Application Hints for Transient Voltage Suppression Diode Circuits

AND8230/D
Application Hints for
Transient Voltage
Suppression Diode Circuits
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Introduction
Transient Voltage Suppression (TVS) diodes provide
a simple solution to increase the EMI and ESD immunity
level of a circuit and only a few guidelines must be followed
to provide effective surge protection. This document will
analyze several important application features of avalanche
TVS and diode arrays. In addition, examples will be
provided to demonstrate the advantages and disadvantages
of uni and bidirectional TVS devices. The following circuit
design considerations will be analyzed:
• Internal IC versus External TVS Protection Circuits
• TVS Diode Turn-on Voltage
• Diode Array Application Hints
• Common Mode Offset Voltages
• Back Drive Protection
• Differential Input and Output Application Guidelines
APPLICATION NOTE
devices are typically a factor of at least ten times larger than
the internal IC TVS devices. External TVS diodes provide
a higher level of surge protection because it is typically not
practical for an IC to incorporate large protection devices. In
addition, the internal protection circuit of most ICs is
designed to handle only a few ESD events while an external
TVS device provides immunity for an indefinite amount of
surges.
Knowledge of the IC’s internal surge protection circuit
can be helpful in selecting an external TVS device with
an appropriate power rating and turn-on voltage.
Unfortunately, the data sheets of most ICs provide only an
ESD rating and do not disclose the internal protection
circuit. Internal IC protection circuits can be created using
high voltage transistors, Zener diodes, diode arrays,
thyristors and overvoltage detection switches. Figure 1
provides two popular IC circuits used to provide internal
surge protection. Guidelines to ensure that a surge event will
not exceed the power rating of the internal protection circuit
will be shown in the following sections.
Internal IC versus External TVS Protection Circuits
Transient Voltage Suppression (TVS) diodes can be used
to supplement the surge immunity level of an IC. Most ICs
contain internal protection circuits that function well at
preventing ESD failures that occur in assembly; however,
they are often inadequate for protecting against surge events
that occur in normal product usage. The surge ability of
a silicon TVS diode is directly related to its size and external
Transceiver IC
Logic IC
VDD
VDD
Transmitter
D_High
I/O
Receiver
D_Low
VSS
Internal IC
Protection
Circuits
GND
Figure 1. Zener Diodes are a Popular Choice for the Internal Protection Circuit of a Transceiver IC that
Requires Power Surge and ESD Protection. Diode Arrays are a Frequent Choice for the Internal ESD
Protection Circuit of a Logic IC
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May, 2016 − Rev. 2
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TVS Diode Array Turn-On Voltage
The main function of an external TVS device is to limit the
current through an IC by virtue of decreasing the magnitude
of the surge voltage. An ideal external TVS device will
turn-on before the IC’s internal circuit and absorb the entire
energy of the surge pulse. In practice both the external and
internal protection circuits will usually turn-on during
a surge event. The IC’s reliability will not be impacted if the
internal protection circuit current is limited to a low value.
In addition, the location of the TVS devices is a key factor
that determines whether the majority of the surge energy is
absorbed by the external protection circuit. Reference [4]
provides PCB layout guidelines that help to ensure that the
surge protection will be provided by the external TVS
devices rather than the IC’s internal protection circuit.
Many ICs incorporate a diode array for ESD protection;
thus, the internal and external protection circuits often have
a similar topology, as shown in Figure 2. A value of 0.7 V and
0.3 V can be used to estimate the turn-on voltage of external
switching and Schottky diodes, respectively. The turn-on
voltage of the internal IC protection circuit is typically equal
to 0.7 V for a bipolar process; however, the value for a CMOS
device is a function of several process variables, as shown
below by the voltage equation of a MOSFET diode.
VMOSFET_Diode + VT )
Ǹ
IDS
ǒ Ǔ
1
m C W
2 o ox L
Often CMOS ICs are designed with MOSFET diodes that
have a VT greater than 0.7 V to increase their immunity
against ground noise; thus, the external array will usually
have a lower turn-on voltage. The VT of a low voltage IC
may be low enough that the turn-on of the MOSFETs can be
lower than the 0.7 value of a standard diode. One solution to
this problem is to use a Schottky diode array, as shown in
Figure 3. Schottky diodes typically have a turn-on voltage
of approximately 0.3 V.
VDD
VDD
VDD
IC
ESD
Circuit
I/O
Input
GND
Figure 3. The Low Turn-on Voltage of a Schottky
Diode can be Used to Provide Surge Protection
for Low Voltage ICs
(eq. 1)
The circuit shown in Figure 4 is another alternative to
solve the potential problem that can occur if the internal and
external arrays have a similar turn-on voltage. The resistor
that is located between the two arrays ensures that the
majority of the surge energy will be dissipated by the
external circuit. The IC’s internal diodes also turn-on, but
the current through these devices will be relatively low
compared to the external diode current.
Where:
VT = Threshold Voltage
IDS = Drain-to-Source Current
mo = Electron Mobility
Cox = Gate Oxide Capacitance Per Unit Area
W/L = Width and Length Dimensions
VDD
VDD
VDD
VDD
IC
Input
VDD
VDD
ESD
Circuit
I/O
I1
IC
ESD
Circuit
I2
I/O
Input
R
I1 >> I2
GND
GND
Figure 2. An External Diode Array often has a Similar
Topology to the IC’s Internal ESD Protection Circuit;
however, the Turn-on Voltages of the Two Arrays can
be Different
Figure 4. R Forces I1 >> I2, which Ensures that the
Majority of the Surge Energy is Diverted by the
External Diode Array rather than by the IC’s Internal
ESD Circuit
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Diode Array Application Hints
Decoupling the Power Supply
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 of one diode drop below ground.
Diodes arrays steer the surge current into the power supply
rails where the energy of the transient voltage pulse is
dissipated, as shown in Figure 5. A positive surge pulse will
VDD
P1
+V
0V
D2
IP2
D1 Clamps Positive Voltages
VC = VDD + VF
D2 Clamps Negative Voltages
VC = −VF
Z1
I/O
−V
0.01 mF
D1
IP1
Z1 Provides Surge Protection for VDD
VBR_Z1 > VDD
P2
Figure 5. Adding a Decoupling Capacitor and Avalanche TVS to a Diode Array Enhances the Ability to Clamp
the Surge Voltage to a Diode Drop above or below the Power Rails
increase the power supply’s output impedance. The change
in impedance produces a peak clamping voltage
significantly larger in magnitude than what Figure 5’s
equations predict. Reference [4] provides additional
recommendations that can be used to maximize the
clamping performance of the diode array.
Decoupling capacitors and avalanche diodes are two
simple solutions to improve the load regulation of a power
supply during a surge effect. Placing an RF ceramic
capacitor of approximately 0.01 to 0.1 mF across the power
pins reduces the magnitude of the surge pulse. Additional
surge protection can be provided by using a diode array with
a built-in avalanche diode that has a breakdown voltage
slightly higher than VDD. Integrating the avalanche diode in
the TVS IC minimizes the inductances associated with the
device connections, which reduces the magnitude of the
surge pulse due to the V = L (DI/Dt) equation.
The diode array clamping equations assume that the
power supply rails VDD and VSS are a constant voltage
source. This is a good assumption for low frequency load
changes, but may not be valid during the high frequency load
demand of a surge pulse. For example, the IEC 61000−4−2
ESD pulse has a rise time of less than 1.0 ns and a peak
current of 30 A. The high peak energy of the ESD pulse can
VDD
Diode Array Surge Ratings
Careful interpretation of a diode array’s data sheet
specifications is required because the surge rating is
a function of the test configuration. Some diode arrays have
a power rating that is measured with power applied to the
VDD pin, while others float the power pin during the surge
test. This issue is especially important with Schottky diodes
because the rating of an unpowered array is much lower than
a powered array. The lower surge capability of an
unpowered Schottky array is due to their relatively poor
reverse bias rating compared to a standard diode.
I/O
D1
D2
I/O
D2
D1
D1 is Forward Biased for Positive Voltages
D2 is Forward Biased for Negative Voltages
A DC Voltage Source
Functions as a Ground to
an AC Signal
VDD = Float
D1
I/O
I/O
D2
D2
D2 is Reverse Biased for Positive Voltages
D2 is Forward Biased for Negative Voltages
Figure 6. The Power Pin (VDD) Functions as a Ground for High Frequency AC Signals.
The Diodes will be Exposed to a Reverse Biased Voltage only if VDD is Floating
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Common Mode Offset Problems
The potential for a common mode offset problem exists in
any system that uses a common ground system and has
remote modules that are connected through long cables.
A common mode voltage is created when there is
a significant difference in the voltage potential between the
ground reference of the transmitting and receiving nodes.
The common mode offset means that the voltage of the data
lines can be offset by several volts above or below their
nominal voltage levels.
The reverse bias surge rating of a diode is typically not an
issue if power is applied to the array. The reverse bias
condition of diode D2 can only occur if the negative surge
pulse is applied to the I/O pin while VDD is in a high
impedance or floating state. If the array is powered, the DC
voltage source functions as a ground to the high frequency
AC signal and the surge pulse will be effectively applied to
two diodes that are in an anti-parallel configuration, as
shown Figure 6. One of the diodes will be forward biased
and this diode will have a turn-on voltage that is well below
the maximum reverse biased voltage of the other diode.
5V
5V
CAN_H
CAN
Transceiver
CAN
Transceiver
CAN_L
Signal GNDB
GNDA
VCM = ±2 V
Chassis GNDA
Chassis GNDB
Figure 7. Bidirectional TVS Devices Solve the +2.0 V Common Mode Offset Voltage (VCM)
Requirement of CAN Transceivers
The Controller Area Network (CAN), shown in Figure 7,
is an example of a system that has a common mode voltage
specification. CAN is a popular serial communication
network for automotive and industrial control applications.
The CAN common mode voltage specification does not
change the differential voltage between the data lines;
however, the absolute value of the CAN_H and CAN_L
VCM = +2 V
7
6
6
CAN_H
5
5
4
4
3
DV
2
1
0
CAN_H
3
DV
2
1
0
CAN_L
−1
VCM = −2 V
7
Voltage (V)
Voltage (V)
signals can vary by up to two volts, as shown in Figure 8.
Bidirectional TVS devices should be used to ensure that the
protection devices do not clamp if the data lines are offset
within the specified common mode range. Unidirectional
avalanche and diode arrays should only be used in systems
where the difference in the voltage potential of the ground
references is small.
CAN_L
−1
−2
−2
Recessive
Dominant
Recessive
Recessive
Dominant
Recessive
Bus Logic States
Recessive DV ≤ 0.5 V
Dominant DV ≥ 0.9 V
Figure 8. CAN Data Lines have a Normal Voltage Range of 0 to +5.0 V; However, the Common
Mode (VCM) Requirement can Shift the Level of the Signals by +2.0 V
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Back Drive Protection
Figure 9 illustrates the potential back drive problem that
can exist with a diode array. Back drive occurs when a path
exists for current to flow through the diode array via the data
line. The data line connecting the two modules can
unintentionally provide power to module 1 if VDD2 is greater
than VDD1. This condition can cause powerup problems with
logic ICs and anomalies such as the illumination of indicator
lights in module 1 when the unit is unpowered.
The back drive problem can be solved with the two
protection circuits shown in Figures 10 and 11. Figure 10’s
circuit uses an avalanche TVS diode to eliminate the current
Module 1
path between the two modules; however, the path will still
exist if the module 1’s IC contains an internal diode array.
A second option to solve back drive, shown in Figure 11, can
be implemented by adding an avalanche and blocking diode
to the diode array. The avalanche diode is used to absorb the
surge event, while the blocking diode serves to break the
current path to VDD1. Locating the external diode array close
to the I/O connector helps to ensure that IPU2 < IPU1. The
lower impedance current path of IPU1 ensures that the
majority of the energy of a surge event will be dissipated by
the external diode array.
Module 2
VDD1
VDD2
VDD2
0V
IPU
Logic IC
Logic IC
Data Line
Figure 9. A Diode Array Creates a Back Drive Current Path (IPU) to Provide Power
through a Data Line when VDD2 > VDD1
Module 1
Module 2
VDD2
VDD1
VDD2
0V
Logic IC
Logic IC
Data Line
Figure 10. A TVS Avalanche Diode Eliminates the Current Path (IPU) for
Back Drive; However, the Problem will Still Exist if Module 1’s Logic IC
Uses a Diode Array for ESD Protection
Module 1
Module 2
VDD1
VDD2
VDD2
IPU1
0V
Logic IC
Logic IC
Data Line
IPU2
Figure 11. Back Drive Protection can be Implemented by Adding an Avalanche and Blocking Diode to the Diode
Array. The Blocking Diode Eliminates the IPU1 Current Path; However, Current Path IPU2 will Exist if the IC has a
Diode Connected between the Output and Power Supply Pins
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Differential Amplifier Circuits
The different clamping voltages of uni and bidirectional
TVS devices sometimes can make a difference in the noise
performance of a differential input or output circuit.
The differences between the two TVS devices can be shown
by reviewing the clamping characteristics of a large sine
wave that is representative of a noise signal, as shown in
Figure 12. The uni and bidirectional TVS devices convert
a sine wave into a square wave with a DC average of
approximately VBR/2 and 0 V, respectively. Amplifiers
typically have better noise rejection specifications if the
average of the input signal is equal to zero volts. Also,
biasing the average of the noise signal to 0 V can reduce the
hum in an audio amplifier that is associated with a DC bias
voltage.
Unidirectional TVS Diode
Bidirectional TVS Diode
Noise
Signal
Voltage
Noise
Signal
Voltage
VBR
VBR
0V
VF
0V
VBR
TVS
Clamping
Voltage
TVS
Clamping
Voltage
VClamp_Avg = (VBR − VF) / 2 ^ VBR / 2
VClamp_Avg = 0 V
Figure 12. The Average Clamping Voltage of a Unidirectional TVS Diode for a Sine Wave Input is VBR/2.
The Symmetrical Breakdown Voltages of a Bidirectional Device has an Average Clamping Voltage
that is Biased at 0 V
The remote sensor amplifier circuit shown in Figure 13
illustrates the advantages of symmetrical clamping.
A remote sensor is connected to the amplifier via long wires,
which often introduces noise into the electronics. The
shielded twisted-pair cable minimizes radiated interference
from inducing a voltage on the signal lines and the
feed-through capacitors in the connector reduce the noise
before the input lines enter the PCB. Next, an amplifier
provides the differential amplification to magnify the sensor
signal and attenuate the noise signal.
Remote Sensing
PCB
SENSOR
AMPLIFIER
Connector
PCB
VREF
+VDD
Sensor
+
−
Amplifier
−VSS
Shielded Cable
Figure 13. Remote Sensor Circuits are an Example of an Application that Can Benefit from
the Symmetrical Clamping Feature of a Bidirectional TVS Diode
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The remote sensor circuit also demonstrates a common
solution for solving the non-ideal frequency response of
a TVS device. Placing an RF capacitor such as
a feed-through capacitor in parallel with the TVS device can
cancel the parasitic inductance term produced by the diode’s
package. The frequency response of a TVS diode can be
modeled by connecting a resistor, inductor and capacitor in
series, as shown in Figure 14. At low frequencies below 500
to 800 MHz for an SMT package, the inductance term is
relatively small and the capacitive impedance is large; thus,
the device functions as a near-ideal capacitor. At high
frequencies, the inductance term becomes large, while the
capacitive term becomes small and the device functions as
an inductor. The change in impedance has the consequences
that the input signal can be rectified and amplified, instead
of attenuated.
An ideal amplifier has an infinite Common Mode
Rejection Ratio (CMRR) and will only amplify the voltage
difference between the two input pins. In contrast, a practical
amplifier has a high CMRR at low frequencies, but the
CMRR decreases as the frequency increases. The error that
occurs from the deviation from the ideal amplifier
characteristics can be minimized by preventing as much
noise as possible from entering the circuit board and by
using a bidirectional TVS device. The ideal amplifier will
reject all signals that are common to the two signal lines;
however, a practical amplifier typically has better
amplification and CMRR characteristics if the signal is
based at the mid-point of the supply voltage, which is ground
for a dual power supply (+VDD, −VSS) device. In contrast,
the optimal bias point of the noise signal is usually equal to
VDD/2 for single power supply (+VDD, VSS = 0 V)
amplifiers.
NZQA6V8
SOT−553
1
Frequency
Response Model
RS
0.8 W
5
CS
84 pF
2
3
LS
650 pH
4
Figure 14. The Frequency Response of an Avalanche TVS Diode can be Modeled by a Resistor, Inductor
and Capacitance that are Connected in Series. This Model is Valid for Signals that are Smaller in
Magnitude than the Clamping Voltage
Bibliography
[1] −; “AP−209 – Design Considerations for ESD
Protection Using ESD Protection Diode Arrays”,
California Micro Devices, 1998.
[2] −, “App. Note 0007 – TVS Device Selection,
Location and Connection for EMC Design”,
Protek, 1997.
[3] Lepkowski, J., “AND8031 − Circuit Configuration
Options for Transient Voltage Suppression Diodes”,
ON Semiconductor, 2005.
[4] Lepkowski, J., “AND8032 − PCB Design Guidelines
that Maximize the Performance of TVS Diodes”,
ON Semiconductor, 2005.
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