Current limiting method and apparatus

Abstract: A method and apparatus for providing a substantially constant current from a voltage source. The apparatus includes a depletion mode transistor connected to a parallel network of resistors. At least one of the resistors is provided with a series fusible link so as to enable removal of one or more of the resistors from the network and adjust the output of the current limiter to a desired value.

Description: BACKGROUND OF THE INVENTION The present invention relates to the field of electronic devices and their manufacture. More specifically, it relates to a method and apparatus for providing a substantially constant current from a voltage source and to a method of adjusting the current output from a current limiter. The invention finds application, for example, in protection, control, signal generation, and the like. Often it is desirable to provide a means and method for limiting current from a variable voltage source in an electronic circuit. Such current limiters (otherwise referred to as current regulators, current sinks, constant current sources, and the like) could be used in a wide variety of applications which include, but are by no means limited to, protection, control, signal generation. Ideally, a current limiter provides an output current which is largely independent of an input voltage. In reality, most such devices do not begin to operate effectively until the voltage between their terminals reaches some minimum voltage, generally referred to as V T Their performance begins to deteriorate above some breakdown voltage, V BD . Fig. 1 illustrates a typical current limiting device 2. The current I s is to be regulated. The current limiting device includes an enhancement mode HVFET 4 which is driven in a closed loop mode via a shunt resistor 6 and an amplifier 8. The current I s will, within limits, be set by the resistance of the shunt resistor 6 and will be largely independent of the voltage applied at the point "D." I s will equal approximately V T /R S where V T is roughly the threshold voltage of the FET 4. Such current limiting devices have the disadvantage of, for example, being complex. In the device shown in Fig. 1, for example, it is necessary to provide a power supply for V cc , an op-amp 8, a transistor 4, a resistor 6, and the like. Fig. 2 illustrates another type of current source in which a depletion mode FET 10 is utilized. In the device shown in Fig. 2, the current is "on" when the gate of the FET is shorted to the source. I D is set by processing conditions in the fabrication of the device and, in particular, the design of the FET and the resistors. Accordingly, users of such devices frequently purchase a transistor 10 and select a discrete resistor 6 to provide the appropriate current I D . Often, for different transistors, even those fabricated in the same production facility, it is found that different values of R s must be provided to obtain a desired value of I D . An improved method and apparatus for providing a constant current is desired. It is an object of the invention to provide such a method and apparatus. SUMMARY OF THE INVENTION An improved method and apparatus for providing a substantially constant current over a wide range of voltages is specified by the features of apparatus claim 1 and method claims 13 and 15. The object is thus satisfied. The current source of the invention provides a precise value of current in mass-produced devices, even when the devices are fabricated under widely varying process conditions. Further, the inventive device is straightforward and inexpensive to manufacture. It is noted that the use of fusible links is known from US 4,788,620 for adjusting a circuit breaker electronic trip unit containing a resistive network arranged as a plurality of trimming resistors electrically in parallel with a base resistor through a corresponding plurality of fusible links. A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a circuit diagram of a prior art current limiter; Fig. 2 illustrates a circuit diagram of a second prior art current limiter; Fig. 3 is a current limiter according to one embodiment of the invention herein; Fig. 4 illustrates the final form and use of the current limiter according to one embodiment of the invention; and Fig. 5 illustrates a current versus voltage curve for one embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 illustrates one embodiment of the invention provided herein. A current limiter 2 is provided with a transistor 10 which in a preferred embodiment is a field effect transistor of the type commonly known to those of skill in the art. In the embodiment shown in Fig. 3, the transistor 10 is a depletion mode transistor, i.e., a transistor in which there is a conducting channel between the source and the drain when the gate and source are at the same potential. While an n-channel device is shown in Fig. 3, it will be readily apparent upon review of this disclosure that current limiters based on p-channel devices could also be fabricated. Also, while a two-pin device is illustrated in Fig. 3, a three-pin device could also be readily fabricated based upon this disclosure. The source of the depletion mode transistor 10 is connected to a parallel network of shunt resistors 12. The parallel network of shunt resistors 12 includes a first group of resistors 14a, 14b, 14c, and 14d. Optionally, the network may also include a second group of fixed resistors 16a and 16b. It is preferred to provide optional resistors 16 so the device will provide a limit on current event if all the fusible links are blown. The gate of the depletion mode transistor is connected to the output of the network 12. Each of the first group of resistors is provided with a series fusible link 18a, 18b, 18c, and 18d while the optional resistors 16a and 16b are not provided with such a link. The invention is illustrated herein with 4 fused resistors and 2 fixed resistors, but of course a wide variety of arrangements could be provided. In general it is desirable to provide 2 or more fused resistors in the network and 1 or more fixed resistors. The selection of the number of resistors provided in the network will be based on the range of current values which the current limiter is intended to cover. Also, while the invention is illustrated herein with regard to discrete resistors 14 and 16, it will be recognized that each of these discrete resistors could actually include one or more resistors or other load devices in series or parallel. The value of resistance to be supplied for each resistor will also vary widely depending upon the application. The resistance values may range from about 10 ohms to 100,000 ohms or more in some applications. More particularly, resistance values may range from about 100 ohms to 10,000 ohms in some applications. Still more particularly, resistance values may range from about 200 ohms to 8,000 ohms in typical applications. These values are meant to be illustrative and in no way limiting on the resistance values used in actual applications. When it is desirable to mass-produce current limiters, all of which provide a nearly identical output, the resistors will be used to account for variations in the fabrication of the device as a result of changing process conditions and the like. In this situation, the values of resistance provided by at least several of the resistors will be closely spaced. For example, resistances varying by between about 10% and 1000% would be used in some embodiments with resistances varying between about 50% and 100% preferred. Of course, the invention will find other applications. For example, the invention may be utilized to fabricate current limiters which may be used to provide widely varying current outputs. In this case, the resistance of the various resistors could vary by 1000% or more. In operation, the resistor network 12 is used to vary the output current by selectively blowing one or more of the fuses. The fuses are blown by applying an appropriate voltage across the fusible links via on-chip trimming pads or pins P 1 , P 2 , P 3 , and/or P 4 , and the ground pad or pin P g . By providing on-chip trim pads, the output of the current limiter may be precisely adjusted in the fab during a wafer testing operation. In some cases, it may be desirable to allow a customer to select an output current of the device after purchase, in which case a multi-pin package (e.g., 6 pins, with four pins connected to trimming pads and two pins for input/output) would be utilized. The customer would, in some embodiments, selectively blow fuses in sequence until the desired current is output. In still further embodiments, fuses may be blown with lasers, ultrasonic pulses, or the like. Fig. 4 provides an illustration of the device 2 in a two-pin configuration. The device is connected to a source of variable voltage 20, but provides a constant current I D . The output characteristics of the device are illustrated in Fig. 5. As shown, the device output reaches I D at V min , after which its output is largely independent of applied voltage up to a breakdown voltage V BD . In common devices fabricated using present fabrication technology, V min is typically 3-4 volts while V BD is 100 to 1200 volts. The device discussed above may be produced by way of a large variety of fabrication sequences. The device may, for example, be fabricated according to the method described in U.S. Patent No. 4,860,072, which is incorporated herein by reference for all purposes. In a preferred embodiment the device is fabricated by way of a process in which resistors for the resistance network may be fabricated without additional masking steps. In this process polysilicon resistors are utilized and the same polysilicon that is used for the gate is used for the resistor network such as in, for example, U.S. Patent No. 4,764,480, which is incorporated herein by reference for all purposes. The preferred resistor is a p-well resistor (for the n-channel device), since the temperature coefficient of the resistor compensates and cancels the effect of the temperature on the gain of the FET thus keeping the current level constant to a high degree with temperature. It is to be understood that the above description is intended to be illustrative and not restrictive. Many variations of the invention will become apparent to those of skill in the art upon review of this disclosure. Merely by way of example, the invention has been illustrated primarily with regard to n-channel transistors, but the invention could readily be adapted to utilize p-channel transistors. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims.

Claims: Apparatus for providing a substantially constant current from a voltage source, the apparatus comprising a transistor (10) for controlling the current from the voltage source, said transistor being fabricated on a die and having an output and a control element as well as a resistive network (12) connected on an input side to said output of the transistor, and on an output side to said control element of the transistor, characterized in that the resistive network (12) is fabricated on the same die as the transistor (10) and comprises at least two resistive elements (14, 16) arranged in parallel, at least one of which (14) having a fusible link (18) arranged in series therewith, said fusible link providing an open circuit upon application of a predetermined voltage across the fusible link. Apparatus in accordance with claim 1, characterized in that the transistor (10) is a field effect transistor. Apparatus in accordance with claim 2, characterized in that said output is a source or drain and in that said control element is a gate. Apparatus in accordance with claim 2 or 3, characterized in that said voltage source is a varying voltage source; in that said field effect transistor is a depletion mode transistor and is connected by its drain to the varying voltage source; and in that said output of the transistor is a source of said transistor. Apparatus in accordance with claim 4, characterized in that the depletion mode transistor is a MOSFET. Apparatus in accordance with one of the claims 2 to 5, characterized in that each fusible link (18) has a first end and a second end and in that at least some of said fusible link or links (18) are provided at their first ends with a connection to a die pad (P n ) for application of said predetermined voltage or with a connection to a package pin. Apparatus in accordance with claim 6, characterized in that at least some of said fusible link or links (18) are provided at their second ends with a connection to ground. Apparatus in accordance with one of the claims 2 to 7, characterized in that, of the said at least two resistive elements, at least one (16) is directly connected between the input and output sides of the resistive network (12), without interposition of a fusible link. Apparatus in accordance with one of the claims 2 to 8, characterized in that the resistance of each of the resistive elements (14, 16) has a value which lies in a range of between about 50% and 800% of a predetermined resistance value. Apparatus in accordance with one of the claims 2 to 9, characterized in that the resistance of each of the resistive elements (14, 16) lies in a range of between about ten ohms and one hundred thousand ohms. Apparatus in accordance with one of the claims 2 to 10, characterized in that said fusible link or links (18) are blown by applying an electric current, a laser, or an ultrasonic thereto. Apparatus in accordance with one of the claims 2 to 11, characterized in that said resistive network (12) and a gate of said field effect transistor (10) are portions of a single polysilicon layer. Method of adjusting to a desired predetermined value the current output from a current limiter (2), said current limiter comprising a depletion mode transistor (10) connected to a voltage source at a first end and a resistive network (12) at a second end, said resistive network comprising at least two resistors (14) connected in parallel, each of said resistors being connected in series with a fusible link (18), wherein said resistive network (12) has an output connected to a gate of said depletion mode transistor (10), said method comprising the steps of : applying a voltage across a fusible link (18) associated with a resistive element (14) of the resistive network (12) to selectively remove the contribution of that resistive element from the resistance of the resistive network; and, if the desired predetermined value of the current has not thereby been reached, repeating the above step on a further resistive element or further resistive elements until the desired predetermined current value is reached. Method of claim 13, characterized in that the voltage is applied to each fusible link (18) via a pad (P n ) on the die connected thereto. Method of adjusting to a desired predetermined value the current output by a voltage source provided with an apparatus in accordance with one of the claims 1 to 12, said method comprising the steps of: applying a voltage across a fusible link associated with a resistive element of the resistive network to selectively remove the contribution of that resistive element from the resistance of the resistive network; and, if the desired predetermined value of the current has not thereby been reached, repeating the above step on a further resistive element or further resistive elements until the desired predetermined value of the current is reached.

Inventors: ZOMMER NATHAN, ZOMMER, NATHAN
Applicants: IXYS CORP, IXYS CORPORATION
Application year: 1991
Main classification: H02H9
IPC codes: G05F3, H02H3, H02H9