Temperature independent reference circuit

This application is a continuation of application Ser. No. 12/931,377, filed Jan. 31, 2011, now U.S. Pat. No. 7,999,606, which is a continuation of application Ser. No. 12/587,204, filed Oct. 2, 2009, now U.S. Pat. No. 7,893,754, both of which are entitled, “TEMPERATURE INDEPENDENT REFERENCE CIRCUIT”, both of which are assigned to the assignee of the present application.

The present disclosure generally relates to the field of temperature independent reference circuits, more particularly, to temperature independent voltage reference and temperature independent current reference circuits manufactured on a semiconductor chip.

Temperature independent reference circuits have been widely used in integrated circuits (ICs) for many years. The purpose of a temperature independent reference circuit is to produce a reference voltage and/or a reference current that are substantially constant with temperature. In prior art ICs, a temperature-compensated reference voltage and a temperature-compensated reference current are sometimes generated on the same silicon chip using separate circuits. Typically, a temperature independent voltage reference is first derived and then a temperature independent current is derived using the temperature independent voltage. A drawback of this approach, however, is that the circuitry utilized to separately generate the reference voltage and reference current is usually complex and typically occupies a large area of the semiconductor (e.g., silicon) die.

The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, wherein:

FIG. 1 illustrates a circuit schematic diagram of a temperature independent reference circuit for simultaneously generating both a temperature-compensated reference voltage and a temperature-compensated reference current on an integrated circuit (IC).

FIG. 2 illustrates another example circuit schematic diagram of a temperature independent reference circuit for simultaneously generating both a temperature-compensated reference voltage and a temperature-compensated reference current on an integrated circuit (IC).

In the following description specific details are set forth, such as device types, conductivity types, voltages, component values, configurations, etc., in order to provide a thorough understanding of the present invention. However, persons having ordinary skill in the relevant arts will appreciate that these specific details may not be needed to practice the embodiments described.

It should be appreciated that although an IC utilizing specific transistor types in certain circuit configurations is disclosed (e.g., N-channel field-effect transistors), different transistor types (e.g., P-channel) may also be utilized in alternative embodiments. In still other embodiments, some or all of the metal-oxide-semiconductor field-effect transistor (MOSFET) devices show by way of example may be replaced with bipolar junction transistors (BJTs), insulated gate field effect transistor (IGFETs), or other device structures that provide a transistor function. Furthermore, those of skill in the art of integrated circuits and voltage and/or current reference circuits will understand that transistor devices such as those shown by way of example in the figures may be integrated with other transistor device structures, or otherwise fabricated or configured in a manner such that different devices share common connections and semiconductor regions (e.g., N-well, substrate, etc.). For purposes of this disclosure, “ground” or “ground potential” refers to a reference voltage or potential against which all other voltages or potentials of a circuit or IC are defined or measured.

FIG. 1 illustrates a circuit schematic diagram of a temperature independent reference circuit 100 for generating both a temperature-compensated reference voltage and a temperature-compensated reference current at the same time on an IC. (In the context of the present application, the term “IC” is considered synonymous with a monolithic device.) Temperature independent reference circuit 100 includes NPN bipolar transistors Q 1 , Q 2 , Q 3 and Q 4 . Transistors Q 1 & Q 2 are matched devices with Q 1 having an emitter size ratio of “a” with respect to emitter size of Q 2 , where “a” is an integer greater than 1. The emitter of Q 2 is shown coupled to ground. The emitter of Q 1 , node V x , is coupled to ground through series-connected resistors R 1 and R 2 . In the embodiment shown, a temperature independent current reference I REF flows through resistors R 1 and R 2 , where I REF =V x /(R 1 +R 2 ). The collector of Q 1 , node 102 , is coupled to the base of Q 3 and an end of resistor R 3 . The other end of R 3 , node 103 , is connected to the emitter of transistor Q 4 . Node 103 provides a temperature independent voltage reference V REF that is derived from the temperature independent current reference I REF , as described in more detail below.

Continuing with the example of FIG. 1 , the base of transistor Q 4 is commonly coupled to the collector of Q 3 , resistor R 4 , and the drain of p-channel metal-oxide-semiconductor field-effect transistor (PMOS) MP 1 . The other end of R 4 and the source of MP 1 are connected to the voltage supply potential VDD. The gate of MP 1 is coupled to receive a power-up (PU) signal that ensures the proper operation of the circuit. At power-up, VDD ramps up from ground potential and PU is initially low to drive current into the base of Q 4 . When VDD reaches a potential high enough for circuit 100 to operate, power-up signal PU transitions to high, thereby turning off MP 1 .

Temperature independent reference circuit 100 further includes PMOS transistor MP 2 coupled between VDD and the collector of Q 4 . The gate and drain of MP 2 are commonly coupled to the gates of matched PMOS transistors MP 3 and MP 4 in a current mirror configuration with NPN transistors Q 1 & Q 2 so as to reflect the temperature independent current reference I REF through MP 4 for output elsewhere on the IC. Practitioners in the art will appreciate that the circuit of FIG. 1 generates a temperature compensated current I REF , which current is then utilized to generate a temperature compensated voltage V REF at node 103 . To achieve this result, resistors R 3 and R 1 have a ratio of M and are matched, meaning that they have the same temperature coefficient of resistance due to the fact that they are fabricated of the same material on the IC. In one embodiment, R 1 and R 3 comprise a semiconductor material implanted or diffused with P type dopant.

A temperature coefficient TC may be defined as the relative change of a physical property when the temperature is changed by one degree C. The temperature coefficient of resistors R 3 and R 1 , TC 3 , is positive and larger than the positive temperature coefficient of ΔV BE , TC 1 . In particular, ΔV BE is the difference between the voltage across base to emitter of transistors Q 1 and voltage across base to emitter of transistor Q 2 . Resistor R 2 is fabricated of a different material type (e.g., polysilicon) as compared to resistors R 3 and R 1 . The temperature coefficient, TC 2 , of R 2 is also positive but smaller than TC 1 . When this circuit is operating properly, the currents flowing thru Q 1 and Q 2 are forced to be equal by the current mirror transistors MP 2 and MP 3 , resulting in a ΔV BE across the series connected resistors R 1 and R 2 . The resistance ratio of R 1 /R 2 is chosen such that, TC 1 =TC 2 ×(R 2 /(R 1 +R 2 ))+TC 3 ×(R 1 /(R 1 +R 2 )). This makes the change over temperature in the combined resistance, R 1 +R 2 , the same as the change over temperature in ΔV BE , resulting in a current I REF flowing thru R 1 and R 2 that is constant over temperature.

To better understand the operation of temperature independent reference circuit 100 , temperature independent current reference I REF may be expressed mathematically by the equation:

To achieve temperature independent current reference I REF , the percent change in ΔV BE should be equal to the percent change in total resistance (R 1 +R 2 ). As further shown, the percent change in ΔV BE may be calculated by the equation (2) below:

Percent ⁢ ⁢ change ⁢ ⁢ in ⁢ ⁢ Δ ⁢ ⁢ V BE = ( Δ ⁢ ⁢ V BEF - Δ ⁢ ⁢ V BEI Δ ⁢ ⁢ V BEI ) · 100 ⁢ % ( 2 )

where ΔV BEF represents the difference in base-to-emitter voltage between Q 1 & Q 2 at a final temperature and ΔV BEI represents the difference in base-to-emitter voltage between Q 1 & Q 2 voltage at an initial temperature.

It is known to one skilled in the art that ΔV BE may be determined based on the following equation:

Δ V BE =V BE2 −V BE1 =V T ·ln a   (3)

where ln is the natural logarithm, “a” is the relative sizing ratio of Q 1 with respect to Q 2 , and V T is a constant that varies only as temperature varies. This leads into equation (4), shown below, which gives the percent change of ΔV BE in terms of V T :

Percent ⁢ ⁢ change ⁢ ⁢ in ⁢ ⁢ Δ ⁢ ⁢ V BE = ( V TH · ln ⁢ ⁢ a - V TI · ln ⁢ ⁢ a V TI · ln ⁢ ⁢ a ) · 100 ⁢ % ( 4 )

where V TF is the value of the constant V T at a final temperature and V TI is the value of the constant V T at an initial temperature.

As shown, the percent change in (R 1 +R 2 ) may be calculated by the equation (5) below:

The above equation can be realized by setting R 1 and R 2 depending on the percent change of the resistance of each resistor such that the total percent change over temperature of the total resistance matches the total percent change over temperature of ΔV BE . As explained above, in one embodiment, resistors R 1 and R 2 are manufactured of different materials, so the percentage change in resistance value over temperature is different between the two resistors.

By way of example, if we assume that ΔV BE varies by 33% over 100° C. (e.g., ΔV BEF =48 mV, ΔV BEI =36 mV), and R 1 and R 2 vary respectively by 60% and 6% over the same temperature range, then the ratio of R 1 to R 2 may be 50/50, meaning that R 1 provides 30% and R 2 provides 3% of the temperature compensation that substantially cancels out the 33% change of ΔV BE . In other words, the change in percentage over temperature in the combined resistance, R 1 +R 2 , is set to be the same as the change in percentage over temperature in ΔV BE , resulting in a current I REF flowing thru R 1 and R 2 that is substantially constant over temperature.

Turning now to the temperature independent voltage reference aspect of temperature independent reference circuit 100 , the output reference voltage V REF generated at node 103 is related to the voltage across resistor R 3 , V R3 , which is established by I REF (e.g., V R3 =R 3 ×I REF ). Since I REF does not substantially vary with temperature as discussed above, the voltage V R3 possesses the same temperature coefficient as R 3 (i.e., TC 3 ). As shown, the output reference voltage V REF is the sum of the V BE of Q 3 (V BE3 ), which typically has a temperature coefficient −2 mV/° C., plus the voltage V R3 which has a positive temperature coefficient of TC 4 . Stated in different mathematical terms,

V REF =V BE3 +V R3   (6)

Equation (6) shows that to achieve a temperature independent voltage, V REF , the change in voltage drop V R3 over temperature must substantially equal to the absolute value of the change in V BE3 over temperature. That is, the temperature variation of V R3 is set to be approximately +2 mV/° C. to substantially cancel out the temperature variation of the V BE3 .

Another way to look at it is that change in resistance R 3 is made to cancel out the change in voltage V BE3 over a given temperature range, as represented in equation (7) below, where V BE3F and V BE3I are the final and initial base-emitter voltages, and V R3F and V R3I are the final and initial voltages across R 3 , at high and low temperatures, respectively.

V BE3F −V BE3I =−( V R3F −V R3I )  (7)

For example, let us assume that the temperature coefficient of V BE3 is exactly −2 mV/° C., so that over a 100° C. increase in temperature the voltage drop across V BE3 decreases by 200 mV. To achieve a temperature independent output reference voltage V REF , the voltage drop V R3 must also increase by 200 mV over the same 100° C. increase in temperature. Since R 3 and R 1 are matched resistors (i.e., made of the same material) their resistance values both change in the same percentage over a unit temperature. The reference output current I REF is set in accordance with the description provided above, which means that R 3 may be determined by the following equation.

R 3 = Δ ⁢ ⁢ V R ⁢ ⁢ 3 Δ ⁢ ⁢ V R ⁢ ⁢ 1 · R 1 ( 8 )

where ΔV R3 =V R3F −V R3I and ΔV R1 =V R1F −V R11 . The change in V R1 is set due to the resistance value of R 1 and I REF . In the example, the change in V R3 is 200 mV. Therefore, R 3 may be determined such that the decrease of voltage V BE3 is the same as the increase of voltage drop V R3 over a change in unit temperature.

FIG. 2 illustrates another example circuit schematic diagram of a temperature independent reference circuit 200 for simultaneously generating both a temperature-compensated reference voltage and a temperature-compensated reference current on an integrated circuit (IC). Temperature independent reference circuit 200 is identical to circuit 100 of FIG. 1 in every respect, except that resistor R 4 in temperature independent reference circuit 100 is replaced by PMOS transistor MP 5 in temperature independent reference circuit 200 . PMOS transistor MP 5 functions as another current mirror transistor, which ensures the current flowing thru NPN transistor Q 3 remains constant over temperature. In addition, another advantage for replacing resistor R 4 with transistor MP 5 is to reduce total area of temperature independent reference circuit 200 . Practitioners in the art will understand that this improvement eliminates a relatively minor error term in V REF present in the embodiment of FIG. 1 . This error term tends to cause a slight change in V REF due to current density changes in the voltage V BE3 .

Although the present invention has been described in conjunction with specific embodiments, those of ordinary skill in the arts will appreciate that numerous modifications and alterations are well within the scope of the present invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.