AN3050 Datasheet, PDF (1/18 Pages) STMicroelectronics – STBP120 overvoltage protection device

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
Doc ID 16207 Rev 1
1/18
www.st.com