Abstract:
An electrical circuit and a procedure for tracking at least one input pulse width applied to the electrical circuit. The electrical circuit includes a threshold component (e.g., a comparator) arranged to provide an output pulse width based on whether an input to the threshold component exceeds a threshold. The circuit also includes a controller arranged to control the threshold of the threshold component, based on the at least one input pulse width applied to the electrical circuit, such that the output pulse width of the threshold component tracks the at least one input pulse width applied to the electrical circuit. The controller includes at least a switch, and the output pulse width tracks the at least one input pulse width by following or anticipating the pulse width. In one example embodiment the tracking is performed for a series of pulses of varied widths.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    Example aspects described herein relate generally to integrated circuits, and, in particular, to methods, apparatuses and systems that can follow and/or anticipate a provided voltage pulse width. 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 1  represents a prior art circuit in which a resistor R is connected between one terminal of a switch  101  and a voltage source Vcc, and another terminal of the switch  101  is connected to ground. Operation of the switch  101  can cause generation of a voltage pulse train (PT) at node N 1 . 
         [0005]    Typically, the generated pulse train (PT) has essentially equal time periods, but the widths of the individual pulses can vary. In general, the pulse widths often change very little from cycle to cycle, perhaps a small fraction of 1%, but over a long amount of time they can change substantially. The change in pulse width might be the result of 60 Hz modulating the pulse width of a much higher sampling frequency (e.g., 100 kHz). 
       SUMMARY 
       [0006]    According to example aspects herein, an electrical circuit and a procedure are provided for tracking at least one input pulse width applied to the electrical circuit. 
         [0007]    According to one example embodiment herein, the electrical circuit includes a threshold component arranged to provide an output pulse width based on whether an input to the threshold component exceeds a threshold. The circuit also includes a controller arranged to control the threshold of the threshold component, based on at least one input pulse width applied to the electrical circuit, such that the output pulse width of the threshold component tracks the at least one input pulse width applied to the electrical circuit. 
         [0008]    In accordance with one example embodiment herein, the threshold component includes a comparator, and the controller includes both a storage element arranged to store charge upon which the threshold is based, and a switch arranged to charge or discharge the storage element based on the at least one input pulse width applied to the electrical circuit. 
         [0009]    In accordance with a further example embodiment herein, the controller further includes at least one logic element arranged to control the switch based on the at least one input pulse width applied to the electrical circuit, and the at least one input pulse width is applied to the electrical circuit by another switch. 
         [0010]    According to an example aspect herein, the output pulse width of the threshold component tracks the at least one input pulse width by following and/or anticipating a low-going end of the at least one input pulse width. 
         [0011]    In still a further example embodiment herein, the electrical circuit further comprises a further threshold component arranged to provide a further output pulse width based on whether an input to the further threshold component exceeds a further threshold. In this example, the controller also is arranged to control the further threshold of the further threshold component, based on the at least one input pulse width applied to the electrical circuit, such that the further output pulse width of the further threshold component tracks the at least one input pulse width applied to the electrical circuit. The further output pulse width of the further threshold component tracks the at least one input pulse width by anticipating a low-going end of the at least one input pulse width, in one example. 
         [0012]    In still a further example embodiment herein, the circuit further comprises at least one voltage divider coupled between a voltage source and the storage element, and arranged to set the threshold based on an amount of charge stored in the storage element. 
         [0013]    The at least one input pulse width may include a plurality of varying input pulse widths, and, in one example embodiment herein, the output pulse width of the threshold component tracks at least one of the input pulse widths having a minimum pulse width. 
         [0014]    The circuit herein can be useful when used in a specialized AC PWM (pulse-width modulator). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The teachings claimed and/or described are further described in terms of exemplary embodiments. These exemplary embodiments are described in detail with reference to the drawings. These embodiments are non-limiting exemplary embodiments, in which like reference numerals represent similar structures throughout the several views of the drawings, and wherein: 
           [0016]      FIG. 1  represents a prior art circuit in which operation of a switch can cause generation of a voltage pulse train. 
           [0017]      FIG. 2  shows a circuit that can follow a provided input pulse width, according to an example embodiment herein. 
           [0018]      FIG. 3  shows a circuit that can anticipate a pulse width, according to another example embodiment herein. 
           [0019]      FIG. 4  comprises  FIGS. 4   a  to  4   d , and represents example waveforms and switching cycles for components of the circuit of  FIG. 2  for a case in which the provided input pulse width is narrow. 
           [0020]      FIG. 4   a  shows a voltage Vc 1  of a capacitor  121 , a voltage V+ on a positive (+) input of a comparator  131 , and a voltage Vc 2  of a capacitor  122  of the circuit of  FIG. 2 , over a cycle. 
           [0021]      FIG. 4   b  shows an example of an on-off cycle of a switch  102  of the circuit of  FIG. 2 , over the cycle of  FIG. 4   a , wherein each pulse shown in  FIG. 4   b  represents a time period in which the switch  102  is closed. 
           [0022]      FIG. 4   c  shows an output U 1  of comparator  131  of the circuit of  FIG. 2 , and a voltage  101   a  across a switch  101 ′ of the circuit of  FIG. 2 , over the cycle of  FIG. 4   a.    
           [0023]      FIG. 4   d  shows a representation of a current I 1  which discharges a capacitor  122  through a resistor  117  and a switch  103  of the circuit of  FIG. 2 , over the cycle of  FIG. 4   a.    
           [0024]      FIG. 5  comprises  FIGS. 5   a  to  5   d , and represents example waveforms and switching cycles for components of the circuit of  FIG. 2  for a case in which the provided input pulse width is wide. 
           [0025]      FIG. 5   a  shows a voltage Vc 1  of a capacitor  121 , a voltage V+ on a positive (+) input of a comparator  131 , and a voltage Vc 2  of a capacitor  122  of the circuit of  FIG. 2 , over a cycle. 
           [0026]      FIG. 5   b  shows an example of an on-off cycle of a switch  102  of the circuit of  FIG. 2 , over the cycle of  FIG. 5   a , wherein each pulse shown in  FIG. 5   b  represents a time period in which the switch  102  is closed. 
           [0027]      FIG. 5   c  shows an output U 1  of comparator  131  of the circuit of  FIG. 2 , and a voltage  101   a  across a switch  101 ′ of the circuit of  FIG. 2 , over the cycle of  FIG. 5   a.    
           [0028]      FIG. 5   d  shows a representation of a current I 1  which discharges a capacitor  122  through a resistor  117  and a switch  103  of the circuit of  FIG. 2 , over the cycle of  FIG. 5   a.    
           [0029]      FIG. 6  comprises  FIGS. 6   a  to  6   d , and represents example waveforms and switching cycles for components of the circuit of  FIG. 3  for an example case in which the provided input pulse width is wide. 
           [0030]      FIG. 6   a  shows a voltage Vc 1  of a capacitor  121 , a voltage V+ on a positive (+) input of a comparator  131 , a voltage V 2 + on a positive (+) input of a comparator  137 , and a voltage Vc 2  of a capacitor  122  of the circuit of  FIG. 3 , over a cycle. 
           [0031]      FIG. 6   b  shows an example of an on-off cycle of a switch  102  of the circuit of  FIG. 3 , over the cycle of  FIG. 6   a , wherein each pulse shown in  FIG. 6   b  represents a time period in which the switch  102  is closed. 
           [0032]      FIG. 6   c  shows an output U 1  of comparator  131  of the circuit of  FIG. 3 , an output U 2  of comparator  137  of the circuit of  FIG. 3 , and a voltage  101   a  across a switch  101 ′ of the circuit of  FIG. 3 , over the cycle of  FIG. 6   a.    
           [0033]      FIG. 6   d  shows a representation of a current I 1  which discharges a capacitor  122  through a resistor  117  and a switch  103  of the circuit of  FIG. 3 , over the cycle of  FIG. 6   a.    
       
    
    
       [0034]    It should be noted that different ones of the Figures may include the same reference numerals to identify the same components, and thus a description of each such component may not be provided herein with respect to each particular Figure. 
       DETAILED DESCRIPTION 
       [0035]    In accordance with an example aspect herein, methods, apparatuses, and systems are provided that can follow and/or anticipate a provided voltage pulse. 
         [0036]      FIG. 2  shows a circuit  201  according to an example embodiment herein. The circuit  201  can follow a next pulse width of a switch  101 ′ by switching low slightly after the ending of a pulse width generated by the switch  101 ′. Circuit  201  comprises switches  101 ′,  102  and  103 , resistors  111 ,  112 ,  113 ,  114 ,  115 ,  116 , and  117 , storage elements such as capacitors  121  and  122 , a threshold component such as a comparator  131 , AND gate  132 , an inverter  133 , and a positive-supply voltage source  134 . At least one of the elements  122 ,  112 ,  103 ,  132 , and  133  forms a controller. 
         [0037]    Switch  101 ′ represents the driver of a pulse train with varying pulse widths, such as the pulse train (PT) of  FIG. 1 . In an example embodiment herein, switch  101 ′ is controlled by an external control source, and the pulse width therefore is so controlled as well, and may or may not be known within the system of circuit  201 . The switch  101 ′ operates by opening when the pulse is high. When switch  101 ′ closes (at some time within the pulse period), the pulse goes low, signaling the pulse width. Switch  102  also is controlled by an external control source, and the pulse width therefore may or may not be known within the system of circuit  201 . 
         [0038]    Resistor  111  is interposed between one terminal of switch  102  and a node  142 , and capacitor  121  is coupled between nodes  141  and  142 . Both of the resistors  113  and  114  are connected at one end thereof to a node  144 , which is connected to voltage source  134 . Another end of resistor  113  is connected to node  143 , and the other end of resistor  114  is connected to a node  146 . A negative (−) input of the comparator  131  is connected to node  143 , and a positive (+) input of the comparator  131  is connected to node  146 . The comparator  131  also has an output connected to a node  147 . The comparator  131  operates such that, when non-inverting positive (+) input is at a higher voltage than the inverting, negative (−) input, the comparator  131  outputs a positive voltage. When a voltage at the positive (+) input drops below a voltage at the negative (−) input, then the comparator  131  outputs a negative voltage. 
         [0039]    Referring again to node  146 , that node is connected to one end of resistor  115 , and another end of that resistor  115  is connected to node  149 . The resistors  114  and  115  form a voltage divider on the positive (+) input of the comparator  131 . Capacitor  122  is coupled between nodes  149  and  150  and resistor  112  is coupled between node  149  and one terminal of switch  103 . Another terminal of that switch  103  is connected to node  148 . The capacitor  122  preferably is large enough such that its voltage does not change much from cycle to cycle of the pulse train. 
         [0040]    Referring to resistor  117 , that resistor  117  is connected between nodes  145  and  147 , and node  147  is connected to a first input of the AND gate  132 . Resistor  116  is connected between node  145  and a first terminal of switch  101 ′. Each of the nodes  141 ,  148 , and  150  is connected to ground. Although for convenience those nodes are shown as being separate, in actuality they may form a same, single node. Inverter  133  is connected between the first terminal of switch  101 ′ and a second input of the AND gate  132 , and a second terminal of the switch  101 ′ is connected to node  148 . An output of the AND gate  132  controls the switch  103 . 
         [0041]    The manner in which the AND gate  132  controls the switch  103  will now be described. The gate  132  provides a high output when both of its inputs receive a high input (and when the inverter  133  provides a high output in response to a low input being applied to its input when switch  101 ′ is closed). A high output from the gate  132  causes the switch  103  to become closed. On the other hand, when one input of AND gate  132  is low and another input of AND gate  132  is high, the gate  132  provides a low output, which results in switch  103  being open. In other example embodiments, the logic functions performed by the inverter  133  and AND gate  132  are accomplished using other logic devices besides those devices. 
         [0042]    The manner in which the overall circuit  201  operates will now be described. Prior to reaching steady-state, the circuit  201  functions as follows. At the end of each pulse period, switch  102  momentarily closes to quickly discharge capacitor  121 . Initially, assume that capacitor  122  is fully discharged such that its voltage is zero. Resistors  114  and  115  form a voltage divider on the positive (+) input of comparator  131 , and since the initial voltage on capacitor  121  is zero, the output U 1  of comparator  131  is high because its positive (+) input voltage is greater than zero. 
         [0043]    When switch  102  opens, it remains open for the entire duration of the pulse period, and resistor  113  charges capacitor  121  during that time. As capacitor  121  charges up, its voltage will eventually (i.e., sometime within the pulse period) rise to be equal to or exceed the voltage on the positive (+) input, and the output U 1  of comparator  131  will go low as a result. 
         [0044]    So long as the voltage on capacitor  122  is kept at ground, the pulse width of output U 1  of comparator  131  will remain constant over all subsequent cycles. 
         [0045]    Now assume that the voltage on capacitor  122  is increased (e.g., manually). This increase causes the voltage at the positive (+) input of comparator  131  to increase, and thus, in such a case, it will take somewhat longer time for the voltage on capacitor  121  to reach this threshold and hence the output U 1  of comparator  131  will have a longer pulse width. 
         [0046]    In the pre-steady-state, in a first part of the pulse cycle, output U 1  of comparator  131  is high and switch  101 ′ is open so its voltage also is high. Under this condition, the second input of AND gate  132  is low (owing to inverter  133 ) so the output of AND gate  132  also is low, and, as a result, switch  103  remains open. Next, assuming that, as time progresses within the pulse cycle, the output U 1  of comparator  131  goes low before switch  101 ′ closes. As soon as output U 1  goes low, the AND gate  132  is locked low even when switch  101 ′ eventually goes low during this pulse cycle and switch  103  remains open throughout this particular pulse cycle. 
         [0047]    Since switch  103  did not close during this example pulse cycle, resistors  114  and  115  slightly charge capacitor  122 , giving a small increase in the voltage at the positive (+) input of comparator  131 . This results in a slight increase in the pulse width of the comparator output U 1 . Thus, if the next pulse cycle still has the output U 1  going low before switch  101 ′ goes low, then, again the voltage on capacitor  122  slightly increases. This process repeats itself until the voltage on capacitor  122  rises high enough such that the pulse widths of output U 1  of the comparator  131  and switch  101 ′ are identical. When that occurs, operation of the circuit  201  has reached the edge of being in steady-state. 
         [0048]    Referring now to  FIG. 4  in conjunction with  FIG. 2 , operation of the circuit  201  for subsequent pulses will now be described, according to an example aspect herein. As can be appreciated in view of  FIGS. 4   a  and  4   b , at the start (time T 1 ) of each cycle switch  102  momentarily closes to completely discharge the voltage of capacitor  121 . For example,  FIG. 4   b  shows a representation (in the form of a pulse, for convenience)  102   a  of the switch  102  closing at time T 1 , and  FIG. 4   a  shows a corresponding drop D 1  of the voltage Vc 1  of capacitor  121  representing the discharge starting at time T 1 , wherein the capacitor  121  discharges through the resistor  111  and switch  102  until time T 2 . 
         [0049]    After the short closure  102   a  ( FIG. 4   b ) of switch  102  between times T 1  and T 2 , the switch  102  opens at time T 2 , and switch  101 ′ opens at time T 2  as well (wherein when open the switch  101 ′ has a voltage pulse  101   a  across it, represented in  FIG. 4   c ). The opening of the switch  101 ′ causes the voltage on resistor  116  to become positive. Also at time T 2  when switch  102  opens, the voltage Vc 1  on capacitor  121  begins to charge positive because of the current through resistor  113 . 
         [0050]    Switch  102 , upon opening at the beginning of each pulse period (e.g., at time T 2 ), remains open (e.g.,  102   b ) for the entire duration of the pulse period ( FIG. 4   b ) while resistor  113  charges capacitor  121  for providing a rising voltage starting at time T 2 . Voltage Vc 2  of capacitor  122  also is shown in  FIG. 4   a , as is a voltage V+ at the positive (+) input to comparator  131 . That latter voltage V+ is greater than voltage Vc 1  of capacitor  121  between times T 2  to T 4 . 
         [0051]    At time T 3  the switch  101 ′ closes, which causes a low input to be provided to the inverter  133  and a resultant high output to be provided from the inverter  133  to the second input of AND gate  132 . Because at time T 3  the output U 1  from the comparator  131  also is high, the input provided to the first input of AND gate  132  also is high. Thus, with both its inputs being high, the gate  132  outputs a high to cause closing of the switch  103 . Closing of the switch  103  results in a slight discharging (D 2 ) ( FIG. 4   a ) of capacitor  122 .  FIG. 4   d  shows discharge current (I 1 ) that passes through resistor  112  and switch  103  as a result of such discharging. 
         [0052]    The capacitor  121 &#39;s voltage Vc 1 , which began rising at time T 2 , eventually rises such that, at a time T 4  occurring momentarily after time T 3  when capacitor  122  begins discharging, the voltage Vc 1  becomes equal to (and then exceeds, until later in the cycle) the voltage V+ on the positive (+) input terminal of the comparator  131 , as represented by L 1  in  FIG. 4   a , and, as a result, the output of the comparator  131  goes low at that time T 4  ( FIG. 4   c ). 
         [0053]    During a small interval (G 1 ) between when the voltage on switch  101 ′ goes low at time T 3  and the time T 4  when the output of comparator  131  goes low, the inverter  133  and the logic AND gate  132  cause switch  103  to momentarily close as described above, which results in a slight discharging (D 2 ) ( FIG. 4   a ) of capacitor  122  through resistor  112  and switch  103  ( FIG. 4   d ). Eventually, the time during which switch  103  is closed grows large enough and reaches stability such that the amount of charge deposited on capacitor  122  during the first part of the pulse cycle becomes removed again during the short time interval when the switch  103  is in a closed position. 
         [0054]    If the value of resistor  112  is small, it does not take much of a pulse overlap for switch  103  to remove substantial charge from capacitor  122 , so the overlap can be made quite small as compared to the pulse cycle. For this situation the output U 1  of comparator  131  goes low slightly after the voltage of switch  101 ′ goes low. 
         [0055]    The circuit  201  reaches steady-state when the charge deposited into capacitor  122  from resistors  114  and  115  during the cycle exactly equals the charge removed during the narrow discharge of switch  103  closing. When the circuit  201  reaches steady-state, the voltage on capacitor  122  is nearly constant from cycle-to-cycle because capacitor  122 , in one example, is chosen to be large enough such that it has a long-term memory (compared to several cycles of the switch  101 ′ waveforms). The larger the capacitor  122  is, the more it tends to reach a stable state based on the shortest of many pulse cycles. Through a voltage divider formed by resistors  114  and  115 , the voltage on capacitor  122  determines the higher voltage on the positive (+) input of comparator  131 . 
         [0056]    In example embodiments such as those in which it may be desired to remember the shortest pulse in a long, but slowly repeating, ensemble of varying pulse widths of pulses at switch  101 ′, capacitor  122  can be selected to be very large. 
         [0057]    At time T 5  the cycle repeats again through times T 5  to beyond T 8 , where operations like those that occurred from time T 1  up to just before time T 5  occur again, etc. 
         [0058]      FIGS. 5   a - 5   d  show the relevant waveforms when the pulse widths of switch  101 ′ are relatively long. The circuit  201  functions exactly the same way as described above in the case where the pulse widths are relatively narrow, except that now the steady-state voltage Vc 2  of capacitor  122  is higher owing to the switch  103  being closed for a slightly longer period of time (owing to the longer pulse widths  101   a  ( FIG. 5   c ) which give a larger total charge into capacitor C 122 ) relative to that in the case of narrower pulse widths, and hence, the voltage V+ ( FIG. 5   a ) on the positive (+) input of comparator  131  also is higher in this case as well. 
         [0059]    Since the voltage V+ on the positive (+) input of comparator  131  is higher ( FIG. 5   a ), it takes longer for the voltage Vc 1  on capacitor  121  ( FIG. 5   a ) to build up to this higher voltage V+ and hence the pulse widths of the output U 1  of comparator  131  are wider (i.e., between times T 2  and T 4 ′ in  FIG. 5   c ) relative to those in  FIG. 4   c , for example. 
         [0060]    Again, for the case of wide pulse widths, the circuit  201  reaches steady-state when the input charge to capacitor  122  from resistors  114  and  115  during the cycle exactly equals the charge removed during the large but narrow discharge that results from switch  103  closing. 
         [0061]    As can be appreciated in view of the above description, the circuit  201  is able to reliably and accurately follow the slowly changing input pulse widths of switch  101 ′, with only slight delay, owing to the output U 1  of comparator  131  following the pulse of switch  101 ′ with only slight delay, as represented in  FIGS. 4   c  and  5   c . The circuit  201  does this by having the output U 1  of comparator  131  pulse low slightly after the switch  101 ′ pulse goes low. 
         [0062]    In addition to the circuit  201 , the inventor also has discovered that, by having accurate placement of the timing of the output of a comparator, the above circuit  201  can be modified so as to enable it to anticipate the next pulse width and produce a pulse which goes low slightly before the input pulse width of switch  101 ′. In accordance with an example aspect herein, and referring now to  FIG. 3 , the inventor has discovered that this can be accomplished by adding one or more additional comparators to the above-described circuit of  FIG. 2 , and by subdividing resistor  115  and biasing the positive (+) input voltage of the added comparator(s) below that of the positive (+) input of comparator  131 , as will be described below. In such a modified circuit, it takes less time for the voltage on capacitor  122  to reach the threshold of the added comparator(s) and hence the output of the added comparator(s) can be made to go low before the pulse from switch  101 ′ goes low. Therefore the output of the added comparator(s) can provide a low-going pulse which accurately “anticipates” the low-going pulse from switch  101 ′. 
         [0063]    Referring now to  FIG. 3 , a circuit  301  according to this example embodiment will now be described. Circuit  301  comprises the same components as the circuit  201  of  FIG. 2 , but also comprises an additional threshold component such as a comparator  137  having a negative (−) input coupled to node  175 , wherein node  175  is connected between nodes  142  and  143 . Comparator  137  also has a positive (+) input coupled to a node  170 , and the resistor  115  is connected between nodes  146  and  170 . The circuit  301  also includes a resistor  160  connected between nodes  149  and  170 , and a resistor  180  connected between a node  182  and a node  190 , wherein node  182  is connected to voltage source  134 , and node  190  is connected to the output of the comparator  137 . The comparator  137  operates in a similar manner as comparator  131  described above. The circuit  301  has an output  185  connected to the node  190 , through which an output U 2  of the comparator  137  is provided to indicate a pulsewidth. 
         [0064]    In the circuit  301 , the comparator  131  output U 1  goes low slightly after switch  101 ′ closed, as described above for the case of circuit  201 . In the circuit  301 , however, because the positive (+) input of comparator  137  is biased at a lower voltage than the positive (+) input of comparator  131 , it takes the voltage Vc 1  of capacitor  121  slightly less time to reach the threshold of comparator  137  as compared to reach the threshold for comparator  131 , and thus, as a result, the width of output pulse U 2  of comparator  137  is shorter than that of output pulse U 1  of comparator  131 , and output U 2  goes low prior to the pulse  101   a  of switch  101 ′ going low. This can be seen in  FIG. 6 , for example, wherein output U 2  is shown going low at time T 2 ′, before time T 3 ′ when pulse  101   a  of switch  101 ′ goes low ( FIG. 6   c ). Also shown is a voltage V 2 + representing the voltage at the positive (+) input of comparator  137  over a cycle ( FIG. 6   a ), as well as a representation L 2  ( FIG. 6   a ) of when voltage Vc 1  of capacitor  122  becomes equal to that voltage V 2 + on the positive (+) input terminal of the comparator  137  to cause the output U 2  of comparator  137  to go low at time T 2 ′ ( FIG. 6   c ). The remaining elements represented in  FIG. 6  are the same as those in  FIG. 5 . (Although an example of a case in which a narrow pulse width is applied to circuit  301  by way of switch  101 ′ is not represented in the Figures, it will be appreciated by one skilled in the art in view of this description that even in such a case the output U 2  would go low prior to pulse  101 a going low, although the pulses would be narrower than those shown in  FIG. 6 .) 
         [0065]    Thus, in circuit  301 , the output U 1  from comparator  131  follows the low-going pulse  101   a  of switch  101 ′, and the output U 2  of the comparator  137  anticipates that low-going pulse  101   a.    
         [0066]    In one example embodiment, the bias voltage set by the ratio of resistors  115  and  160  can be set such that the output U 2  of comparator  137  always properly anticipates the next pulse width and goes low slightly before the input pulse of switch  101 ′ goes low. This can be set even to account for any inherent delays of the circuit elements. 
         [0067]    As for the case of circuit  201  described above, in circuit  301  of  FIG. 3  capacitor  122  preferably is large enough such that it has a memory of at least several (perhaps tens or hundreds of) pulse widths. Thus its steady-state voltage does not change much from cycle-to-cycle. In another example embodiment, the capacitor  122  of  FIG. 3  is substantially larger than that in the case of circuit  201  of  FIG. 2 , such that it deliberately does not fully keep up with the changing pulse widths of switch  101 ′. For example, in an example situation in which the pulse widths of switch  101 ′ vary (over several 100s or 1000s of pulses), a repeating pattern may be present. That is, when the pulse widths are at a minimum, then over many thousands of cycles they widen and then narrow back to the minimum, only to repeat this long pattern over and over again. The repeating pattern is referred to herein for convenience as “period Tx”. If the value of capacitor  122  is very large and the resistors  114  and  115  are also large, then capacitor  122  has such a long term memory that its voltage does not change much over period Tx. In one example embodiment herein, capacitor  122  may have a capacitance of 20 microfarads, and the sum of the resistances of resistors  114  and  115  is 100 kilohms. In other words, the RC time constant is very long compared to Tx. Also, the resistance of resistor  112  is small, in one example embodiment. (Of course, these foregoing examples are not exclusive, and the scope of the invention is not limited only thereto.) 
         [0068]    With the above conditions, the capacitor  122  remembers at least the minimum pulse width of switch  101 ′ for each of the Tx periods. Upon learning the shortest pulse width, the circuit  301  will continue outputting an indication of a narrow pulse even though the input pulses continue to vary widely. During times when the pulse widths are greater than the minimum, switch  103  does not close and the voltage on capacitor  122  increases very slowly by, for example, a tiny amount over the Tx period. Then when the pulses return to minimum width, switch  103  closes (during perhaps several of the minimum pulse widths) and returns the voltage on capacitor  122  back to a desired value. 
         [0069]    This feature is especially useful for controlling the pulse widths of a PWM (pulse width monitor) which gives an excellent power factor for a power converter operated from an AC power supply. 
         [0070]    In the above descriptions, various aspects of the invention have been described with reference to specific example embodiments. The specification and drawings are accordingly to be regarded in an illustrative rather than in a restrictive sense. It will, however, be evident that various modifications and changes may be made without departing from the broader spirit and scope of the present invention. 
         [0071]    By example only, resistor  113  can be replaced with a current source, resistors  111  and  112  also can include an internal impedance of switches  102  and  103 , respectively, and may not be literally populated in cases in which a low-impedance discharge path is desired. Moreover, although the above description is described in the context of the period of the pulse train being constant, in other embodiments the period of the pulse train need not be constant since the circuit  201  and/or  301  can work properly with moderate variations. Moreover, one or more additional comparators may be added to circuit  301  by further sub-dividing resistors  115  and  160  to provide output pulses which are even shorter than that of the output U 2  of comparator  137 . Furthermore, other types of threshold or comparing devices may be used in the above circuits in lieu of the comparators described above, and other types of storage elements besides capacitors may be used in the circuits as well. 
         [0072]    In addition, it should be understood that the figures illustrated in the attachments, which highlight the functionality and advantages of the present invention, are presented for example purposes only. The architecture of the example aspect of the present invention is sufficiently flexible and configurable such that it may be utilized (and navigated) in ways other than that shown in the accompanying figures. 
         [0073]    Although example aspects of this invention have been described in certain specific embodiments, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that this invention may be practiced otherwise than as specifically described. Thus, the present example embodiments, again, should be considered in all respects as illustrative and not restrictive.