Projection device against electrostatic discharges

A device for the protection of an integrated circuit input/output pin against electrostatic discharges includes a first diode between a positive power supply line and an internal connection node for connection to the pin, and a second diode between the internal node and a second negative or zero supply line. The device also includes a protection transistor series-connected between the positive power supply line and the first diode, and a stack of N diodes, where N is equal to one or more, series-connected between the control electrode of the protection transistor and the first diode.

FIELD OF THE INVENTION 
The present invention relates to the field of integrated circuits, and, 
more particularly to protecting integrated circuits against electrostatic 
discharges. 
BACKGROUND OF THE INVENTION 
A need to protect integrated circuits against electrostatic discharges is 
known. These electrostatic discharges occur in particular when the 
integrated circuits are not under power, for example when they are being 
manually handled. The discharges occur between two external pins. This is 
why protection devices are usually provided. They are placed on each 
input/output pin of the integrated circuit, and enable both the clamping 
of the voltage and the flow of the charges without destroying the 
integrated circuit or allowing the integrated circuit to be damaged. 
A prior art protection device as shown in FIG. 1 comprises, on each 
input/output pin of the integrated circuit, two series-connected diodes D1 
and D2 as shown on the pin P1 in the example. One of the diodes, D1, is 
between an internal point N1 of connection to the pin and an internal 
positive power supply line A. The other diode, D2, is between an internal 
negative or zero power supply line B and the same internal connection 
point N1. 
If the power supply potentials applied to the lines A and B are 
respectively Vcc, between 3 and 5 volts for example, and Vss, equal to 
zero volts, and if Vd1 and Vd2 denote the threshold voltage of the diodes 
D1 and D2, the voltages applied to the pin P are limited by the protection 
device to Vcc+Vd1 in positive terms and Vss-Vd2 in negative terms. This 
limiting, while it is satisfactory in the context of protection against 
electrostatic discharges, may prove to be highly inconvenient in 
operation. 
Indeed, this limiting also takes place in an operational mode. In certain 
applications, the voltage applied in the operational mode to certain 
input/output pins of an integrated circuit may go beyond the level of a 
power supply voltage. An exemplary application in which voltage levels 
going beyond the level of a power supply voltage are used operationally, 
relates to the field of radio frequencies. 
In certain radio-frequency applications, a reference signal is used in 
reception with respect to a virtual ground. For reasons of coupling 
capacitance, this virtual ground may be chosen in particular to be equal 
to the level of the positive supply voltage, Vcc in the example. A signal 
is then received at a pin of the integrated circuit. The DC value of this 
signal is linked to the level of the positive supply voltage Vcc and its 
AC value oscillates around this DC value. In the operational mode, the 
instantaneous value of the voltage at the associated pin will therefore 
regularly go beyond the level of the positive supply voltage Vcc. If the 
device of FIG. 1 for protection against electrostatic discharges is placed 
on this pin in the operational mode, then this device will get activated. 
The integrated circuit therefore cannot fulfill its function. The prior 
art protection device is therefore not compatible with applications of 
this type. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a device which protects against 
electrostatic discharges that does not have the drawbacks of the prior 
art. 
The invention includes placing a protection transistor between the diode D1 
and the positive power supply line, and connecting a stack of N 
series-connected diodes between the control electrode of the transistor 
and the diode D1, where N is at least equal to one. In practice, the 
number N of diodes of the stack depends on the threshold with respect to 
Vcc at which it is desired that the protection transistor should get 
activated. Preferably, the protection transistor used is a bipolar type 
transistor. 
The invention therefore relates to a device for the protection of an 
input/output pin of an integrated circuit against electrostatic 
discharges. The device includes a first diode D1 between a positive power 
supply line and an internal connection node for connection to the 
input/output pin and a second diode between the internal node and a second 
negative or zero supply line. According to the invention, the device 
further includes a protection transistor series-connected between the 
positive power supply line and the first diode. Also, a stack of N diodes, 
where N is at least equal to one, are series-connected between the control 
electrode of the protection transistor and the first diode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2a shows a protection device according to the present invention 
applied to an integrated circuit input/output pin P1. It comprises a diode 
D2 between the internal connection node N1 of the pin P1 and the negative 
or zero power supply line B. This diode is normally reverse-biased. It has 
its cathode connected to the node N1 and its anode connected to the line 
B. 
The device further comprises a protection transistor T1 and a diode D1 
series-connected between the positive power supply line A and the internal 
connection node N1. Preferably, and as shown, the protection transistor T1 
is a bipolar transistor, for example, an NPN type transistor. Its 
collector is connected to the positive power supply line A and its emitter 
is connected to the cathode of the diode D1. The diode D1 is normally 
reverse-biased. Its anode is connected to the internal node N1. 
A stack 1 of N series-connected diodes Di, Dj, Dz are connected between the 
diode D1 and the control electrode 2. These N diodes are, like the diode 
D1, normally reverse-biased. The first diode Di has its anode connected to 
the cathode of the diode D1 while the last diode Dz has its cathode 
connected to the control electrode 2 of the protection transistor T1. In 
the example, since it is a bipolar transistor, this control electrode is 
the base. A resistor 3 is provided between the positive power supply line 
A and the control electrode 2. The diodes of the stack 1 are in practice 
identical, with a threshold voltage referenced Vd. 
In normal operation, so long as the voltage at the pin P1 is below 
Vcc+Vd1+N*Vd, no current flows into the series circuit formed by the 
resistor 2, the stack 1 and the diode D1. The transistor T1 is off. In the 
event of an electrostatic discharge on the input/output pin P1, the 
voltage at this pin goes beyond the level Vcc+Vd1+N*Vd. The stack 1 of the 
N diodes and the diode D1 then let through the current. This leads to 
conditions on the control electrode 2 that are sufficient to activate the 
protection transistor T1. The discharge current then goes through this 
protection transistor T1, which offers very low resistance, and the diode 
D1. In practice, the transistor T1 is sized to withstand a typical 
discharge current. Also, there is a capacitor 4 provided and 
parallel-connected to the resistor 3. In the event of discharge, it 
assists in the activation of the structure formed by the diodes Di, Dj, Dz 
and the transistor T1. 
In practice, the diodes D1, D2 and the N diodes of the stack are not 
necessarily made with the same technology. With regard to the diodes D1 
and D2, a technology is chosen that preferably provides the lowest 
possible equivalent capacitance so as to minimize the equivalent 
capacitance of the pin P1 as experienced from the internal circuitry of 
the integrated circuit. This requirement can also be found for example in 
radio-frequency applications. It is possible, for example, to make the 
diode D2 by a PN junction of the bipolar transistor base type. The diode 
D1 could be made according to a similar technique. 
The diodes of the stack do not have the same requirements. In practice, it 
is possible to make them of bipolar transistors as shown in FIG. 2b, each 
diode being formed by a bipolar transistor whose base and emitter are 
short circuited. FIG. 2b also shows a alternative embodiment of the 
protection device in which there is provided a current limitation resistor 
RE placed between the pin P1 and the internal connection node N1. 
FIG. 3 shows the general principle of protection of an integrated circuit 
against electrostatic discharges. A discharge takes place between two 
input/output terminals of the integrated circuit. The input/output 
terminals P1 and P2 have an associated protection device added to each, 
identical to that of FIG. 2a. We thus have the device DP1 for the pin P1 
and the device DP2 for the pin P2. There is further provided a clamping 
device DP3, connected between the two power supply lines A and B. The role 
of this clamping device is to provide a path of least resistance in the 
integrated circuit in the event of electrostatic discharge. Thus, it is 
certain that the discharge current will flow through this path of least 
resistance and not damage the integrated circuit. 
For example, if the electrostatic discharge results in a positive surge 
voltage on the pin P1 and a negative surge voltage on the pin P2, the 
discharge current will flow through the diode D1 and the transistor T1 of 
the device DP1, through the clamping device DP3 and then through the diode 
D2 of the device DP2, as indicated by the arrows in FIG. 3, without 
damaging the integrated circuit. The protection device associated with 
each input/output pin of the integrated circuit according to the invention 
thus makes it possible to adjust the voltage clamping threshold by 
enabling the choice, for the stack 1, of an appropriate number N of 
diodes, at least equal to one, while at the same time dictating a path for 
the passage of the discharge current with only one transistor and one 
diode. 
Finally, the equivalent capacitance experienced by the internal circuitry 
is only that due to the diodes D1 and D2, and to the metals used to make 
the contact on the pin, because the rest of the circuitry (transistor T1, 
stack 1 of N diodes) is series-connected with the diode D1. Thus, the 
protection device according to the invention makes no change in the 
capacitance experienced by the internal circuitry and therefore does not 
cause any deterioration in the characteristics of the integrated circuit. 
Hence, a very low equivalent capacitance is kept. This is very important 
for RF applications. 
FIG. 4 shows an exemplary embodiment of the clamping device DP3 of FIG. 3. 
The function of the clamping device is to activate when a voltage 
threshold is crossed. The activation corresponds to a short-circuiting of 
the two power supply lines A and B. In this way, the discharge current 
necessarily passes through this short circuit. This prevents the 
integrated circuit from being damaged by a strong current in the 
substrate. 
The example shown in FIG. 4 pertains to a dynamic clamping device, namely 
one that will activate only on a very fast voltage variation. It is also 
possible to make use of static clamping devices which are well known. The 
advantage of using a dynamic clamping device lies in its speed of 
activation. The dynamic clamping device shown in FIG. 4 thus comprises an 
activation stage comprising a capacitor C1 and a resistor R1, 
series-connected between the two power supply lines A and B, the 
connection point NC1 between the capacitor and the resistor providing an 
activation signal to a cascade structure of transistors. In the example, 
this cascade structure comprises three transistors T2, T3 and T4. A first 
transistor T2 is series-connected with a resistor R2 between the two power 
supply lines A and B. Its control electrode is connected to the connection 
point NC1. A second transistor T3 is series-connected with a resistor R3 
between the two power supply lines A and B. Its control electrode is 
connected to the connection point NC2 between the first transistor T2 and 
the resistor R2. A third transistor T4 is connected between the two power 
supply lines A and B. Its control electrode is connected to the connection 
point NC3 between the second transistor T3 and the resistor R3. 
In practice, the second transistor of the cascade structure, T3 in the 
example, is very wide. It is this transistor which, when it is on, offers 
the least resistance and will therefore be the main element that bears the 
discharge current. 
In the example, and as shown in the figure, these transistors are NPN type 
bipolar transistors. The capacitor C1 then has a pin connected to the 
positive supply voltage. The other pin is connected to one pin of the 
resistor R1, the other pin of R1 being connected to the negative or zero 
power supply line B. Similarly, the transistors T2, T3, T4 each have their 
emitter connected to the positive power supply line and their collector 
connected to the negative supply voltage, by a resistor as the case may 
be. With MOS type transistors, an identical structure working in the same 
way is obtained. This structure is particularly efficient in terms of 
short circuits. The activation signal given by the connection point NC1 
between the capacitor C1 and the resistor R1 almost instantaneously 
activates all the transistors in cascade.