SPV1050 datasheet(25/36 Pages) STMICROELECTRONICS

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SPV1050

Functional description

MPPRATIO = R3 / (R2 + R3)

In boost mode if the electrical characteristics of the selected source and battery are such

that VOC-MAX  VUVP(min), then the resistor R1 can be replaced by a short-circuit.

Consequently, only R2 and R3 have to be selected for a proper setting of MPPRATIO.

In a PV panel the VMPP is typically within 70% ÷ 80% of VOC.

In a TEG the VMPP is typically about 50% of VOC.

The MPPT accuracy can be strongly affected by an improper selection of the input

capacitor. The input capacitance CIN = 4.7 F generally covers the most typical use cases.

The energy extracted from the harvested source, and stored in the input capacitance, is

transferred to the load by the DC-DC converter through the inductor. The energy extracted

by the inductor depends by the sink current: the higher input currents cause higher voltage

drop on the input capacitance and this may result a problem for low voltage (< 1 V) and high

energy (> 20 mA) sources. In such application cases the input capacitance has to be

increased or, alternatively the L1 inductance has to be reduced.

During the TSAMPLE time frame the input capacitor CIN is charged up to VOC by the source

with a T1 time constant resulting from the capacitance and the equivalent resistance REQ of

the source.

In case of the PV source, assuming IMPP the minimum current at which the MPP must be

guaranteed, the REQ can be calculated as following:

Equation 8

REQ = (VOC - VMPP) / IMPP = VOC • (1 - MPPRATIO) / IMPP

Thus CIN is calculated by the following formula:

Equation 9

CIN T1 /REQ

The following plots in Figure 20 and Figure 21 show the effect of different CIN values on the

time constant. If the capacitance is too high, the capacitor may not be charged within the

TSAMPLE = 400 ms time window, thus affecting the MPPT accuracy.

Figure 20. Energy harvester equivalent circuit

Figure 21. Voltage vs. time at different C values

and fixed current

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