Abstract:
Apparatus and methods are described for providing an adaptive trip point detector circuit that receives an input signal at an input signal node and generates an output signal at an output signal node, the output signal changing from a first value to a second value when the input signal exceeds a trip point reference value. In particular, the trip point reference value is adjusted to compensate for variations in process or temperature, without requiring an externally-supplied reference signal.

Description:
BACKGROUND 
   Most electronic circuits, such as integrated circuits, receive power from an externally-supplied power supply. For example, an electronic system may include a power supply (e.g., V 33 ) that supplies power to one or more integrated circuits included in the system. At system start-up, V 33  may start at an initial value (e.g., 0 volts), and then gradually increase to its full-scale value (e.g., 3.3 volts). Many integrated circuits, however, include chip configuration circuits or other circuits that require a minimum power supply voltage (e.g., 1.5 volts) for normal operation. If a power supply signal less than the minimum is applied to such configuration circuits, the chip may not operate properly. As a result, many integrated circuits use power-on reset (“POR”) circuitry to sense the voltage level of the power supply signal, and generate a control signal that indicates when V 33  exceeds the minimum power supply voltage. 
   To accomplish this task, POR circuits typically compare the power supply signal with a reference signal that has a voltage level equal to the minimum power supply voltage, and generate a control signal that indicates when V 33  is greater than the reference voltage. If the reference signal is an external signal (i.e., off-chip) that is always available, this task is quite straightforward. In most instances, however, an external reference signal is not available, but instead must be generated internally. Previously known POR circuits typically generate such reference signals by using properties of semiconductor devices, such as the threshold voltages of transistors and diodes. 
   For example, referring now to  FIG. 1 , a previously known POR circuit is described. POR circuit  10  includes trip detector circuit  12  and filtering circuit  14 . Trip detector circuit  12  has an input coupled to V 33 , and generates an output signal X HI  that may be used to indicate when V 33  is greater than an internally-generated trip-point reference signal V REF . Filtering circuit  14  smoothes and further processes signal X HI , and generates an output control signal POR OUT  our that may be used to indicate when power supply signal V 33  is sufficiently high for normal circuit operation. 
   Referring now to  FIG. 2 , an exemplary previously known trip detector circuit  12  is described. Trip detector circuit  12  includes diode-connected p-channel transistor  16  having its source terminal coupled to power supply V 33 . and its drain and gate terminals coupled together at node V x . Node V x  also is coupled to ground via resistor  20 , and to the gate of n-channel transistor  18 . N-channel transistor  18  has its drain coupled to output node X HI , which also is coupled to power supply V 33  via resistor  22 . P-channel transistor  16  has a threshold voltage V TP  having a nominal magnitude of about 0.8V, and n-channel transistor  18  has a threshold voltage V TN  having a nominal value of about 0.8V. For simplicity, the symbol V TP  will be used to refer to the magnitude of the threshold voltage of a p-channel transistor. 
   Referring now to  FIGS. 2 and 3 , the operation of exemplary trip detector circuit  12  is described. In particular,  FIG. 3  illustrates V 33 , V x  and X HI  as a function of time. At t=0, V 33 =0V, transistor  16  is OFF, and no current flows through resistor  20 . As a result, V x =0V, transistor  18  is OFF, no current flows through resistor  22 , and X HI =V 33 =0V. For 0≦t&lt;T 1 , V 33  increases, but remains below V TP . As a result, transistor  16  remains OFF, and V X =0. At t=T 1 , V 33  exceeds V x  by the threshold voltage V TP , and transistor  16  begins to conduct. If resistor  20  is very large, the drain current of transistor  16  is very small, and V x  remains one V TP  below V 33 . For T 1 ≦t&lt;T 2 , the voltage on node V X  increases with increasing V 33 , but remains below the threshold voltage V TN  of transistor  18 . Accordingly, transistor  18  remains OFF, no current flows through resistor  22 , and thus X HI =V 33 . At t=T 2 , V x  is greater than V TN , and transistor  18  begins to conduct. If resistor  22  is large, the drain current of transistor  18  is small, and transistor  18  pulls X HI  to ground. Thus, X HI  changes from a positive non-zero voltage to 0V when V 33  exceeds trip-point reference signal V REF =V TP +V TN . 
   Threshold voltages V TP  and V TN , however, may vary significantly with variations in processing and temperature. For example, over normal process and temperature variations, threshold voltages V TP  and V TN  may have values between 0.6V to 1.2V. As a result, trip-point reference signal V REF  may vary between V REFL =1.2V to V REFH =2.4V. For some circuit applications, such a wide variation in V REF  may be unacceptable. For example, as described above, if a chip configuration circuit requires that V 33  be at least 1.5V, such a circuit may fail if threshold voltages V TP  and V TN  are low (e.g., V TN =V TP =0.6V, and thus V REF =1.2V). Likewise, if threshold voltages V TP  and V TN  are both high (e.g., V TN =V TP =1.7V, and thus V REF =3.4V), X HI  may never change state, and thus the POR circuit would fail. 
   In view of the foregoing, it would be desirable to provide methods and apparatus that reduce the sensitivity of trip point detection circuits to process and temperature variations. 
   It also would be desirable to provide methods and apparatus that increase the trip point reference V REF  of trip point detection circuits when transistor threshold voltages are lowered as a result of process or temperature conditions. 
   It additionally would be desirable to provide methods and apparatus that decrease the trip point reference V REF  of trip point detection circuits when transistor threshold voltages are raised as a result of process or temperature conditions. 
   SUMMARY 
   In view of the foregoing, it is an object of this invention to provide methods and apparatus that reduce the sensitivity of trip point detection circuits to process and temperature variations. 
   It also is an object of this invention to provide methods and apparatus that increase the trip point reference V REF  of trip point detection circuits when transistor threshold voltages are lowered as a result of process or temperature conditions. 
   It additionally is an object of this invention to provide methods and apparatus that decrease the trip point reference V REF  of trip point detection circuits when transistor threshold voltages are raised as a result of process or temperature conditions. 
   These and other objects of this invention are accomplished by providing adaptive trip point detection circuits that adjust the trip point reference signal value to compensate for variations in process or temperature, without requiring an externally-supplied reference signal. In a first exemplary embodiment, a controlled current source is coupled to an internal node of a trip point detection circuit, and the controlled current source conducts a current that varies based on process and temperature conditions. For nominal or slow processes or nominal or low temperature conditions, the trip-point reference signal value equals a sum of two threshold voltages. For fast processes or high temperature conditions, in contrast, the trip-point reference signal value is increased. 
   In a second exemplary embodiment, a controlled current source is coupled to the output node of a trip point detection circuit, and the controlled current source conducts a current that varies based on process and temperature conditions. For nominal or slow processes or nominal or low temperature conditions, the trip-point reference signal value equals a sum of two threshold voltages. For fast processes or high temperature conditions, in contrast, the trip-point reference signal value is increased. 
   In a third exemplary embodiment, a first controlled current source is coupled to an internal node of a trip point detection circuit, a second controlled current source is coupled to an output node of the trip point detection circuit, and the first and second controlled current sources conduct currents that vary based on process and temperature conditions. For nominal or slow processes or nominal or low temperature conditions, the trip-point reference signal value equals a sum of two threshold voltages. For fast processes or high temperature conditions, in contrast, the trip-point reference signal value is increased. 
   In a fourth exemplary embodiment a first transistor having a nominal threshold voltage and a second transistor having a high threshold voltage are coupled to an output node of a trip point detection circuit, and the first and second transistors are switched in or out of the trip point detector circuit based on process and temperature conditions. For nominal or slow processes or nominal or low temperature conditions, the first transistor is switched into the trip point detector circuit. For fast processes or high temperature conditions, in contrast, the second transistor is switched into the trip point detector circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned objects and features of the present invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which the same reference numerals denote the same elements throughout, and in which: 
       FIG. 1  is a diagram of a previously known power-on reset circuit; 
       FIG. 2  is diagram of a previously known trip detector circuit; 
       FIG. 3  is a diagram of signal response values of the circuit of  FIG. 2 ; 
       FIG. 4  is a diagram of an exemplary trip-point detector circuit in accordance with this invention; 
       FIG. 5  is a diagram of signal response values of the circuit of  FIG. 4 ; 
       FIG. 6  is a diagram of an exemplary implementation of the circuit of  FIG. 4 ; 
       FIG. 7  is a diagram of an alternative exemplary trip-point detector circuit in accordance with this invention; 
       FIG. 8  is a diagram of signal response values of the circuit of  FIG. 7 ; 
       FIG. 9  is a diagram of an exemplary implementation of the circuit of  FIG. 7 ; 
       FIG. 10  is a diagram of an exemplary V BE  detector circuit of  FIG. 9 ; 
       FIG. 11  is a diagram of another alternative exemplary trip-point detector circuit in accordance with this invention; 
       FIG. 12  is a diagram of signal response values of the circuit of  FIG. 11 ; 
       FIG. 13  is a diagram of still another alternative exemplary trip-point detector circuit in accordance with this invention; and 
       FIG. 14  is a diagram of signal response values of the circuit of  FIG. 13 . 
   

   DETAILED DESCRIPTION 
   The present invention provides methods and apparatus that reduce the sensitivity of trip point detection circuits to process and temperature variations. In some embodiments, methods and apparatus in accordance with this invention increase the trip point reference V REF  when transistor threshold voltages are lowered as a result of process or temperature conditions. In other embodiments, methods and apparatus in accordance with this invention decrease the trip point reference V REF  when transistor threshold voltages are raised as a result of process or temperature conditions. As used herein, a semiconductor process is characterized as “nominal,” “slow” or “fast,” based on the value of transistor threshold voltages produced by the process. In particular, a process is characterized as nominal, slow or fast if the transistors produced by the process have nominal, high or low threshold voltages, respectively. 
   Persons of ordinary skill in the art will understand that because p-channel and n-channel transistors are produced by different process steps, the threshold voltages of p-channel and n-channel transistors may not necessarily track one another. Thus, wafers produced by a single process may have “slow” p-channel transistors and “fast” n-channel transistors. As a result, methods and apparatus in accordance with this invention may adjust the trip point reference V REF  based on detecting process-induced shifts in the threshold voltages of p-channel transistors only, n-channel transistors only, or both p- and n-channel transistors. 
   Referring now to  FIG. 4 , an exemplary trip point detector circuit in accordance with this invention is described. Trip point detector circuit  12   a  includes the same circuit elements as trip point detector circuit  12  of  FIG. 2 , but also includes controlled current source  24  coupled between node V x  and ground. As described in more detail below, controlled current source  24  conducts a current I 1  that varies based on process and temperature conditions. The following table illustrates an exemplary output response of controlled current source  24  as a function of process and temperature conditions: 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Process/Temperature 
               I 1   
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               slow process or low temperature 
               0 
             
             
                 
               nominal process or nominal temperature 
               0 
             
             
                 
               fast process or high temperature 
               &gt;0 
             
             
                 
                 
             
           
        
       
     
   
   That is, for slow or nominal processes, or low or nominal temperature, controlled current source  24  conducts no current. As a result, controlled current source  24  is effectively disconnected from node V x , and trip point detector circuit  12   a  behaves like previously known trip point detector circuit  12  of  FIG. 2 . In contrast, for fast processes or high temperature, controlled current source  24  conducts current I 1 &gt;0, and effectively increases trip point reference signal V REF . 
   Referring now to  FIGS. 4 and 5 , the operation of trip detector circuit  12   a  is described for fast processes or high temperature conditions that result in low threshold voltages (e.g., V TN =0.6V or V TP =0.6V). Persons of ordinary skill in the art will understand that threshold voltages V TN  and V TP  may not necessarily have equal values, and that methods and apparatus in accordance with this invention do not require that the two threshold voltages be equal. At t=0, V 33 =0V, transistor  16  is OFF, and no current flows through resistor  20 . As a result (assuming V x  cannot go below ground), V x =0V, transistor  18  is OFF, no current flows through resistor  22 , and X HI =V 33 =0V. For 0≦t&lt;T 1 ′, V 33  increases, but remains below V TP . As a result, transistor  16  remains OFF, and V X =0. At t=T 1 ′, V 33  exceeds V x  by the threshold voltage V TP , and transistor  16  begins to conduct. Because resistor  20  is large, transistor  16  tries to supply almost all of current I 1  required by controlled current source  24 . As a result, V x  remains at ground. 
   For T 1 ′≦t&lt;T 2 ′, V 33  increases, but V x  remains at ground as transistor  16  continues to try to supply current I 1 . At t=T 2 ′, transistor  16  is fully saturated, which occurs at a V 33  value of:
 
 V   33   =|V   GS   |=V   TP   +ΔV   a   (1)
 
where ΔV a  is given by:
 
                   Δ   ⁢           ⁢     V   a       =         2   ⁢     I   1         β   16                 (   2   )                 β   16     =         (     W   L     )     16     ⁢       μ   ⁢           ⁢     C   ox       2               (   3   )               
where
 
             (     W   L     )     16         
is the ratio of the width to length of transistor  16 , μ is a constant and C ox  is a process parameter.
 
   For T 2 ′≦t&lt;T 3 ′, V x  continues to track V 33 , but remains below the threshold voltage V TN  of transistor  18 . Accordingly, transistor  18  remains OFF, and X HI =V 33 . At t=T 3 ′, when V x  equals V TN , transistor  18  turns ON, and pulls X HI  to ground. In this example, X HI  changes from a positive non-zero voltage to 0V when V 33  exceeds trip-point reference signal V REFa =V TP +V TN +ΔV a . Thus, trip point detector circuit  12   a  has a trip-point reference signal V REFa  that adapts to process and temperature conditions, as indicated in the following table: 
                               TABLE 2                       Process/Temperature   V REFa                             slow process or low temperature   V TP  + V TN             nominal process or nominal temperature   V TP  + V TN             fast process or high temperature   V TP  + V TN  + ΔV a                          
For nominal or slow processes or nominal or low temperature conditions (i.e., when threshold voltages V TN  and V TP  are nominal or high), trip-point reference signal V REFa  equals the sum of threshold voltages V TN  and V TP . However, for fast processes or high temperature conditions (i.e., when threshold voltages V TN  and V TP  are low), trip-point reference signal V REFa  equals the sum V TN +V TP +ΔV a .
 
   Controlled current source  24  may be implemented using any circuit that has an output current that varies with process and temperature as shown in Table 1. Referring now to  FIG. 6 , an exemplary embodiment of such a circuit is described. In particular, trip point detector circuit  12   a   1  includes native n-channel transistor  24   a  having its drain terminal coupled to node V x , and its gate and source terminals coupled to ground. Native n-channel transistor  24   a , sometimes referred to as a depletion-mode transistor, has a threshold voltage V TZ  having a nominal value of approximately 0V. If native n-channel transistor  24   a  is fabricated on the same die as n-channel transistor  18 , the threshold voltage of both transistors often will track with temperature conditions and n-channel process conditions, as illustrated in the following table: 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
               N-Process/Temperature 
               V TN   
               V TZ   
             
             
                 
                 
             
           
           
             
                 
               slow process or low temperature 
               high 
               high 
             
             
                 
               nominal process or nominal temperature 
               nominal 
               nominal 
             
             
                 
               fast process or high temperature 
               low 
               low 
             
             
                 
                 
             
           
        
       
     
   
   Thus, if V TZ  has a nominal value of 0V, for nominal or low temperatures, or slow or nominal n-processes, native n-channel transistor  24   a  never turns ON because the transistor&#39;s gate-to-source voltage V GS =0. Under such conditions, trip point detector circuit  12   a   1  behaves like trip point detector circuit  12  of  FIG. 2 . However, for fast n-processes or high temperatures, V TZ  is less than 0V, and native n-channel transistor  24   a  turns ON when V x  is above 0V. Thus, native n-channel transistor  24   a  acts like a controlled current source whose current varies with n-process and temperature conditions, as in Table 1, above. As a result, trip point detector circuit  12   a   1  has a trip-point reference signal V REFa  that adapts to process and temperature conditions, as in Table 2, above. Persons of ordinary skill in the art will understand that trip point detector circuit  12   a   1  alternatively may be configured to have a trip-point reference signal V REFa  that adapts to p-process and temperature conditions. 
   Referring now to  FIG. 7 , an alternative exemplary trip point detector circuit in accordance with this invention is described. Trip point detector circuit  12   b  includes the same circuit elements as trip point detector circuit  12  of  FIG. 2 , but also includes controlled current source  26  coupled between V 33  and node X HI . As described in more detail below, controlled current source  26  conducts a current I 2  that varies based on process and temperature conditions. The following table illustrates an exemplary output response of controlled current source  26  as a function of process and temperature conditions: 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 4 
             
             
                 
                 
             
             
                 
               Process/Temperature 
               I 2   
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               slow process or low temperature 
               0 
             
             
                 
               nominal process or nominal temperature 
               0 
             
             
                 
               fast process or high temperature 
               &gt;0 
             
             
                 
                 
             
           
        
       
     
   
   That is, for slow or nominal processes, or low or nominal temperature, controlled current source  26  conducts no current. As a result, controlled current source  26  is effectively disconnected from node X HI , and trip point detector circuit  12   b  operates like previously known trip point detector circuit  12  of  FIG. 2 . In contrast, for fast processes or high temperature, controlled current source  26  conducts current I 2 &gt;0, and effectively increases trip point reference signal V REF . 
   Referring now to  FIGS. 7 and 8 , the operation of trip detector circuit  12   b  is described for fast processes or high temperature conditions that result in low threshold voltages (e.g., V TN =0.6V or V TP =0.6V). At t=0, V 33 =0V, transistor  16  is OFF, V x =0V, transistor  18  is OFF, and X HI  equals V 33 =0V. For 0≦t&lt;T 1 ′, V 33  increases, but remains below V TP . As a result, transistor  16  remains OFF, V X =0, and X HI =V 33 . At t=T 1 ′, V 33  exceeds V x  by the threshold voltage V TP , and transistor  16  therefore begins to conduct. 
   For T 1 ′≦t&lt;T 2 ′, V X  remains one V TP  below V 33 . Because V X  is less than V TN , transistor  18  remains OFF, and X HI =V 33 . At t=T 2 ′, V 33 =V TP +V TN , V x =V TN , and transistor  18  begins to conduct. However, a higher gate-to-source voltage is required to turn ON transistor  18  and sink the current I 2  from controlled current source  26 . As a result, X HI =V 33 . At t=T 3 ″, transistor  18  is fully saturated, and pulls X HI  to ground. This occurs when V 33  has a value of:
 
 V   33   =V   TP   +V   GS18   =V   TP +( V   TN   +ΔV   b )  (4)
 
where ΔV b  is given by:
 
                   Δ   ⁢           ⁢     V   b       =         2   ⁢     I   2         β   18                 (   5   )                 β   18     =         (     W   L     )     18     ⁢       μ   ⁢           ⁢     C   ox       2               (   6   )               
where
 
             (     W   L     )     18         
is the ratio of the width to length of transistor  18 , μ is a constant and C ox  is a process parameter. In this example, X HI  changes from a positive non-zero voltage to 0V when V 33  exceeds trip-point reference signal V REFb =V TP +V TN +ΔV b .
 
   Thus, trip point detector circuit  12   b  has a trip-point reference signal V REFb  that adapts to process and temperature conditions, as indicated in the following table: 
                               TABLE 5                       Process/Temperature   V REFb                             slow process or low temperature   V TP  + V TN             nominal process or nominal temperature   V TP  + V TN             fast process or high temperature   V TP  + V TN  + ΔV b                          
For nominal or slow processes or nominal or low temperature conditions (i.e., when threshold voltages V TN  and V TP  are nominal or high), trip-point reference signal V REFb  equals the sum of threshold voltages V TN  and V TP . However, for fast processes or high temperature conditions (i.e., when threshold voltages V TN  and V TP  are low), trip-point reference signal V REFb  equals the sum V TN +V TP +ΔV b .
 
   Controlled current source  26  may be implemented using any circuit that has an output response as shown in Table 4. Referring now to  FIG. 9 , an exemplary embodiment of such a circuit is described. Trip point detector circuit  12   b   1  includes p-channel transistor  26   b  having its drain terminal coupled to node X HI , its gate terminal coupled to signal X FAST , and its source terminal coupled to node V 33 . As described in more detail below, V BE  detector circuit  28  provides signal X FAST  whose value depends on process and temperature conditions. In particular, for nominal or slow processes, or nominal or low temperatures, X FAST  is HIGH, and transistor  26   b  is OFF. Under such conditions, trip point detector circuit  12   b   1  behaves like trip point detector circuit  12  of  FIG. 2 . In contrast, for fast processes or high temperatures, X FAST  is LOW, and transistor  26   b  injects current into node X HI . Thus, transistor  26   b  acts like a controlled current source whose current varies with process and temperature conditions, as in Table 1, above. As a result, trip point detector circuit  12   b   1  has a trip-point reference signal V REFb  that adapts to process and temperature conditions, as in Table 4, above. 
   Referring now to  FIG. 10 , an exemplary V BE  detector circuit is described for generating X FAST . In particular, V BE  detector circuit  28  includes PNP transistor  30  having its base and collector terminals coupled to ground, and its emitter terminal coupled to V 33  via current source  32 . The emitter terminal of PNP transistor  30  is also coupled to the gate of n-channel transistor  34 , which has its source coupled to ground, and its drain terminal (node X FAST ) coupled to V 33  via current source  36 . Thus, the base-emitter voltage of PNP transistor  30  equals the gate-source voltage of n-channel transistor  34 . 
   The base-emitter voltage V BE  of PNP transistor  30  and the threshold voltage V TN  of n-channel transistor  34  tend to shift in the same direction with variations in n-process and temperature. However, variations in V BE  typically are much less than variations in V TN , and V BE  typically remains very close to 0.7V. Thus, if V TN  has a nominal value of 0.8V, for nominal or slow n-processes and nominal or low temperatures, V BE  is less than V TN . In contrast, for fast n-processes or high temperatures, V BE  is greater than V TN . Thus, for nominal or slow n-processes and nominal or low temperatures, the V BE  of PNP transistor  30  is less than V TN , transistor  34  is OFF, and X FAST  is HIGH. In contrast, for fast n-processes or high temperatures, the V BE  of PNP transistor  30  is greater than V TN , transistor  34  is ON, and X FAST  is LOW. Persons of ordinary skill in the art will understand that if V TN  has a nominal value other than 0.8V, V BE  may be compared to a scaled version of V TN  to generate X FAST . Persons of ordinary skill in the art will understand that V BE  detector circuit  28  alternatively may be configured to provide a signal X FAST  that varies based on p-process and temperature conditions. 
   Referring now to  FIG. 11 , another exemplary trip point detector circuit in accordance with this invention is described. In this example, the techniques illustrated in exemplary trip detector circuits  12   a   1  and  12   b   1  are combined. In particular, trip detector circuit  12   c  includes native n-channel transistor  24  coupled between node V x  and ground, and p-channel transistor  26   b  coupled between V 33  and node X HI .  FIG. 12  illustrates the response of trip detector circuit  12   c  for fast processes or high temperature conditions that result in low threshold voltages (e.g., V TN =0.6V or V TP =0.6V). Using an analysis similar to that described above, persons of ordinary skill in the art will understand that trip point detector circuit  12   c  has a trip-point reference signal V REFc  that adapts to process and temperature conditions, as indicated in the following table: 
                       TABLE 6               Process/Temperature   V REFc                     slow process or low temperature   V TP  + V TN         nominal process or nominal temperature   V TP  + V TN         fast process or high temperature   V TP  + V TN  + ΔV a  + ΔV b                      
where ΔV a +ΔV b  have values as specified in equations (2) and (3), and (5) and (6), respectively.
 
   Referring now to  FIG. 13 , another exemplary trip point detector circuit in accordance with this invention is described. In particular, trip point detector circuit  12   d  includes n-channel transistors  38  and  40  having drain terminals coupled to node X HI , and source terminals coupled to the drain terminals of transistors  18  and  18 F, respectively. In addition, transistor  38  has a gate terminal coupled to signal X FAST , and transistor  40  has a gate terminal coupled to signal FAST (i.e., the logical inverse of X FAST ). Transistor  18 F is similar to transistor  18 , but has a higher nominal threshold voltage V TNH  than the threshold voltage V TN  of transistor  18 . For example, if V TN  has a nominal threshold voltage of 0.8V, V THN  may have a nominal value of 1.0V. The difference in threshold values may be achieved, for example, by adjusting the dimensions of transistor  18 F relative to the dimensions of transistor  18 , or by adjusting the processing steps that affect the threshold voltages of the two transistors. 
   Transistors  38  and  40  are sized to operate as switches that alternately switch transistors  18  or  18 F in or out of the circuit based on process and temperature conditions. In particular, for nominal or slow processes, or nominal or low temperatures, X FAST  is HIGH, FAST is LOW, the drain of transistor  18  is coupled to node X HI , and transistor  18 F is effectively disconnected from the rest of the circuit. Under such conditions, trip point detector circuit  12   d  behaves like trip point detector circuit  12  of  FIG. 2 . In contrast, for fast processes or high temperatures, X FAST  is LOW, FAST is HIGH, the drain of transistor  18 F is coupled to node X HI , and transistor  18  is effectively disconnected from the rest of the circuit. Thus, for fast processes or high temperatures, trip point detector circuit  12   d  swaps nominal threshold transistor  18  with high threshold transistor  18 F. 
   If transistors  18  and  18 F are fabricated on the same die, the threshold voltage of both transistors often will track with process and temperature conditions, an example of which is illustrated in the following table: 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 7 
             
             
                 
                 
             
             
                 
               N-Process/Temperature 
               V TN   
               V TNH   
             
             
                 
                 
             
           
           
             
                 
               slow process or low temperature 
               1.0 
               1.2 
             
             
                 
               nominal process or nominal temperature 
               0.8 
               1.0 
             
             
                 
               fast process or high temperature 
               0.6 
               0.8 
             
             
                 
                 
             
           
        
       
     
   
   Referring now to  FIGS. 13 and 14 , the operation of trip point detector circuit  12   d  is described for fast processes or high temperature conditions that result in low threshold voltages. In this example, V TN =V TP =0.6V, V TNH =0.8V, X FAST  is LOW, and FAST is HIGH. As a result, transistor  18  is effectively switched out of the circuit, and transistor  18  F is effectively switched into the circuit. At t=0, V 33 =0V, transistor  16  is OFF, V x =0V, transistor  18 F is OFF, and X HI  equals V 33 =0V. For 0≦t&lt;T 1 ′, V 33  increases, but remains below V TP . As a result, transistor  16  remains OFF, V X =0, and X HI =V 33 . At t=T l ′, V 33  exceeds V x  by the threshold voltage V TP , and transistor  16  therefore begins to conduct. For T 1 ′≦t&lt;T 5 , V X  remains one V TP  below V 33 . Because V X  is less than V TNH , transistor  18 F remains OFF, and X HI =V 33 . At t=T 5 , V 33 =V TP +V TNH , V x =V TNH , and transistor  18 F turns ON and pulls X HI  to ground. In this example, X HI  changes from a positive non-zero voltage to 0V when V 33  exceeds trip-point reference signal V REFd =V TP +V TNH . 
   The exemplary circuits described above illustrate techniques used to increase the trip point reference V REF  when transistor threshold voltages are lowered as a result of process or temperature conditions. Persons of ordinary skill in the art will understand that methods and apparatus in accordance with this invention also may be used to decrease the trip point reference V REF  when transistor threshold voltages are raised as a result of process or temperature conditions. For example, in trip point detector circuit  12   b1  illustrated in  FIG. 9 , the gate of p-channel transistor  26   b  may be coupled to a control signal SLOW that is LOW for nominal or fast processes, or nominal or high temperatures, and HIGH for slow processes or low temperature conditions. In that regard, current I 2  would be injected into the drain of transistor  18  except if process or temperature conditions tended to increase threshold voltages V TP  and V TN . Under such circumstance, I 2  would turn OFF, which would decrease the trip point reference V REF . 
   Alternatively, in trip point detector circuit  12   d  illustrated in  FIG. 13 , the gate terminals of transistors  38  and  40  may be coupled to X SLOW  (i.e., the logical inverse of SLOW) and SLOW, respectively, and transistor  18 F may be fabricated to have a lower nominal threshold voltage V TNL  than the threshold voltage V TN  of transistor  18 . Thus, for nominal or fast processes, or nominal or high temperatures, X SLOW  is HIGH, SLOW is LOW, the drain of transistor  18  is coupled to node X HI , and transistor  18 F is effectively disconnected from the rest of the circuit. In contrast, for slow processes or low temperatures, X SLOW  is LOW, SLOW is HIGH, the drain of transistor  18 F is coupled to node X HI , and transistor  18  is effectively disconnected from the rest of the circuit. Thus, for slow processes or low temperatures, trip point detector circuit  12   d  swaps nominal threshold transistor  18  with high threshold transistor  18 F, which would decrease the trip point reference V REF . 
   The foregoing merely illustrates the principles of this invention, and various modifications can be made by persons of ordinary skill in the art without departing from the scope and spirit of this invention.