SLUSAH0F October 2011 – November 2019 BQ25504
The energy harvesting source (e.g., solar panel, TEG, vibration element) must provide a minimum level of power for the IC to operate as designed. The IC's minimum input power required to exit cold start can be estimated as:
where I-STR_ELM_LEAK@1.8V is the storage element leakage current at 1.8V and
RSTOR(CS) is the equivalent resistive load on VSTOR during cold start and 0.05 is an estimate of the worst case efficiency of the cold start circuit.
Once the IC is out of cold start and the system load has been activated (e.g., using the VBAT_OK signal), the energy harvesting element must provide the main boost charger with at least enough power to meet the average system load. Assuming RSTOR(AVG) represents the average resistive load on VSTOR, the simplified equation below gives an estimate of the IC's minimum input power needed during system operation:
where ηEST can be derived from the datasheet efficiency curves for the given input voltage and current and VBAT_OV. The simplified equation above assumes that, while the harvester is still providing power, the system goes into low power or sleep mode long enough to charge the storage element so that it can power the system when the harvester eventually is down. Refer to spreadsheet SLUC462 for a design example that sizes the energy harvester.