ST AN4123 APPLICATION NOTE

AN4123

Application note

STEVAL-ISV015V1 up to 2.5 W solar USB charger

By Domenico Ragonese

Introduction

The STEVAL-ISV015V1 is a demonstration board mounting the SPV1040 device (solar energy harvester) as input stage and the LD39050PUR device (low noise and low quiescent current voltage regulator) as output stage. It targets any portable application powered by USB supply and merges the capability of the SPV1040 device to maximize the power extraction from solar modules with the high precision voltage regulation of the LD39050 device. It is shown in Figure 1.

Figure 1. STEVAL-ISV015V1 demonstration board

The board has been designed to harvest power from PV panels and to supply loads requiring up to 2.5 W (5 V, 500 mA) through a mini-USB (B type) connector. Between the two stages, a 440 mF super capacitor stores the harvested energy even when load is not connected or, if connected, it needs less power than that available from the source.

The application components at the input stage have been selected to optimize the energy harvesting from polycrystalline PV panels composed of 2, 3 and 4 PV cells in series and able to supply up to 900 mA. The trimmer VR3 is connected between the PV panel and the MPP-SET pin of the SPV1040 device and allows the maximum power extraction from the selected PV panel to be fine tuned. Setting the VR3 = 1 kΩ is recommended to cover most application cases.

So, other PV panels can also be used but it may be necessary to replace some of the application components in order to make the system work in the most efficient way.

The PV panel and main application components can be replaced, but the following guidelines must be carefully considered:

■The PV panel can be selected as long as VOC < 5.5 V 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 (1.65 A) does not have to be triggered

■The maximum output current can be limited by inserting the current sensing resistor RS1 (0 Ω by default)

For further details on component selection, please refer to Section 6 “External component selection” of the AN3319 application note. For details on the SPV1040 device and the LD39050 device features, please refer to the related datasheets.

July 2012

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Contents	AN4123

Contents

1	SPV1040 operating description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 3
2	LD39050 operating description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 6
3	Reference design description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 7
4	Schematic and bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	PCB silkscreen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16

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AN4123	SPV1040 operating description

1 SPV1040 operating description

The SPV1040 device is a high efficiency, low power and low voltage DC-DC converter that provides a single output voltage up to 5.2 V. The combination of the SPV1040 device and the LD39050 device provides an optimal solution to supply devices requiring a regulated voltage.

The SPV1040 device 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 overall application safety and its own safety by stopping the PWM switching in the case of 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

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The SPV1040 device acts as an impedance adapter between the PV module and the output load. In fact, the equivalent circuit can be shown as in Figure 3.

Figure 3. SPV1040 equivalent circuit

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SPV1040 operating description	AN4123

The MPPT algorithm sets up the DC working point properly by guaranteeing ZIN = ZM (assuming ZM the impedance of the supply source). In this way, the power extracted from the supply source (PIN = VIN x IIN) is maximum (PM = VM x 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 plotting the product V x I for each voltage generated. For further details on the MPPT algorithm, please refer to the SPV1040 device datasheet.

Figure 4. MPPT working principle

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Figure 5. SPV1040 internal block diagram

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AN4123	SPV1040 operating description

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 triggers the 1.25 V internal reference

● Output current limitation: RS1 x (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 triggering of the above thresholds.

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LD39050 operating description	AN4123

2 LD39050 operating description

The LD39050 device is an ultra low dropout linear regulator with low quiescent current and low noise features that make it suitable for low power battery powered applications. It provides up to 500 mA with a low 200 mV dropout. The input voltage range is from 1.5 V up to 5.5 V. For this application the device is used in its adjustable output version, with

a reference voltage of 0.8 V.

The regulator is equipped with internal protection circuitry, such as short-circuit current limiting and thermal protection.

The power supply rejection is 65 dB at low frequencies and starts to roll off at 10 kHz. An Enable logic control function puts the LD39050 device in shutdown mode allowing a total current consumption lower than 1 µA.

An internal thermal feedback loop disables the output voltage if the die temperature rises to approximately 160 °C. This feature protects the device from excessive temperature and allows the user to push the limits of the power handling capability of a given circuit board without risk of damaging the device.

Figure 6. LD39050 basic application schematic

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