8 Detailed Description
SLUSAH0C – OCTOBER 2011 – REVISED JUNE 2015
The bq25504 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
bq25504 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 bq25504 starts with a
DCDC boost charger that requires only microwatts of power to begin operating.
Once the VSTOR voltage is above VSTOR_CHGEN (1.8V 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 outputing voltages down to VIN(DC) (130mV minimum). When
starting from VSTOR=VBAT < 100mV, the cold start circuit needs at least VIN(CS), 330 mV typical, to charge
VSTOR up to 1.8V.
The bq25504 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 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 re-chargeable 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 user programmable undervoltage (VBAT_UV) and overvoltage (VBAT_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 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
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