Electronic driver circuit that utilizes resonance with load circuitry in combination with timed switching to reduce power consumption

[0002] This is a continuation of U.S. patent application Ser. No. 09/029,707, filed Mar. 6, 1998, which corresponds to, and claims benefit of, International Application PCT/GB96/02199, filed Sep. 6, 1996.

[0003] This invention relates to electronic circuits. In particular, this invention relates to reducing power consumption in electronic circuits and conditioning signals in electronic circuits.

[0004] An electronic circuit consumes power in various ways. For example, power is consumed when input signals to elements of the circuit change state. The power consumption occurs as the result of the charging and discharging of parasitic capacitances associated with the inputs to the circuit elements and with electrical conductors that provide the input signals to circuit elements. Power dissipation does not occur in the parasitic capacitance associated with a signal, but in the output resistance of the driver circuit which is the source of the signal.

[0005] A large portion of the power consumption in a modern integrated circuit (“IC”) occurs in the signal drivers which drive the output pins of the IC and thus also the inputs to any other circuits or ICs connected to the output pins, typically by way of electrically conductive tracks formed on a printed wiring board (“PWB”). The parasitic capacitance associated with (i.e., driven by) each output signal driver is typically much higher than that associated with the internal signals within the IC. Consequently, the power dissipation is high.

[0006] A further problem associated with the output drivers of an IC results from the fact that the associated wiring, both within the IC package and on the PWB, is relatively long and therefore has high inductance. This inductance commonly causes undesirable signal characteristics such as “ringing” and “overshoot”.

[0007] Referring to the drawings, FIG. 1aillustrates a conventional CMOS inverting output driver 10 formed as part of an IC. Output driver 10 generates an inverted driver output voltage signal V_DOin response to a driver input voltage signal V_I. Driver 10 is connected through electrical conductor 12 of a PWB to load circuitry 14. Specifically, electrical conductor 12 converts driver output voltage V_DO) into a conductor output voltage signal V_BOthat drives a group of digital CMOS ICs 16 in load 14.

[0008] Driver circuit 10 is formed with N-channel insulated-gate field-effect transistor (“FET”) QA and P-channel insulated-gate FET QB whose gate electrodes receive driver input voltage V_I. The sources of FET QA and QB are respectively connected to a source of a low supply voltage V_SS, typically ground reference (0 volt), and a source of a high supply voltage V_DD. The QA and QB drains are connected together to provide driver output voltage V_DO.

[0009] N-channel FET QA is turned on by raising input voltage V_Ito a suitably high level. On the other hand, FET QB is turned on by reducing voltage V_Ito a suitably low level. Accordingly, only one of FETs QA and QB is conductive during steady-state operation. If input voltage V_Iis high, FET QA is turned on to pull output voltage V_DOto a low value close to V_SS. Conversely, output voltage V_DOis at a high value close to V_DDwhen input voltage V_Iis low and causes FET QB to be turned on.

[0010] The “on” resistance of each of FETs QA and QB is normally quite low. Consequently, output voltage V_DOmakes a rapid transition from V_SSto V_DDwhen input voltage V_Imakes a high-to-low transition. Likewise, output voltage V_DDmakes a rapid transition from V_DDto V_SSwhen input voltage makes a low-to-high transition. During a transition, there is typically a brief period when both of FETs QA and QB are conductive.

[0011] PWB electrical conductor 12, commonly referred to as an interconnect, consists of a length of copper track and a ground plane at the V_SSpotential. The steps shown in the line passing through conductor 12 in FIG. 1aqualitatively represent the changes in direction that conductor 12 makes on the PWB. The ground plane is represented by the block in slanted shading. CMOS ICs 16 in load 14 are also variously connected to the V_SSsupply.

[0012] FIG. 1adoes not explicitly show the various parasitic circuit elements which typically exist in any such circuit arrangement. For example, PWB conductor 12 is typically inductive and is also coupled to the nearby ground plane by parasitic capacitance. When driver 10 is formed as part of an IC within an IC package, the conductors internal to the package introduce further parasitic inductance and capacitance. Likewise, when ICs 16 are contained within IC packages, ICs 16 introduce further parasitic inductance and capacitance.

[0013] FIG. 1bshows a simplified electrical model of the circuitry in FIG. 1a. Inverting driver 10 is modeled by a switch SW in series with an output resistor R_ON. Switch SW is controlled by input voltage V_I. Resistor R_ONrepresents the source-drain resistance of each FET QA or QB when it is turned on. PWB conductor 12 is modeled by parasitic inductance LB with distributed parasitic capacitance CB. Inductance LB and capacitance CB also model the parasitic inductance and capacitance of conductors internal to IC packages when driver 10 is formed as part of an IC contained in an IC package and/or when ICs 16 are contained within IC packages. Load 14 is modeled by parasitic capacitance CL representing the combined capacitances of the inputs to ICs 16 in load 14.

[0014] Consider a typical case in which conductor capacitance CB is much less than load capacitance CL. When input voltage V_Icauses switch SW of FIG. 1bto change state, output voltages V_DOand V_BOchange in the manner generally illustrated by the waveforms of FIGS. 2a-2c. Since capacitance CB is much less than capacitance CL, the combination R_ON, CL, CB, and LB approximates a series LC resonant circuit which can be (a) underdamped, (b) critically damped, or (c) overdamped according to the value of on resistance R_ONcompared to the reactance of inductance LB.

[0015] The waveforms shown in FIG. 2acorrespond to the underdamped case in which resistance R_ONis very low. FIG. 2aillustrates how output voltages V_DOand V_BOgenerally vary when input voltage V_Imakes a high-to-low transition. Driver output voltage V_DOrises quickly from V_SSto V_DD. The effect of conductor inductance LB is to limit the initial flow of current from driver 10. Consequently, conductor output voltage V_BOchanges slowly at first. However, once current has started flowing through inductance LB, the current continues to flow even when conductor output voltage V_BOhas reached V_DD. This leads to overshoot in conductor output voltage V_BOand consequent ringing.

[0016] FIG. 2bshows waveforms generally representative of the critically damped case in which resistance R_ONis moderately (but not very) low. The behavior is similar to the underdamped case except that just enough energy is dissipated in resistance R_ONso that there is little overshoot in conductor output voltage V_BO, substantially no ringing, and the transition speed for both of output voltages V_BOand V_DOis moderately high.

[0017] FIG. 2cshows waveforms corresponding to the overdamped case where resistance R_ONis relatively high. Here, the transition speed for both of output voltages V_DOand V_BOis low. In particular, conductor output voltage V_BOtakes a comparatively long time to reach the desired V_DDlevel.

[0018] Both overshoot and low transition speed are generally undesirable signal characteristics. Consequently, the waveforms shown in FIG. 2boften represent the best case for conventional driver 10. For any of the circuit behaviors depicted in FIGS. 2a-2c, the ringing frequency, the transition speed, and the degree of overshoot depend on the values of elements R_ON, LB, CB, and CL. While the value of on resistance R_ONcan be controlled during driver design, the values of elements LB, CB, and CL vary from one PWB conductor to another and from one load to another, making it very difficult to achieve the often desired waveforms of FIG. 2bin the typical case where driver design is completed without knowledge of the specific load characteristics.

[0019] The energy drawn from the V_DDpower supply approximately equals CV², where V is the potential difference V_DD−V_SS, and C is the sum of capacitances CB and CL. Approximately half the supplied energy—i.e., ½CV²—is stored in load capacitance CL. The remainder of the supplied energy—i.e., also approximately ½CV²—is dissipated in on resistance R_ON.

[0020] Similar waveforms occur when input voltage V_Imakes a low-to-high transition, causing output voltages V_DOand V_BOto make high-to-low transitions. In this case, no energy is drawn from the V_DDsupply. However, the energy stored in load capacitance CL is substantially dissipated. Accordingly, the average power drawn from the V_DDsupply when input voltage V_Imakes transitions at an input frequency f_Iis f_ICV². The majority of the power is dissipated in driver output resistance R_ON. The load itself typically contains relatively insignificant loss mechanisms.

[0021] In the case where on resistance R_ONis very low, only a small amount of power dissipation occurs during the initial output transition. Unfortunately, the ringing persists for a relatively long time. Power is gradually dissipated in driver output resistance RON, causing the amplitude of the ringing to decay gradually. This is quite wasteful of power. It is desirable to reduce the power consumption in a driver circuit without sacrificing other desirable performance characteristics and without reducing the capability to accept loads of greatly varying types.

[0022] The present invention furnishes a split-level technique for reducing power consumption in an electronic driver circuit. By enabling the output signal of the driver circuit to be switched in a timely manner to an intermediate voltage level between the two extreme voltage levels of the driver output signal, the split-level technique of the invention harnesses the natural resonance of inductive/capacitive load circuitry as an energy-storage mechanism that assists in driving the load. The energy stored in the resonance-produced energy storage mechanism is re-used, thereby reducing power consumption in the driver. Up to 75% savings in power consumption can readily be achieved.

[0023] Re-use of the stored energy serves to reduce ringing that could otherwise occur in the driver output waveforms. Consequently, signal integrity is improved.

[0024] The present invention also furnishes a self-adjusting technique in which the driver circuit automatically adjusts its timing according to different load characteristics. The driver circuit of the invention can thus be utilized in IC designs having a wide variety of load characteristics. The self-adjustment technique also allows driver output transitions to occur at a largely optimal high speed while still avoiding overshoot and undershoot. Hence, the invention provides a large advance over the prior art.

[0025] More particularly, a driver circuit configured in accordance with the invention generates a circuit output signal which is typically provided to an electrical conductor that furnishes a conductor output signal provided to a load. The circuit and conductor output signals respectively make corresponding transitions between a pair of output voltage levels. The conductor and load have inductance and capacitance which, in the absence of the invention, could cause the conductor output signal to overshoot during the transition from one of the output levels to the other output level.

[0026] The invention avoids the overshoot by utilizing an intermediate voltage level between the two output levels. During the course of a circuit output transition, the driver circuit of the invention holds the circuit output signal approximately at the intermediate voltage level for a non-zero holding period. During the holding period, resistive losses within the driver circuit due to the current of the driver output signal are kept low. Simultaneously, underdamped resonance produced by the inductance and capacitance of the conductor and the load cause the conductor output signal, which is the signal of interest insofar as the load is concerned, to overshoot the intermediate voltage level and end up approximately at the desired final output level. In essence, the voltage overshoot in the present driver circuit starts and ends at earlier points than in a prior art driver that exhibits overshoot, the ending point in the invention being controlled so that the conductor output signal approximately reaches, but does not go significantly beyond, the desired final output voltage level.

[0027] When the holding period is over, the circuit output signal goes rapidly to the desired final output voltage level. This causes the conductor output signal to be held at the desired final output voltage level without significant ringing.

[0028] A mechanical analogy is helpful in understanding how the present driver circuit re-uses energy which, in the absence of the invention, would be dissipated. The conductor has an inductance L which acts like a flywheel that stores energy E_I: $E_I=\frac{1}{2}{\mathrm{LI}}_c^2$

[0029] where I_cis the conductor current. During the first half of a conductor output transition from a low voltage level to a high voltage level, current and therefore energy are supplied from the source of the intermediate supply voltage. Approximately half of this energy charges the load capacitance and, because resistive energy loss is kept low, a large part of the remaining energy is stored in the inductive flywheel. Only a relatively small amount of energy is dissipated.

[0030] During the second half of the conductor output transition, energy stored in the inductive flywheel is added to further energy supplied from the source of the intermediate supply voltage to enable the conductor output signal to substantially overshoot the intermediate voltage level. Thus, a large part of the energy supplied from the source of the intermediate voltage supply is used to charge the load capacitance with a very low resistive energy loss.

[0031] During the next conductor output transition in which the conductor output signal is returned to the low voltage level, the inductive flywheel works in reverse to return a large part of the energy stored in the load capacitance to the source of intermediate voltage supply with a low resistive energy loss. Consequently, over a complete operational cycle, only a small amount of energy is dissipated compared to what occurs with a conventional driver.

[0032] The driver circuit of the invention contains control circuitry and switching circuitry. The control circuitry normally generates a plurality of control signals in response to the circuit input signal. The switching circuitry then generates the circuit output signal in response to the control signals.

[0033] The present driver circuit is normally coupled between sources of first, second, and third supply voltages respectively equal to one of the output voltage levels, the intermediate voltage level, and the other output voltage level. The switching circuitry typically contains three switches.

[0034] One of the switches has a first flow electrode coupled to the source of the first supply voltage, a second flow electrode coupled to an output node from which the circuit output signal is provided, and a control electrode responsive to one of the control signals for controlling current flow between the flow electrodes. Another of the switches has a first flow electrode coupled to the source of the second supply voltage, a second flow electrode coupled to the output node, and a control electrode responsive to another of the control signals for controlling current flow between that switch's flow electrodes. The third switch has a first flow electrode coupled to the source of the third supply voltage, a second flow electrode coupled to the output node, and a control electrode responsive to a third of the control signals for controlling current flow between the third switch's flow electrodes.

[0035] The second switch typically has an on resistance of sufficiently low value that, over a wide range of inductive-capacitive load characteristics, transitions of the conductor output voltage are substantially underdamped relative to the intermediate voltage level. Operation of the present driver circuit is relatively insensitive to the on resistances of the first and third switches. The on resistances of the first and third switches are typically chosen such that, in the absence of the second switch, the conductor output transitions would be critically damped relative to the two (extreme) output voltage levels for average load characteristics. Nonetheless, the on resistances of the first and third switches could be chosen so that, again in the absence of the second switch, the conductor output transitions would be overdamped or underdamped relative to the output voltage levels for an average load. The on resistance of the second switch is typically less than half the on resistance of the first switch.

[0036] The control circuitry preferably adjusts the intermediate-level holding period dependent on when the circuit output signal substantially completes a circuit output transition compared to when the second control signal substantially completes a corresponding control transition. This self-adjusting capability enables the driver circuit of the invention to be employed with loads having a wide variety of different capacitive and inductive characteristics. The self-adjustment capability can, for example, be achieved by implementing the control circuitry with time-comparison circuitry and adjustment circuitry. The time-comparison circuitry makes comparisons between the circuit output signal and the second control signal. The adjustment circuitry adjusts the intermediate-level holding period depending on the results of the comparisons. The self-adjustment capability enables conductor output transitions to occur at largely optimal high speed without ringing.

[0037] Opposite circuit output transitions cause largely equal, but opposite, current flows from the source of the second (intermediate) supply voltage. Thus the net current flow over a complete operational cycle is nearly zero. This allows the intermediate supply voltage to be provided by a reservoir capacitor rather than a power supply.

[0038] FIGSs. 1aand 1brespectively are a circuit diagram and a simplified electrical model of a conventional driver circuit that drives a load through an electrical conductor of a printed wiring board.

[0039] FIGS. 2a, 2b, and 2care simplified timing diagram graphs of voltage waveforms that arise in the circuitry of FIGS. 1aand 1bwhen the load in combination with the conductor and driver output resistance respectively approximate underdamped, critically damped, and overdamped series LC resonant circuits.

[0040] FIG. 3 is a block diagram of circuitry that contains a driver circuit which utilizes resonance to assist in making output transitions according to the invention.

[0041] FIG. 4 is a circuit diagram of circuitry containing a CMOS embodiment of the driver circuit in FIG. 3.

[0042] FIGS. 5aand 5bare simplified timing diagrams of respective voltage and current waveforms that would occur in the circuitry of FIGS. 3 and 4 if ringing were permitted to occur.

[0043] FIG. 6 is a simplified timing diagram of voltage waveforms that arise in the circuitry of FIGS. 3 and 4 when the driver circuit is operated in its intended mode.

[0044] FIGS. 7a, 7b, and 7care simplified timing diagrams of voltage waveforms that arise in the circuitry of FIGS. 3 and 4 when the timing respectively is too fast, just right, and too slow.

[0045] FIG. 8 is a block/circuit diagram of an embodiment of the control circuitry in FIG. 4.

[0046] FIG. 9 is a block/circuit diagram indicating how the intermediate supply voltage for the driver circuit of FIG. 3 is provided from a reservoir capacitor.

[0047] Like reference signals are employed in the drawings and in the description of the preferred embodiments to represent the same, or very similar, item or items.

[0048] Referring to FIG. 3, it illustrates how a driver circuit 18 is configured in accordance with the teachings of the invention for reducing power consumption in driving load circuitry 14 through electrical conductor 12 formed on a PWB. Conductor 12 and load 14 are typically configured in the manner described above in connection with FIG. 1. The slanted shading in conductor 12 of FIG. 3 again indicates the ground plane maintained at low supply voltage V_SS. Load 14 again contains digital CMOS ICs 16.

[0049] Conductor voltage output signal V_BO, which drives ICs 16, is supplied from one end of conductor 12 in response to driver output voltage signal V_DOsupplied to the other end of conductor 12. Conductor output voltage V_BOmakes conductor output transitions between low supply voltage V_ssand high supply voltage V_DD.

[0050] Driver circuit 18 is connected between the V_SSand V_DDsupplies. Low supply voltage V_SSagain typically is ground reference. Driver circuit 18 is also coupled to a source of an intermediate supply (or reference) voltage V_HHthat lies between voltages V_SSand V_DD. Intermediate supply voltage V_HHis typically halfway between V_SSand V_DD. Instead of being provided from a power supply, intermediate voltage V_HHcan be provided from a reservoir capacitor as described below in connection with FIG. 9.

[0051] Driver 18 generates driver output voltage V_DOin response to driver input voltage signal V_I. Driver output voltage V_DOmakes circuit output transitions between V_SSand V_DD. The V_DOoutput transitions follow the V_Iinput transitions inversely or directly. That is, driver 18 functions as an inverting or non-inverting driver.

[0052] Driver circuit 18 is formed with control circuitry 20 and switching circuitry 22. Control circuitry 20 provides control voltage signals V_C1, V_C2, and V_C3in response to input voltage V_I. Control voltages V_C1, V_C2, and V_C3are also typically generated in response to driver output voltage V_DOas a feedback signal. An example of control circuitry 20 is given below in FIG. 8. Switching circuitry 22 generates driver output voltage V_DOin response to control voltages V_C1-V_C3. Driver output voltage V_DOis supplied from a driver output node NO.

[0053] Switching circuitry 22 consists of three switching elements (referred to here simply as switches) S1, S2, and S3. Switch S1 has a first flow electrode E1 connected to the V_SSsupply, a second flow electrode E2 connected to output node NO, and a control electrode CE responsive to control voltage V_C1for controlling the current flow between the S1 flow electrodes. Switch S2 has a first flow electrode E1 connected to the V_HHsupply, a second flow electrode E2 connected to node NO, and a control electrode CE responsive to control voltage V_C2for controlling the current flow between the S2 flow electrodes. Switch S3 has a first flow electrode E1 connected to the V_DDsupply, a second flow electrode E2 connected to node NO, and a control electrode CE responsive to control voltage V_C3for controlling the current flow between the S3 flow electrodes.

[0054] Each of switches S1-S3 has a low on resistance. In particular, switches S1 and S3 have moderately low on resistances such that, in the absence of switch S2, transitions of conductor output voltage V_BObetween the V_SSand V_DDlevels would be approximately critically damped for average inductive-capacitive load characteristics. Provided that the on resistances of switches S1 and S3 are sufficiently low to achieve desired switching speed, circuit performance is relatively insensitive to the precise values of the on resistances for switches S1 and S3. Consequently, the on resistances of switches S1 and S3 can be of such values that, in the absence of switch S2, V_BOtransitions between the V_SSand V_DDlevels would be overdamped or underdamped. Switch S2 has a very low on resistance such that low-to-high transitions in conductor output voltage V_Bare underdamped relative to intermediate voltage V_HHover a wide range of inductive-capacitive load conditions when switch S3 is closed.

[0055] Each of switches S1-S3 (collectively the “S” switches) can be configured in various ways. For example, each of the S switches can consist of a single transistor. In the case of a field-effect transistor of either the insulated-gate or junction type, the two source/drain electrodes of the FET are the first and second flow electrodes while the FET's gate electrode is the control electrode. For a bipolar transistor, its emitter and collector are the flow electrodes while its base is the control electrode.

[0056] Each S switch could be formed with multiple transistors in parallel to achieve lower on resistance. The number of transistors in parallel can be controlled by self-adjusting circuitry for adapting to the load characteristics.

[0057] FIG. 4 illustrates a CMOS example of the driver circuit of FIG. 3 in which each of the S switches is implemented with an enhancement-mode insulated-gate FET. In particular, switch S1 consists of an N-channel FET Q1 whose source and drain are respectively connected to the V_SSsupply and output node NO. Switch S2 is formed with another N-channel FET Q2 whose source/drain elements are respectively connected to the V_HHsupply and node NO. Switch S3 consists of a P-channel FET Q3 whose source and drain are respectively connected to the V_DDsupply and node NO. Control voltages V_C1-V_C3are respectively supplied to the gate electrodes of FETs Q1-Q3.

[0058] FET Q2 could also be a P-channel FET even though the on resistance of a P-channel FET is normally somewhat greater than that of an N-channel FET. In this case, control voltage V_C2would typically make transitions opposite to those described below.

[0059] As a further alternative, switch S2 could be formed with a P-channel FET whose source/drain elements are respectively connected to the V_HHsupply and node NO in parallel with an N-channel FET also with source/drain elements connected respectively to the V_HHsupply and node NO. In this case, control voltage V_C2is replaced with a pair of complementary control voltages, one for the gate electrode of each of the P-channel and N-channel FETs in switch S2.

[0060] The example in FIG. 4 also illustrates how PWB conductor 12 and load circuitry 14 are modeled for the purpose of analysis. As in FIG. 1, conductor 12 and any conductors internal to associated IC packages—i.e., packages present when driver 18 is formed as part of an IC contained in an IC package and/or when ICs 16 are contained in IC packages—are modeled by inductance LB and distributed capacitance CB connected between inductance LB and the V_SSsupply. Load 14 is modeled with capacitance CL connected to the V_SSsupply. To simplify the analysis, conductor capacitance CB can be viewed as part of load capacitance CL. Although the exact distribution of the total load capacitance between capacitances CL and CB affects the V_BOwaveform shapes, the essential resonant characteristics of the inductive-capacitive loading are present across a wide range in the ratio of capacitance CL to capacitance CB.

[0061] The operation of the driver circuit in FIGS. 3 and 4 is now explained using the timing diagrams of FIGS. 5a, 5b, and 6 which apply specifically to the example of FIG. 4. FIG. 5aillustrates how output voltages V_DOand V_BOwould vary if, element S1/Q1 having previously been turned on, is switched off at the same time that element S2/Q2 is switched permanently on, element S3/Q3 being maintained permanently off. FIG. 5bcorrespondingly illustrates how the S2/Q2 current would vary. The upper waveforms in FIG. 6 illustrate how output voltages V_DOand V_BOvary when all of elements S1-S3/Q1-Q3 operate in the desired manner. The lower waveforms in FIG. 6 depict the variations of input voltage V_Iand control voltages V_C1-V_C3.

[0062] Assume that input voltage V_Iis at such a value that driver output voltage V_DOis initially at low value V_SS. Conductor output voltage V_BOis likewise initially at V_SS. If driver 18 is an inverter, these conditions are achieved by placing input voltage V_Iat a high level. This case is illustrated in FIG. 6. Conversely, input voltage V_Iis initially at a low level when driver 18 is a non-inverting circuit.

[0063] With driver output voltage V_DOat V_SSelement S1,/Q1 is turned on so as to connect output node NO to the V_SSsupply. A current I₁flows from node NO through element S1/Q1 to the V_SSsupply. Current, constitutes any leakage currents through the inputs to CMOS ICs 16 and is normally quite small. When switch S1 is implemented with N-channel FET Q1, control voltage V_C1is at a high value close to V_DDas shown in FIG. 6.

[0064] Elements S2/Q2 and S3/Q3 are initially off. For the case in which switches S2 and S3 are respectively implemented with N-channel FET Q2 and P-channel FET Q3, control voltage V_C2is initially at a low value close to V_SSwhile control voltage V_C3is at a high value close to V_HH. Again, see FIG. 6.

[0065] Input voltage V_Inow goes through a suitable input voltage transition, causing driver output voltage V_DOto initiate a low-to-high voltage transition from V_SSto V_DDat a time t₁as indicated in FIG. 5a. In particular, control voltage V_C1makes a transition that causes element S1/Q1 to turn off. When switch S1 is implemented with N-channel FET Q1, control voltage V_C1therefore drops to a low value close to V_SSas indicated in FIG. 6. Current I₁goes to zero.

[0066] Simultaneous with the V_C1transition, control voltage V_C2makes a transition that causes element S2/Q2 to turn on. A current I₂starts to flow from the V_HHsupply through element S2/Q2 to output node NO. Driver output voltage V_DOrapidly rises to V_HH. In the case where switch S2 is implemented with N-channel FET Q2, control voltage V_C2is raised to a high value close to V_DDas shown in FIG. 6. Control voltage V_C3remains at its original value for the time being. Element S3/Q3 thus remains off at this stage.

[0067] As driver output voltage V_DOrises rapidly to V_HH, conductor output voltage V_BOalso moves upward. Capacitances CL and CB begin to charge. However, the presence of inductance LB causes conductor output voltage V_BOto rise considerably more slowly than driver output voltage V_DOas indicated in FIGS. 5aand 6. Due to the very low on-resistance of element S2/Q2 and the inductive effect of inductance LB, conductor output voltage V_DOovershoots V_HHand reaches a maximum voltage close to V_DDat a time t₂.

[0068] If element S3/Q3 were maintained permanently off and element S2/Q2 were maintained permanently on as represented by the voltage waveforms of FIG. 5a, conductor output voltage V_BOwould ring approximately sinusoidally with a frequency f_R:

[0069] f_R=½π{square root over (L_B+L C)}

[0070] where L_Bis the value of inductance LB, and C again is the sum of the values of capacitances CL and CB. Current I₂through element S2/Q2 would alternate in direction as depicted in FIG. 5b. Inasmuch as a small amount of power is dissipated in element S2/Q2 and also in the various loss mechanisms associated with load 14, the amplitudes of the V_BOand I₂oscillations gradually die out.

[0071] A small voltage drop occurs across element S2/Q2. This causes driver output voltage V_DOto deviate slightly from V_HHin the manner shown in FIGS. 5aand 6.

[0072] During the oscillations, energy is continually exchanged in an efficient manner between the V_HHsupply, load capacitances CL and CB, and inductance LB. When conductor output voltage V_BOis at a peak high level, capacitances CL and CB store a maximum of energy. The V_HHsupply stores a maximum of energy when output voltage V_BOis at a peak low level. When output voltage V_BOis at the intermediate V_HHlevel, inductance LB stores a maximum of energy.

[0073] In the actual operation of the present driver circuitry, the travel of conductor output voltage V_BOis arrested when it first reaches a value quite close to V_DD. This value corresponds approximately to a maximum V_BOexcursion and is somewhat less than V_DDbecause some energy has been dissipated in various loss mechanisms. The arrest in the V_BOtravel is performed approximately at time t₂by causing control voltage V_C3to make a transition so that element S3/Q3 turns on. A current I₃now begins to flow from the V_DDsupply through element S3/Q3 to output node No. Control voltage V_C2is simultaneously adjusted to a value that causes element S2/Q2 to turn off. Current I₂goes to zero.

[0074] The simultaneous changes in control voltages V_C2and V_C3disconnect output node NO from the intermediate V_HHsupply and connect node NO to the higher V_DDsupply. Driver output voltage V_DOthus goes rapidly up to V_DDas shown in FIG. 6. Capacitances CL and CB are further charged, replenishing energy dissipated by various loss mechanisms. The connection of node NO to the V_DDsupply holds conductor output voltage V_BOat a value close to V_DD. A small current constituting the leakage currents through inputs to ICs 16 is drawn from the V_DDsupply. Substantially no ringing occurs in conductor output voltage V_BO. For the case in which switches S2 and S3 are respectively implemented with N-channel FET Q2 and P-channel FET Q3, control voltages V_C2and V_C3both drop to a low level close to V_SS. This completes a first half cycle of the operation of driver 18.

[0075] Inductance LB, by acting as a temporary energy storage mechanism during the full V_BOtransition, allows energy supplied from the V_HHsupply to charge capacitances CB and CL in a very efficient, substantially non-resistive, non-dissipative manner. Consequently, a large part of the energy supplied by the V_HHsupply ends up stored in capacitances CL and CB.

[0076] Largely the reverse occurs when input voltage V_Imakes a transition that causes driver output voltage V_DOto initiate a high-to-low output voltage transition from V_DDto V_SSat a time t₃as indicated in FIG. 6. Control voltage V_C3makes a transition that causes element S3/Q3 to turn off. Current I₃goes to zero as output node NO is disconnected from the V_DDsupply.

[0077] Simultaneous with the V_C3transition, control voltage V_C2makes a transition that causes element S2/Q2 to turn on. Output node No is connected to the V_HHsupply, thereby enabling current I₂to flow from node NO through element S2/Q2 to the V_HHsupply. In the second half of a full operational cycle for driver 18, current I₂thus flows in the opposite direction from the first half of the cycle. Driver output voltage V_DOdrops rapidly to V_HH.

[0078] Similar to what happens in the first half of the operational cycle, conductor output voltage V_BOattempts to follow driver output voltage V_DOdownward. However, the presence of inductance LB causes conductor output voltage V_BOto fall considerably slower than driver output voltage V_DO. Due to the very low on resistance of element S2/Q2 and the inductive effect of inductance LB, conductor output voltage V_BOovershoots V_HHand reaches a minimum value close to V_SSat a time t₄.

[0079] The travel of conductor output voltage V_BOis arrested at time t₄by enabling control voltage V_C1to make a transition that causes element S1/Q1 to turn on. Output node NO is then connected to the V_SSsupply, enabling current I₁to flow from node NO through element S1/Q1 to the V_SSsupply. A transition in control voltage V_C2is simultaneously made that causes element S2/Q2 to turn off, thereby disconnecting node NO from the V_HHsupply. Current I₂goes to zero. Driver output voltage V_DOdrops rapidly to V_SS.

[0080] Capacitances CB and CL complete their discharge during which a small amount of residual energy is lost. Inductance LB, by acting as a temporary storage mechanism during the transition, allows energy stored in capacitances CL and CB to be returned to the V_HHsupply in a very efficient, substantially non-resistive, non-dissipative manner. In particular, a large part of the energy stored by capacitances CL and CB is returned to the V_HHsupply. Thus, over a complete operational cycle, only a small amount of energy is dissipated. The connection of output node NO to the V_SSsupply holds connector output voltage V_BOat a value close to V_SS. Again, substantially no ringing occurs in conductor output voltage V_BO. This completes the second half of the operational cycle.

[0081] The transitions in control voltages V_C2and V_C3during the first half cycle when conductor output voltage V_BOfirst reaches V_DDneed to be made at a point in time having a relatively small tolerance in order for the inductive/capacitive resonance mechanism to provide power savings. The same applies to the transitions in control voltages V_C2and V_C1during the second half cycle when conductor output voltage V_BOfirst reaches V_SS. In other words, the timing sequence produced by control circuitry 20 needs to be quite precise for disconnecting element S2/Q2 at the point of maximum or minimum load voltage.

[0082] The timing depends particularly on inductance LB, conductor capacitance CB, and load capacitance CL. Ideally, the timing should be determined from an a priori external examination of conductor output voltage V_BO. While this can be done when an implementation of driver circuit 18 is developed for a specific application, conductor output voltage V_BOis not generally available a priori when driver 18 is developed for general usage. Nonetheless, various techniques can be employed to provide precise adjustment of the control timing to accommodate particular applications.

[0083] For example, the point of maximum or minimum load voltage closely corresponds to the point at which current I₂from the V_HHsupply through element S2/Q2 falls to zero. Therefore, the small voltage drop across element S2/Q2 also drops to zero. Consequently, the voltage drop can be used in conjunction with a voltage comparator to determine when to switch element S2/Q2 off. Since element S2/Q2 has a very low on resistance, the comparator needs high speed and high gain to achieve the desired objective.

[0084] Alternatively, control circuitry 20 can directly monitor current I₂to determine when its value reaches zero. Upon determining that current I₂is zero, control circuitry 20 causes element S2/Q2 to turn off.

[0085] Another technique for adjusting the timing depends on the fact that the timing, although generally different for each different circuit design, remains substantially constant in any particular application. Therefore, by making driver 18 to be self-adjusting and able to memorize the required timing sequence, the timing sequence can be determined by observing driver output voltage V_DOduring a driver output transition immediately after element S2/Q2 is turned off.

[0086] A fundamental characteristic of an inductor is that it acts to maintain any existing current flowing through it. While element S2/Q2 is conductive, current I₂flows to or from the V_HHsupply through element S2/Q2 and through inductance LB. When element S2/Q2 is turned off, current continues to flow through inductance LB at substantially the same value of current I₂that existed just before element S2/Q2 was turned off. During the V_HH-to-V_DDpart of a V_BOoutput transition, element S3/Q3 is turned on when element S2/Q2 is turned off. During the V_HH-to-V_SSpart of the subsequent V_BOoutput transition, element S1/Q1 is turned on when element S2/Q2 is turned off. Since each of elements S1/Q1 and S3/Q3 has a higher on resistance than element S2/Q2, the current flow through inductance LB causes a larger and more easily detected voltage drop across either element S1/Q1 or S3/Q3 than across element S2/Q2 after element S2/Q2 is turned off and either element S1/Q1 or S3/Q3 is turned on.

[0087] FIGS. 7a-7cpresent waveforms that provide a foundation for using a technique of the foregoing type to develop the control timing. These three timing diagrams respectively illustrate the cases in which the choice of time t₄in the second half of the operational cycle occurs (a) too early, (b) at the right time, and (c) too late. If time t₄occurs too early, current flows from load 18 through inductance LB when element S2/Q2 is turned off, causing an inductive kick to delay the movement of output driver voltage V_DOfrom V_HHto V_SSrelative to the corresponding transition in control voltage V_C2. See FIG. 7a. Driver output voltage V_DOmakes a rapid, unhindered transition from V_HHto V_SSwhen time t₄is at the right value as shown in FIG. 7b. If time t₄occurs too late, current flows through inductor LB and to load 14 when element S2/Q2 is turned off. This causes the movement of driver output voltage V_DOto be speeded up relative to the corresponding transition in control voltage V_C2as indicated in FIG. 7c.

[0088] FIG. 8 depicts an implementation of control circuitry 20 that takes advantage of the foregoing to produce control voltages V_C1-V_C3, particularly control voltage V_C2, at substantially optimum times without relying on an a priori external examination of conductor output voltage V_BO. Control circuitry 20 in FIG. 8 contains a race resolver 30, an up-down counter 32, a multiplexer 34, an AND gate 36, an exclusive OR gate 38, a NOR gate 40, and sixteen CMOS inverters I1, I2, . . . I16 connected as shown. Race resolver 30 is formed with NOR gates 42 and 44 arranged in a cross-connected configuration.

[0089] Control circuitry 20 in FIG. 8 operates in the following way. Race resolver 30 provides an output signal RP to up/down counter 32 in response to driver output voltage V_DOand control voltage v_C2. When both of voltages V_DOand V_C2are low, race resolver 30 can be in one of two stable states depending on which of voltages V_DOand V_C2went low first. Consequently, the state of resolver 30 immediately after time t₄indicates whether element S2/Q2 was switched off too early or too late. Resolver output signal RP is supplied at a value corresponding to the resolver state.

[0090] Up/down counter 32 is sampled on every rising edge of input voltage V_I. Resolver output signal RP causes counter 32 to count up one or down one depending on the RP value, and thus depending on whether control voltage V_C2went low before driver output voltage V_DOwent low or vice versa.

[0091] If element S2/Q2 was turned off at precisely the right value of time t₄so that resolver 30 is in an indeterminant state, resolver output signal RP is at an arbitrary value. This causes counter 32 to count up one or down one. However, if counter 32 counts downs one as the result of an arbitrary value of signal RP during one cycle, counter 32 will normally count up one during the next cycle, and vice versa.

[0092] Inverters I1-I16 form a delay line driven by input voltage V_I. Output voltage signals V_D1taken from every other one of inverters I1-I16 are supplied as delay input signals to multiplexer 34. Up/down counter 32 provides a count signal CT that determines which of delay signals V_D1is supplied from multiplexer 34 as its output voltage signal V_M. Consequently, the combination of multiplexer 34 and inverters I1-I16 provide a variable delay controlled by counter 32. Input voltage V_Iand multiplexer output voltage V_Mare supplied to logic gates 36, 38, and 40. The length of the delay determined by up/down counter 32, and thus by the difference between the times that driver output voltage V_DOand control voltage V_C2go low, thereby determines the timing of control voltage V_C2as well as the timing of control voltages V_C1and V_C3.

[0093] Inasmuch as control voltage V_C2controls when element S2/Q2 is switched off, driver circuit 18 as embodied with control circuitry 20 in FIG. 8, implements a control loop that adjusts the driver output timing according to the requirements of load 14 on every rising edge of driver input signal V_I. After several transitions of input signal V_I, driver 18 becomes adjusted to the requirements of load capacitance CL, conductor inductance LB, and conductor capacitance CB to within one increment of the timing provided by inverters I1-I16. This self-adjustment technique avoids the necessity to perform an a priori external examination of conductor output voltage V_BO.

[0094] The time-averaged value of current I₂flowing to and from the V_HHsupply is approximately zero. Referring to FIG. 9, intermediate voltage V_HHmay thus be supplied from one plate of a reservoir capacitor CR. The other plate of reservoir capacitor CR is typically connected to the V_SSsupply. The value of reservoir capacitor CR is at least as large as, preferably larger than, the value of the sum of capacitances CL and CB. In this case, the intermediate level V_HHchanges throughout a transition due to the current flow into or out of reservoir capacitor CR.

[0095] A further method of determining the correct timing for disconnecting element S2/Q2 can be utilized when intermediate voltage V_HHis supplied from reservoir capacitor CR. In this case, current I₂flowing to or from reservoir capacitor CR causes its voltage to change in the opposite direction to conductor output voltage V_BO. The rate of change of voltage across reservoir capacitor CR, and thus also of intermediate voltage V_HH, is proportional to current I₂flowing into or out of capacitor CR via element S2/Q2. In light of this, an electronic differentiator monitoring the reservoir capacitor voltage can be used to indicate the point of zero current flow. Alternatively, a voltage comparator can be used to detect the point of minimum or maximum voltage deviation of reservoir capacitor CR, corresponding to the point of zero current flow through element S2/Q2 and thus also indicating the correct point to turn off element S2/Q2.

[0096] While the invention has been described with reference to particular embodiments, this description is solely for the purpose of illustration and is not to be construed as limiting the scope of the invention claimed below. For example, the number of inverters in control circuit 20 in FIG. 8 could differ from sixteen. Inverters I1-I16 could be replaced with a monostable multivibrator. A control loop for providing self-adjustment of the timing sequence used to generate control voltage V_C2along with control voltages V_C1and V_C3, can be implemented in a manner considerably different than that shown for control circuitry 20 in FIG. 8.

[0097] Part or all of ICs 16 in FIG. 3 can be connected to PWB conductor 12 at various points along its length so as to produce a distributed load. With capacitance CL representing the parasitic capacitance of the IC input(s) at the end of conductor 12, capacitance CB would be effectively increased while capacitance CL is effectively reduced. Alternatively, conductor 12 can be a branching structure with each branch terminating in one or more of ICs 16. A branching structure for conductor 12 can be combined with a distributed load.

[0098] Driver 18 and load 14 could be formed as part of one IC, and conductor 12 could then be an interconnect within that IC. Various modifications and applications may thus be made by those skilled in the art without departing from the true scope and spirit of the invention as defined in the appended claims.