Programmable Current Mirror

The present invention relates generally to reference current circuits and more specifically to an improved programmable reference current circuit.

A reference current on an integrated circuit typically has a tolerance of .+-.20%. Many complicated circuits are provided to assure a reference current which is indepenent of voltage supply, temperature, processing variations and other physical as well as environmental characteristics. Similarly, adjustability using variable resistance and/or laser trimming have been suggested.

It is an object of the present invention to provide a unique reference current circuit which is programmable.

Another object of the present invention is to provide a reference current circuit which is readily programmable without sophisticated equipment or processing time.

These and other objects of the invention are attained by providing a current mirror which receives the actual reference current and is programmable to provide the desired reference current as a function of the actual reference current. The current mirror includes a controlled leg having a transistor and a plurality of controlling legs including a plurality of parallel connected transistors in a current mirror configuration. In addition to the primary controlled transistor, a plurality of secondary control transistors are connected with a programming element in its current path. The method of programming includes measuring the input reference current in the controlling leg and programming the individual secondary controlled legs to provide a desired output reference current which is a function of the input reference current. By controlling the current handling capacity of the plurality of controlled legs, the desired output reference current is provided. A diode is provided between the reference input current and the controlling transistor such that the input reference current can be measured without activating the current mirror. The programming of the secondary controlled legs is achieved by a programming terminal for each of the secondary legs used in combination with a reference terminal of the current mirror. Preferably, the current capacity of the controlling leg and the primary controlled leg are equal whereas the current capacity of the secondary controlled legs have a binary weighted current capacity.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

FIG. 1 is a schematic of a current mirror incorporating the principles of the present invention.

FIG. 2 is a table illustrating the relationship between the current ratio of the current mirror and the programming states of the programmable legs.

A current mirror 10 as illustrated in FIG. 1 includes an input terminal 12 receiving reference input current I.sub.REFIN and an output terminal 14 providing an output reference current I.sub.REFOUT. The controlling leg 16 of the current mirror 10 includes a controlling transistor Q.sub.1 having a current path connected in series with the input terminal 12 and a reference terminal 18. In series with the current path of Q.sub.1 is a diode D. A sensing or measuring terminal 20 is provided between the current input terminal 12 and the diode D and which may be connected a current meter M as shown. The meter M is not part of the present circuit and is connected only during a measurement. The controlled legs 22, 23, 24 and 25 of the current mirror 10 each include a transistor Q.sub.2, Q.sub.3, Q.sub.4 and Q.sub.5 respectively having their current paths connected between the output current terminal 14 and the reference terminal 18. The control terminals of the transistors Q.sub.2, Q.sub.3, Q.sub.4 and Q.sub.5 are connected to the control terminal of transistor Q.sub.1 of the controlling leg. A transdiode connection connects the control terminals of the transistors to the current path of the controlling leg.

Connected in the current path of each of the secondary controlled legs includes a programmable element illustrated as fuses F.sub.3, F.sub.4 and F.sub.5. Connected between the transistors Q.sub.3, Q.sub.4 and Q.sub.5 and the programmable elements F.sub.3, F.sub.4 and F.sub.5 are programming terminals P.sub.3, P.sub.4 and P.sub.5 respectively. Programming current source IP is shown which may be selectively connected to the programming terminals P.sub.3, P.sub.4 and P.sub.5. As is evident from FIG. 1, by applying a current to the programming terminals P.sub.3, P.sub.4 and P.sub.5 and a voltage to reference source 18, the programmable elements F.sub.3, F.sub.4 and F.sub.5 may be selectively programmed. Although fuses are shown for the programmable elements, other types of the programmable elements may be used which have a low initial resistance state and a high or substantially open resistance state. The programming current source IP is not a part of the present circuit, but is connected only during programming.

As is well known for current mirrors, the output current is a function of the input current as defined by the ratio of the current capacity of the controlled legs 22, 23, 24 and 25 to the current capacity of the controlling leg 16. Thus, by selecting which of the controllable legs are in the circuit, a specific ratio of current capacity of the output to input currents can be obtained. Since it is desirable in an integrated circuit to have a specific reference current, the current mirror of FIG. 1 provides a method of assurring a specific reference current in spite of manufacturing tolerances. The current mirror of FIG. 1 provides the desired reference current I.sub.REFOUT at the terminal 14 by measuring the reference current I.sub.REFIN at the input terminal 12 via sensing terminal 20. The diode D is reverse biased and, thus, the current mirror 10 is not operable with a meter connected at the sensing or measuring terminal 20. Once the input reference current I.sub.REFIN is measured, the controlled legs 23, 24 and 25 are programmed to give the desired current ratio such that the output reference current I.sub.REFOUT at terminal 14 is the desired reference current.

Preferably, the current carrying capacity of Q.sub.1 and Q.sub.2 of the controlling legs 16 and the primary controlled leg 22 are equal. The current carrying capacity of the secondary controlling transistors Q.sub.3, Q.sub.4 and Q.sub.5 of secondary controlling legs 23, 24 and 25 are smaller than the current capacity of Q.sub.1 or Q.sub.2 and vary in a binary weighted fashion. To be more specific, if the current carrying capacity of transistor Q.sub.1 and Q.sub.2 are in, the current carrying capacity of Q.sub.3, Q.sub.4 and Q.sub.5 are 1, 2 and 4 respectively when "n" is greater than the largest area of the secondary transistors. FIG. 2 provides the ratio of the output reference current I.sub.REFOUT to the input reference current I.sub.REFIN as a function of the states of the programmable elements F.sub.3, F.sub.4 and F.sub.5.

Although the transistors Q.sub.1, Q.sub.2, Q.sub.3, Q.sub.4 and Q.sub.5 are shown as MOS transistor, it is obvious that they may be bipolar transistors. Similarly, it should be noted that even though only three secondary controlled transistor legs are shown, any number of legs may be used. The greater number of legs, the finer gradation of variance of the ratio of the output to input current may be obtained. It should also be noted that although a binary weighted ladder is described, any mathematical progression may be used for example but not claimed to exponential, square root, .mu.-law etc.

From the preceding description of the preferred embodiments, it is evident that the objects of the invention are attained, and although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the invention are to be limited only by the terms of the appended claims.