External component selection

AN3319

14/25

DocID18265 Rev 8

6

External component selection

SPV1040 requires a set of external components and their proper selection guarantees both

the best chip functionality and system efficiency.

Input voltage capacitor

C

IN

is the input capacitor connected to the input rail in order to reduce the voltage ripple.

According to the maximum current (I

SC

) provided by the PV panel connected at the input,

the following formula should be considered to select the proper capacitance value for a

specified maximum input voltage ripple (V

IN_rp_max

):

Maximum voltage of this capacitor is strictly dependent on the input source (typically

between 1 V and 3 V).

Low-ESR capacitors are a good choice to increase the whole system efficiency. In order to

reduce the ESR effect, it is suggested to split the input capacitance into two capacitors

placed in parallel.

Input voltage partitioning

V

MPP-SET

is the pin used to monitor the voltage generated by the solar cells.

The V

MPP-SET

pin can be directly connected to PV+ rail through a 1 k

Ω R

3

resistor.

With regard to the V

MPP-SET

pin, two constraints must be taken into account:

•

When SPV1040 is off, V

MPP-SET

voltage must be

≥0.3 to turn-on the device

•

When SPV1040 is in operating mode, it enters BURST MODE if V

MPP-SET

decreases

triggering the 450 mV threshold.

Input voltage sensing capacitor

C

4

is placed as close as possible to the V

MPP-SET

pin to reject noise on V

MPP-SET

voltage.

However, V

MPP-SET

must be able to follow the V

IN

waveform to allow SPV1040 to monitor

input voltage variations.

It means that the time constant R

3

*C

4

must be chosen according to system properties,

which is the MPPT tracking time (T

MPP

≅1 ms). The rule below must be followed in order to

select C

4

capacitance:

Assuming R

3

= 1 k

Ω then:

C

IN

I

SC

F

SW

V

IN

⋅

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

≥

_rp_max

C

4

T

MP P

1

R

3

-------

10

3

–

1

10

3

---------

⋅

=

⋅

≤

C

4

10

μF

≤