Circuit Configuration Options for TVS Devices

AND8231/D
Circuit Configuration
Options for TVS Diodes
Prepared by: Jim Lepkowski
ON Semiconductor
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APPLICATION NOTE
INTRODUCTION
TVS Diode Protection Options
Figure 1 shows a schematic representation of avalanche
TVS diodes and diode arrays that provide surge protection.
Both types of diode devices can be used for surge
suppression; however, each option offers unique protection
features. Tables 1 and 2 provide a summary of the features
of avalanche TVS diode and diode array protection circuits.
Transient Voltage Suppression (TVS) protection is
important because EMI and ESD can disturb the operation of
the system, produce permanent damage or cause latent
damage that will eventually cause a failure. Avalanche TVS
diodes and diode arrays are available in a number of different
circuit configurations to protect electronic circuits from surge
voltages. This document will analyze the attributes and
trade−offs of different circuit configurations created with
avalanche TVS and diode array protection devices.
Bidirectional Avalanche TVS Diodes
Diode Array
NUP2301
Unidirectional Avalanche TVS Diodes
Diode Array with Avalanche Diode
NUP1105L
NUP2201
Figure 1. Schematic Representation of TVS Diode Protection Options
© Semiconductor Components Industries, LLC, 2005
July, 2005 − Rev. 0
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Avalanche TVS Diodes
Avalanche diodes are a good TVS device for applications
that require power line surge immunity and ESD protection.
These devices provide protection by clamping a surge voltage
to a safe level. They function as a variable impedance to
directly absorb the surge energy and maintain a constant
clamping voltage. The avalanche TVS diode’s current and
voltage characteristics are similar to a Zener diode; however,
there are significant differences between these devices. A
TVS diode has a junction that is optimized to absorb the high
peak energy of a transient event, while a standard Zener diode
is designed to clamp a steady state voltage.
device for data line ESD protection. In addition, the low
clamping voltage of diode arrays is an advantage for
protecting low voltage ICs. The effective minimum
operating voltage of a diode array is limited only by the
forward voltage drop of a diode. Diode arrays can also be
used as line terminators to remove overshoot or ringing on
high speed data lines. Diode data line termination circuits
are often called Thevenin networks.
Diodes arrays steer the surge current into the power supply
rails, as shown in Figure 2. A positive surge pulse will be
clamped to a voltage that is equal to a forward diode voltage
drop above the supply voltage (VDD). Typically the VSS pin
is grounded; thus, a negative pulse will be clamped to a
voltage level one diode drop below ground. The energy of
the positive and negative surge pulses is dissipated through
the PCB’s power planes.
Diode Arrays
Diode arrays typically have a moderate power rating and
low capacitance. These features make this a popular TVS
D1 clamps positive voltages
VC = VDD + VF
D2 clamps negative voltages
VC = VSS − VF
(VSS is typically connected to Ground)
Figure 2. Diode arrays clamp the surge voltage to a diode drop above or below the power rails.
Diode arrays are constructed by combing switching,
avalanche and Schottky diodes. Most diode arrays are built
with switching diodes in a stacked configuration; however,
a number of products are available that include an avalanche
diode to increase the surge rating. The switching diodes
provide a low capacitance load for data lines, while the
avalanche diode offers energy dissipation ability for the
power line. An avalanche diode is selected that has a
breakdown voltage slightly higher than the nominal power
supply voltage. The relatively large capacitance load of the
avalanche diode is on the power lines and has only a minor
effect on the frequency response of the data lines. Another
popular option for diode arrays is to use low turn−on voltage
Schottky diodes to create an effective TVS device for low
voltage applications.
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Table 1. Avalanche TVS Diode Surge Protection Circuits
Bidirectional
Unidirectional
Schematic
Low Capacitance
Bidirectional
Low Capacitance
Unidirectional
Data Line
Data Line
Z1
Z1
Z2
Clamping Voltage (VC)
Positive Surge
Negative Surge
Attributes
VF_Z1 + VBR_Z2
−(VBR_Z1 + VF_Z2)
•
•
•
Low negative
clamping voltage
Z2
D1
Z1
D2
Z1
VF_D1 + VBR_Z1
−(VF_D2 + VBR_Z2)
VBR_Z1
−VF_Z1
Solves common
mode offset issues
Direct replacement
for varistors
D1
•
•
D2
VF_D1 + VBR_Z1
−VF_D2
D1 and D2 lower the
capacitance
Z1 and Z2 increase
power rating
•
•
•
D1 lowers capacitance
Z1 increases power
rating
Use with short cables
Trade−Offs
•
High capacitance
compared to a
diode array
•
High capacitance
compared to a
diode array
•
Requires four
diodes
•
Requires three diodes
Applications
•
Differential data
lines
Use with long
cables
DC power lines
High frequency
applications
•
•
High−speed
differential data
lines
•
High−speed single
ended data lines
•
•
Single−ended data
lines
Use with short
cables
DC power lines
Digital logic ICs
NUP2105L
NUP4102
•
•
NUP1105L
NZQA5V6
•
•
SL05
SL15
•
•
•
ON Products
•
•
•
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AND8231/D
Table 2. Diode Array Surge Protection Circuits
Diode Array
Schematic
Diode Array Plus TVS
VDD
VDD
D1
I/01
D2
Schottky Diode Array
D1
I/01
D2
D3
I/02
D4
Z1
VDD
D1
I/01
D2
D3
I/02
D4
D3
I/02
D4
I/O1
Clamping
I/O1
I/O2
I/O2
I/O2
I/O1
Voltage (VC)
Positive Surge
VF_D1 + VDD VF_D3 + VDD VF_D1 + VDDvVCvVZ1_BR VF_D3 + VDDvVCvVZ1_BR VF_D1 + VDD VF_D3 + VDD
Negative Surge −VF_D2
−VF_D4
−VF_D2
−VF_D4
−VF_D2
−VF_D4
Low capacitance
Good capacitive
matching (small DC
I/O1−to−I/O2)
Low clamping voltage
•
•
•
•
Low capacitance, with moderate power rating
Z1 increases power rating
Z1 has minor effect on I/O line capacitance
Z1 functions as a decoupling capacitor
•
•
•
Low clamping voltage
Low VF ( ^ 0.3 V)
Low VC ensures surge
event is clamped by
external protection
circuit
•
•
Z1 is large compared to diodes – which increases
package size
VF ^ 0.7 V for D1 – D4
•
•
Poor power rating
compared to avalanche
TVS diodes
VF ^ 0.7 V
Power rating is poor
compared to a
switching diode
Relatively poor reverse
bias surge rating
Applications
•
•
•
Differential data lines
Use with short cables
ESD protection
•
•
•
Use with single−ended data line
Use with short cables
ESD protection
•
•
•
•
Differential data lines
Use with short cables
ESD protection
Low voltage ICs
ON Products
•
•
NUP1301
NUP4301
•
•
NUP2201
NUP4201
•
NUP4302
Attributes
•
•
•
Trade−Offs
•
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Uni− Versus Bidirectional Protection
Avalanche TVS diodes are available in either a uni− or
bidirectional configuration. In contrast, diode arrays are
typically used only as a unidirectional protection device. Uni−
and bidirectional devices both provide protection against
positive and negative surges; however, the magnitudes of the
breakdown voltages are different, as shown in Figure 3. A
unidirectional device has a clamping voltage equal to the
breakdown voltage when reversed biased and a forward diode
drop if forward biased. A bidirectional device typically has a
symmetrical VBR for both positive and negative voltages.
Diode arrays can be connected to a positive and negative
power supply to create a bidirectional device; however, most
applications connect the bottom diode to ground which forms
a unidirectional device.
Unidirectional TVS Device
Bidirectional TVS Device
Definitions
VBR = Breakdown Voltage
= Voltage at Test Current IT
VF = Forward Bias Voltage at Current IF
VF < VBR
Figure 3. Definition of a Uni− and Bidirectional TVS Circuit
Although both uni− and bidirectional devices can often be
used in the same application, there are many applications
where one of the clamping options provides a distinct
advantage. In applications such as the protection of a DC
power supply or a logic IC, a unidirectional diode device
offers a lower clamping voltage (i.e. −VF) for negative surge
voltages. Bidirectional TVS devices offer several advantages,
including solving a common mode offset voltage problem.
Often bidirectional TVS diodes are selected simply because
they are replacing metal oxide varistors (MOVs) which are
inherently bidirectional. Figure 4 provides an overview of
typical applications of uni− and bidirectional TVS devices.
Unidirectional Applications
•
•
DC power supply lines
Data lines with short cables
(i.e. GndA ^ GndB)
•
Logic device protection
Bidirectional Applications
•
•
AC power supply lines
Data lines with long cables
(i.e. GndA 0 GndB)
Figure 4. Typical Applications of Uni− and Bidirectional TVS Devices
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AND8231/D
Bidirectional
TVS Diode
Creating a Bidirectional Diode
TVS diodes are inherently unidirectional. A bidirectional
device can be created by combining two unidirectional
diodes, as shown in Figure 5. The electrical characteristics
of a common cathode and common anode device are
typically equivalent. The clamping voltage (VC) of the
composite bidirectional device is equal to the breakdown
voltage (VBR) of the diode that is reversed biased, plus the
diode drop of the second diode that is forwarded biased.
Common
Cathode
Common
Anode
Figure 5. A bidirectional TVS diode is created by
combining two unidirectional diodes to form a
common cathode or common anode device.
Bibliography
1. −; “AP−209 – Design Considerations for ESD
Protection Using ESD Protection Diode Arrays”,
California Micro Devices, 1998.
2. −, “SI99−01 – PCB Design Guidelines for ESD
Suppression”, Semtech, 2002.
3. Lepkowski, J., “AND8232 − PCB Design
Guidelines that Maximize the Performance of TVS
Diodes”, ON Semiconductor, 2005.
4. Lepkowski, J., “AND8230 − Application Hints for
Transient Voltage Suppression Diode Circuits”,
ON Semiconductor, 2005.
General TVS Diode Selection Guidelines
The following guidelines can be used to select a TVS
diode.
1. Select a device with a breakdown voltage greater
than the maximum specified voltage to ensure that
the TVS device does not turn−on during normal
operation.
2. A bidirectional TVS device maybe required
for a circuit that has a common mode voltage
requirement. The common mode specification is
required when there is a significant difference in
the voltage potential between the ground reference
of the transmitting and receiving nodes.
3. Choose a TVS device that is capable of dissipating
the energy of the surge pulse.
4. The power rating of most TVS devices decreases
with temperature and a derating of the TVS’s
energy specification maybe necessary.
5. The capacitance of the TVS devices should be
minimized for high speed circuits in order
to reduce signal distortion. In addition, the
capacitance of two differential signals must be
matched in order to maintain pulse width integrity
in the amplifier’s output signal.
6. Some systems bundle power and data lines in
the same cable and require the unit to survive a
short between the power and data lines. This
requirement means that the breakdown voltage of
the data line protection device must be higher than
the maximum value of the supply voltage.
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