L6924D operation in solar powered applications

AN4050

8/19

DocID022816 Rev 2

During the fast-charge phase, the output voltage of the SPV1040 (VIN of L6924D) goes

down to the battery voltage (V

BAT

) plus the voltage drop across the Power MOSFET

(

Δ

V

MOS

) of the charger.

Consequently, the internal MOSFET works in saturation mode with a voltage drop given by

the following formula:

Equation 1

where

Equation 2

I

LIM

is the current limit of the SPV1040, which depends on solar irradiation.

Neglecting the voltage drop across the charger (

Δ

VMOS) when the device operates in this

condition, its input voltage is equal to the battery one, and so a very low operating input

voltage (down to 2.5 V) is required. The power dissipated by the device during this phase is:

Equation 3

The advantage of the quasi-pulse charging method allows the energy harvested by few

solar cells to be maximized.

Note that the STEVAL-ISV0012V1 mounts two LEDs, D1 and D2, which indicate (when ON)

whether the charge is in progress or is completed, respectively.

R14, and consequently I

LIM

, must be set up according to the power provided by the PV

panel at the maximum irradiation, but it is possible that at lower irradiations D1 starts

flickering (or appearing ON), while D2 is ON as well.

This is due to the battery charger which tries to charge the battery at 4.2 V (or 4.1 V,

depending on the V

OPRG

setting) and I

LIM

, but the required power can be sustained only if

enough irradiation is available at the PV panel side. If the irradiation is not sufficient, the

input voltage of the L6924D drops down to the battery voltage, causing the battery charging

to stop and D1 turns ON. Shortly after, the voltage rises back to 4.2 V (or 4.1 V) and the

battery charge starts again (D1 turns OFF).

In these low irradiation conditions the battery is charged by current packets anyway.

The plots below show behavior in the case of low irradiation:

V

IN

V

AD P

V

BA T

ΔV

MOS

+

=

=

ΔV

MOS

R

DS

o n

()

I

LIM

×

=

P

CH

R

DS

o n

()

I

LIM

2

×

=