FEEDBACK CIRCUIT FOR CMOS HIGH VOLTAGE GENERATOR TO PROGRAM (E)EPROM-MEMORY CELLS

Abstract: Feedback circuit for a high voltage generator comprising several voltage multiplying stages connected in series, wherein an oscillator generates at least two clock pulses being 180 DEG out of phase to one another, controlling alternately successive voltage multiplying stages to provide a high voltage pulse at the output of the high voltage generator, the high voltage output being connected to the feedback circuit generating a control signal supplied to the oscillator, so that the two clock pulses are modified in dependence on the high voltage output voltage, wherein the feedback circuit comprises a high voltage feedback circuit provided with a capacitive input stage (CP, CR), the output signal (VCTRLHV) of the high voltage feedback circuit controlling the current of a controlled current source, and at least the oscillator generating the clock pulses receives said current as control signal and in dependence thereon controls the frequency of the clock pulses.

Description: Feed-back circuit for CMOS high voltage generator to program (E)EPROM-memory cells. The invention relates to a high voltage feed-back circuit for a high voltage generator comprising several voltage multiplying stages connected in series, wherein an oscillator generates at least two clock pulses being 180° out of phase to one another, con¬ trolling alternately successive voltage multiplying stages to pro¬ vide a high voltage pulse at the output of the high voltage gener- ator, the high voltage output being connected to the high voltage feed-back circuit comprising a capacitive input stage and gener¬ ating a control signal supplied to the oscillator, so that the oscillator output is modified in dependence on the high voltage output voltage, said capacitive input stage comprising at least a first capacitor receiving at one terminal the high voltage output and its other terminal being connected to one terminal of a second capacitor. Such a high voltage feed-back circuit is known from EP-A- 0.323.156. Said known feed-back circuit is applied to a high volt- age generator comprising a series connection of voltage multipliers being alternately controlled by clock signals Φ and ϊ. The output voltage of this high voltage generator is capacitively measured and compared with a reference voltage by a comparator. The output sig¬ nal of the comparator is supplied to a ring oscillator generating the clock signals Φ and ϊ. When the measured high voltage exceeds a certain threshold level the output of the comparator becomes low and as a consequence the ring oscillator stops oscillating and, therefore, the generation of clock pulses is stopped. When the high voltage level decreases below said threshold level the output of the comparator becomes high restarting the ring oscillator and the generation of clock pulses. The high voltage feed-back circuit as known shows many dis¬ advantages, the most important being discontinuities in the deriva¬ tive of the high voltage output of the high voltage generator to which the feed-back circuit is applied. This is caused by the application of the comparator in the feed-back circuit, the output of the comparator being only capable of starting or stopping the ring oscillator and, therefore, the multiplying stages in the high voltage generator suffer from sudden stops or starts in the receipt of controlling clock pulses. Such discontinuities in the derivative of the high voltage output reduces the lifetime of EEPROM cells. Moreover, the comparator in the known feed-back circuit must be rather fast. Otherwise the programming voltage will show too large overshoots and undershoots, which will also reduce the lifetime of the EEPROM cells as well as of the high voltage generator itself, because of break-down effects. However, a rather fast comparator . consumes substantial power. Furthermore, in EP-A-0.323.156 the reference voltage being supplied to the comparator is not constant, but is set to a low value before starting the generation of the programming pulse. During the generation of the programming pulse this reference volt¬ age is increased proportional to the desired increase of the pro- gramming pulse. A complex network of capacitors and control cir¬ cuitry is required to realise the slow increase of the reference voltage. Another high voltage feed-back circuit used in a high voltage generator is known from US-A-4.527.180. The output voltage of the high voltage generator is capacitively measured to generate a feed¬ back signal. The feed-back signal is supplied to one input of an operational amplifier, the other input of which receives a refer¬ ence voltage. The output of the operational amplifier, being used as an comparator, supplies a driving signal to a driver stage in the high voltage generator. The reference voltage is not constant, but is set to a low value before the generation of a programming pulse by the high voltage generator. During the generation of the programming pulse the reference voltage is slowly increased propor¬ tional to the desired increase of the programming pulse. To realize a slow rise-time of the programming pulse in this way a rather com¬ plex circuitry is used. Moreover, an RC network is used to realize said slow rise-time. However, the high-oh ic resistor as used in¬ troduces an inaccuracy not desired. The object of the present invention is to overcome the prob- lems associated with the prior art as mentioned above. Therefore, the feed-back circuit according to the invention is characterized in that the other terminal of the second capacitor is connected both to a point of common voltage through a first cur- rent source and to a first reference voltage through a first switch controlled by a control signal , the connecting point of.the first and second capacitor being connected to a second reference voltage through a second switch controlled by the control signal, which connecting point provides the output signalof the high voltage feed-back circuit, which control signal only then brings the first and second switch into a conducting state when no programming pulse is to be generated. In the feed-back circuit no comparator is used. An analog control circuit is used instead, by means of which the generation of discontinuities in the derivative of the output voltage of the high voltage generator can be avoided, thereby significantly im¬ proving the lifetime of EEPROM cells to be programmed. Besides, power consumption is reduced. No complex network of capacitors and control circuitry is required to realize a slow rise-time of the output of the high voltage generator. Here, a simple current source connected to the series connection of the first and second capa¬ citors suffices. In a preferred embodiment the junction of the first and second capacitor is connected to an amplifying unit, the output of which provides the output signal of the high voltage feed-back circuit. The amplifying unit preferably comprises an operational amplifier the output of which provides the output signal and is fed back to its inverting input through a third capacitor, which in- verting input is also connected to the point of common voltage through a fourth capacitor, a third switch also controlled by the control signal being arranged parallel to the third capacitor and the non-inverting input being connected to the junction of the first and second capacitors. So, instead of the comparator of the prior art, an operational amplifier is used in a analog regulating circuit. It is, therefore, easily possible to amplify the control voltage, i.e. the voltage at the junction of the first and second capacitors, in a linear way. Furthermore the voltages at the inputs of the operational amplifier are almost constant which simplifies the common mode input range requirement of the regulator. The invention will be hereinafter explained by reference to a drawing. In the drawing figure 1 shows an embodiment of a voltage multiplying circuit in which the feed-back circuit according to the invention may be applied; figure 2 shows a first embodiment of a high voltage feed-back circuit having a capacitive input stage; figure 3 shows a second embodiment of a high voltage feed¬ back circuit having a capacitive input stage; figure 4 shows a third embodiment of a high voltage feed-back circuit having a capacitive input stage; figure 5 shows a representation of a high voltage pulse cap- able to program a (E)EPRO -memory cell; figure 6 shows a controlled current source being able to generate a control current to an oscillator; figure 7 shows a realization of the arrangement of figure 6 in CMOS-technology. In figure 1 a possible application of the feed-back circuit according to the invention is shown. The figure shows a prior art voltage multiplier consisting of several charge pumps connected in series. Each charge pump comprises a diode Di and a capacitor Ci+1 connected to its cathode. At its other side each capacitor receives a clock pulse (Φ1 or Φ2) generated by an oscillator 3. The oscil¬ lator 3 generates two clock pulses being 180° out of phase to one another and being supplied alternately to the successive capacitors Ci. In this prior art voltage multiplier in each stage a voltage increase being at most equal to the step magnitude of the clock pulse less the threshold voltage of the diodes occurs. The diodes DO ... Dn may be MOS-transistors connected as diodes. In the arrangement according to figure 1 the last capacitor Cn is not controlled by one of both clock pulses Φ1 or Φ2, but by a control current 11. This provides the possibility to reduce spikes on the high voltage output during switching of the clock pulses Φ1 and Φ2. Because the principle of current control of the last capa¬ citor is described in the copending Dutch patent application 9200056 here no further explanation will be given. Important to the present invention is that the amplifier A controls the magnitude of the current 11 and, therefore, among other things controls the oscillator 3, by means of which by modifying the frequency of the clock pulses Φ1 and Φ2 the rise time of the slope of the high volt¬ age pulse VPP is determined. Reference is made to figure 5 which shows an example of the wave-form of a programming pulse VPP occur- ' ring at the high voltage output. Application of the feed-back circuit according to the inven¬ tion is, however, not limited to the arrangement of figure 1; the feed-back circuit may also be applied in such cases where only a control current is supplied to the oscillator, by means of which the frequency of the clock pulse Φ1 and Φ2 is controlled and where¬ in the last capacitor of the voltage multiplying circuit also receives one of both clock pulses Φ1 and Φ2, and consequently the output of the high voltage generator is not current controlled. Figure 2 shows a high voltage feed-back circuit having a capacitive input stage. The high voltage input HVIN is supplied to a capacitor CP connected in series with a capacitor CR. The con¬ necting point of both capacitors is connected to a reference volt- age Vref2 through a switch controlled by a signal RS . RST is a pulse-shaped signal having the value "1" (high) when no high volt¬ age pulse is to be generated and having the value "0" (low) when there is to be generated a programming pulse. The other terminal of the capacitor CR is connected both to a reference voltage Vrefl through a switch also controlled by RST and to a point of common voltage VSS (ground) through a current source I dlβch . As long as RST = 1 both controlled switches are closed and the output voltage of the high voltage feed-back circuit VCTRLHV = Vref2. Vref2, therefore, determines the zero-level of the output of the feed-back circuit. The capacitors CP and CR, respectively, are then charged to a voltage HVIN βt -Vref2 and Vrefl-Vref2, respective¬ ly. Here, HVIN Bt is the starting level of the programming pulse VPP at the high voltage generator (see figure 5). Mostly HVIN βt = VDD, where VDD is the positive supply voltage. The output voltage VCTRLHV of the high voltage feed-back cir¬ cuit, for example by means of an arrangement as shown in figure 6, is converted to a control current I 0 . c controlling the oscillator and controlling the frequency of the clock pulses Φ1 and Φ2. The voltage VCTRLHV, therefore, is compared to the reference voltage Vref2. In dependence on the result thereof part of the current l mΛX (see figure 6) is led away and, therefore, the current I OMC . is con¬ trolled. Figure 7 shows an embodiment of the arrangement of figure 6 in CMOS-technology. In the arrangement of figure 7 the current ϊ ∞ - nn , is generated by means of a MOS-transistor M1, the gate elec¬ trode of which receives a predetermined voltage VB1. The current ^ • . ^ s partly led away from the output current I ββ< _ through an other MOS-transistor M2, the gate electrode of which receives the voltage VCTRLHV. In this situation the threshold voltage of M2 has to be equal to Vref2. Then, I ββc then is controlled by the magni¬ tude of VCTRLHV when it exceeds the magnitude Vref2. When a programming pulse has to be generated RST becomes zero and both switches controlled by RST in figure 2 are opened. Consequently the magnitude of the voltage VR decreases, in depend¬ ence on the magnitude of I AΛ . ma*Λ * The magnitude of VCTRLHV, there¬ fore, will only then retain the same value as the rising edge of the voltage HVIN (= VPP) rises with a value of CR/CP times the falling edge of VR. If VPP has a rise time of t rl . β then: I.*, . -. * = Vrefl * (CR/t =iM ) (1) At the end of t__ iM VPP will reach a maximum value VPP mIljc . This value VPP...-. is determined by: VPP..... = (CR/CP) * Vrefl + HVIN -t (2) The magnitude of Vrefl, therefore, determines the maximum value of the programming pulse. Figure 3 shows a preferred embodiment of the high voltage feed-back circuit according to the invention. The capacitive input stage is identical to the one of figure 2. The output of the capa¬ citive input stage, however, is amplified to generate the signal VCTRLHV. This amplification reduces the inaccuracy of the circuit caused by a (small) difference between Vref2 and the real threshold voltage of transistor M2 from figure 7. It is observed that the entire amplifying stage is, strictly spoken, not necessary when VCTRLHV is only loaded by a (small) capacitive load, as would be the case if the load would be transistor M2 from figure 7. The amplifying stage is exclusively utilized in the circuit to enhance the accuracy of the controller. The amplifier comprises an operational amplifier the output of which is fed back to its inverting input through a capacitor C1 , said inverting input being also connected to ground (VSS) through a capacitor C2. A switch controlled by RST is connected parallel to C1. Before a programming pulse has to be generated RST = 1 and C1 is discharged. Consequently, as in the circuit according to figure 2, then VCTRLHV = Vref2. When a programming pulse has to be gener¬ ated all switches controlled by RST are opened and: VCTRLHV = (C2/C1) * VP (3) where VP is the output voltage of the capacitive input stage. It is observed that a programming pulse generally lasts no longer than 20 ms. Therefore, the drift of the voltage VP at the non-inverting in¬ put of the operational amplifier A, when the programming pulse has reached his maximum value VPP mβ>c , is small enough not to interfere with the operation of the feed-back circuit. Consequently, the voltages at the capacitive junctions may be properly defined before starting a programming pulse. The voltages at the inputs of operational amplifier A are both about equal to VCTRLHV. VCTRLHV is set to the reference volt- age Vref2 before the start of the generation of the programming pulse by the high voltage generator. During the whole period of generation of the programming pulse the voltages at the inputs of the regulator are constant, i.e. equal to Vref2. This is possible, because capacitor CR is not directly connected to ground but to Vrefl just before the generation of the programming pulse, whereafter it is discharged slowly to ground by the current source I dllιeh , Any voltage can be chosen for Vref2, for example the thres¬ hold voltage of an NMOST. Figures 6 and 7 show how this can be used to realize a very simple controlled output current to drive a cur- rent controlled oscillator, while in copending Dutch patent appli¬ cation 9200056 it is described how this technique can be extended to get even the output current of the charge pump 3 under control, resulting in an extreme smooth and accurate programming voltage. This significantly increases the life time of the EEPROM cells to be programmed and that of the high voltage generator itself. More¬ over, the control voltage present at the junction of the capacitors CP, CR can be easily amplified in a linear way. At last, since the voltages at the inputs of the operational amplifier A are almost constant, the common mode input range requirement of the regulator is simplified. The circuit according to figure 1, which combines the present feed-back circuits with the control circuit of copend- ing Dutch patent application 9200056 dissipates extremely low power. It will be clear that the capacitors C1 and C2 in the ampli¬ fier part may be substituted by resistors R1 and R2, as shown in figure 4. The switch controlled by RST between the output and the inverting input of the operational amplifier may, then, be omitted.

Claims: Claims 1. High voltage feed-back circuit for a high voltage gener¬ ator comprising several voltage multiplying stages connected in series, wherein an oscillator generates at least two clock pulses being 180° out of phase to one another, controlling alternately successive voltage multiplying stages to provide a high voltage pulse at the output of the high voltage generator, the high voltage output being connected to the high voltage feed-back circuit com- prising a capacitive input stage (CP, CR) and generating a control signal supplied to the oscillator, so that the oscillator output is modified in dependence on the high voltage output voltage, said capacitive input stage (CP, CR) comprising at least a first capa¬ citor (CP) receiving at one terminal the high voltage output and its other terminal being connected to one terminal of a second capacitor (CR), characterized in that the other terminal of the second capacitor (CR) is connected both to a point of common volt¬ age (VSS) through a first current source (I dl , ch ) and to a first reference voltage (Vrefl ) through a first switch controlled by a control signal (RST), the junction of the first and second capacitor being connected to a second reference voltage (Vref2) through a second switch controlled by the control signal (RST), which junction provides the output signal (VCTRLHV) of the high voltage feed-back circuit, which control signal (RST) only then brings the first and second switch into a conducting state when no programming pulse is to be generated. 2. High voltage feed-back circuit according to claim 1, wherein said junction (VP) of the first and second capacitor (CP, CR) is connected to an amplifying unit, the output of which pro¬ vides the output signal (VCTRLHV) of the high voltage feed-back circuit. 3. High voltage feed-back circuit according to claim 2, wherein the amplifying unit comprises an operational amplifier (A) the output of which provides the output signal (VCTRLHV) and is fed back to its inverting input through a third capacitor (C1), which inverting input is also connected to the point of common voltage (VSS) through a fourth capacitor (C2), a third switch also control¬ led by the control signal (RST) being arranged parallel to the third capacitor and the non-inverting input being connected to the junction (VP) of the first and second capacitor (CP, CR) . 4. High voltage feed-back circuit according to any of the preceding claims, wherein the output signal (VCTRLHV) of the high voltage feed-back circuit controls the current (I oβe _) of a con¬ trolled current source, and at least the oscillator (3) generating the clock pulses receives said current as control signal and in dependence thereon controls the frequency of the clock pulses (Φ1, Φ2), the controlled current source comprising a current source of a predetermined current I m x ) > which predetermined current (I mΛX ) is divided into two current branches, the first one conducting the current (I om< -.) of the controlled current source and the second one conducting a current determined by the output signal (VCTRLHV) of the high voltage feed-back circuit. 5. High voltage feed-back circuit according to claim 4, wherein said controlled current source comprises a series connec¬ tion of a first MOS-transistor M1 , the gate electrode of which receives a predetermined voltage (VB1 ) and a second MOS-transistor M2, the gate electrode of which receives the output signal (VCTRLHV) of the high voltage feed-back circuit, and the junction of both MOS-transistors is connected to said first current branch conducting said current (I oβc .) . 6. High voltage generator comprising a high voltage feed-back circuit according to claim 4 or 5, wherein the capacitor (Cn) of the last voltage multiplying stage receives a control current (11 ) determined by the feed-back circuit instead of one of both clock pulses. *****

Inventors: SEESINK PETRUS HENDRIKUS, SEESINK, PETRUS, HENDRIKUS
Applicants: SIERRA SEMICONDUCTOR BV, SIERRA SEMICONDUCTOR B.V.
Application year: 1992
Main classification: H02M3
IPC codes: G11C16, G11C17, H02M3