Introduction
1
Introduction
1.1
EVM Features
•
Evaluation module for bq25505
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Ultra-low power boost converter/charger with battery management for energy harvester applications
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Resistor-programmable settings for overvoltage providing flexible battery management
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Programmable push-pull output indicator for battery status (VBAT_OK)
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Test points for key signals available for testing purpose – easy probe hook-up
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Jumpers available – easy to change settings
1.2
General Description
The bq25505 is an integrated energy harvesting Nano-Power management solution that is 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 milliwatts (mW) of power generated from a variety
of high output impedance (Hi-Z) DC sources like photovoltaic (solar) or thermal electric generators; or with
an AC/DC rectifier, a piezoelectric generator. The bq25505 implements a highly efficient, pulse-frequency
modulated (PFM) boost converter/charger targeted toward products and systems, such as wireless sensor
networks (WSN) which have stringent power and operational demands. Assuming a depleted storage
element has been attached, the bq25505 DC-DC boost converter/charger that requires only microwatts of
power to begin operating in cold-start mode. Once the boost converter output, VSTOR, reaches ~1.8 V
and can now power the converter, the main boost converter can now more efficiently extract power from
low voltage output harvesters such as thermoelectric generators (TEGs) or single- and dual-cell solar
panels. For example, assuming the Hi-Z input source can provide at least 5 µW typical and the load on
VSTOR (including the storage element leakage current) is less than 1 µA of leakage current, the boost
converter can be started with VIN_DC as low as 330 mV typical, and once VSTOR reaches 1.8 V, can
continue to harvest energy down to VIN_DC
≃
120 mV.
Hi-Z DC sources have a maximum output power point (MPP) that varies with ambient conditions. For
example, a solar panel's MPP varies with the amount of light on the panel and with temperature. The MPP
is listed by the harvesting source manufacturer as a percentage of its open circuit (OC) voltage. Therefore,
the bq25505 implements a programmable maximum power point tracking (MPPT) sampling network to
optimize the transfer of power into the device. The bq25505 periodically samples the open circuit input
voltage every 16 seconds by disabling the boost converter for 256 ms and stores the programmed MPP
ratio of the OC voltage on the external reference capacitor (C2) at VREF_SAMP. Typically, solar cells are
at their MPP when loaded to ~70–80% of their OC voltage and TEGs at ~50%. While the storage element
is less than the user programmed maximum voltage (VBAT_OV), the boost converter loads the harvesting
source until VIN_DC reaches the MPP (voltage at VREF_SAMP). This results in the boost charger
regulating the input voltage of the converter until the output reaches VBAT_SEC_OV, thus transferring the
maximum amount of power currently available per ambient conditions to the output.
The battery undervoltage, VBAT_UV, threshold is checked continuously to ensure that the internal battery
FET, connecting VSTOR to VBAT_SEC, does not turn on until VSTOR is above the VBAT_UV threshold
(2 V).The overvoltage (VBAT_OV) setting initially is lower than the programmed value at startup (varies on
conditions) and is updated after the first ~32 ms. Subsequent updates are every ~64 ms. The VBAT_OV
threshold sets maximum voltage on VSTOR and the boost converter stops switching when the voltage on
VSTOR reaches the VBAT_OV threshold. The open circuit input voltage (VIN_OC) is measured every ~16
seconds in order for the Maximum Power Point Tracking (MPPT) circuit to sample and hold the input
regulation voltage. This periodic update continually optimizes maximum power delivery based on the
harvesting conditions.
PowerPAD is a trademark of Texas Instruments.
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User's Guide for bq25505 Battery Charger Evaluation Module for Energy
SLUUAA8 – September 2013
Harvesting
Copyright © 2013, Texas Instruments Incorporated
