Vcc-compensated bus-hold circuit

[0002] This application claims priority under 35 U.S.C. §119 (e)(1) of provisional application No. 60/184,523, filed Feb. 24, 2000.

[0003] The present invention relates to the field of hold circuits, such as bus-hold circuits. More particularly, it relates hold circuits that may be used with power supplies of differing supply voltage.

[0004] An IGFET circuit used as a digital switch can be left with a floating input either by design or in bus applications when all devices driving the bus are in 3-state. In this state the transistors on both sides of the driving circuits are turned off leaving them with high output impedance. A floating input of an IGFET circuit allows the capacitance of the IGFET gate to gradually charge. If the gate is sufficiently charged, the resulting voltage on the gate causes static current to flow through the IGFET, causing wasted power dissipation. Further, if the gate voltage rises even further, it can reach a certain threshold where a more serious consequence can occur. In the worst case condition, it can draw enough current from the supply to ground to destroy the metal lead and render the device useless.

[0005] Initially, external static pull-up or pull-down resistors were used to prevent floating of unused gates. These static resistors connect the gate either to the supply voltage, V_CC, or ground, but still cause wasted power dissipation. For bus applications where a bus driver must drive the input, bus-hold circuits have been developed. The function of a bus-hold circuit is to hold the input of the digital switch, i.e., the gate of the IGFET, to the state previously set by the bus driver. The hold must be strong enough to prevent the input from floating (charging) but weak enough to allow the input to be driven by the bus driver. Initially, these were external circuits. Now bus-hold circuits are integrated into the IGFET (including CMOS) digital IC.

[0006] In the known art, a bus-hold circuit is a digital signal feedback path from the output of a digital switch to its input, consisting of an inverter and a current-limiting resistor. The purpose of the feedback is to hold the input to its last driven state until it is driven to the alternate state. Current flows through the resistor only during the switching period when, due to propagation delays, there is a voltage difference between the output of the inverter and the input of the digital switch. Since voltage drops across the resistor occur only for brief intervals during state changes, bus-hold circuits significantly reduce the power dissipation.

[0007] The holding current through the resistor becomes the critical parameter for power dissipation by a bus-hold circuit. The magnitude of the holding current depends upon the values of the resistor and V_CC. If the bus-hold circuit is used with a higher value of V_CCthen the magnitude of the hold override current is proportionately higher and the circuit dissipates more power in proportion to the square of the current.

[0008] Historically, the values of V_CCfor digital switches on IGFET integrated circuits have decreased as the state of the technology progresses. For CMOS, they have diminished from 5V to 3.3V, to 2.5V and to 1.8V. It is likely that this progression to still lower values of V_CCwill continue. An important benefit of evolving the technologies to operate from a lower value of V_CCis that the magnitudes of the hold override currents also are reduced. That lowers the power consumption of the bus-hold circuit. This evolution to lower V_CCcreates the need for bus-hold circuits that can operate at two different values of V_CCso it can be used with either version of the technology.

[0009] The present invention provides a hold circuit for holding a digital switch, having an input and an output, at the level of the last driven state substantially independently of the output impedance of a circuit driving the digital switch. The hold circuit includes an inverter having an input connected to the output of the digital switch and having an output, and a variable resistor having a port connected to the output of the inverter and having a port connected to the input of the digital switch.

[0010] In accordance with a preferred embodiment of the present invention, a V_CC-compensated bus-hold circuit is provided that can operate from either of two adjacent values of V_CCwithout increasing the holding currents at the higher value of V_CC. This duality in circuit behavior results from introducing a variable resistor (whose resistance depends on which of the two values of V_CCis applied) between the bus-hold inverter and the current limiting resistor. The dual-valued variable resistor is implemented using two transistors of different strengths connected in parallel. The transistor with the smaller channel width and therefore higher resistance is biased on continuously. When the lower value of V_CCis applied, the transistor with the larger channel width and therefore lower resistance is also turned on and the parallel combination has a lower resistance. When the higher value of V_CCis applied, the transistor with the larger channel width is turned off and the parallel combination then has a higher resistance. A bias circuit with V_CCas input supports the operation of this embodiment. This bias circuit uses the V_CCvalue to control the state (ON or OFF) of that transistor in the parallel pair that has the larger channel width. If the lower value of V_CCis applied then the transistor with the larger channel width is turned on and the resistance is lowered. If the higher value of V_CCis applied then that transistor is turned off and the resistance is increased to a value that keeps the magnitudes of the holding currents approximately the same as for the lower value of V_CC.

[0011] FIG. 1 illustrates a prior art bus-hold circuit.

[0012] FIG. 2 shows an example of a prior art bus-hold circuit implemented in CIGFET technology.

[0013] FIG. 3 is a graph of the typical holding current characteristics of a prior art bus-hold circuit.

[0014] FIG. 4 illustrates a first preferred embodiment of the present invention.

[0015] FIG. 5 illustrates a second preferred embodiment of the present invention, in a CIGFET digital switch IC.

[0016] FIG. 6 shows a third preferred embodiment of the present invention.

[0017] FIG. 7 illustrates the variation in holding currents of a preferred embodiment, for various conditions.

[0018] FIG. 1 illustrates a known art bus-hold circuit consisting of an inverter 11 and a resistor 13 that are connected in series from the output to the input of an IGFET switch 10. If the input 14 is in a low state then the output 15 is in a high state. This puts the output of inverter 11 in a low state, and resistor 13 becomes a pull-down resistor with current flowing through it only during the switching period when there is a voltage difference between the output of 11 and the input of 10. Conversely, when the input 14 is high, the output 15 is low and the output of inverter 11 is high. In this state, resistor 13 is a pull-up resistor and current flows through it only during the switching period when there is a voltage difference between the output of 11 and the input of 10. Since voltage drops across resistor 13 occur only for brief intervals during state changes, as mentioned above bus-hold circuits significantly reduce the power dissipation.

[0019] FIG. 2 illustrates a Complementary IGFET (CIGFET) implementation of the known art bus-hold circuit 16. CMOS is a special case of the more general class of CIGFET circuits. CIGFET transistor pair 17 & 18 forms the inverter of the bus-hold circuit. The digital switch 3 is also a pair of CIGFET transistors 19 & 20.

[0020] FIG. 3 is a graphical plot of input current (I_IN) versus input voltage (V_IN) that characterizes the operation of a typical bus-hold circuit. Referring now also to FIG. 1, I_INis defined as the current that flows from the output of the inverter through resistor 13 to the digital switch input 14. When the input is in the low state, V_IN<V_THL(the threshold voltage at which the output of digital switch 10 remains in the high state). As the voltage at the input is driven from zero toward V_THLthe value of I_INslowly begins to fall from zero toward I_LHO, the low hold override current (also called the holding current, I_HOLD). Near V_IN=zero, the magnitude of the current is set by the value of resistor 13 (or 23, FIG. 2), and by the voltage difference between the input 14 (or 24, FIG. 2) and the output of inverter 11. As V_INapproaches V_THL, I_INreaches its maximum negative value I_LHO. Increasing V_INfurther strengthens the n-channel transistor and weakens the p-channel transistor of the digital switch, identified in FIG. 2 as transistors 20 and 19, respectively, thus forcing 15 (or 25, FIG. 2) to a low state and the output of the bus-hold circuit to a high state. I_INreverses and rises rapidly until V_INreaches V_THH(the threshold voltage at which the output of digital switch 10 remains in the low state). At this point, I_INreaches its maximum positive value I_HHO, the high hold override current. As V_INapproaches V_CC, I_INfalls back to zero at a rate determined by the value of resistor 13 and the voltage difference between the output of inverter 11 and the input 14 to the digital switch. As the input 14 is driven from a high state to a low state, a similar process occurs as determined by this same I_IN-V_INcurve.

[0021] Referring to FIG. 2, the magnitudes of the hold override currents (I_LHOand I_HHO) depend upon the values of resistor 23 and V_CC. Therefore, when the bus-hold circuit 16 is used with a higher value of V_CC(for example, V_CC=5V, instead of V_CC=3.3V), the magnitudes of the hold override currents are proportionately higher and more power is dissipated by the circuit.

[0022] FIG. 4 shows a bus-hold circuit according to a first preferred embodiment of the present invention. A variable resistor 12 is provided in the bus-hold circuit between the inverter output and the resistor 13′, allowing the circuit to be operated with either of two prescribed values of supply voltage (V_CC). The resistance of 12 switches between two values according to the value of the applied V_CC. If the lower value of V_CCis applied then 12 has the lower value of resistance. If the higher value of V_CCis applied then 12 has the higher value of resistance. The holding currents, I_LHOand I_HHO, are thus made substantially independent of which of the two values of V_CCis applied.

[0023] FIG. 5 shows a bus-hold circuit according to a second preferred embodiment of the present invention, in a CIGFET bus-hold circuit. In a circuit of this type, the variable resistance is divided into two components, 31 and 32. When V_CCis set at the higher value (V_CCH), 31 and 32 have higher resistance values. When V_CCis set at the lower value (V_CCL), they have lower resistance values.

[0024] FIG. 6, shows a third preferred embodiment of the present invention, which is a further application of the principles of the present invention to a CIGFET bus-hold circuit, but with enhancements over the circuit shown in FIG. 5. In the circuit shown in FIG. 6, four CIGFET switches (MPL, MNL, MPH and MNH) implement the two-state variable resistors 31 and 32 shown in FIG. 5. The drains of the digital switch CIGFET pair MPS and MNS at the output 45 are connected to the input gates of the bus-hold inverter CIGFET pair MPB and MNB. The drains of MPB and MNB are tied together at the node 41 on the bus-hold side of a current-limiting resistor 43 through two CIGFET pairs MPL/MNL, MPH/MNH, connected in parallel. The opposite side of resistor 43 is connected to the input 44 of the digital switch. The sources of MPL and MPH are connected to the drain of MPB. The sources of MNL and MNH are connected to the drain of MNB. The drains of MPL, MPH, MNL and MNH are connected to the node 41. The source of MPB and the gate of MNH are connected to V_CC. The source of MNB and the gate of MPH are connected to ground. The gate of MPL is connected to a first bias source B1, while the gate of MNL is connected to a second bias source B2.

[0025] A V_CC-dependent bias circuit provides the bias sources B1 and B2. This bias circuit has two outputs, B1 and B2, where B1 and B2 have the following characteristics:

[0026] 1. If V_CC=V_CCLthen B1=0.0V and B2=V_CC

[0027] 2. If V_CC=V_CCHthen B1=V_CCand B2=0.0V

[0028] As mentioned above, the gate of MPL is connected to the bias circuit output B1, while the gate of MNL is connected to bias circuit output B2. Since B2 is, in a sense, the complement or inverse of B1, CIGFET MPL and CIGFET MNL are either turned on or turned off in unison.

[0029] Transistors MPL and MNL are fabricated to have larger channel widths than transistors MPH and MNH. Thus they provide a lower resistance path for current when they are turned on. As such, their function can be considered in this context to be, effectively, that of resistors. If the value of V_CCis at V_CCL, then MPL and MNL are enabled. This creates current paths through these two effective resistors. Resistor 31 is formed by the parallel combination of the lower channel resistance of MPL and the higher channel resistance of MPH. Resistor 32 is formed by the parallel combination of the lower channel resistance of MNL and the higher channel resistance of MNH. When V_CCis at V_CCH, then MPL and MNL are switched off and the values of the effective resistors 31 and 32 become the higher channel resistances through MPH and MNH, respectively. This keeps the bus-hold currents for the higher value of V_CCapproximately the same as those for the lower value of V_CC. Thus, effective resistors 31 and 32 together comprise an effective variable resistor, wherein the variability is controlled by the switching on and off of MPL, MPH, MNL and MNH, as described above.

[0030] FIG. 7 shows the results of a transistor-level SPICE simulation of the embodiment shown in FIG. 6 for the specific V_CCvalues: 3.0V and 4.5V. It shows two families of I_IN-versus-V_INcharacteristic curves for a variety of transistor strengths and device temperature conditions. The family of curves on the left side of FIG. 7 (which terminate in a common left end point and a common right end point) were obtained for V_CC=3.0V. The family of curves on the right side were obtained for V_CC=4.5V. The curves in FIG. 7 show that in each case (I through V) of transistor strength and device temperature, the values of the hold override currents (I_LHOand I_HHO) for V_CC=4.5V are very nearly the same as those for 3.0V.

[0031] Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.