ST STEVAL-ISV012V1 Installation and operating instructions

December 2017
DocID022816 Rev 3
1/17
www.st.com
AN4050
Application note
STEVAL-ISV012V1 lithium-ion solar battery charger
Domenico Ragonese; Alessandro Nicosia; Giovanni Conti
Introduction
The STEVAL-ISV012V1 evaluation board mounts an SPV1040 (solar energy harvester) for the input
stage and an L6924D (Li-Ion battery charger) as the output stage. It targets any portable application
powered by lithium-ion batteries and merges the SPV1040 power extraction capacity of the solar
module with the linear regulation of the L6924D for optimum battery charging load protection while
reducing the power dissipation at the bottom.
Figure 1: STEVAL-ISV012V1 evaluation board
The board is designed to charge lithium-ion and lithium-polymer batteries with VBATT_max = 4.1 or 4.2 V
and it includes a 400 mWpk polycrystalline PV panel (SZGD6060-4P from NBSZGD) with VOC = 2.2 V
and ISC = 220 mA.
According to specific application requirements, some components may be replaced
a
:
The PV panel can be replaced as long as VOC < VBATT_max and IS< 1.65 A.
The inductor L1 can be replaced, but consider its effect on the maximum peak current to ensure
that the input overcurrent limit is not triggered.
The maximum output current can be limited by replacing the current sensing resistor RS(0 0Ω by
default).
Resistor R14, which limits the charge current threshold (500 mA by default).
a
For more details on component selection, refer to Application note AN3319, section “external component selection”

Contents
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Contents
1SPV1040 operation..........................................................................4
2L6924D operation ............................................................................6
2.1 L6924D operation in solar powered applications...............................6
3Reference design description ......................................................10
4Schematic diagrams......................................................................12
5Bill of materials..............................................................................14
6Revision history ............................................................................16

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List of figures
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List of figures
Figure 1: STEVAL-ISV012V1 evaluation board..........................................................................................1
Figure 2: Typical application circuit.............................................................................................................4
Figure 3: SPV1040 equivalent circuit..........................................................................................................4
Figure 4: MPPT working principle...............................................................................................................5
Figure 5: SPV1040 internal block diagram.................................................................................................5
Figure 6: Basic application schematic ........................................................................................................6
Figure 7: Typical charge curve in Quasi-pulse mode.................................................................................7
Figure 8: Battery charging at low irradiation...............................................................................................8
Figure 9: Battery charging at low irradiation (zoom)...................................................................................8
Figure 10: Maximum available current vs. Pin, 200 mW peak PV panel....................................................9
Figure 11: Maximum available current vs. Pin, 2 W peak PV panel...........................................................9
Figure 12: Application set-up....................................................................................................................10
Figure 13: V-I and P-V plot diagrams .......................................................................................................10
Figure 14: Partial charge ..........................................................................................................................11
Figure 15: Full charge...............................................................................................................................11
Figure 16: STEVAL-ISV012V1 schematic, battery charge section ..........................................................12
Figure 17: STEVAL-ISV012V1 schematic, solar power optimizer section ...............................................13

SPV1040 operation
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1 SPV1040 operation
The SPV1040 device is a low power, low voltage, monolithic step-up converter with an
input voltage range from 0.3 V to 5.5 V, capable of maximizing the energy generated by a
single solar cell (or fuel cell), where low input voltage handling capability is important.
When combined with the L6924D, it provides an ideal solution for charging lithium battery
packs with energy harvested from a very small solar panel.
The SPV1040 is a 100 kHz, fixed-frequency pulse width modulation (PWM) step-up
converter able to maximize the energy harvested by a few solar cells. It employs a
maximum power point tracking (MPPT) algorithm which continuously tracks its output
voltage and current. The converter guarantees the safety of the overall application and its
own by stopping PWM switching in case of an overvoltage, overcurrent or overtemperature
condition. The IC integrates a 120 mΩ N-channel MOSFET power switch and a 140 mΩ P-
channel MOSFET synchronous rectifier.
Figure 2: Typical application circuit
The SPV1040 acts as an impedance adapter between the PV module and the output load.
The equivalent circuit is shown below.
Figure 3: SPV1040 equivalent circuit
The MPPT algorithm sets up the correct DC working point by ensuring Zin = Zm(assuming
Zmis the impedance of the supply source). In this way, the power extracted from the supply
source (Pin = Vin * Iin) is maximum (Pm= Vm* Im).
Lx RS
LVBATT
XSHUT
GND
MPP-SET
VPV
R1 R3COUT
RF1
CFRF2
R2
CINsns COUTsns
CIN DOUT
ICTRL_MINUS
ICTRL_PLUS
VCTRL
VOUT
SPV1040
I I
R
C
I
V V
gm V
C
I
Z DC
R
V V
PV
Panel
IN
IN
IN
IN OUT
OUT OUT
OUT

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SPV1040 operation
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The voltage-current curve shows all the available working points of the PV panel at a given
solar irradiation. The voltage-power curve is derived from the voltage-current curve by
plotting the product V*I for each voltage generated
a
.
Figure 4: MPPT working principle
Figure 5: SPV1040 internal block diagram
The duty cycle set by the MPPT algorithm can be overwritten if one of the following events
is triggered:
Input overcurrent protection (OVC): inductor peak current ≤1.65 A
Overtemperature protection (OVT): internal temperature ≤155 °C
Output voltage regulation: VCTRL pin triggers the 1.25 V internal reference
Output current limitation: RS* (ICTRL_PLUS - ICTRL_MINUS) ≤50 mV
MPP-SET voltage VMPP-SET ≤300 mV at startup and VMPP-SET ≤450 mV in
running mode.
Application components must be carefully selected to avoid any undesired triggering of the
above thresholds.
a
For more details regarding the MPPT algorithm, refer to the SPV1040 datasheet.
IMP PMAX
[A]
[W]
urrent
ower
C
P
Voltage [V]
0VOC
VMP
STARTSTART SSIGNALIGNAL
Lx
XSHUT
MPP BLOCK
DETECTOR
-
Burst Ref
CLOCK
+
-
GND
OVER CURRENT
OVER TEMPERATURE
REVERSE POLARITY
+
BURST MODE DIGITAL
DAC CODE CORE MPP-SET
VREF
V
-
CTRL_PLUS
I
CLOCK
CTRL
V
CTRL_MINUS
MPP-REF
PWM
DRIVERS
CONTROL
MPP-SET
V
ZEROZERO CROSSINGCROSSING
OUT
V
ANALOG BLOCK
MPP-REF
I
+
Iout Reg
Vin Reg
Vout Reg VREF
+
-

L6924D operation
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2 L6924D operation
The L6924D is a fully monolithic battery charger dedicated to single-cell Li-Ion/polymer
battery packs. It is designed with BCD6 technology and integrates all of the power
elements (Power MOSFET, reverse blocking diode and sense resistor) in a small
VFQFPN16 3 mm x 3 mm package.
It normally works as a linear charger when powered from an external voltage regulated
adapter. However, thanks to its very low minimum input voltage (down to 2.5 V) the
L6924D can also work as a quasi-pulse charger when powered from a current limited
adapter, dramatically reducing the power dissipation.
The L6924D charges the battery in three phases:
Pre-charge constant current: a deeply discharged battery is charged with a low
current.
Fast-charge constant current: the device charges the battery with the maximum
current.
Constant voltage: when the battery voltage is close to the selected output voltage, the
device starts to reduce the current until the charge termination has completed.
Regardless of the charging approach, a closed loop thermal control features protects the
device from overheating. The L6924D allows the user to program many parameters, such
as pre-charge current, fast-charge current, pre-charge voltage threshold, end-of-charge
current threshold and charge timer.
The L6924D offers two open collector outputs for diagnostic purposes, which can be used
to either drive two external LEDs or communicate with a host microcontroller.
Finally, the L6924D also provides other battery related functions, such as checking for
battery presence, monitoring and protection from unsafe thermal conditions.
Figure 6: Basic application schematic
2.1 L6924D operation in solar powered applications
Thanks to its very low minimum input voltage (down to 2.5 V), the L6924D can also work as
a quasi-pulse charger when powered from a current limited adapter such as a PV panel or
a current limiting device such as the SPV1040 step-up.
BATTERY
SHDN
ON
OFF
GND VOPRGIPRE
TPRG
VPRE
IPRG
IEND
VOSNS
VOUT
TH
VREF
VINSNS
VIN
ST1
ST2
L6924D
CHARGER
Vref
L6924D
R3 R9
C4
C1
R1 R2
LD1 LD2 C2
R7 R8
R4
R5
R6 R10
J5

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L6924D operation
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To work in this condition, set the device charging current (with R14) higher than the
maximum peak current of the PV panel. During the fast-charge phase, the output voltage of
the SPV1040 that supplies the L6924D drops down to the battery voltage plus the voltage
drop across the power MOSFET of the charger.
In this mode, the L6924D charges the battery with the same three phases as in linear
mode, but power dissipation is greatly reduced, as shown in the following figure.
Figure 7: Typical charge curve in Quasi-pulse mode
During the fast-charge phase, the output voltage of the SPV1040 (VIN of L6924D) drops
down to the battery voltage (VBAT) plus the voltage drop across the Power MOSFET
(ΔVMOS) of the charger.
Consequently, the internal MOSFET works in saturation mode with a voltage drop given by:
Equation 1

        

L6924D operation
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where
Equation 2

  󰇛󰇜 
ILIM is the current limit of the SPV1040, which depends on solar irradiation.
Neglecting the voltage drop across the charger (ΔVMOS) when the device operates in this
condition, its input voltage is equal to the battery’s, and therefore 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.
The STEVAL-ISV0012V1 LEDs D1 and D2 indicate (when ON) whether the
charge is in progress or is completed, respectively.
R14, and consequently ILIM, must be set up according to the power provided by the PV
panel at the maximum irradiation, but it is possible that D1 starts flickering (or appearing
ON) at lower irradiation levels, 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 VOPRG setting) and ILIM, but the required power can only be sustained if
enough irradiation is available on the PV panel side. If the irradiation is not sufficient, the
input voltage of the L6924D drops down to the battery voltage, causing battery charging to
stop and D1 to turn 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 demonstrate the behavior in the event of low irradiation.
Figure 8: Battery charging at low irradiation
Figure 9: Battery charging at low irradiation
(zoom)
The plots below show the maximum available current that can be provided to the battery
charger according to the input power.

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L6924D operation
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Figure 10: Maximum available current vs. Pin, 200 mW peak PV panel
Figure 11: Maximum available current vs. Pin, 2 W peak PV panel
0
10
20
30
40
50
60
70
80
0 50 100 150 200 250 300 350 400
Pin [mW]
Iout max[mA]
Vout =4.5V
0
50
100
150
200
250
300
350
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Pin [mW]
Iout max[mA]
Vout =4.5V

Reference design description
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3 Reference design description
The set-up used for measurements is shown below.
Figure 12: Application set-up
A solar array simulator (SAS, SAS-FL05/01 from CBL Electronics) to simulate the PV
module with VOC = 2.5 V, ISC = 210 mA, Vmp = 2.0 V, Imp = 200 mA (@ 1000 W/m²
irradiance) and a Li-Ion battery 3.7 V-700 mAh, are used. Figure 13: "V-I and P-V plot
diagrams" shows the I-V and P-V curves generated by the SAS, obtained using a PV
module analyzer (ISM490 from ISOTECH).
Figure 13: V-I and P-V plot diagrams
Figure 14: "Partial charge" and Figure 15: "Full charge" show the partial and full charge
curves respectively. The partial charge curve shows charge current and voltage within a
one hour time frame at full irradiation starting from a 3.4 V condition. The full charge curve
shows charge current and voltage until the fully charged status is triggered, starting from a
3.4 V condition. After the one hour charge period time, the battery voltage reaches 3.8 V.
Different results can be obtained if a different PV panel and/or battery are used
a
.
a
Visit the support section on www.st.com if you require help regarding the use of different PV panels or batteries.