PASSIVE SOLUTIONS

Set-Top Boxes Require Special Protection

With a broad portfolio of circuit protection devices and expertise, Littelfuse provides safeguards against all of the electrical threats to Internet Protocol TV (IPTV) Set-Top Boxes (STBs). In addition, the company provides application-level testing to facilitate the qualification of protection solutions as well as debugging for troublesome protection issues.

IPTV is expanding the application scope and flexibility of the existing multimedia experience. Using a broadband connection, IPTV provides interactivity for applications such as Video on Demand, web browsing and e-mail checking, as well as the core function of viewing broadcast television.

For the consumer, the key piece of hardware will be the Set-Top Box, which connects the service provider’s network to the customer’s multimedia hardware. Because of its numerous inputs (power and I/O), the STB will be besieged with electrical threats including ESD (Electro-Static Discharge) and lightning transients as well as short circuit and overload conditions (see figure 1).


Figure 1 : Identification of Electrical Threats to an IPTV Set-Top Box

In order to ensure the long-term reliability of the STB – and hence the product’s market success – the design engineer should include circuit protection provisions to provide a robust defence against the electrical threats. The aim is to prevent damage to the STB and maintain the consumer’s experience.

Even amongst the most common circuit protection devices (see table) there are often several options available to solve the same problem. In these cases, the characteristics of the protection devices (capacitance, form factor) should be reviewed to help narrow down the selection.

Linking the Protection Device to the Electrical Threat

Circuit / Threat	Relevant Protection Devices
AC POWER INPUT
- Lighting surges	Metal oxide varistor
- Switching transients	Metal oxide varistor
- Fault current	Leaded/cartridge fuse
DC POWER INPUT
- Lightning surges	Metal oxide varistor TVS diode Multilayer varistor
- Fault current	Leaded/cartridge fuse Resettable PTC
USB 1.1/2.0
- System transients, Vbus	Multilayer varistor
- Fault current, Vbus	Resettable PTC
- ESD, 1.1 D+/D-	Multilayer varistor Diode array
- ESD, 2.0 D+/D-	Polymer ESD suppressor
xDSL INPUT
- Lightning surges	Protection thyristor Gas discharge tube
- Power cross	Leaded/cartridge fuse
RF INPUT (CATV) – Lightning surges	Gas discharge tube
10/100 ETHERNET - ESD	Multilayer varistor Diode array Polymer ESD suppressor
HDMI PORT - ESD	Polymer ESD suppressor
AUDIO PORTS - ESD	Multilayer varistor Diode array
VIDEO OUTPUT - ESD	Multilayer varistor Diode array
SERIAL PORT - ESD	Multilayer varistor Diode array
SCART PORT - ESD	Multilayer varistor Diode array

For example, USB ports have both power and data circuits. On the DC bus, surge and ESD transients are to be expected. The best solution is a single-line device capable of handling both such events. In this case, the recommendation is a multilayer varistor. For fault current conditions, a resettable Positive Temperature Coefficient (PTC) device is the preferred choice.

On the data line (D+/D-), the data rate drives the selection. If the STB is using USB 1.1, then the possibilities include multilayer varistors (single-line) and diode arrays (multi-channel). The capacitance value should be kept below 50pF. On the other hand, if USB 2.0 is used, the data rate is 480Mbit/s and this value of capacitance is unacceptable. A polymer suppressor with capacitance of less than 0.1pF is recommended.

Solutions

• Leaded/cartridge fuses
• Resettable PTC
• MOV
• TVS diode
• Protection thyristor
• Gas discharge tube
• Multilayer varistor
• Diode array
• Polymer ESD suppressor

For further information, including Datasheets, please click here