LTC3108 – Ultralow Voltage Step-Up Converter and Power Manager
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The LTC3108 is a highly integrated step-up converter and power manager intended for energy harvesting applications. It can operate from extremely low input voltages as low as 20mV from sources such as thermoelectric generators, thermopiles, and small solar cells. It efficiently harvests and manages surplus energy, providing selectable and regulated output voltages while also storing excess energy for later use. The LTC3108 integrates logic-controlled outputs, a reserve energy output, and an LDO to power low-power processors or sensors from harvested energy in a compact package.
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LTC3108 – Ultralow Voltage Step-Up Converter and Power Manager
- 11. LTC3108: Output Current vs. dT and TEG Size
- 12. LTC3108: Output Current vs. VOC and Rsource
- 13. LTC3108: Output Current vs. VOC and Rsource
Editor's Notes
- Welcome to the training module on LTC3108 – Ultralow Voltage Step-Up Converter and Power Manager.
- This training module will introduce energy harvesting technology and LTC3108 step-up DC/DC regulator.
- Electronic circuits are now capable of operating at microwatt levels, and it is feasible for them to be powered using non-traditional sources. This has led to energy harvesting, which provides the power to charge, supplement, or replace batteries in systems where battery use is inconvenient, impractical, expensive, or even dangerous. It can also eliminate the need for wires to carry power or transmit data. Energy harvesting can power smart wireless sensor networks to monitor and optimize complex industrial processes, remote field installations, and building HVAC systems. In addition, otherwise wasted energy from industrial processes, solar panels, or internal combustion engines can be harvested for useful purposes.
- In most cases, the “fuel” for energy harvesters is naturally present and may be considered free. Using natural sources in remote areas for energy harvesting is an attractive alternative to inconvenient utility and battery power. These natural energy sources may be available maintenance-free for a lifetime. Energy harvesting can also be an alternative energy source that supplements the primary power source and enhances its reliability.
- Energy harvesters are intended for applications that require very low average power but also need periodic pulses of higher load current. For example, in many wireless sensor applications the circuitry is only powered to make measurements and transmit data periodically at a low duty cycle. Energy harvesting is becoming more feasible today because of the increased efficiency of devices used to capture, store, and produce electrical energy. This can be accomplished with very efficient, very-low-voltage input step-up converters. Also, improved low-voltage, high-efficiency microprocessors may allow them to become participants in energy-harvesting systems.
- Linear Technology’s LTC®3108, a highly integrated dc-dc converter, is intended for energy harvesting. It can harvest surplus energy from extremely low-input-voltage sources such as thermoelectric generators (TEGs), thermopiles, and small solar cells. The circuit in the figure uses a small step-up transformer to boost the input voltage to an LTC3108, providing a complete power management solution for wireless sensing and data acquisition.
- The LTC3108 is designed to manage 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. The AC voltage produced on the secondary winding of the transformer is boosted and rectified using an external charge pump capacitor (from the secondary winding to pin C1) and the rectifiers internal to the LTC3108. The rectifier circuit feeds current into the VAUX pin, providing charge to the external VAUX capacitor and the other outputs. The internal 2.2-V LDO can support a low-power processor or other low-power ICs. The LDO is powered by the higher value of either VAUX or VOUT. This enables it to become active as soon as VAUX has charged to 2.3 V, while the VOUT storage capacitor is still charging. In the event of a step load on the LDO output, current can come from the main VOUT capacitor if VAUX drops below VOUT. The LDO requires a 1-μF ceramic capacitor for stability. The VSTORE output can be used to charge a large storage capacitor or rechargeable battery after VOUT has reached regulation. Once VOUT has reached regulation, the VSTORE output will be allowed to charge up to the VAUX voltage. The VSTORE capacitor may be a very large value to provide holdup at times when input power may be lost.
- The LTC3108 uses a MOSFET switch to form a resonant step-up oscillator using an external step-up transformer and a small coupling capacitor. This allows it to boost input voltages as low as 20 mV, to levels that high enough to provide multiple regulated output voltages for powering other circuits. The frequency of oscillation is determined by the inductance of the transformer’s secondary winding and is typically in the range of 20 to 200 kHz. For input voltages as low as 20 mV, a primary-secondary turns ratio of about 1:100 is recommended. For higher input voltages, this ratio can be lower. These transformers are standard off-the-shelf components.
- Once power harvesting has been completed, the main outputs and a switch output are all available. If the input power fails, operation can still continue by operating of VSTORE capacitor.
- The main output voltage on VOUT is charged from the VAUX supply, and is user programmed to one of four regulated voltages using the voltage select pins VS1 and VS2. Its output can be programmed to one of four (1.8, 2.5, 3.3, or 3.6 V) fixed voltages to power a wireless transmitter or sensor. Although the logic threshold voltage for VS1 and VS2 is 0.85V typical, it is recommended that they be tied to ground or VAUX. The internal programmable resistor divider sets VOUT, eliminating the need for very high value external resistors that are susceptible to board leakage.
- It can be seen from this graph of the LTC3108’s output current vs. Delta T and TEG size, a 1.5” square TEG performs as well as a 2” square TEG. This is mostly due to the higher ESR of the large device. Notice that a 20 º C dT yields 1mA of output current.
- Most TEGs are in the 0.5 Ω to 10 Ω range of ESR or source resistance. This graph illustrates how the LTC3108’s output current varies with input voltage and TEG source resistance. Thus with a 1:100 turns ratio transformer, the LTC3108 is perfectly matched to the source when the ESR is proximately 3 Ω , although it is fairly close to optimum at other ESRs as well.
- Here is a similar graph to the one just shown, however, in this case the transformer has a lower turns ration of 1:20. This gives more output current when larger deltaTs are present.
- In summary, the LTC3108 thermal energy harvesting, DC to DC step-up converter and system manager is a revolutionary device that extracts energy from solar cells, thermo-electric generators or other similar thermal sources. Its unique resonant power converter topology allows it to start up at an extremely low 20mV input voltage. Its high integration, including power management control, and off-the-shelf external components make it small, simple and easy-to-use solution available.
- Thank you for taking the time to view this presentation on “ LTC3108 – Ultralow Voltage Step-Up Converter and Power Manager” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link right beside the play button on the TechCast portal, or simply call our sales hotline. For more technical information you may either visit the LINEAR TECHNOLOGY site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility. You may visit element14 e-community to post your questions.