Datasheet LTC3108 (Analog Devices) - 12

link to page 5 link to page 5 LTC3108
APPLICATIONS INFORMATION Introduction
Refer to the IIN vs VIN curves in the Typical Performance The LTC3108 is designed to gather energy from very Characteristics section to see what input current is low input voltage sources and convert it to usable output required from the source for a given input voltage. voltages to power microprocessors, wireless transmitters For a given transformer turns ratio, there is a maximum and analog sensors. Such applications typically require recommended input voltage to avoid excessively high much more peak power, and at higher voltages, than secondary voltages and power dissipation in the shunt the input voltage source can produce. The LTC3108 is regulator. It is recommended that the maximum input volt- designed to accumulate and manage energy over a long age times the turns ratio be less than 50. period of time to enable short power bursts for acquiring and transmitting data. The bursts must occur at a low Note that a low ESR bulk decoupling capacitor will usu- enough duty cycle such that the total output energy dur- ally be required across the input source to prevent large ing the burst does not exceed the average source power voltage droop and ripple caused by the source’s ESR integrated over the accumulation time between bursts. and the peak primary switching current (which can reach For many applications, this time between bursts could be hundreds of milliamps). The time constant of the filter seconds, minutes or hours. capacitor and the ESR of the voltage source should be much longer than the period of the resonant switching The PGD signal can be used to enable a sleeping micro- frequency. processor or other circuitry when VOUT reaches regula- tion, indicating that enough energy is available for a burst.
Peltier Cell (Thermoelectric Generator) Input Voltage Sources
A Peltier cell (also known as a thermoelectric cooler) is made up of a large number of series-connected P-N junc- The LTC3108 can operate from a number of low input tions, sandwiched between two parallel ceramic plates. voltage sources, such as Peltier cells, photovoltaic cells Although Peltier cells are often used as coolers by apply- or thermopile generators. The minimum input voltage ing a DC voltage to their inputs, they will also generate required for a given application will depend on the trans- a DC output voltage, using the Seebeck effect, when the former turns ratio, the load power required, and the inter- two plates are at different temperatures. The polarity of nal DC resistance (ESR) of the voltage source. Lower ESR the output voltage will depend on the polarity of the tem- will allow the use of lower input voltages, and provide perature differential between the plates. The magnitude of higher output power capability. the output voltage is proportional to the magnitude of the 1000 100 TEG: 30mm 127 COUPLES V TEG MAXIMUM P OC R = 2Ω (mV) 100 10 OUT MAX P —IDEAL (mW) OUT OPEN_CIRCUIT (IDEAL) 10 1 TEG V 1 0.1 1 10 100 dT (°C) 3108 F02
Figure 2. Typical Performance of a Peltier Cell Acting as a Thermoelectric Generator
Rev. D 12 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts