FEATURES

APPLICATIONS

DESCRIPTION

TYPICAL APPLICATION

ABSOLUTE MAXIMUM RATINGS  (Note 1)

PIN CONFIGURATION
ORDER INFORMATION

ELECTRICAL CHARACTERISTICS

PIN FUNCTIONS   (DFN/SSOP)

OPERATION (Refer to the Block Diagram)

OPERATION
Oscillator

Charge Pump and Rectifier

VAUX

Voltage Reference

Synchronous Rectifiers

Low Dropout Linear Regulator (LDO)

Vout

OPERATION

PGOOD

Vout2

VSTORE

Short-Circuit Protection

Output Voltage Sequencing

Introduction
The LTC3108 is designed to gather energy from very low input voltage sources and convert it to usable output volt- ages to power microprocessors, wireless transmitters and analog sensors. Such applications typically require much more peak power, and at higher voltages, than the input voltage source can produce. The LTC3108 is designed to accumulate and manage energy over a long period of time to enable short power bursts for acquiring and transmit¬ ting data. The bursts must occur at a low enough duty cycle such that the total output energy during the burst does not exceed the average source power integrated over the accumulation time between bursts. For many applications, this time between bursts could be seconds, minutes or hours. The PGD signal can be used to enable a sleeping micro- processor or other circuitry when Vout reaches regulation, indicating that enough energy is available for a burst.
Input Voltage Sources
The LTC3108 can operate from a number of low input voltage sources, such as Peltier cells, photovoltaic cells or thermopile generators. The minimum input voltage required for a given application will depend on the transformer turns ratio, the load power required, and the internal DC resistance (ESR) of the voltage source. Lower ESR will allow the use of lower input voltages, and provide higher output power capability. Refer to the l IN vs V IN curves in the Typical Performance Characteristics section to see what input current is required from the source for a given input voltage. For a given transformer turns ratio, there is a maximum recommended input voltage to avoid excessively high secondary voltages and power dissipation in the shunt regulator. It is recommended that the maximum input voltage times the turns ratio be less than 50. Note that a low ESR bulk decoupling capacitor will usually be required across the input source to prevent large voltage droop and ripple caused by the source’s ESR and the peak primary switching current (which can reach hundreds of milliamps). The time constant of the filter capacitor and the ESR of the voltage source should be much longer than the period of the resonant switching frequency.
Peltier Cell (Thermoelectric Generator)
A Peltier cell (also known as a thermoelectric cooler) is made up of a large number of series-connected P-N junc- tions, sandwiched between two parallel ceramic plates. Although Peltier cells are often used as coolers by apply- ing a DC voltage to their inputs, they will also generate a DC output voltage, using the Seebeck effect, when the two plates are at different temperatures. The polarity of the output voltage will depend on the polarity of the tem¬ perature differential between the plates. The magnitude of the output voltage is proportional to the magnitude of the temperature differential between the plates. When used in
APPLICATIONS INFORMATION
this manner, a Peltier cell is referred to as a thermoelectric generator (TEG). The low voltage capability of the LTC3108 design allows it to operate from a TEG with temperature differentials as low as 1°C, making it ideal for harvesting energy in applications in which a temperature difference exists between two surfaces or between a surface and the am¬ bient temperature. The internal resistance (ESR) of most cells is in the range of 1Ω to 5Ω, allowing for reasonable power transfer. The curves in Figure 2 show the open- circuit output voltage and maximum power transfer for a typical Peltier cell (with an ESR of 2Ω) over a 20°C range of temperature differential.
TEG Load Matching
The LTC3108 was designed to present a minimum input resistance (load) in the range of 2Ω to 10Ω, depending on input voltage and transformer turns ratio (as shown in the Typical Performance Characteristics curves). For a given turns ratio, as the input voltage drops, the input resistance increases. This feature allows the LTC3108 to optimize power transfer from sources with a few ohms of source resistance, such as a typical TEG. Note that a lower source resistance will always provide more output current capability by providing a higher input voltage under load.
Peltier Cell (TEG) Suppliers
Peltier cells are available in a wide range of sizes and power capabilities, from less than 10mm square to over 50mm square. They are typically 2mm to 5mm in height. A list of Peltier cell manufacturers is given in Table 3.

Thermopile Generator
Thermopile generators (also called powerpile generators) are made up of a number of series-connected thermo¬ couples enclosed in a metal tube. They are commonly used in gas burner applications to generate a DC output of hundreds of millivolts when exposed to the high tem¬ perature of a flame. Typical examples are the Honeywell CQ200 and Q313. These devices have an internal series resistance of less than 3Ω, and can generate as much as 750mV open-circuit at their highest rated temperature. For applications in which the temperature rise is too high for a solid-state thermoelectric device, a thermopile can be used as an energy source to power the LTC3108. Because of the higher output voltages possible with a thermopile generator, a lower transformer turns ratio can be used (typically 1:20, depending on the application).
Photovoltaic Cell
The LTC3108 converter can also operate from a single photovoltaic cell (also known as a PV or solar cell) at light levels too low for other low input voltage boost convert- ers to operate. However, many variables will affect the performance in these applications. Light levels can vary over several orders of magnitude and depend on light- ing conditions (the type of lighting and indoor versus outdoor). Different types of light (sunlight, incandescent, fluorescent) also have different color spectra, and will produce different output power levels depending on which type of photovoltaic cell is being used (monocrystalline, polycrystalline or thin-film). Therefore, the photovoltaic cell must be chosen for the type and amount of light avail¬ able. Note that the short-circuit output current from the cell must be at least a few milliamps in order to power the LTC3108 converter
Non-Boost Applications
The LTC3108 can also be used as an energy harvester and power manager for input sources that do not require boosting. In these applications the step-up transformer can be eliminated. Any source whose peak voltage exceeds 2.5V AC or 5V DC can be connected to the C1 input through a current- limiting resistor where it will be rectified/peak detected. In these applications the C2 and SW pins are not used and can be grounded or left open. Examples of such input sources would be piezoelectric transducers, vibration energy harvesters, low current generators, a stack of low current solar cells or a 60Hz AC input. A series resistance of at least 100Ω/V should be used to limit the maximum current into the VAUX shunt regulator.
COMPONENT SELECTION
Step-Up Transformer
The step-up transformer turns ratio will determine how low the input voltage can be for the converter to start. Using a 1:100 ratio can yield start-up voltages as low as 20mV. Other factors that affect performance are the DC resistance of the transformer windings and the inductance of the windings. Higher DC resistance will result in lower efficiency. The secondary winding inductance will deter- mine the resonant frequency of the oscillator, according to the following formula.
Where L is the inductance of the transformer secondary winding and C is the load capacitance on the secondary winding. This is comprised of the input capacitance at pin C2, typically 30pF, in parallel with the transformer secondary winding’s shunt capacitance. The recommended resonant frequency is in the range of 10kHz to 100kHz. See Table 5 for some recommended transformers.

APPLICATIONS INFORMATION
C1 Capacitor
The charge pump capacitor that is connected from the transformer's secondary winding to the C1 pin has an ef- fect on converter input resistance and maximum output current capability. Generally, a minimum value of 1nF is recommended when operating from very low input volt- ages using a transformer with a ratio of 1:100. Too large a capacitor value can compromise performance when operating at low input voltage or with high resistance sources. For higher input voltages and lower turns ratios, the value of the C1 capacitor can be increased for higher output current capability. Refer to the Typical Applications schematic examples for the recommended value for a given turns ratio.
Squegging
Certain types of oscillators, including transformer-coupled oscillators such as the resonant oscillator of the LTC3108, can exhibit a phenomenon called squegging. This term refers to a condition that can occur which blocks or stops the oscillation for a period of time much longer than the period of oscillation, resulting in bursts of oscillation. An example of this is the blocking oscillator, which is designed to squegg to produce bursts of oscillation. Squegging is also encountered in RF oscillators and regenerative receivers. In the case of the LTC3108, squegging can occur when a charge builds up on the C2 gate coupling capacitor, such that the DC bias point shifts and oscillation is extinguished for a certain period of time, until the charge on the capacitor bleeds off, allowing oscillation to resume. It is difficult to predict when and if squegging will occur in a given ap¬ plication. While squegging is not harmful, it reduces the average output current capability of the LTC3108. Squegging can easily be avoided by the addition of a bleeder resistor in parallel with the coupling capacitor on the C2 pin. Resistor values in the range of 100k to 1MΩ are sufficient to eliminate squegging without having any negative impact on performance. For the 330pF capacitor used for C2 in most applications, a 499k bleeder resistor is recommended. See the Typical Applications schematics for an example.
Using External Charge Pump Rectifiers
The synchronous charge pump rectifiers in the LTC3108 (connected to the C1 pin) are optimized for operation from very low input voltage sources, using typical transformer step-up ratios between 1:100 and 1:50, and typical C1 charge pump capacitor values less than 10nF. Operation from higher input voltage sources (typically 250mV or greater, under load), allows the use of lower transformer step-up ratios (such as 1:20 and 1:10) and larger C1 capacitor values to provide higher output cur¬ rent capability from the LTC3108. However, due to the resulting increase in rectifier currents and resonant oscil¬ lator frequency in these applications, the use of external charge pump rectifiers is recommended for optimal performance. In applications where the step-up ratio is 1:20 or less, and the C1 capacitor is 10nF or greater, the C1 pin should be grounded and two external rectifiers (such as 1N4148 or 1N914 diodes) should be used. These are available as dual diodes in a single package. Avoid the use of Schottky rectifiers, as their lower forward voltage drop increases the minimum start-up voltage. See the Typical Applications schematics for an example.
Vout and VSTORE Capacitor
For pulsed load applications, the Vout capacitor should be sized to provide the necessary current when the load is pulsed on. The capacitor value required will be dictated by the load current, the duration of the load pulse, and the amount of voltage droop the circuit can tolerate. The capacitor must be rated for whatever voltage has been selected for Vout by VS1 and VS2.
Note that there must be enough energy available from the input voltage source for Vout to recharge the capacitor during the interval between load pulses (to be discussed in the next example). Reducing the duty cycle of the load pulse will allow operation with less input energy.
APPLICATIONS INFORMATION
The VSTORE capacitor may be of very large value (thou- sands of microfarads or even Farads), to provide holdup at times when the input power may be lost. Note that this capacitor can charge all the way to 5.25V (regardless of the settings for Vout), so ensure that the holdup capacitor has a working voltage rating of at least 5.5V at the tem¬ perature for which it will be used. The VSTORE capacitor can be sized using the following:
Where 6μA is the quiescent current of the LTC3108, lo is the load on Vout in between bursts, I」 no is the load on the LDO between bursts, IBURST is the total load during the burst, t is the duration of the burst, f is the frequency of the bursts, TSTORE is the storage time required and Vout is the output voltage required. Note that for a programmed output voltage of 5V, the VSTORE capacitor cannot provide any beneficial storage time. To minimize losses and capacitor charge time, all capaci¬ tors used for Vout and VSTORE should be low leakage. See Table 6 for recommended storage capacitors.
Storage capacitors requiring voltage balancing are not recommended due to the current draw of the balancing resistors.
PCB Layout Guidelines
Due to the rather low switching frequency of the resonant converter and the low power levels involved, PCB layout is not as critical as with many other DC/DC converters. There are, however, a number of things to consider. Due to the very low input voltage the circuit may operate from, the connections to V IN , the primary of the trans- former and the SW and GND pins of the LTC3108 should be designed to minimize voltage drop from stray resistance and able to carry currents as high as 500mA. Any small voltage drop in the primary winding conduction path will lower efficiency and increase capacitor charge time. Also, due to the low charge currents available at the out¬ puts of the LTC3108, any sources of leakage current on the output voltage pins must be minimized. An example board layout is shown in Figure 3. VIAS TO GROUND PLANE Figure 3. Example Component Placement for Two-Layer PC Board (DFN Package)
Design Example 1
This design example will explain how to calculate the necessary storage capacitor value for Vout in pulsed load applications, such as a wireless sensor/transmitter. In these types of applications, the load is very small for a major¬ ity of the time (while the circuitry is in a low power sleep state), with bursts of load current occurring periodically during a transmit burst. The storage capacitor on Vout supports the load during the transmit burst, and the long sleep time between bursts allows the LTC3108 to recharge the capacitor. A method for calculating the maximum rate
APPLICATIONS INFORMATION
at which the load pulses can occur for a given output cur- rent from the LTC3108 will also be shown. In this example, Vout is set to 3.3V, and the maximum allowed voltage droop during a transmit burst is 10%, or 0.33V. The duration of a transmit burst is 1ms, with a total average current requirement of 40mA during the burst. Given these factors, the minimum required capacitance on Vout is:
Note that this equation neglects the effect of capacitor ESR on output voltage droop. For most ceramic or low ESR tantalum capacitors, the ESR will have a negligible effect at these load currents. A standard value of 150μF or larger could be used for Cout in this case. Note that the load current is the total current draw on Vout, Vout2 and VLDO, since the current for all of these outputs must come from Vout during a burst. Current contribution from the holdup capacitor on VSTORE is not considered, since it may not be able to recharge between bursts. Also, it is assumed that the charge current from the LTC3108 is negligible compared to the magnitude of the load current during the burst. To calculate the maximum rate at which load bursts can occur, determine how much charge current is available from the LTC3108 Vout pin given the input voltage source being used. This number is best found empirically, since there are many factors affecting the efficiency of the converter. Also determine what the total load current is on Vout during the sleep state (between bursts). Note that this must include any losses, such as storage ca¬ pacitor leakage. Assume, for instance, that the charge current from the LTC3108 is 50μA and the total current drawn on Vour in the sleep state is 17µA, including capacitor leakage. In addition, use the value of 150μF for the Vout capacitor. The maximum transmit rate (neglecting the duration of the transmit burst, which is typically very short) is then given by:
Therefore, in this application example, the circuit can sup- port a 1ms transmit burst every 1.5 seconds. It can be determined that for systems that only need to transmit every few seconds (or minutes or hours), the average charge current required is extremely small, as long as the sleep current is low. Even if the available charge current in the example above was only 10μA and the sleep current was only 5μA, it could still transmit a burst every ten seconds. The following formula enables the user to calculate the time it will take to charge the LDO output capacitor and the Vout capacitor the first time, from OV. Here again, the charge current available from the LTC3108 must be known. For this calculation, it is assumed that the LDO output capacitor is 2.2μF.
If there were 50μA of charge current available and a 5μA load on the LDO (when the processor is sleeping), the time for the LDO to reach regulation would be 107ms. If Vout were programmed to 3.3V and the Vout capacitor was 150μF, the time for Vout to reach regulation would be:
If there were 50μA of charge current available and 5μA of load on Vout, the time for Vout to reach regulation after the initial application of power would be 12.5 seconds.
Design Example 2
In many pulsed load applications, the duration, magnitude and frequency of the load current bursts are known and fixed. In these cases, the average charge current required from the LTC3108 to support the average load must be calculated, which can be easily done by the following:
Where lo is the sleep current on Vout required by the ex¬ ternal circuitry in between bursts (including cap leakage), lburst is the total load current during the burst, t is the 3108fa 17
APPLICATIONS INFORMATION
duration of the burst and T is the period of the transmit burst rate (essentially the time between bursts). In this example, lo = 5μA, lburst = 100mA, t = 5ms and T = one hour. The average charge current required from the LTC3108 would be:
Therefore, if the LTC3108 has an input voltage that allows it to supply a charge current greater than 5.14μA, the application can support 100mA bursts lasting 5ms every hour. It can be determined that the sleep current of 5μA is the dominant factor because the transmit duty cycle is so small (0.00014%). Note that for a Vout of 3.3V, the average power required by this application is only 17μW (not including converter losses). Note that the charge current available from the LTC3108 has no effect on the sizing of the Vout capacitor (if it is assumed that the load current during a burst is much larger than the charge current), and the Vout capacitor has no effect on the maximum allowed burst rate.
TYPICAL APPLICATIONS
Peltier-Powered Energy Harvester for Remote Sensor Applications
Li-Ion Battery Charger and LDO Powered by a Solar Cell Supercapacitor Charger and LDO Powered by a Thermopile Generator DC Input Energy Harvester and Power Manager AC Input Energy Harvester and Power Manager
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REVISION HISTORY

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