Datasheet LTC3109 (Analog Devices) - 9

LTC3109
operaTion (Refer to the Block Diagram)
The LTC3109 is designed to use two small external
Charge Pump and Rectifier
step-up transformers to create an ultralow input voltage The AC voltage produced on the secondary winding of step-up DC/DC converter and power manager that can the transformer is boosted and rectified using an external operate from input voltages of either polarity. This unique charge pump capacitor (from the secondary winding to capability enables energy harvesting from thermoelectric pin C1A or C1B) and the rectifiers internal to the LTC3109. generators (TEGs) in applications where the temperature The rectifier circuit feeds current into the V differential across the TEG may be of either (or unknown) AUX pin, provid- ing charge to the external VAUX capacitor and the other polarity. It can also operate from low level AC sources. It outputs. is ideally suited for low power wireless sensors and other applications in which surplus energy harvesting is used to
VAUX
generate system power because traditional battery power is inconvenient or impractical. The active circuits within the LTC3109 are powered from VAUX, which should be bypassed with a 1µF minimum The LTC3109 is designed to manage the charging and capacitor. Once VAUX exceeds 2.5V, the main V regulation of multiple outputs in a system in which the OUT is al- lowed to start charging. average power draw is very low, but where periodic pulses of higher load current may be required. This is typical of An internal shunt regulator limits the maximum voltage wireless sensor applications, where the quiescent power on VAUX to 5.25V typical. It shunts to ground any excess draw is extremely low most of the time, except for transmit current into VAUX when there is no load on the converter pulses when circuitry is powered up to make measure- or the input source is generating more power than is ments and transmit data. required by the load. This current should be limited to 15mA max. The LTC3109 can also be used to trickle charge a standard capacitor, super capacitor or rechargeable battery, using
Voltage Reference
energy harvested from a TEG or low level AC source. The LTC3109 includes a precision, micropower reference,
Resonant Oscillator
for accurate regulated output voltages. This reference becomes active as soon as VAUX exceeds 2V. The LTC3109 utilizes MOSFET switches to form a reso- nant step-up oscillator that can operate from an input of
Synchronous Rectifiers
either polarity using external step-up transformers and small coupling capacitors. This allows it to boost input Once VAUX exceeds 2V, synchronous rectifiers in paral- voltages as low as 30mV high enough to provide multiple lel with each of the internal rectifier diodes take over the regulated output voltages for powering other circuits. The job of rectifying the input voltage at pins C1A and C1B, frequency of oscillation is determined by the inductance improving efficiency. of the transformer secondary winding, and is typically
Low Dropout Linear Regulator (LDO)
in the range of 10kHz to 100kHz. For input voltages as low as 30mV, transformers with a turns ratio of about The LTC3109 includes a low current LDO to provide a 1:100 is recommended. For operation from higher input regulated 2.2V output for powering low power proces- voltages, this ratio can be lower. See the Applications sors or other low power ICs. The LDO is powered by Information section for more information on selecting the higher of VAUX or VOUT . This enables it to become the transformers. active as soon as VAUX has charged to 2.3V, while the 3109fb For more information www.linear.com/LTC3109 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts