bq25505
SLUSBJ3B –AUGUST 2013–REVISED JANUARY 2014
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
DESCRIPTION (CONTINUED)
Once started, the boost charger can effectively extract power from low voltage output harvesters such as TEGs
or single or dual cell solar panels. The boost charger can be started with VIN as low as 330 mV, and once
started, can continue to harvest energy down to VIN = 100 mV.
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. 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 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 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 internally programmed undervoltage (UV) and user programmable overvoltage (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 thresholds are programmed independently.
In addition to the boost charging front end, bq25505 provides the system with an autonomous power multiplexor
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 multiplexor 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, e.g. rechargable battery or super capacitor or a primary non-rechargeable battery
(e.g. 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.
All the capabilities of bq25505 are packed into a small foot-print 20-lead 3.5 mm x 3.5 mm QFN package.
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