Electronic integrated circuit including trimming means and method therefor

Abstract: The invention relates to a method and an electronic circuit for carrying out a trimming operation on integrated circuit portions (2) which are provided with at least first (D) and second (E) access terminals. The circuit of this invention comprises: memory elements; a circuit means for modifying, either temporarily or permanently, the state of the memory elements, which means includes: an error amplifier (4) having an input connected to the second terminal (E) for comparing an output voltage from the circuit portion (2) with an internal voltage reference (Vref), a comparator (5) having an input connected to the output of the error amplifier, a direct connection of the first terminal (D) to the comparator (5) input; a controlled switch (S2) between the output of the error amplifier (4) and the input of the comparator (5); a second comparator (15) having an input connected to the second terminal (E) and the output connected to control said controlled switch (S2); a control logic (18) for controlling the trimming operation; a serial interface (25) adapted to receive, from said first terminal (D), a data sequence relating to the trimming operation for said control logic (18), through another controlled switch (S1).

Description: Field of the Invention This invention relates to an electronic circuit for carrying out a trimming operation in integrated circuits. In particular, the invention relates to an electronic integrated circuit for carrying out a trimming operation on circuit portions which are provided with at least first and second access terminals, comprising: memory elements; a circuit means for modifying, either temporarily or permanently, the state of said memory elements, said means including: an error amplifier having an input connected to the second terminal for comparing an output voltage from the circuit portion with an internal voltage reference, a comparator having an input connected to the output of said error amplifier, a direct connection of the first terminal to said comparator input. As is well known in this specific technical field, whenever a high accuracy level is required of an integrated circuit, additional circuit portions must be associated with the circuit for the purpose of modifying certain circuit parameters at the integrated circuit testing stage. This modifying operation is commonly referred to as "trimming". The parameter modifications are usually performed at a wafer testing stage. In fact, integrated circuits are fabricated in wafers of a semiconductor material, and an EWS (Electrical Wafer Sort) test is conducted on the wafer prior to separating the circuits for subsequent packaging operations. In special cases, trimming may also have to be carried out at the device final testing stage. One example of a circuit that requires this type of modification would be a step-down switching voltage regulator as largely employed in a variety of applications on account of its good efficiency and accuracy. To fill the market's demand, new-generation regulators are to exhibit higher switching frequencies and allow for the use of smaller-size external components, so as to keep their space requirements and cost low. In particular, the latter requirements involve of necessity a minimum presence of external components. Also required is a high accuracy throughout the operational range (supply voltage, temperature, etc.), and by reason of the requisite accuracy, certain basic parameters of a device must be further trimmed after the device packaging. Prior Art Prior approaches to modifying certain circuit parameters after the packaging step have been known. These approaches essentially amount to using memory elements whereinto the information needed for trimming is saved. Such memory elements may be, for example: Zener diodes of the ZAP type, a feature of these components being that they behave as open circuits so long as the voltage thereacross does not exceed a predetermined threshold value; upon this threshold value being exceeded and a large current (a few hundred milliamperes) flowed, the ZAP diode changes its characteristic permanently, turning in particular into a resistive (a few Ohms) component; integrated fuses which change their state, from short circuit to open circuit, upon an appropriate current being flowed therethrough; integrated non-volatile memories (EEPROMs, EPROMs, etc.), whereinto an appropriate sequence of bits is stored for later use in forcing certain integrated microswitches to the closed or the open state by a suitable circuitry. A system and method for calibrating a chip after packaging using a non-volatile storage memory is described in the US patent No. 5,671,183 issued on September 23, 1997. The above approaches do have merits, but also some deficiencies. The two approaches first mentioned above have the advantage of requiring no complicated managing circuitry; in fact, the fuses can be set by the mere forcing of a voltage and/or a current, but they need dedicated contact pins for addressing individual components at the EWS testing stage; in particular, additional pins must be specially There also have been proposals for minimizing the number of additional pins. For example, European Patent Application No. 98830521.5 with publication number EP-A1-0 986 105 published on 15.03.2000, by this Applicant discloses a method whereby the pins can be reduced in number, compared to other prior proposals, down to that of the trimming circuits plus one. Said document merely constitute prior art under Article 54(3) EPC. It should be noted that the methods based on the use of integrated memories actually require less pins. In fact, only two additional pins are needed: one each for a serial data input and for forcing the device to enter the trimming mode. In some cases, the additional two inputs can be obtained by utilizing a third logic level on a pin of the integrated circuit. For example, by raising the voltage at a given pin above its specified upper limit, a comparator is caused to operate and place the circuit in the trimming mode. The internal data line can then be connected to the given pin, once this is disconnected from the line to which it was connected in normal operation; in this way, all the circuitry needed for managing the trimming operations can be enabled. This operation is usually carried out through the Reset and Enable pins. For example, RESET is taken away from the managing logic, and ENABLE is given to the affected analog portion. Such methods, although keeping the required number of pins low, have disadvantages as specified herein below. They require more silicon area for the trim and memory write managing circuitry. This can be a problem where a large number of parameters are to be trimmed. With devices fabricated with mixed technologies, such as BCD technologies, the fabrication of integrated memories involves the addition of at least three extra masks to the basic process, which is cost-intensive. With integrated circuits having a small number of pins, selecting the two pins to be used from the available ones and checking the state of the others may prove difficult, especially when it is considered that the whole operation is to be carried out on a live device. Finally, there is the problem of "simulating" the effects of trimming. After the data is entered, but before it is permanently written into the memory, the effects of the trimming operation about to be carried out must be simulated. The "simulation" consists of closing and/or opening internal switches and measuring the values of the parameters to be trimmed. The measurement often requires that the device condition be that of normal operation, so that a pin being used for entering the trim mode must be allowed to operate as normal. But this results in the trim managing circuitry going out of control. The underlying technical problem of this invention is to provide a novel trimming system with suitable structural and functional features for the trimming operations to be carried out in a simple manner, similar to the fuse arrangement but with no need for additional pins to those already provided in the integrated circuit to be trimmed. Summary of the Invention The principle of the present invention is one of using, as integrated memory elements in the electronic circuit and in order to carry out the trimming operation, a pair of pins already present in the circuit, without losing control of the pin normal operation during the trimming step. Based on this principle, the technical problem is solved by an electronic integrated circuit as defined in the characterizing portion of Claim 1 . Further enhancements are provided by the subclaims. The technical problem is also solved by a method as defined in Claim 9. The features and advantages of an electronic circuit and a method according to the invention can be more clearly understood by reading the following description of embodiments thereof, given by way of non-limitative examples with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 shows in schematic block form a prior art trimming device associated with an integrated circuit; Figure 2 is a schematic view of a trimming device embodying this invention; Figure 3 shows schematically a detail of the device in Figure 2; Figure 4 illustrates the trimming method of this invention, in block form; Figure 5 is a schematic diagram of an analog actuator incorporated to the electronic circuit of this invention; Figure 6 shows diagramatically a detail of the actuator in Figure 5. Detailed Description Referring to the drawing views, and particularly to the example of Figure 2, an electronic circuit according to this invention for carrying out a trimming operation in an integrated circuit 2 is generally shown at 1 in schematic form. The integrated circuit 2 may be a DC/DC step-down converter having a regulating loop 3. However, the device of this invention can be associated with other types of integrated circuits. Figure 1 is a schematic block diagram of the regulating loop 3 of the voltage controlled converter 2. The integrated circuit 2 has five pins, hereinafter referenced A, B, C, D and E. These pins have the following functions: A: output OUT pin; B: power supply input pin at a voltage Vin; C: connecting pin for connection to a ground reference GND; D: signal COMP pin; E: sense pin for detecting the output voltage Vout. The loop 3 comprises an error (E/A) amplifier 4 operative to compare the output voltage, or a division thereof, with an internal voltage reference Vref. The amplifier 4 delivers a signal COMP through its output U, which signal is applied to a non-inverting input of a PWM comparator 5. The comparator 5 has a second, inverting input which is applied a sawtooth signal from an oscillator 6 within the circuit 2. The comparator 5 has an output UC presenting a logic signal whose duty cycle varies with the magnitude of the error detected by the amplifier 4. This logic signal is used to drive a power switch M1 connected between the pins A and B of the circuit 2, through a series of a latch and a drive circuit portion (Driver). Provided externally of the circuit 2 are: a load 7, referenced Load, an input filter 8 represented by a capacitor Cin, an output filter 9 represented by a circuit portion LCi, a one-direction element 11 represented by a loop-back diode D, and an optional resistive divider 12 to divide the input voltage Vin. This divider 12 is formed from a series of resistors R1 and R2. Also provided is a compensation network ZC for the whole regulating loop 3. Advantageously in this invention, the pins D and E of the integrated circuit 2 are used to have the circuit 2 enter the trim mode and for entering the related data serially, which data, once processed within the circuit 2, will also allow a simulation of the trimming sought, its associated measurements, and the actual trimming to be made. Shown schematically in Figure 2 is the circuit arrangement provided by this invention, whereby two pins of the circuit 2 can be utilized for trimming purposes, without losing control of their operation and with the power switch positively held in the off state through the duration of the trimming step. According to the invention, the error amplifier 4 output is connected to the comparator 5 via a normally closed controlled switch S2. The switch S2 is driven by the output UT from a comparator 15 having a first, non-inverting input connected to the pin E, and having a second, inverting input maintained at a threshold voltage value Vth. Another controlled switch S1 is provided between the output U of the amplifier 4 and a serial interface 25, which interface is inside the circuit 2 and adapted to receive the data sequence pertaining to the trimming operation. The switch S1 is placed in a line 20 connecting the output U to the interface 25. This switch S1 is normally open and is driven by the same signal as is used for driving the switch S2. The switches S1 and S2 are essentially in phase opposition. In the normal mode of operation, the potential at the pin E would never reach the threshold value Vth, which value is advantageously set, at the designing stage, to a higher safe value than the reference voltage Vref. Accordingly, the signal TR-MODE at the output U of the comparator 15 will be low throughout. In this condition, the switch S1 will be open, the signal DIN low, and S2 closed to connect the compensation pin D to the non-inverting input of the comparator 5. In this way, one can be assured of the trim managing circuitry being fully transparent to the application. In fact, the signal TR-MODE represents an asynchronous resetting of the entire trim managing system. To enter the trim mode, the potential at the pin E should be raised, from outside, to a value above the threshold Vth. In this case, the switch S2 opens while the other switch S1 closes, since the signal TR-MODE is high. The pin D is connected to the internal line 20 representing the serial data input DIN for the system which is to manage the trimming, as described hereinafter. During this step, control of the state of the power switch M1 is not lost because the open switch S2 would force to ground the non-inverting input of the comparator 5, the latter forcing the power transistor M1 to the off state. According to this invention, the structure just described is cooperating with a control logic, generally referenced 18, to drive an analog trim actuator 19. Also shown in Figure 2 is a block diagram of the circuit portions which are parts of the control logic 18 and arranged to manage the data sequence being delivered to the circuit for the simulated or actual trimming operation. The serial interface 25 is input the externally issued data, from the pin D over the internal line 20. The signal TR-MODE, driven from the pin E, represents the resetting of the whole trimming circuitry. A synchronous state machine 26 is provided for managing and coordinating all the trimming operations. The state machine 26 is connected bidirectionally to an eight-bit counter 27, provided for decoding the data sequence received from the pin D, as well as to a shift register 28 whereinto the decoded bits are stored. The shift register 28 is an n-bit, preferably but not necessarily fifteen-bit, shift register whereinto the serial input data to the line 20 are stored. The machine 26 output drives a simulation block 29, managing the simulated trimming, and a trimming block 30 which executes the actual trimming. The respective outputs from the blocks 29 and 30 are addressed to an OR node 31 for re-constructing a single trim output signal. The data receive timing, and that of the subsequent operations, is provided by an internal clock of the integrated circuit 2, since the simulation or trimming operations are all carried out with the circuit live. More particularly, as shown schematically in Figure 4, the state machine 26 carries out sequentially operations which are clocked by a clock period CLOCK supplied by the integrated circuit 2. The machine 26 is held in a WAIT_EVENT initial state until it receives a first positive edge of the input signal. A sequence of N bits is entered from outside, through the pin D, according to the rules of the trim protocol being implemented. In essence, the values 0, 1, and of sequence end (STOP) are discriminated according to the distance between the leading edges contained in the input signal. Of the N bits in the sequence, M (M<N) bits represent the state sought for the fuses, K are intended for the user to select simulated or actual trimming, and T for an optional safety system (parity, error correction code, etc.). Of course, it should be N = M+K+T. In this instance, the assumed values are: M=12, K=2, T=1. At the initial stage, the signals delivered to the input D are checked, or rather decoded, to indicate if a logic 0 or 1, or a STOP value has been entered. Thus, the counter 27 is enabled to count, as indicated by the state IDLE in Figure 4. The count is kept going until a new edge of the input data is detected. The state DECODE is then attained in which the counter 27 is stopped and the input data decoded. If the number detected by the counter 27 is less than 64, then a logic high (H) is attributed to a signal IS_ZERO. As the counter 27 counts beyond 64, the signal IS_ZERO is brought to a logic low (L) and a signal IS_ONE to a logic high. This data is transferred to the input of the shift register 28. On the other hand, if a stop is detected, a signal IS_STOP would be brought to a logic high, and control handed down to a parity check incorporated to the control logic 18. A pair of flags IS_SIMUL and IS_TRIMM allow the control logic 18 to manage the simulated or actual trimming steps. Dependent on the logic values of these flags and the outcome of the parity check performed on the contents of the register 28, control is handed down to either the block 29 or the block 30. If the flag IS_SIMUL has a high logic value and the flag IS_TRIMM a low logic value, the block 29 is enabled to store the associated information into an internal register and carry on with the trimming step. Since for the fuse trimming step one switch must be closed at a time, as required to hold the transistor M1 closed for a longer time than the clock period, the signal DIN on the line 20 will be acting as a clock for the operations within the block 30. The duration of the ON state on the line 20 is same as the duration of a switch closure. At the end of the trimming operations, control is restored to the initial state WAIT_EVENT, after the signal TR_MODE is forced to a logic low. To complete the description, it should be remarked that a theoretical diagram of the analog actuator 19 which enables the trimming is shown in Figure 5 by way of example. In the simulation aspect, the signal SIMUL has a high logic value and, accordingly, the switch represented by the NMOS transistor M1 will be closed, regardless of the comparator 35 output, to permit of a temporary modification of the parameter considered. On the other hand, when a trimming operation is to be carried out, the signal TRIMM will be at a logic high and the switch M1 closed to flow a sufficient current, of about 170mA, to blow the fuse FUSE. The output UC of the comparator 35 will change from a low into a high state, in a permanent fashion. To prevent reliability problems from employing fuses, the comparator circuit structure 35 shown in Figure 6 is charged with discriminating between an intact fuse and a blown one. In fact, with BCDV technology, a blown fuse is not always indicative of an open circuit, and occasionally may represent a resistance of a few hundred kiloOhms. In addition, this value is likely to deteriorate, more often decrease, over the life span of the device. The comparator circuit structure 35 of Figure 6 ensures a correct logic level for the output, even when the fuse is not an open circuit. A first pair of parallel-connected DMOS transistors M3 and M4 are provided for the purpose. A first transistor M3 in the pair is connected to ground GND via a resistive divider 34 which comprises at least a pair of resistors of equal resistance, e.g. 5 kOhms. The first transistor M3 in said pair is further connected to the supply via a series of a transistor M11 in diode configuration and a current I generator. A transistor M5 connects the point of interconnection of the divider resistors to ground GND, and receives a signal RES10K on its control terminal. The second transistor M4 in said pair is connected to ground GND via the fuse FUSE. The second transistor M4 in said pair is further connected to the supply via a series of a transistor M21 and a second current I generator. The control terminal of the transistor M21 is connected to the interconnection node between the transistor M11 and the first current I generator. The comparator output OUT is picked up from the interconnection node between the transistor M21 and the second current I generator, through a series of two inverters 32 and 33. During the trimming step, the signal RES10K would be low, and the current I through the diode M11 and the series of the divider resistors generate a voltage reference level for all the fuses. The DMOS transistors M3 and M4 function to protect the circuitry as the fuse FUSE is being blown, since during this event, the supply voltage is applied across the fuse. The voltage drop Vgs through the transistor M4 is effective to protect the whole circuitry. When the fuse FUSE is intact, its resistance amounts to a few Ohms, e.g. 5 Ohms. The transistor M21 exhibits a higher voltage drop Vgs than that of the corresponding transistor M11, and will force the input of the first inverter 32 to a logic low, thereby to ensure an output OUT of 0 Volts. When, on the contrary, the fuse FUSE is badly blown, the circuit structure 35 only outputs a high logic value if the resistance of the fuse exceeds that of the series resistors in the divider 34. Thus, based on this indication provided by the value of the output OUT, any such pieces as may become a source of problems from the potential deterioration of the fuse resistances can be rejected at the trimming stage. The signal RES10K, by controlling the resistive value of the divider 34, allows the resistance of the fuse FUSE to be compared, at the trimming stage, with a higher resistive value of the divider, whereas in normal operation of the circuit, the comparison can be made with a lower resistive value. The advantages of this innovative solution can be summarized as follows: an ability to perform the trimming after the packaging step, without having to introduce additional pins and while keeping the state of the power switch under control; an ability to also use the system with EWS, without any additional pads inside the device, which affords considerable savings in silicon area because the entire system can be implemented using logic components at a high level of integration density; an ability to have the outcome of the trimming simulated.

Claims: An electronic integrated circuit (1) for carrying out a trimming operation on portions (2) of said circuit which are provided with at least first (D) and second (E) access terminals, comprising: memory elements; a circuit means for modifying, either temporarily or permanently, the state of said memory elements, said means including: an error amplifier (4) having an input connected to the second terminal (E) for comparing an output voltage from the circuit portion (2) with an internal voltage reference (Vref), a comparator (5) having an input connected to the output of said error amplifier, a direct connection of the first terminal (D) to said comparator (5) input; characterized in that it further comprises: a controlled switch (S2) between the output of the error amplifier (4) and the input of the comparator (5); a second comparator (15) having an input connected to the second terminal (E) and the output connected to control said controlled switch (S2); a control logic (18) for controlling the trimming operation; a serial interface (25) adapted to receive, from said first terminal (D), a data sequence relating to the trimming operation for said control logic (18), through another controlled switch (S1) inserted between the output of the error amplifier (4) and said serial interface (25) and essentially in phase opposition with respect to said controlled switch (S2). A trimming circuit according to Claim 1, characterized in that said controlled switch (S2) is normally closed. A trimming circuit according to Claim 1, characterized in that the output of the second comparator (15) is also connected to said serial interface (25). A trimming circuit according to Claim 1, characterized in that said another controlled switch (S1) is driven by the output signal from said second comparator (15). A trimming circuit according to Claim 1, characterized in that said another controlled switch (S1) is normally open. A trimming circuit according to Claim 1, characterized in that said control logic (18) controlling the trimming operation includes a synchronous state machine (26) connected downstream of said interface (25). A trimming circuit according to Claim 6, characterized in that said control logic (18) further includes a counter (27) and a shift register (28), both connected to the state machine (26). A step-up or step-down switching voltage regulating device, characterized in that it incorporates at least one electronic circuit as claimed in Claim 1. A method of carrying out a trimming operation on integrated circuit portions (2) of an elastromic integrated circuit which are provided with at least first (D) and second (E) access terminals, by means of a trimming circuit which comprises at least: a number of memory elements; a circuit means for modifying, either temporarily or permanently, the state of said memory elements, said means including: an error amplifier (4) having an input connected to the second terminal (E) for comparing an output voltage from the circuit portion (2) with an internal voltage reference (Vref), a comparator (5) having an input connected to the output of said error amplifier, a direct connection of the first terminal (D) to said comparator (5) input; characterized in that it comprises: providing a control logic (18) for the trimming operation; breaking the connection between the output of the error amplifier (4) and the input of the comparator (5) by means of a controlled switch (S2) which is driven by a second comparator (15) having an input connected to the second terminal (E); transmitting, to said control logic (18), a data sequence relating to the trimming operation, through a serial interface (25) connected to said first terminal (D) and another controlled switch (S1) inserted between the output of the error amplifier (4) and said serial interface (25) and essentially in phase opposition with respect to said controlled switch (S2).

Inventors: CAPICI SALVATORE, MARINO FILIPPO, CAPICI, SALVATORE, MARINO, FILIPPO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1999
Main classification: G06F11
IPC codes: G01R31