7.1 Overview

The bq25505 is the first of a new family of intelligent integrated energy harvesting Nano-Power management solutions that are well suited for meeting the special needs of ultra low power applications. The product is specifically designed to efficiently acquire and manage the microwatts (µW) to miliwatts (mW) of power generated from a variety of DC sources like photovoltaic (solar) or thermal electric generators (TEGs). The bq25505 is a highly efficient boost charger targeted toward products and systems, such as wireless sensor networks (WSN) which have stringent power and operational demands. The design of the bq25505 starts with a DCDC boost charger that requires only microwatts of power to begin operating.

The main boost charger is powered from the boost output, VSTOR. Once the VSTOR voltage is above VSTOR_CHGEN (1.8 V typical), for example, after a partially discharged battery is attached to VBAT, the boost charger can effectively extract power from low voltage output harvesters such as TEGs or single or dual cell solar panels outputting voltages down to VIN(DC) (100 mV minimum). When starting from VSTOR = VBAT < 100 mV, the cold start circuit needs at least VIN(CS), 600 mV typical, to charge VSTOR up to 1.8 V.

The bq25505 implements a programmable maximum power point tracking (MPPT) sampling network to optimize the transfer of power into the device. Sampling of the VIN_DC open circuit voltage is programmed using external resistors, and that sample voltage is held with an external capacitor connected to the VREF_SAMP pin.

For example solar cells that operate at maximum power point (MPP) of 80% of their open circuit voltage, the resistor divider can be set to 80% of the VIN_DC voltage and the network will control the VIN_DC to operate near that sampled reference voltage. Alternatively, an external reference voltage can be applied directly to the VREF_SAMP pin by a MCU to implement a more complex MPPT algorithm.

The bq25505 was designed with the flexibility to support a variety of energy storage elements. The availability of the sources from which harvesters extract their energy can often be sporadic or time-varying. Systems will typically need some type of energy storage element, such as a rechargeable battery, super capacitor, or conventional capacitor. The storage element will make certain constant power is available when needed for the systems. The storage element also allows the system to handle any peak currents that can not directly come from the input source. To prevent damage to the storage element, both maximum and minimum voltages are monitored against the internally programmed undervoltage (VBAT_UV) and user programmable overvoltage (VBAT_OV) levels.

To further assist users in the strict management of their energy budgets, the bq25505 toggles the battery good flag to signal an attached microprocessor when the voltage on an energy storage battery or capacitor has dropped below a pre-set critical level. This should trigger the shedding of load currents to prevent the system from entering an undervoltage condition. The OV and battery good (VBAT_OK) thresholds are programmed independently.

In addition to the boost charging front end, bq25505 provides the system with an autonomous power multiplexer gate drive. The gate drivers allow two storage elements to be multiplexed autonomously in order to provide a single power rail to the system load. This multiplexer is based off the VBAT_OK threshold which is resistor programmable by the user. This allows the user to set the level when the system is powered by the energy harvester storage element, for example, rechargable battery or super capacitor or a primary nonrechargeable battery (for example, two AA batteries). This type of hybrid system architecture allows for the run-time of a typical battery powered systems to be extended based on the amount of energy available from the harvester. If there is not sufficient energy to run the system due to extended "dark time", the primary battery is autonomously switched to the main system rail within 8 µsec in order to provide uninterrupted operation.