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USING THE TL7726 HEX CLAMPING CIRCUIT TL7726
Hex Clamping Circuit
SLAA004
June 1994 IMPORTANT NOTICE
Texas Instruments (TI) reserves the right to make changes to its products or to discontinue any
semiconductor product or service without notice, and advises its customers to obtain the latest version
of relevant information to verify, before placing orders, that the information being relied on is current.
TI warrants performance of its semiconductor products and related software to the specifications
applicable at the time of sale in accordance with TIs standard warranty. Testing and other quality
control techniques are utilized to the extent TI deems necessary to support this warranty. Specific
testing of all parameters of each device is not necessarily performed, except those mandated by
government requirements.
Certain applications using semiconductor products may involve potential risks of death, personal
injury, or severe property or environmental damage (Critical Applications).
TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED, INTENDED, AUTHORIZED, OR
WARRANTED TO BE SUITABLE FOR USE IN LIFE-SUPPORT APPLICATIONS, DEVICES OR
SYSTEMS OR OTHER CRITICAL APPLICATIONS.
Inclusion of TI products in such applications is understood to be fully at the risk of the customer. Use
of TI products in such applications requires the written approval of an appropriate TI officer. Questions
concerning potential risk applications should be directed to TI through a local SC sales office.
In order to minimize risks associated with the customers applications, adequate design and operating
safeguards should be provided by the customer to minimize inherent or procedural hazards.
TI assumes no liability for applications assistance, customer product design, software performance,
or infringement of patents or services described herein. Nor does TI warrant or represent that any
license, either express or implied, is granted under any patent right, copyright, mask work right, or other
intellectual property right of TI covering or relating to any combination, machine, or process in which
such semiconductor products or services might be or are used.
Copyright
©
1994, Texas Instruments Incorporated iii
Contents
Title
Page
Introduction
1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parasitic Transistors in Complementary MOS Circuits
1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Latch-Up
2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Guard Rings
2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CMOS Internal-Input-Protection Circuitry
3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
External Clamping Diodes
3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TL7726 Hex Clamping Circuit
4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Device Description
5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Circuit Operation
5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Application Examples Using the TL7726
8
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Current Flow
9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary
9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
List of Illustrations
Figure
Title
Page
1
Parasitic Bipolar Transistors in CMOS Circuits
1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
Structure of Parasitic Thyristor
2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
Guard Rings in CMOS Circuits
2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
Parasitic Transistors in Input-Protection Circuits
3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
Protection Circuit With External Clamping Diodes
4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
Simplified Circuit of the TL7726
4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Current and Voltage Characteristics for Various Devices
5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Characteristics of the TL7726 (V
ref
= 5 V)
6
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
Characteristics of the TL7726 at Low Current (V
ref
= 5 V)
6
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
Current and Voltage Limits of TL7726
7
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
Behavior of TL7726 With Rapid Voltage Changes
7
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
Measurement Circuit
7
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
Settling Time at the Input of the Hex Clamping Circuit
8
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
Typical Application of the Hex Clamping-Circuit TL7726
8
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
Current Flow Paths
9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Introduction
This application report describes both the parasitic effects present in integrated circuits (ICs) and the problems that can
result when protecting precision analog components with conventional methods. Specifically illustrated are how these
problems can be overcome by using the TL7726 hex clamping circuit.
All semiconductor ICs, regardless of function and manufacturer, are vulnerable to voltages and currents exceeding the
absolute maximum ratings. Although semiconductor manufacturers often build in protection features such as
electrostatic-discharge (ESD) protection, voltage clamping, and current limitation, the devices may fail if operated
outside the manufacturers absolute maximum ratings.
Main failure mechanisms result from overvoltage stress of the semiconductor material. CMOS devices are particularly
vulnerable in this regard, even at low-voltage levels, due to inherent parasitic structures. The best understood parasitic
effect is latch-up, which is caused by parasitic thyristor action caused by overvoltage stress. If sufficient current is
injected into either the input or output pins of the device, the thyristor triggers and a short circuit results between the
supply rails (latch-up). This usually results in catastrophic device failure.
Through careful semiconductor design and by using the device within the manufacturers absolute maximum voltage
ratings, the effects of overvoltage stress can be greatly reduced. For precision analog circuits, this externally applied
voltage level should be tightly controlled; the voltage should be no more than 0.3 V above the positive supply or
0.3 V below ground. Since it is difficult to predict if an applied voltage falls within these limits, external clamping circuits
in the form of silicon diodes are often employed.
Zener diodes seem to be an obvious choice for this function. Unfortunately, due to poor voltage tolerance and
asymmetrical clamping, the protected circuitry may not only be inadequately clamped but may suffer from reduced
performance. For example, the dynamic range of an analog-to-digital converter may be reduced.
The preferred use of Schottky diodes proves similarly inadequate. The forward voltage of a Schottky diode is 400 mV.
While this can protect the device for the majority of fault conditions, it still allo