1. Introduction to SLIC Overvoltage Protection Devices
This module will introduce you to the Littelfuse portfolio of SLIC overvoltage protection devices. The estimated time to complete this module is 10 minutes.

2. Outline Littelfuse SLIC Protection The SLIC & Ringing Functions
Fixed Negative Solutions
P0641SC Family
LCAS Solutions
P1200CC / P2000CC / A1220
Negative Tracking
B1101UC Family
VoIP Solutions
P0641DF-1E Family
Positive / Negative Tracking
B3104UC Family
This module is arranged in a more or less the chronological order of the development of slick protection.
After a brief discussion of the purpose of the slick and ringing functions, we will look at the progression of slick protection in the central office. Starting with the fixed negative solutions, moving to the el cass solutions and then the negative tracking solutions.
Then we will jump to the customer premises end and look at the voice over I-P application and its protection solution.
Finally, we will look at the rather sophisticated positive negative tracking solution.

3. SLIC Overvoltage Protection SLIC & Ringing Functions
POTS (Plain Old Telephone Service) requires two distinct signal types for telephones:
Subscriber Line Interface Circuit
-48V
H.V. Ring
∿∿∿∿∿∿
Generator
Ring Relay
Telephone
Line
SLIC Protection
POTS Protection
Lets begin by looking at a traditional telephone central office arrangement. As we look back into the central office from the telephone line we can see two basic circuits to which the phone line may be connected. The first is called the slick. That is an acronym that stands for subscriber line interface circuit. The slick provides all of the functions you normally associate with a telephone line. Things like dial tone, busy signals, the DMTF touch tone interface and of course the voice signal of the person with whom you’re having a conversation. The slick is a low power circuit made up of some delicate electronic components. As such, it needs specialized electrical protection against lightning and other transient voltages that may appear on the telephone line.
The one function that a traditional slick circuit does not provide is the high voltage ringing signal used to ring a house full of phones on the end of a long telephone line. The high voltage AC ringing signal is provided by a dedicated ring generator that continuously puts out a ringing signal.
A ring relay controls whether the slick circuit or the ring generator is connected to the line. The ring relay is controlled by the slick circuit. Through the ring relay, the slick controls the duration and sound pattern of the ring. Notice that the high voltage ring generator is never connected to the slick circuit.
The telephone line has another protection layer. This is standard pots protection. Pots is another acronym that stands for plain old telephone service. The pots protection layer must not interfere with the ringing signal. Typically, pots protection is set between 230 and 310 volts.
SLIC provides “Off-Hook” services.
Ring Generator provides high voltage ringing signal.

4. SLIC Overvoltage Protection Fixed Negative Solutions
POTS (Plain Old Telephone Service) requires two distinct signal types for telephones:
Positive Surge Pulse
-48V
Negative Surge Pulse
Now lets focus on the specialized slick protection requirements.
All telephone circuits use negative voltages for their operation. The reason for this is that positive voltages would lead to unwanted and sometimes dangerous corrosion of telephone line equipment – particularly the grounding systems used. Even small positive voltages will attract oxygen ions in the soil and greatly accelerate the corrosion of ground rods used by telephone engineers to keep their systems safe.
So we see here that the slick is powered by the minus forty eight volt battery of the central office. The protection requirements on the slick interface is that no positive voltage can ever be allowed on either conductor of the telephone line. Also, the negative voltages on the telephone lines should not exceed the battery voltage by more than a few volts. So we can see that there are different requirements for positive and negative voltages. Standard SIDACtor devices are symmetric bidirectional devices and are not appropriate for traditional slick protection.
Instead, Littelfuse offers an asymmetric slick protection device. Shown here, it contains a SIDACtor element and a diode. You can see that positive voltage surges, shown here in red, will go through the diode to ground. Negative surges, shown in blue, will activate the SIDACtor if they exceed the breakdown voltage which is chosen to be just beyond the maximum battery voltage.
Positive Surges Taken to Ground by the Diode.
Negative Surges (beyond VBO) Taken to Ground by the SIDACtor.

5. SLIC Overvoltage Protection Fixed Negative Solutions – P0641SC Family
The P0641SC Family of Fixed Negative SIDACtor Solutions:
VBO
SIDACtor
A: ITU Surge
C: GR-1089 Surge
e.g. P1101CA2
e.g. P0641SC
Optional Circuit Count
P0641CA2
e.g. P1701AC2
e.g. P0901UC
SLIC
A: TO-220
C: Compak
S: SMB Pkg
Q22: QFN
U: MS-013
The fixed negative voltage protection solutions for Littelfuse are anchored by the P zero 6 41 SC device. But the family comes in a wide range of configurations. Lets look at them one at a time.
The standard P zero 6 41 SC shown in the upper left corner is an example of the standard slick protector housed in a D-O 2 14 or SMB package. An alternative package for the same chip is shown in center bottom of the slide. These are packaged in tiny leedless QFN packages.
The upper right corner shows the twin slick version. These are specially modified SMB packages where one lead has been split into two. Inside are two slick protection circuits so only one package is needed for a slick interface.
The bottom left photo shows a through hole version of a two circuit protector. The package is a modified T-O 2 20 without an exposed heat sink or mounting tab.
The bottom right picture shows the six pin MS oh 13 package. Each of these contains four slick protection circuits arranged in pairs.
The Littelfuse part numbering scheme for these devices is pretty straightforward. The part number always starts with a P since it is a SIDACtor device. The next three numbers indicate the nominal breakdown voltage. The red one in the fifth position is key. It indicates the presence of the diode so you know it is a slick protector. The next letter indicates the package used. That is followed by the surge current rating. A-Rated devices are appropriate for ITU K dot 20 and K dot 21 applications found around the globe. The C-rated devices are can withstand the higher surge currents found in GR This is a telephone equipment specification used primarily in North America. Finally, the part number may have a circuit count indicator.
e.g. P0641Q22C

6. LCAS (Solid-State Relay) Protection
LCAS (Line Circuit Access Switch) is a solid-state relay that replaces the mechanical ring relay.
SLIC Protection
Subscriber Line Interface Circuit
Tip
-48V
LCAS
LCAS Protection
Ring
Tip
120V
Telephone
Line
H.V. Ring
∿∿∿∿∿∿
Generator
Ring
As you may have guessed, the weak point of the traditional slick arrangement is the reliability of the ring relay. To address this, a specialized solid-state relay was developed to replace the mechanical ring relay. The custom solid-state circuits are call el cass devices. El cass is an acronym that stands for line circuit access switch. The technology behind the el cass relay is not as robust as the mechanical ring relay. This puts additional restrictions on the telephone line protection scheme.
As you can see in this diagram, the blue slick box is unchanged. The same fixed voltage negative slick protection device is used. The ring generator itself is also the same. But because the el cass switch now replaces the ring relay, the pots protection scheme has been changed to an el cass protection scheme that is custom fitted to the needs of the el cass switch.
Notice that we have now labeled the slick telephone conductors as tip and ring. These are old names for the two wires in a telephone line that date back to when telephone lines terminated in a phone plug and a person actually plugged your phone line into a patch panel to connect you to the other party to make a phone call. Tip referred to the connection to the tip of the phone plug and usually has very little voltage on it. Ring referred to the connection to the conductive ring behind the tip on the phone plug and typically will have nearly the entire battery voltage on it when the phone line is not being used.
Tip and ring only matter here because the el cass switch is asymmetric device that must be connected properly to the slick. This asymmetry is also found in the el cass protection scheme. The tip conductor will have a nominal 120V protection voltage. The ring conductor will have either a 200V or a 250V nominal protection voltage.
200V or 250V
LCAS Relays are more delicate than mechanical ring relays.
LCAS Relays require specialized VBO protection devices.

7. SLIC Overvoltage Protection LCAS Solutions
VBO
Littelfuse Fixed Voltage SIDACtor Solutions:
VBO#1 10
·l·
SIDACtor
SMB Pkg
MS-013 Pkg
Asymmetric SIDACtor
A1220UC
P2500SC
A: ITU Surge
B: TIA-968-A Surge
C: GR-1089 Surge
VBO#2 10
·l·
Bidirectional
A: ITU Surge
B: TIA-968-A Surge
C: GR-1089 Surge
e.g. P1200SC
e.g. A1220UC4
Littelfuse offers two approaches to fulfilling the need for el cass protection.
On the left you can see that Littelfuse offers symmetric bidirectional devices with nominal voltages that are carefully aligned with the needs of the el cass switch. They use the standard part numbering system as shown here.
On the right is shown a custom device tailor made for el cass applications. These are housed in the 6 pin MS oh 13 package and contain two pairs of SIDACtor devices. The part numbering scheme is a bit different than others in that it begins with an “A” which stands for asymmetric. The next two places are the breakdown voltage of the tip protector divided by ten followed by the breakdown voltage of the ring protector divided by ten.
So the solution on the left requires two devices per port whereas the solution on the right will protect two ports with a single device.
2 Devices per Port
2 Ports per Device

8. Self-Ringing SLICs Battrax® Negative Tracking Devices
Self-Ringing SLICs require protection that tracks the variable DC supply voltage:
Telephone
Line
Battrax Protection
Self-Ringing SLIC -V
Variable DC Supply +
Positive Surge Pulse
Gate Current
The next improvement in slick technology was the development of the self ringing slick. While not powerful enough to replace the discrete ring generator in the central office, the self ringing slick is capable of ringing fairly short telephone lines. This technology meshes well with the change in the telephone network that placed intelligent remote terminal cabinets away from the central office.
Remote terminals are placed in neighborhoods to provide some of the central office functions without having to run long telephone lines all the way beck to the central office. Self ringing slicks are then used in these remote terminals because they are powerful enough to ring the shorter telephone lines.
To conserve energy use in the remote terminal, the slick operates from a low voltage supply while it is idle. However, in order to deliver the high voltage ringing signal, the voltage of the DC supply powering the slick is increased during the ringing cycle.
Looking at the circuit here, you can see that it is drastically simplified by eliminating the ring generator, the ring relay or the el cass switch. Also, there is now only a single level of overvoltage protection. However, that protection scheme has become a bit more sophisticated. The protection used now is a Battrax negative tracking protection device. The diodes to protect the slick from positive voltages are still there. What has changed is the negative voltage protection.
Instead of using SIDACtor devices, the Battrax uses specially designed SC arz. Let’s take a closer look at the blue negative surge pulse scenario. In the event of a negative surge, once the voltage on the telephone line goes below the variable DC supply voltage, a current will flow into the gate of the SCR and out the cathode to the telephone line. This current will trigger the SCR on which will clamp the line to ground.
As the voltage of the variable DC supply changes, so does the negative protection voltage. However, positive surges are always taken to ground through the diode. -V
Negative Surge Pulse
Positive Surges Taken to Ground by the Diode.
Negative Surges (beyond -V) Taken to Ground by the SCR.

9. SLIC Overvoltage Protection Negative Tracking Battrax® Solutions
The B1101UC Family of Fixed Negative SIDACtor Solutions:
IH 10
·l·
Neg Tracking
C: GR-1089 Surge
Battrax®
Optional Circuit Count
B1101UC4
e.g. B1161UC
SLIC
MS-013
e.g. B1201UC4
Littelfuse offers the Battrax line of negative tracking protectors.
The part numbering scheme is as shown. The initial letter is “B” which stands for Battrax. The next number is a one which indicates it is a negative tracking device. The next two numbers, shown here in blue, is the holding current in milly amps divided by 10. In this example, the holding current would be one hundred milly amps. The next number is a one, shown here in red, which has the same meaning as before. It indicates the presence of the positive surge diode. The you package designation is for the MS oh 13 package and is followed by the surge rating code. Finally, there is an optional circuit count indicator.
That’s all pretty complicated sounding, but in fact the actual offerings are pretty simple. On the left is shown the single port version. This has two SCR circuits so only one part is need per slick port. On the right is a dual port version.

10. Fixed Voltage SLIC Protection
VoIP SLICs
VoIP (Voice over Internet Protocol) SLICs create telephone lines at the customer’s premises but connect to the PSTN via an internet connection.
Subscriber Line Interface Circuit
Fixed Voltage SLIC Protection
8 – 40V
Telephone
Line
Integrated DC-DC Converter
The SLIC provides ringing signal from a low voltage battery using an integrated DC-DC converter.
The ring voltage may be software controlled, but is usually fixed for any particular application.
Therefore, fixed voltage protection is the best solution.
One recent advancement in slick technology is the voice over eye pea slick. These are designed to create a phone line at the customers premises and connect to the public switched telephone network over an internet connection. Because the loop is known to be very, very short, the ringing voltages can be quite low.
Because this slick is located at the customers site, there is no standard central office battery or ring generator. The voice over eye pea slick has an on board dc to dc converter that will generate all of the needed high voltages from a wide range low voltage dc input. Often times the dc supply to one of these slicks can range from eight to forty volts.
The output voltage of the dc to dc converter is usually under software control, but a single set of voltages is usually selected in the firmware and the voltages settings won’t change once the slick ships from the factory. This means that the protection voltage is also fixed. In this situation, the exposure to external surge events is fairly minimal, so a very light duty protector is all that is needed.

11. SLIC Overvoltage Protection VoIP SLICs - P0641DF-1E Family
The P0641DF Family of Fixed Negative SIDACtor Solutions:
ITU K.20 / K.21 &
GR-1089 Intra-Building Surge Rating
VBO
SIDACtor
Pin-to-Pin compatible with industry standard SO-8 tracking devices.
Littelfuse offers a family of voice over eye pea protectors. These are pin for pin compatible with the industry tracking S-O-8 devices that are their main competitors. This means that they can be sold into solutions that currently use the competitive parts. The D-F series is sold with a variety of breakdown voltages that make them the right choice for almost any voice over eye pea system.
The part numbering scheme is straight forward. New in this family of protectors is the D package which is the MS-O-12 package which shares a board footprint with the S-O-8 package. Also, the F surge rating is very light but yet compatible with the I-T-U specs as well as GR intra building requirements.
One other note, there are two versions of each device. A standard version and an enhanced version. The difference is that the enhanced device uses an enhanced diode design that has a lower voltage peak during the positive surges. Some customers may appreciate this performance benefit.
P0641DF-1E
Optional Enhanced Diode Performance
SLIC
Single
MS012 (SO-8) Package

12. Positive / Negative Ringing SLICs For Long Telephone Lines
Ringing long telephone lines requires extra voltage.
Sometimes a positive / negative ringing SLIC is used:
Telephone
Line
-48V
Battrax Protection
Self-Ringing SLIC +V
Positive DC Supply -
Positive Surge Pulse
Gate Current +
Negative DC Supply -V
Slick technology has now advanced to where the central office ring generator can now be eliminated. This advancement required the development of slick chips that not only had an enhanced negative voltage range, but whose outputs could also swing substantially positive during the ring cycle. This mimics the discrete ring generator performance so that ring relays and el cass switches can now be eliminated in the central office.
In a manner similar to the remote terminal self ringing slick, the central office self ringing slick changes its power supply voltage when it is in the ring mode. This requires not only a a variable negative DC supply, but a variable positive DC supply as well. It also means some changes for the protection scheme.
As you can see, the positive negative ringing slick protection scheme no longer has the diodes to protect against positive voltages. In fact, positive voltages are required during the ring cycle so they must now be tolerated by the protection scheme. The negative protection scheme is just like that found in the remote terminal. As you can see in the diagram in blue, the SCR is triggered by a negative surge that exceed the voltage of the variable negative DC supply.
A very special reverse polarity SCR is used in the positive protection circuit. It functions just like a regular SCR but all of the voltages and currents are reversed from a conventional SCR. If a positive surge is encountered that goes beyond the positive variable DC supply, this special SCR is triggered on and the excess energy is shunted to ground as you can see in red in the diagram. -V
Negative Surge Pulse
Gate Current

13. SLIC Overvoltage Protection Positive / Negative Tracking Solutions
Discrete Pos / Neg Tracking Battrax® Solutions
IH 10
·l·
IH 10
·l·
Neg Tracking
Compak
Pos Tracking
Compak
Battrax®
B1100CC
B2050CC
Single
Battrax®
A: ITU Surge
C: GR-1089 Surge
A: ITU Surge
C: GR-1089 Surge
Single
The B3000 Family of Pos / Neg Tracking Battrax® Solutions
IH 10
·l·
e.g. B3164UC
Pos / Neg Tracking
The Littelfuse offering of Battrax protection for positive and negative ringing slicks are available as discrete devices or an integrated unit.
The positive protection discrete option is the B C C. The B indicates it is a Battrax tracking device. The two indicates it is for positive protection. The rest of the part number follows the standard Battrax numbering convention.
The negative protection discrete option is numbered in a similar way. Notice that the fifth character in red is a zero in both cases. This indicates that the diode is not present in these devices, making them suitable to be used together to protect positive negative ringing slicks.
Littelfuse also offers a full port positive negative solution in a single six pin MS oh 13 package. This is a very elegant and convenient solution. The part number contains a three after the B to indicate that it is both positive and negative protection. The four, shown here in red, indicates there are four SCR devices in the package.
MS-013
Battrax®
B3104UC
SCR Count
A: ITU Surge
C: GR-1089 Surge

14. Introduction to SLIC Overvoltage Protection
Thank You!
Thank you. This completes the Introduction to SLIC overvoltage Protection Devices module.