ST AN3050 Application note

AN3050

Application note

STBP120 overvoltage protection device

1 Introduction

In order to reduce the power consumption, size and cost of electronic devices, most semiconductor components are manufactured using a “low voltage” process resulting in a maximum operating voltage of 7 V or even less. Any overvoltage causes an excessive power dissipation on the chip quickly leading to damage, or even electrical breakdown.

Portable electronic devices, such as cell phones, PDAs, MP3 players, digital cameras, etc. are normally powered from a Li-Ion or Li-Pol battery pack, which is recharged by an internal charger controller supplied by an external AC adapter, USB hub, etc. The failure of this adapter or accidental usage of another adapter with a higher voltage can damage the charger controller and lead to overvoltage on the battery, which may be very dangerous for the device and potentially even for the user.

To avoid this, some type of protective device is absolutely necessary.

One well known solution is a device known as a Transil™. It can be understood as a Zener diode, capable of withstanding high power dissipation. Although Transils are easy to use and relatively inexpensive, their threshold voltage is not very accurate and is dependent upon the current flowing through the Transil. For heavy overload, the voltage on the Transil can still be too large and, in addition, the high power dissipation can lead to high junction temperature, and in extreme cases the Transil and surrounding circuit board can be damaged. For this reason, there is often a fuse connected between the supply connector and Transil to break the circuit in case of extreme overload.

Other, modern and safe devices, are integrated circuits known as “Overvoltage Protection” (OVP) devices. The OVP device can be understood as a “firewall” between the application and the external world represented by the power supply (AC adapter, USB, etc.), allowing only the correct voltage to reach the application and preventing malfunction or damage resulting from the use of an illegal or broken power supply. It contains a voltage comparator and either a driver for external Power MOSFET, or even the Power MOSFET itself. In the event of overvoltage, the comparator turns off the MOSFET, disconnecting the application from the power supply. No excessive power dissipation is generated during overvoltage.

The first member of the STMicroelectronics™ OVP devices family is the STBP120, which will be described in this application note.

November 2009

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www.st.com

Contents	AN3050

Contents

1	Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 1
2	Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
4	Powering peripherals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
5	STBP120 highlights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
6	STBP120 versus Transil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
7	Compatibility of STBP120 with other OVP devices . . . . . . . . . . . . . . .	15
8	PCB layout recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
9	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
10	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17

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AN3050	List of tables

List of tables

Table 1. Key benefits of STBP120 device over Transils. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Table 2. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

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List of figures	AN3050

List of figures

Figure 1. STBP120 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 2. Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 3. STBP120 turn-off delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 4. Startup into overvoltage condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 5. Startup delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 6. Typical STBP120 soft-start performance, CLoad = 22 µF . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 7. Typical STBP120 soft-start performance, CLoad = 47 µF . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. Typical STBP120 soft-start performance, CLoad = 100 µF . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 9. Pin to pin compatibility of the STBP120 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 10. Example of PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

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AN3050	Description

2 Description

The STBP120 provides robust protection for positive input voltage up to +28 V and is

capable of supporting current up to 2 A using a built-in low RDS(on) N-channel MOSFET and charge pump. The overvoltage thresholds currently available are 5.375 V, 5.50 V, 5.90 V and

6.02 V.

As additional protection, the STBP120 also monitors its own junction temperature and turns off the internal MOSFET if the temperature exceeds the specified threshold.

The STBP120 is equipped with the undervoltage lockout function preventing unreliable operation of the protected application for low input voltage.

The STBP120 device can be controlled (enabled / disabled) by the microcontroller and provides status information (input overvoltage, input undervoltage, thermal shutdown) to the microcontroller.

The STBP120 requires only one external component (1 µF input capacitor improving the ESD immunity and stability under input transients conditions), other components are optional (see Figure 2).

It is offered in a small, RoHS compliant 10-lead TDFN package of 2.5 x 2 mm dimensions. For more details, please refer to the STBP120 datasheet available on www.st.com.

Figure 1. STBP120 block diagram

	IN						OUT
	ESD	Core	SUPPLY	OSCILLATOR	CHARGE PUMP
		negative				OFF
	protection		REGULATOR		MOSFET DRIVER
		protection
			VCC
		VREF	VOLTAGE	COUNTERS
			REFERENCE
			Input overvoltage
					MCU		FLT
				CONTROL LOGIC
					INTERFACE
			Input undervoltage
							EN
		Temperature	Thermal shutdown
		detector			ESD		ESD
					protection		protection
				GND			AM00306

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Application circuit	AN3050

3 Application circuit

Figure 2. Typical application circuit

			PERIPHERAL
	SYSTEM	SUPPLY CURRENT				DC-DC
		CHARGING CURRENT
AC	CONNECTOR					EN	BATTERY
adapter
			IN	OUT		CHARGER	PACK
	OR	C1					SUPPLY
					C2
		1 µF	STBP120			IC	CIRCUITS
					1 µF	ENABLE
							RPU
POWERED						RFLT
PERIPHERALS				FLT			CONTROLLER
				EN		REN
				GND
							APPLICATION
							AM00314a

As shown above, the right place for the OVP device is just after the system supply connector (see Figure 2).

The input capacitor C1 plays an important role for improving the OVP functionality under fast transients caused by hot insertion / disconnection of power supply, ESD events, etc. For fast overvoltage transients, this capacitor acts initially as a short-circuit requiring some current to be charged, thus making the transients slower. It also acts as a reservoir of energy in case of fast undervoltage transients caused by the supply cable impedance when the current drawn by the application increases, preventing the input voltage from falling below the undervoltage threshold and cutting off the power.

After the OVP device, there is usually the charge controller IC managing proper charging and protection of the battery back.

Normally a set of supply circuits are present to convert the battery pack voltage to the voltage levels necessary for particular parts of the application.

A connection of STBP120 status output (FLT) and enable input (EN) to the application controller is also shown on Figure 2. The open-drain FLT output is connected by an external pull-up resistor RPU to the controller supply voltage. The resistors RFLT and REN are optional. They increase the safety of the controller in case of extreme voltage or current condition, leading to possible damage of the STBP120, limiting the current flowing to the controller I/O ports to a safe value (the absolute maximum voltage on the STBP120 is 30 V).

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