SLUSAH0F October 2011 – November 2019 BQ25504
Maximum power point tracking (MPPT) is implemented in order to maximize the power extracted from an energy harvester source. The boost converter 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 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 harvester's open-circuit voltage (VOC). For solar harvesters, the maximum power point is typically 70%-80% of VOC and for thermoelectric harvesters, the MPPT is typically 50%. The exact ratio for MPPT can be optimized to meet the needs of the input source being used by connecting external resistors ROC1 and ROC2 between VIN_DC and GND with mid-point at VOC_SAMP.
Spreadsheet SLUC484 provides help on sizing and selecting the resistors.
The internal MPPT circuitry and the periodic sampling of VIN_DC can be disabled by tying the VOC_SAMP pin to VSTOR. 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 (i.e. the input source is a low-impedance output battery or power supply instead of a high impedance output energy harvester), VREF_SAMP can be tied to GND.