Circuit arrangement for protection against electrostatic discharges and method for diverting electrostatic discharges

This patent application claims the priority of the German patent application no. 10 2006 021 847.7 filed May 10, 2006, the disclosure content of which is hereby explicitly incorporated by reference.

The invention relates to a circuit arrangement for protection against electrostatic discharges and to a method for diverting electrostatic discharges.

In the case of electrostatic discharges, ESD, high voltages occur for example between terminals of an electrical circuit. This can lead to high currents through the circuit. In the case of integrated circuits, in particular, it is possible for the circuit to be destroyed thereby.

In order to protect a circuit against electrostatic discharges, it is possible to provide protection circuits which, in the case of an electrostatic discharge, that is to say when a high voltage occurs, can divert a current and thus protect the electrical circuit against being destroyed.

FIG. 11 shows an exemplary embodiment of a conventional circuit for protection against electrostatic discharges for positive signal voltages. A PNP bipolar transistor T 1 is connected as a diverting element between terminals K 1 , K 2 . In addition, the terminals K 1 , K 2 are coupled via a series circuit comprising a resistor R 1 and a Zener diode D 1 . The connecting node between the components in the series circuit is connected to the control terminal or base terminal B of the transistor T 1 .

Particularly in the case of circuits in the semiconductor area which operate with relatively high signal voltages or operating voltages, PNP transistors are often used as diverting elements in protection circuits since, in contrast to NPN transistors, they do not have a voltage snapback. A diverting element having voltage snapback goes into a low-impedance state after triggering and causes the voltage present across the diverting element to fall. This is shown by way of example in a voltage-current diagram of a diverting element in FIG. 12 . In the diagram, a current I through a diverting element is represented as a function of a voltage V across the diverting element. Proceeding from a low voltage, the diverting element attains a conducting state in the event of an increase in the voltage to or above a breakdown voltage VBP, whereby a current starts to flow. At the same time, however, the voltage across the diverting element firstly falls to a holding voltage VH. Current and voltage can rise further from this point on.

In applications with lower operating voltages, for example an operating voltage VNV, the holding voltage VH lies above the operating voltage VNV of the circuit arrangement and does not lead to a rise in the current through the diverting element. At higher operating voltages, the holding voltage VH may lie below a different operating voltage VHV, for example. In the event of triggering of a diverting element having a voltage snapback, the supply voltage VHV may be present across the low-impedance diverting element and bring about a high current flow that may lead as far as the destruction of the diverting element.

If, in FIG. 11 , the voltage between emitter E and collector C of the transistor T 1 exceeds the Zener voltage of the Zener diode D 1 , a current can flow from the base of the transistor T 1 , whereby the transistor T 1 is turned on and starts to conduct. However, a Zener diode generally supplies relatively little triggering current for turning on the transistor. The Zener diode D 1 should therefore be given sufficiently large dimensions in order to supply a necessary triggering current for a reliable turn-on of the transistor T 1 .

FIG. 13 shows a further exemplary embodiment of a conventional circuit for protection against electrostatic discharges for positive and negative signal voltages. The PNP transistor T 1 is connected between the terminals K 1 , K 2 . The base terminal B of the transistor T 1 is not connected. In order that a circuit arrangement of this type can be used for positive and negative signal voltages, it is expedient that the base B of the transistor T 1 can float with regard to its potential, that is to say is not connected to a fixed potential. This is intended to have the effect that an internal diode of the bipolar transistor T 1 is forward-biased neither in the positive direction nor in the negative direction.

In normal operation, that is to say where no electrostatic discharge occurs, the transistor T 1 should be switched off, that is to say non-conducting. In the case of an electrostatic discharge, the voltage between emitter E and collector C of the transistors T 1 exceeds a normal operating voltage of the transistor, whereby it becomes conducting. By way of example, in the case of a positive pulse of an electrostatic discharge between emitter E and collector C, the PN junction between base B and collector C can break down and thus turn on the transistor T 1 . As an alternative, by way of example, in the case of a negative pulse of an electrostatic discharge, the PN junction between base B and emitter E can break down, whereby the transistor T 1 is turned on. However, the base current generated by the pulse of the electrostatic discharge in the transistor T 1 is usually small, such that the effectiveness of an arrangement of this type is generally inadequate.

Consequently, the arrangements described cannot ensure that a diverting element is reliably turned on. Accordingly, a current caused by an overvoltage on account of an electrostatic discharge cannot reliably be diverted from a circuit to be protected.

In one embodiment a circuit arrangement for protection against electrostatic discharges comprises a diverting element, which is connected between a first and a second terminal and has a control input, via which the diverting element can be controlled into the conducting state. Moreover, the circuit arrangement comprises a first trigger element which has a first trigger output for outputting a first trigger signal in a manner dependent on a voltage between the first and the second terminal, and a first amplifier unit, which is coupled to the first trigger output on the input side and to the control input on the output side.

If, by way of example, an electrostatic discharge occurs between the first and the second terminal, this can be identified by the first trigger element. In this case, the first trigger element can output a trigger signal, which is amplified by the first amplifier unit. The amplified trigger signal can be fed to the diverting element via the control input in order to put the diverting element into a conducting, low-impedance state. A current brought about by the electrostatic discharge can flow away via the diverting element. Thus, by way of example, components of a circuit that is connected to the first and second terminals can be protected against being destroyed. The amplification of the trigger signal makes it possible to ensure that even in the case of a small trigger signal of the first trigger element, for example a small triggering current, the diverting element is reliably put into a conducting state and electrostatic discharges can be diverted with improved reliability.

The diverting element may comprise a transistor, the controlled path of which is connected between the first and the second terminal. By way of example, the diverting element comprises a PNP bipolar transistor or a p-channel field effect transistor for this purpose. As a result of the turn-on of the controlled path of the transistor in the diverting element into a low-impedance state, a current on account of an electrostatic discharge can be diverted via the transistor.

In one embodiment, the first amplifier unit comprises at least one transistor having a control terminal coupled to the first trigger output. The trigger signal can therefore be amplified by means of a transistor-based amplifier. For the amplification of the trigger signal, it is also possible for a plurality of transistors to be connected in series for the purpose of amplification. By way of example, an amplifier cascade is formed from a plurality of transistors.

In a further embodiment, the circuit arrangement comprises a second trigger element, which has a second trigger output for outputting a second trigger signal in a manner dependent on the voltage between the first and the second terminal, and a second amplifier unit, which is coupled to the second trigger output on the input side and to the control input on the output side.

The provision of a second trigger element with amplifier unit connected downstream makes it possible, in the circuit arrangement, to output trigger signals for different voltages between the first and the second terminal. By way of example, the first trigger element is set up for outputting the first trigger signal if the voltage between the first and the second terminal exceeds a positive threshold value. The second trigger element may be set up for outputting the second trigger signal if the voltage between the first and the second terminal falls below a negative threshold value.

In the event of a positive voltage spike on account of an electrostatic discharge with a positive voltage having a magnitude greater or more positive than the positive threshold value, the first trigger element outputs the first trigger signal. The second trigger element outputs the second trigger signal if a negative voltage spike occurs between the first and the second terminal on account of an electrostatic discharge with a negative voltage having a magnitude greater or more negative than the negative threshold value. Therefore, a trigger signal is in each case output if the magnitude of the voltage between the first and the second terminal is greater than the respective magnitude of the threshold value.

In one embodiment, the second amplifier unit comprises at least one transistor having a control terminal coupled to the second trigger output. Consequently, in the second amplifier unit, too, the trigger signal can be amplified by means of one or a plurality of transistors.

The first trigger element may be connected between the first and the second terminal. Moreover, the first trigger element may comprise a Zener diode.

The second trigger element may also be connected between the first and the second terminal. The second trigger element may likewise comprise a Zener diode. Consequently, the outputting of the respective trigger signals may depend on the Zener voltage of the respective Zener diode.

In an embodiment of a method for diverting electrostatic discharges, a trigger signal is generated in a manner dependent on a voltage between a first and a second terminal. The trigger signal is amplified. The first and the second terminal are conductively connected in a manner dependent on the amplified trigger signal.

If the voltage between the first and the second terminal exceeds or falls below specific threshold values, a trigger signal can be generated. In order to ensure a sufficient magnitude of the trigger signal, the trigger signal is amplified. The first and second terminals are conductively connected to one another in a manner dependent on the amplified trigger signal, with the result that a current flow that occurs on account of the high voltage during an electrostatic discharge is effected via this conductive connection. As a result, components of a circuit that is connected to the first and second terminals can be protected against being destroyed by excessively high currents on account of electrostatic discharge. The amplification of the trigger signal improves a reliability when diverting electrostatic discharges.

By way of example, the conductive connection of the first and second terminals may be effected by means of a PNP bipolar transistor or a p-channel field effect transistor.

The trigger signal may be generated if the voltage between the first and the second terminal exceeds a positive threshold value or if the voltage between the first and the second terminal falls below a negative threshold value. Consequently, both electrostatic discharges with a positive pulse and electrostatic discharges with a negative pulse can be reliably diverted. An amplification of the trigger signal may be effected by means of at least one transistor.

The invention is explained in detail below on the basis of exemplary embodiments with reference to the drawings.

FIG. 1 shows a first exemplary embodiment of a circuit arrangement,

FIG. 2 shows a second exemplary embodiment of a circuit arrangement,

FIG. 3 shows a third exemplary embodiment of a circuit arrangement,

FIG. 4 shows a fourth exemplary embodiment of a circuit arrangement,

FIG. 5 shows a first exemplary voltage-current diagram,

FIG. 6 shows a fifth exemplary embodiment of a circuit arrangement,

FIG. 7 shows a second exemplary voltage-current diagram,

FIG. 8 shows a sixth exemplary embodiment of a circuit arrangement,

FIG. 9 shows a seventh exemplary embodiment of a circuit arrangement,

FIG. 10 shows a third exemplary voltage-current diagram,

FIG. 11 shows a first exemplary embodiment of a conventional circuit arrangement for protection against electrostatic discharge,

FIG. 12 shows a voltage-current diagram of a diverting element, and

FIG. 13 shows a second exemplary embodiment of a conventional circuit arrangement for protection against electrostatic discharge.

Components that are functionally identical or identical in terms of their effect bear identical reference symbols.

FIG. 1 shows an exemplary embodiment of a circuit arrangement for protection against electrostatic discharges. For this purpose, a PNP bipolar transistor is provided as diverting element T 1 , said transistor being connected between a first terminal K 1 and a second terminal K 2 . The emitter terminal E and control input or base terminal B are coupled via a resistor R 1 . In this case, the resistor R 1 may also be integrated in the transistor T 1 in parasitic fashion. Moreover, a first trigger element TR 1 is provided, which is connected between the control input B and the second terminal K 2 . The first trigger element comprises a Zener diode D 1 and a resistor R 2 . A connecting node between the Zener diode D 1 and the resistor R 2 forms a first trigger output TA 1 . Connected thereto is a first amplifier unit V 1 , which is connected to the control input B on the output side.

The terminals K 1 , K 2 may be arbitrary terminals of an electronic circuit to be protected, in particular of an integrated circuit. The terminals K 1 and K 2 may be used by the circuit to be protected as terminals for supplying or outputting signals or else as supply terminals for an operating voltage. In the exemplary embodiment shown, it is assumed that the circuit to be protected operates with positive signal voltages at the terminals K 1 , K 2 , that is to say that a potential at the first terminal K 1 is generally higher than a potential at the second terminal K 2 . The circuit to be protected is not depicted in FIG. 1 .

In the normal case, that is to say when no electrostatic discharge occurs, no current flows via the resistor R 1 and through the first trigger circuit TR 1 . The resistor R 1 makes it possible to ensure that base B and emitter E of the transistor T 1 have no potential difference. Since, moreover, no base current flows via the control input B in the transistor T 1 , the transistor T 1 is therefore not in a conducting state in the normal case.

In the case of an electrostatic discharge, the voltage between the first and second terminals K 1 , K 2 rises significantly. This leads to a Zener breakdown of the Zener diode D 1 in the first trigger element TR 1 , which brings about a current flow from the first terminal K 1 via the resistor R 1 , the Zener diode D 1 and the resistor R 2 to the second terminal K 2 . As a result, a trigger signal is output at the trigger output TA 1 , said trigger signal being amplified by the amplifier unit V 1 . Via the control input B, the amplifier unit V 1 puts the diverting element T 1 into a conducting state with the amplified trigger signal. A current flow between the first and the second terminal K 1 , K 2 is thus essentially effected via the conductive connection of the terminals K 1 , K 2 through the diverting element T 1 . This takes place until the electrostatic discharge has been diverted.

FIG. 2 shows a further exemplary embodiment of the circuit arrangement. In this case, the first amplifier unit V 1 comprises a resistor R 3 , a normally off n-channel field effect transistor, the control input or gate of which is coupled to the trigger output TA 1 , and a normally off p-channel field effect transistor T 2 , the gate terminal of which is connected between the resistor R 3 and the transistor T 3 . As output of the amplifier unit V 1 , the transistor T 2 is coupled to the control input B.

In normal operation, the Zener voltage of the Zener diode D 1 is not exceeded. Therefore, owing to the low voltage across the resistor R 2 , the transistor T 3 remains in a switched-off, non-conducting state, whereby the transistor T 2 is also kept in a non-conducting state.

In the case of an electrostatic discharge, that is to say when the voltage between emitter E and collector C of the diverting element T 1 starts to rise considerably, the Zener voltage of the Zener diode D 1 is exceeded and a current starts to flow through it. Said current causes a voltage drop at the resistor R 2 , which, as trigger signal, turns on the gate of the transistor T 3 . The higher the voltage generated by the electrostatic discharge becomes, the higher the current through the Zener diode D 1 becomes. The voltage drop at the resistor R 3 also rises proportionally to this, whereby the conductivity of the transistor T 3 is increased further.

As a result of the current flow via the transistor T 3 and the resistor R 3 , the gate of the p-channel field effect transistor T 2 is discharged and the transistor T 2 is thereby controlled in low-impedance fashion. The low-impedance connection of the control input B to the second terminal K 2 thus results in the generation of the base current of the diverting element T 1 via the transistor T 2 as amplified trigger signal. As a result, the diverting element T 1 is controlled into the conducting state and the electrostatic discharge can be diverted via the diverting element T 1 with improved reliability.

The conductivity of the transistors T 1 , T 2 , T 3 usually increases in the event of rising voltage as a result of the electrostatic discharge, whereby the electrostatic discharge is diverted better.

In the exemplary embodiments shown, instead of being connected to the control input B, the Zener diode D 1 could also be connected directly to the first terminal K 1 .

FIG. 3 shows a further exemplary embodiment of a circuit arrangement for protection against electrostatic discharges, which can be used also to protect electronic circuits in which positive and negative signal voltages or operating voltages can occur at the terminals to be protected. For this purpose, alongside the first trigger element TR 1 with amplifier unit V 1 connected downstream, a second trigger element TR 2 and a second amplifier unit V 2 are provided. The second trigger element TR 2 comprises a resistor R 4 and a Zener diode D 2 , which is provided with opposite polarity in comparison with the diode D 1 . The first and the second trigger element TR 1 , TR 2 are connected in series between the first and the second terminal K 1 , K 2 .

In this exemplary embodiment, the PNP transistor T 1 is preferably constructed symmetrically, that is to say that it has p-type regions that are essentially of identical size and doped identically.

Independently of a polarity of a voltage between the terminals K 1 , K 2 , one of the Zener diodes D 1 , D 2 is forward-biased. Since, however, in normal operation, that is to say without electrostatic discharge, the respective other Zener diode is in a turned-off state, a current flow via the trigger elements TR 1 , TR 2 nevertheless does not occur in this case. It is only when an electrostatic discharge occurs that a Zener breakdown occurs in one of the Zener diodes D 1 , D 2 and a current flow via the Zener diodes D 1 , D 2 thus occurs.

If a high positive voltage that is greater than a positive threshold value is produced on account of an electrostatic discharge with a positive pulse between the first and second terminals K 1 , K 2 , the Zener breakdown occurs in the Zener diode D 1 , whereby the first trigger signal is output via the trigger output TA 1 to the first amplifier unit V 1 and is amplified by the latter. The amplified first trigger signal controls the diverting element T 1 into a conducting state, with the result that the electrostatic discharge is diverted via the diverting element T 1 .

If a high negative voltage that is less than a negative threshold value occurs on account of a negative pulse of an electrostatic discharge between the first and second terminals, a Zener breakdown takes place in the Zener diode D 2 . As a result, the second trigger signal is generated and output via the second trigger output TA 2 to the second amplifier unit V 2 . The amplified second trigger signal serves in turn for turning on the diverting element T 1 into a conducting state.

FIG. 4 shows a further exemplary embodiment of a circuit arrangement. In this case, the first trigger element TR 1 and the first amplifier unit V 1 correspond to the arrangement shown in FIG. 2 . The second amplifier unit V 2 comprises a normally off p-channel field effect transistor T 4 , the gate terminal of which is coupled to the second trigger output TA 2 . On the output side, the second amplifier unit V 2 is coupled to the control input B.

In the event of a positive pulse of an electrostatic discharge and a resultant positive voltage between the terminals K 1 , K 2 , a Zener breakdown of the Zener diode D 1 can occur. The mode of operation of the circuit arrangement for this case corresponds to the mode of operation of the circuit arrangement shown in FIG. 2 .

For a negative voltage between the first and second terminals K 1 , K 2 on account of an electrostatic discharge, the Zener voltage of the Zener diode D 2 is exceeded and a current starts to flow through it. Said current causes a voltage drop at the resistor R 4 , by means of which the gate of the p-channel field effect transistor T 4 is turned on. Since the p-channel field effect transistor T 4 is connected to the control input B of the diverting element T 1 , the latter is also turned on, with the result that the terminals K 1 , K 2 are connected in low-impendence fashion via the diverting element T 1 . Consequently, the electrostatic discharge can once again be reliably diverted via the diverting element.

FIG. 5 shows an exemplary voltage-current diagram in accordance with a circuit arrangement according to FIG. 3 or FIG. 4 . The illustration shows the current via the diverting element T 1 as a function of the voltage between the terminals K 1 , K 2 . For positive voltages, an appreciable current flow is discernable only starting from a positive threshold value VBP. Likewise, an appreciable current flows for negative voltages only starting from a negative threshold value VBN. As a result, it is possible for a circuit that is to be protected to be operated at the terminals K 1 , K 2 with signal voltages or operating voltages that have positive or negative polarity and in this case lie between the negative threshold value VBN and the positive threshold value VBP. A triggering of the circuit arrangement according to one of the embodiments shown, that is to say a low-impedance connection of the first and second terminals K 1 , K 2 , is therefore effected only if higher or lower voltages than the respective threshold values are present at the terminals K 1 , K 2 , for example on account of an electrostatic discharge.

FIG. 6 shows a further exemplary embodiment of the circuit arrangement. In addition to the arrangement shown in FIG. 4 , a resistor R 5 and an NPN bipolar transistor T 5 are provided. The resistor R 5 is connected between the first terminal K 1 and the transistor T 4 . The transistor T 5 is coupled to the first terminal K 1 and to the control input B. The control terminal of the transistor T 5 is connected to the connecting node between the resistor R 5 and the transistor T 4 .

If the Zener diode D 2 undergoes transition to the conducting state for a high negative voltage between the terminals K 1 , K 2 and the second trigger signal turns on the transistor T 4 into the conducting state, then a voltage drop across the resistor R 5 is also generated by means of the current flow via the transistor T 4 . The transistor T 5 is turned on by the ever increasing voltage drop across the resistor R 5 and helps to turn on the diverting element T 1 . The NPN transistor T 5 used can be dimensioned in such a way that it has a voltage snapback in the case of a specific voltage value or a current value resulting therefrom and can thereby divert even large quantities of current. The transistor T 5 thus supports the transistor T 4 in the driving of the diverting element T 1 . In the event of a voltage snapback, the transistor T 5 acquires very low impedance and turns on the diverting element T 1 in such a way that the latter can divert an ESD pulse even better. Since the voltage snapback does not take place directly when the trigger voltage is reached, this may also be referred to as a voltage snapback that occurs in a delayed fashion.

FIG. 7 shows a further exemplary voltage-current diagram for application of the circuit arrangement shown in FIG. 6 . For positive voltages, the diagram corresponds to the diagram shown in FIG. 5 . For negative voltages, a current flow is discernable for voltages below a first negative threshold value VBN 1 . At negative voltages of greater magnitude, starting from a second negative threshold value VBN 2 a voltage snapback occurs, starting from which part of the current flow between the first and second terminals K 1 , K 2 is effected via the transistor T 5 . The dashed line indicates that the second negative threshold value VBN 2 can be varied by corresponding dimensioning of the transistor T 5 and/or of the resistor R 5 . Likewise, the transistor T 5 and/or the resistor R 5 can be dimensioned in such a way that no voltage snapback occurs via the transistor T 5 and the latter acts only to support the transistor T 4 in the driving of the diverting element T 1 .

When using a circuit arrangement in an embodiment with a voltage snapback that occurs in a delayed fashion, care should be taken to ensure that generally no signal voltages that are greater in magnitude than the negative holding voltage VH to which the voltage snapback takes place occur at the terminals K 1 , K 2 . The diverting element T 1 or the transistor T 5 may otherwise be destroyed owing to permanently occurring high currents.

FIG. 8 shows a further exemplary embodiment of the circuit arrangement. The second trigger element TR 2 and the second amplifier unit V 2 correspond in terms of construction and function to the embodiment shown in FIG. 4 . The first amplifier unit for amplifying the first trigger signal comprises only the p-channel field effect transistor T 2 . In the event of a Zener breakdown of the Zener diode D 1 , the potential at the gate terminal of the transistor T 2 is pulled in the direction of the potential at the second terminal K 2 . A voltage corresponding to the Zener voltage of the Zener diode D 1 remains, however. Consequently, the transistor T 2 can be turned on to a somewhat lesser extent than shown in FIG. 4 , for example. This may nevertheless be sufficient, depending on the requirement made of the circuit arrangement for protection against electrostatic discharges. Particular advantages arise on account of a somewhat lower outlay on circuitry.

FIG. 9 shows an exemplary embodiment of a circuit arrangement, in which an n-channel field effect transistor T 6 is provided for the triggering for negative voltages on account of an electrostatic discharge with a negative pulse, the controlled path of said transistor being connected between the first terminal K 1 and the control input B and the gate terminal of said transistor being coupled to the first terminal K 1 via a resistor R 6 .

In this embodiment transistor T 6 serves both as a trigger element and as an amplifier unit. The trigger function is realized by means of a reverse-biased junction within the field effect transistor T 6 and a respective break-down of said junction. Amplifying is performed by a lateral NPN-transistor comprised by transistor T 6 , wherein also snapback effects can be utilized. Transistor T 6 can be a conventional n-channel field effect transistor or as a non-limiting example a gate coupled n-channel field effect transistor.

FIG. 10 shows an exemplary voltage-current diagram for a circuit arrangement having this construction. For positive voltages, the current profile once again corresponds to the current profile shown in FIG. 5 . For negative voltages it is evident that when a negative threshold value for the voltage is undershot, a breakdown with simultaneous voltage snapback to a holding voltage VH occurs, that is to say that the transistor T 6 remains conducting up to the holding voltage VH. When using a circuit of this type, care should once again be taken to ensure that generally no signal voltages that are more negative than the holding voltage VH are present at the terminals K 1 , K 2 , in order to prevent destruction of the transistor T 6 and/or the diverting element T 1 .

By way of example, n-metal oxide semiconductor, NMOS transistors can be used for the n-channel field effect transistors. PMOS transistors can likewise be used for the p-channel field effect transistors. Furthermore, PNP bipolar transistors can also be used instead of PMOS transistors, and vice versa. Equally, NMOS transistors and NPN bipolar transistors can also be mutually interchanged.

Instead of Zener diodes, it is possible to use in the trigger elements field effect transistors, for example, which have a corresponding field threshold voltage by means of which a positive or negative threshold value can be set.

An interconnection of a trigger element with an amplifier unit connected downstream as shown in the exemplary embodiments may also be regarded as a cascaded trigger.

The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in each novel characteristic and each combination of characteristics, which includes every combination of any features which are stated in the claims, even if this feature or combination of features is not explicitly stated in the examples.