Patent Application
20170269624
The invention relates to current reference generators, and more particularly, to current reference generators that mix currents to generate a reference current with relatively low temperature and process coefficients.
A current reference circuit is an essential part of an autonomous Input/Output (I/O) limited integrated circuit. An approach to generate a stable current is to employ an external (e.g., off-chip) precision resistor and produce a fixed voltage across this resistor through internal (e.g., on-chip) circuitry. Off-chip resistors are used since on-chip resistors suffer from relatively large (e.g., 20-30%) tolerances and therefore are not very suitable for generating a stable reference current using this technique. In certain I/O-limited applications, current variations in a simplistic on-chip current reference circuit due to process voltage temperature (PVT) variations lead to specification violation or functional failure.
With complementary metal-oxide semiconductor (CMOS) processes in the deep submicron regime, second-order effects (e.g., drain-induced-barrier-lowering) have reduced transistors intrinsic drain-to-source resistance and have pushed transistors towards highly non-ideal current source behaviors. A temperature compensation technique includes generating a proportional to absolute temperature (PTAT) and a complementary to absolute temperature (CTAT) current and adding them up to achieve a smaller temperature coefficient. This, however, does not address process variations, which are especially problematic for deep submicron technologies.
Another technique to address temperature compensation is based on passively mixing components having opposite temperature and process coefficients. This approach, however, provides a very limited freedom as different components have different geometrical and structural issues. Also, this approach leads to further issues of reducing sensitivities without adding any extra fabrication or structural sensitivities.
In an aspect of the invention, a current reference generator includes a first voltage reference configured to generate a first current through a first resistor; a second voltage reference configured to generate a second current; and a first current mirror configured to subtract the second current from the first current to generate a temperature invariant current.
In an aspect of the invention, a system comprises: a first voltage reference configured to generate a first current through a first resistor; a second voltage reference configured to generate a second current; a first current mirror configured to mix the first current and second current to generate a temperature invariant current; a third voltage reference configured to generate a third current via a second resistor; and a second current mirror configured to: mix the third current and the temperature invariant current to produce a process-temperature invariant current, and output the process-temperature invariant current.
In an aspect of the invention, a system comprises: a current reference generator configured to output a current-temperature invariant current.
The present invention is described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention.
The invention relates to current reference generators, and more particularly, to current reference generators that mix currents to generate a reference current with relatively low temperature and process coefficients. Aspects of the present invention provide a process voltage temperature (PVT) tolerant compensated precision current reference for application specific integrated circuits. In embodiments, the precision current reference generator exhibits relatively smaller scattering in bias current value for PVT variations without needing an external precision resistor.
In embodiments, the current reference generator circuit mixes three different temperature and process coefficients with a relatively high-degree of insulation from supply voltage to considerably reduce the current variations in the output bias current. In embodiments, the circuit may mix and match different sets of temperature and process coefficients available within a process design kit (e.g., design libraries).
As described herein, the current reference generator circuit first subtracts two currents to achieve a near zero temperature coefficient but still with a large process coefficient. Another current is generated which natively has a relatively small temperature coefficient. This current is mixed with the difference of the previous two current to minimize the process coefficient. The currents are generated in a manner such that they are isolated from the power supply using components of a relatively high impedance, therefore, also achieving voltage tolerance. In this manner, complete PVT tolerance is achieved across all process corners.
As described herein, three currents are employed in generating the reference current:
As shown in
The rppolyh resistor 110 (also referred to as an rphpoly resistor) may include a precision P+ polysilicon resistor without salicide. The current output after the voltage is provided through the rppolyh resistor 110 is a current reference, referred to as I1. The current reference I1 may be proportional to an absolute temperature (PTAT) current that is generated from the rppolyh resistor 110. As further shown in
As an illustrative, non-limiting example, the temperature and process coefficients can be used to express currents I1 and I2 as following for a particular bias point.
I
1(T,p)=97.8809+p*103.8716+T*(0.2638408+p*0.2888912) (1)
I
2(T,p)=88.6093+p*37.4134+T*(0.3816264+p*0.161782) (2)
where T is absolute temperature, p is process coefficient (0 for min corner and 1 for max corner).
While particular values are provided in the above example, in practice, the values may vary based on the properties of the rppolyh resistor 110 and of the band gap 125. That is, the values may be known based the known properties of the rppolyh resistor 110 and of the band gap 125.
The current reference I1 is provided to a current gain amplifier 115, which applies a gain A to the current reference I1. As described herein, the gain A is applied in order to match the temperature coefficients of I1 and I2 such that when the currents I2 and gainA*I1 are subtracted, the resulting current is a temperature invariant current.
In embodiments, the gain A is based on the properties and attributes of the rppolyh resistor 110 and of the band gap 125. For example, to determine the gain A, the temperature coefficients of I1 and I2 are matched, and the difference of the currents I1 and I2 is taken (e.g., using equation 3 below).
δ/δT(A*I1−I2)=0 (3)
Solving the partial derivate by substituting I1 in equation 3 with I1 in equation 1, and substitution I2 in equation 3 with I2 in equation 2 produces the result:
97.8809*A*(0.0026955+0.00295154*p)−88.6093*(0.004306+0.0018258*p)=0 (4)
Equation 4 is then solved with respect to A for both process corners (e.g., when p=0 and p=1). Solving equation 4 for A when p=0 produces the result:
A=1.4461 (5)
Solving equation 4 for A when p=1 produces the result:
A=0.9830 (6)
In embodiments, the two values for A may be averaged in order to ensure that the current change over temperature is minimal for both process corners. Averaging the values for A as shown in equations 5 and 6 produce the result:
A=1.21455 (7)
The amplified current (e.g., the current GainA*I1) is subtracted from the current reference I2 to produce the output current I4. For example, the current GainA*I1 and the current reference I2 are mixed (e.g., subtracted) by a current mirror 120, as shown in
I
4(T,p)=28.84778+87.234606*p−T*(0.06494−p*0.1848799 (8)
where T is absolute temperature, p is process coefficient (0 for min corner and 1 for max corner).
As shown in
As an illustrative, non-limiting example, the temperature and process coefficients can be used to express current I3 as following for a particular bias point.
I
3(T,p)=28.0352+p*11.97+T*(0.00492944+p*0.00386) (9)
While particular values are provided in the above example, in practice, the values may vary based on the properties of the rppolyl resistor 210. That is, the values may be known based the known properties of the rppolyl resistor 210.
The current I3 is provided to a current gain amplifier 215, which applies a gain B to the current I3. As described herein, the gain B is applied in order to match the process coefficient of I4 (e.g., the temperature invariant current produced by the circuit 100 of
δ/δp(B*I3−I4)=0 (10)
Substituting I3 in equation 10 with I3 in equation 9 and I4 in equation 10 with I4 in equation 8 and subsequently solving the partial derivate of equation 10 produces the following result:
B*(11.9699+0.0038599*T)−87.234606+0.1848799*T=0 (11)
Setting T=0 in equation 11 to eliminate the temperature coefficient and solving for B yields the result:
B=7.2878 (12)
The current I4 (which is the temperature-invariant current produced by the circuit 100 of
As described herein, aspects of the present invention may mix different components to nullify temperature and process coefficients. However, instead of performing mixing and matching passively, aspects of the present invention generate currents from each component and subsequently mix the currents using an active current-mirroring technique. The current-mirroring allows the circuit to have a large of current-ratio(s) so that the three different currents can be mixed with the optimally required coefficients in a power and area efficient manner. Due to its active nature, this approach itself consumes a particular amount of power to achieve a relatively high-accuracy current matching.
As described herein, the current reference generator, in accordance with aspects of the present invention, include the circuit 100 of
The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.
