BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to integrated circuits, and more specifically to a method for carrying out a trimming operation in a closed condition and to a corresponding trimming circuit.
2. Description of Related Art
This present invention relates, particularly but not exclusively, to an improved method for carrying out a trimming operation on integrated circuits having a trimming circuit portion which includes memory elements and means of modifying the state of such memory elements, at least a first input or supply pin, an output pin, and a second supply pin. The present invention also relates to an electronic circuit for carrying out a trimming operation on portions of integrated circuits having at least a first input or supply pin, an output pin, and a second supply pin, the electronic circuit including at least memory elements and a means of modifying the state of such memory elements.
On completion of a conventional integrated circuit fabrication process (e.g., after the packaging operation), a circuit calibrating or “trimming” operation is carried out. For example, it may be necessary to “trim” the output voltage Vout of a voltage regulator. The trimming of an integrated circuit is usually performed at the EWS (Electrical Wafer Sort) test stage, that is while the circuit is still held to the wafer of semiconductor material. However, in certain cases, especially those commanding high precision, the trimming is also applied at the final testing stage, that is after the packaging operation.
Pins must be made available to access the integrated circuit for trimming purposes, either in the form of specially arranged pins or of multi-function pins which are biased to outside their normal range of operating values and allow the trimming operation to be selected and activated. However, some integrated circuits often have a very limited number of pins. For example, a linear voltage regulator may include no more than a supply pin, an output pin and a ground pin. With so small a number of pins, it is practically impossible to carry out post-packaging trimming operations, and in such cases the trimming must be performed on-wafer.
To better appreciate the problems involved in a trimming operation, reference can be made to the accompanying FIG. 1, in which a linear voltage regulator incorporating a prior art trimming circuit is depicted schematically.
The regulator of FIG. 1 is intended for applications with a small voltage drop between the input and the output. This regulator includes: a power transistor PT representing the regulating element; an error amplifier OPAMP driving the power transistor; a charge pump voltage booster circuit CHARGE PUMP providing a boosted voltage to the control terminal of the power transistor; an internal voltage reference BANDGAP REFERENCE; and a voltage divider which comprises a pair of resistors R1 and R2 and is used to fix the value of the output voltage. This voltage regulator further includes a trimming circuit for on-wafer application. The voltage divider and error amplifier form a negative feedback regulating loop.
The trimming circuit comprises a selection portion SELECTION LOGIC based on a combinational logic, and a memory portion. The selection portion acts directly on the calibrating elements, and the memory elements can be accessed from zapping pins through which a voltage or current can be forced.
The memory elements may be, for example: ZAP-type Zener diodes, being components which perform in an open circuit mode until the voltage across them exceeds a predetermined threshold value, and when this threshold is exceeded while a large current (such as a few hundred mA) is being flowed, the ZAP diode changes its characteristic permanently and, in particular, becomes a resistive component (such as of a few Ohms); integrated fuses which change their state from short circuit to open circuit upon an appropriate current being passed therethrough; and integrated non-volatile memories (EEPROMS, EPROMs, etc.) wherein a suitable sequence of bits is stored for subsequent use, by appropriate circuitry, to force integrated microswitches to the closed or the open state.
The solutions based on the use of diodes or fuses have an advantage in that they involve no complex managing circuitry. In fact, the forcing of a voltage and/or a current is all that is required in order to set the memory elements. However, they do require dedicated contact pins for addressing each component at the EWS testing stage. Thus, if a post-packaging trimming is to be carried out, additional pins must be provided for the purpose.
The solution based on the use of integrated memories can indeed work with a smaller number of pins. In fact, no more than two pins need to be added, one for a serial data input and another for placing the device in the trimming mode. In some instances, these two additional inputs can be obtained exploiting a third logic level at some pin which are already in the integrated circuit. Where at least two pins are available in the circuit, this operation would be carried out using the Reset and Enable pins; for example, the RESET function is taken away from the managing logic and assigned to the affected analog part.
These methods do afford savings in the number of pins required, but have the following disadvantages. They require a huge silicon area for the circuitry arranged to manage the trimming and writing into memory; this becomes a significant problem where the number of parameters to be trimmed is large. In devices realized with combined technologies, such as BCD technologies, the realization of integrated memories requires at least three additional masks to the basic process, thereby dilating manufacturing costs. It would be difficult, with integrated circuits having a small number of pins, to find two available pins among those provided and check the condition of the rest, considering that the entire operation is to be carried out with the device alive.
There is also a problem of “simulating” the effects of trimming; in fact, once data have been inserted and before writing them permanently into the memory, it would be possible to simulate the effects of the intended trimming operation. The “simulation” consists in closing and/or opening internal switches and measuring the values of the parameters to be trimmed. The measurements frequently need the device in a normal condition of its operation, so that the pin being used to enter the trimming mode must be allowed to operate as normal. However, this results in lost control of the trim handling circuitry.
SUMMARY OF THE INVENTION
In view of these drawbacks, it is an object of the present invention to overcome the above-mentioned drawbacks and to provide a new trimming method and circuitry with suitable functional and structural features to enable trimming operations to be carried out in a simple manner, without requiring pins other than those already present on the integrated circuit to be trimmed.
Another object of the present invention is to allow a single pin to be used to force an integrated circuit into a trimming mode and input serially both the simulation data and the trimming data proper. In this way, a single pin (e.g., used outside its operating voltage range) can be shared to enable the trimming circuitry and to input the data serially.
One embodiment of the present invention provides a method for carrying out a trimming operation on an integrated circuit having a trimming circuit portion which includes memory elements and a modification circuit for modifying the state of the memory elements, at least a first input or supply pin, an output pin, and a second supply pin. According to the method, a single pin is enabled to receive trimming data by biasing the pin to outside its operating range. A clock signal is obtained from a division of the bias potential of the pin, and the logic value of the trimming data is obtained from a different division of the bias potential of the pin. Serial acquisition of the data is enabled in accordance with the clock signal, and the data is transferred to the modification circuit. In a preferred embodiment, the data is transferred to selection logic with the modification circuit bypassed on the occurrence of a simulated trimming operation.
Another embodiment of the present invention provides an electronic circuit for carrying out a trimming operation on circuit portions of an integrated circuit having at least a first input or supply pin, an output pin, and a second supply pin. The circuit includes memory elements, means for modifying the state of the memory elements, and first and second comparators having their inputs connected to a trimming pin to produce a clock signal for the trimming operation and a sequence of trimming data. A single trimming pin is used. In one preferred embodiment, the first comparator has an input coupled to the trimming pin via a voltage divider, and the second comparator has an input coupled to the trimming pin via a different node of the voltage divider.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only and various modifications may naturally be performed without deviating from the present invention.