PHOTOVOLTAICS FEATURE


single inductor to generate a user- selectable fixed regulated output voltage through seamless transition between either of the two power inputs. If input power is available (VIN


) the buck-boost regulator will operate from VIN


up to 300mA to the load. Should the VIN


/VCAP , providing source become


unavailable, the regulator will select VSTORE


as its input delivering up to


50mA to the load. If a rechargeable battery is used as the backup source, a low current recharge power path is also provided allowing use of excess input energy to charge the backup source if the output voltage is in regulation. User selectable upper and lower charge/


discharge thresholds are available to handle multiple battery chemistries and to protect the battery from overcharge/ deep discharge. Charging can be externally disabled using the PRI when a primary battery is used as the backup source. The main input voltage, VIN


, can be


configured to operate over a voltage range from 850mV to 5.1V without a back-up source and from 330mV to 5.1V with a back-up source, like a primary battery. This range accommodates multiple power source types including high impedance sources such as a small solar panel. To ensure maximum power extraction,


the LTC3106 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 favour 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.


Figure 4:


Application load profile for schematic in Figure 2


/MICROMATTERS Figure 3:


Measured I-V and P-V curves under variable light conditions for AM- 1816


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. 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.


(VIN


The main input voltage of the LTC3106 ) 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). 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 VIN(OV)


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 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. For this application the load is a low power RF device and its load profile is shown in Figure 4. The total average operating power loss


for the load profile is 37µW. The resister divider load adds an additional 5µW of input power loss for a total input power requirement of 207µW. The calculated average efficiency, including the resistive divider is η = 165µW/207µW which is 80%. The available power from the AM- 1816 at 200lux is about 400µW. With a converter efficiency of about


80%, the 400µW powers the total 207µW average load with some margin, drawing no power from the battery. If the light conditions become less favourable, the available input power may drop below that needed to maintain the output voltage. This device will then operate in “hiccup” mode turning on as VIN


if VIN


increases above 4.2V and turning off drops below 3.8V. With VIN


off,


power is then taken from VSTORE (primary battery) until VIN


recovers and increases above the 4.2V threshold. If the light


conditions become more favourable, VIN will rise to the open-circuit voltage of the harvested source and once again provide all of the load power. Even though some energy harvesting


sources only provide low levels of useable power, they are usually enough to power most wireless sensors. The LTC3106 buck-boost DC/DC converter is optimised for multiple input sources commonly found in low power systems, and provides the necessary feature set for a broad range of energy harvesting applications.


Linear Technology (UK) Ltd. Now part of Analog Devices www.linear.com 01628 477 066


MICROMATTERS | WINTER 2017 21


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