Abstract:
A switching circuit that transitions using a switch transistor. During the initial stages of a high-low transition, a control loop provides supplemental pre-charge to the gate of the switch transistor to reduce high-low switching delays. Once the current flowing through the switch transistor rises to a level causing the output voltage to change at specified speed threshold, a loop opening mechanism opens the loop. Further opening of the switch transistor in the high-low transition is taken care of by a relatively constant current source. At that point, no or negligible feedback current is used to charge the gate of the switch transistor. Low-high transitions may be performed in a similar complementary manner.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. The Field of the Invention  
         [0002]     The present invention relates generally to switching circuits. More specifically, the present invention relates to switching circuits that have controlled switching of the switching transistor for reduced Electro-Magnetic Interference (EMI), and that precharge the switching transistor for reduced switching delay.  
         [0003]     2. Background and Relevant Art  
         [0004]     Electronic circuitry has contributed enormously to the advancement of civilization. One of the fundamental building blocks of electronic circuitry is the switching circuit, in which a node of the circuitry is switched from one voltage to another in response to a switch control signal.  
         [0005]     Designers of switching circuits take various constraints into account when conceiving such circuits. One of these constraints is the electromagnetic compatibility (or EMC) of the silicon chip they are designing. While operating, the chip should not radiate excessive electromagnetic energy in order not to interfere with the proper operation of other surrounding circuits. Such interference is often termed Electro-Magnetic Interference (EMI).  
         [0006]     Electro-Magnetic energy will radiate to some extent every time a time-dependent voltage signal is present on chip. In general, the faster that voltage signal varies, the more electro-magnetic energy is radiated. Since switching circuits by their very nature involve the transition of a voltage signal from one voltage to another, switching circuits also involve rapid changes in current to sustain the voltage signal transition at a particular node. These rapid changes in current cause the switching circuits to radiate EMI. EMI may be of particular concern in Pulse Width Modulation (PWM) drivers, which often generate sharp transition edges.  
         [0007]     One conventional solution to limit EMI is to limit the slope of the transition edge when switching the switching circuit. An example of such a conventional switching circuit is illustrated in  FIG. 7  as switching circuit  700 . The slope of the transition edge is controlled by the current sources IQ 1  and IQ 2  and the capacitor C 1 . Accordingly, this switching circuit emits less EMI than switching circuits that do not implement slope control.  
         [0008]     However, a main disadvantage of that switching circuit is that it has a very long delay between the command to perform a switch to the actual start of the switch. The turn-on delay is caused by the necessity for the gate to source voltage of switch transistor M 1  to reach the threshold voltage of the switch transistor M 1  before the switch transistor M 1  starts conducting. The turn-off delay is caused by the necessity for the gate to source voltage of switch transistor M 1  to drop before exiting the linear region.  FIG. 8  illustrates a timing signal diagram  800  of the transition control signal VON and the switch voltage VDS across the switch transistor M 1 . Note the turn on delay t 1  and the turn off delay t 2 .  
         [0009]     Some conventional switching circuits reduce switching delay by pre-charging the gate terminal of the switch transistor prior to an off-on transition, and pre-discharging the gate terminal of the switch transistor prior to the on-off transition. However, the gate terminal should not be pre-charged to a point where the switch transistor conducts significant current. Likewise, the gate terminal should not be pre-discharged to a point where the switch transistor reduces significantly the amount of current conducted. Otherwise, the output voltage would be altered despite not being in a transition. To avoid this, a significant safety factor is built into the pre-charge and pre-discharge. This safety factor may be quite large due to the variability in transistor performance. This large safety factor often prevents much of any possible pre-charge or pre-discharge. Accordingly, such conventional circuits may still have significant switching delay.  
         [0010]     Furthermore, this and other conventional circuits may use a closed loop to pre-charge and pre-discharge. Closed loops can be quite unstable if not designed properly. Accordingly, significant time is often expended in designing such closed loop pre-charge switching circuits.  
         [0011]     What would thereby be advantageous are switching circuits that have slope control during transitions to reduce EMI, that perform pre-charging to reduce switching delay, and that do not need significant design and/or testing to ensure stability during operation.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     The foregoing problems with the prior state of the art are overcome by the principles of the present invention, which relate to a switching circuit that transitions using a switch transistor. Prior to a high-low transition, a control loop pre-charges the gate of the switch transistor to reduce high-low switching delays. Once the current flowing through the switch transistor reaches a value for which the rate of decline in output voltage (dV/dt) lowers to a specified threshold, a loop opening mechanism opens the control loop. Further opening (in the case of an n-type pull down switch transistor) or closing (in the case of a p-type pull up switch transistor) of the switch transistor in the high-low transition is taken care of by a relatively constant current source. At that point, no or negligible feedback current is used to charge the gate of the switch transistor.  
         [0013]     Prior to the low-high transition, the control loop pre-discharges the gate of the switch transistor to reduce low-high switching delays. Once the current flowing through the switch transistor reaches a value for which the rate of increase of the output voltage dV/dt rises to a specified threshold, the loop once again opens. Further closing (in the case of an n-type pull down switch transistor) or opening (in the case of a p-type pull up switch transistor) of the switch transistor in the low-high transition is also taken care of by a constant current source, with no more than negligible current being drawn through the control loop.  
         [0014]     The switch transistor is opened and closed at controlled speeds, resulting in reduced Electro-Magnetic Interference (EMI). Furthermore, the gate of the switch transistor is pre-charged prior to a high-low transition, and is pre-discharged prior to a low-high transition. This reduces switching delays that occur between the time a switch signal is provided to the time that the switching is actually initiated. Furthermore, even though a closed control loop is used to pre-charge and pre-discharge, the control loop is opened in order to perform the actual switching operation. Since the switching occurs in open loop, stability is achieved without having to rely on a well-designed stable closed loop for such switching.  
         [0015]     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0017]      FIG. 1A  is a schematic diagram of a switching circuit having an n-type field effect switch transistor in accordance with one embodiment of the present invention;  
         [0018]      FIG. 1B  is a schematic diagram of a switching circuit having a p-type field effect switch transistor in accordance with a second embodiment of the present invention;  
         [0019]      FIG. 2  illustrates the control loop component of  FIGS. 1A and 1B  in further detail including a first stage and a second stage;  
         [0020]      FIG. 3A  illustrates a first embodiment of the first stage of  FIG. 2  in which the transistors are n-type field effect transistors;  
         [0021]      FIG. 3B  illustrates a second embodiment of the first stage of  FIG. 2  in which the transistors are p-type field effect transistors;  
         [0022]      FIG. 3C  illustrates a third embodiment of the first stage of  FIG. 2  in which the transistors are NPN bi-polar transistors;  
         [0023]      FIG. 3D  illustrates a fourth embodiment of the first stage of  FIG. 2  in which the transistors are PNP bi-polar transistors;  
         [0024]      FIG. 4A  illustrates a first embodiment of the second stage of  FIG. 2  in which the transistors are field-effect transistors;  
         [0025]      FIG. 4B  illustrates a second embodiment of the second stage of  FIG. 2  in which the transistors are bi-polar transistors;  
         [0026]      FIG. 5  illustrates a complete circuit diagram of an embodiment of the switching circuit of  FIG. 1A  using the first stage of  FIG. 3A , and the second stage of  FIG. 4A ;  
         [0027]      FIG. 6  illustrates an alternative embodiment of a control loop component suitable as the control loop component of  FIGS. 1A and 1B ;  
         [0028]      FIG. 7  is a circuit diagram of a conventional switching circuit; and  
         [0029]      FIG. 8  is a signal timing diagram associated with the conventional switching circuit of  FIG. 7 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     The principles of the present invention relate to a switching circuit that transitions using a controlled voltage slope at the drain terminal of the switch transistor to thereby reduce Electro-Magnetic Interference (EMI). In addition, a closed control loop pre-charges and pre-discharges the gate voltage of the switch transistor to thereby reduce switching delay. Furthermore, unlike conventional switching circuits, the closed control loop is configured to open during the actual switching of the switch transistor. Accordingly, the switching circuit is stable during the actual switching operation, which is the operation of greatest susceptibility to instability. Therefore, the closed control loop need not be stable and thus is more easily designed and tested.  
         [0031]      FIG. 1A  is a schematic diagram of a switching circuit  100 A having an n-type switch field effect transistor  131 A in accordance with one embodiment of the present invention. The n-type switch field effect transistor  131 A has a source terminal electrically connected to a first circuit node  135 A that is configured to draw current when the n-type switch field effect transistor  131 A is conducting. The drain terminal of the n-type switch field effect transistor  131 A is electrically connected to the output terminal  141 A of the switching circuit  100 A shown as carrying voltage V OUT . A resistor  134 A having resistance R LOAD  is shown between the output terminal  141 A and a circuit node  136 A that provides current. This resistance may be, but need not be, a load resistance.  
         [0032]     When the output voltage V OUT  is to transition from high to low, a control signal S 1  transitions high, and a complementary control signal S 2  transitions low. Accordingly, the switch  111 A is closed and the switch  121 A is opened permitting a substantially constant current I 1  to be provided by the current source  112 A to the gate terminal of the n-type switch field effect transistor  131 A. This opens the switch field effect transistor  131 A to thereby draw current from the circuit node  136 A, through resistor  134 A, and through switch field effect transistor  131 A. As the resistance through the channel region of the switch transistor  131 A decreases, the output voltage V OUT  accordingly lowers.  
         [0033]     Conversely, when the output voltage V OUT  is to transition from low to high, the control signal S 2  transitions high, and a complementary control signal S 1  transitions low. Accordingly, the switch  121 A is closed and the switch  111 A is opened permitting a substantially constant current I 2  to be drawn by the current sink  122 A from the gate terminal of the n-type switch field effect transistor  131 A. This increases the resistance of (and ultimately closes) the switch field effect transistor  131 A to thereby increase the output voltage V OUT  to a high value.  
         [0034]     In this configuration, there is an implicit capacitance looking into the gate terminal of the n-type switch field effect transistor  131 A. This capacitance means that it takes some time for the current source  112 A to fully turn the switch transistor  131 A on, and that it takes some time for the current source  122 A to fully turn the switch transistor  131 A off. In order to increase this transition time. The capacitance may be increased by capacitor  133 A, which capacitively couples the drain and gate terminals of the n-type switch field effect transistor  131 A. This controlled rate of transition reduces the amount of EMI emitted by the switching circuit  100 A.  
         [0035]     For example, immediately before a low to high transition, the gate voltage of switch transistor  131 A is high, and the output voltage at the drain terminal of switch transistor  131 A is low. As mentioned above, a low to high transition is initiated when switch  121 A closes, and switch  111 A opens. From this time, the current I 2  discharges the combination of the gate terminal capacitance of the switch transistor  131 A (based on its gate to source capacitance) as well as the capacitor  133 A. The split of these two discharging currents is based on capacitor ratio of these two capacitors.  
         [0036]     When the gate voltage of switch transistor  131 A reaches a voltage for which the drain current through switch transistor  131 A starts to be smaller than approximately V DD /R LOAD , the output voltage Vout starts to increase. As the switch transistor  131 A is in saturation at this moment, very small variations of gate voltage cause large variation in the drain current. In this state, the gate voltage of switch transistor  131 A may be considered to be constant through the rest of the transition. As the change in voltage across the gate to source capacitance of switch transistor  131 A is relatively small, substantially all of the discharging current I 2  flows through  133 A. Because the gate voltage of switch transistor  131 A has a relatively constant voltage, and since the current I 2  flowing through capacitor  133 A is relatively constant, a voltage slope of dV OUT /dt=I 2 /C (where C is the capacitance of capacitor  133 A) is seen on the output voltage Vout. This is of course true if the loop composed of switch transistor  131 A and capacitor  133 A has sufficient gain (or in other words, if the gate voltage of switch transistor  131 A varies only a little during the transition).  
         [0037]     A control loop component  132 A performs different functions depending on whether the switching circuit is transitioning to a low state (with V OUT  being low), or transitioning to a high state (with V OUT  being high). In particular, when the output voltage V OUT  is in the initial stages of a high-to-low transition, the control loop component  132 A partially or fully pre-charges the gate terminal of the n-type switch field effect transistor  131 A through the output terminal  142 A of the control loop component  132 A using a closed control loop. This pre-charging supplements the relatively constant charge current I 1  to accelerate the pre-charging process. Once the rate of decline of the output voltage dV OUT /dt increases to above a certain value, the control loop component  132  opens, leaving much of the rest of the transition to occur in open loop with a relatively constant current I 1  driving the transition. As a secondary effect, towards the end of the transition, the rate of decline dV OUT /dt will once again decline below the threshold value, thereby closing the control loop component  132 A. This has the effect of accelerating the end of the transition, which is not susceptible to instabilities in the control loop.  
         [0038]     Conversely, when the output voltage V OUT  is in the initial stages of a low-to high transition, the control loop component  132 A partially or fully pre-discharges the gate terminal of the n-type switch field effect transistor  131 A by drawing current through the terminal  142 A of the control loop component  132 A using a closed control loop. This pre-discharging supplements the relatively constant discharge current I 2  to accelerate the pre-charging process. Once the rate of increase of the output voltage dV OUT /dt increases to above a certain value, the control loop component  132  opens, leaving the rest of the transition to occur in open loop with a relatively constant current I 2  driving the transition. As a secondary effect, towards the end of the transition, the rate of increase dV OUT /dt will once again decline below the threshold value, thereby closing the control loop component  132 A. Once again, this has the effect of accelerating the end of the transition, which is not susceptible to instabilities in the control loop.  
         [0039]     This pre-charging and pre-discharging of the n-type field effect transistor  131 A significantly reduces switching delay. Switching delay is the amount of time between the time that the control signal for the transition is received by the switching circuit (i.e., when the switches  111 A and  121 A change state) until the time the transition of the output voltage V OUT  actually begins. During non-transitioning moments, the switching circuit is not vulnerable to any instability of the closed loop in the control loop component  132 A. This is because control loop component  132 A allows either only positive or negative output currents, and also because the switch transistor  131 A operates in either the linear or “on” mode, or the deep subthreshold or “off” mode, and thus has no gain.  
         [0040]      FIG. 1B  illustrates a switching circuit  100 B, which is similar to the switching circuit  100 A described with reference to  FIG. 1A . However, in  FIG. 1B , the n-type switch field effect transistor  131 A is replaced by a p-type field effect transistor  131 B. Circuit components  111 B,  112 B,  121 B,  122 B,  131 B,  132 B,  133 B,  134 B,  135 B and  136 B,  141 B and  142 B of  FIG. 1B  may be similar to circuit components  111 A,  112 A,  121 A,  122 A,  131 A,  132 A,  133 A,  134 A,  135 A and  136 A,  141 A and  142 A of  FIG. 1A . In the case of  FIG. 1B , however, circuit node  135 B is configured to provide current, whereas circuit node  136 B is configured to draw current. A high to low transition is accomplished by turning off the p-type switch field effect transistor  131 B. Conversely, a low to high transition is accomplished by turning on the p-type switch field effect transistor.  
         [0041]     While any configuration for the control loop component that at least partially opens during the transition will suffice,  FIG. 2  schematically illustrates an embodiment  200  of the control loop component  132 A of  FIG. 1A , or of the control loop component  132 B of  FIG. 1B  in further detail.  
         [0042]     The control loop component  200  includes a first stage  210  having an input terminal  141  electrically connected to the drain terminal of the switch field effect transistor. The first stage  210  is configured to change a first stage output voltage applied to the output terminal  211  of the first stage depending on the rate of change of the first stage input voltage on the input terminal  141  of the first stage  210 . The first stage output voltage is a function of the rate of change of the input voltage (i.e., dV OUT /dt) such that if the rate of change increases above a certain rate, the first stage output voltage decreases below a first reference voltage (referred to below as V REFN ), and such that if the rate of change decreases below a certain rate, the first stage output voltage increases above a second reference voltage (referred to below as V REFP ).  
         [0043]     The control loop component  200  also includes a second stage  220  that has an input terminal  211  electrically connected to the output terminal of the first stage  210 , and an output terminal  142  that is electrically coupled to the gate terminal of the switch field effect transistor  131 .  
         [0044]     If the first stage output voltage applied to the input terminal  211  of the second stage  220  is below the second reference voltage V REFP  and the transition of the output voltage V OUT  is from high-to-low, a current is provided through the output terminal  142  of the second stage to the gate terminal of the switch field effect transistor  131 A or  131 B to thereby facilitate pre-charge. If the first stage output voltage applied to the input terminal  211  of the second stage  220  is above the first reference voltage V REFN  and the transition of the output voltage V OUT  is from low-to-high, a current is drawn from the gate terminal of the switch field effect transistor  131 A through the output terminal  142  of the second stage  220  to thereby facilitate pre-discharge. This configuration supports the pre-charging and pre-discharging operation of the control loop component  200 .  
         [0045]     If the first stage output voltage is below the first reference voltage V REFN  and the switching circuit is transitioning from low to high, or if the first stage output voltage is above the second reference voltage V REFP  and the switching circuit is transitioning from high to low, the output terminal of the first stage is substantially electrically disconnected from the gate terminal of the switch field effect transistor.  
         [0046]      FIGS. 3A  shows an example first stage circuit embodiment  300 A of the first stage  210  of  FIG. 2 . Just as the first stage  210  of  FIG. 2 , the first stage circuit  300 A receives a voltage signal from input terminal  141  and provides a resulting voltage signal on the output terminal  211 . The first stage circuit  300 A includes a capacitor  301 A that capacitively couples the input terminal  141  to gate terminals of two n-type field effect transistors  302 A and  304 A and to the drain terminal of the n-type field effect transistor  302 A. The source terminal of the n-type field effect transistor  302 A is electrically connected to a circuit node  306 A (such as ground) that is configured to draw current when the n-type field effect transistor  302 A is conducting. A current source  303 A provides a substantially constant current I 3  from the circuit node  308 A (e.g., V DD ) to the drain terminal of the n-type field effect transistor  302 A. The n-type field effect transistor  304 A has a drain terminal electrically connected to the output terminal  211  of the first stage, and a source terminal electrically connected to a circuit node  307 A (such as ground) that is configured to draw current when the n-type field effect transistor  304 A is conducting. A resistor  305 A resistively couples the output terminal  211  to another circuit node  309 A that is configured to provide current through the resistor  305 A. The node  309 A is connected to a voltage source V DD  to which the voltage references V REFP  and V REFN  (described hereinafter) are referred for reasons that will be described hereinafter. The operation of  FIG. 3A  will be described with respect to  FIG. 5 .  FIGS. 3B through 3D  are alternative embodiments for the first stage  210  and will be described subsequent to the descriptions of  FIGS. 5 and 6 .  
         [0047]      FIG. 4A  illustrates a second stage circuit  400 A that represents an embodiment of the second stage  220  of  FIG. 2 . Just as the second stage  220  of  FIG. 2 , the second stage circuit  400 A receives a voltage signal from input terminal  211  and provides a resulting current on the output terminal  142 .  
         [0048]     The second stage circuit  400 A includes a transconductance amplifier  401 A having a negative input terminal configured to receive a reference voltage V REFP , and a positive input terminal electrically connected to the input terminal  211 . The output terminal of the transconductance amplifier  401 A is electrically connected to the gate terminals of the p-type field effect transistors  402 A and  403 A and to the drain terminal of p-type field effect transistor  402 A. The source terminals of the p-type field effect transistors  402 A and  403 A are electrically connected to a circuit node (e.g., V DD ) that is configured to provide current through the p-type field effect transistors  402 A and  403 A when conducting. The drain terminal of the p-type field effect transistor  403 A is selectively connected to the output terminal  142  via a switch  405 A that is controlled by a signal S 1 B that is controlled together with signal S 1   
         [0049]     The second stage circuit  400 A also includes a transconductance amplifier  411 A having a negative input terminal configured to receive a reference voltage V REFN , and a positive input terminal electrically connected to the input terminal  211 . The output terminal of the transconductance amplifier  411 A is electrically connected to the gate terminals of the n-type field effect transistors  412 A and  413 A and to the drain terminal of n-type field effect transistor  412 A. The source terminals of the n-type field effect transistors  412 A and  413 A are electrically connected to a circuit node (e.g., ground) that is configured to draw current through the n-type field effect transistors  412 A and  413 A when conducting. The drain terminal of the n-type field effect transistor  413 A is selectively connected to the output terminal  142  via a switch  415 A that is controlled by a signal S 2 B that is controlled together with signal S 2 .  FIG. 4B  illustrates an alternative embodiment for the second stage  220  and will be explained in further detail after the description of  FIGS. 5 and 6 .  
         [0050]      FIG. 5  illustrates a first particular embodiment of a switching circuit  500  in accordance with the principles of the present invention. The switching circuit  500  is the same as the switching circuit  100 A of  FIG. 1A , with the control loop component  132 A being structured as shown in  FIG. 2 , and with the first stage of the control loop being as illustrated in  FIG. 3A , and with the second stage of the control loop being as illustrated in  FIG. 4A . The operation of the switching circuit  500  will now be described.  
         [0051]     A driver circuit for slope limited pulse switching of the load  134 A comprises a transistor switch  131 A. The capacitor  133 A is connected between the drain and the gate terminals of transistor  131 A. The gate terminal of transistor  131 A is connected to the two current sources  112 A and  122 A respectively activated by the switches  111 A (controlled by signal S 1 ) and  121 A (controlled by signal S 2 ). Signals S 1  and S 2  are complementary signals such that when switch  111 A is open, the switch  121 A is closed, and vice versa. The capacitor  301 A is used to sense the slope dV OUT /dt.  
         [0052]     The current I S  flowing through the sense capacitor  301 A is given by the following Equation (1): I S =pC S (V D2 −V OUT ), where “V D2 ” is the voltage at the drain of transistor  302 A, “C S ” is the capacitance of sense capacitor  301 A, and “p” is the Laplace variable.  
         [0053]     In a first approximation, Equation (1) may be rewritten as Equation (2) as follows: I S =−pC S V OUT . In this case, the sense current I S  is approximately proportional to the slope dV OUT /dt, where I S  will be positive for falling edges (dV OUT /dt being negative) and negative for rising edges (dV OUT /dt being greater than zero). This approximation is justified by the fact that the interval of variation for V OUT  is greater than the interval of variation of V D2 , and by the fact that V D2  does not vary faster than V OUT . Equation (2) is a good approximation of Equation (1) since the impedance of the drain of transistor  302 A (which is approximately equal to the transconductance “gm” of the transistor  302 A) is much smaller than the impedance of the output voltage (which is approximately equal to the resistance of resistor  305 A).  
         [0054]     During the initial stage of a high-to-low transition when switches  111 A and  405 A are initially closed, and switches  121 A and  415 A are initially opened, the gate terminal of the switching transistor  131 A is pre-charged using a closed loop. In this state, the current I 1  starts to charge up the gate of the transistor  131 A. As long as the gate voltage required for conduction is not reached, the switch transistor  131 A remains closed in this initial stage of the high-low transition. Thus, V OUT  does not vary and the sense current I S  is zero.  
         [0055]     The current magnitude  13 , the transistors  302 A and  304 A, and the resistor  305 A are dimensioned so that if the magnitude of the sense current I S  is smaller than a given threshold, the voltage at terminal  211  will be smaller than V REFP . This balance is important to properly open and close the control loop at the appropriate times, and so the voltage reference V REFP  is tied to the voltage V DD , such that variations in the supply voltage VDD will not adversely affect this balance. These same elements are also sized so that in that case, the output of transconductance amplifier  401 A is sufficiently low to have current flowing through the transistor  402 A. The current through transistor  402 A is mirrored by transistor  403 A. The mirrored current (referred to herein as I 9 ) passes through closed switch  405 A, and is added to current I 1  at the gate terminal of switch transistor  131 A.  
         [0056]     The gate voltage of switch transistor  131 A now increases at a rate (I 1 +I 9 )/C instead of I 1 /C, where C is the capacitance seen at the gate of transistor  131 A. The current I 9  may be designed to be large compared to current I 1  so that the switch transistor  131 A quickly pre-charges. As soon as switch transistor  131 A starts conducting, V OUT  will vary, and thus sense current I S  will be non-zero. The current through transistor  302 A is now I 3  minus I S  instead of I 3 . That current is mirrored by the transistor  304 A. Variations of the drain current of transistor  304 A are converted into voltage variations at terminal  211  using resistor  305 A. As the voltage at terminal  211  increases, the output of the transconductance amplifier  401 A increases. Consequently, the gate to source voltage of transistor  402 A decreases and the current sourced by transistor  403 A decreases. The system is designed so that the transconductance amplifier  401 A rapidly saturates (i.e. its output is limited by the supply voltage of the amplifier). Transconductance amplifier  401 A saturates in the moment when the voltage at node  211  becomes greater than V REFP .  
         [0057]     When the transconductance amplifier  401 A saturates, no current flows through transistors  402 A and  403 A. The switch  405 A is still closed, but transistor  403 A is now open. Therefore, the control loop is likewise open. Once the control loop is open, the slope of V OUT  is determined by I 1  alone without using the control loops. Thus there are no stability issues during the high to low transition.  
         [0058]     A similar reasoning can be done for a low to high transition. In the initial stages of a low-to-high transition, the gate terminal of the switching transistor  131 A is pre-discharged using a closed loop. In the low state, the switches  121 A and  415 A are closed, whereas switches  111 A and  405 A are open. In this state, the current I 2  starts to discharge the gate of the transistor  131 A. As long as the transistor  131 A remains closed, V OUT  remains low and does not vary, and thus the sense current I S  is zero.  
         [0059]     As previously mentioned, the current magnitude I 3 , the transistors  302 A and  304 A, and the resistor  305 A are dimensioned so that if the magnitude of the sense current I S  is less negative than a given threshold, the voltage at terminal  211  will be larger than V REFN . This balance is important to properly open and close the control loop at the appropriate times, and so the voltage reference V REFN  is also tied to the voltage V DD , such that variations in the supply voltage VDD will not adversely affect this balance. These same elements are also sized so that in that case, the output of transconductance amplifier  411 A is sufficiently high to have current flowing through the transistor  412 A. The current through transistor  412 A is mirrored by transistor  413 A. The mirrored current (referred to herein as I 10 ) is drawn through closed switch  415 A, and is added to the current I 2  drawn from the gate terminal of switch transistor  131 A.  
         [0060]     The gate voltage of switch transistor  131 A now decreases at a rate (I 2 +I 10 )/C instead of I 2 /C. Once again, the current I 10  may be designed to be large compared to current I 2  so that the switch transistor  131 A quickly pre-discharges. As soon as switch transistor  131 A increases resistance, V OUT  will vary, and thus sense current I S  will be non-zero. The current through transistor  302 A is now I 3  minus I S  (where I S  is negative) instead of I 3 . That current is mirrored by the transistor  304 A. Variations of the drain current of transistor  304 A are converted into voltage variations at terminal  211  using resistor  305 A. As the voltage at terminal  211  decreases, the output of the transconductance amplifier  411 A decreases. Since the transconductance amplifier has an output lout equal to its transconductance gm times the differential input voltage Vin, the transconductance amplifier saturates. Consequently, the gate to source voltage of transistor  412 A decreases and the current drained by transistor  413 A decreases. The system is designed so that the transconductance amplifier  411 A rapidly saturates at the point where the voltage at node  211  becomes less than V REFN .  
         [0061]     When the transconductance amplifier  411 A saturates, no current flows through transistors  412 A and  413 A. The switch  415 A is still closed, but transistor  413 A is now open. Therefore, the control loop is likewise open. Once the control loop is open, the slope of V OUT  is determined by I 2  alone without using the control loops. Thus there are no stability issues during the low to high transition.  
         [0062]     The switching circuit  500  provides a driver (such as, but not limited to, a PWM driver) with reduced turn on and turn off delays. The circuit  500  may be used in a wide variety of operating conditions (i.e. specified R L  and dV OUT /dt) without requiring extensive stability analysis. The circuit  500  more quickly brings the switch transistor to the verge of conduction regardless of the process tolerances by operating in closed loop. Once the switch is starting to conduct, the regulation loop opens and the gate is charged in open loop without the risk that a closed loop would become unstable as in the prior art.  
         [0063]     Having described the general principles of the present invention with respect to a specific circuit embodiment, those of ordinary skill in the art will recognize various modification and variations that be apparent from this description. For instance,  FIGS. 3B through 3D  represent alternative embodiments of the first stage circuit, in which the polarity of the circuit is reversed, and in which the possibility of using bipolar transistors is shown. The elements of  FIGS. 3B through 3D  have a similar function as described for the corresponding elements of  FIG. 3A . Likewise,  FIG. 4B  shows that bi-polar transistors may be used in the second stage of the control loop as well.  
         [0064]     More specifically,  FIG. 3B  shows a first stage circuit embodiment  300 B of the first stage  210  of  FIG. 2 . The first stage circuit  300 B includes a capacitor  301 B that capacitively couples the input terminal  141  to gate terminals of two p-type field effect transistors  302 B and  304 B and to the drain terminal of the p-type field effect transistor  302 B. The source terminal of the p-type field effect transistor  302 B is electrically connected to a circuit node  306 B (such as V DD ) that is configured to provide current when the p-type field effect transistor  302 B is conducting. A current source  303 B draws a substantially constant current I 3  to the circuit node  308 B (e.g., ground) from the drain terminal of the p-type field effect transistor  302 B. The p-type field effect transistor  304 B has a drain terminal electrically connected to the output terminal  211  of the first stage, and a source terminal electrically connected to a circuit node  307 B (such as V DD ) that is configured to provide current when the p-type field effect transistor  304 B is conducting. A resistor  305 B resistively couples the output terminal  211  to another circuit node  309 B that is configured to draw current through the resistor  305 B.  
         [0065]      FIG. 3C  shows yet another example first stage circuit embodiment  300 C of the first stage  210  of  FIG. 2 . The first stage circuit  300 C includes a capacitor  301 C that capacitively couples the input terminal  141  to base terminals of two NPN bipolar transistors  302 C and  304 C and to the collector terminal of the NPN bipolar transistor  302 C. The emitter terminal of the NPN bipolar transistor  302 C is electrically connected to a circuit node  306 C (such as ground) that is configured to draw current when the NPN bipolar transistor  302 C is conducting. A current source  303 C provides a substantially constant current I 3  from the circuit node  308 C (e.g., V DD ) to the collector terminal of the NPN bipolar transistor  302 C. The NPN bipolar transistor  304 C has a collector terminal electrically connected to the output terminal  211  of the first stage, and an emitter terminal electrically connected to a circuit node  307 C (such as ground) that is configured to draw current when the NPN bipolar transistor  304 C is conducting. A resistor  305 C resistively couples the output terminal  211  to another circuit node  309 C that is configured to provide current through the resistor  305 C.  
         [0066]      FIG. 3D  shows a final example first stage circuit embodiment  300 D of the first stage  210  of  FIG. 2 . The first stage circuit  300 D includes a capacitor  301 D that capacitively couples the input terminal  141  to base terminals of two PNP bipolar transistors  302 D and  304 D and to the collector terminal of the PNP bipolar transistor  302 D. The emitter terminal of the PNP bipolar transistor  302 D is electrically connected to a circuit node  306 D (such as V DD ) that is configured to provide current when the PNP bipolar transistor  302 D is conducting. A current source  303 D draws a substantially constant current I 3  to the circuit node  308 D (e.g., ground) from the collector terminal of the PNP bipolar transistor  302 D. The PNP bipolar transistor  304 D has a collector terminal electrically connected to the output terminal  211  of the first stage, and an emitter terminal electrically connected to a circuit node  307 D (such as VDD) that is configured to provide current when the PNP bipolar transistor  304 D is conducting. A resistor  305 D resistively couples the output terminal  211  and another circuit node  309 D that is configured to draw current through the resistor  305 D.  
         [0067]      FIG. 4B  illustrates an alternative second stage embodiment  400 B of the second stage  220  of  FIG. 2 . The second stage  400 B is structured similar to the second stage  400 A described with respect to  FIG. 4A , except that PNP bipolar transistors replace p-type bi-polar transistors, and in which NPN bipolar transistors replace n-type bi-polar transistors.  
         [0068]     More specifically, the second stage  400 B includes a first transconductance amplifier  401 B having a negative input terminal configured to receive the reference voltage V REFP , and a positive input terminal electrically connected to the input terminal  211 . A PNP bi-polar transistor  402 B has base and collector terminals electrically connected to the output terminal of the first transconductance amplifier  401 B, and an emitter terminal electrically connected to circuit node  404 B (such as V DD ) that is configured to provide current through the PNP bi-polar transistor  402 B. A second PNP bi-polar transistor  403 B has a base terminal electrically connected to the output terminal of the first transconductance amplifier  401 B, an emitter terminal electrically connected to the circuit node  404 B, and a collector terminal selectively electrically connected to the output terminal  142  of the second stage through switch  405 B controlled by signal S 1 B.  
         [0069]     A second transconductance amplifier  411 B has a negative input terminal configured to receive the reference voltage V REFN , and a positive input terminal electrically connected to the input terminal  211 . An NPN bi-polar transistor  412 B has base and collector terminals electrically connected to an output terminal of the second transconductance amplifier  411 B, and an emitter terminal electrically connected to a circuit node  414 B (such as ground) that is configured to drawn current through the NPN bi-polar transistor  412 B. A second NPN bi-polar transistor  413 B has a base terminal electrically connected to the output terminal of the second transconductance amplifier  411 B, an emitter terminal electrically connected to the circuit node  414 B, and a collector terminal selectively electrically connected to the output terminal  142  of the second stage through switch  415 B controlled by signal S 2 B.  
         [0070]      FIG. 6  illustrates a second particular embodiment  600  of a control loop component  132 A of  FIG. 1A  or  132 B of  FIG. 1B . During the high-to-low transition, when the rate of decline dV OUT /dt of the output voltage starts to accelerate, the current I S  increases in the positive direction. This causes the current passing through transistor  302 A to decrease from I 3  by increasing amounts. This decreasing current is passed through transistor  604  sized with ratio N with respect to the transistor  302 A. More importantly for high-to-low transitions, however, the decreasing current is passed through transistor  603  sized with ratio N with respect to transistor  302 A. Before the current passing through transistor  603  (equal to N times (I 3 −I S )) drops below the current I refp  provided by current source  601 , there is a current drawn from the gate terminals of p-type transistor  402 A, causing transistor  402 A to conduct. That current is mirrored by mirror ratio M (where M may be much larger than 1) to provide a supplemental pre-charge current through switch  405 A, to the terminal  142  and to the gate of the switch transistor. However, once the current passing through transistor  603  drops below the current I refp  provided by current source  601  (due to a more rapid decline in the output voltage), there is a current provided to the gate terminal of p-type transistor  402 A, causing transistor  402 A to turn off. That opens the control loop for much of the remainder of the transition, until the rate of decline dV OUT /dt once again drops below the threshold.  
         [0071]     During the low-to-high transition, when the rate of increase dV OUT /dt of the output voltage starts to accelerate, the current I S  increases in the negative direction. This causes the current passing through transistor  302 A to increase from I 3  by increasing amounts. This increasing current is passed through transistor  604  sized with ratio N with respect to the transistor  302 A. Before the current passing through transistor  604  (equal to N times (I 3 −I S ) (where I S  is negative)) increases above the current I refn  provided by current source  602 , there is a current provided from the gate terminals of n-type transistor  412 A, causing transistor  412 A to conduct. That current is mirrored by mirror ratio M (where M may be much larger than 1) to provide a supplemental pre-discharge current through switch  415 A, to the terminal  142  and to the gate of the switch transistor. However, once the current passing through transistor  604  increase to be above the current I refn  provided by current source  602  (due to a more rapid increase in the output voltage), there is a current drawn from the gate terminal of n-type transistor  412 A, causing transistor  412 A to turn off. That opens the control loop for much of the remainder of the transition, until the rate of increase dV OUT /dt once again drops below the threshold.  
         [0072]     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes, which come within the meaning and range of equivalency of the claims, are to be embraced within their scope.