Method and circuit to perform a trimming phase on electronic circuits

Abstract: The invention relates to a method and a circuit for carrying out a trimming operation on integrated circuits (2) having a trimming circuit portion (1) which includes memory elements (10) and a means (8) of modifying the state of said memory elements (10), at least a first input or supply pin (IN), an output pin (OUT), and a second supply pin (GND). The method comprises the following steps: enabling a single pin (OUT) to receive trimming data by biasing the pin to outside its operating range; to acquire such data, obtaining a clock signal from a division of the bias potential of the trimming pin (OUT); obtaining the logic value of the trimming data from a different division of the bias potential of said pin (OUT); enabling serial acquisition of the data according to the clock signal; and transferring the data to the means (8) of modifying the state of the memory elements (10). Advantageously, the data are also transferred into a selection logic (11), by-passing the means (8) for modifying the state of the memory elements (10), on the occurrence of a simulated trimming operation.

Description: Field of the Invention This invention relates to a method for carrying out a trimming operation in a closed condition, and to its relevant trimming circuit for implementing the method. The invention relates, particularly but not exclusively, to a new 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 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. Prior Art As is well known in this technical field, on completion of an integrated circuit fabrication process, e.g. after the packaging operation, a circuit calibrating or "trimming" operation is to be 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 Figure 1, where a linear voltage regulator incorporating a prior art trimming circuit is depicted schematically. The regulator of Figure 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, 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; when this threshold is exceeded while a large current (a few hundred mA) is being flowed, the ZAP diode changes its characteristic permanently and, in particular, becomes a resistive (a few Ohms) component; integrated fuses which change their state from short circuit to open circuit upon an appropriate current being passed therethrough; 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. EP 0 578 252 A discloses such a solution. 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 measurement 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. The underlying technical problem of this invention is to provide a new trimming method and circuitry with suitable functional and structural features to enable the trimming operations to be carried out in a simple manner, without requiring other pins than those already present on the integrated circuit to be trimmed. Summary of the Invention The concept behind this invention is one of using a single pin to force the integrated circuit into a trimming mode and input serially both the simulation data and the trimming data proper. In this way, a single pin, used outside its operating voltage range, can be shared to enable the trimming circuitry as well as to input the data serially. Based on this concept, the technical problem is solved by a trimming method as previously indicated and defined in the characterising portions of Claim 1 foll. The technical problem is further solved by a trimming circuit as defined in Claim 9 foll. The features and advantages of a circuit according to the invention will be apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is a block diagram of an integrated circuit including a trimming circuit portion according to the prior art; Figure 2 shows diagramatically an integrated circuit having a trimming circuit portion according to the invention. Detailed Description Referring to the drawing views, and in particular to the example of Figure 2, a trimming circuit portion for carrying out a trimming operation on an integrated circuit 2 incorporating the circuit portion, according to the invention, is generally shown at 1 in schematic form. The integrated circuit 2 is a linear voltage regulator 3 having three access pins: a first input or supply pin IN, an output pin OUT, and a second supply pin which is a ground biased pin GND in the example. A different type of integrated circuit could be used instead, and the following discussion will take in no consideration the actual function of the integrated circuit 2. A voltage regulator 3 as indicated has been chosen for convenience of illustration only. Advantageously in this invention, the trimming mode is enabled by biasing the output pin OUT to outside its operating range, above a threshold voltage which is set by a comparator. In detail, the voltage regulator 3 comprises: a power transistor PT which represents the regulating element; an error amplifier OPAMP driving the power transistor PT; a charge pump type of booster circuit 4 supplying a necessary boosted voltage to the control terminal of the power transistor; and an internal voltage reference 5, e.g. a bandgap reference. Unlike the conventional voltage regulator shown in Figure 2, this regulator 3 includes a voltage divider 6 connected between the output pin OUT and the ground pin GND and comprising a series of resistors R1a, R1b, R1c, R2. The value of the output voltage Vout is set by the divider 6. The construction of the trimming circuit portion 1 associated with the voltage regulator 3 will now be described in detail. This portion 1 comprises a first comparator COMP1 and a second comparator COMP2, each having an inverting (-) input and a non-inverting (+) input. The inverting inputs (-) of the comparators are held at a potential Vbg supplied by the bandgap reference 5 output from the regulator 3. The non-inverting input of the first comparator COMP1 is connected to a node of the resistive divider 6, specifically to the interconnect node between the resistors R1b and R1c. The non-inverting input of the second comparator COMP2 is connected to a node of the resistive divider 6, specifically to the interconnect node between the resistors R1a and R1b. The output of the first comparator COMF1 is connected to a clock input of a shift register 7 of "n+2" order or length. The output of the second comparator COMP2 is connected to a data input of the shift register 7. This shift register 7 has a set of n data outputs (0,..., n-1) connected directly to corresponding inputs of a block 8 of zapping switches. These n outputs are also connected to corresponding inputs of a multiplexer 9. The last two n and n+1 outputs of the register 7 are connected to the blocks 8 and 9. In particular, the n-th output is connected to a pretrimming simulation input of the multiplexer 9, and the (n+1)-th output is connected to a selection input of the zapping switch block 8. Memory elements or fuses 10, zap diodes, or any other suitable memory elements, are placed between the zapping switch block 8 and the multiplexer 9. Located downstream of the multiplexer 9 is a selection logic 11 having its output connected to the circuit portion to be calibrated, specifically to the bandgap reference 5. The way of applying the inventive method will now be described. As previously mentioned, the trimming mode is enabled by suitably biasing the output pin OUT to outside its normal operating range, thereby surpassing the threshold set by the first comparator COMP1. Exceeding this first threshold generates moreover a front of the clock signal which is useful to enable the shift register 7 to acquire data through the data input downstream of the second comparator COMP2. The clock signal is thus generated by reiterate crossings of the threshold of the first comparator COMP1. The data sequence is input through the pin OUT and the second comparator COMP2. Exceeding the threshold of this comparator COMP2 requires in a higher potential being attained than the threshold of the first comparator COMP1, which enables the logic one and logic zero to be discriminated. More particularly, for a logic "zero" to be input, only the lower threshold that also clocks the clock signal need to be crossed, whereas to input a logic "one", both thresholds need to be exceeded. The trimming operation data are stored into the shift register. The first n bits represent the result itself, that is the state sought for the memory elements 10, and the last two bits are used to select between the trimming simulation and the actual trimming operation. In particular, the n-th output of the block 8 selects the simulated trimming function through the corresponding input of the multiplexer 9, whereas the (n+1)-th output of the block 8 selects the actual trimming operation by operating the zapping switches in the block 8. The zapping switches fuses or zap diodes, act directly on the memory elements 10, which by forcing a voltage and/or current dependent on the type of element being used. In this way, the structure and state of the memory element are modified permanently. The selection logic 11 has the same function as that illustrated in Figure 1. This logic is usually a combinational logic acting directly on the calibrating elements of the circuitry being calibrated which, in this exemplary embodiment, is represented by the bandgap reference block 5. As previously described this method can be applied to any integrated circuits having at least one pin different from the input or supply pin and the ground pin. Thus, the circuitry and method of this invention do solve the technical problem, offering several advantages. The advantages of the method according to the invention can be summarized as follows: the trimming operation can be carried out after the packaging operation, even on devices having a limited number of pins for connection to the outside world; the device can be made to enter the trimming mode, and trimming data can be simultaneously input, by single-pin sharing; the trimming circuit portion is quite simple and requires no internal clock signal to be generated. Illustratively, it should be noted that the circuit portion of this invention was implemented by the Applicant in combination with a linear voltage regulator having an output voltage that could be calibrated at different values, namely: 1.25V, 1.5V, 1.8V, 2.5V, 3.3V and 5.0V. The clock signal generation threshold and the data input threshold were selected at 180% and 220% of the output voltage value. For example, with Vout=2.5V, the threshold of the first comparator COMP1 was set at 4.5V, and the threshold of the second comparator COMP2 set at 5.5V.

Claims: A method of carrying out a trimming operation on integrated circuits (2) having a trimming circuit portion (1) which includes memory elements (10) and means (8) for modifying the state of said memory elements (10), as well as at least a first input or supply pin (IN), an output pin (OUT), and a second supply pin (GND), the method being characterized by : enabling a single pin, other than the first input or supply pin (IN) or the second supply pin (GND), to receive trimming data by biasing said single pin to outside its operating range; to acquire said trimming data, obtaining a clock signal from a division of the bias potential of said single pin; obtaining the logic value of the trimming data from a different division of the bias potential of said single pin; enabling serial acquisition of said trimming data according to said clock signal; transferring said trimming data to said means (8) for modifying the state of said memory elements (10). A method according to Claim 1, characterized in that said trimming data are also transferred to selection logic (11) with said means (8) by-passed on the occurrence of a simulated trimming operation. A method according to Claim 1, characterized in that the transfer operation is carried out by means of a shift register (7). A method according to Claim 3, characterized in that said shift register (7) receives said trimming data on one input and said clock signal on another input. A method according to Claim 1, characterized in that said clock signal is obtained by comparing a division of the bias potential on said single pin with a reference potential (Vbg). A method according to Claim 1, characterized in that the logic value of the trimming data is obtained by comparing a division of the bias potential on said single pin with a reference potential (Vbg). A method according to Claims 5 and 6, characterized in that the trimming data logic value comparation threshold is higher than the comparation threshold whereat the clock signal can be obtained. An electronic circuit (1) for carrying out a trimming operation on circuit portions (3) of integrated circuits (2) having at least a first input or supply pin (IN), an output pin (OUT), and a second supply pin (GND), the electronic circuit including at least memory elements (10) and a means (8) of modifying the state of said memory elements (10), characterized in that a single trimming pin, other than said first input or supply pin (IN) or said second supply pin (GND), is used, and that it further comprises first (COMP1) and second (COMP2) comparators having their inputs connected to said single trimming pin to obtain a clock signal for the trimming operation and a sequence of trimming data, respectively. A circuit according to Claim 8, characterized in that said first comparator (COMP1) has an input coupled to said single trimming pin via a voltage divider (6), and that said second comparator (COMP2) has an input coupled to said single trimming pin via a different interconnect node of said voltage divider (6). A circuit according to Claim 9, characterized in that it comprises a shift register (7) connected downstream of said comparators (COMP1, COMP2). A circuit according to Claim 10, characterized in that said shift register (7) is of the n+2 order. A circuit according to Claim 11, characterized in that said means (8) of modifying the state of said memory elements (10) is connected downstream of said shift register (7). A circuit according to Claim 11, characterized in that said shift register (7) receives trimming data on one input from the output of the second comparator (COMP2), and receives a clock signal on another input from the output of the first comparator (COMP1). A circuit according to Claim 11, characterized in that said shift register (7) has its output connected to selection logic (11), by-passing said means (8) of modifying the state of said memory elements (10) on the occurrence of a simulation of the trimming results. A circuit according to Claim 14, characterized in that it includes a multiplexer (9) between the outputs of said shift register (7) and said selection logic (11). A circuit according to Claim 15, characterized in that an n-th output of said shift register (7) is connected to an input of the multiplexer (9) to select a simulated trimming operation. A circuit according to Claim 14, characterized in that said selection logic has its output connected to a voltage reference (5) of said circuit portion (3) producing a reference voltage signal for the integrated circuit (2).

Inventors: PULVIRENTI FRANCESCO, PULVIRENTI, FRANCESCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2000
Main classification: H01L21
IPC codes: G11C17, G01R31