BQ25504 Datasheet, PDF (16/24 Pages) Texas Instruments – Ultra Low Power Boost Converter with Battery Management for Energy Harvester Applications

bq25504
SLUSAH0 – OCTOBER 2011
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The sum of the resistors must be 10 MΩ i.e., R_OK1 + R_OK2 + R_OK3= 10 MΩ. The logic high level of this signal is
equal to the VSTOR voltage and the logic low level is ground. The logic high level has ~20 KΩ internally in series
to limit the available current to prevent MCU damage until it is fully powered. The VBAT_OK_PROG threshold
must be greater than or equal to the UV threshold. For the best operation of the system, the VBAT_OK should
be setup to determine when a load can be applied or removed to optimize the storage element capacity.
Thermal Shutdown
Rechargeable Li-ion batteries need protection from damage due to operation at elevated temperatures. The
application should provide this battery protection and ensure that the ambient temperature is never elevated
greater than the expected operational range of 85°C.
The bq25504 uses an integrated temperature sensor to monitor the junction temperature of the device. If the
OT_PROG pin is tied low then the temperature threshold for thermal protection is set to TSD_ProtL which is
65°C typically. If the OT_PROG is tied high, then the temperature is set to TSD_ProtH which is 120°C typically.
Once the temperature threshold is exceeded, the boost converter/charger is disabled and charging ceases. Once
the temperature of the device drops below this threshold, the boost converter/charger can resume operation. In
order to avoid unstable operation near the overtemp threshold, a built-in hysteresis of approximately 5°C has
been implemented. Care should be taken to not over discharge the battery in this condition since the boost
converter/charger is disabled. However, if the supply voltage drops to the VBAT_UV setting, then the switch
between VBAT and VSTOR will open and protect the battery even if the device is in thermal shutdown.
APPLICATION INFORMATION
INDUCTOR SELECTION
For the bq25504 to operate properly, an inductor of appropriate value must be connected between Pin # 16
(LBST) and Pin #2 (VIN_DC) for the boost converter.
For the boost converter / charger, the inductor must have an inductance = 22 µH and have a peak current
capability of ≥250 mA with the minimum series resistance to keep high efficiency.
CAPACITOR SELECTION
In general, all the capacitors need to be low leakage. Any leakage the capacitors have will reduce efficiency,
increase the quiescent current and diminish the effectiveness of the IC for energy harvesting.
Sampled Reference Storage Capacitance:
The MPPT operation depends on the sampled value of the open circuit voltage and the input regulation follows
the voltage stored on the CREF capacitor. This capacitor is very sensitive to leakage since the holding period is
around 16 seconds. As the capacitor voltage drops due to any leakage, the input regulation voltage will also drop
and this can prevent proper operation from extraction the maximum power from the input source. Therefore, it is
recommended that the leakage be less than 2 nA at 3 V bias.
Input capacitor:
Operation of the BQ25504 requires a capacitor to be connected between Pin 15 (VSTOR) and ground. A
capacitor of 4.7 µF should be connected between Pin 15 and ground to assure stability of the boost converter,
especially when the battery is fully charged and the converter in output voltage limiting mode.
Energy from the energy harvester input source is initially stored on a capacitor CHVR tied to Pin 2 (VIN_DC) and
ground (VSS, Pin 1). For energy harvesters which have a source impedance which is dominated by a capacitive
behavior, the value of the harvester capacitor should scaled according to the value of the output capacitance of
the energy source , but an initial value of 4.7 µF is recommended.
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