LTC3108 Linear Technology, LTC3108 Datasheet - Page 11

• Manufacturer Part Number: LTC3108
• Description: Ultralow Voltage Step-Up Converter and Power Manager
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
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-
processor or other circuitry when V
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.
Figure 2. Typical Performance of a Peltier Cell Acting as a Thermoelectric Generator
OUT
reaches regulation,
0.3
0.2
0.1
0
0
R
S
= 2Ω
5
MAX P
10
ΔT (°C)
OUT
V
OC
15
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 fi lter 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).
20
3108 F02
25
10
1
0.1
0.01
LTC3108
11
3108p