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December 2017
DocID022816 Rev 3
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www.st.com
AN4050
Application note
STEVAL-ISV012V1 lithium-ion solar battery charger
Domenico Ragonese; Alessandro Nicosia; Giovanni Conti
a
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 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 replaceda:
The PV panel can be replaced as long as VOC < V
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).
For more details on component selection, refer to Application note AN3319, section “external component selection”
BATT_max
and IS < 1.65 A.
BATT_max
= 4.1 or 4.2 V
Page 2
Contents
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Contents
1 SPV1040 operation .......................................................................... 4
2 L6924D operation ............................................................................ 6
2.1 L6924D operation in solar powered applications............................... 6
3 Reference design description ...................................................... 10
4 Schematic diagrams ...................................................................... 12
5 Bill of materials .............................................................................. 14
6 Revision history ............................................................................ 16
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List of figures
DocID022816 Rev 3
3/17
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
Page 4
SPV1040 operation
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Lx
R
S
L
V
BATT
XSHUT
GND
MPP-SET
V
PV
R
1
R
3
C
OUT
R
F1
C
FRF2
R
2
C
INsns
C
OUTsns
C
IN
D
OUT
I
CTRL_MINUS
I
CTRL_PLUS
V
CTRL
V
OUT
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
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 Pchannel 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
Zm is the impedance of the supply source). In this way, the power extracted from the supply
source (Pin = Vin * Iin) is maximum (Pm = Vm * Im).
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SPV1040 operation
DocID022816 Rev 3
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a
I
MP
P
MAX
[A]
[W]
urren
t
ower
C
P
Voltage [V]
0
V
OC
V
MP
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
+
-
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 generateda.
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
Application components must be carefully selected to avoid any undesired triggering of the
above thresholds.
running mode.
For more details regarding the MPPT algorithm, refer to the SPV1040 datasheet.
Page 6
L6924D operation
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BATTERY
SHDN
ON
OFF
GND
V
OPRGIPRE
T
PRG
V
PRE
I
PRG
I
END
V
OSNS
V
OUT
TH
V
REF
V
INSNS
V
IN
ST1
ST2
L6924D
CHARGER
Vref
L6924D
R3
R9
C4
C1
R1
R2
LD1 LD2
C2
R7 R8
R4
R5
R6
R10
J5
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.
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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 (V
(ΔV
) of the charger.
MOS
Consequently, the internal MOSFET works in saturation mode with a voltage drop given by:
Equation 1
) plus the voltage drop across the Power MOSFET
BAT
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L6924D operation
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The STEVAL-ISV0012V1 LEDs D1 and D2 indicate (when ON) whether the
charge is in progress or is completed, respectively.
Figure 8: Battery charging at low irradiation
Figure 9: Battery charging at low irradiation
(zoom)
where
Equation 2
I
is the current limit of the SPV1040, which depends on solar irradiation.
LIM
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.
R14, and consequently I
, must be set up according to the power provided by the PV
LIM
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 V
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.
The plots below show the maximum available current that can be provided to the battery
charger according to the input power.
setting) and I
OPRG
, but the required power can only be sustained if
LIM
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AN4050
L6924D operation
DocID022816 Rev 3
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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
Figure 10: Maximum available current vs. Pin, 200 mW peak PV panel
Figure 11: Maximum available current vs. Pin, 2 W peak PV panel
Page 10
Reference design description
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DocID022816 Rev 3
a
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 useda.
Visit the support section on www.st.com if you require help regarding the use of different PV panels or batteries.
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AN4050
Reference design description
DocID022816 Rev 3
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0
20
40
60
80
100
120
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0
50 100 150 200 250 300
Output Current [mA]
Output Voltage [V]
Time [m]
90
92
94
96
98
100
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 10 20 30 40 50 60
Output Current [mA]
Output Voltage [V]
Time[m]
The average overall power efficiency is approximately 85% (94% for SPV1040 and 90% for
L6924D).
Figure 14: Partial charge
Figure 15: Full charge
Page 12
Schematic diagrams
AN4050
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DocID022816 Rev 3
L6924D
VFQFPN16
SW3, SW4:
default is 0 Ohm on LED (2-3 closed)
ST1
ST2
Tprg
Vin
Vin_sns
default closed
default closed
default open
PV-
SHDWN
TH
Voprg
Vosns
Vo
Iend
Vpre
Iprg
Ipre
PV-
VOUT_SPV1040
mc2
mc1
VREF
VBAT
NTC
PV-
PV-
VREF
VBAT
PV-
R16
DNM
R9
470 Ohm
SW3
1
3
2
R15
DNM
SW4
1
3
2
D1D1
R6 1k Ohm
R6 1k Ohm
4.7uF
C9
R12 DNM
TP9TP9
TP7TP7
R14 24k Ohm
1J1J
1
2
C7
10nFC710nF
C6
47uFC647uF
2J2J
1
2
3J3J
1
2
C8
1nFC81nF
R10 3.3k Ohm
R10 3.3k Ohm
R13 DNM
R7 1k Ohm
R7 1k Ohm
TP8TP8
R8
1k Ohm
R8
1k Ohm
J28J28
123
4
5
6
7
8
9
10
14
16
15
13
12
11
D2D2
4 Schematic diagrams
Figure 16: STEVAL-ISV012V1 schematic, battery charge section
Page 13
AN4050
Schematic diagrams
DocID022816 Rev 3
13/17
SPV1040
Vctrl =1.25V
TSSOP8
@Vout = 3.45V
VRs=50mV @ Imax
Vout
SMM4F5.0A
VOUT_SPV1040
VIN_sns
PV-
LX_1040
Vctrl
Ictrl-
Ictrl+
X_SHUT
Vctrl
Vrs+
Vrs-
Ictrl+
Ictrl-
Vrs+
Vrs-
VIN_sns
PV +
RF2 1kRF2 1k
C2
1nFC21nF
TP1TP1
CF1
1uF
CF1
1uF
Cout1
10uF
Cout1
10uF
TP2TP2
R2
820kR2820k
R4
DNM
R4
R31R3
1k
J26
CON8B
J26
CON8B
123
4
5
6
7
8
Rs1 0 OhmRs1 0 Ohm
C4C4
1nF
R1
2.2MR12.2M
RF11kRF1
1k
Dout1
TRISIL
Dout1
TRISIL
TP5TP5
R5 0 OhmR5 0 Ohm
Figure 17: STEVAL-ISV012V1 schematic, solar power optimizer section
Page 14
Bill of materials
AN4050
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DocID022816 Rev 3
Item
Q.ty
Ref.
Part/Value
Description
Manufacturer
Order code
1
1
PV1
(polycris
talline)
400 mW, Vmp =
1.92 V; Imp = 200
mA; Voc = 2.2 V;
Isc = 220 mA
Solar panel
NBSZGD
SZGD60604P 2 1
Cin1
47 µF, 6.3 V, 0805
Multilayer
ceramic
capacitor
Kemet
C0805C476M
9PAC7800
3 2 C2, C4
1 nF, 50 V, 0805
Ceramic
capacitors
Kemet
C0805C102K
5RAC
4 1 Cout1
10 µF, 16 V, 0805
Multilayer
ceramic
capacitor
Kemet
C0805C106K
4PAC7800
5 1 R3
1 kΩ, 0805
Resistor
Vishay
CRCW08051
K00FKEA
6 1 R4
3.3 mΩ, 63M
Resistor
DNM
7 1 L1
10 µH, Isat > 1.5 A
at vmp = 2 V,
2220(EIA)
Power inductor
Coilcraft
MSS7341103ML
EPCOS
B82442T110
3K050
8 1 VRS
50 mV at Iout_max,
0805
Thick film
resistor
Vishay
CRCW08050
000Z0EA
9 1 R1
2.2 mΩ, 0805
Resistor
Multicomp
MCHV05WAJ
0225T5E
10 1 R2
820 kΩ, 0805
Resistor
Vishay
CRCW08058
20KFKEA
11 1 R5
0805
Resistor
Vishay
CRCW08050
000Z0EA
12 1 J26
SPV1040, TSSOP8
High efficiency
solar battery
charger with
embedded
MPPT
ST
SPV1040T
13 1 Dout1
Vbr = 5 V, Vcl = 9
V, STmite Flat,
SMM4F
400 W Transil™
ST
SMM4F5.0
14 1 J28
L6924D,
VFQFPN16
Battery charger
system with
integrated power
switch for LiIon/Li-Polymer
ST
L6924D
5 Bill of materials
Table 1: STEVAL-ISV012V1 bill of materials
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AN4050
Bill of materials
DocID022816 Rev 3
15/17
Item
Q.ty
Ref.
Part/Value
Description
Manufacturer
Order code
15
2
RF1,
RF2
1 kΩ, 0805
Thick film
resistors
Vishay
CRCW08051
K00FKEA
16 1 CF1
1 µF, 10 V, 0805
Multilayer
ceramic
capacitor
Murata
GRM21BR71
C105KA01L
17 2 D1, D2
SMD, 2.5 V, 25 mA,
0805
Green LED
Kingbright
KP-2012SGC
18
3
R6, R7,
R8
1 kΩ, 0805
Resistors
Vishay
CRCW08051
K00FKEA
19 1 C6
47 µF, 6.3 V, 0805
Ceramic
capacitors
Kemet
C0805C476M
9PAC7800
20 1 C7
10 nF, 50 V, 0805
Ceramic
capacitors
Kemet
C0805C103K
5RAC
21 1 C8
1 nF, 50 V, 0805
Multilayer
ceramic
capacitor
Kemet
C0805C102K
5RAC
22 1 C9
4.7 µF, 0805
Ceramic
capacitor
Murata
GRM21BF51
A475ZA01L
23 1 R10
3.3 kΩ
Resistor
Bourns
CR0805-FX3301GLF
24 1 R9
470 Ω, 0805
Resistor
Bourns
CR0805-FX4700GLF
25 1 R14
24 kΩ, 0.1 W, 0805,
± 1%
Resistor
Multicomp
C2012C0G2A
103J125AA
26
3
J1, J2,
J3
Jumper100
Jumpers
Any 27
2
SW3,
SW4
0 Ω, 0805, SMD,
1/8 W
Thick film
resistors
Vishay
CRCW08050
000Z0EA
28 2 J29
3-position wire
to board terminal
block
Phoenix
Contact
1935174
Page 16
Revision history
AN4050
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Date
Version
Changes
11-Jun-2012
1
Initial release.
21-Mar-2013
2
Updated Figure 5: SPV1040 internal block diagram.
05-Dec-2017
3
Text and formatting changes throughout document.
Updated Section 5: "Bill of materials"
6 Revision history
Table 2: Document revision history
Page 17
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