BQ25504 Datasheet, PDF (14/24 Pages) Texas Instruments – Ultra Low Power Boost Converter with Battery Management for Energy Harvester Applications

bq25504
SLUSAH0 – OCTOBER 2011
www.ti.com
Internally, the boost converter modulates the effective impedance of the energy transfer circuitry to regulate the
input voltage (VIN_DC) to the sampled reference voltage (VREF_SAMP). A new reference voltage is obtained
every 16s by periodically disabling the charger for 256ms and sampling a ratio of the open-circuit voltage. The
reference voltage is set by the following expression:
æ
VREF_SAMP = VIN_DC(OpenCircuit)_ç
R_OC1
ö
÷
è ^ROC1 ^{+ R}OC2 ø
(1)
The internal MPPT circuitry and the periodic sampling of VIN_DC can be disabled by tying the VOC_SAMP pin
to VSTOR. When disabled an external reference voltage can be fed to the VREF_SAMP pin. The boost
converter will then regulate VIN_DC to the externally provided reference. If input regulation is not desired,
VREF_SAMP can be tied to GND.
Storage Element
The storage elements should be connected to VBAT pin. Many types of elements can be used, such as
capacitors, super capacitors or various battery chemistries. If a capacitor is selected it needs to meet the
minimum capacitance of 100 µF. If a battery is used it should be selected to have a minimum capacity equivalent
to 100 µF capacitance. To take full advantage of the battery management, the load is normally tied to the
VSTOR pin. Also, if there is large load transients or the storage element has impedance then it is necessary to
add a low ESR by-pass capacitor to prevent a droop in voltage.
Battery Management
In this section the battery management functionality of the bq25504 integrated circuit (IC) is presented. The IC
has internal circuitry to manage the voltage across the storage element and to optimize the charging of the
storage element. For successfully extracting energy from the source, three different threshold voltages must be
programmed using external resistors, namely the under voltage (UV) threshold, battery good threshold
(VBAT_OK) and over voltage (OV) threshold. The three threshold voltages determine the region of operation of
the IC. Figure 22 shows a plot of the voltage at the VSTOR pin and the various threshold voltages. For the best
operation of the system, the VBAT_OK should be used to determine when a load can be applied or removed. A
detailed description of the three voltage thresholds and the procedure for designing the external resistors for
setting the three voltage thresholds are described next.
device absolute max = 5.5 V
over voltage (user programmable) = 3.1 V
over voltage – hyst (internal)
Charger off
VBAT_OK + hyst (user programmable) = 2.8 V
VBAT_OK (user programmable) = 2.4 V
Main Boost
Charger on
under voltage + hyst (internal)
under voltage (user programmable) = 2.2 V
charger enable = 1.8 V
chip enable = 1.4 V
ground
Cold start
Figure 22. Figure Shows the Relative Position of Various Threshold Voltages
(Threshold Voltages are From Typical Solar Application Circuit in Figure 2)
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