DocID025515 Rev 1

13/19

AN4397

Component selection

19

4.2

Input capacitance

Every 16 seconds (typical) the SPV1050 device stops switching for 400 ms. During this time

according to the time constant (T1), which depends both on its capacitance and on the

Equation 5

Consequently:

Equation 6

Figure 10 and Figure 11 show the effect of different value of the C1 on the time constant: too

high capacitance might be not charged within the 400 ms time window affecting the MPPT

precision.

The default C1 = 4.7

F capacitance covers the most typical application range. The energy

extracted from the harvested source, and stored in the input capacitance, is transferred to

the load by the DC-DC converter though the inductor. The energy extracted by inductor

depends by the sink current: the higher input currents causes 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 inductance has to be reduced.

The SPV1050 performances might be further improved by reducing the time constant

(e.g. reducing input capacitance) at very low input power.

–

--------------------------------

–





-------------------------------------------------------------

==

C1

T1

-----------



Figure 10. Input stage equivalent circuit

Figure 11. Effect of C1 on sampled voltage