A charge pump (CP) is an electronic circuit that uses capacitors as energy storage elements to create either a higher or lower voltage power source. Charge pump circuits are capable of high efficiencies, sometimes as high as 90-95% while being electrically simple circuits. A charge pump circuit is sometimes referred to as a “pipe”.
Charge pumps use some form of switching device(s), such as transistors, to control the connection of voltages to the capacitor. A capacitor is an electronic component that can store charges. For instance, to generate a higher voltage, the first stage involves the capacitor being connected across a voltage and charged up. In the second stage, the capacitor is disconnected from the original charging voltage and reconnected with its negative terminal to the original positive charging voltage. Because the capacitor retains the voltage across it (ignoring leakage effects) the positive terminal voltage is added to the original, effectively doubling the voltage. The pulsing nature of the higher voltage output is typically smoothed by the use of an output capacitor.
This is known as the “charge pumping action”, which typically operates at tens of MegaHertz (MHz) to minimize the amount of capacitance required. The capacitor used as the charge pump is typically known as the “flying capacitor”.
A common application for charge pump circuits is in integrated circuit (IC) level shifters where they are used to derive positive and negative voltages (often +10 V and −10 V) from a single 5 V, 3 V power supply rail or any other supply voltage. Charge pumps can also be used as drivers for LCDs (liquid crystal displays) or white LEDs (light emitting diodes), generating high bias voltages from a single low-voltage supply, such as a battery. Charge pumps are also used to generate high voltage (5 kv and up) for modern neon signs (older signs use a transformer).
Semiconductor non-volatile memories (NVM), such as erasable, programmable read only memory (EPROM), electrically erasable, programmable read only memory (EEPROM) and flash memories, typically require voltage values which are higher than a supply voltage reference (VDD), and lower than a ground voltage reference (GND). Such voltages are referred to as “overvoltages”. For example, the erasing operation of a non-volatile memory cell may requires a positive overvoltage equal to about 10 volts (V) as well as a negative overvoltage equal to −8V, both generated from a supply voltage reference VDD ranging between 1.8V and 5V and a ground voltage reference GND, that is conventionally equal to 0V. To generate these positive and negative overvoltages, charge pump (“CP”) circuits are typically utilized. These CP circuits are generally realized by cascading (connecting, one after another) a plurality (N) of “basic stages”.
Charge pumps may be a key component in Flash RAM (random access memory) devices. These devices may, for example, require a high voltage pulse to “clean out” any existing data in a particular memory cell before it can be written with a new value. Modern Flash RAMs generally operate at 1.8 or 3.3V, but may require about 10V to write.
For all kind of integrated circuits (ICs), and particularly for Flash and similar non-volatile memory (NVM) circuits, (called generally herein as “Flash” or “Flash Memory Circuits”) the average current consumption and supply peak current are important electrical parameters, limited by the product specification.
In Flash memory circuits, all internal voltages, which are higher than the supplied voltage, are usually generated by Charge Pump (CP) circuits.
Well-known and widely used charge pump (CP) circuits convert supply voltage power (Vsupply) to output high voltage power with power efficiency (η), where:                input low voltage power can be presented as a product of total charge pump (Icp_in) current consumption and applied supply voltage (Vsupply);        output high voltage power can be calculated as a product of output charge pump (Icp_out) current and output charge pump voltage (Vcp_out). Thus,η*Icp_in*Vsupply=Icp_out*Vcp_out  (1)        
Icp_in is the total current delivered to the charge pump by the low voltage supply. If all stages of the charge pump are identical, and the voltage supply to the stage's phase drivers (see FIG. 1A) equals to Vsupply, the total charge pump current consumption can be calculated:Icp_in=I1+N*I2.   (See equation (4), below).
In FIG. 1A, Iout (meaning Icp_out) and Vout (meaning Vcp_out) are presented.
and respectively,Icp_in=1/η*Icp_out*Vcp_out/Vsupply  (2)
From equation (2) it is evident that charge pump (CP) current consumption (Icp_in) depends on the CP's efficiency (η), supply voltage (Vsupply) and output current (Icp_out) supplied to the load. Each of these parameters appears to depend to some extent on the process, temperature, and supply range, including the charge pump's load (Icp_out).
As a result of the possible variations of the above parameters, the current consumption of the charge pump (Icp_in) could exceed the limit of the average current and/or maximal peak current for a Flash memory circuit, as defined by specifications.
NROM Modes of Operation
The following table presents exemplary conditions for programming, erasing and reading a nitride read-only memory (NROM) cell. The program (PGM) operation may utilize channel hot electron (CHE) injection. The erase (ERS) mode of operation may utilize hot hole injection (HHI)
TABLE 1Exemplary NROM Operating ConditionsVsVgVdVbtimeProgram+0.5v8-10v+4-5v0v0.1-1μsErase2v−7v5v0v100usRead1.3v5v0v0v10-100ns
From the table above, it is evident that Vg for the program operation may require a charge pump outputting an overvoltage. Generally, several NROM cells may be programmed at once, which demands capable and robust performance from a charge pump supplying the overvoltage.
Some examples of NROM memory cells may be found in commonly-owned U.S. Pat. Nos. 5,768,192 and 6,011,725, 6,649,972 and 6,552,387.
Some examples of methods of operation of NROM and similar arrays, such as algorithms related to programming, erasing, and/or reading such array, may be found in commonly-owned U.S. Pat. Nos. 6,215,148, 6,292,394 and 6,477,084.
Some examples of methods of operation for each segment or technological application, such as: fast programming methodologies in all flash memory segments, with particular focus on the data flash segment, smart programming algorithms in the code flash and EEPROM segments, and a single device containing a combination of data flash, code flash and/or EEPROM, may be found in commonly-owned U.S. Pat. Nos. 6,954,393 and 6,967,896.
A more complete description of NROM and similar cells and devices, as well as processes for their development may be found at “Non Volatile Memory Technology”, 2005 published by Saifun Semiconductor and materials presented at and through http://siliconnexus.com, both incorporated by reference herein in their entirety.
Glossary
Unless otherwise noted, or as may be evident from the context of their usage, any terms, abbreviations, acronyms or scientific symbols and notations used herein are to be given their ordinary meaning in the technical discipline to which the disclosure most nearly pertains. The following terms, abbreviations and acronyms may be used throughout the descriptions presented herein and should generally be given the following meaning unless contradicted or elaborated upon by other descriptions set forth herein. Some of the terms set forth below may be registered trademarks (®).
ACshort for alternating current, and sometimes written lower-case as “ac”. Alternating current is voltage or current in acircuit that is alternating in polarity at a set frequency, mostoften 50 or 60 Hz, as typified by current coming out ofstandard household wall sockets. The “other” type ofcurrent that we are familiar with is Direct Current (DC),typified by current coming out of standard householdbatteries.Capacitorusually abbreviated “C”. A capacitor is a device that storesenergy in an electric field created between a pair of con-ductors on which equal but opposite electric charges havebeen placed. A capacitor is occasionally referred to usingthe older term condenser. The amount of charge which canbe stored by a capacitor is measured in Farads (F).Capacitors resist changes in voltage, and are an essentialcomponent in (for example) power supplies for filtering outunwanted AC noise from a DC signal. A capacitor canrelease the energy that it has stored.ChargeA power supply which uses capacitors to store and transferPumpenergy to the output, often stepping the voltage up or down.Charge is transferred from one capacitor to another undercontrol of regulator and switching circuitry.CMOSshort for complementary metal oxide semiconductor.CMOS consists of n-channel and p-channel MOS tran-sistors. Due to very low power consumption and dissipationas well minimization of the current in “off” state CMOS isa very effective device configuration for implementation ofdigital functions. CMOS is a key device in state-of-the-artsilicon microelectronics.CMOS Inverter: A pair of two complementary transistors (ap-channel and an n-channel) with the source of then-channel transistor connected to the drain of the p-channelone and the gates connected to each other. The output(drain of the p-channel transistor) is high whenever theinput (gate) is low and the other way round. The CMOSinverter is the basic building block of CMOS digitalcircuits.NMOS: n-channel CMOS.PMOS: p-channel CMOS.comparatorA comparator, or “voltage comparator” is an amplifierwhich is configured to compare the magnitude of thevoltages at its two inputs (+IN and −IN). Thecomparator outputs a “high” level if +IN > −IN, or a“low” level if +IN < −IN. A voltage comparator can beconsidered to be a 1-bit ADC (Analog-to-DigitalConverter).Currentusually abbreviated “i”, or “I”. Electric current is bydefinition the flow of electric charge. The SI unit of electriccurrent is the ampere (A, or simply “amp”), which is equalto a flow of one coulomb of charge per second. In semi-conductor devices, current is often a number of milliamps(mA). There are 1000 milliamps to an amp. By way ofanalogy, a large river represents a lot of current, a smallriver represents a small amount of current. The speed atwhich the river is flowing is analogous to voltage,which exerts a pressure to move the water. Power, isrelated to the pressure and the volume.DCshort for direct current, and sometimes written lowercase as“dc”. An example of direct current is the current suppliedby 1.5 volt batteries for common appliances, such asfor a flashlight.DiodeA two-terminal semiconductor device that allows current toflow in one direction only; an essential component used inpower supplies to convert ac into dc (a processcalled rectification)Duty-CycleThe ratio (or averaged ratio) between the time phase wherea signal is considered active and the time phase where thesignal is considered non-active.EEPROMshort for electrically erasable, programmable read onlymemory. EEPROMs have the advantage of being able toselectively erase any part of the chip without the needto erase the entire chip and without the need to removethe chip from the circuit. The minimum erase unit is 1 Byteand more typically a full Page. While an erase and rewriteof a location appears nearly instantaneous to the user, thewrite process is usually slightly slower than the readprocess; the chip can usually be read at full system speeds.EPROMshort for erasable, programmable read only memory.EPROM is a memory cell in which information (data) canbe erased and replaced with new information (data).FETshort for field effect transistor. The FET is a transistor thatrelies on an electric field to control the shape andhence the conductivity of a “channel” in a semiconductormaterial. FETs are sometimes used as voltage-controlledresistors. The terminals of FETs are called gate,drain and source.FlashFlash memory is a form of non-volatile memorymemory(EEPROM) that can be electrically erased andreprogrammed. Flash memory architecture allows multiplememory locations to be erased or written in oneprogramming operation.Frequencyusually abbreviated “F”. Frequency refers to the number oftimes per second that a signal (or wave) alternates (oroscillates), typically from positive to negative and backagain, and is expressed in Hertz (Hz). Sometimes,frequency is expressed in cycles per second (cps). 1 cps =1 Hz. Exemplary sound waves which a human ear can hearare from a few Hz up to about 20,000 Hz. The electro-magnetic wave corresponding to the visible color red has afrequency of 428,570 GHz (gigahertz).Kirchhoff'sKirchhoff's circuit laws are a pair of laws that deal with thelawconservation of charge and energy in electrical circuits, andwere first described in 1845 by Gustav Kirchhoff. Widelyused in electrical engineering, they are also calledKirchhoff's rules or simply Kirchhoff's laws.Kirchhoff's Current Law: also called Kirchhoff's first law,Kirchhoff's point rule, Kirchhoff's junction rule, andKirchhoff's first rule. This law states that at any point in anelectrical circuit where charge density is not changing intime, the sum of currents flowing towards that point isequal to the sum of currents flowing away from that point.Generally, in layman's terms, current out (Iout) equalscurrent in (Iin).Kirchhoff's Voltage Law: also called also calledKirchhoff's second law, Kirchhoff's loop rule, andKirchhoff's second rule. This law states that the directedsum of the electrical potential differences around a circuitmust be zero. (Otherwise, it would be possible to build aperpetual motion machine that passed a current in a circlearound the circuit.)Micro-A highly integrated chip that contains all of the componentscontrollerneeded--a central processing unit (CPU), random accessmemory (RAM), some form of read-only memory (ROM),input/output ports, and timers--to control a system.Unlike a general-purpose computer, which also includesall of these components, a microcontroller is designedfor a very specific task-to control a particular system. As aresult, the parts can be simplified and reduced, which cutsdown on costs.Micro-(1) A central processing unit (CPU) fabricated on one orprocessormore chips, containing the basic arithmetic, logic, andcontrol elements of a computer that are required forprocessing data; (2) An integrated circuit that acceptscoded instructions, executes the instructions received,and delivers signals that describe its internal status. Theinstructions may be entered or stored internally. Also called“MPU” (microprocessor unit). Widely used as controldevices for household appliances, business machines, toys,etc., as well as for microcomputers.Ohm' LawOhm's law expresses a simple mathematical relationshipbetween voltage (abbreviated “V”, or “E”) and current(abbreviated “I”), based on the resistance (abbreviated “R”)of a conductor through which the current is flowing.According to Ohms' law, E = I*R. To push a givencurrent through a conductor, when there is more resistance,more voltage is needed. At a given voltage, more resistancemeans that less current will flow through the conductor.RAMshort for random access memory. RAM refers to datastorage formats and equipment that allow the stored datato be accessed in any order - that is, at random, not just insequence. In contrast, other types of memory devices(such as magnetic tapes, disks, and drums) can accessdata on the storage medium only in a predetermined orderdue to constraints in their mechanical design.resistorusually abbreviated “R”. A resistor is a two-terminalelectrical or electronic component that resists (impedes) theflow of current, producing a voltage drop between itsterminals in accordance with Ohm's law (E = IR, orI equals E over R). The electrical resistance is equalto the voltage drop across the resistor divided by the currentthat is flowing through the resistor. Resistors are usedas part of electrical networks and electronic circuits.transistorusually abbreviated “Q”. The transistor is a solid statesemiconductor device which can be used for amplification,switching, voltage stabilization, signal modulation andmany other functions. It acts as a variable valve which,based on its input voltage, controls the current it drawsfrom a connected voltage source. Transistors are madeeither as separate components or as part of an integratedcircuit (IC). By way of analogy - the transistor functionslike a water tap. The water tap knob controls the flow ofthe water. In transistors the control tap is called the Basein BJT (Bipolar Junction Transistor), or Gate in FET (FieldEffect Transistor) or IGBT (Insulated Gate BipolarTransistor). Transistors control the flow of electroniccurrent.voltageusually abbreviated “V”, or lowercase “v”. Sometimesabbreviated “E”. Voltage is measurement of the electro-motive force in an electrical circuit or device expressedin volts. It is often taught that voltage can be thoughtof as being analogous to the pressure (rather than thevolume) of water in a waterline. Generally speaking,voltage is the electrical “pressure” that causescurrent to flow in a circuit.SI unitsThe SI system of units defines seven SI base units:fundamental physical units defined by an operationaldefinition, and other units which are derived from theseven base units, including:kilogram (kg), a fundamental unit of masssecond (s), a fundamental unit of timemeter, or metre (in), a fundamental unit of lengthampere (A), a fundamental unit of electrical currentkelvin (K), a fundamental unit of temperaturemole (mol), a fundamental unit of quantity of a substance(based on number of atoms, molecules, ions, electrons orparticles, depending on the substance)candela (cd), a fundamental unit luminous intensitydegrees Celsius (° C.), a derived unit of temperature.t ° C. = tK − 273.15farad (F), a derived unit of electrical capacitancehenry (H), a derived unit of inductancehertz (Hz), a derived unit of frequencyohm (Ω), a derived unit of electrical resistance, impedance,reactanceradian (rad), a derived unit of angle (there are 2π radiansin a circle)volt (V), a derived unit of electrical potential (electromotiveforce)watt (W), a derived unit of powerVoltageabbreviated v, or V. A voltage can be positive or negative(or zero). Usually, a negative voltage is preceded by aminus sign (−). Sometimes a positive voltage is precededby a plus sign (+), or no sign at all. A number of voltagesare relevant with regard to operating a memory cell, andare typically designated by the capital letter “V”,followed by another letter or letters.Some exemplary voltages for NVM (such as NROM)memory cells are:Vtshort for threshold voltageVsshort for source voltageVdshort for drain voltageVgshort for gate voltageVdsthe potential difference between source and drain(or drain and source)Vdpshort for drain potentialVbshort for bulk (or substrate) voltage. sometimeswritten VsubVbishort for built-in potential (n+ to p− typically ~1 V)Vblshort for bitline voltage. (the bitline may functionas source or drain)Vwlshort for wordline voltage (which typically isthe same as Vg)