Booster circuit

Abstract: A booster circuit comprises first to third MOS-­ FETs, and a capacitance element. In the first MOS-FET, a gate is connected to a first signal terminal, a drain is connected to an output terminal, and a source is connected to a first power supply. In the second MOS-­ FET, a gate is connected to a predetermined point, a drain is connected to a signal terminal, to which a first signal being an inverted signal of a signal applied to the first signal terminal is applied, and a source is connected to the predetermined point. In the third MOS-FET, a gates is connected to a second power supply, a drain is connected to a second signal terminal, and a source is connected to the predetermined point. The capacitance element is connected at one terminal to a signal terminal, to which a second signal being an inverted signal of a signal applied to the second signal terminal is applied, and at the other terminal to the output terminal. In operation, a predetermined voltage is obtained with a high speed at the output terminal.

Description: FIELD OF THE INVENTION This invention relates to a booster circuit, and more particularly to, a booster circuit operating with a high speed to provide a high voltage. BACKGROUND OF THE INVENTION One type of a conventional booster circuit is shown in Fig. 1, and comprises first and second N channel MOS-FETs (called "N₁₁" and "N₁₂" hereinafter), an inverter IN₁₁, and a capacitance element C. The MOS-FET N₁ has a drain connected to an output terminal OUT, a gate connected to a first signal terminal T₁₁, and a source connected to a first power supply voltage of a ground potential GND. The MOS-FET N₂ has a gate connected through the inverter IN₁₁ to the first signal terminal T₁₁, a drain connected to a second power supply terminal of a voltage Vcc and a source connected to the output terminal OUT. The capacitance element C is connected at one terminal to a second signal terminal T₁₂ and at the other terminal to the output terminal OUT. In operation, when a first signal of "high" is applied to the first signal terminal T₁₁, the N₁₁ is "on" (conductive), while the N₁₂ is "off" (non-conductive), so that an output signal V OUT of "low" is obtained at the output terminal OUT, as shown in Fig. 2. Next, when the first signal becomes "low", the N₁₁ is turned off, while the N₁₂ is turned on, so that the output signal V OUT of "Vcc-V TN " is obtained at the output terminal, as shown in Fig. 2. The voltage "V TN " is a threshold voltage of an N channel MOS-FET. When the second signal becomes "high", the output signal V OUT is increased at the output terminal OUT to "2Vcc-V TN " due to the presence of the capacitance element C. In the conventional booster circuit, the N₁₂ and the capacitance element C may be replaced by third to fifth N channel MOS-FETs (called "N₁₃", "N₁₄" and N₁₅" hereinafter) and first and second capacitance elements C₁₁ and C₁₂, as shown in Fig. 3. The MOS-FET N₁₃ has a gate connected to the inverter IN₁₁ which is connected to the first signal terminal T₁₁, a drain connected to the second power supply terminal of the voltage Vcc, and a source connected to a first connecting point 11 for connecting a drain and a gate of the N₁₄ and the first capacitance element C₁₁. The source of N₁₄ is connected to a second connecting point 12 for connecting a drain and a gate of the N₁₅ and the second capacitance element C₁₂, and the source of N₁₅ is connected to the output terminal OUT. In addition, the first capacitance element C₁₁ is connected at one terminal to a first clock signal terminal , and the second capacitance element C₁₂ is connected at one terminal to a second clock signal terminal  . Consequently, the third to fifth N channel MOS-FETs (N₁₃ to N₁₅), and the first and second capacitance elements C₁₁ and C₁₂ thus connected provide a charge pump circuit. A clock signal applied to the second clock signal terminal  is an inverted signal of a clock signal applied to the first clock signal terminal . In operation, a voltage of the first connecting point 11 is charged up to "Vcc-V TN " by the N₁₃, and this voltage is pushed up to "2Vcc-V TN " by the first capacitance element C₁₁. The pushed-up voltage of "2Vcc-V TN " is charged up to "2Vcc-2V TN " at the second connecting point 12 by the N₁₄. This charge-up voltage of "2Vcc-2V TN " is further pushed up to "3Vcc-2V TN " by the second capacitance element C₁₂, and this voltage of "3Vcc-2V TN " is charged up to "3Vcc-3V TN " at the output terminal OUT by the N₁₅. In the second conventional booster circuit, the charge pump circuit includes a two-stage structure consisting of the N₁₄ and the first capacitance element C₁₁, and the N₁₅ and the second capacitance element C₁₂. As understood from the above, if the charge pump circuit includes an N-stage structure, where N is an integer, a voltage is increased to (N+1)·(Vcc-V TN ). However, the first type of the conventional booster circuit has a disadvantage in that the maximum voltage is limited to a voltage of "2Vcc-V TN " at the output terminal OUT, because the charge-up voltage of the N₁₂ is limited to a voltage of "Vcc-V TN ", although a voltage of the output terminal is increased simultaneously, when the signal of the second signal terminal T₁₂ becomes "high". On the other hand, the second type of the conventional booster circuit can boost the output terminal OUT up to a voltage twice or more as large as Vcc. However, this circuit has a disadvantage in that it takes a considerable time to obtain a predetermined voltage at the output teminal OUT, because the clock signals  and  has to be employed. For instance, where a period of the clock signals is 1 µsec, it takes 10 µsec equivalent to ten periods of the clock signals to provide the predetermined voltage at the output terminal OUT. Increasing the number of stages in the charge pump circuit to make an output voltage higher makes the time longer proportionally. SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a booster circuit operating with a high speed in providing a predetermined voltage. It is a further object of this invention to provide a booster circuit in which a high output voltage is obtained. According to this invention, a booster circuit, comprises a first field effect transistor having a gate connected to a first signal terminal supplied with a first signal and a source-drain path connected between an output terminal and a first power supply, a second field effect transistor having a gate connected to a first node and a source-drain path connected between said output terminal and a second node, the second node being supplied with an inverted signal of the first signal, a third field effect transistor having a gate connected to a second power supply and a source-drain path connected between a second signal terminal and the first node, the second signal terminal being supplied with a second signal that is a delayed version of the first signal, and a capacitance element connected between a third node and the output terminal, the third node being supplied with a signal being inverted relative to the second signal. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be described in more detail in conjunction with appended drawings, wherein: Fig. 1 is a circuitry diagram showing the first type of the conventional booster circuit, Fig. 2 is a timing chart showing operation in the first type of the conventional booster circuit, Fig. 3 is a circuitry diagram showing the second type of the conventional booster circuit, Fig. 4 is a circuitry diagram showing a booster circuit in a first preferred embodiment according to the invention, Fig. 5 is a timing chart showing operation in the first preferred embodiment, and Fig. 6 to 8 are circuitry diagrams showing booster circuits in second to fourth preferred embodiments according to the invention, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS A booster circuit in the first preferred embodiment according to the invention will be explained in Fig. 4, wherein it comprises first to third N channel MOS-FETs N₁, N₂ and N₃, first and second inverters IN₁ and IN₂, and a capacitance element C. In the booster circuit, the N₁ is connected at a drain to an output terminal OUT, at a gate to a first signal terminal T₁, and at a source to a first power supply of a ground potential GND, the N₂ is connected at a drain through the first inverter IN₁ to the first signal terminal T₁ and at a source to the output terminal OUT, the N₃ is connected at a drain to a second signal terminal T₂, at a source to a gate of the N₂ to provide a connecting point 1, and at a gate to a second power supply of a voltage Vcc, and the capacitance element C is connected at one terminal through the second inverter IN₂ to the second signal terminal T₂, and at the other terminal to the output terminal OUT. The MOS-FET N₂ has a parasitic capacitance C GD between the drain and gate thereof and another parasitic capacitance C GS between the source and gate thereof. Each of the inverters IN₁ and IN₂ receives Vcc and GND as operating voltages and thus "high" and "low" outputs thereof are substantially equal to levels of Vcc and GND, respectively. Operation of this booster circuit will be explained in conjunction with Fig. 5. When the signals at the first and second signal terminals T₁ and T₂ are "high", the N₁ is "on" (conductive), so that a voltage V OUT at the output terminal OUT is the ground potential. At this time, the connecting point 1 is charged up to a level of "Vcc-V TN " by the N₃. VTN represents threshold voltages of the MOS-FETs N₁, N₂ and N₃. When the signal at the first signal terminal T₁ becomes "low", the N₁ is turned off and the inverter IN₁ provides an output of "high". Consequently, the voltage at the connecting point 1 is boosted up to the level of "2Vcc-V T " by the parasitic capacitance C GD . The MOS-FET N₃ is reverse-biased to be in off state. Since the voltage at the point 1 is greater than a level of "Vcc+V TN ", the output terminal OUT is charged up to a level of "Vcc" independently of the threshold value "V TN ". The capacitor C is also charged up to Vcc. Then, the signal at the second signal terminal T₂ is inverted to "low", so that the N₃ is turned on to discharge the connecting point 1 to GND. The N₂is thereby turned off. On the other hand, the inverter IN₂ outputs "high" (Vcc) , so that the output voltage V OUT is boosted from "Vcc" to "2vcc" by the capacitance element C. A time period t₁₂, during which the signals at the first and second signal terminals T₁ and T₂ are in the low and high levels, respectively, is required to increase the output voltage from the ground potential GND to the second power supply voltage Vcc. This time period t₁₂ can be as short as 10 to 20 n sec, however. As understood from this explanation, the voltage V OUT is boosted to the level of "2Vcc" in a very short time period of 10 to 20 n sec after the low state of the signal applied to the first signal terminal T₁. When the signal at the first signal terminal T₁ is returned to "high" level, the N₁ is turned on to change the output voltage V OUT to the ground potential. When the signal at the second signal terminal T₂ is thereafter changed to the high level, the connecting point 1 is charged again up to a level of "Vcc-V TN " by the N₃. The inverter IN₂ produces the low output. In this state, the output voltage V OUT is lowered to a level less than the ground potential for the presence of the capacitance element C. The lowered voltage is represented by "V US " in Fig. 5 and is defined as follows, because the N₁ is turned on for this period. V US = Vcc x Z₁ / (Z₁ + Zc), where Z₁ is an impedance of the N₁, and Zc is that of the capacitance element C. By setting a relation between values of Z₁ and Zc to be "Z₁«Zc" in the above equation, the lowered voltage V US can be suppressed as small as expected. Thus, the booster circuit of Fig. 1 can boost the output terminal OUT(V OUT ) up to the level of 2Vcc at a high speed without employing clock signals () shown in Fig. 3. Fig. 6 shows a booster circuit in the second preferred embodiment, wherein like parts are indicated by like reference numerals and symbols as used in Fig. 4 to omit further discription thereof. In this embodiment, a delay circuit 20 is provided between the first and second signal terminals T₁ and T₂. If it is designed that the delay circuit 20 has a delay time equal to the time t₁₂ as shown in Fig. 5, the same operation as in the first, preferred embodiment is carried out in the second preferred embodiment. Even more, it provides an additional advantage in that the second signal is not necessary to be applied to the second input terminal T₂. Fig. 7 shows a booster circuit in the third preferred embodiment, wherein like parts are indicated by like reference numerals and symbols as used in Fig. 4. A NOR circuit NOR 1 is provided in place of the second inverter IN₂ which is connected at a first input terminal to the first signal terminal T₁, at a second input terminal to the drain of the N₃ and the second signal terminal T₂, and at an output terminal to the capacitance element C. The operation of this booster circuit is the same as that of the circuit of Fig.4, until the voltage at the output terminal OUT is boosted up to 2Vcc. When the signal at the first signal terminal T₁ is changed to "high", the N₁ is turned on to discharge the output terminal OUT. Simultaneously, the output signal of the NOR 1 is changed from "high" (Vcc) to "low" (the ground potential) to discharge the capacitor C. Thus, the capacitor C is applied with the ground voltage at both ends thereof. Therefore, if it is assumed that a timing, at which the output signal of the NOR 1 is changed from "high" to "low", is t D , it is avoided that the voltage is lowered to a level less than the ground potential due to the presence of the capacitance element C at the output terminal OUT by setting a current flowing from the output terminal OUT through the N₁ to the first power supply of the ground potential GND to be less than a value I as defined below. I = Cc x Vcc / t D , where Cc is a capacitance value of the capacitance element C. This is a particular advantage in the third preferred embodiment. Fig. 8 shows a booster circuit in the fourth preferred embodiment, wherein like parts are indicated by like reference numerals and symbols as used in Fig. 4. In this embodiment, a fourth N channel MOS-FET N₄ is additionally provided to be connected at a drain and a gate to the output terminal OUT, and at a source to the second power supply of the voltage Vcc. In operation, when the voltage at the output terminal OUT is increased to a level greater than "Vcc+V TN ", the N₄ is turned on to clamp the output voltage to be the level of "Vcc+V TN ". This is an advantage in the fourth preferred embodiment in that the output voltage is avoided to be too high, when a power supply voltage is high. As a matter of course, two preferred embodiments may be combined to provide the same advantage among the second to fourth preferred embodiments. Further, the capacitance element may be replaced by a gate capacitance of a MOS-FET, a capacitance of a three layer structure including a polycrystalsilicon, oxidation film, and a polycrystalsilicon, etc.

Claims: A booster circuit, comprising: a first field effect transistor (N₁) having a gate connected to a first signal terminal (T₁) supplied with a first signal and a source-drain path connected between an output terminal (OUT) and a first power supply (GND); a second field effect transistor (N₂) having a gate connected to a first node (1) and a source-drain path connected between said output terminal and a second node; first means (IN₁) for supplying said second node with an inverted signal of said first signal; a third field effect transistor (N₃) having a gate connected to a second power supply (V cc ) and a source-drain path connected between a second signal terminal (T₂) and said first node, said second signal terminal being supplied with a second signal being a delayed version of said first signal; a capacitance element (C) connected between said output terminal and a third node; and second means (IN₂, NOR₁) for supplying said third node with a third signal being inverted relative to said second signal. A booster circuit, according to claim 1, further comprising: a delay circuit (20) inserted between said first and second signal terminals, said delay circuit delaying said first signal to produce said second signal. A booster circuit, according to claim 1, wherein: said second means comprises an inverter (IN₂) circuit for producing an inverted signal of said second signal as said third signal. A booster circuit, according to claim 1, wherein: said second means comprises a NOR circuit (NOR₁) having a first input terminal connected to said first signal terminal and a second input terminal connected to said second signal terminal. A booster circuit, according to claim 1, further comprising: a fourth field effect transistor (N₄) having a gate connected to said output terminal and a source-drain path connected between said second power supply and said output terminal.

Inventors: KOBATAKE HIROYUKI C O NEC CORP, KOBATAKE, HIROYUKI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: H03K17
IPC codes: H02M3, H03K19, H03K5