ST AN4050 APPLICATION NOTE

AN4050

Application note

STEVAL-ISV012V1: lithium-ion solar battery charger

By Domenico Ragonese, Alessandro Nicosia and Giovanni Conti

Introduction

The STEVAL-ISV012V1 is a demonstration board that mounts the SPV1040 (solar energy harvester) as the input stage and the L6924D (Li-Ion battery charger) as the output stage. It targets any portable application powered by lithium-ion batteries and merges the capability of the SPV1040 to maximize the power extraction from the solar module with the linear regulation of the L6924D, to optimize the battery charge and to protect the load while reducing the power dissipation at the bottom. It is shown in Figure 1.

Figure 1. STEVAL-ISV012V1 demonstration board

The board has been 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 as per the following guidelines:

●The PV panel can be replaced by a different one as long as Voc < VBATT_max and Isc < 1.65 A.

●The inductor L1 can be replaced by considering that it affects the maximum peak current and that an input overcurrent limit must not be triggered.

●The maximum output current can be limited by replacing the current sensing resistor Rs (0 Ω by default).

●The resistor R14, which limits the charge current threshold, by default is set to 500 mA.

For further details on component selection, please refer to the section “external component selection” of the AN3319 application note. For details about SPV1040 and L6924D features please refer to the related datasheets.

June 2012

Doc ID 022816 Rev 1

1/19

www.st.com

Contents	AN4050

Contents

1	SPV1040 operation description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 4
2	L6924D operation description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
3	L6924D operation in solar powered applications . . . . . . . . . . . . . . . . . .	7
4	Reference design description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
5	Schematic and bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
6	Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18

2/19	Doc ID 022816 Rev 1

AN4050	List of figures

List of figures

Figure 1. STEVAL-ISV012V1 demonstration 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 9. Battery charging at low irradiation, zoomed in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 12. Application set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 13. V-I and P-V plot diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 14. Partial charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 15. Full charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 16. STEVAL-ISV012V1 schematic, battery charge section . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 17. STEVAL-ISV012V1 schematic, solar power optimizer section. . . . . . . . . . . . . . . . . . . . . . 14 Figure 18. STEVAL-ISV012V1 PCB top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 19. STEVAL-ISV012V1 PCB bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Doc ID 022816 Rev 1

3/19

SPV1040 operation description	AN4050

1 SPV1040 operation description

The SPV1040 is a high efficiency, low power and low voltage DC-DC converter that provides a single output voltage up to 5.2 V. If combined with the L6924D, it provides the ideal solution for charging lithium battery packs by harvesting energy from a very small solar panel.

The SPV1040 is a 100 kHz fixed frequency PWM step-up converter able to maximize the energy harvested by few solar cells. thanks to the embedded MPPT algorithm which maximizes the power generated from the panel by continuously tracking its output voltage and current. The converter guarantees the safety of the overall application and its own by stopping the 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

L

Lx

VOUT

RS

VBATT

RF1

VPV

XSHUT

ICTRL_PLUS

R3

GND

ICTRL_MINUS

CF RF2

R1

CIN

MPP -SET

VCTRL

COUT

DOUT

CINsns

COUTsns

R2

AM11733v1

The SPV1040 acts as an impedance adapter between the PV module and the output load. In fact, the equivalent circuit can be seen below:

Figure 3. SPV1040 equivalent circuit

) ).

# ).

6 ).

306

) /54

).

#/54 2 /54 6/54

!- V

The MPPT algorithm properly sets up the DC working point by guaranteeing 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).

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

4/19	Doc ID 022816 Rev 1

AN4050	SPV1040 operation description

plotting the product V*I for each voltage generated. For further details of the MPPT algorithm, please refer to the SPV1040 datasheet.

Figure 4. MPPT working principle

I MP									PMAX
trrenuC									owerP
[A]									[W]

0

Voltage [V] VMP

VOC

AM11735v1

Figure 5. SPV1040 internal block diagram

Lx	VOUT
	I CTRL_PLUS
XSHUT
	I CTRL_MINUS
MPP-SET
GND	VCTRL
	AM11736v1

The duty cycle set by the MPPT algorithm can be overwritten if one of the following conditions 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 the startup and VMPP-SET ≤450 mV in running mode.

Application components must be carefully selected to avoid any undesired trigger of the above thresholds.

Doc ID 022816 Rev 1

5/19

L6924D operation description	AN4050

2 L6924D operation description

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 (the Power MOSFET, reverse blocking diode and the 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 is completed.

Regardless of the charging approach, a closed loop thermal control avoids device 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		2
		6	2%&	4(
#(!2'%2	6).				6/54		"!44%29
	#				6/3.3
2	6).3.3						#
	2				) %.$
	4	02'
	#
,$	,$		, $		) 02'
							2
	34
					602%	*
	34						2
		3($. '.$		6	2	2
					/02') 02%
	/.
	/&&
		6REF	2		2
							!- V

6/19	Doc ID 022816 Rev 1