Abstract:
To monitor the state of an antifuse capacitor, a transistor is connected to the capacitor such that it saturates only when the capacitor is not blown. Monitoring the base current of the transistor allows the state of the capacitor to be monitored without needing to use a conventional high-voltage comparator.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a trimming circuit, and in particular to a circuit which includes an antifuse, and means for detecting whether or not the antifuse has blown. 
     DESCRIPTION OF RELATED ART 
     Trimming circuits are known which include at least one antifuse, that is a fuse which, when blown, presents a short circuit across its terminals. 
     For example, in order to blow an antifuse capacitor, it may be necessary to apply a zapping voltage of several tens of volts across the terminals of the capacitor, whilst limiting the current. There is then a problem in sensing that the capacitor has blown. This arises because the sensing circuitry will then need to be able to distinguish between two voltages several volts apart and survive the zapping voltage of several tens of volts. Although high voltage comparator circuits are available, they are not easily incorporated into integrated circuit designs, because they require a relatively large area. This is a particular problem because a trimming circuit will typically include several antifuses, each requiring its own comparator, and so problems of size are multiplied accordingly. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome the problem of sensing when an antifuse capacitor has blown, by connecting a transistor to the capacitor, and relying on the fact that, in one state, the transistor will be saturated, and then detecting when saturation occurs. Specifically, the transistor may be connected to the antifuse capacitor such that, when the antifuse capacitor is not blown, the transistor is saturated and draws a large base current, whereas, when the antifuse capacitor is blown, the transistor is not saturated and draws a smaller base current. There is then provided means for monitoring the base current of the transistor, to determine whether the transistor is saturated, and hence to determine whether the antifuse capacitor is blown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a trimming circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a trimming circuit, based on an antifuse capacitor C, which is a poly1 to poly2 capacitor, with a value of 13.8 fF. The capacitor acts as an antifuse, which means that, when it blows, which is when a voltage in the region of 45-55 V is applied across its plates, it forms an essentially short circuit resistance of typically 80 Ω. The capacitor C is connected between a ground rail GND of the circuit and a node I1. Before the capacitor C is blown, this node is pulled up towards the positive supply voltage +V1 by a transistor T5. In order to blow the capacitor, a voltage of at least about -50 V must be applied at the node I1, and thereafter the node I1 is essentially at earth voltage, 0 V. 
     The circuit used to blow the capacitor consists of transistors T1, T2, T3 and T4 and resistors R1 and R2. The transistor T1 is connected in series with a resistor R1, and with the base-emitter junction of a transistor T2, between a positive voltage supply rail +V1 and the ground rail GND. The positive supply rail +V1 supplies a voltage in the region of +5 V. The collector of transistor T2 is connected to a current mirror circuit comprising transistors T3 and T4 and resistor R2, the emitters of the transistors and one end of the resistor being connected to the negative supply voltage -V2, which is greater than 60 V below GND, for example -70 V. The collector of transistor T4 is connected to the node I1. 
     Thus, with I2 at GND, transistor T1 is conducting, and there is a voltage drop across the resistor R1. The resulting current flows through transistor T2 to the current mirror circuit, resulting in a corresponding current flowing through transistor T4, which pulls the node Ii down towards -V2. 
     With the current flowing through T4, and I1 approaching -V2, there is a sufficient voltage across the antifuse capacitor C to blow it. 
     Once blown, the capacitor is equivalent to an 80Ω resistor, i.e. effectively a short circuit. The node I1 is thus pulled to GND. The current through T4 is then switched off by switching I2 to +V1. 
     Thus, before the capacitor C blows, the node I1 is at a potential close to +V1, during zapping it is several tens of volts below GND, and, after the capacitor C has blown, it is at GND, zero volts. The remainder of the circuitry is for the purpose of detecting this difference. Although this could be achieved using a conventional high voltage comparator, such comparators have considerable surface areas, and are thus unsuitable for integration, particularly since a device may include several such trimming circuits. 
     Therefore, in order to sense that the capacitor C has blown, the transistor T5, which is a PNP transistor connected at its collector terminal to the node I1, and at its emitter terminal to the positive supply voltage +V1, is used. 
     Resistor R3 has a voltage of approximately 3 V applied across it, which, with suitable component values, causes a current of approximately 5 μA to flow through it. A current mirror circuit comprising transistors T6 and T7 causes a similar current of 5 μA to flow through transistor T8, and this current then acts as a reference current. Transistors T5 and T8 form a further current mirror circuit, with their emitters connected to the positive supply rail +V1, and their bases connected together. The bases of transistors T5 and T8 are also connected to the emitter of a transistor T9, which acts as a protection diode, with its base and collector terminals connected to the positive supply rail +V1. This prevents the bases of transistors T5 and T8 from being pulled much above the positive supply voltage +V1, which might otherwise happen when the terminal I1 is pulled down to the negative supply voltage -V2, and the capacitor C suddenly blows and pulls the terminal I1 up to GND. In the absence of the protection diode T9, this would run the risk that the voltage change at I1 would be coupled through the collector-base capacitance of T5, and similarly through T8, and damage the transistor T6. 
     The bases of transistors T5 and T8 are also connected to the emitter of a transistor T10. Thus, the base currents of transistors T5 and T8 flow through transistor T10, and then normally flow through transistor T11 to GND, T11 being configured as a 0.5 μA current sink. The collector of transistor T10 is connected to the base of a further transistor T12, which has its emitter connected to the ground rail GND. Transistor T13 is connected between transistor T12 and the positive supply rail +V1, and its gate is connected to the gate of transistor T14, which is connected between the resistor R3 and the positive supply rail +V1. 
     When the capacitor C is blown, and the node I1 is at GND, the transistor T5 is unsaturated, and has a relatively small base current, which is able to flow through transistor T10 and the current sink T11 to GND. With the combined base currents of transistor T5 and T8 flowing through transistor T11 to GND, the collector of the transistor T10 is pulled close to GND. Thus, in this situation, transistor T12 is turned off. With transistor T12 switched off, current flows through transistor T13, and an output terminal I3 is pulled up towards the positive supply voltage. 
     By contrast, if the capacitor C is not blown, it will be charged by transistor T5, such that I1 is pulled close to +V1 and therefore T5 will be saturated, and will therefore draw a much larger base current, for example up to 5 μA. This is much larger than can flow through the current sink T11. The excess current therefore flows into the base of the transistor T12, which is therefore turned on, and so the output terminal I3 is pulled low towards the ground rail. 
     It is therefore possible to monitor the voltage at terminal I3 to determine whether transistor T5 is saturated, and hence whether antifuse capacitor C has blown. This allows the state of the antifuse capacitor to be monitored in a simple way, using a relatively small number of components, so that the circuitry can be included in an integrated circuit design.