BQ25570_14 Datasheet, PDF (23/41 Pages) Texas Instruments – Ultra Low Power Harvester Power Management IC with Boost Charger, and Nano-Powered Buck Converter

bq25570
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SLUSBH2C – MARCH 2013 – REVISED JANUARY 2014
Boost Charger Cold-Start Operation (VSTOR < VSTOR_CHGEN and VIN_DC > VIN(CS) )
If the attached storage element does not charge CSTOR above VSTOR_CHGEN, VIN_DC ≥ VIN(CS) and EN =
0, the cold-start circuit turns on. The cold-start circuit is essentially an unregulated boost converter with lower
efficiency compared to the main boost charger. The energy harvester must supply sufficient power for the IC to
exit cold start. See the Energy Harvester Selection applications section for guidance.
When the CSTOR voltage reaches VSTOR_CHGEN, the main boost charger starts up. The VSTOR voltage from
the main boost charger is compared against the battery undervoltage threshold (VBAT_UV). When the VBAT_UV
threshold is reached, the PMOS switch between VSTOR and VBAT turns on, which allows the energy storage
element attached to VBAT to charge up. Cold start is not as efficient as the main boost charger. If there is not
sufficient input power available, the cold start circuit may run continuously and the VSTOR output may never
increase above VSTOR_CHGEN for the main boost charger to start up. The battery management thresholds are
explained later is this section. See the Energy Harvester Selection applications section for guidance on minimum
input power requirements.
Main Boost Charger Operation (VSTOR > VSTOR_CHGEN and VIN_DC > VIN(DC) )
The main boost charger charges the storage element attached at VBAT with the energy available from the high
impedance input source. For the first 32 ms (typical) after the boost charger is turned ON (assuming EN is low),
the charger is disabled to let the input rise to its open-circuit voltage. This is needed to obtain the reference
voltage which will be used for the remainder of the charger operation until the next MPPT sampling. The boost
charger employs pulse frequency modulation (PFM) mode of control to regulate the voltage at VIN_DC close to
the desired reference voltage. The reference voltage is set by the MPPT control scheme as described in the next
section. Input voltage regulation is obtained by transferring charge from the input to VSTOR only when the input
voltage is higher than the voltage on pin VREF_SAMP. The current through the inductor is controlled through
internal current sense circuitry. The peak current in the inductor is dithered internally to pre-determined levels in
order to maintain high efficiency of the charger across a wide input current range. The charger transfers up to a
maximum of 100 mA average input current (230mA typical peak inductor current). The boost charger is disabled
when the voltage on VSTOR reaches the OV condition to protect the battery connected at VBAT from
overcharging. In order for the battery to charge to VBAT_OV, the input power must exceed the power needed for
the load on VSTOR. See the Energy Harvester Selection applications section for guidance on minimum input
power requirements.
Maximum Power Point Tracking
Maximum power point tracking (MPPT) is implemented in order to maximize the power extracted from an energy
harvester source. The boost charger indirectly modulates the input impedance of the main boost charger by
regulating the charger's input voltage, as sensed by the VIN_DC pin, to the sampled reference voltage, as stored
on the VREF_SAMP pin. The MPPT circuit obtains a new reference voltage every 16 s (typical) by periodically
disabling the charger for 256 ms (typical) and sampling a fraction of the open-circuit voltage (VOC). For solar
harvesters, the maximum power point is typically 70%-80% and for thermoelectric harvesters, the MPPT is
typically 50%. Tying VOC_SAMP to VSTOR internally sets the MPPT regulation point to 80% of VOC. Tying
VOC_SAMP to GND internally sets the MPPT regulation point to 50% of VOC. If input source does not have
either 80% or 50% of VOC as its MPP point,, the exact ratio for MPPT can be optimized to meet the needs of the
input source being used by connecting external resistors R_OC1 and R_OC2 between VRDIV and GND with mid-
point at VOC_SAMP.
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)
Storage Element / Battery Management
In this section the battery management functionality of the bq25570 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, two different threshold voltages must be
programmed using external resistors, namely battery good threshold (VBAT_OK) and over voltage (OV)
threshold. The two user programmable threshold voltages and the internally set undervoltage threshold
determine the IC's region of operation. Figure 40 show plots of the voltage at the VSTOR pin and the various
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