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EETE SEP 2015

Table 1: Typical light conditions. integrates an accurate RUN pin and an optional maximum power point function. Both can be used to control regulator turn on at the maximum power point of an input source. For higher power input sources the accurate RUN pin function is ideal to program predictable regulator turn on at a specific input voltage. In the event that the input harvestable energy voltage is lost, a primary or secondary battery may be connected from VSTORE to GND to power the system. In the case of a rechargeable battery, current will be sourced from this pin to trickle charge the battery, up to the maximum selected voltage. The LTC3106 will start up from either input voltage source but gives priority to VIN. The AUX output is initially charged with the synchronous rectifiers disabled. Once VAUX has reached its terminal voltage the output voltage is then also charged asynchronously until VOUT reaches approximately 1.2V. The converter then leaves the asynchronous mode in favor of a more efficient synchronous start-up mode until VOUT is in regulation and the part enters normal operation. It is normal for the output voltage to rise as VAUX is charging. The main output voltage is user programmed to one of four pre-set regulated voltages of 1.8V, 2.2V, 3.3V or 5V. A real-life design example In a traditional battery powered-only wireless network node, the main control unit (MCU) is connected directly to the battery. Energy Harvesting Several factors contribute to reduced battery capacity in these applications. Typically these wireless systems poll the node at a very low frequency with long low power inactive periods with occasional high current bursts when communicating with the node. The peak current during the pulsed load can be much greater than the nominal drain current given by the battery manufacturer, reducing capacity beyond that specified at the typical static drain current. Further, the usable input voltages for most MCUs (2V min typ) limit the usable battery capacity. The application circuit in Figure 2 shows the LTC3106 interfaced with the AM-1816 solar cell with an overall dimension of 9.8cm X 5.7cm (size of business card) and is supplemented with a CR2032 primary battery configured to deliver power to a pulsed load output. Though an energy harvesting system can eliminate the need for batteries, it can also serve to supplement and increase battery life. When sufficient ambient energy is available, the battery is unloaded and is only used when the ambient source is inadequate to service the load. This not only extends battery life but improves reliability by extending battery life and also reducing service cost. The main input voltage of the LTC3106 (VIN) is designed to accommodate high impedance solar cells over a wide voltage range. Solar cells are classified according to their output power level, material employed (crystal silicon, amorphous silicon, compound semiconductor) and application space (indoor or outdoor lighting). Sanyo Electric’s Amorton product line (a subsidiary of Panasonic) offers a variety of solar cells for various light conditions (see Table 1 for typical light conditions) and power levels as well the ability to customize cells for specific application size and shapes. Additional companies that manufacture small solar cells suitable for use with the LTC3106 include PowerFilm, G24 Power and Alta Devices. The I-V and P-V curves for the AM-1816 panel are shown in Figure 3. The maximum power from the cell (PMAX) changes with light level but the voltage at PMAX changes only slightly. The VIN threshold voltage in this application example is set to equal the voltage at PMAX using the resistive divider on the RUN pin. The input voltage rising UVLO threshold VIN(OV) set point was chosen to be 4.2V. With internal hysteresis, the VIN(UV) is then 3.8V, so the average VIN voltage of ~4V is at the maximum power point from the manufacturer I-V and P-V data on the AM-1816 solar cell. Table 2: Application load profile power budget for figure 2. 32 Electronic Engineering Times Europe September 2015 www.electronics-eetimes.com


EETE SEP 2015
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