

to the following formula.

Frequency

2•

π • L(sec)• C

Where L is the inductance of the transformer secondary

winding and C is the load capacitance on the secondary

winding. This is comprised of the input capacitance at pin

C2, typically 30pF, in parallel with the transformer secondary

winding’s shunt capacitance. The recommended resonant

frequency is in the range of 10kHz to 100kHz. See Table 5

for some recommended transformers.

Table 5. Recommended Transformers

VENDOR

PART NUMBER

Coilcraft

www.coilcraft.com

LPR6235-752SML (1:100 Ratio)

LPR6235-253PML (1:20 Ratio)

LPR6235-123QML (1:50 Ratio)

Würth

www.we-online

S11100034 (1:100 Ratio)

S11100033 (1:50 Ratio)

S11100032 (1:20 Ratio)

APPLICATIONS INFORMATION

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