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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 VOUT 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 VOUT, 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 VOUT is typically less than 20mV peak-to-peak.
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In a typical application, a reservoir capacitor (typically a few hundred microfarads) is connected to VOUT. As soon as VAUX exceeds 2.5V, the VOUT capacitor begins to charge up to its regulated voltage. The current available to charge the capacitor depends on the input voltage and transformer turns ratio, but is limited to about 15mA typical. For very low input voltages, this current may be in the range of 1μA to 1000μA. The LTC3109’s <7μA quiescent current and high efficiency ensure fast charge times for the output reservoir capacitor.
An additional output, VOUT2, can be turned on and off by the host using the VOUT2_EN pin. When enabled, VOUT2 connects to VOUT through a 1Ω P-channel MOSFET switch. You can use this output, controlled by a host processor, to power external circuits such as sensors and amplifiers, that don’t have a low power “sleep” or shutdown capability.
VOUT2 limits its peak current to 0.3A, typical. The VOUT2 enable input has a typical threshold of 1V with 100mV of hysteresis, making it logic compatible. If VOUT2_EN (that has an internal 5MΩ pull-down resistor) is low, VOUT2 will be off. Driving VOUT2_EN high turns on the VOUT2 output.
Stored Power
The VSTORE output can be used to charge a large storage capacitor or rechargeable battery. Once VOUT has reached regulation, the VSTORE output is allowed to charge up to the clamped VAUX voltage (5.25V typical). The storage element on VSTORE can then be used to power the system in the event that the input source is lost, or is unable to provide the current demanded by the VOUT, VOUT2 and LDO outputs.
Maximum charging current available at the VSTORE output is limited to about 15mA, so it can safely be used to trickle charge NiCd or NiMH batteries for energy storage when the input voltage is lost. VSTORE is not intended to supply high pulse load currents to VOUT. Any pulse load on VOUT must be handled by the VOUT reservoir capacitor.
A power good indicator signals that the main output is within regulation by using a comparator to monitor VOUT. The PGOOD pin is an open-drain output with a weak pull-up (1MΩ) to the LDO voltage. Once VOUT has charged to within 7.5% of its programmed voltage, the PGOOD output goes high. If VOUT drops more than 9% from its programmed voltage, PGOOD goes low. The PGOOD output can drive a microprocessor or other chip I/O and is not intended to drive a higher current load such as an LED. The PGOOD pin can also be pulled low in a wire-OR configuration.
An integral low current LDO provides a regulated 2.2V output to supply low power processors or other low power ICs. LDO power originates from the higher value of VAUX or VOUT. This enables it to become active as soon as VAUX has charged to 2.3V, 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 reservoir capacitor. The LDO requires a 2.2μF ceramic capacitor for stability. Larger capacitor values can be used without limitation, but increase the time it takes for all the outputs to charge up. The LDO output is current limited to 5mA, minimum.
PCB Layout
The resonant converter has a rather low switching frequency and power levels are low, so PCB layout is not as critical as it might be with other dc-dc converters. However, the converter operates with low input voltages. Therefore, connections to VIN, the primary of the transformers and the SW, VIN and GND pins of the LTC3109 should be configured to minimize voltage drop from stray resistance, and be able to carry up to 500mA. Any small voltage drop in the primary winding conduction path will lower efficiency and increase start-up voltage and capacitor charge time. Plus, due to the low charge currents at the LTC3109’s outputs, sources of leakage current on the output voltage pins must be minimized to ensure efficient operation.
The combination of the LTC3109’s leadless 4mm x 4mm QFN-20 package (or leaded SSOP-20) and very small external components ensure a highly compact solution for energy harvesting applications.
LTC3588-1
Another of Linear Technology’s “harvesting-oriented” ICs is the LTC3588-1, an ultralow quiescent current power supply designed specifically for energy harvesting and/or low current step-down applications. The part is designed to interface directly to a piezoelectric or alternative A/C power source, rectify a voltage waveform and store harvested energy on an external capacitor, bleed off any excess power via an internal shunt regulator, and maintain a regulated output voltage by means of a nanopower high efficiency synchronous buck regulator.
The LTC3588-1 has an internal full-wave bridge rectifier accessible via the differential PZ1 and PZ2 inputs that rectifies AC inputs such as those from a piezoelectric element. The rectified output is stored on a capacitor at the VIN pin and can be used as an energy reservoir for the buck converter. The low-loss bridge rectifier has a total drop of about 400mV with typical piezo generated currents (~10μA). The bridge is capable of carrying up to 50mA. One side of the bridge can be operated as a single-ended DC input. PZ1 and PZ2 should never be shorted together when the bridge is in use.
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