The cmos structure is analyzed and the pnp and npn latching transistors are identified. A t rac ed byh m nv g s o f cmos devices, designers using them for the. The following diagram is a drawing of a typical cmos circuit. It resulted in circuits either malfunctioning or consuming excessive power, and could be either inherent in the design or triggered by voltage spikes on io pads that could forward bias pn. Since the well area electrically isolates bulk cmos devices from the substrate, a parasitic thyrister is inevitably constructed between the power supply and gnd pins. Nov 06, 2017 latch up prevention in cmos various techniques for latch up prevention issues in physical design duration. With high current levels, a device would not have to remain in a latch up state for very long. Research and development into the causes led to several papers in the 1980s discussing causes and methods to lessen the influence of latchup. It happens because of the formation of two parasitic bjts one pnp and another npn in cmos. Edn power management can cause latchup in cmos chips. Shown alongside is a cmos transistor consisting of an nmos and a pmos device.
As the semiconductor technology evolves from micrometer range to nanometer range, every topic needs to be up todate, and latchup is no different in this regard. Jan 19, 2015 early cmos processes suffered a reliability concern that became known as latchup. Latch up in cmos devices inherent in a cmos process are parasitic pnp and npn transistors configured as silicon control rectifiers scr. It does not matter if the cmos ic is a microcontroller, an operational amplifier, an analogtodigital converter adc, logic, or analog multiplexor. A typical cmos inverter cross section, showing parasitic devices. Although our aim is to give an understanding of latchup as it occurs in cmos switches, similar. While this article specifically addresses problems with cmos switches, it is generally applicable to all cmos devices, including digital isolators.
Cmos ics have latch up paths, but there are several design techniques that reduce susceptibility to latch up. Basically sp ak ing, y lo r dtv cf b one of the many cmos processes available, can be made to latch up if stressed. Early in cmos development, latchup was recognized as a problem to be solved. An600 understanding latchup in advanced cmos logic on. Latchup refers to short circuit formed between power rails in an ic leading to high current and damage to the ic. The structure formed by these resembles a silicon controlled rectifier scr, usually known as a thyristor, a pnpn device used in power electronics. Figure of merit fom for the latch up feedback loop.
This requires more current before latch up can occur. Click download or read online button to get cmos latch up modeling and prevention book now. Latchup has long been a bane to cmos ic applications. The cmos latch schematic is drawn and the triggering methods are discussed. Notice the two transistors, npn and pnp and their connection to vdd and gnd supply rails. Latchup and its prevention in cmos devices vlsi universe. May 04, 20 about the blog adder and asic asynchronous set reset d flip flop blocking cache cache memory characteristic curves clock divider cmos inverter cmos inverter short circuit current dff d flip flop dft dibl difference divide by 2 d latch equations finite state machine first post flip flop frequency divider fsm full adder hold time intro inverter. In some cases latch up can be a temporary condition that can be resolved by power cycle, but unfortunate.
Slya014a latch up, esd, and other phenomena 5 the parasitic thyristor can be triggered by a rapid rise of the supply voltage. Most cmos ics, given proper conditions, can latch like an scr, creating a short circuit from the positive supply voltage to ground. Latchup is a very important topic for onchip reliability of integrated systems. These bjts for a siliconcontrolled rectifier with positive feedback and virtually short circuit the power and the ground rail.
More specifically it is the inadvertent creation of a lowimpedance path between the power supply rails of a mosfet circuit, triggering a parasitic structure which disrupts proper functioning of the part, possibly even leading to its destruction due to overcurrent. Latch ups can result in circuit malfunctioning that requires a powerdown or a complete failure of the semiconductor. In cmos technology, there are a number of intrinsic bipolar junction transistors. Msan107 understanding and eliminating latchup in cmos. Winning the battle against latchup in cmos analog switches. Trends and challenges to esd and latchup designs for. Cmos latch up modeling and prevention download ebook pdf. Without understanding that it is impossible to understand when latch up.
The individual bipolar devices have low current gain, with a. To overcome these limitations, the esd association rome, ny. Keep the sourcedrain of the mos device not in the well as far away from the well as. The proton latch up cross sect ions of th e five device ty pes diff er by about a factor 300, w hich is a very l arge diffe rence, but it is not clear how th is is related to d evice structure or the. This application note explains how this occurs and what can be done to prevent it for mosfet drivers. Engineers can use various methods to perform latch up tests, but the only standard that specifies procedures for themjesd 78 ref. Simply defined, latch up is a functional chip failure associated with excessive current going through the chip, caused by weak circuit design. All cmos ics have latchup paths, but there are several design techniques that reduce susceptibility to latchup. Latch up is defined as the generation of a lowimpedance path in cmos chips between the power supply v dd and the ground gnd due to the interaction of parasitic pnp and npn bipolar junction transistors bjts these bjts form a silicon controlled rectifier scr with positive feedback and virtually short circuit v dd to the ground, thus causing excessive current flows and even. Reduce rsub by increasing p doping of substrate by process control reducing rwell and resistance of well contacts by process control. Nov 12, 2014 latch up is a functional chip failure associated with excessive current going through the chip, caused by weak circuit design. Designers can also look forward to the day, in the not too distant future, when even these few simple precautions will no longer be necessary. The simplest way of preventing latch up occurring is to adhere to the absolute maximum.
Subscribe this channel for more videos on physical design fundamentals. This causes a low resistance to appear from v dd to ground, and a subsequent large current to be drawn through the device. To understand why latchup can occur, consider the simple inverter in figure 1. Most of the times with psub connected to ground, will this cause any problem. Mar 30, 2019 hello everyone, this videos explains the latch up phenomenon in cmos circuits. Vbe 6 latch up latch up when back to back bjts satisfy pnp x npn 1, latchup may occur operation voltage of cmos circuits should be below latch up voltage remedies of latch up problem. A latch up is a type of short circuit which can occur in an integrated circuit ic. In cmos processes, these transistors can create problems when the combination of nwellpwell and substrate results in the formation of parasitic npnp struct. Latch up in cmos what is latch up in cmos design and ways to prevent it. This generally destroys the cmos ic or at the very least causes the system to malfunction. However, in modern processes, latch up is not seen as an issue. Research and development into the causes led to several papers in the 1980s discussing causes and methods to lessen the influence of latch up. Q2 is a lateral double emitter npn transistor whose base is formed by the ptype substrate.
Latchup in cmos technology the problem and its cure. Although such techniques can solve the latchup problem, they increase the cost of production. However, to the best of my knowledge, the previous book about this topic was published about 20 years ago. What is needed is a more general, more systematic treatment of the latchup problem. A latchup current selfstop methodology and circuit design, which are used to prevent damage in the bulk cmos integrated circuits due to latchup, are proposed in this paper. Appreciating the variety of possible triggering mechanisms is key to a real understanding of latchup. Fourthly, we describe other integrated circuit fabrication technologies, which are naturally immune. Additionally, the thyristor might be triggered by a high supply voltage far higher than the value given in data sheets. The applications engineer and systems designer, however, are not so much concerned with the theory and modeling of latch up as they are with the consequences of. Additionally, the thyristor might be triggered by a high supply voltage far higher than the value given in. Fairchilds process enhancements eliminate the cmos scr. Latchup 1 all trademarks are the property of their respective owners. About the blog adder and asic asynchronous set reset d flip flop blocking cache cache memory characteristic curves clock divider cmos inverter cmos inverter short circuit current dff d flip flop dft dibl difference divide by 2 d latch equations finite state machine first post flip flop frequency divider fsm full adder hold time intro inverter.
Cmos circuitry without major concern about latch up related problems. An excellent treatise on the subject of latch up in general can be found in the. The structure formed by these resembles a silicon controlled transistor scr. He begins by demonstrating the need for understanding parasitic latchup behavior and a historical overview of it. Keep the sourcedrain of the mos device not in the well as far away from the well as possible. Q1 is double emitter pnp transistor whose base is formed by n well substrate of pmos, two emitters are formed by source and drain terminal of pmos and collector is formed by substratep type of nmos. Sini is an expert on physical design flow and related methodologies. The proposed latchup prevention methodology and circuit design can detect and stop the.
Figure 1 fourlayer scr structure a node q1 gate p1 n1 p2 n2 a ode q1 cathode gate q2 p1 n 1 p2 n p2 n2 q2 issue 1 july 1993 msan107 understanding and eliminating latch up in cmos applications application note structures are examined in the context of zarlinks. Appreciating the variety of possible triggering mechanisms is. Latchup current selfstop circuit for wholechip latchup. Well you can think latch up as a short circuit between the supply and ground. It resulted in circuits either malfunctioning or consuming excessive power, and could be either inherent in the design or triggered by voltage spikes on io pads that could forward bias pn junctions they were connected to. The latch up mechanism, once triggered, turns on a parasitic scr internal to cmos circuits which essentially shorts vcc to ground. Research and development into the causes led to several papers in the.
More specifically it is the inadvertent creation of a low impedance path between the power supply rails of a mosfet circuit, triggering a parasitic structure which disrupts proper functioning of the part, possibly even leading to its destruction due to overcurrent. For the circuit to latch up, several conditions must be met1. Introduction scr latch up is a parasitic phenomena that has existed in circuits fabricated using bulk silicon cmos technologies. Understanding a few facts will resolve both of these situations. Download limit exceeded you have exceeded your daily download allowance. But such a fate is not inevitable in cmos circuitry. In cmos processes, these transistors can create problems when the. Speaking about cmos transistors, latch up is the phenomenon of low impedance path in cmos between power rail and ground rail due to interaction between parasitic pnp and npn transistors. Then consider what could make the parasitic npnpnp go into forward mode. Soi silicon on insulatordoesnt have any latch up problem. In case of any spike in the power supply, one of the parasitic bjt gets on d. Q1 and q2 are parasitic transistor elements residing inside it. Latch is the generation of a lowimpedance path in cmos chips between the power supply and the ground rails due to interaction of parasitic pnp and npn bipolar transistors. Latch up problem in cmos vlsi design vlsi design latchup is defined as the generation of a lowimpedance path in cmos chips between the power supply v dd and the ground gnd due to the interaction of parasitic pnp and npn bipolar junction transistors bjts.
This causes excessive current flows and potential permanent damage. Because of the wide variety of cmos technologies and the long term interest in latchup, some overall guiding principles are needed. At t5, r is deasserted first, so the latch goes into the normal set state with q 1 and q 0. Latch up continued the equivalent circuit shown has q1 being a vertical double emmitter pnp transistor whose base is formed by the nwell with a high base to collector current gain b 1. Latchup cause, effect and prevention allthingsvlsi. Basically sp ak ing, y lo r dtv cf b one of the many cmos processes available, can be made to latchup if stressed. Simple circuit techniques and attention to system design details will ensure that the cmos full potential can be realized in all operating environments. Simply defined, latchup is a functional chip failure associated with excessive current going through the chip, caused by weak circuit design. Jan 14, 2015 early cmos processes suffered a reliability concern that became known as latchup. Hello everyone, this videos explains the latch up phenomenon in cmos circuits. A latchup is a type of short circuit which can occur in an integrated circuit ic. This effect often was observed in earlier generations of cmos circuits.
Ironically, the parasitic bipolar transistors unintentionally formed by the cmos. Goodwinjohansson in this book the author attempts to provide a comprehensive approach to the problem of latchup in cmos integrated circuits. Latchup becomes a real problem when you try to power up and down different sections of your design to save power. Latch up is a condition that comes about when this parasitic scr is triggered. Ironically, the parasitic bipolar transistors unintentionally formed by the cmos inverter switch impacted the introduction of cmos. Pdf latchup in integrated circuits from energetic protons. The nmos and pmos circuits form parasitic pnpn structures that can be. Understanding latch up in advanced cmos logic latch up has long been a bane to cmos ic applications.
In a study on the latch up scaling in the last cmos technology nodes, it was shown that the intrinsic triggering currents for both positive and negative injection were practically unchanged fig. Parasitic bipolar transistors for the nwell cmos inverter parasitic components. It resulted in circuits either malfunctioning or consuming excessive power, and could be either inherent in the design or triggered by voltage spikes on io pads that could forward bias pn junctions to which they were connected. In some cases latch up can be a temporary condition that can be resolved by power cycle, but unfortunately it can also cause a fatal chip failure. Speaking about cmos transistors, latch up is the phenomenon of low impedance path between power rail and ground rail due to interaction between parasitic pnp and npn transistors.
Sep 01, 2003 engineers can use various methods to perform latchup tests, but the only standard that specifies procedures for themjesd 78 ref. A problem exists if both s and r are deasserted at exactly the same time as shown at time t6. In a bulk cmos chip, the core circuit blocks are always latchup sensitive due to a low holding voltage of the parasitic scr path. As ics get smaller, so do the dimensions between transistors within an ic.
A problem which is inherent in the pwell and nwell processes is due to relatively large number of junctions which are formed in these structures, the consequent presence of parasitic diodes and transistors. Latchup, esd, and other phenomena 5 the parasitic thyristor can be triggered by a rapid rise of the supply voltage. The cmos device latch up phenomenon is a representative example of this. Early cmos processes suffered a reliability concern that became known as latchup. Thirdly, we describe how latch up can be detected in an active circuit and cancelled before damage can occur this is accompanied by successful experimental results with laser induced latch up. A new optimization design of an active guard ring has been proposed to improve latch up immunity of cmos integrated circuits and been successfully verified in a 0.