Integrated circuit current reference

[0001] 1. Field of the Invention

[0002] This invention relates generally to integrated circuit (IC) current references, and more particularly, to IC current references which provide reference currents for use both internal and external to an IC.

[0003] 2. Description of the Related Art

[0004] Integrated circuits (ICs) often require the use of a reference current. Such currents are used, for example, as a reference for a current limit detection circuit, to generate bias currents, or for any of a number of other purposes.

[0005] A reference current is typically generated by applying a reference voltage generated internally to the IC across the terminals of an internal resistor. Such an arrangement is illustrated in FIG. 1. Here, an IC 10 receives a reference voltage V_ref from a voltage source (not shown). V_ref is buffered with an amplifier 12, and the buffered output 14 is used to drive a transistor 16, shown here as an NMOS FET, though other transistor types could also be used. An internal resistor 18 having a resistance R is connected between the source of FET 16 and one of the IC's input/output (I/O) pins 20, and the FET's drain is connected to the input 22 of a current mirror 24 which is referred to a positive supply voltage V+. I/O pin 20 would typically be connected to a circuit common point 26.

[0006] In operation, V_ref drives FET 16 to conduct a current I_ref which is given by V_ref/R. Current mirror 24 then produces copies (28) of I_ref, which can be used for other circuits internal to IC 10 as needed.

[0007] Ideally, resistor 18 is a high quality resistor having a low temperature coefficient (TC) and voltage coefficient (VC) and a known resistance value. For example, some IC processes allow the fabrication of thin film resistors, which have both low TC and VC and may have an initial accuracy of 15% or better. However, for some low cost IC processes, thin film resistors are not available, and other types such as polysilicon or diffused resistors must be used. In these cases, initial accuracy may be no better than 30% or more and TCs can be on the order of 1000 ppm/° C. or larger, leading to unacceptably large errors over temperature. If a higher accuracy reference current is needed, errors due to the internal resistor must be corrected and/or compensated for using correction circuits, on-die trimming, additional test time, external circuitry requiring additional pin count, etc. Expensive laser or fuse-blow trimming can improve initial accuracy, but have little to no effect on the internal resistor's non-ideal temperature and voltage characteristics.

[0008] Reference voltages and currents generated within an IC are often used by other circuitry which is external to the IC. For example, in FIG. 1, IC 10 is used to provide regulation for a negative supply voltage channel V−. A negative voltage is often generated by means of a charge pump; in FIG. 1, some of the charge pump circuitry (30a) is internal to the IC (such as switches, control circuitry, etc.) and some (30b) is external (feedback resistors, switches, capacitors, etc.), with the internal and external portions coupled together via an I/O pin 31. Regulation is provided by means of an error amplifier 32, one input of which is connected to circuit common point 26 and the other of which is connected to an I/O pin 34. Pin 34 is connected to the junction 36 of two resistors, one of which (38) is connected between junction 36 and V−, and the other of which (40) is connected between junction 36 and V_ref via an I/O pin 42. Resistors 38 and 40 are chosen such that the voltages at the inputs of error amplifier 32 are equal when V− is at a desired value. While the operation of the regulation scheme shown in FIG. 1 is satisfactory, it does require the use of three I/O pins (31, 34, 42) and two resistors (38,40).

[0009] Extreme cost pressures on some ICs push manufacturers to use inexpensive IC processes and lower pin count packaging. But as noted above, these inexpensive processes typically lack high quality resistors, so that an internally generated reference current may be unacceptably inaccurate. And a limited pin count may make an application of a high quality current reference for internal bias generation or current limit detection impractical.

[0010] An IC current reference is presented which overcomes the problems noted above, by creating an accurate reference current without requiring a high quality internal resistor. The present invention also makes a copy of the reference current available to circuitry external to the IC.

[0011] The present current reference requires an IC having a plurality of I/O pins. The IC contains a voltage source which outputs a reference voltage V_ref, a current mirror which mirrors a current received at an input to at least one output, and a transistor having its current circuit connected between the current mirror input and a first I/O pin and which is driven by V_ref.

[0012] To create a reference current, a resistor external to the IC and having a resistance R1 is coupled to the first I/O pin such that it conducts a current I_ref which is proportional to V_ref/R1. The current mirror provides currents which are proportional to I_ref at respective outputs, at least one of which is provided at a second I/O pin for use external to the IC. Moving the reference current-generating resistance outside of the IC in this way allows a high quality, low TC and VC resistor to be used, and thereby provide a highly accurate and stable reference current.

[0013] The reference current made available at the second I/O pin can be used by external circuitry as needed. For example, the externally-available reference current can be used as part of a negative voltage supply regulation circuit which regulates a negative supply voltage channel, without requiring any more I/O pins or external resistors than previous designs. In this way, a highly accurate reference current is generated for use internal and external to an IC, while maintaining the same number of I/O pins and external components as conventional designs.

[0014] These and other features, aspects, and advantages of the present invention will become better understood with reference to the following drawings, description, and claims.

[0018] One possible embodiment of an IC current reference in accordance with the present invention is shown in FIG. 2. An IC 100 receives a reference voltage V_ref from a voltage source (not shown). V_ref is preferably buffered with an amplifier 102, and the buffered output 104 is used to drive a transistor MN1, shown here as an NMOS FET, though other transistor types could also be used. The IC also contains a current mirror 106 referred to a positive supply voltage V+, which has an input 108 and at least one output (110, 112, 114). The transistor's current circuit is connected between the current mirror and one of the IC's I/O pins 116; in this example, MN1's drain is connected to current mirror input 108 and its source is connected to I/O pin 116.

[0019] To generate a reference current, an external resistor 118 is connected between I/O pin 116 and a fixed voltage, typically circuit common (120). Resistor 118 has a resistance R1. In operation, V_ref drives FET MN1 to conduct a current I_ref which is proportional to V_ref/R1, and is equal to V_ref/R1 when node 120 is at zero volts. Because there are no other current-consuming connections made to current mirror input 108 or pin 116, all the current through R1 flows in current mirror 106, such that currents proportional to I_ref are provided at its outputs (110, 112, 114). The proportionality between I_ref and the mirrored currents is determined by the current mirror ratios; assuming 1:1 ratios, the mirrored currents are effectively copies of I_ref. The mirrored currents can be distributed as needed for use by other internal and/or external circuits. At least one of the mirrored currents is provided at a second I/O pin 122, for use by circuitry external to IC 100.

[0020] By arranging the present current reference such that the reference current-generating resistance (R1) is outside of the IC, a user can select a resistor having a desired set of characteristics. Typically, a high quality, low TC and VC resistor would be used. This enables a highly accurate and stable reference current to be generated for use internal and external to IC 100 (assuming that V_ref is well-controlled), even if the IC is fabricated using low cost processes that cannot provide high quality resistances. Because a high quality current reference is now available, IC designers can further reduce cost by, for example, decreasing transistor size and the test and/or trim time needed to achieve desired design safety margins.

[0021] The ability to employ a low TC and VC resistor to generate a highly accurate and stable reference current, as described herein, can be very advantageous. For example, buck and boost power converters typically include current limit circuits which require a high quality current reference for comparison to a load current. Without such a current reference, both the IC designer and the designer of the printed circuit board (PCB) on which the IC resides may need to over-design their circuits to provide a safety margin which accommodates undesirable operating possibilities. For instance, generating a reference current using a resistor having a TC of 1000 ppm/° C. results in a 10% increase/decrease in output current limit over a 100° C. operating range. Consequently, an IC designer would need to increase a device's metal and silicon die area by 10% to accommodate the possible increase in maximum current. These problems can be largely avoided by using a high quality, low TC and VC external resistor to generate a reference current, in accordance with the present invention.

[0022] An IC current reference and one primary application of the externally-available copy of reference current I_ref is shown in FIG. 3. The reference current generation as is shown in FIG. 2: a buffered version of V_ref is applied across resistor 118 to produce a reference current I_ref given by V_ref/R1 (when node 120 is at zero volts). Current mirror 106 produces copies of I_ref at its outputs (110, 112, 114) (assuming 1:1 ratios), at least one of which (110 in this example) is provided at second I/O pin 122.

[0023] In this application, the reference current at I/O pin 122 is used to regulate a negative supply voltage V−. The negative voltage may be generated by many different means; in this example, a charge pump is used. In FIG. 3, some of the charge pump circuitry (130a) is internal to IC 100 (such as switches, control circuitry, etc.) and some (130b) is external (feedback resistors, switches, capacitors, etc.), with the internal and external portions coupled together via an I/O pin 132; the charge pump is arranged to vary voltage V− in response to a control signal. Regulation is provided by means of an error amplifier 134, the output of which provides the charge pump control signal, one input of which is connected to circuit common via an I/O pin 136, and the other input of which is connected to I/O pin 122. A second resistor 138 having a resistance R2 is connected between I/O pin 122 and V−. Resistance R2 is selected such that the voltages at the inputs of error amplifier 134 are equal when V− is at a desired value.

[0024] Note that, in addition to providing an accurate current reference, the circuit arrangement shown in FIG. 3 provides regulation of a negative supply voltage channel without increasing pin count: the circuit of FIG. 3 requires the same number of I/O pins and external resistors as the conventional arrangement shown in FIG. 1. In effect, one of the two external resistors required by the conventional regulation scheme is no longer needed. This enables a low TC/VC external resistor to be employed to generate a much more accurate reference current, without increasing the required component or I/O pin count.

[0025] Thus, using an arrangement such as that shown in FIG. 3, and IC manufacturer can use low pin count packaging and inexpensive IC processes with low quality internal resistors, and still meet the accuracy requirements presented by critical circuitry such as timing, clock generation, precise current limiting and bias current generation.

[0026] The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.