DC/DC converter ideal for harvesting and managing surplus energy from extremely low input voltage sources such as TEGs (thermoelectric generators), thermopiles and small solar cells. The step-up topology operates from input voltages as low as 20mV. It is a power source for wireless sensor applications. Whether outside where a single solar cell can be used, or inside where a Peltier element can be used on the input side.

Operation

The DC/DC converter uses a small transformer to create an ultralow input voltage step-up. It is ideally suited for low power wireless sensors and other applications in which surplus energy harvesting is used to generate system power because traditional battery power is inconvenient or impractical. The LTC3108 manages the charging and regulation of multiple outputs in a system in which the average power draw is very low, but there may be periodic pulses of higher load current required. This is typical of wireless sensor applications, where the quiescent power draw is extremely low most of the time, except for transmit bursts when circuitry is powered up to make measurements and transmit data. The converter can also be used to trickle charge a standard capacitor, supercapacitor or rechargeable battery, using energy harvested from a Peltier or photovoltaic cell. A storage capacitor provides power when the input voltage source is unavailable.

It has three power outputs:

• a LDO voltage regulator with fixed 2.2V output
• Vout with selectable voltage and
• Vout2 which follows Vout in terms of voltage

Output voltage level is selected on the two solder jumpers where the following combination is used:

1+3 Vout = 5V

1+4 Vout = 4.1V

2+3 Vout = 3.3V

2+4 Vout = 2.35V

In the above example 5V is selected.

There are also control lines:

• Vout2_en is an input to the converter which is used to turn on the Vout2 output.
• PGood indicator signals the host microcontroller that the main output voltage is within regulation.

A tipical application

Selectable input for voltages higher than 500mV

It is used for voltages higher than 500mV that can range up to 6V. Like a 5V solar cell, for example. Use 100ohm of resistance per every volt on the input (500ohm for 5V).