Patent Document

BACKGROUND 
     Complex programmable logic devices (CPLDs) are well-known integrated circuits that may be programmed to perform various logic functions. Numerous types of memory elements may be used in CPLD architectures to provide programmability. One such memory element, known as a flash memory cell, is both electrically programmable and erasable. Program and erase are performed on a plurality of flash memory cells using either Fowler-Nordheim tunneling or hot electron injection for programming and Fowler-Nordheim tunneling for erasing. Flash memories can also be in-system programmable (ISP). An ISP device can be programmed, erased, and can have its program state verified after it has been connected, such as by soldering, to a system printed circuit board. Some CPLDs do not have ISP capability and must be programmed externally (outside the system) by programming equipment. 
     Continuous advances in integrated-circuit process technology have dramatically reduced device feature size. The reduction in feature size improves device performance while at the same time reducing cost and power consumption. Unfortunately, smaller feature sizes also increase a circuit&#39;s vulnerability to over-voltage conditions. Among the more sensitive elements in a modern integrated circuit are the gate oxide layers of MOS transistors. These layers are very thin in modern devices, and are consequently easily ruptured by excessive voltage levels. Modern circuits with small feature sizes therefore employ significantly lower source voltages than was common only a few years ago. For example, modern 0.18-micron processes employ supply voltages no greater than 2 volts. 
     The voltages required to program and erase flash memory cells are dictated by physical properties of the materials used to fabricate memory cells. Unfortunately, these physical properties have not allowed the voltages required to program, erase, and verify the program state of a memory cell to be reduced in proportion to reductions in supply voltages. For example, modern flash memory cells adapted for use with a 0.18-micron processes require program and erase voltages as high as 14 volts, a level far exceeding the required 1.8-volt supply level. For a more detailed treatment of program, erase, and verify procedures, see U.S. Pat. No. 5,889,701, which is incorporated herein by reference. 
     FIG. 1 (prior art) depicts a conventional CPLD  100 . The circuitry within CPLD  100  is instantiated on an integrated circuit chip  105 , which is later wire bonded to pins  110  of a device package  115  using a number of bond wires  120 . Bond wires  120  connect to respective bond pads  125 , some of which extend to respective input/output circuits  130 . Input/output circuits  130  convey signals to and from other programmable logic and interconnect resources (not shown). The logic of input/output circuits  130  and these configurable elements is dictated by the program state of a collection of configuration memory cells  135 . 
     Integrated circuits, including CPLDs, undergo substantial test procedures. Among these tests, memory cells are programmed, erased, and their states verified to insure proper device operation. To accomplish this, a sophisticated test apparatus, or “tester,” applies and receives signals via pads on the integrated circuit. These pads might be bond pads, like bond pads  125 , or dedicated test pads used only to make contact with the tester. 
     Chip  105  depicts two test-specific pads  145 , sometimes called “octal pads,” connected to a ramp-up circuit  150 . A pair of test pins  155  extends from an external tester (not shown) to pads  145  to convey a relatively high programming voltage VPP and a control signal CTRLB to circuit  150 . Circuit  150  uses these two external test signals to develop a ramped version VPP_R of the programming voltage VPP to steering logic  160 . While VPP is referred to herein as a “programming” voltage, it is to be understood that the applied voltage on terminal VPP might also be used to erase memory cells. Moreover, as with other designations in the present disclosure, VPP refers to both the signal and the corresponding circuit node. Whether a given designation refers to a node or a signal will be clear from the context. 
     Steering logic  160  selectively applies the ramped up program voltage VPP_R to the bitlines of memory cells within the box labeled memory cells  135 . Though shown in FIG. 1 as a discrete area, memory cells  135  are typically distributed throughout chip  105  to control the various programmable resources. A power line  165  conveys a power-supply voltage VDD from one of external supply pins  110  to I/O circuits  105  and the other internal components (not shown). 
     FIG. 2 (prior art) depicts a more detailed schematic of ramp-up circuit  150  of FIG.  1 . Ramp-up circuit  150  receives as input the relatively high programming voltage VPP on octal pad  145 . EEPROM cells can be damaged if programming and erase voltages are applied too quickly. Ramp-up circuit  150  is therefore provided to raise the external program or erase voltages on the respective pad  145  gradually to the appropriate voltage level. 
     Ramp-up circuit  150  includes a clock terminal  200  adapted to receive a clock signal generated either internally or externally to CPLD  100 . Control signal CTRLB is shown here associated with an octal pad  145 , but the control signal can also be generated internally, or can be received externally via a different type of pad. The last letter of the designation CTRLB indicates that the control signal is an active low (i.e., the B is for “bar”), and this convention is used throughout the present application. 
     The clock and control signals are fed into a circuit  205  that divides the clock signal into a pair of complimentary clocks C 1  and C 2 . These clocks are then each fed via respective capacitors to an output circuit  210 . Output circuit  210  receives the externally generated high-voltage signal VPP as an additional input, and also receives the compliment CTRL of control signal CTRLB. 
     When control signal CTRLB is brought low, output circuit  210  ramps up the voltage on the gate of a transistor  215  from zero volts to a level above VPP. The output VPP_R of ramp-up circuit  150  thus gently approaches the requisite program voltage VPP to be directed to the bit line of one or more memory cells. The output VPP_R ramps up over a time RT determined primarily by the clock signal CLK and the values of the capacitors between circuits  205  and  210 . The output VPP_R returns to zero when control signal CTRLB is brought high. 
     The trouble with ramp-up circuit  150  is that the voltage on the gate of transistor  215  must rise above the voltage level VPP. As noted above, modern integrated circuits are becoming ever more sensitive to high voltages, so it is beneficial to keep all voltages presented to CPLD  100  as low as possible. 
     SUMMARY 
     The present invention is directed to a ramp-up circuit that receives a relatively high program voltage for changing the program state of a memory cell. The ramp-up circuit gradually raises the program voltage to provide a ramped up version of the programming signal to the memory cells. The gradual ramping of the program voltage prevents damage to the memory cells. 
     The ramp-up circuit includes a pass gate and associated control circuitry that provides a controlled, ramped-up version of the program voltage to the memory cells without raising internal circuit nodes above the program voltage. This aspect of the invention reduces the maximum voltage required on nodes within the circuit, and therefore protects sensitive components from potentially damaging over-voltage conditions. 
     This summary does not define the scope of the invention, which is instead defined by the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 (prior art) depicts a conventional CPLD  100 . 
     FIG. 2 (prior art) depicts a more detailed schematic of ramp-up circuit  150  of FIG.  1 . 
     FIG. 3 depicts a ramp-up circuit  300  configured in accordance with the present invention. 
     FIG. 4 is a detailed schematic of level shifters  305  of FIG.  3 . 
     FIG. 5 details input ramp-up sub-circuit  310  of FIG.  3 . 
     FIG. 6 depicts an embodiment of booster sub-circuit  315  of FIG. 3, another type of ramp-up circuit similar to ramp-up sub-circuit  310  of FIG.  5 . 
     FIG. 7 depicts output stage  320  of FIG.  3 . 
     FIG. 8 is a waveform diagram depicting the operation ramp-up circuit  300  of FIG. 3, as detailed in FIGS. 4-7. 
     FIG. 9 depicts an output stage  900  similar to output stage  320  of FIG.  7 . 
     FIG. 10 depicts a circuit  1000  used to generate the signal VBON, which controls transistor  705  in FIG.  9 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 is a block-level depiction of a ramp-up circuit  300  configured in accordance with the present invention. Like ramp-up circuit  150  of FIG. 1, circuit  300  receives a programming (erase) voltage level VPP on an octal pin  145  and creates from that voltage a ramped-up program voltage VPP_R. Unlike circuit  150 , however, circuit  300  performs this function without raising internal circuit nodes above the voltage level VPP. This and other advantages are discussed below. 
     Ramp-up circuit  300  includes a bank of level shifters  305 , an input ramp-up sub-circuit  310 , a booster sub-circuit  315 , and an output stage  320 . These elements are detailed below in FIGS. 4-8. 
     FIG. 4 is a detailed schematic of level shifters  305  of FIG.  3 . Level shifters  305  receive a control input CTRL and a pair of clock inputs CLK 1  and CLK 2 . These three inputs can be provided externally, developed using CPLD resources, or a combination of the two. The control and clock signals are logic signals that alternate from zero volts to VDD, or between e.g. zero volts and approximately 1.8 volts for CPLDs manufactured using a 0.18-micron process. Level shifters  305  alter the logic levels of these signals, shifting the voltage level representative of a logic one to the programming voltage VPP. Level shifters  305  also develop complimentary clock signals for both CLK 1  and CLK 2 . The level shifted signals are terminated with the letter “S” to indicate that these signals are sourced from level shift circuit  305 . The signal VPP_S is essentially control signal CTRL level-shifted to transition between zero volts and VPP. 
     FIG. 5 details ramp-up sub-circuit  310  of FIG.  3 . Ramp-up sub-circuit  310  receives control signal CTRL, programming voltage VPP, the controlled programming voltage VPP_S from level shift bank  305 , and the complementary clocks CLK 1 _S and CLK 1 B_S, also from level shift bank  305 . Complementary clock signals CLK 1 _S and CLK 1 B_S connect to respective transistors  500  and  505 . These and other transistors with similarly depicted gate structures are pull-back-drain transistors, which are more voltage tolerant than more typical MOS transistors. 
     Clock signal CLK 1 _S periodically turns on transistor  500  to charge a capacitor  510 . Clock signal CLK 1 B_S then turns on transistor  505  to dump the charge collected on capacitor  510  onto a second capacitor  515 . Capacitor  510  is substantially smaller than capacitor  515  (400 times smaller in one embodiment), so the output signal on a terminal VPPR 1  rises gradually from zero to VPP. The frequencies and duty cycles of clock signals CLK 1 _S and CLK 1 B_S can be adjusted to alter the rise time RT 1  of signal VPPR 1 . 
     Ramp-up sub-circuit  310  includes some control circuitry  520  that removes the charge collected on capacitors  510  and  515  when control signal CTRL is brought low. Control circuit  520  additionally includes an output terminal RGND that grounds the output terminal VPP_R of the entire ramp-up circuit  300 , as discussed below in connection with FIG.  7 . 
     FIG. 6 depicts an embodiment of booster sub-circuit  315  of FIG. 3, another type of ramp-up circuit similar to ramp-up sub-circuit  310  of FIG.  5 . Booster sub-circuit  315  receives the programming voltage VPP, the complimentary clocks CLK 2 _S and CLKK 2 B_S from level shift bank  305 , and the ramped programming voltage VPPR 1  from input ramp-up sub-circuit  310 . Booster  315  includes a pair of high voltage OR gates  600  and  605 , a series of inverters  610 , and a ramp-up circuit  615  similar to the ramp-up portion of input ramp-up sub-circuit  310  of FIG.  5 . 
     Inverters  610  conventionally include both PMOS transistors and NMOS transistors. The first PMOS transistor in the series is wider than the first NMOS transistor, so the threshold voltage Vth of the first inverter is close to the threshold voltage of the first PMOS transistor. The second and third inverters are added to sharpen the edge of the resulting inverted version of signal VPPR 1  (VPPR 1 B). The falling edge of signal VPPR 1 B is delayed from the beginning of the rising edge of ramped up signal VPPR 1  by the time required for VPPR 1  to rise to within a threshold voltage Vth of programming voltage VPP. In one embodiment, a single inverter takes the place of inverters  610  to save area. 
     OR gates  600  and  605  OR the inverted ramp signal VPPR 1 B and respective clock signals CLK 2 _S and CLK 2 B_S and provide the resulting complementary output signals to a pair of transistors  620  and  625  within ramp-up circuit  615 . Ramp-up circuit  615  functions in the same manner as the similar portion of ramp-up sub-circuit  310 , except capacitors  630  and  635  within ramp-up circuit  615  have a ratio of approximately 1 to 225, which is to say that capacitor  630  is 225 times smaller than capacitor  635 . Ramp-up sub-circuit  315  produces a second ramp up signal VPPR 2 , the rise time RT 2  of which can be modified by changing the frequencies and duty cycles of clock signals CLK 2 _S and CLK 2 B_S. The capacitor values here and in FIG. 5 can also be modified to change the rise time of the various ramped-up voltages. 
     FIG. 7 depicts output stage  320  of FIG.  3 . Output stage  320  receives the first and second ramp-up signals VPPR 1  and VPPR 2 , the program voltage VPP, the controlled program voltage VPP_S sourced from level shift block  305 , and the signal RGND from ramp-up sub-circuit  310 . Output stage  320  includes two N-type transistors  700  and  705  and four P-type transistors  710 ,  715 ,  720 , and  715 . Transistors  700  and  710  are connected together in parallel to form a pass gate  717  capable of pulling terminal VPP_R substantially to the programming voltage VPP without requiring any node within ramp-up circuit  300  to rise above VPP. The operation of output stage  320  and the remaining circuits within ramp-up circuit  300  is explained below in connection with FIG.  8 . 
     FIG. 8 is a waveform diagram depicting the operation of ramp-up circuit  300  of FIG. 3, as detailed in FIGS. 4-7. The process begins when the programming voltage VPP is brought high, to 13 volts for example (edge  802 ). Next, a control signal CTRL, typically brought in externally from a tester, is brought high to enable the various circuits within ramp-up circuit  300  (edge  804 ). As a result of the control signal being brought high, level shift circuit  305  produces the controlled version VPP_S of the programming voltage VPP. VPP_S transitions between zero and programming voltage VPP when control signal CTRL transitions between zero and VDD. Although not depicted in FIG. 8, level shifter  305  produces complimentary clock signals that oscillate between approximately zero volts and the programming voltage VPP. 
     Turning now to FIG. 5, the voltage on terminal VPP_S and the complementary clock signals CLK 1 _S and CLK 1 B_S cause the voltage on output terminal VPPR 1  (the first ramp-up voltage) to gradually climb from zero volts to approximately VPP, as indicated by arrow  805  of FIG.  8 . 
     Turning next to FIG. 6, inverter chain  610  transitions when the first ramp-up signal VPPR 1  rises to approximately within one threshold voltage Vth of the first PMOS transistor of the programming voltage VPP (arrow  810  of FIG.  8 ). The resulting low voltage on line VPPR 1 B enables both of OR gates  600  and  605  (FIG.  6 ), causing their respective outputs to begin oscillating as defined by clocks CLK 2 _S and CLK 2 B_S. These clocks then periodically and alternately enable transistors  620  and  625  of ramp-up circuit  615  so that terminal VPPR 2  (the second ramp-up voltage) gradually rises from approximately zero volts to VPP. The falling level on line VPPR 1 B thus initiates the gradual rise of output terminal VPPR 2  (arrow  815  of FIG.  8 ). 
     Referring now to FIG. 7, the rising edge on the first ramp-up signal VPPR 1  gradually turns on transistor  700 , thus pulling output terminal VPP_R up toward programming voltage VPP. This transition is depicted in FIG. 8 using arrow  820 . Because terminal VPPR 1  rises only as high as VPP, transistor  700  cannot, by itself, raise output terminal VPP_R to the level of the programming voltage VPP. However, terminal VPPR 2  begins going high after VPPR 1 , gradually turning on transistor  705  to pull the gate of transistor  710  toward ground potential. Grounding the gate of transistor  710  turns on transistor  710 , which then pulls output terminal VPP_R the rest of the way to programming voltage VPP (arrow  825  of FIG.  8 ). Ramp-up circuit  300  thus achieves the goal of providing a substantially undiminished programming voltage on terminal VPP_R to bit lines of selected memory cells without requiring any internal node on the CPLD to rise above programming voltage VPP. 
     Returning to FIG. 3, control terminal CTRL is brought low each time steering logic  160  (FIG. 1) is to convey the programming voltage to a different bit line. Returning the control signal CTRL to ground disables each of the elements in FIG. 3, and control circuit  520  of FIG. 5 pulls output terminal VPP_R to ground. The entire cycle then begins again with the next assertion of control signal CTRL. 
     FIG. 9 depicts an output stage  900  similar to output stage  320 , like-numbered elements being the same. Output stage  900 , employed in place of output stage  320  in one embodiment, provides better control over the turn-on time of transistor  705 . Output stage  900  differs from output stage  320  in that the gate of transistor  705  receives a control signal VBON, where “BON” stands for “booster on.” VBON is a ramped-up signal that weakly follows the rising voltage transitions on terminals VPPR 1  and VPPR 2 , and consequently turns transistor  705  on more slowly than does output stage  320 . Also different from output stage  320 , a signal VPUB, where “PUB” stands for “pull-up bar,” is taken from the node connected to the gate of transistor  710 . The source of signal VPUB is depicted below in FIG.  10 . 
     FIG. 10 depicts a circuit  1000  used to generate the signal VBON, which controls transistor  705  in FIG.  9 . Circuit  1000  includes a pair of parallel-connected NMOS transistors  1005  and  1010 , the gates of which connect to respective control signals VPPR 2  and VPPR 1  (FIGS.  5  and  6 ). Circuit  1000  also includes a pair of transistors  1015  and  1020 , the gates of which connect to terminal VPUB of FIG. 9, and a transistor  1025 , the gate of which connects to terminal VPPR 1 B of FIG.  5 . 
     In operation, signal VBON rises toward VPP as ramp-up signals VPPR 2  and VPPR 1  turn on respective transistors  1005  and  1010 . Transistors  1005  and  1010  are relatively weak, so the maximum voltage on terminal VBON is several threshold voltages below VPP. The weak transistors  1005  and  1010  provide a slow rise time on the gate of transistor  705 . As signal VBON rises, transistor  705  pulls node VPUB toward ground, eventually turning on transistors  710  and  1015 . As in the embodiment of FIG. 7, transistor  710  pulls output terminal VPP_R all the way to programming voltage VPP; transistor  1015  likewise pulls terminal VBON all the way to programming voltage VPP, and consequently turns on transistor  705  completely. When terminal VPP_S returns to ground, transistors  1020  and  1025  pull terminal VBON to ground. In one embodiment, transistors  1020  and  1025  are replaced with a single transistor controlled by either signal VPPR 1 B or signal VPUB. 
     While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, application of the invention is not limited to the above-described CPLD architecture, or even to CPLDs. Moreover, some components are shown directly connected to one another while others are shown connected via intermediate components. In each instance, the method of interconnection establishes some desired electrical communication between two or more circuit nodes, or terminals. Such communication may often be accomplished using a number of circuit configurations, as will be understood by those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.

Technology Category: 5