1. Introduction to SDP / SEP SIDACtor Overvoltage Protection Devices
This module will introduce you to the Littelfuse family of SDP and SEP SIDACtor overvoltage protection devices. The estimated time to complete this module is 10 minutes.

2. Outline SDP / SEP Protectors High Surge Capability
Breakthrough Package Size
Low Capacitance
Capacitance Independent of Line Voltage
Perfect Balance Bridge
Transformer Coupled Apps
SDP vs GDT
Notes on Biasing
Notes on the QFN Package
Demo PCBs Available
In this module we will first focus on the key features and benefits of the new SDP and S-E-P Protectors for broadband protection from Littelfuse.
These will include the high surge capability, the breakthrough package size and the low and uniquely constant capacitance of these exciting new offerings.
We will take a few minutes to dive into some detail on the benefits of the perfectly balanced bridge circuit and compare the SDP to Gas Discharge Tubes.
We will wrap it up with few comments on the biasing circuit and a look at the package details.
Finally, we will discuss the demo p c boards available for customer evaluation and qualification.
Lets get started with some features and benefits.

3. All SDP / SEP Devices are RoHS Compliant
High Surge Capability
Surge Regulatory Requirement
(without external resistance)
Voice, ISDN, T1/E1, T3,
ADSL, VDSL
Ethernet
Power Over Ethernet
GR-1089 Interbuilding
ITU K.20/K.21 Enhanced
(500A 2x10)
SDPxxx0Q38CB
(8V-350V)
SEPxxx0Q38CB
(8V -90V)
ITU K.20/K.21 Basic
(100A 10x560)
SDPxxx0Q38B
(64V-350V)
All SDP / SEP Devices are RoHS Compliant
and UL recognized.
(File# E133083)
This chart lists some of the key regulatory surge requirements in the first column. In green we see the highest surge requirements placed on telecommunications equipment. These are the Telcordia GR inter-building requirements that apply to service providers in North America and a few other places around the globe. Also listed here is the I-T-U K dot 20 and K dot 21 enhanced requirements which are called out is some places in Europe and Asia. These requirements include being able to survive a 500 amp surge on a 2 by 10 wave form which simulates the current induced in a telephone cable by a nearby lightning stroke.
In the chart, the middle column shows solutions for traditional telephone circuits from voice only all the way up to VDSL2. In green we see the SDP Q38CB family is the correct choice for protection that complies with the standards in the left column. The right column shows the S-E-P Q38CB series is the proper choice for Ethernet circuits including the emerging power over Ethernet technology.
In the blue row we see the basic requirements for the I-T-U K dot 20 and K dot 21 standards. With a much lower surge current requirement, the SDP Q38B series is the best choice to meet this global standard.
Note that all SDP and S-E-P devices are R-O-H-S compliant and U-L recognized.
If you were curious, the prefix SDP stands for semiconductor DSL protector and S-E-P stands for Semiconductor Ethernet Protector.
These patented products are the only solutions that meet these surge standards and are compatible with these high bandwidth signals while offering semiconductor protection performance.
SDP: Semiconductor DSL Protector
SEP: Semiconductor Ethernet Protector

4. Breakthrough Package Size
Tip & Ring Protection in one package
5 x 6 x 1.5mm Surface Mount Package
Standard SO-8 Footprint
Feed-thru PCB Layout
Package Size Comparison
The SDP and S-E-P protectors include protection for two communication wires. In telephony these are called tip and ring. The package is a 5 by 6 millimeter leadless QFN body that is only one and a half millimeter high. While the connection pads form a unique pattern on the bottom of the package, it is compatible with a standard S O 8 footprint. We recommend that customers lay out the S-O-8 pad arrangement so that the SDP package bridges the communication lines as shown in the diagram. This allows for the best transmission characteristics for the broadband signal.
As you can see in the package size comparison, the SDP series would be a big hit with customers just based on package size alone. It is by far the smallest package available for GR tip and ring protection available in the industry. With the never ending quest for higher circuit density in telephone circuits, the size alone would drive significant sales even without the superior protection performance on broadband signals.
SDP / SEP

5. Broadband Overview
Broadband encompasses many technologies and protocols.
For this module we will be focusing on the xDSL technololgies.
xDSL uses complex coding schemes to pack more data into less bandwidth.
Broadband encompasses many technologies and protocols. The most popular ones are shown in the two charts here. The chart on the left shows the spectral content of the various types of signals. They range from ADSL, which is a one point one megahertz signal to 100 base T Ethernet which uses a 125 megahertz clock. But the clock speeds can be deceiving. The chart on the right shows the protocols in the same order, but now instead of the spectra we are looking at the data rates present on each pair of the protocols. You can see that the DSL signals have much more data in them. This is because the complex DSL coding schemes pack a lot more information in each clock cycle than the simpler T1 E1 or Ethernet coding schemes.
These complex coding schemes also present a challenge to the protection products used on the data lines which we will soon see.

6. Typical Capacitance Values
Low Capacitance
High Capacitance will rob power from the signal.
Capacitance gains in importance as the data rate of the signal increases.
Non-Issue for Voice, T1/E1
Minor Issue for T3, ADSL
Major Issue for VDSL, Ethernet
SDP & SEP devices have capacitance values that are low enough to preserve signal power.
Typical Capacitance Values
(Zero Line Voltage)
SDPxxx0Q38CB
VBIAS = 50V
11 pF
SDPxxx0Q38B
23 pF
SEPxxx0Q38CB
VBIAS = 5V
20 pF
SP03-3.3
9 pF
P0080SCMC
73 pF
P0640SCMC
93 pF
P1800SCMC
57 pF
P3100SCMC
50 pF
GDT
<1 pF
The SDP and S-E-P protectors offer low capacitance to the broadband signal. This is key because high capacitance will rob power from the signal, limiting bandwidth of the signal or the distance the signal can be transmitted or both. The effect of high capacitance becomes a larger and larger issue as the data rate increases.
It is not a real issue for voice or T1-E1 signals and is only a minor concern for T3 and the early ADSL technologies. But it is a major issue for VDSL and Ethernet applications.
In the table you can see the nominal capacitance loads of several protection devices. The devices with acceptably low capacitance for broadband signals are highlighted in green. Note that even the micro capacitance line of SIDACtor devices do not make the cut. The SP-O-3 series is low capacitance, but cannot meet the high surge requirments. Gas Discharge Tubes have very low capacitance, but cannot offer the protection performance of semiconductor devices.

7. Constant Capacitance / Changing Voltage
Error-Free Broadband Requires Unchanging Capacitance As Line Voltage Varies
Changing Capacitance Causes Data Errors
Constant Capacitance Becomes More Critical as Data Rates Increase
Only the Patented Circuit of the SDP / SEP Can Offer Constant Capacitance in a Semiconductor Protector. (US Patent # )
Increasing Bias Voltage
Another issue with conventional semiconductor device protectors is that their capacitance value changes as the voltage applied to them changes. You can see by the chart on the left that standard SIDACtor devices have a significant change in capacitance as the line voltage changes.
This causes a type of intermodulation distortion in the signal that the even the sophisticated equalization and error correction circuitry in the DSL and Ethernet receivers cannot handle.
As you may have guessed, the need for a constant capacitance increases with the data rate used.
The SDP and S-E-P use a patented circuit to force a constant capacitance to be presented to the data stream. This is accomplished by applying a fixed bias voltage to the SIDACtor element in the bridge circuit. This preloads the SIDACtor with a fixed voltage so that changing line voltages on the tip and ring lines have almost no effect on the capacitance.
The diagram on the right is taken from the SDP data sheet. The tan arrow shows that as the bias voltage is increased, the capacitance decreases. Since the bias voltage is fixed, the capacitance will be fixed as well. You can see by the blue line that the capacitance does not change as the line voltage increases from left to right.
Ideal Constant Capacitance

8. A Perfectly Balanced Bridge
One Robust SIDACtor Device
One VS (T-R, T-G, R-G)
Optimal Capacitance Balance to Ground
Minimal Longitudinal Conversion Loss
No Conversion of Common-Mode Events to Differential Mode
It is worth looking at the unique patented circuit used in the SDP and S-E-P protectors. The device is actually built from three silicon chips. The SIDACtor element in the middle of the bridge is rated to handle simultaneous surges from the tip and ring lines. This means that the SIDACtor in the Q38CB versions, which are rated for 500 amp 2 by 10 surges can actually handle more than 1000 amps.
The two other chips are diode chips. One is divided up to create four common cathode diodes as shown on the top of the diagram. The other is divided up to create four common anode diodes as shown on the bottom of the bridge. Each of the four diodes are sized to handle the current requirements of each position in the bridge. Because these diodes are all perfectly matched to each other, the bridge is perfectly balanced and symmetric.
Because there is a single SIDACtor device in the bridge, the protection voltage is exactly the same between any two pins.
While the circuit analysis is beyond the scope of this module, the bridge minimizes the difference between tip and ring to ground capacitance and tip to ring capacitance. This minimizes another type of distortion on the broadband signal called longitudinal conversion loss.
The bridge also prevents the conversion of common mode events to differential mode. This important feature is worth a closer examination.

9. Transformer Coupled Applications
No Conversion of Common-Mode Events to Differential Mode
2-Element Solution
OVP Switches Close when VBO is present.
3-Element Solution
Here are three diagrams that show a common mode surge applied to various protection schemes on a DSL interface. All DSL interfaces use a coupling transformer as do all Ethernet, T3 and T1-E1 circuits. Whenever an interface uses a coupling transformer there is a danger that common mode surges will be converted to differential mode by the protection scheme. This is a problem because while transformers will not couple very much energy to the driver circuits during a common mode surge, they will couple the maximum amount of energy during a differential mode surge.
The first circuit shows simple tip to ground and ring to ground protection. This could be semiconductor or GD tease. Both are essentially voltage activated switches. In this example, the breakdown voltages of the two devices are not perfectly matched. In reality, they never would be either. As the surge voltage increases, the upper switch, which is activated at 300 volts will close first and the current represented by the blue arrow will circulate. However, since the upper switch is closed, the purple current will also begin to circulate. In fact, the lower switch may never close since the voltage may never reach 310 volts. Two bad things happen in this scenario. First, the upper switch is probably only rated for the blue current and adding in the purple current may destroy the upper protector. The second bad thing that happens the purple current flows through the windings of the transformer, coupling the maximum amount of energy into the driver side of the transformer. This is shown by the red arrow and this current presents a real threat to the driver circuitry.
The second circuit shows an improvement to the first circuit. Here, three protection devices that operate at half the voltage of the first circuit are used in a wye arrangement. This represents a wye configured SIDACtor arrangement or a three element gas tube arrangement. Lets look at what happens in this circuit during a common mode surge. As the voltage reaches 300 volts, the top and center switches will close and, again, the blue current will circulate as designed. This time, however, since the center switch is closed, the lower protector is now grounded at its upper terminal. This means that the 300 volts is now across the 160 volt switch, forcing it to close. However, this takes a few micro seconds to occur. During that time the purple current will still be drawn through the transformer winding. Fortunately, this time the current only exists for a micro second or two until the lower switch closes, meaning much less energy is coupled through to the sensitive driver circuitry.
The bottom diagram shows what happens when an SDP device is used. Here there is only one SIDACtor and so there is only one voltage controlled switch. When it closes the diodes will conduct in perfect tandem to take both the tip and ring lines to ground simultaneously and virtually no current will circulate in the transformer meaning almost no energy will get coupled into the driver circuits.
While shown for DSL here, this perfectly balance bridge advantage is useful in any transformer coupled application.
SDP/ SEP Solution
While shown for DSL here, this perfectly balance bridge advantage is useful in any transformer coupled application.

10. SDP vs GDT Before After Comparison SDP / SEP GDT
Acceptable Capacitance
Yes
Constant Capacitance
AC Power Cross Dissipation
Very Low
Very High
Low (< 75V) Breakover Voltages Available No
Voltage Overshoot
Board Space Req’d
Minimal
Substantial
Backside PCB Mount
Not Likely
After
Some customers are considering the use of gas discharge tubes for broadband protection. For those customers that are determined to use Gas discharge tubes, they certainly can purchase them from Littelfuse. However, a direct comparison of the GDT with the SDp is warranted.
While both have acceptable capacitance and constant capacitance, the SDP device has significant advantages during AC power cross events. This is because in their on state, GDT’s have an on-state voltages of 20 to 30 volts. In comparison silicon solutions have on-state voltages of 2 or 3 volts. This means that during the GR point 2 Amp tests, the power dissipation by a GDT will be about 55 Watts compared with the power dissipation by silicon solutions will be about 6 watts.
Since the GDT power is about ten times more than the silicon power, GDT devices will generate very high temperatures that can damage PCB’s and surrounding components.
Using an SDP device in conjunction with the Enhanced TeleLink fuse can limit damage by opening during the 2.2 Amp tests in 5 minutes or less with no damage to the SDP, the PC board or surrounding components.
For Ethernet applications, there really is no GDT solution since they are not offered with a breakdown voltage below about 75 volts.
Also, the voltage overshoot of GDT devices is much higher that that of silicon solutions. This is especially true for the lowest voltage GDT devices.
When we compare the board space requirements, we find that, while there are some fairly small surface mount GDT devices available, they must have considerable empty board space around them to prevent issues during the AC power cross tests.
Finally, with a package height of only one and a half milly meters, the SDP and S-E-P devices can be placed on the back side of almost any PC board. GDT devices almost always must take up valuable top side PC board real estate.

11. Seven SDP / SEP Biasing Rules
The bias voltage MUST be less than the SDP / SEP stand-off voltage. (Vbias < Vdrm)
The bias supply may be ground referenced.
The bias supply may “float” with respect to ground.
The higher the bias supply voltage, the better the performance of the SDP / SEP. Just watch out for Rule #1…
We recommend 1 MΩ resistors for most applications.
The bias supply isolation resistors must be voltage rated to VS to avoid flash-over during a surge event.
(Usually 1206 for SDP or 0805 for SEP)
Several devices can share bias resistors.
Bias supply current will be < 5µA if these rules are followed.
Even though the biased bridge presents a breakthrough in performance, many designers will not be familiar with their use and will have questions about biasing the Q38CB versions of the SDP and S-E-P devices. Let’s look at seven simple rules for biasing.
First, the bias voltage must be less that the V-D-R-M of the device being used. This prevents the bias supply from triggering the SIDACtor device into conduction.
Rules two and three note that the bias supply can be ground referenced, but doesn’t have to be. The bias voltage cannot ever get to the tip and ring lines because the diode arrangement prevents any current flow.
Rule four states that the higher the bias supply voltage, the better. This is because higher bias voltages will further depress the capacitance presented to the broadband circuit.
Rule five states that we recommend that a minimum one megohm resistor be placed in each bias supply line for isolation in most applications.
Rule six notes that the bias resistors should be voltage rated for the V-S to avoid any flash over during a surge event. For most SDP applications this means a 12-O-6 size resistor is a minimum size and for Ethernet applications, the resistor must be at least an O-8-O-5 size.
Rule seven says that several SDP or S-E-P devices can be biased from a single pair of resistors.

12. FAQ: Will My CM Have Trouble Mounting Your QFN Package??
Not Likely!
While some CM’s have had issues with other QFN packages, those concerns are usually associated with a central ground pad in the center of the package. Our QFN package does not utilize such a pad.
Littelfuse QFN
Another issue that sometimes pops up as customers look at this new technology is the QFN package itself.
Customers may note the some contract manufacturers have warned them that the QFN package presents significant problems for them during board soldering. These concerns are usually associated with a large central ground pad used by many QFN eye seas. They need to understand that the Littelfuse QFN does not use a large central pad on the package and, in fact, should be placed on a standard S-O-8 footprint.
Non-Littelfuse QFN w/ pad

13. SDP / SEP Demo PCB SDP Demo PCB SEP Demo PCB Four RJ11 Channels
Variety of SDP’s Can be Specified
One Reference Channel
On-board 3 – 35V Bias Supply
External Bias Option
Enhanced TeleLink Fuses
SEP Demo PCB
One 4-Pair Ethernet Channel
Variety of SEP’s Can be Specified
On-board 3-35V Bias Supply
To help customers get familiar and more comfortable with this revolutionary technology, a couple of demo PC boards have been developed for the customers use.
The SDP version has four RJ-11 channels. Each of the channels can use a different SDP device, but we recommend that one channel not contain an SDP device at all so that is can be used as a reference channel. This reference channel allows the customers to separate the impact of the SDP on the circuit from the interference caused by the exact board layout and other connector issues that may impact the signal, but not be representative of their own circuit.
There is an on board bias supply that can provide an adjustable three to thirty five volts of bias. Or the customer can connect their own external bias supply as well. Each channel is has Enhanced TeleLink overcurrent protection. Full documentation and instructions are included with each demo board.
For Ethernet customers, the RJ45 Ethernet Demo PCB is equipped with RJ45 jacks so that it can be simply and quickly inserted into the customer’s interface. SEP devices provide the overvoltage protection. It can be ordered with any of the SEP devices installed. Also, Enhanced TeleLink fuses provide overcurrent protection.
There are two identical protection circuits on the board. As you can see in the photo, the circuits on the left side have SEP devices installed but the circuits on the right side do not have the SEP devices installed. This is intentional as it allows the customer to compare the performance of the two circuits. The difference will be the effect of the SEP device on the performance. This allows customers to ignore the effects of the “loop-through” connections and focus only on the effects of the SEP itself.
The board is equipped with its own on-board bias supply. The customer is also able to provide their own bias, if desired. Full documentation and instructions for the demo board are available.

14. SDP / SEP SIDACtor Overvoltage Protection Devices
Thank You!
Thank you. This completes the Introduction to the SDP and SEP SIDACtor overvoltage Protection Devices module.