Step-Up Converter /Power Manager Harvests Energy from ±30mV Inputs

Aug 1, 2010 12:00 PM
Sam Davis, Editor-in-Chief, PET

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The LTC3109 and two external compact step-up transformers produce an ultra-low input voltage step-up dc-dc converter and power manager (Fig. 1). The LTC3109 has the unique ability to promote energy harvesting from thermoelectric generators (TEGs) in applications where the temperature differential across the TEG may be of either (or unknown) polarity. Also, it can operate from low-level ac sources and is ideally suited for other applications in which energy harvesting generates system power. Fig. 2 is a plot of the output current from the VOUT pin vs. the input from a thermoelectric generator. These energy harvesters are well suited for applications requiring low average power, even with periodic pulses of higher load current.

Step-up transformer turns ratio determines the usable input voltage. To achieve auto-polarity operation, two identical step-up transformers should be used, unless the temperature drop across the TEG is significantly different in one polarity, in which case the ratios may be different. A 1:100 primary-secondary ratio yields start-up voltages as low as 30mV. Other factors that affect performance are transformer winding resistance and inductance. Higher dc resistance results in lower efficiency and higher start-up voltages. For operation from higher input voltages, this ratio can be lower.

The IC’s proprietary auto-polarity topology allows it to generate usable power from ±30mV input voltages, enabling temperature differences as low as ±1°C to provide enough current for harvesting. This makes it ideal for applications in which the input voltage polarity is unknown or is subject to reversal.

Configured with internal MOSFET switches that form a resonant step-up oscillator, the LTC3109 can boost the input voltage high enough to provide multiple regulated output voltages to power other circuits. The transformer’s secondary winding inductance determines the oscillation frequency, which is typically in the 10kHz to 100kHz range.

In operation, external charge pump capacitors (from the secondary winding to pin C1A or C1B) boost the ac voltages produced on the secondary windings of the input transformers. Then, the LTC3109 internal rectifiers produce the dc output. The rectifier circuit feeds current into the VAUX pin, providing charge to the external VAUX capacitor and the other outputs.

Charge pump capacitors C1A and C1B affect converter input resistance and maximum output current capability. Generally a minimum value of 1nF is recommended when operating from very low input voltages using a transformer with a ratio of 1:100. Capacitor values of 2.2nF to 10nF provide higher output current at higher input voltages, however; larger capacitor values can compromise performance when operating at low input voltage or with high resistance sources.

For most applications, the recommended capacitor value is 470pF for C2A and C2B. Smaller capacitor values tend to raise the minimum start-up voltage, and larger capacitor values can lower efficiency. Note that the C1 and C2 capacitors must have a voltage rating greater than the maximum input voltage times the transformer turns ratio.

VAUX (bypassed with a 1μF capacitor) powers active circuits within the LTC3109. An internal shunt regulator limits the maximum voltage on VAUX to a typical 5.25V. It shunts to ground any excess current into VAUX when there is no load on the converter or the input source is generating more power than required by the load. This current has a 15mA limit.

Output Voltage

The main output voltage on V_OUT is charged from the VAUX supply, and is user-programmed to one of four regulated voltages: 2.35, 3.3, 4.1 or 5V. Pins VS1 and VS2 set the output voltages by connecting either pin to VAUX or ground. Internal programmable resistor dividers controlled by VS1 and VS2 set V_OUT, eliminating the need for very high value external resistors that are susceptible to noise pickup and board leakages.

When the output voltage drops slightly below the regulated value, the charging current will be enabled as long as VAUX is greater than 2.5V. Once VOUT has reached the proper value, the charging current turns off. The resulting ripple on V_OUT is typically less than 20mV peak-to-peak.

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