Abstract:
A constant current source for generating a constant reference current that is relatively temperature insensitive. The constant current source comprises: i) first circuitry for generating a first output current that increases with increases in temperature; ii) second circuitry for generating a second output current that decreases with increases in temperature; and iii) a current combiner circuit that combines the first and second output currents to thereby generate the constant reference current. A change in the first output current caused by a temperature change is at least partially offset by a change in the second output current caused by the temperature change.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is generally directed to current sources for use in integrated circuits, and more specifically, to a reference current generating circuit that is highly stable and temperature insensitive across a defined operating range of supply voltage. 
     BACKGROUND OF THE INVENTION 
     There are a myriad number of applications in which an integrated circuit (IC) requires a constant current source (or reference current generator). These applications encompass both digital devices and analog devices. Many constant current sources are based on the principle of creating a constant voltage drop across a fixed on-chip resistor, thereby getting a constant current source. These types of constant current sources are suitable for most of the common application. 
     However, many applications require high precision and operate under extreme temperature conditions. One typical example is a cell phone, which typically includes high-precision components and must operate in sub-zero temperatures and in 100+ degree temperatures. For these types of applications, a constant current source based on a fixed on-chip resistor cannot be used. This is because the on-chip resistor is temperature sensitive. As the resistance increases with temperature, the current through the on-chip resistor must be reduced in order to maintain a constant voltage across the on-chip resistor. Thus, the supposedly constant current decreases with increasing temperature. This problem is often addressed by adding cooling equipment that maintains the integrated circuit in a narrow operating temperature range, thereby minimizing the variation in the desired constant current. 
     Therefore, there is a need in the art for improved constant current sources. In particular, there is a need for a reference current generator that is highly temperature insensitive across a relatively wide range of operating temperatures. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems inherent in the prior art by generating a constant current that is highly temperature-insensitive and very stable across a relatively wide operating range of supply voltages. The present invention has the advantage that process variations have only a very minimum effect on the temperature-insensitiveness of the constant current source. This is achieved by combining a positive temperature coefficient dependent current with a negative temperature coefficient dependent current in correct proportions to get a desired constant output current. 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a constant current source for generating a constant reference current that is relatively temperature insensitive. According to an advantageous embodiment of the present invention, the constant current source comprises: i) first circuitry for generating a first output current that increases with increases in temperature; ii) second circuitry for generating a second output current that decreases with increases in temperature; and iii) a current combiner circuit that combines the first and second output currents to thereby generate the constant reference current, wherein a change in the first output current caused by a temperature change is at least partially offset by a change in the second output current caused by the temperature change. 
     According to one embodiment of the present invention, the first circuitry comprises a first reference current generator for generating a first reference current that increases with increases in temperature. 
     According to another embodiment of the present invention, the first circuitry further comprises a first current mirror circuit capable of multiplying the first reference current by a factor m to thereby generate the first output current that increases with increases in temperature. 
     According to still another embodiment of the present invention, the second circuitry comprises a second reference current generator for generating a second reference current that decreases with increases in temperature. 
     According to yet another embodiment of the present invention, the second circuitry further comprises a second current mirror circuit capable of multiplying the second reference current by a factor n to thereby generate the second output current that decreases with increases in temperature. 
     According to a further embodiment of the present invention, the factor m and the factor n are determined based on the relative proportions of the first and second reference currents. 
     According to a still further embodiment of the present invention, the factor m and the factor n are selected according to the relative proportions of the first and second reference currents required to negate the temperature dependency of the constant reference current. 
     According to a yet further embodiment of the present invention, the first and second current mirror circuits comprise P-channel transistors. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIG. 1  illustrates a cellular telephone containing a reference current generator according to the principles of the present invention; 
         FIG. 2  illustrates the reference current generator in  FIG. 1  in greater detail according to an exemplary embodiment of the present invention; 
         FIG. 3  illustrates the positive temperature coefficient current generator in the reference current generator in greater detail according to an exemplary embodiment of the present invention; and 
         FIG. 4  illustrates the negative temperature coefficient current generator in the reference current generator in greater detail according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 through 4 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged electronic device that requires a stable, temperature-insensitive constant current source. 
       FIG. 1  illustrates cellular telephone  100 , which contains reference current generator  140  according to the principles of the present invention. Cellular telephone  100  contains printed circuit board (PCB)  101 , which comprises analog-to-digital converter (ADC)  105 , low-drop-out (LDO) voltage regulator  110 , audio amplifiers  115 , codec  120 , controller  125 , battery  130 , and reference current generator  140 . Reference current generator  140  provides a highly stable, temperature-insensitive constant reference current for one or more of ADC  105 , LDO voltage regulator  110 , audio amplifiers  115  and codec  120 , among other circuits. 
     According to an exemplary embodiment of the present invention, controller  130  of cellular telephone  100  is capable of conserving power and prolonging the operating life of battery  120  by periodically shutting down reference current generator  140 , and many of the other electrical circuits in cellular telephone  100 . According to an exemplary embodiment of the present invention, reference current generator  140  may be a part of a band-gap reference circuit (or similar circuit) that also provides a constant reference voltage to ADC  105 , LDO voltage regulator  110 , audio amplifiers  115  and codec  120 , among other circuits. 
     In the embodiment shown in  FIG. 1 , reference current generator  140  is disposed in a cell phone. However, this is by way of illustration only and should not be construed so as to limit the scope of the present invention. Those skilled in the art will recognize that reference current generator  140  or a similar constant current generator according to the principles of the present invention may be implemented in a myriad number of integrated circuit applications. 
       FIG. 2  illustrates selected portions of reference current generator  140  in greater detail according to an exemplary embodiment of the present invention. Reference current generator  140  comprises positive coefficient current generator  210 , negative coefficient current generator  220 , P-channel transistor  230 , P-channel transistor  240 , and N-channel transistor  250 . The gate of N-channel transistor  250  is controlled by the biasing voltage, V(bias). The purpose of reference current generator  140  is to generate an output current, I(out), that is very stable with respect to temperature variations from about −40° C. to about 120° C. and also over a significant supply voltage range. The present invention minimizes the process dependency so that technology transfer between processes becomes less complicated. 
     According to the principles of the present invention, positive coefficient current generator  210  generates a first reference current, I(x), that increases with respect to increases in temperature and negative coefficient current generator  220  generates a second reference current, I(y), that decreases with respect to increasing temperature. The current I(x) is mirrored by P-channel transistor  230  as the current mI(x), where the scaling factor m is determined by the width (W) and length (L) dimensions of P-channel transistor  230 . The current I(y) is mirrored by P-channel transistor  240  as the current nI(y), where the scaling factor n is determined by the width (W) and length (L) dimensions of P-channel transistor  240 . Since the current mI(x) mirrors the first reference current, I(x), the current mI(x) also increases with temperature increases. Since the current nI(y) mirrors the second reference current, I(y), the current nI(y) also decreases with temperature increases. 
     Finally, N-channel transistor  250  combines the current mI(x) and the current nI(y) to form the output current, I(out). As temperature increases, the increase in the current mI(x) is offset by the decrease in the current nI(y), so that I(out), the drain current in N-channel transistor  250 , remains constant. The currents I(x) and I(y) are added in the correct proportions to cancel out any temperature dependency. When the currents mI(x) and nI(y) are generated on the same integrated circuit (IC), this also ensures that the currents mI(x) and nI(y) are independent of the supply. Therefore, the output current, I(out), is also supply independent. The biasing voltage, V(bias) can be used as the gate biasing voltage for all of the N-channel transistors anywhere on the IC chip to reflect the required amount of current. 
       FIG. 3  illustrates selected portions of positive temperature coefficient current generator  210  in reference current generator  140  in greater detail according to an exemplary embodiment of the present invention. Positive temperature coefficient current generator  210  comprises P-channel transistor  310 , P-channel transistor  320 , n-p-n bias-junction transistor  330 , n-p-n bias junction transistor  340  and resistor  350 . According to an exemplary embodiment of the present invention, transistor  340  has a size (A 1 ) that is ten times larger than the size (A 2 ) of transistor  330  (i.e., A 1 = 10 A 2 ). 
     Positive temperature coefficient current generator  210  is a conventional proportional-to-absolute-temperature (PTAT) current generator that generates a current, I(x), that has a linearly increasing dependency on the absolute temperature. P-channel transistors  310  and  320  form a current mirror. The gates of transistors  310  and  320  are coupled together and the sources of transistors  310  and  320  are coupled to the VDD power supply, so that the gate-to-source voltage (Vgs) for transistor  310  is identical to the gate-to-source voltage for transistor  320 . Since Vgs is the same for transistors  310  and  320 , the drain currents are the same for transistors  310  and transistor  320 . The gate voltage, V 1 , of transistors  310  and  320  is an output that is used to generate the mirror current mI(x) in N-channel transistor  230 . 
     The identical currents from transistors  310  and  320  ensure that the collector currents of bias-junction transistors  330  and  340  are also identical. In a preferred embodiment of the present invention, a cascode mirror arrangement of transistors may be used to minimize the effect of supply variation. 
     The collector current, I(x) of transistor  340  is given by: 
                       I   ⁡     (   x   )       =       ⁢       (       V   T     /   R350     )     ⁢     ln   ⁡     (     A2   /   A1     )           ;                 =       ⁢       (     kT   /     gR   ⁢   350       )     ⁢     ln   ⁡     (   10   )           ;                 =       ⁢       (   C1   )     ⁢     T   /   R350         ,               
where R 350  is the resistance value of resistor  350  and C 1  is a constant equal to (kln10/q). For all practical purposes, I 1  may be considered to be proportional to absolute temperature.
 
       FIG. 4  illustrates selected portions of negative temperature coefficient current generator  220  in reference current generator  140  in greater detail according to an exemplary embodiment of the present invention. Negative temperature coefficient current generator  220  comprises P-channel transistor  410 , P-channel transistor  420 , N-channel transistor  430 , N-channel transistor  440 , and resistor  450 . According to an exemplary embodiment of the present invention, transistor  440  has a W/L ratio that is z times larger than the W/L ratio of transistor  430 . 
     Negative temperature coefficient current generator  220  generates a current, I(y), that has an approximately linearly decreasing dependency on the absolute temperature. P-channel transistors  410  and  420  form a current mirror. The gates of transistors  410  and  420  are coupled together and the sources of transistors  410  and  420  are coupled to the VDD power supply, so that the gate-to-source voltage (Vgs) for transistor  410  is identical to the gate-to-source voltage for transistor  420 . Since Vgs is the same for transistors  410  and  420 , the drain currents are the same for transistors  410  and transistor  420 . The gate voltage, V 2 , of transistors  410  and  420  is an output that is used to generate the mirror current nI(y) in N-channel transistor  240 . 
     The identical currents from transistors  410  and  420  ensure that the drain currents of N-channel transistors  430  and  440  are also identical. As noted above, transistor  440  has a W/L ratio that is larger than the W/L ratio of transistor  430  by the multiplicative factor, z. For z&gt;1, it can be shown that the current, I(y), depends on the resistance value (R 450 ) of resistor  450  and the channel mobility (μ n ) such that:
 
 Iα 1/[( R 450) 2 (μ n )],
 
where α is an operator meaning “is proportional to”.
 
     The current, I(y), also depends on W/L, C ox , and z, which are constant with respect to temperature. The resistance value (R 450 ) of resistor  450  has a positive temperature coefficient and the channel mobility (μ n ) has a negative temperature coefficient, such that:
 
RαT; and
 
μ n αT −3/2 .
 
     Therefore, the output current, I(y), has a weak negative temperature coefficient. In practice, this current can be chosen in the design so as to offset the increase due to the +ve temperature coefficient current in the temperature range of interest. 
     If P-channel transistor  320  has the dimension ratio (W 1 /L 1 ) and generates the current I(x), then P-channel transistor  230  has the dimension ratio (mW 1 /L 1 ) in order to generate the current mI(x). Similarly, if P-channel transistor  420  has the dimension ratio (W 2 /L 2 ) and generates the current I(y), then P-channel transistor  240  has the dimension ratio (nW 1 /L 1 ) in order to generate the current nI(y). The values of the multiplying factors m and n are determined by the proportion of I(x) and I(y) required to nullify the temperature dependency of the output current, I(out). 
     This process makes the design simpler in the sense that a circuit designer only needs to make sure that the currents are added in the correct proportion to make the output current independent of temperature. In actual implementation, these current mirrors are cascode current mirrors to minimize the variation due to supply voltage. 
     Additionally, positive temperature coefficient current generator  210  and negative temperature coefficient current generator  220  both have two operating points. One operating point is trivial—zero current flows even when the supply has built up to a nominal value. Therefore, a start-up circuit (not shown) ensures that positive temperature coefficient current generator  210  and negative temperature coefficient current generator  220  both reaches the desired operating point when the VDD power supply is applied. 
     Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.