Rush current prevention circuit

Abstract: A second circuit (4) to which back-up power is supplied, such as a RAM, is connected downstream of a first circuit (3) to which no back-up power is supplied, such as a CPU. A gate circuit fixs the output signal of the first circuit at a predetermined level when the output signal of the first circuit is at an indefinite level and prevents a penetration current from flowing through the second circuit (6).

Description: This invention relates to rush current prevention technology and, more particularly, to a technology for prohibiting the production of a penetration current in circuitry when an input signal is at an indefinite level. A system is widely known wherein back-up power is supplied to some circuit such as a memory when the power source is off. In such a system, back-uped circuits and non back-uped circuits coexist in the circuitry and an output signal from the non back-uped circuits goes to an indefinite level when the power source is turned on and off. Where the system is of a type in which an output from the non back-uped circuit is supplied to the back-uped, penetration, surge current or rush current flows through the back-uped circuit when the power source is turned on and off. This increases power consumption of the back-up battery and, hence, shortens a time period available for back-up. Furthermore, rapid changes in current flow decrease the back-up voltage, so that it is likely that a normal back-up voltage is not obtained. It is known that, when the input signal changes to an indefinite level, a large rush current flows through a CMOS transistor. Therefore, the above drawback becomes particularly critical where the back-uped circuit is formed of a CMOS transistor. Prior art document US-A-4831595 discloses a low voltage power down logic control circuit employing a diode and capacitor to develop a voltage from a main power supply which normally holds a transistor in an "off" state. Upon disconnection of the main supply, the transistor switches "on" and activates a second transistor. Upon activation, the second transistor produces an inhibit signal to tri-state memory control circuitry and thereby preserve the memory data during transition of the main supply voltage to a standby voltage provided by a battery. In this way latch-up effects caused by input voltages exceeding the supply voltage during power down of the main power supply are avoided. This invention has been accomplished with a view to eliminating the above drawback and has for its object to minimize the flow of penetration current through a circuit in which a back-up circuit and a non back-up circuit coexist. Another object of the invention is to minimize the flow of penetration current from a power supply trough circuits. To solve this object the present invention provides a system as specified in claim 1. It may happen that an output signal from the non back-uped circuit is rendered to be at an indefinite level after the power supply means is turned on or off. The above arrangement of the invention prevents signals of an indefinite level from being supplied to the circuit to which back-up power is brought in. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram of the arrangement of the system according to the first embodiment of the invention; Figs. 2A to 2D are timing charts of signals from various parts of the circuit shown in Fig. 1; Fig. 3 shows a circuit diagram showing an example of the arrangement of input and output buffers of the RAM controller shown in Fig. 1; Fig. 4 is a circuit diagram illustrating one example of the arrangement of the detection circuit shown in Fig. 1; and Fig. 5 is a block diagram showing another embodiment according to this invention. The system according to one embodiment of this invention will be described with reference to the accompanying drawings. First, the arrangement of the system will be discussed in connection with Fig. 1. A main power supply 1 is provided with aswitch SW and, when it is turned on, supplies operating power of a potential of approximately Vcc to each component of the system. The main power supply 1 is formed by an AC power source, a battery, etc. A backup battery 2 acts as a back-up power supply which brings backup power of a potential of about VBK to a memory, etc. The output terminals of the main power supply 1 and the back-up battery 2 are connected to power lines 11 via reverse current blocking diodes D1 and D2. The output voltage VCC of the main power source 1 is set slightly higher than the output voltage VBK of the back-up battery 2, so that power is fed to the power lines 11 mainly from the main power supply 1 when the switch SW is on, and mainly from the back-up battery 2 when the switch SW is off. The output terminal of the main power supply 1 is also connected to power lines 12. A CPU (central processing unit) 3, a ROM (read only memory) 4, an I/O port (input/output port) 5 and an address bus 14 are connected to the power lines 12. The address bus 14 transfers address data output from the CPU 3 to the ROM 4, I/O port 5, and to a RAM controller 6 to be described later. To the power lines 11 are connected the RAM controller 6 and a RAM 7. The RAM controller 6 is constantly operative by receiving power fed through the power lines 11. For example, the RAM controller 6 receives an address signal from the address bus 14 and makes access to the RAM 7 by supplying the address signals and control signals thereto, or generates an address signal by itself and supplies the address signals and control signals to the RAM 7 to effect refreshment thereof. The RAM 7 operates constantly upon feeding of power from the power lines 11 and stores data therein. The RAM controller 6 comprises input gates IB formed of NAND gates, control circuit 6A and output gates OB formed of inverters. For ease of understanding, only a single input gate IB is shown in Fig. 1. It should be understood, however, that the input gate IB is arranged in a number corresponding to the number of signals that are fed to the RAM controller 6 and the RAM 7 from the CPU 3, ROM 4, I/O port 5, etc. Also, a plurality of output gates are provided. Input signals are fed to one of the input terminals of each input gate IB and detection signals from a detection circuit to be described hereinafter are fed to the other input terminal of the input gate IB. Address data, control signals, etc. output by the RAM controller 6 are supplied to the RAM 7. The CPU 3, ROM 4, I/O port 5 and the address bus 14, already mentioned, are illustrative of circuits to which no back-up power is supplied. The above mentioned RAM controller 6 and RAM 7 are an example of circuits which are disposed downstream of the circuits to which no back-up power is supplied, and which operates upon receipt of back-up power and signals from circuits located upstream thereof. To the power lines 12 is connected the detection circuit 8 which detects the on/off state of the main power supply 1 and outputs a detection signal of a binary level, whose level changes rapidly. The detection signal is fed via a line 13 to the other of the input terminals of the input buffer IB. The function of the Fig. 1 circuit will be explained with reference to Figs. 2A to 2D. When the main power supply 1 is off, the output voltage VCC of the main power supply 1 is 0 V as shown in Fig. 2A, so that the CPU 3, ROM 4, I/O port 5, power supply on-off detection circuit 8 and the address bus 14 are inoperative. In contrast, to the RAM control circuit 6 and RAM 7 are, as shown in Fig. 2D, supplied a back-up current IBAT from the back-up battery 2 via the power lines 11. The back-up current IBAT causes the RAM control circuit 6 and RAM 7 to perform predetermined functions, such as, refreshment. When the main power supply 1 is turned on, the output voltage thereof gradually rises as shown in Fig. 2A. As a result, the non back-uped circuits, such as, the CPU 3 and the address bus 14, are rendered electrically unstable. In other words, the voltage of the input signal (voltage on the bus 14) of, for example, the RAM controller 6 is, as is apparent from Fig. 2B, put into an indefinite state, i.e., into an unstable state where the voltage is neither at a logic "1" level nor a logic "0" level. On the other hand, the detection circuit 8 starts functioning when the output voltage VCC of the main power supply 1 is still at a low level and, as shown in Fig. 2C, continues to output detection signals of a low level (ground level) till the voltage VCC reaches such a level as enables the CPU 3, bus 14, etc. to operate stably. The NAND gates forming the input gates IB are closed in response to the detection signal. The uncertain level signal from the address bus 14 is inhibited and prevented from being fed to the control circuit 6A. The above arrangement prevents penetration current from flowing through the control circuit 6A and RAM 7, so that the back-up current IBAT is maintained approximately at a constant level as shown in Fig. 2D. As the output voltage VCC of the main power supply 1 further increases, the CPU 3, bus 14, etc. starts normal functions and an input signal to the RAM control circuit 6 is also at a stable voltage level (of a logic 1 or logic 2). When the output voltage VCC subsequently reaches the level of a reference voltage, the detection circuit 8 rapidly changes the level of the detection signal into a high level as shown in Fig. 2C, whereby the NAND gates forming the input buffer IB are opened permitting the signal on the bus 14 to be inverted and the signal so inverted to be fed to the control circuit 6, which then controls the RAM 7 on the basis of the signal received. As the power supply voltage VCC rises, the current IBAT flowing from the back-up battery 2 to the power lines 11 decreases and terminates as shown in Fig. 2D. While the main power source 1 is on, the detection signal maintains its high level. The CPU 3 drives the bus 14 and, as shown in Fig. 2B, sends, for example, the address signal to the control circuit 6. The level of the address signal is substantially stable so that no penetration current flows into the control circuit 6A, RAM 7, etc. On the other hand, when the main power supply 1 is switched off, its output voltage VCC gradually drops as shown in Fig. 2A. When the voltage VCC decreases to a reference value before the CPU 3 and bus 14 start unstable operation, the detection circuit 8 outputs a detection signal of a low level, in response to which the input gates IB are closed and the address signal on the bus 14 ceases to be fed to the control circuit or chip 6A. Thereafter, the signal levels of output signal from the CPU 3 and the address signal on the address busses 14 are brought to unstable levels with decrease of the power supply voltage VCC. These unstable level signals are, however, blocked by the NAND gates. With a drop of the power supply voltage VCC, a supply of current IBAT from the back-up battery 2 to the power lines 11 takes place as shown in Fig. 2D, whereby the RAM controller 6 and RAM 7 continue to operate. On the other hand, circuits to which no back-up power is supplied stop operation. As has been described above, according to this embodiment, the non backed up circuits 3 and 14 to this embodiment are located upstream of the circuit 6A which is backed up by the back-up power supply 2. Further, owing to the provision of the input buffer IB, the indefinite-level signal resulting from turn-on and turn-off of the main power supply 1 is inhibited from being supplied to the back-uped circuit 6A, thus eliminating the known drawback that the back-up current IBAT is rapidly increased immediately after turn-on and turn-off of the power supply (or that surge current is produced). In other words, a rush current prevention circuit is obtained. One example of the arrangement of the input buffer IB will now be explained with reference to Fig. 3. Fig. 3 illustrates a specific arrangement of the RAM control circuit. As shown, the input buffer is formed of a first P-type MOS transistor 21, a first N-type MOS transistor 22, a second N-type MOS transistor 23 and a second P-type MOS transistor. The first P-type MOS transistor 21 is connected to the power lines 11 at one end of its current path. The first N-type MOS transistor 22 has a current path connected at one end to the other end of the current path of the first P-type MOS transistor 21. The second N-type MOS transistor 23 is connected at one end of its current path to the other end of the current path of the first N-type MOS transistor 22, and grounded at the other end of its current path. The second P-type MOS transistor 24 has a current path coupled at one end to one end of the current path of the first P-type MOS transistor 21 and at the other end to the other end of the current path of the second P-type MOS transistor 24. The first P-type MOS transistor 21 and the first N-type MOS transistor 22 constitute an inverter, and are supplied at respective gates with an input signal coming from the address bus 14. To the gates of the second N-type MOS transistor 23 and the second P-type MOS transistor 24 are supplied a detection signal. The current driving ability of the second P-type MOS transistor 24 and the second N-type MOS transistor 23 is greater than that of the first P-type MOS transistor 21 and the first N-type MOS transistor 22. Voltage at the other end of the current path of the first P-type MOS transistor 21 is supplied as an output of the buffer circuit IB to a circuit (control circuit 6A) located downstream thereof. The input buffer OB shown in Fig. 3 is an inverter circuit of a known type. The operation of the circuit shown in Fig. 3 will be explained. Upon receipt of a detection signal of a high level, the second P-type MOS transistor 24 turns off and the second N-type MOS transistor 23 turns on, so that the inverter formed of the transistors 21 and 22 turns on and outputs a signal from the address bus 14 in an inverted form. When a low level detection signal is supplied, the second P-type MOS transistor 24 turns on and the second N-type MOS transistor turns off, so that the output terminal of the input buffer IB is set at a high level, with the result that an input signal of an indefinite level is disabled. A specific arrangement of the detection circuit 8 will be described in conjunction with Fig. 4. In the figure, the power supply voltage VCC and ground voltage are applied to an operational amplifier 31. A capacitor 32 is connected between the output terminal of the operational amplifier 31 and ground potential. Between the output terminal of the operational amplifier 31 and the power input terminal thereof is connected a diode 33 for discharging the charge of the capacitor 32 at the power-off time, and a resistor 34. The output terminal of the operational amplifier is connected to inverters (HCl 4 ) 36 and 37 cascade connected to each other in two stages, via a resistor 35 for inhibiting the latch-up of a CMOS circuit. Between the output terminal of the inverter 37 and the ground is connected a resistor 38. The above circuit outputs a high level signal when the power voltage VCC exceeds a predetermined value, say, 4.5 V and outputs a low level signal when the voltage VCC becomes 4.5 V or less. Gate arrays, Schmitt trigger cells, or the like may be employed for the inverters 36 and 37. As has been described in detail, the input signal according to this embodiment is gated using a detection signal from the detection circuit 8. Accordingly, even if the input signal is at an indefinite level, a signal of an uncertain level is not inputted into the back-uped circuit. In other words, no penetration current flows through the back-uped circuit, thus affording back-up function by a battery over a long period of time and avoiding a back-up voltage drop due to a rapid increase in the back-up current. This invention is not limited to the embodiments described. For example, the arrangement of the input buffer IB is not limited to the one shown in Fig. 3. In Fig. 3, for instance, the transistors to which the input signal and detection signal are fed may be interchanged. In this case, it is preferred that the current driving ability of the transistors 21, 22 is set to be greater than that of the transistors 23, 24. The input gate IB need not be formed of a NAND gate. For example, a two-terminal OR gate may be used as the input buffer IB. In this case, a high level detection signal is supplied to the OR gate while the input signal remains at an uncertain level. Further, an optional circuit which is capable of fixing the input signal at a predetermined level while the level of the input signal stays uncertain may be used as the input buffer IB. This invention is not limited to computer devices. For example, the invention is for a wide range of circuitry in which both a back-uped circuit 51 and a non back-uped circuit 52 exist as shown in Fig. 5. During a period of unstability of the power-supply voltage, transmission of signals (output signals, control signals, data, addresses) from the non back-uped circuit 52 to the back-uped circuit 51 is inhibited by the inhibiting circuit 53. According to the preceding embodiment, diodes are used as a reverse current blocking circuits of the back-up battery 2. The invention is not limited thereto. For example, elements, such as FETs or transistors, may be used instead. The arrangement of the detection circuit is not limited to the one described in connection with the foregoing embodiments. For example, the detection signal may be generated in the main power supply. Further, it may be generated manually prior to turn-on and turn-off of the main power supply. Still alternatively, a transiendt contact or intermediate contact may be set up in the main power supply, so that the generation of a detection signal is accomplished by using a signal from such transient or intermediate contact. In a computer, etc. where a CPU is used exclusively for the control of power supply, the main power supply may be turned on or off after the level of detection signals has been set. Detection signals need not be generated by measuring the magnitude of the power supply voltage VCC. For example, turn-on and turn-off of the power supply switch may trigger calculating setting a predermined period of time to spend, to inhibit unstable-level signals from flowing through a back-uped circuit for such set period of time during which it is anticipated that the power supply voltage is at an unstable level.

Claims: A system comprising: a main power supply (1) for outputting a main power supply voltage; a back-up power supply (2) for outputting a back-up power supply voltage at least when said main power supply (1) is off; first circuit means (52) connected to said main power supply (1); second circuit means (51) connected to said back-up power supply (2); detection means (8) for generating an inhibit signal indicating that the output voltage (VCC) of said main power supply (1) is equal to or lower than a predetermined reference voltage, whereby an output signal of said first circuit means is at an indefinite level; and inhibiting means (6; 53) for inhibiting the output signal from said first circuit means (52) from being sent to said second circuit means (51), when said inhibit signal is generated; said inhibiting means comprising supply means (53) connected to said first circuit means (52) and said second circuit means (51), for supplying an output signal from said first circuit means (52) to said second circuit means (51), when said inhibit signal is not generated; characterized in that said supply means (53) includes fixing means (8, 53) for fixing said input signal for said second circuit means (51) at a predetermined level when said inhibit signal is generated and for releasing said input signal from said predetermined level when said input signal is not generated. The system according to claim 1, wherein said inhibit signal is generated after said power supply means (1) is turned on or turned off. The system according to claim 1 or 2, characterized in that said fixing means (8, 53) comprises: a gate circuit (IB) including a first input terminal which receives the input signal and a second input terminal which receives a control signal, a signal level of the output signal becoming a fixed level when a signal level of at least one signal is said fixed level; and voltage detection means (8) for outputting said control signal which sets an output of said gate circuit (IB) at a predetermined level when the output voltage (VCC) at said main power supply (1) is equal to or lower than a predetermined reference voltage. The system according to any one of claims 1 to 3, characterized in that said inhibiting means comprises a gate circuit comprising: a first MOS transistor (21) of a first conductivity type having a current path connected at one end to a first voltage; a first MOS transistor (22) of a second conductivity type having a current path connected at one end to the other end of the current path of said first MOS transistor (21) of the first conductivity type; a second MOS transistor (23) of the second conductivity type having a current path connected at one end to the other end of the current path of said first MOS transistor (22) of the second conductivity type, and connected at the other end to a second voltage; a second MOS transistor (24) of the first conductivity type having a current path connected between said one and the other end of the current path of said first MOS transistor (21) of the first conductivity type; gates of said first MOS transistor (21) of the first conductivity type and of said first MOS transistor (22) of the second conductivity type being connected to receive the input signal or a control signal; gates of said second MOS transistor (23) of the second conductivity type and of said second MOS transistor (24) of the first conductivity type being connected to receive the control signal or the input signal; and said fixing means further comprises: voltage detection means (8) for outputting said control signal, said control signal fixing an output from said gate circuit at the predetermined level when the output signal from said first circuit means (52) is at the indefinite level. The system according to claim 4, characterized in that said first voltage is an output voltage of said back-up power supply (2); said second voltage is ground voltage; and said first conductivity type is a P-type and said second conductivity type is an N-type. The system according to claim 4, further comprising means (D1, D2, 11) for selectively applying one of said first voltage and said back-up power supply voltage to said second circuit means (51). The system according to claim 1, characterized in that said second circuit means (51) is formed of a CMOS circuit. The system according to claim 3, characterized in that said gate circuit includes NAND gates.

Inventors: TAKENAKA TSUTOMU, TAKENAKA, TSUTOMU, TAKENAKA, TSUTOMU, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: G06F1
IPC codes: G06F1, H03K19, G11C11, H03K17