Abstract:
Plural bootstrap capacitors are coupled to an output stage of a MOSFET driver. A conventional bootstrap driver is preceded by one or more additional bootstrap stages. Each one includes a capacitor, a tri state inverter and a delay section. When the output stage is off all capacitors are discharged. To turn the output stage on, all capacitors, including the output gate capacitance, are charged in parallel. Then each capacitor in turn is caused to pump its charge into the gate of the output stage, with the last capacitor pumping the output stage gate voltage to a level well in excess of the applied power supply voltage.

Description:
BACKGROUND OF THE INVENTION 
     Bootstrap drivers are well known in the prior art. In particular metal oxide semiconductor field effect transistor (M0SFET) circuits employ bootstrapping to good effect. Such devices require substantial channel width in order to conduct an appreciable load current such as those commonly used in practice. In general, high capacitance loads require substantial charging currents to improve the speed performance. In the case of lamp drivers, most commonly light emitting diode (LED) arrays, a substantial excitation current is drawn. When such drivers are to be used at low voltage, the problems are aggravated. When MOS devices are operated in the 15-30 volt range, no real drive problems are encountered and conventional circuits can be employed. However, the 9-volt battery has long been an economical power source and at this level MOS driver problems become apparent. At this level bootstrapping is commonly employed. 
     At the 6-volt level, another common economical power source voltage, driver problems become more acute and bootstrapping becomes very important. At still lower voltages severe problems set in and bootstrapping becomes necessary. In a typical MOSFET enhancement mode device a threshold voltage of about 1.5 volts is common. This is an appreciable fraction of 6 volts. Since, in a conventional driver, the output voltage limits at about one threshold below the supply voltage, the output voltage swing becomes limited. In driver applications substantial areas are involved in obtaining currents in the tens-of-milliamperes range. Bootstrapping will overcome the output swing limit so that the output swing will not be limited by the threshold voltage. Also with bootstrap drive, area consumption can be reduced significantly. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to use multi bootstrap drive to improve the area efficiency in MOSFET output stages. 
     It is a further object of the invention to divide a bootstrap drive capacitor into a plurality of separate capacitors, each one being timed to pump its charge into the output transistor gate so that a cumulative drive signal increase is realized. 
     These and other objects are achieved in the following arrangement. The output stage inverter is preceded by a plurality of sections, each one comprising a capacitor, a tri sate inverter, and a delay element. Each capacitor is coupled to the output stage gate and the tri state inverters and delay elements operated in cascade. In the quiescent state, or output zero, all capacitors are discharged. When a one is desired, all capacitors, including the output stage gate capacitance, are first charged in parallel and then each bootstrap capacitor sequentially pumps charge into the output gate and is then disconnected. Thus the cumulative sequential charge pump action causes the output gate voltage to be driven to a high level. It is necessary that the cumulative delay be less than the desired time of the on state and desirable that each delay be long enough to allow substantial charge transfer. The delay elements can be made up of cascaded inverters to provide static delay or they can be composed of clocked or dynamic delay devices. By using the multi bootstrap approach for low voltage drivers, surprisingly large device area economies can be realized. 
    
    
     BREIF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a logic diagram of a double bootstrap driver; 
     FIG. 2 is a waveform diagram showing the waveforms of various points in the circuit of FIG. 1; 
     FIG. 3 is a schematic diagram of a form of the circuit of FIG. 1; and 
     FIG. 4 is a logic diagram of a triple bootstrap driver. 
    
    
     DESCRIPTION OF THE INVENTION 
     In reference to FIG. 1, output terminal 10 is taken from the juncture of transistors 11 and 12. When the gates of transistors 12 and 11 are taken to V DD  and V SS  respectively, terminal 10 will be at substantially V SS  or ground. If the gates of transistors 11 and 12 are taken to V DD  and V SS  respectively, terminal 10 will be pulled toward V DD . Without bootstrapping, the output will approach to one threshold below V DD . In high voltage circuits, for example those using 15-volt V DD  supplies, the output swing of terminal 10 will be close percentage wise to 15 volts. In a low voltage system, for example 6 volts, the threshold voltage will be a substantial fraction thereof. (A 1.5 volt threshold represents 25% of V DD .) Thus,if terminal 10 is to be driven very close to V DD , the gate of transistor 11 will have to be driven at least one threshold above V DD . 
     When terminal 10 is connected to a load device 13 that requires a substantial current, transistor 11 will have to be of sufficient channel width to pass the required current. It has been found that channel width can become excessive when loads requiring tens of milliamperes are present. This is particularly true in low voltage systems, in which case transistor 11 can dominate chip area. However, if the gate of transistor 11 can be driven well in excess of V DD , the width of transistor 11 can be greatly reduced with the attendant virtue of much more efficient utilization of chip area. 
     While bootstrap amplifiers are known in the prior art and are quite useful, we have discovered that multiple bootstrap circuits have surprisingly useful performance characteristics. It might be expected that the use of multiple bootstrapping would provide little, if any, performance benefit. However, as will be shown hereinafter, we have found that by proper partitioning of the bootstrap action, great improvements in chip area utilization can be realized. Using a single bootstrap action, a 30 pf capacitor was used to bootstrap a 96 mil transistor. With double bootstrap action, as obtained with the circuit of FIG. 1, two 15 pf capacitors could bootstrap a 60 mil transistor to equivalent performance. A triple bootstrap with three 10 pf capacitors bootstrapped a 50 mil transistor to the same performance. Thus, using the same total cpacitor area, the double bootstrap produced a 37% reduction in output transistor area and the triple bootstrap an additional 12% improvement. Clearly further bootstrapping would provide even greater benefits, but the benefits are less pronounced. 
     The bootstrap action of FIG. 1 will now be described. Reference will be made to the waveforms of FIG. 2 which show the signals at various points of FIG. 1. 
     In the output zero state, terminal 10 is close to ground potential and the input potential of terminal 14 is close to V DD . Since all of the transistors are of p-channel variety, it will be noted that V DD  is negative with respect to V SS . For this state enhancement transistors 12, 15, 16, 17, 45, and 46 will be on and capacitors 18 and 19 will be discharged. Transistor 11 will be off. Transistors 20, 21, and 22 are depletion devices, as indicated by the triangular symbols. While depletion mode devices are used as load elements as shown, it is to be understood that enhancement mode devices could be used if desired. 
     Waveform 24 of FIG. 2 is the signal at terminal 14. It is shown going to V SS  at time A when a logic one is required at output terminal 10. This turns transistors 12, 45 and 46 off. Waveform 26, which is the signal at the gate of transistor 11, is the delayed output of the input inverter comprising transistors 15 and 20 starting at time B. Since the gate node of transistor 11 is a high capacitance node, the voltage swing is exponential as node capacitance charges toward V DD  through transistor 20. 
     Waveform 24 is also applied to delay buffer 25 which produces an output starting at time C and labeled waveform 27. Waveform 27 acts to turn transistor 16 off at time D, whereupon transistor 21 starts to pull the upper end of capacitor 18 to V DD . At about the same time waveform 27 turns transistor 20 off, thereby disconnecting the input inverter from the gate node of transistor 11. Since capacitor 18 had previously been charged to almost V DD  in the interval B-D, and its upper end is now pulled to V DD , capacitor 18 will pump its charge into the gate of transistor 11, thereby charging capacitor 19 to well in excess of V DD . 
     It will be noted that transistors 15 and 20 comprise a tri state inverter. Initially both conduct, but transistor 15 is designed to conduct substantially more than transistor 20, thereby pulling the output to V SS  or logic zero. Then the input goes to logic zero and transistor 20 pulls the output to V DD  or logic one. Then, after a time delay (B-D), transistor 20 is turned off, thereby disconnecting the inverter entirely from the remainder of the circuitry to invoke the off version of the tri state inverter. Transistors 16, 21, and 45 comprise a second such tri state inverter. 
     Delay inverter 28, which has its input connected to the upper end of capacitor 18, has an output shown as waveform 29. At time E the output of delay inverter 28 goes toward V SS  and at time F turns transistor 21 off, thereby disconnecting the upper end of capacitor 18 and removing it from the circuit (tri state off). Waveform 29 also turns transistor 17 off and transistor 22 will then pull the left hand terminal of capacitor 19 to almost V DD . Transistors 17, 22, and 46 comprise a conventional NOR gate. Since the gate of transistor 46 is at logic zero, it is off and the NOR gate operates as a simple tri state inverter. Capacitor 19, which was previously charged in excess of V DD , will now pump its charge into the gate of transistor 11, still further driving its potential over the level of V DD  in the interval F-G. Thus transistor 11 is conducting with its gate substantially in excess of V DD . This will drive the potential at terminal 10 very close to V DD  or permit substantial current flow in load 13. 
     The input waveform 24 goes back toward V DD  at time G. This turns on transistor 12, 15, 45, and 46, and discharges the gate node of transistor 11 to V SS , as shown in waveform 26. Due to the delay of delay buffer 25, waveform 27 returns to V DD  at time H to turn transistor 16 on. Then at time I waveform 29 goes back to V DD  (due to the delay in delay inverter 28) and transistors 21 and 17 are turned on. This completes the cycle and, as can be seen from waveform 26, transistor 11 was turned on hard with its gate voltage well in excess of V DD  in the interval F-G to execute the logic one output function. 
     FIG. 3 is a complete schematic diagram of the circuit of FIG. 2. Transistors 30-33 comprise delay buffer 25 and transistors 34-39 comprise delay inverter 28. 
     EXAMPLE 
     A double bootstrap driver was constructed in accordance with the schematic of FIG. 3. The potential applied between V DD  and V SS  was 6 volts. The following chart gives the size of the various transistors; the first number is the channel width in mils and the second number is the channel length in mils. The capacitors are rated in picofarads. 
     
         ______________________________________ELEMENT            SIZE (MILS/MILS)______________________________________Transistor 11      60/0.3Transistor 12      7/0.3Transistor 15      6.5/0.3Transistor 16      7/0.3Transistor 17      1/0.3Capacitor 18       16 pfCapacitor 19       16 pfTransistor 20      1.8/0.3Transistor 21      0.7/0.3Transistor 22      0.3/1Transistor 30      0.7/0.3Transistor 31      0.2/3.4Transistor 32      0.3/0.6Transistor 33      0.2/2Transistor 34      0.2/0.9Transistor 35      0.2/1.4Transistor 36      1/0.3Transistor 37      0.2/3.4Transistor 38      0.3/0.6Transistor 39      0.2/1.4Transistor 45      1.0/0.3Transistor 46      1.0/0.3______________________________________ 
    
     In order to achieve the output drive capabilities of the circuit using a single bootstrap driver, a 32 pf capacitor would have to be used driving a transistor of 96/0.3 mils. Thus using the double bootstrap driver results in a surprisingly large reduction of total area consumed by the circuit. 
     FIG. 4 shows a triple bootstrap circuit. Where the elements correspond to those of FIG. 1, the same designations are used. It can be seen that between delay inverter 28 and the inverter comprising transistors 17 and 22 an additional stage has been inserted. Another tri state inverter comprising transistors 40, 41, and 47 drives another delay inverter 42 and capacitor 43. 
     The circuit action is an extension of that of FIG. 1 except that an additional capacitor charge pumping interval is employed. Instead of three exponential charge increments, as shown in waveform 26, there would be four. Further additional stages incorporating elements like those of 40-43 can further be cascaded. Each cascade would result in reduced area for transistor 11 and in each case all of the bootstrap capacitors could be made equal. 
     Our invention has been described and its operation explained for double and triple bootstraps. Clearly the number of bootstraps could be expanded as long as the input logic signal duration exceeds the combined delays. However, as the number increases, the area reduction of the output transistor will be offset by the area consumption of the related delay and inverter circuits. Therefore the actual number is a matter of choice. 
     One important aspect of the invention is the use of delay elements to time the switching of bootstrap capacitors and the use of tri state inverters. While static means are shown, dynamic delays could be used. While not shown, clocked dynamic delay elements could be employed in place of elements 25, 28, and 42 of FIGS. 1, 3, and 4. The delay values are selected to achieve the desired charging characteristics, as illustrated in the waveforms of FIG. 2 and described above. 
     In view of the foregoing, it is clear that there are numerous alternatives and equivalents for our invention that will occur to a person skilled in the art. For example, while p-channel examples are taught and detailed, n-channel devices could be employed. In this case the signal and supply voltage polarities would be inverted. Accordingly, it is intended that the scope of our invention be limited only by the following claims.