APPLICATION
NOTES
Set Top Box Over-Voltage and
Over-Current Protection
AC Input
OC
OV
NTSC Analog Audio/Video
Processor/Descrambler
Analog
IF/AGC
Linear/Switch Mode
Power Supply
Audio
Switch
O
V
Audio Vol.
Control
1
RF
Modulator
6
Audio
Output
O
V
RF
Output
OV
2
QAM/VSB
Demod
A/D
Tuner
Video D/A
Video
Output
O
V
Video Switch
OV
3
O
V
7
MPEG2/
Video/Audio
Decoder
RF Input
OC
OV
Digital
Audio
4
Control
Demand
Microcontroller
RF mod
DRAM
O
V
Display
(Optional)
8
Keypad
9
O
V
OC
Telephone Dialer
Modem
O OC
V
O
V
OV
Telephone
Return
Path
5
Flash/ROM
Expansion
Port
10
O OC
V
O
V
I/O Port
O
V
PCMCIA
11
1)
2, 7)
3)
4)
IR Receiver
O
V
Overcurrent and overvoltage protection provided for the audio
output by Littelfuse suppression devices. See Figure 4 for
additional information.
RF output and input need to be protected by overvoltage
suppression devices.
Video outputs are protected from overvoltage transients. See Figure 5.
Digital audio outputs are protected from overcurrent and overvoltage
transients. See Figure 4.
5) Telephone connections require overvoltage and overcurrent transient
protection. See Figure 6 and Figure 7 for additional details.
6) AC input requires overcurrent and overvoltage transient protection.
See Figure 2.
8) Display requires overvoltage transient protection. See Figure 3 for
additional details.
9, 10) Keyboard and expansion port require overvoltage transient protection
and overcurrent protection. See Table 1 for Type of Device Use.
11)
PCMCIA interfaces require overvoltage transient protection.
Figure 1. Block Diagram of a typical Set Top Box
In the last few years the number of set top
boxes has increased significantly. Initially,
when the words “set top box” were
mentioned, one would think of the cable set
top box (CATV).These days the same
phrase could refer to satellite, cable, HDTV,
Web TV, pay per view, etc. set top boxes. A
block diagram of a typical Set Top Box is
illustrated in Figure 1. With the ever increasing number of set top boxes being
introduced into the market-place, more and
more consumers are coming in contact with
them.This introduces different types of transients into the environment in which the set
top box is placed. More consumers will be
handling the controls and cable connections
on the set top boxes.This will increase the
incidence of ESD being applied to the user
controls and to the input and output ports
that are available on the different set top
boxes.Today’s set top boxes contain more
sensitive components than ever before and
need to be protected from ESD and other
transients.The different transients (lightning
strikes, inductive load switching, commutative spikes, and ESD) have many sources
and have many forms.They adversely effect
electronic products.
A sudden change in the electrical condition
of any circuit can cause a transient voltage
to be generated from the energy stored in
the circuit elements.Transient over-voltage
protection requires that the impulse energy
be clamped or redirected by the suppressor
device to a voltage low enough as not to
damage sensitive components. A detailed
description of over-voltage and over-current
stresses are provided in the Index (which is
located in the back of this document).
tions can exhibit a number of symptoms
from lockups to catastrophic failures.
Standard
Description
MIL-STD-461
Requirements for the control of electromagnetic interference
characteristics of subsystems and equipment
IEC 801
Test method details. (Has recently been replaced by
IEC61000-4.)
EIAJ IC121
ESD Machine Model Test Conditions
ANSI/IEEE C62.41
AC Line Surges.
IEC 61000-4
Immunity Requirements,Test Methodology
Telecommunication Standards
Telcordia (Bellcore)
GR-1089-CORE
Safety standards for wired TELECO apparatus.
UL 1950
UL Telecommunication equipment safety standard
FCC 47 Part 68
Requirements for protection and safety
on the phone network
ITU-T K.20 & K.21
International telecommunication Union. European safety
standards for wired telecommunication apparatus.
Industry Standards
A number of standards are in existence
that provide a description of transients, test
methods, and test criteria. Some of these
are as follows:
The IEC61000-4 standard has become a
compulsory requirement for all commercial
electrical and electronic devices that are
sold in Europe from 1996.The specifications
are subdivided into different sections
that address the performance of electrical
components and equipment that is
subject to transients and Radio Frequency
Interference.The IEC61000-4 specification
includes the following:
Regarding Telecom standards, the set top
box falls under the Customer Premise
Equipment category. If it contains a telephone line connector, then the set top box
needs to comply with the UL1950 standard
and FCC 47 part 68.
Protection Devices
The Set Top Box has a great number of
input and output points, such as I/O
Connectors and Keypads, A/C Line,
Telephone Line, Audio,Video, etc.These
entry points provide a means for the ESD,
surges, and other transients to enter the
Set Top Box and cause serious damage.The
Set Top Box when exposed to these condi-
IEC61000-4 -2
• An ESD test, which may be applied as
a contact or air discharged test at four
different voltage levels.
IEC61000-4-3
• This part tests for susceptibility to
Electromagnetic Interference (EMI).
Devices under test are subject to RF
frequencies of 27 to 500 MHz.This
test may be applied at three different
field strengths.
IEC61000-4-4
• Electrically Fast Transients (EFT) are
applied at four voltage levels.
FUSE
PRIMARY MOV PROTECTION
There are a number of protection devices
that are available to the designer that will
protect the Set Top Box from incurring serious damage.These protection devices fall
into two categories, over-current protection
or over-voltage.
Fuses and PPTC (Polymer Positive
Temperature Coefficient) devices are part
of the overcurrent protection category.The
over-voltage suppression devices produced
by Littelfuse are as follows; MOV (Metal
Oxide Varistors), MLV’s (Multilayer Varistors),
SCR (Silicon Controlled Rectifiers)/ Diode
Arrays (SP72x family),Voltage Variable
Material ESD Suppression Devices
(PulseGuard® Suppressors), and TVS
Thyristors/Zener (Surgector devices).
Additional information on the Overvoltage
suppression and Overcurrent protection
devices can be found on the Littelfuse web
site (www.littelfuse.com).
AC Line Input
When a Set Top Box is plugged into an AC
outlet, it will be exposed to line transients
(voltage and current). A fuse provides overcurrent protection and an MOV provides
overvoltage suppression of the AC input to
the set top box. Figure 2 illustrates the use
of MOVs in protecting the Set Top Box
from line transients.To provide protection
from line transients, three MOV devices are
used on the primary side of the AC input
line following the line fuse.This configuration provides the maximum amount of
protection for the set top box. In many
SECONDARY MOV
PROTECTION
LOAD
1
2
3
N
G
Figure 2. AC input protection from line transients using MOV’s
In order to control the set top box, the
user will be using the keypad that is located
on the set top box or the remote control.
While interacting with the set top box, the
user may discharge an ESD arc. In order to
protect the set top box circuitry from ESD
discharges, transient (ESD) suppressors are
placed on the lines that lead from the
keypad to the microcontroller.The suppression devices should be placed as close
as possible to the point where the ESD
will enter.
For a single line, the Littelfuse ML series
offers voltage surge suppressors that will
protect the microcontroller from ESD transients. If a large number of lines need
protection and space is at a premium, then
the Littelfuse MLN series or the SP72X
series can be used to protect the microcontroller from ESD transients.The MLN
series is available in a package that contains
four individual devices in one 1206 package
and is similar to the ML and MLE series of
discrete leadless chips. If a leaded MOV is
needed, then Littelfuse ZA series is available. Figure 3 illustrates the use of
V5.5MLA1206 in the suppression of ESD
pulses.The ML Series devices are available
in 1210, 1206, 0805, and 0603 and 0402
packaging sizes. Additional information is
available in the Littelfuse Suppression
Products catalog.
Video Input/Output
The left and right audio outputs on the set
top box use cables that have RCA plugs to
make the interconnection between the set
top box and other devices such as televisions,VCRs, and stereo amplifiers.These
outputs need ESD protection, since the
user handles them very frequently during
the setup of the equipment. Some set top
boxes have Dolby® Digital (AC-3) 5.1-channel surround sound audio outputs. Data
stream does not need protection—equipment does. Figure 4 shows the left and right
audio outputs being protected by Littelfuse
There are a number of different connectors
that are used in the set top box to
input/output video information to the television set.The most common connector
that is used on set top boxes and other
consumer type of products is the RCA jack
(see figure 5a).The RCA jack (connector) is
used to output composite and component
V18MLE0603
PIN
CONNECTOR
Shield/Chasis
Ground
Figure 4. ESD protection of Set Top Box Audio output
circuitry using littelfuse® V18MLE0603 Multilayer Surface
Mount ESD Suppressor/Filter devices.
V18MLE0603 Multilayer Surface Mount
Transient Voltage Suppressors.The MLE
Series operating voltage range VM(DC) =
up to 18V (maximum continuous working)
was specifically designed to protect against
ESD transients.The devices capacitance
characteristic suitable for high frequency
attenuation/low-pass filter circuit functions,
thereby providing suppression and filtering
in a single device.These devices are characterized for Impedance and Capacitance.
MENU
OK
INFO
V5.5MLA1206
CHASSIS
GROUND
PIN
CONNECTOR
Video
Output
Shield/Chasis
Ground
Figure 5. ESD protection of Set Top Box Video output
circuitry using Littelfuse® devices
video signals. Set top box providers are
using the S-Video connector more
frequently to output video.The S-Video
connector comes in a four or seven pin
configuration. As can be seen (figure 5a)
the RCA plugs are exposed and can be
touched by a person easily. A person can
easily discharge an ESD arc to the
input/output RCA jacks, which will result in
damaged circuitry. Figure 5 shows a portion
of a typical composite video output circuit
that is protected by a PulseGuard
Suppressor device from ESD. PulseGuard
Suppressors have a very low capacitance
(˜ 0.05 pF at 1MHZ).The SP724 (3pf)
or V5.5MLA0603 (660 pF) device can
also be used if higher capacitance levels
are acceptable.
Telephone Connections
CHANNEL
ON
OFF
Audio
Output
PGB0010603
Video Signal Processing IC
Keypad
Audio Input/Output
Audio Amp IC
cases only Varistor 1(across Load and
Neutral) is used or Varistor 1 and 2 are
selected. Secondary protection is shown for
transients that need to be reduced further
to protect more sensitive devices.
MICROCONTROLLER
S-Video/S VHS (4 Position)
Figure 3. ESD protection of Set Top Box Keypad using
Littelfuse MLA series Multilayer Surface Mount Transient
Surge Suppressor (V5.5MLA 1206) devices.
RCA Plug
Figure 5A.
An ever-increasing number of set top boxes
are including telephone connections on
their boxes.The telephone connection is
used to keep accounting information
current, upgrade software, allow pay-per
view, and provide special programming
features.The phone connection needs to be
protected from a number of over-voltage
and over-current conditions (Lightning and
power-cross) that can damage the set
top box.
Lightning strikes the earth 100 times every
second. Lightning bolts can go from cloud
to ground, within and between clouds or
Input/Output
ESD Lighting Transients Power
(other)
Cross
AC Input
Display (Optional)
Keyboard
X
X
X
Expansion Port
PCMCIA
Audio Output
RF Input
X
X
X
X
RF Output
Video Output
S-Video
USB(2.0)
Digital Audio
Telephone Connection
X
Fuse
X
PPTC (Polymer Positive
Temperature Coefficient)
PPTC
PPTC
MOV
MLA, MLE
MLA, MLE, SP72X
X
X
X
X
X
X
PPTC
X
It does not have to strike the telephone
line to cause damage to equipment. Lighting
strikes generate electromagnetic fields,
which are coupled into the telephone
line and cause damage to the attached
equipment. Figure 6 illustrates the minimum
protection that should be used at the
telephone input of the set top box. In this
scheme, the designer is trying to allow
Battery and Ring Voltages and Currents
to pass unimpeded, meet transient requirements called out by agency standards
(UL1950, FCC Part 68, and ITU); and have
the fuse clear under Power Cross agency
testing without fire or fragmenting.The TIP
line is protected by a Littelfuse‚ 436 or 461
(newest addition) series surface mount fuse
or 2AG Glass fuse (through hole), which
X
X
Fuse
provides overcurrent protection from
lightning and power cross transient.The
Littelfuse‚ SGT3100SBT Surgector suppression provides Longitudinal (Tip to Ring)
protection (over-voltage). Figure 7 illustrates
an alternative protection scheme. Since
many SLIC’s can’t handle large positive
voltages, a diode bridge is used to keep
the voltage level to less than +5V. An
SGTB3100SBT is used to clamp voltages
that exceed –275V. Additional information
on the telecommunication standards, terminology, and application examples can be
found in the “Littelfuse‚ Solutions for
Telecom Applications” application note.
Fuse 436 or 461
or 2AG
Tip
To SLIC
or Circuit
(~)
100A, 100V
Diode Bridle
(–)
(+)
Ring
SGT0640 or
SGT0720SCT
(~)
Figure 7.
Alternate Telephone circuitry using Littelfuse® Devices.
PulseGuard® suppressors
PulseGuard® suppressors
MLA, MLE
MOV, Gas Discharge
Tubes, PulseGuard® suppressors
PulseGuard® suppressors
MLN, MLA, PulseGuard® suppressors
MLN, MLA, PulseGuard® suppressors
PulseGuard® suppressors
MLA, PulseGuard® suppressors
Diode Bridge, Surgector™ suppressors
Summary
This article addresses the different threats
that are posed to the set top box both
internally and externally, there are numerous suppression devices that can be used
to protect set top boxes from ESD, line
transients, Lightning, Power Cross, and other
transients that were listed in this article and
are summarized in Table 1. General solutions are presented for each threat that the
set top box would experience in the real
world. In order to select specific parts for a
particular design a number of parameters
(working voltage, surge energy, clamping
voltage, capacitance, operating temperature,
etc.) need to be known.These parameters
are design-specific and the designer needs
to review the data sheets in the Littelfuse‚
Suppression Products Data Book. For
additional information, or contact Littelfuse‚
Technical Support at (800) 999-9445
or visit the Littelfuse website at
www.littelfuse.com.
Fuse- 436 or 461
SGT3100SBT
Ring
Overvoltage
X
X
from ground to cloud, and can carry
charges in excess of 100 million volts and
can be very deadly. Lightning in the United
States every year kills approximately one
hundred people, hundreds more are seriously injured and property damage
amounts to millions of dollars.
Tip
Overcurrent
To CPE Equipment,
Ex. Modems, Telephones,
Fax Machines, Caller ID Boxes,
Answering Machines
Figure 6.
Basic Telephone circuitry protection using Littelfuse devices.
Littelfuse, Inc.
800 E. Northwest Highway
Des Plaines, IL 60016 USA
(847) 824-1188
www.littelfuse.com
FORM NO. EC609 Printed in U.S.A. MARCH 2001 Copyright © 2001 Littelfuse, Inc., All Rights Reserved