The BQ25504 device 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 device 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. The BQ25504 is the first device of its kind to implement a highly efficient boost converter/charger targeted toward products and systems, such as wireless sensor networks (WSNs) which have stringent power and operational demands. The design of the BQ25504 starts with a DC-DC boost converter/charger that requires only microwatts of power to begin operating.
Once started, the boost converter/charger can effectively extract power from low-voltage output harvesters such as thermoelectric generators (TEGs) or single- or dual-cell solar panels. The boost converter can be started with VIN as low as 600 mV, and once started, can continue to harvest energy down to VIN = 130 mV.
The BQ25504 also implements a programmable maximum power point tracking sampling network to optimize the transfer of power into the device. Sampling the VIN_DC open-circuit voltage is programmed using external resistors, and held with an external capacitor (CREF).
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 provide by a MCU to produce a more complex MPPT algorithm.
The BQ25504 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 ensures that constant power is available when needed for the systems. The storage element also allows the system to handle any peak currents that cannot directly come from the input source.
To prevent damage to a customer’s storage element, both maximum and minimum voltages are monitored against the user programmed undervoltage (UV) and overvoltage (OV) levels.
To further assist users in the strict management of their energy budgets, the BQ25504 toggles the battery good flag to signal an attached microprocessor when the voltage on an energy storage battery or capacitor has dropped below a preset critical level. This warning should trigger the shedding of load currents to prevent the system from entering an undervoltage condition. The OV, UV, and battery good thresholds are programmed independently.
All the capabilities of BQ25504 are packed into a small-footprint, 16-lead, 3-mm x 3-mm VQFN package.