ST AN4069 APPLICATION NOTE

AN4069

Application note

5 V and 12 V power line protection based on STEF05 and STEF12 electronic fuses

Introduction

This demonstration board is based on the STEF05 and STEF12 electronic fuses (E-fuses). It is designed to help developers to evaluate the benefit of the complete protection offered by the E-fuse family and to customize their own application. The E-fuse family of hot-swap converters is designed to replace the mechanical fuses on low voltage power lines.

The STEF05 and STEF12 are integrated electronic fuses optimized for monitoring the output current and input voltage. Connected in series to the 5 V/12 V rails, they are capable of protecting the electronic circuitry on their output from overcurrent and overvoltage.

The turn-on time is programmable by means of an external capacitor, allowing the control of the inrush current at startup and during hot-swaps.

When an overload condition occurs, the E-fuse limits the output current to a predefined safe value. If the anomalous overload condition persists, it goes into an open state, disconnecting the load from the power supply. If a continuous short-circuit is present on the board, when power is re-applied the E-fuse initially limits the output current to a safe value, and then again goes into an open state. Both devices are equipped with a thermal protection circuit. The intervention of thermal protection is signaled to the board monitoring circuits through a signal on the ENABLE/FAULT pin that can be connected to other parts belonging to the same family to cause a simultaneous shutdown during failure events.

Unlike mechanical fuses, which must be physically replaced after a single event, E-fuses do not degrade in their performance after short-circuit/thermal protection interventions and can be reset either by re-cycling the supply voltage or by using the ENABLE pin.

Figure 1. Board image

March 2012

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Contents	AN4069

Contents

1	Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 4
2	Board characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	Board testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
4	ENABLE/FAULT pin, thermal latch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
5	Output dV/dt circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
6	Current limit circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
7	Overvoltage clamping circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
8	Demonstration board schematic and BOM list . . . . . . . . . . . . . . . . . .	12
	8.1 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
9	Device order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
10	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15

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AN4069	List of figures

List of figures

Figure 1. Board image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Demonstration board layout - top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. Demonstration board layout - bottom layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 4. Demonstration board connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 5. ENABLE/FAULT pin status. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 6. STEF05 delay time and VOUT ramp-up time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 7. STEF12 delay time and VOUT ramp-up time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 8. STEF05: current limit vs. R-limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 9. STEF12: current limit vs. R-limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 10. Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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Features	AN4069

1Features

●Input voltages: 5 V and 12 V

●Continuous current: 3.6 A typ. each

●Adjustable slew rate for output voltage

●Overvoltage clamp

●Undervoltage lockout

●Programmable short-circuit current limit

●Overload current limit

●Controlled output voltage ramp

●Thermal latch protection

●Fault condition flag

●ENABLE pin.

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AN4069	Board characteristics

2 Board characteristics

The demonstration board is provided with a STEF05PUR for the 5 V rail, a STEF12PUR for the 12 V rail, and a MOSFET switch connected to the ENABLE/FAULT pins to reset the E- fuses in case of thermal fault.

The board provides for separate or linked ENABLE pins, in order to allow either the observation of each single E-fuse or the complete 5 V/12 V protection application.

The short-circuit current limit is programmed to the standard values (2.9 A for the STEF05, 4.4 A for the STEF12), by means of a 22 Ω resistor placed on each channel. This resistor can be replaced by the user in order to set the desired current level.

The dV/dt control capacitor, used to set up the turn-on time and slew rate, is optional.

Figure 2. Demonstration board layout - top layer

Figure 3. Demonstration board layout - bottom layer

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