Through currents minimisation in electronic circuits

Abstract: A low power, break before make output circuit includes an output transistor pair 12 and 14, a first control circuit 20, a second control circuit 22, a first comparator 16, and a second comparator 18. First control circuit 20 has a first input coupled to a first digital control input and an output coupled to a control terminal of a first transistor 12 in the output transistor pair. Second control circuit 22 has a first input coupled to a second digital control input and an output coupled to a control terminal of a second transistor 14 in the output transistor pair. First comparator 16 has an input connected to the output of first control circuit 20 and an output connected to the second input of second control circuit 22. First comparator 16 compares a voltage at the control terminal of first transistor 12 to a first predetermined voltage and formulates a voltage at its output in response to the comparison. Second comparator 18 has an input connected to the output of second control circuit 22 and an output connected to the second input of first control circuit 20. Second comparator 18 compares a voltage at the control terminal of second transistor 14 to a second predetermined voltage and formulates a voltage at its output in response to the comparison. The voltages at the output of first comparator 16 and second comparator 18 influence the outputs of first control circuit 20 and second control circuit 22 thereby ensuring that first transistor 12 is not conducting when second transistor 14 is activated.

Description: FIELD OF THE INVENTION This invention relates to electronic circuits and more particularly relates to control circuitry for driving output devices. BACKGROUND OF THE INVENTION When using totem pole output circuit configurations (sometimes called push-pull outputs) as illustrated in prior art circuit 99 of FIG.1, one often encounters a phenomena called "shoot-through." Because two transistors are driven in unison, there is a short period of time during which both transistors are conducting simultaneously. As is shown in FIG. 1a, as the gate voltage drops from Vcc to ground, M1 gradually turns on as M2 is gradually turning off. During the time when the gate voltage is at an intermediate level, a pulse of current passes from supply Vcc through M1 and M2 to circuit ground. This pulse of current is known as "shoot-through" or "through current." Because transistors in totem pole output configurations are designed for minimum on resistance, "through currents" can exceed the working current levels in a circuit. For example, a 1A totem pole circuit could see "through currents" of 5A. Although "shoot through" pulses are narrow, substantial efficiency losses may result if the currents are substantially large. Losses in efficiency are unacceptable in micro-power applications. In addition, the lost power is dissipated in the output transistors and requires larger heat sinks which are costly in applications that require space-efficient packaging. In addition to lost efficiency, "through currents" may cause premature device failures which cause reliability problems. "Through currents" also produce substantial noise which may result in erratic circuit operation and undesired RFI/EMI (radio frequency interference/electromagnetic interference) problems. There are several prior art approaches to controlling through currents. One prior art solution uses external resistive limiting. Unfortunately, this solution only reduces, but does not eliminate "through currents." A resistor is a lossy element and thus adds dissipation losses of its own, thus reducing efficiency. Another prior art solution to "through currents" involves inductive snubbing, which eliminates the "through currents." However, the inductive elements reduce the circuit's performance and require extra external components which are undesirable. Yet another prior art solution uses non-overlapping timing signals. In this solution each of the transistors in the totem pole output are driven with a different signal, created by a non-overlapping clock generator as illustrated in prior art FIG.2. This is often called a "break-before-make" configuration because one transistor is assured to be non-conducting before the other transistor begins conduction. The problems with this solution is that "dead time" introduced by the non-overlapping clock generator diminishes the circuit's performance. Additionally, the "dead time" must be made long enough to operate without "through currents" under worst case process conditions. Such guardbanding reduces circuit performance. Still another prior art solution utilizes the concept of non-overlapping timing signals in adaptive "dead time" controllers. These circuits compute the duration of "dead time" by monitoring the voltage waveforms so that only the minimal "dead time" necessary is inserted to prevent "through currents" from occurring. The solution has been to use a comparator circuit to detect voltage levels of the driving transistor as shown in prior art FIG.3, but this solution requires DC quiescent current. DC quiescent current, even at low levels, is unacceptable for many applications such as portable applications requiring battery operated equipment or any other micro-power application. It is an object of this invention to provide a "break-before-make" adaptive "dead time" output control circuit that is independent of output loading and requires no DC quiescent current and no external components. Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings herein. JP 62 166615A, JP 02 114718A and JP 57 050133A each disclose a CMOS output driver that prevents both of the transistors of the output driver from being turned on at the same time when the output of the circuits changes from one state to another. They each disclose coupling the gate input of the PMOS and NMOS transistors of the output driver to control circuits for the NMOS and PMOS transistors respectively. The control logic for the PMOS transistor having a lower voltage threshold than that for the NMOS transistor, thereby ensuring that during output transitions, both transistors are turned off for a brief period of time. SUMMARY OF THE INVENTION A low power, break-before-make output circuit includes a push-pull output transistor pair, a control means coupled to control terminals of the output transistor pair, and comparing means coupled to the control terminals of the output transistor pair and the control means. The comparing means compares voltages at the control terminals of the output transistor pair to predetermined voltage levels and communicates the comparison to the control means which manipulates the voltages at the control terminals of the output transistor pair in response to the comparison without any static power dissipation, thereby ensuring that a first transistor of the output transistor pair is not conducting before a second transistor of the output transistor pair begins to conduct. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating a prior art totem pole output circuit 11. FIG.1a is a graph illustrating "shoot-through" current in prior art totem pole output circuit 11 of FIG.1. FIG.2 is a combined graph and schematic diagram illustrating a prior art adaptive "dead time" control circuit 13. FIG.3 is a combined block diagram/schematic diagram illustrating in greater detail a prior art adaptive "dead time" control circuit 10 for decreasing shoot-through current. FIG.4 is an embodiment of the invention, an adaptive "dead time" control circuit utilizing comparators 16 and 18 of FIG.7. FIG. is a logic diagram illustrating control circuit 20 of FIG.4. FIG.5a is a logic diagram illustrating control circuit 22 of FIG.4. FIG.6 is a timing diagram illustrating the timing relationships of various nodes in comparator circuit 10 of FIG.4 utilizing novel comparators 16 and 18 of FIG.7. FIG.7 is a combined block diagram/schematic diagram illustrating the preferred embodiment of the invention, comparators 16 and 18 which may operate in comparator circuit 10 in FIG.4 to produce adaptive "dead time" control with zero static (quiescent) current. FIG.8 is a combined block diagram/schematic diagram illustrating an alternative embodiment of the invention, comparators 16 and 18 which may be used in circuit 10 of FIG.4. FIG.9 is a logic diagram illustrating control circuit 42 of FIG.8. DETAILED DESCRIPTION FIG.3 is a prior art combined block diagram/schematic diagram illustrating an adaptive "dead time" control circuit 11 for decreasing shoot-through current on power MOS switches. Circuit 11 utilizes standard, prior art comparators COMP1 and COMP2 and exhibits substantial power dissipation through quiescent current conduction through comparators COMP1 and COMP2 which is highly undesirable in micro-power applications. FIG.4 is a combined block diagram/schematic diagram illustrating a novel adaptive "dead time" control circuit 10 utilizing novel comparators 16 and 18. Although the description of the invention is in the context of power MOS switches it should be understood that the invention is not limited to power MOS applications, but is applicable to any totem pole (push-pull) output configuration using high input impedance (voltage driven) switches. A totem pole output transistor configuration exists in FIG.4 with a PMOS transistor 12 connected in series with an NMOS transistor 14 between a voltage source Vcc and circuit ground. The output transistors 12 and 14 may be either internal or external devices (integrated onto the same substrate or may be independent). A first control circuit 20 is connected to a gate terminal of PMOS transistor 12 and an input of a first comparator 16. First control circuit 20 also is connected to a digital input HSDin(bar) and to an output of a second comparator 18. A second control circuit 22 is connected to a gate terminal of NMOS transistor 14 and an input of second comparator 18. Second control circuit 22 also is connected to a digital input LSDin and an output of first comparator 16. FIG.5 is a logic diagram illustrating an embodiment of first control circuit 20 of FIG.4. First control circuit 20 has a NOR gate 24 having a first input connected to digital input HSDin(bar) and a second input connected to the output of second comparator 18 (NODE1). An output of NOR gate 24 is connected to an input of an inverting power driver 26. Inverting power driver 26 has an output which forms the output (P-gate) of first control circuit 20 and is connected to the gate terminal of PMOS transistor 12. FIG.5a is a logic diagram illustrating an embodiment of second control circuit 22 of FIG.4. Second control circuit 22 has a NAND gate 28 having a first input connected to digital input LSDin and a second input connected to the output of first comparator 16 (NODE2). An output of NAND gate 28 is connected to an input of an inverting power driver 30. Inverting power driver 30 has an output which forms the output (N-gate) of second control circuit 22 and is connected to the gate terminal of NMOS transistor 14. FIG.6 is a timing diagram illustrating the voltage waveform timing relationship between various nodes in comparator circuit 10 of FIG.4. The voltage waveforms illustrated are digital inputs HSDin(bar) and LSDin, the gate terminal voltages of PMOS transistor 12 and NMOS transistor 14 (P-gate and N-gate) and the outputs of comparators 16 and 18 (NODE1 and NODE2). FIG.7 is a combined block diagram/schematic diagram illustrating novel comparators 16 and 18 within circuit 10 of FIG.4. Both comparator 16 and comparator 18 have the same internal circuitry with their only difference being where their output is taken. Comparator 16 has its output taken from an output of inverter 50 while comparator 18 has its output taken from an output of inverter 48. Both comparators 16 and 18 have an input connected to a gate terminal of a PMOS transistor M1 and an input connected to a gate terminal of an NMOS transistor M4. M1 is connected in series between a first resistor R1 (which in turn is connected to a supply voltage Vcc) and a second resistor R2. An NMOS transistor M2 is connected in series between R2 and circuit ground. M4 is connected in series between a third resistor R3 and a fourth resistor R4 which in turn is connected to circuit ground. A PMOS transistor M3 is connected in series between R3 and supply voltage Vcc. The junction between Ml and R2 is connected to inverter 50 which in turn is connected to a gate terminal of M3. The junction between R3 and M4 is connected to inverter 48 which in turn is connected to a gate terminal of M2. Comparators 16 and 18 of FIG.4 and shown in detail in FIG.7 develop their trip points internally and have no need of a second input. FIG.8 is a combined block diagram/schematic diagram illustrating comparators 16 and 18 of FIG.4. Both comparator 16 and comparator 18 have the same internal circuitry. Both comparators 16 and 18 have an input connected to a gate terminal of a PMOS transistor M1 and to a gate terminal of an NMOS transistor M4. M1 is connected in series between a first resistor R1 (which in turn is connected to a supply voltage Vcc) and a second resistor R2. An NMOS transistor M2 is connected in series between R2 and circuit ground. M4 is connected in series between a third resistor R3 and a fourth resistor R4 which in turn is connected to circuit ground. A PMOS transistor M3 is connected in series between R3 and supply voltage Vcc. A first inverter 46 has an input coupled to a source terminal of M1 and is connected in series with a second inverter 44 which in turn is connected to a third control circuit 42. Inverter 44 also is connected to a third inverter 50 which in turn is connected to a gate terminal of M3. A fourth inverter 52 has an input coupled to a drain terminal of M4 and is connected in series with a fifth inverter 54 which in turn is connected to third control circuit 42. Inverter 54 also is connected to a sixth inverter 48 which in turn is connected to a gate terminal of M2. As stated for FIG.7, comparators 16 and 18 develop their trip points internally and have no need of a second input. FIG.9 is a logic diagram illustrating third control circuit 42 of FIG. 8. Third control circuit 42 has a NOR gate 72 having a first input connected to the output of inverter 54 of FIG.8 and a second input that is connected to an output of a second NOR gate 76. NOR gate 72 has an output that forms the output of third control circuit 42 and also forms a first input to NOR gate 76. An inverter 74 has an input connected to the output of inverter 44 of FIG.8 and an output that forms the second input of NOR gate 76. The following is a functional description of the invention of FIG.7 within circuit 10 of FIG.4. In this description of circuit 10 of FIG.4, digital control inputs HSDin(bar) and LSDin are connected together. However circuit 10 could operate with HSDin(bar) and LSDin operating independently. When HSDin(bar) (and therefore LSDin) goes high and NODE1 is low, the output of first control circuit 20, the gate terminal of PMOS transistor 12 (hereinafter called P-gate) goes high as shown in FIG.6 (point 1). The slope of P-gate is less than that of HSDin(bar) because of the substantial gate capacitance of PMOS transistor 12. At some predetermined voltage level (called trip point A) the output of first control circuit 20 triggers comparator 16. The value of trip point A is able to be manipulated by the designer and will be discussed in conjunction with FIG.7. When comparator 16 triggers, its output (NODE2) goes high (point X of FIG.6). The digital input combination of NODE2 being high and LSDin being high causes the output of second control circuit 22, the gate terminal of NMOS transistor 14 (hereinafter called N-gate) to go high. The value of trip point A (see point 2 of FIG.6) should be set greater than the difference between Vcc and the threshold voltage (Vt P ) of PMOS transistor 12 (Vcc-Vt P ) to ensure that PMOS transistor 12 is off before NMOS transistor 14 is turned on. Similar to the slope of P-gate, the slope of N-gate is less than that of NODE2 because of the substantial gate capacitance of NMOS transistor 14. When N-gate reaches a predetermined voltage level the output (NODE1) of second comparator 18 triggers and goes high. The combination on NODE1 being high and HSDin(bar) being high results in no change to the output of first control circuit 20 (P-gate); therefore P-gate remains high thereby ensuring PMOS transistor 12 stays off. When HSDin(bar) and LSDin go low, NODEI and NODE2 are still high. The combination of HSDin(bar) being low and NODE1 being high results in no change to the output of first control circuit (P-gate); therefore PMOS transistor 12 stays off. The combination of LSDin being low and NODE2 being high results in the output of second control circuit 22 (N-gate) going low (point 3 of FIG.6). When N-gate falls below trip point B the output of comparator 18 trips and NODE1 goes low (point Y of FIG.6). When NODE1 goes low, the combination of NODE1 being low and HSDin(bar) being low causes the output of first control circuit 20 (P-gate) to go low. PMOS transistor 12 will begin conducting when P-gate falls a threshold voltage (Vt P ) below the supply voltage V cc . It is important to note that circuit 10 of FIG.4 exhibits a "break before make" circuit configuration provided that internal trip points (trip point A and trip point B of comparators 16 and 18) are set appropriately. For example, trip point A of comparator 16 should be set high enough to ensure that PMOS transistor 12 is off(not conducting) before comparator 16 triggers high. This can be done by setting trip point A above the difference between supply voltage Vcc and the threshold voltage (Vt P ) of PMOS transistor 12 (Vcc-Vt P ). Similarly, trip point B of comparator 18 should be set low enough to ensure that NMOS transistor 14 is off (not conducting) before comparator 18 triggers low. This can be done by setting trip point B below the threshold voltage (Vt N ) of NMOS transistor 14. The operation of comparator 16 of FIG.7, an embodiment of the invention, is now described in detail. Comparator 18 has the same schematic as comparator 16 except that resistor values and transistor sizings may differ depending upon the desired trip points and the output of comparator 18 is taken from a different node. In FIG.7, when the input is low, M4 is off and M1 is on. When M1 is on, node N14' is pulled high and node N12' is pulled low through inverter 50. With N12' low, M3 is on, however no current is conducting through M3 because M4 is off. When M3 is on, node N19' is pulled high thereby causing node N8' to be low, which means OUT P (18) is low. As the input to comparator 16 transitions from low to high, comparator 16 goes through five phases of operation. In phase 1, the input voltage is less than the threshold voltage Vt 4 of M4. Therefore M4 remains off and the comparator requires no supply current. In phase 2, the input voltage exceeds Vt 4 and current begins to flow through M3, R3, M4 and R4. M4 now acts as a source follower, setting the current flow through R4, and so the voltage at node N19' is roughly equal to: V(node 19') = Vcc - (R3/R4)(Vin-Vgs M4 ). Since R3 = N*R4, where N is a constant, the equation above has a gain factor "N" that allows one to minimize the change in input voltage during phase 2. In phase 3, the voltage at node N19' drops below the transition threshold for inverter 48, node N8' goes high, turning on M2. Now current begins to flow through R1, M1, R2 and M2. M1 acts as a source follower, setting the current flow through R1, and so the voltage at N14' is roughly equal to: V(node 14') = (R2/R1)(Vcc - Vin - Vgs M1 ). Since R2 = M*R1, where "M" is a constant, the equation above has a gain factor "M" that allows one to minimize the change in input voltage during phase 4 (to be discussed below). In phase 4, the voltage at node N14' drops below the transition threshold for inverter 50, and node N12' goes high. This causes the output of comparator 16 to transition from low to high. Simultaneously, M3 is turned off. This stops current flow through M3, R3, M4 and R4. However, M4 remains on, and pulls N19' to ground, so the comparator's operation is unaffected. In phase 5, the input voltage continues to rise. Eventually it exceeds (Vcc-Vt M1 ) where Vt M1 is the threshold voltage of M1. Current now stops flowing through R1, M1, R2 and M2 and comparator 16 (or 18) stops drawing supply current. The trip point (V trip16 ) of comparator 16 is, as mentioned above, the point where the voltage on N14' crosses the transition threshold (V trans50 ) of inverter 50. Therefore, this trip point is: V trip16 = Vcc - Vgs M1 - (R1/R2)V trans50 , where R2/R1 is equal to "M". If "M" is made large, the last term in the above equation becomes small, and the threshold V trip16 moves closer to Vcc-Vgs M1 . By appropriately selecting the value of "M", trip point A of comparator 16 in FIG.6 can be set as desired. In a similar manner, the trip point (V trip18 ) of comparator 18 is the point where the voltage on N19' crosses the transition threshold V trans48 of inverter 48. Therefore, this trip point is: V trip18 = (R4/R3)(Vcc - V trans48 ) + Vgs M4 . where R3/R4 is equal to "N". If "N" is made large the first term in the above equation becomes small, and the threshold V trip18 moves closer to Vgs M4 . By appropriately selecting the value of "N", trip point B of comparator 18 in FIG.6 can be set as desired. The transition thresholds (V trans48 and V trans50 ) can be computed given the sizes of the transistors inside inverters 48 and 50. With this knowledge, resistors R1, R2, R3 and R4 can be set to provide desired thresholds V trip16 and V trip18 . By making "M" and "N" large (for example M=N=9), the thresholds are not only adjusted to desirable voltages, but the sensitivity of the thresholds to changes in transistor parameters in the inverters is minimized. The following is a functional description of the invention with regard to FIG.8. In this description of circuit 10 of FIG.4, digital control inputs HSDin(bar) and LSDin are connected together. However circuit 10 could operate with HSDin(bar) and LSDin operating independently. When HSDin(bar) (and therefore LSDin) goes high and NODE1 is low, the output of first control circuit 20, the gate terminal of PMOS transistor 12 (hereinafter called P-gate) goes high as shown in FIG.6 (point 1). The slope of P-gate is less than that of HSDin(bar) because of the substantial gate capacitance of PMOS transistor 12. At some predetermined voltage level (called trip point A) the output of first control circuit 20 triggers comparator 16. The value of trip point A is able to be manipulated by the designer and will be discussed in conjunction with FIG.8. When comparator 16 triggers, its output (NODE2) goes high (point X of FIG.6). The digital input combination of NODE2 being high and LSDin being high causes the output of second control circuit 22, the gate terminal of NMOS transistor 14 (hereinafter called N-gate) to go high. The value of trip point A (see point 2 of FIG.6) should be set greater than the difference between Vcc and the threshold voltage (Vt P ) of PMOS transistor 12 (Vcc - Vt P ) to ensure that PMOS transistor 12 is off before NMOS transistor 14 is turned on. Similar to the slope of P-gate, the slope of N-gate is less than that of NODE2 because of the substantial gate capacitance of NMOS transistor 14. When N-gate reaches a predetermined voltage level the output (NODE1) of second comparator 18 triggers and goes high. The combination on NODE1 being high and HSDin(bar) being high results in no change to the output of first control circuit 20 (P-gate); therefore P-gate remains high thereby ensuring PMOS transistor 12 stays off. When HSDin(bar) and LSDin go low, NODE1 and NODE2 are still high. The combination of HSDin(bar) being low and NODE1 being high results in no change to the output of first control circuit (P-gate); therefore PMOS transistor 12 stays off. The combination of LSDin being low and NODE2 being high results in the output of second control circuit 22 (N-gate) going low (point 3 of FIG.6). When N-gate falls below trip point B the output of comparator 18 trips and NODE1 goes low (point Y of FIG.6). When NODE1 goes low, the combination of NODE1 being low and HSDin(bar) being low causes the output of first control circuit 20 (P-gate) to go low. PMOS transistor 12 will begin conducting when P-gate falls a threshold voltage (Vt P ) below the supply voltage Vcc. It is important to note that circuit 10 of FIG.4 exhibits a "break before make" circuit configuration provided that internal trip points (trip point A and trip point B of comparators 16 and 18) are set appropriately. For example, trip point A of comparator 16 should be set high enough to ensure that PMOS transistor 12 is off (not conducting) before comparator 16 triggers high. This can be done by setting trip point A above the difference between supply voltage Vcc and the threshold voltage (Vt P ) of PMOS transistor 12 (Vcc - Vt P ). Similarly, trip point B of comparator 18 should be set low enough to ensure that NMOS transistor 14 is off (not conducting) before comparator 18 triggers low. This can be done by setting trip point B below the threshold voltage (Vt N ) of NMOS transistor 14. The operation of comparator 16 of FIG.8, an embodiment of the invention, is now described in detail. Comparator 18 has the same schematic as comparator 16 except that resistor values and transistor sizings may differ depending upon the desired trip points. In FIG.8, when the input is low, M4 is off and M1 is on. When M1 is on, node N14 is pulled high and node N12 is pulled low through inverters 44, 46 and 50. With N12 low, M3 is on, however no current is conducting through M3 because M4 is off. When M3 is on, node N19 is pulled high thereby causing node N11 to be high. Therefore, the logic values at the two inputs of third control circuit 42 (nodes N10 and N11) when the input is low is: N10 high and N11 high. When both inputs to third control circuit 42 are high the output is low. As the input to comparator 16 transitions from low to high, comparator 16, again, goes through five phases of operation. In phase 1, the input voltage is less than the threshold voltage V t4 of M4. Therefore M4 remains off and the comparator requires no supply current. In phase 2, the input voltage exceeds V t4 and current begins to flow through M3, R3, M4 and R4. M4 now acts as a source follower, setting the current flow through R4, and so the voltage at node N19 is roughly equal to: V(node 19) = Vcc - (R3/R4)(Vin-Vgs M4 ). Since R3 = N*R4, where N is a constant, the equation above has a gain factor "N" that allows one to minimize the change in input voltage during phase 2. In phase 3, the voltage at node N19 drops below the transition threshold for inverter 52, node N8 goes high, turning on M2. Inverters 48 and 54 provide additional gain, ensuring a sharp threshold for inverter 52. Now current begins to flow through R1, M1, R2 and M2. M1 acts as a source follower, setting the current flow through R1, and so the voltage at N14 is roughly equal to: V(node 14) = (R2/R1)(Vcc - Vin - Vgs M1 ). Since R2 = M*R1, where "M" is a constant, the equation above has a gain factor "M" that allows one to minimize the change in input voltage during phase 4 (to be discussed below). In phase 4, the voltage at node N14 drops below the transition threshold for inverter 46, and node N12 goes high, turning M3 off. Inverters 44 and 50 provide additional gain, ensuring a sharp threshold for inverter 46. When node N12 goes high, current flow through M3, R3, M4 and R4 stops. However, M4 remains on, and pulls N19 to ground, so the comparator's operation is unaffected. In phase 5, the input voltage continues to rise. Eventually it exceeds (Vcc - Vt M1 ), where Vt M1 is the threshold voltage of M1. Current now stops flowing through R1, M1, R2 and M2 and comparator 16 (or 18) stops drawing supply current. The trip point (V trip16 ) of comparator 16 is, as mentioned above, the point where the voltage on N14 crosses the transition threshold (V trans46 ) of inverter 46. Therefore, this trip point is: V trip16 = Vcc - Vgs M1 - (R1/R2)V trans46 , where R2/R1 is equal to "M". If "M" is made large, the last term in the above equation becomes small, and the threshold V trip16 moves closer to Vcc - Vgs M1 . By appropriately selecting the value of "M", trip point A of comparator 16 in FIG.6 can be set as desired. In a similar manner, the trip point (V trip18 ) of comparator 18 is the point where the voltage on N19 crosses the transition threshold V trans52 of inverter 52. Therefore, this trip point is: V trip18 = (R4/R3)(Vcc - V trans52 ) + Vgs M4, where R3/R4 is equal to "N". If "N" is made large, the first term in the equation above becomes small, and the threshold V trip18 moves closer to Vgs M4 . By appropriately selecting the value of "N", trip point B of comparator 18 in FIG.6 can be set as desired. The transition thresholds (V trans46 and V trans52 ) can be computed given the sizes of the transistors inside inverters 46 and 52. With this knowledge, resistors R1, R2, R3 and R4 can be set to provide desired thresholds V trip16 and V trip18 . By making "M" and "N" large (for example M=N=9), the thresholds are not only adjusted to desirable voltages, but the sensitivity of the thresholds to changes in transistor parameters in the inverters is minimized. Third control circuit 42 obeys the following truth table: N10 N11 Output 1 0 Hold 0 0 1 1 1 0 where "Hold" indicates that control circuit 42 will "remember" (or hold) the previous state of the output. It is comprehended that any form of logic circuitry that provides the truth table above would fall within the scope of this invention. FIG.9 is a logic diagram illustrating a logic circuit 42 that will produce the truth table shown above. Third control circuit 42 allows the use of one output and a single circuit to generate the signals supplied to Node 1 and Node 2 of FIGs.4 and 6. Comparators 16 and 18 of FIG.7 have several advantages over the prior art. Firstly, comparators 16 and 18, utilized within circuit 10 of FIG.4, prevent shoot-through current independently of capacitive loading. When the output of control circuits 20 and 22 have large capacitive loads, the gate terminals of PMOS transistor 12 and NMOS transistor 14 transition more slowly. However, circuit 10 does not "make" before it "breaks." Since the gate terminals of transistors 12 and 14 must cross either trip point A or trip point B before the other transistor conducts and since the trip points can be made to be above Vcc-Vt P or below Vt N , it ensures that one transistor is off before the other transistor turns on. This can be achieved in a majority of applications because power MOSFETs often have threshold voltages which are deliberately set higher than those of the signal devices used in comparators 16 and 18. Even if the thresholds of the power transistors are equal to those of the signal transistors, this circuit can nearly eliminate through currents, for the time allowed for these currents to flow can be made very small by selecting large values for the gain factors "M" and "N" discussed above. Therefore a "break before make" configuration exists independent of capacitive loading. Comparators 16 and 18 of FIG.7 also advantageously allows one to vary both the high trip point and the low trip point. The high trip point may be varied by simply altering the resistor ratios of R1 and R2 of comparator 16 while the low trip point may be varied by simply altering the resistor ratios of R3 and R4 of comparator 18. This allows circuit 10 of FIG.4 utilizing comparators 16 and 18 of FIG.7 to be portable across various semiconductor processes that utilize MOS transistors having varying threshold voltages. Another advantage of utilizing comparators 16 and 18 of FIG.7 within circuit 10 of FIG.4 is that circuit 10 draws supply current only during switching transients. More specifically, circuit 10 draws current only when Vt N < Vin < Vcc - Vt P , where Vt N is the threshold voltage of an NMOS transistor and Vt P is the threshold voltage of a PMOS transistor, as described in the description of the operation of comparator 16 of FIG.7. This greatly reduces power dissipation, thereby allowing circuit 10 to advantageously operate in myriads of micro-power applications. This reduction in power dissipation represents a substantial advantage over prior art solutions. Comparators 16 and 18 of FIG. 8 are an alternative embodiment to that of FIG.7. The presence of multiple inverters 44, 46 and 50 and 48, 52 and 54 provide additional gain to sharpen the transition thresholds of the lead inverters, increasing the accuracy possible with this circuit configuration.

Claims: An output driver circuit comprising: an output transistor pair comprising first and second transistors coupled together in series between a first and a second voltage potential operable to switch an output of the output driver circuit between the first and second voltage potentials; first and second control circuits each having an output coupled to control terminals of the first and second transistors respectively and each having a first input coupled to first and second inputs of the output driver circuit respectively; first and second comparators each having an input coupled to the control terminal of the first and second transistors respectively and each having an output coupled to a second input of the second or first control circuits respectively; wherein: the first and second comparators are respectively adapted to compare the voltages at the control terminals of the first and second transistors with predetermined voltage levels and communicate via an output the result of those comparisons to the second and first control circuits respectively; and the first and second control circuits are arranged to manipulate the voltages at the control terminals of the first and second transistors under the influence of the outputs of the second and first comparators so that when the output of the output driver circuit changes state, one transistor in the output transistor pair is turned off before the other transistor in the output transistor pair is turned on, characterised in that the first or second comparator, or each of those, comprises: an upper trip point circuit having a first input coupled to the input of that comparator, a second input, and an output; a lower trip point circuit having a first input coupled to the input of the comparator, a second input coupled to the output of the upper trip point circuit, and an output coupled to the second input of the upper trip point circuit; and wherein the output of the upper trip point circuit is responsive to its input rising above a predetermined voltage value, the output of the lower trip point circuit is responsive to the input rising above a predetermined voltage value, and one or both of the outputs of those trip point circuits is connected to the output of the comparator so as to influence the respective control circuit to ensure that when the output of the output driver changes state, one transistor in the output transistor pair is turned off before the other transistor in the output pair is turned off. The output driver circuit of claim 1, wherein the output transistor pair comprises: a PMOS transistor having a source terminal coupled to a first voltage source, a gate terminal coupled to the first control circuit and the first comparator, and a drain terminal; and a NMOS transistor having a source terminal coupled to a second voltage source, a gate terminal coupled to the second control circuit and the second comparator, and a drain terminal coupled to the drain terminal of the PMOS transistor, wherein a junction between the drain terminals of the PMOS transistor and NMOS transistor forms an output of the output driver circuit. The output driver circuit of claim 2 wherein the first voltage source has a voltage potential greater than the voltage potential of the second voltage source. The output driver circuit of any preceding claim, wherein the first control circuit outputs a digital low value when the first input of the output driver circuit and the output of the second comparator are digital low values and outputs a digital high value otherwise. The output driver circuit of any preceding claim, wherein the first control circuit comprises an OR gate. The output driver circuit of any preceding claim, wherein the first control circuit comprises: a dual input NOR gate having a first input coupled to the first input of the output driver circuit, a second input coupled to the output of the second comparator and an output; and an inverter having an input coupled to the output of the dual input NOR gate and an output coupled to the control terminal of the first transistor in the output transistor pair. The output circuit of any preceding claim, wherein the second control circuit outputs a digital high value when the second input of the output driver circuit and the output of the first comparator are a digital high value and outputs a digital low value otherwise. The output driver circuit of any preceding claim, wherein the second control circuit comprises an AND gate. The output circuit of any preceding claim, wherein the second control circuit comprises: a dual input NAND gate having a first input coupled to the second input of the output driver circuit, a second input coupled to the output of the first comparator and an output; and an inverter having an input coupled to the output of the NAND gate and an output coupled to the control terminal of the second transistor of the output transistor pair. The output driver circuit of any preceding claim wherein the upper trip point circuit outputs a high digital value when the input rises above its predetermined voltage value and a low digital value otherwise and, the lower trip point circuit outputs a high digital value when the input rises above its predetermined voltage value and a low digital value otherwise. The output driver circuit of any preceding claim wherein the first comparator comprises the said upper trip point circuit having its first input coupled to the input of the first comparator, a second input, and an output, and the said lower trip point circuit having its first input coupled to the input of the first comparator, a second input coupled to the output of the upper trip point circuit, and an output coupled to the second input of the upper trip point circuit; and wherein the output of the upper trip pint circuit forms the output of the first comparator. The output driver circuit of any preceding claim wherein the second comparator comprises the said upper trip point circuit having its first input coupled to the input of the second comparator, a second input, and an output, and the said lower trip point circuit having its first input coupled to the input of the second comparator, a second input coupled to the output of the upper trip point circuit, and an output coupled to the second input of the upper trip point circuit; and wherein the output of the lower trip point circuit forms the output of the second comparator. The output circuit of any preceding claim wherein the upper trip point circuit comprises: a first resistor connected between a first voltage source and a source terminal of a PMOS transistor; the PMOS transistor having a gate terminal connected to the input of the comparator, and a drain terminal; a second resistor connected between the drain terminal of the PMOS transistor and a drain terminal of an NMOS transistor; the NMOS transistor having a gate terminal connected to the output of the lower trip point circuit, and a voltage source terminal connected to the second voltage source; and an inverter having an input connected to the drain terminal of the PMOS transistor and an output of which forms and output of the upper trip point circuit. The output driver circuit of any preceding claim, wherein the lower trip point circuit comprises: a PMOS transistor having a source terminal connected to the first voltage source, a gate terminal connected to an output of the upper trip point circuit, and a drain terminal; a first resistor connected between the drain terminal of the PMOS transistor and a drain terminal of an NMOS transistor; the NMOS transistor having a gate terminal connected to the input of the first comparator, and a source terminal connected to a second resistor; a second resistor connected between the source terminal of the NMOS transistor and the second voltage source; and an inverter having an input to the drain of the NMOS transistor and an output which forms an output of the lower trip point circuit. The output circuit of claim 13 or claim 14, wherein the inverter comprises a plurality of inverter elements. The output circuit of any preceding claim, wherein the upper trip point circuit outputs a high digital value when its input rises above its predetermined voltage value and a low digital value otherwise and, the lower trip point circuit outputs a high digital value when its input rises above its predetermined voltage value and a low digital value otherwise; and wherein the said comparator further comprises: a third control circuit coupled to the upper trip point circuit and the lower trip point circuit that outputs, as the output of the comparator, a high digital value when the upper trip point circuit and lower trip point circuit output high digital values, outputs a digital low value when the upper trip point circuit and lower trip point circuit output digital low values and maintains its digital value when the upper trip point circuit outputs a digital low value and the lower trip point circuit outputs a digital high value. The output circuit of claim 16, wherein the third control circuit comprises: a first dual input NOR gate having a first input coupled to the output of the lower trip point circuit, a second input and an output, wherein the output forms the output of the third control circuit; a second dual input NOR gate having a first input coupled to the output of the first dual input NOR gate, a second input and an output coupled to the second input of the first dual input NOR gate; and and an inverter having an input coupled to the output of the upper trip point circuit and an output coupled to the second input of the second dual input NOR gate. An output driver circuit comprising: an output transistor pair comprising first and second transistors coupled together in series between a first and a second voltage potential operable to switch an output of the output driver circuit between the first and second voltage potentials; first and second control circuits each having an output coupled to control terminals of the first and second transistors respectively and each having a first input coupled to first and second inputs of the output driver circuit respectively; a comparator having an input coupled to the control terminal of the first or second transistors and having an output coupled to a second input of the second and first control circuits respectively; wherein: the comparator is adapted to compare the voltages at the control terminals of the first or second transistors with predetermined voltage levels and communicate via an output the result of those comparisons to the second and first control circuits respectively; and the first and second control circuits are arranged to manipulate the voltages at the control terminals of the first and second transistors under the influence of the output of the comparator so that when the output of the output driver circuit changes state, one transistor in the output transistor pair is turned off before the other transistor in the output transistor pair is turned on, characterised in that the comparator comprises an upper trip point circuit having a first input coupled to the input of the comparator, a second input, and an output; a lower trip point circuit having a first input coupled to the input of the comparator, a second input coupled to the output of the upper trip point circuit, and an output coupled to the second input of the upper trip point circuit; and wherein the upper trip point circuit outputs a digital high value when the input rises above a predetermined voltage, the lower trip point outputs a digital high value when the input rises above a predetermined voltage value, and the comparator further comprises: a third control circuit coupled to the upper and lower trip point circuits that outputs, as the output of the comparator, a high digital value when the upper trip point circuit and lower trip point circuit output high digital values, outputs a digital low value when the upper trip point circuit and lower trip point circuit output digital low values and maintains its digital value when the upper trip point circuit outputs a digital low value and the lower trip point circuit outputs a digital high value. The output circuit of claim 18, wherein the third control circuit comprises: a first dual input NOR gate having a first input coupled to the output of the lower trip point circuit, a second input and an output, wherein the output forms the output of the third control circuit; a second dual input NOR gate having a first input coupled to the output of the first dual input NOR gate, a second input and an output coupled to the second input of the first dual input NOR gate; and and an inverter having an input coupled to the output of the upper trip point circuit and an output coupled to the second input of the second dual input NOR gate.

Inventors: HASTINGS ROY A, SALAMINA NICOLAS, HASTINGS, ROY A., SALAMINA, NICOLAS
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1995
Main classification: H03K19
IPC codes: H03K19, H03K17