Datasheet LTC3107 (Analog Devices) - 9

LTC3107
operaTion
The LTC3107 is an ultra-low input voltage step-up DC/DC The LTC3107 is designed to use the battery to start-up converter and power manager for extending the battery the IC and power VOUT and VLDO with or without any life of low power wireless sensors and other low power harvested energy available. If there is no harvester input, applications that utilize a primary battery. The LTC3107 or insufficient harvester input to power the load, then VOUT intelligently manages harvested energy from sources will be provided by the battery through a current-limited such as TEGs (Thermo-Electric Generators) to service the switch internal to the LTC3107, and will be hysteretically output while minimizing battery drain, thereby maximizing regulated to a voltage 230mV below the battery voltage. battery life. The VLDO output will be fixed at 2.2V unless VBAT is below To simplify the adoption of energy harvesting by applica- 2.2V, in which case it will track VBAT. When no harvested tions presently powered by a primary battery, the LTC3107 energy is available, the LTC3107 average quiescent current is designed to use the voltage on the V draw from the battery is typically 6µA. BAT pin not only as an energy source to power the outputs in the absence of If there is sufficient harvested energy available, then VOUT harvested input energy, but also as a voltage reference will be regulated to a voltage approximately 30mV below to regulate VOUT. In this way, the LTC3107 automatically the VBAT voltage, and the battery will not be used to power adapts VOUT to track whatever battery voltage the applica- VOUT. In this case, the battery current draw will drop to tion is already designed for, in the range of 2V to 4.0V. just 80nA typical. The LTC3107 is suitable for extending the battery life in
Oscillator
applications where the average power draw is very low, but where periodic pulses of higher load current may be The LTC3107 utilizes a MOSFET switch to form a resonant required. This is typical of wireless sensor applications, step-up oscillator using an external step-up transformer where the quiescent power draw is extremely low a high and a small coupling capacitor. This allows it to boost percentage of the time, except during transmit bursts input voltages as low as 20mV up to values high enough when circuitry is powered up to make measurements and to provide multiple regulated output voltages for powering transmit data. other circuits. The frequency of oscillation is determined by the inductance of the transformer secondary winding, The LTC3107 can also be used to trickle charge a standard and is typically in the range of 10kHz-100kHz. For input capacitor or supercapacitor to store excess harvested voltages as low as 20mV, a primary-secondary turns ratio energy when it is available. This further extends the life of about 1:100 is recommended. For higher input voltages, of the primary battery, by allowing the converter to ride- this ratio can be lower. See the Applications section for through periods of heavier load, or times when no harvested more information on selecting the transformer. energy is coming in by drawing from this reservoir before switching over to the battery.
Charge Pump and Rectifier VBAT Input
The AC voltage produced on the secondary winding of the transformer is boosted and rectified using an external The VBAT input should be connected to a primary battery charge pump capacitor (from the secondary winding to pin with a voltage between 2V and 4.0V. Typical examples are C1) and the rectifiers internal to the LTC3107. The rectifier 2 Alkaline cells, a single 3V Lithium coin cell, or a 3.6V circuit feeds current into the VAUX pin, providing charge Li-SOCl2 battery. These are representative batteries that to the external VAUX capacitor. Once VAUX exceeds 2V, would normally power the application without the benefit synchronous rectifiers in parallel with each of the diodes of energy harvesting. take over the job of rectifying the input voltage, improv- ing efficiency. 3107f For more information www.linear.com/LTC3107 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts