Abstract:
An improved word line boost circuit suitable for use on integrated circuits such as flash memory devices includes a two step boosting circuit with a floating circuit node. A first circuit provides an initial boost of the output voltage from a precharged voltage. Part of the first circuit is floated, lessening a load on a second circuit. Then, the second circuit provides a second boost of the output voltage with increased power efficiency. A time delay separates the onset of the second boosting operation from the onset of the first boosting operation so as to define a two-step boost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of voltage boost circuits. In particular, the invention relates to integrated circuits using word line boost circuits to produce on-chip voltages outside the range of the off-chip voltage supply. 
     2. Description of the Related Art 
     The electronics industry has continued to define standard power supply voltages of decreasing magnitudes. Decreasing power supply voltages, such as 5 volts, 3 volts, and 1.8 volts, raise the demands on modern circuits to provide sufficiently high on-chip voltages despite a lower off-chip supply voltage. Flash memory is an example of an application that would welcome more efficient boosting of a low off-chip supply voltage to an on-chip voltage sufficiently high to access flash memory cells. Therefore, what is needed is a word line boost circuit having higher boosting efficiency. 
     SUMMARY OF THE INVENTION 
     An improved word line boost circuit is disclosed that increases boosting efficiency. The improved word line boost circuit can be implemented in an integrated circuit that includes a memory array with word lines powered by word line drivers. Boosting efficiency is increased by floating a part of a first circuit that initially boosts an output voltage of the word line boost circuit. Floating part of the first circuit obviates the need for a diode to isolate the first circuit, and decreases the load on a second circuit that further boosts the output voltage of the word line boost circuit, thereby increasing efficiency. 
     A boost circuit includes an output, a precharge circuit connected to the output, a first capacitor with a first terminal connected to the output, a first circuit connected to a second terminal of the first capacitor, a second capacitor, and a second circuit connected to the output through the second capacitor. The second terminal of the first capacitor can be in a floating state, set to a first supply voltage, or a second supply voltage. An onset of a first boost operation performed by the first circuit is followed after a time delay by an onset of a second boost operation performed by the second circuit. 
     In some embodiments, one of the first supply voltage and the second supply voltage is a ground; the precharge circuit has a switching circuit connected to the output of the first voltage supply and the second voltage supply; and the second terminal of the first capacitor switches between i) a floating state, ii) being set to a first supply voltage, and iii) being set to a second supply voltage in response to one or both of a first signal and a second signal. 
     In a further embodiment, the word line boost circuit is part of an integrated circuit device with a substrate. In yet another embodiment, the word line boost circuit is part of an integrated circuit memory device with a substrate, a memory array, and word lines. 
     A method for reducing energy consumption of a boost circuit to achieve higher boosting efficiency for the above mentioned word line boost circuit comprises: precharging an output to a precharge voltage, changing the output to a first voltage with a first coupling circuit that is connected to the output, floating a part of the first coupling circuit, and changing the output to a second voltage with a second coupling circuit connected to the output. 
     In some embodiments, the method for reducing energy consumption of a boost circuit to achieve higher boosting efficiency for the above mentioned word line boost circuit comprises: changing an output from a precharge voltage to a first voltage with a first circuit, setting a node in the first circuit to a floating voltage, and changing the output from the first voltage to a second voltage with an energy expenditure that is lower than if the node were not floating. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a block diagram of word line boost circuit representing an embodiment of the invention. 
     FIG. 2 is a circuit diagram of a precharge circuit. 
     FIG. 3 is a circuit diagram of a first boost circuit. 
     FIG. 4 is a circuit diagram of a second boost circuit. 
     FIG. 5 is a block diagram of a word line boost circuit representing an embodiment of the invention. 
     FIG. 6 is a circuit diagram of a precharge circuit. 
     FIG. 7 is a circuit diagram of a first boost circuit. 
     FIG. 8 is a circuit diagram of a second boost circuit. 
     FIG. 9 is a timing diagram of a first signal and a second signal supplied to a word line boost circuit. 
     FIG. 10 is a timing diagram of voltages supplied by word line boost circuits representing embodiments of the invention. 
     FIG. 11 is a simplified block diagram of an integrated circuit utilizing an improved word line boost circuit. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a word line boost circuit  100 . The word line boost circuit  100  includes a first precharge circuit  200 , a first boost circuit  300 , a second precharge circuit  338 , a diode  370 , a second boost circuit  400 , and an output  150 . The first precharge circuit  200  and the second precharge circuit  338  each serves as a node charging circuit that charges a node from a starting voltage to another voltage. 
     FIG. 2 schematically illustrates the first precharge circuit  200 . The first precharge circuit  200  includes a NOR gate  210 , a first transistor  220 , a second transistor  230 , a third transistor  240 , and a switching transistor  250 . Switch circuit  222  includes the first transistor  220  and the third transistor  240 . The NOR gate  210  has a first input terminal  202  receiving a first signal  206 , a second input terminal  204  receiving a second signal  208 , and an output connected to a node  215 . The first transistor  220  is an n-channel transistor with a gate connected to the node  215 , a source connected to a ground  225 , and a drain. The second transistor  230  is an n-channel transistor with a gate connected to a voltage supply  235 , a source connected to the drain of the first transistor  220 , and a drain connected to a node  238 . Node  238  is the output node of the switch circuit  222 . The third transistor  240  is a p-channel transistor with a gate connected to the node  215 , a source connected to the output  150 , and a drain connected to the node  238 . The switching transistor  250  is a p-channel transistor with a gate connected to the node  238 , a source connected to the output  150 , and a drain connected to the voltage supply  235 . The first precharge circuit  200  charges the output  150  to the voltage of the voltage supply  235 . The first precharge circuit  200  then floats the output  150 . 
     FIG. 3 schematically illustrates the first boost circuit  300 , the diode  370 , and the second precharge circuit  338 . The first boost circuit  300  includes a first inverter  310 , a second inverter  315 , a third inverter  320 , a fourth inverter  325 , and a first capacitor  330 . The second precharge circuit  338  includes a fifth inverter  340 , a fourth transistor  350 , a fifth transistor  355 , a sixth transistor  360 , and a seventh transistor  365 . The second precharge circuit  338  charges a node  335  to the voltage of the voltage supply  235 . The second precharge circuit  338  then floats the node  335 . 
     An input of the first inverter  310  receives the first signal  206 . The first inverter  310 , the second inverter  315 , the third inverter  320 , and the fourth inverter  325  are connected in series. The first capacitor  330  has a first terminal connected to an output of the fourth inverter  325 , and a second terminal connected to the node  335 . The fifth inverter  340  has an input that receives the first signal  206 , and an output that is connected to a node  345 . The fourth transistor  350  is an n-channel transistor with a gate connected to the node  345 , a source connected to the ground  225 , and a drain. The fifth transistor  355  is an n-channel transistor with a gate connected to the voltage supply  235 , a source connected to the drain of the fourth transistor  350 , and a drain connected to a node  358 . The sixth transistor  360  is a p-channel transistor with a gate connected to the node  345 , a source connected to the node  335 , and a drain connected to the node  358 . The seventh transistor  365  is a p-channel transistor with a gate connected to the node  358 , a source connected to the node  335 , and a drain connected to the voltage supply  235 . The diode  370  has a first terminal connected to the node  335  and a second terminal connected to the output  150 . 
     FIG. 4 schematically illustrates the second boost circuit  400 . The second boost circuit  400  includes a sixth inverter  410 , a seventh inverter  420 , an eighth inverter  430 , a ninth inverter  440 , and a second capacitor  450 . An input of the sixth inverter  410  receives the second signal  208 . The sixth inverter  410 , the seventh inverter  420 , the eighth inverter  430 , and the ninth inverter  440  are connected in series. The second capacitor  450  has a first terminal connected to an output of the ninth inverter  440  and a second terminal connected to the output  150 . 
     When the word line boost circuit  100  operates, the first precharge circuit  200  and the second precharge circuit  338  precharge both terminals of the diode  370 . The first precharge circuit  200  and the second precharge circuit  338  float both terminals of the diode  370 . The first boost circuit  300  boosts the first terminal of the diode  370 . The second boost circuit  400  boosts the second terminal of the diode  370 , or the output  150 . 
     FIG. 5 illustrates a word line boost circuit  500 . The word line boost circuit  500  includes a precharge circuit  600 , a first boost circuit  700 , a second boost circuit  900 , and an output  550 . 
     FIG. 6 schematically illustrates the precharge circuit  600 . Transistors having a thick gate oxide are indicated with a rectangle for a gate. The oxide thicknesses for thick gate oxide devices and thin gate oxide devices are 180 Å and 100 Å respectively for 0.4 micron technology. The precharge circuit  600  includes a first NOR gate  610 , a first transistor  620 , a second transistor  630 , a third transistor  640 , and a switching transistor  650 . The first NOR gate  610  has a first input terminal  602  receiving a first signal  606 , a second input terminal  604  receiving a second signal  608 , and an output connected to a node  615 . The first transistor  620  is an n-channel transistor with a thick gate oxide having a gate connected to the node  615 , a source connected to a ground  625 , and a drain. The second transistor  630  is an n-channel transistor with a thick gate oxide having a gate connected to a voltage supply  635 , a source connected to the drain of the first transistor  620 , and a drain connected to a node  638 . The third transistor  640  is a p-channel transistor with a thick gate oxide having a gate connected to the node  615 , a source connected to the output  550 , and a drain connected to the node  638 . The switching transistor  650  is a p-channel transistor with a thick gate oxide having a gate connected to the node  638 , a source connected to the output  550 , and a drain connected to the voltage supply  635 . 
     FIG. 7 schematically illustrates the first boost circuit  700 . Transistors having a thick gate oxide are indicated with a rectangle for a gate. The first boost circuit  700  includes a first branch  710 , a fourth transistor  720 , a fifth transistor  730 , a first capacitor  740 , and a second branch  800 . The first branch  710  includes a second NOR gate  750 , a first inverter  760 , and a second inverter  770 . The second branch  800  includes a third inverter  810 , a first NAND gate  820 , a sixth transistor  830 , a seventh transistor  840 , an eighth transistor  850 , a fourth inverter  860 , a fifth inverter  870 , a sixth inverter  880 , and a ninth transistor  890 . 
     The second NOR gate  750  has a first input terminal  752  receiving the first signal  606  and a second input terminal  754  receiving the second signal  608 . An output of the second NOR gate  750  is connected to an input of the first inverter  760 . An output of the first inverter  760  is connected to an input of the second inverter  770 . The fourth transistor  720  is an n-channel transistor with a thick gate oxide having a gate connected to an output of the second inverter  770 , a source connected to the ground  625 , and a drain connected to a node  725 . The fifth transistor  730  is an n-channel transistor with a thick gate oxide having a gate connected to a node  735 , a source connected to the node  725 , and a drain connected to the voltage supply  635 . The first capacitor  740  has a first terminal connected to the node  725  and a second terminal connected to the output  550 . The third inverter  810  has an input receiving the second signal  608 . The NAND gate  820  has a first input terminal  822  receiving the first signal  606 , a second input terminal  824  connected to an output of the third inverter  810 , and an output connected to a node  825 . The sixth transistor  830  is an n-channel transistor  830  with a thick gate oxide having a gate connected to the node  825 , a source connected to the ground  625 , and a drain connected to the node  735 . The seventh transistor  840  is a p-channel transistor with a thick gate oxide having a gate connected to the node  825 , a drain connected to the node  735 , and a source connected to a node  845 . The eighth transistor  850  is a diode-connected n-channel transistor with a thick gate oxide having an anode connected to the voltage supply  635  and a cathode connected to the node  845 . The fourth inverter  860  has an input connected to the node  825 . The fifth inverter  870  has an input connected to an output of the fourth inverter  860 . The sixth inverter  880  has an input connected to an output of the fifth inverter  870 . The ninth transistor  890  is a capacitor-connected n-channel transistor with a thick gate oxide having a first terminal connected to an output of the sixth inverter  880  and a second terminal connected to the node  845 . 
     FIG. 8 schematically illustrates the second boost circuit  900 . The second boost circuit  900  includes a seventh inverter  910 , an eighth inverter  920 , a ninth inverter  930 , a tenth inverter  940 , and a second capacitor  950 . An input of the seventh inverter  910  receives the second signal  608 . The seventh inverter  910 , the eighth inverter  920 , the ninth inverter  930 , and the tenth inverter  940  are connected in series. The second capacitor  950  has a first terminal connected to an output of the tenth inverter  940  and a second terminal connected to the output  550 . 
     FIG. 9 is a timing diagram displaying voltage versus time for the first signal  606  and the second signal  608 . The first signal  606  has a low level  609 , a rising edge  610  triggering an onset of a first boost operation, and a high level  611 . The second signal  608  has a low level  612 , a rising edge  613  triggering an onset of a second boost operation, and a high level  614 . 
     FIG. 10 is a timing diagram displaying voltage versus time for an output signal  1000  supplied by the output,  150  and an improved output signal  1100  supplied by the output  550 . Output signal  1000  has a first level  1010  and a second level  1020 . Improved output signal  1100  has a precharge level  1105 , a first level  1110 , and a second level  1120 . 
     With reference to FIG. 6-10, initially, the first signal  606  is at the low level  609  and the second signal  608  is at the low level  612 . The precharge circuit  600  connects the output  550  to voltage supply  635  through the switching transistor  650 . The improved output signal  1   00  has the precharge level  1105  of 2.5 volts. The first branch  710  of the first boost circuit  700  turns on the fourth transistor  720  and the second branch  800  turns off the fifth transistor  730 . The first terminal of the first capacitor  740  is connected to the ground  625  through the fourth transistor  720 . The second boost circuit  900  connects the first terminal of the second capacitor  950  to the ground  625  through the tenth inverter  940 . 
     The rising edge  610  of the first signal  606  triggers the onset of the first boost operation. In the precharge circuit  600 , the switching transistor  650  turns off. The output  550  is no longer connected to the voltage supply  635 . The first branch  710  of the first boost circuit  700  turns off the fourth transistor  720 . The second branch  800  turns on the fifth transistor  730 , connecting the voltage supply  635  to the first terminal of the first capacitor  740 . Capacitive coupling through the first capacitor  740  raises the improved output signal  1100  to the first level  1110 , yielding advantageous results. Specifically, the first level  1110  of the improved output signal  1100  is about 3.5 volts, about 0.3 volts higher than the first level  1010  of the output signal  1000 . This difference is both of significant magnitude and sustained duration. 
     The rising edge  613  of the second signal  608  triggers the onset of the second boost operation. The second branch  800  turns off the fifth transistor  730 . The first terminal of the first capacitor  740  floats. The second boost circuit  900  connects the first terminal of the second capacitor  950  to the voltage supply  635  through the tenth inverter  940 . Capacitive coupling through the second capacitor  950  raises the improved output signal  1100  to the second level  1120 , continuing to yield advantageous results. Specifically, the second level  1120  of the improved output signal  1100  is about 5.1 volts, about 0.3 volts higher than the second level  1020  of the output signal  1000 . This difference between the second level  1120  and the second level  1020  is of significant magnitude and duration. 
     FIG. 11 provides a simplified diagram of an integrated circuit device utilizing the word line boost circuit of the present invention. The integrated circuit  1200  includes a semiconductor substrate. A memory array  1201  on the substrate has word lines  1214  for accessing rows of memory cells in the memory array  1201 . The word lines  1214  utilize an operating voltage which is outside a pre-specified range of a supply potential normally applied to the integrated circuit  1200  at supply terminals  1202  and  1203 , which are adapted to receive a supply potential VDD and ground. The word line boost circuit  1204  supplies the operating potential to the word lines  1214  through word line drivers  1205 . Input signals applied to the integrated circuit  1200  in this example include address signals  1206  applied to the word line drivers  1205  and data signals  1207 . 
     FIG. 11 is representative of a wide variety of integrated circuits which include on-chip circuitry that utilizes the operational voltage outside the pre-specified range of the supply potential. Memory devices such as flash memory devices are one class of integrated circuit devices according to the present invention. 
     Other embodiments of the invention can use different logic in one or more of the precharge branch, the first circuit, and the second circuit to process the signals triggering the onsets of the boosting operations. Another embodiment of the invention uses different signals triggering the onsets of the boosting operations, for example, signals going from high to low; one signal going from high to low and another signal going from low to high; and level triggering signals. Another embodiment of the invention is a word line boost circuit producing a boosted negative voltage. 
     The foregoing description of various embodiments of the invention have been presented for purposes of illustration and description. It is not intended to limit the invention to the precise forms disclosed. Many modifications and equivalent arrangements will be apparent.