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Why do Set Top Boxes need Circuit Protection?

Posted: 29 May 2003     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2003MAY/A/2003MAY29_MSD_POW_AN.PDF

7 Set Top Boxes include cable set top boxes, satellite receivers,

HDTV, Web TV and Pay Per View units and many more.

7 As the functionality increases and more connection ports

are required, the potential for external current and

voltage transients also increases.

7 As the circuits become more sophisticated they become more

sensitive and can be more easily damaged by transients, such

as Electrostatic Discharge (ESD).

What types of transients occur?

7 Both overcurrent and overvoltage transients that affect Set

Top Boxes can be generated externally (lightning strikes,

ESD, inductive load switching, commutative spikes)

or internally (relay switching, capacitor discharges).

Set Top Box Port Circuit Protection

7 Figure 1 shows a block diagram of a typical set top box.

Each port requires different protection depending on the

transient threat.

AC Line Input Protection

7 AC Line input requires both overcurrent and overvoltage

protection. A glass fuse is a suitable overcurrent device

and a radial MOV provides suitable suppression of over-

voltage transients.

7 The MOV would be placed on the primary side of the

AC input line following the line fuse. Additional MOVs

could be required as secondary protection of more

sensitive components.

Keyboard Protection

7 The main threat to the keyboard microcontroller is from

ESD transients produced by the user. Suitable suppression

devices include the MLE Series multilayer varistor on each

line or if space is at a premium a multi-line MLN Surge Array

could be used.

7 The low current levels (200mA to 500mA) transmitted

between the keyboard and host can increase exponentially

in a short circuit condition. A PTC should be used to limit

the current and protect the Set Top Box's circuitry without

needing to be replaced.

Audio Input/Output Protection

7 Audio Input/Output needs to be protected from ESD

transients. Multilayer varistor part V18MLE0603 is recom-

mended for this application because its inherent capacitance

can provide filtering and suppression in one device.

Why do Set Top Boxes need Circuit Protection?

Linear/Switch Mode

Power Supply

I/O Port

AC Input

RF Input

Expansion

Port

PCMCIA

RF

Output

Audio

Output

Video

Output

Digital

Audio

Telephone

Return

Path

AudioVol.

Control

RF

Modulator

Tuner A/D

Flash/ROM

IR receiver

Microcontroller

Display

(Optional)

Keypad

Telephone Dialer

Modem

Audio

Switch

NTSC Analog Audio/Video

Processor/Descrambler

Analog

IF/AGC

MPEG2/

Video/Audio

Decoder

DRAM

Video Switch

Video D/AQAM/VSB

Demod

Control

Demand

RF mod

6

2

8

9

3

1

7

10

11

5

4

OC

OV

OC

OV

OC

OV

OV

OV

O

V

O

V

O

V

O

V

O

V

O

V

O

V

O

V

O

V

O

V

OC

O

V

O

V

OC

Video Input/Output

7 The V5.5MLA0603 (multilayer varistor) or PulseGuard

suppression devices can be used to protect the video line

from transients. Selection is usually based on capacitance

requirements. PulseGuard suppressors are polymer

suppression devices with extremely low capacitance

(55fF at 1MHz) suitable for high data rates.

Telecom Connections

7 Telecom lines need to meet transient requirement standards

UL1950, FCC part 68 and ITU and satisfy power cross. This

can be done by protecting from overcurrent events on the

Tip line with a 2AG glass fuse or 436/461 surface mount

fuse. The Littelfuse SGT3100SBT SurgectorTM

suppressor

can be used to suppress longitudinal (Tip to Ring) over-

voltage transients.

7 A diode bridge can be used to keep the voltage level less

than +5V; an SGT3100SBT is used to clamp voltages that

exceed 275V.

1) Overcurrent and overvoltage protection provided for the audio

output by Littelfuse suppression devices.

2, 7) RF output and input need to be protected by overvoltage

suppression devices.

3) Video outputs are protected from overvoltage transients.

4) Digital audio outputs are protected from overcurrent and

overvoltage transients.

5) Telephone connections require overvoltage and overcurrent

transient protection.

6) AC input requires overcurrent and overvoltage transient protection.

8) Display requires overvoltage transient protection.

9, 10) Keyboard and expansion port require overvoltage transient protection

and overcurrent protection.

11) PCMCIA interfaces require overvoltage transient protection.

Figure 1 - Block Diagram of a Typical Set Top Box

Table 1

Input/Output ESD Lighting Transients Power Overcurrent Overvoltage

(other) Cross

Audio Output X MLA, MLE

RF Output X PulseGuard.

suppressors

Video Output X MLN, MLA, PulseGuard.

suppressors

S-Video X MLN, MLA, PulseGuard.

suppressors

Digital Audio X MLA, PulseGuard.

suppressors

Telephone Connection X X X Fuse Diode Bridge, SurgectorTM

suppressors

AC Input X X Fuse MOV

RF Input X X MOV, Gas Discharge

Tubes, PulseGuard.

suppressors

Display X MLA, MLE

Keyboard X X PPTC (Polymer Positive MLA, MLE, SP72X

Temperature Coefficient)

Expansion Port X X PPTC PulseGuard.

suppressors

PCMCIA X X PPTC PulseGuard.

suppressors

USB(2.0) X PPTC PulseGuard.

suppressors

EC617

800 E. Northwest Highway

Des Plaines, IL 60016 USA

(847) 824-1188

www.littelfuse.com





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