Temperature independent CMOS reference voltage circuit for low-voltage applications

The present invention relates to reference voltage generators, and more particularly relates to a CMOS reference voltage circuit that is temperature-independent for low-voltage applications.

In many electronic applications; a reference voltage having a very low temperature coefficient is required. Band-gap voltage reference circuits have been developed to fulfill this need. The nominal temperature coefficient (TC) of a silicon diode is −2 mV/° C. However, TC is inversely proportional to the current density J in the diode. By manipulating the current densities through two diodes and taking the difference in forward bias voltages, one can create a circuit with a well-defined positive TC. This is then added to the forward bias voltage of a third diode. The positive TC of the diode pair cancels the negative TC of the third diode and one is left with a circuit with zero TC.

Referring to band-gap voltage reference circuits having bipolar transistors, FIG. 1 depicts a conventional bandgap circuit 100 . The circuit 100 contains two bipolar transistors 102 and 104 . The two bipolar transistors 102 and 104 have voltages V BE and ΔV BE in relation. V BE has a negative temperature coefficient, which is −2 mV/C. ΔV BE has a positive temperature coefficient, which depends on the current density of the two bipolar transistors 102 and 104 . The relation is expressed by the following equation:

Δ V BE =( KT/q )*ln( J 2 / J 1 )

The bandgap circuit 100 operates by using PTAT and CTAT currents from two branches to derive a constant current into the resistor, which generates a constant reference voltage. In circuit 100 , three resistors 106 , 108 and 110 must be matched. Thus, the conventional bandgap circuit 100 is a complex circuit.

Further, there are many PMOS and NMOS transistors that need to be matched to obtain a low temperature-dependent reference voltage. Accordingly, a need exists for a simplified temperature-independent reference voltage circuit.

In view of the foregoing, a need exists to overcome the problems with the prior art as discussed above.

In accordance with the present invention, a temperature independent CMOS reference voltage circuit includes a CMOS current mirror circuit containing first and second CMOS transistors of a first polarity. A temperature compensation circuit is coupled to the CMOS current mirror circuit, and contains a first resistor, a second resistor, and third and fourth CMOS transistors of a second polarity. The third and fourth CMOS transistors are configured to operate substantially in a subthreshold region. One of the third and fourth CMOS transistors is diode connected.

Other aspects and features of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 is a schematic diagram illustrating a conventional bandgap circuit having typical bipolar transistors.

FIG. 2 is a schematic diagram of a conventional reference current circuit.

FIG. 3 is a schematic diagram showing a CMOS temperature independent voltage reference circuit according to one embodiment of the present invention.

FIG. 4 is a graph showing simulation results of Vt's temperature coefficient for the circuit of FIG. 3 , according to one embodiment of the present invention.

FIG. 5 is a graph showing simulation results of the temperature coefficient for proportional-to-absolute-temperature (PTAT) voltage for the circuit of FIG. 3 , according to one embodiment of the present invention.

FIG. 6 is a graph illustrating simulation results of Vref versus temperature for the circuit of FIG. 3 , according to one embodiment of the present invention.

While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.

The present invention, according to one embodiment, overcomes problems with the prior art while reducing the size and power required for the operation of the circuit.

Reference throughout the specification to “one embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Moreover, these embodiments are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features, but not to others. In general, unless otherwise indicated, singular elements may be in the plural and visa versa with no loss of generality.

Turning now to FIG. 2 , there is shown a conventional PTAT-current circuit that generates an output current with an accuracy of about +/−25% or higher.

Implementation Embodiment in Hardware

According to one embodiment of the present invention, as shown in FIG. 3 , the CMOS temperature independent voltage reference circuit 300 contains two PMOS transistors MP 1 , MP 2 , two NMOS transistors MN 1 , MN 2 , and two resistors R 1 and R 2 . The current mirror circuit 302 is formed by the two PMOS transistors MP 1 and MP 2 . The temperature compensation circuit 304 contains the two NMOS transistors MN 1 , MN 2 , and the two resistors R 1 and R 2 .

In an embodiment, the resistors R 1 and R 2 are variable. The resistor R 1 is coupled between the sources of the two NMOS transistors MN 1 , MN 2 . The gates of the NMOS transistors MN 1 , MN 2 transistors are interconnected.

In accordance with the present invention, a first side of the resistor R 1 is coupled to the drain of the CMOS transistor MN 2 , and a second side of the resistor R 1 is coupled to the CMOS current mirror circuit 302 .

Further, the temperature compensation circuit 304 is configured so as to generate a reference voltage, V ref , containing a proportional to absolute temperature (PTAT) voltage component, and a threshold voltage of the NMOS transistor MN 2 , as explained below.

The CMOS temperature independent voltage reference circuit 300 is configured so as to operate the two NMOS transistors MN 1 and MN 2 near subthreshold regions. One of the NMOS transistors MN 1 and MN 2 is diode-connected (i.e., drain is connected to gate). For example, in an embodiment, the NMOS transistor MN 2 is diode connected. Accordingly, a small current is needed to operate the transistor MN 2 near the subthreshold region. Therefore, the NMOS transistors MN 1 and MN 2 , which are operated near the subthreshold regions, behave like bipolar transistors, and subthreshold MOS current formula is applied to derive the temperature-independent reference voltage V ref . The derivation is shown below, as set forth in equation (1):

From KVL rules,

V ref =I D2 R 2 +V GS2   Eq(1)

where V ref is the reference voltage, I D2 is the drain current and V GS2 is the gate-to-source voltage of the NMOS transistor MN 2 .

If V GS2 =V t2 (i.e., transistor MN 2 's threshold voltage), it implies that the NMOS transistor MN 2 is very close to the subthreshold region.

Again, from KVL rules:

V ref= I D2 R 2 +V t2   Eq (2)

V GS1 +I D1 R 1 =V GS2   Eq (3)

If the NMOS transistors MN 1 and MN 2 are in the subthreshold region, their drain currents are defined as shown below:

I D = W L ⁢ I D0 ⁢ exp ⁡ ( V GS - V t n ⁢   ⁢ V T ) Eq ⁢   ⁢ ( 4 )

where

V T = k ⁢   ⁢ T q ,

k is Boltzman's constant and Vt is the threshold voltage of MOS transistor.

Assuming

α = W L ⁢ I D0 ,

Equation (4) becomes:

I D = α ⁢   ⁢ exp ⁡ ( V GS - V t n ⁢   ⁢ V T ) Eq ⁢   ⁢ ( 5 ) V GS = n ⁢   ⁢ V T ⁢ ln ⁡ ( I D α ) + V t Eq ⁢   ⁢ ( 6 )

Solving equations (3) and (6) yields:

n ⁢   ⁢ V T ⁢ ln ⁡ ( I D1 α 1 ) + I D1 ⁢ R 1 = n ⁢   ⁢ V T ⁢ ln ⁡ ( I D2 α 2 ) Eq ⁢   ⁢ ( 7 ) I D1 ⁢ R 1 = n ⁢   ⁢ V T ⁢ ln ⁡ ( I D2 ⁢ α 1 I D1 ⁢ α 2 ) Eq ⁢   ⁢ ( 8 )

If I D1 =I D2 =I D , by design, then equation (8) becomes:

I D = n ⁢   ⁢ V T R 1 ⁢ ln ⁡ [ ( W / L ) 1 ( W / L ) 2 ] Eq ⁢   ⁢ ( 9 )

Solving equations (2) and (9) yields:

V ref = n ⁢   ⁢ V T ⁡ ( R 2 R 1 ) ⁢ ln ⁡ [ ( W / L ) 1 ( W / L ) 2 ] + V t2 Eq ⁢   ⁢ ( 10 ) V ref = n ⁢   ⁢ k ⁢   ⁢ T q ⁢ ( R 2 R 1 ) ⁢ ln ⁡ [ ( W / L ) 1 ( W / L ) 2 ] + V t2

where n is a constant, 1<n<2, k is Boltzman's constant, and q is charge.

In equation (10), the first term on the right hand side is “proportional to absolute temperature (PTAT) voltage”, which has a positive temperature coefficient. The second term is threshold voltage of the NMOS transistor, which has a negative temperature coefficient. Thus, equation (10) describes a reference voltage, which has a very small temperature-dependence, similar to a conventional bandgap reference voltage.

Thus, as shown above in equation (10), Vref is determined only by the ratios of NMOS transistors MN 1 , MN 2 and resistors R 1 , R 2 . If the NMOS transistors MN 1 , MN 2 and resistors R 1 , R 2 are matched well, Vref does not change according to the type of NMOS transistors used or the absolute value of the resistance.

Accordingly, the present invention also includes a complementary circuit, in an embodiment, with PMOS transistors working substantially in the sub-threshold regions and NMOS transistors functioning as current mirrors.

Further, the current mirror circuit 302 is a simplified circuit. It can be replaced, in other embodiments, by other circuits such as a cascode circuit and a gain boosted circuit, in order to increase the power supply rejection ratio (PSRR) of the circuits.

In addition, in other embodiments, the circuit 300 of the present invention is coupled with a non-illustrated circuit substrate so as to form an integrated circuit.

Regarding Vt, the threshold voltage of the MOS transistor is defined in equation (11), as shown below:

V t =V to −α( T−T o )  (11)

where V to is the threshold voltage at absolute temperature=0, To=0 K and α is the proportional constant. α changes with process nodes, and a typical example is given below.

Design Example

A real design example in the 1233C027 process is included below for demonstrative purposes. Simulation was also run in the 1833C05 process, and it showed a similar temperature-independent behavior.

FIG. 4 is a graph showing the simulation results of Vt's temperature coefficient for the circuit 300 according to one embodiment of the present invention. FIG. 5 is a graph showing the simulation results of the temperature coefficient for proportional-to-absolute-temperature (PTAT) voltage for the circuit 300 . FIG. 6 is a graph illustrating the simulation results of Vref versus temperature for the circuit 300 .

a) Temperature Coefficients:

Simulations over a temperature range were used to verify the design. For the C027 process, it is found from the simulation that V t 's temperature coefficient is approximately equal to 0.8 mV/C, as shown in FIG. 4 ; and k/q is approximately equal to +0.08 mV/C (i.e., the temperature coefficient for the PTAT voltage), as shown in FIG. 5 .

b) Calculation of Device and Resistor Ratios:

From part a), the temperature coefficients of the Vt and PTAT voltages have been calculated using SPICE simulation, which are applied to equation (10) for calculating the ratios of the NMOS transistors MN 1 -MN 2 and resistors R 1 -R 2 in order to obtain the minimal temperature variation of Vref. In the following design case, circuit specifications are targeted as shown below:

Voltage Reference Specifications:

Vref˜640 mV at 27 degrees C. Current consumption<10 uA Supply Voltage˜1.3 V Temperature coefficient of Vref<50 ppm/C

Accordingly, the drain current is partitioned to be 4 uA in each leg. From FIG. 4 , it can be seen that Vt is 0.395 V at 27 degrees C. Thus, R 2 needs to be around 60 kohms to achieve the desired Vref. (W/L) 1 /(W/L) 2 can be chosen as 8 to keep the NMOS transistor close to the subthreshold region. The ratio of R 2 /R 1 is about 3.53 to get a reference voltage with minimized temperature dependence. From the simulation results, as is illustrated in FIG. 6 , the temperature variation is about 43 ppm over the range of −40 to 125 deg C.

The original layout area for the C035 version is about 400 um*400 um. However, in this design it is only about 110 um*110 um, including the test pad (70 um*60 um). The area is reduced and the current is also reduced from 20 uA to 10 uA within the same specification, +/−5% variation of the Vref. Therefore, the size of the reference circuit and the power are reduced. This is very helpful for low voltage mixed-signal applications.

Thus, advantageously, the present invention realizes a low power and low current circuit, without any bipolar transistor, while generating a stable reference voltage, similar to a bandgap voltage generator.

Non-Limiting Embodiments

In view of the above, it can be seen the present invention presents a significant advancement in the art of reference voltage circuit technology. Further, this invention has been described in considerable detail in order to provide those skilled in the data communication art with the information needed to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should further be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow.