Power on reset circuit with auto turn off

Abstract: The present invention relates to an electronic power on reset circuit (1) of the type comprising a comparator (2) having at least two inputs and one output (A) for receiving a first reference signal from a generator block (11) and a second signal proportional to a supply voltage (Vdd) from a divider block (12) and for producing at output an initialization signal (INTPOR). Advantageously the output (A) is connected to a third turn off enablement input (10) of the comparator (2) through the series of an inverter pair (I1,I2). The generator block (11) and the divider block (12) also comprise respective turn off enablement inputs (15,13) connected downstream of the inverter pair (I1,I2).

Description: Field of application The present invention relates to an electronic power on reset circuit with auto turn-off device. Specifically but not exclusively the present invention concerns a circuit of the above mentioned type and comprising a comparator having at least two inputs and an output for receiving a first reference signal from a generator block and a second signal proportional to a supply voltage from a divider block to produce at output an initialization signal. As known, a common problem with nearly all digital electronic devices is the correct initialization of the device upon starting. Specifically, digital electronic devices are normally associated with a so-called power on circuit which controls turning on of electric power and performs a reinitialization, or reset, of the digital device. In technical jargon the practice of denominating starting circuits assigned to the above mentioned reinitialization 'power on reset' is established. Prior art The main function of the power on reset circuit is to generate a POR signal for a digital device. This signal is generated downstream of a comparison between the rising slope of a supply voltage of the digital device and a reference potential which is generally taken from a node inside the power on reset circuit. It is very important for correct operation of the circuit that the voltage value of this internal node reach the operating value in advance of the supply voltage. The comparison between the two voltage values is performed by a comparator which produces at output an INTPOR signal on which the POR signal depends directly. In the annexed FIG. 1 is shown an example of a power on reset circuit provided in accordance with the prior art. The internal reference node is indicated by the block RIF which is directly connected to an input of the comparator while the block Vdd!div indicates a division of the supply voltage supplied to a second input of the comparator. The output of the comparator is coupled to the supply voltage Vdd through a capacitor C and is also connected to an output buffer which produces the POR signal. The Vdd!div block has the duty of producing at output a voltage having the same linear behaviour as the supply voltage Vdd but with a slope reduced by a factor m. The factor m is for all purposes a division ratio which must be ensured with accuracy and stability. The capacitor C has the function of favouring the coupling of the INTPOR signal with the supply voltage Vdd during the rising transient thereof. In rated operation the value of INTPOR remains fixed from the comparator output. The output buffer has the purpose of decoupling the dynamics of the output signal POR which has a range equal to the power supply Vdd from that of the intermediate signal INTPOR which can have a smaller range. This allows increasing the response speed of the circuit and driving the POR line which has a high fan-out. Although advantageous in some ways the power on reset circuit described above displays the serious shortcoming of excessive consumption. Indeed, all the circuitry must necessarily remain on to be able to promptly regenerate the POR signal in case of a drop in supply voltage. This requires the presence of a rated current other than zero. Such rated behaviour is undesirable for low consumption digital devices, for example those incorporated in cellular telephones. Attention is drawn also to all the flash semiconductor memories powered for example with low voltage between 2.5V and 3.6V and for which is required a virtually null current (maximum 5µA) on stand-by. Also known from the US Patent No. 5,287,011 to Koshikawa et al. (NEC Corporation) is a Power-On detecting circuit equipedd with internal step-down circuit and comprising a timing generating unit and a monitoring unit for producing an enabling signal when a step-down power voltage level reaches a constant level. US-A-5,376,835 discloses a circuit as defined in the preamble of claim 1. The technical problem underlying the present invention is to conceive a power on reset circuit having structural and functional characteristics such as to allow virtually null current consumption in stand-by while maintaining the characteristics of promptness in generating the power on reset signal in case of a drop in the power supply. This would allow overcoming the limitations and shortcomings of the present solutions proposed by the prior art. Summary of the invention The solution idea underlying the present invention is to equip the power on reset circuit with an extinguishing device which would however allow immediate return to the on state following a drop in supply voltage. On the basis of this solution idea the technical problem is solved by a circuit of the type indicated above and defined in the characterizing part of the annexed claims 1 and following. The characteristics and advantages of the circuit in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. Brief description of the drawings FIG. 1 shows a diagrammatic block view of a power on reset circuit provided in accordance with the prior art, FIG. 2 shows a diagrammatic block view of a power on reset circuit provided in accordance with the present invention, FIG. 3 shows a diagrammatic view of a detail of the power on reset circuit of FIG. 2, and FIGS. 4 to 6 show respective diagrammatic view of circuit details of the power on reset circuit provided in accordance with the present invention. Detailed description With reference to the above figures reference number 1 indicates as a whole and diagrammatically a power on reset circuit provided in accordance with the present invention. The circuit 1 is powered by a supply voltage Vdd and comprises a block 11 RIF generating a reference signal having one output y directly connected to a non-inverting input (+) of a comparator 2. The reference voltages is set preferably at 1.8V. A divider block Vdd!div has one output G, visible in FIG. 4, connected to a second inverting input (-) of the comparator 2. The block Vdd!div is also indicated by the number 12 and has the duty of producing at output a voltage proportional to the supply voltage. Specifically the output of the divider block has the same linear behaviour as the supply voltage Vdd but with a slope reduced by a predetermined factor m. The factor m is for all purposes a division ratio which must be ensured with accuracy and stability. The output of the comparator 2 is a circuit node A which is coupled to the supply voltage Vdd through a capacitor C and is also connected to an output buffer 3 which produces the POR signal. The internal structure of the buffer 3 is conventional and a detailed description thereof is accordingly omitted. The capacitor C has the function of aiding the coupling of the signal INTPOR with the supply voltage Vdd during the rise transient of the latter. At rated operation the value of INTPOR remains fixed. The output buffer 3 has the purpose of disconnecting the dynamics of the output signal POR from that of the intermediate signal INTPOR. Advantageously in accordance with the present invention the circuit 1 comprises an inverter pair I1, I2 connected in mutual series between the output node A of the comparator 2 and a third input 10 for enablement of the comparator. The divider block 12 comprises in turn an enablement input 13 connected to the output of the second inverter I2. Even the generator block RIF comprises an enablement input 15 connected downstream of the two inverters I1, I2. The diagram explained up to this point is quite general and solves the technical problem of the present invention. The various enablement inputs 10, 13 and 15 simultaneously receive the enablement signal EN and allow taking the circuit 1 to the off state once the POR signal has been generated. The circuit 1 is also capable of automatically leaving the off state following a drop in the supply voltage. There is discussed below in detail the structure of the individual circuit blocks making up the circuit 1 and how they intervene in the various operating phases of the circuit. With special reference to the example of FIG. 3 there is described in detail the internal structure of the block 11 generating the reference potential. The block 11 comprises a biassing network 5 connected between the power supply Vdd and a reference potential, for example a ground signal GND. The network 5 comprises a first current mirror 6 including MOS P-channel transistors M1 and M3 with their respective gate terminals G1, G3 connected together in a common node X. The mirror 6 is controlled by a transistor M260 inserted between the power supply Vdd and the node X. The gate terminal GP of the transistor M260 corresponds to the input 15 to which is applied an enablement signal EN. The network 5 comprises a second current mirror 7 including MOS N-channel transistors M2 and M4 with their respective gate terminals G2, G4 connected together in a common node Y. This common node Y also represents the output of the block RIF. The mirror 7 is controlled by a transistor M258 inserted between ground GND and the node Y. To the gate terminal GN of the transistor M258 is applied the negated enablement signal EN-N. The transistor M2 was chosen with a threshold having a negative coefficient of variation with respect to the temperature. Between the source terminal of the transistor M2 and the ground GND is inserted the parallel of a capacitor C1 and of a resistance R3. The resistance R3 was provided to compensate the effects due to thermal drift and the value of which has a positive variation coefficient with respect to the temperature. The two current mirrors 6 and 7 are started by the transistors M260 and M258 when the signal EN reaches a high logical value. As regards the divider block 12, with reference to FIG. 4 it can be appreciated that the basic structure comprises a resistive divider 8 comprising the resistances R1 and R2. The interconnection point G between the resistances R1, R2 is connected to ground GND by a capacitor C3. A complementary pair of transistors M5 and M6 forms a first inverter I8 connected to one end of the divider 8 through a second inverter I9. The gate terminals G5, G6 of the transistors M5 and M6 are both connected to the node Y of the network 5. This allows starting the divider 8 only when the reference has reached its final value and thus performs a correct comparison. Under rated operating conditions the voltage signal present on the node Y has low logical value and the resistances R1 and R2 are short circuited to ground. The internal structure of the comparator 2 is illustrated in detail in FIG. 5. This structure is in large part of known type with a differential cell 9 comprising the transistors M8 and M14 having their respective gate terminals operating as inputs of the comparator with one being connected to the output node Y of the network 5 and the other to the node G of the divider 8. The differential cell 9 is biassed towards the power supply Vdd by a current mirror comprising the transistors M9 and M11 while it is connected to ground through a transistor M10 operating virtually as a current generator. The differential cell 9 has an output node E connected to the power supply Vdd through a capacitor C263 capable of supporting high voltages. The node E is also connected to the output node A of the comparator through the series of two inverters I21 and I24. A node F for connection between these inverters is connected to ground through a capacitor C5. The structure of the comparator 2 is distinguished essentially by the presence of a transistor M256 of the MOS N-channel type connecting the node E of the cell 9 to ground. On the gate terminal GM of this transistor M256 is applied the negated enablement signal EN-N. Advantageously the signal output on the node E of the cell 9 is appropriately amplified by the two inverters I21 and I24 which, through the transistor M256, can be taken to a state of null consumption each time rated operating conditions are reached. Lastly, in FIG. 6 is illustrated the structure of the inverter pair I1, I2 which connects the node A with the various enablement inputs of the blocks 2, 11 and 12. The first inverter I1 comprises a complementary pair of transistors M12 and M13 the first of which is in diode configuration. Between the power supply Vdd and the first M12 of these transistors is inserted another MOS P-channel transistor M266. The gate terminal G66 of the latter transistor and the gate terminal G13 of the transistor M13 are both connected to the node A and receive the intermediate signal INTPOR. The output U1 of the first inverter I1 is connected to ground through a high value resistor R28 and produces the negated enablement signal EN-N. The output U1 is connected at input to the second inverter I2 which comprises a complementary pair of transistors M268 and M271. Another transistor M269, of the N-channel type, is connected between the transistor M268 and ground. The gate terminals G69 and G71 of the transistors M269 and M271 are connected at output U1 while the transistor M268 is in diode configuration and has a drain terminal corresponding to the output U2 of the series of inverters I1, I2 and produces the enablement signal EN. It is important to note that the two inverters I1 and I2 do not have full dynamics between the power supply Vdd and ground GND with 0V. The signals applied thereto are already brought in the direction of restarting to aid the power on reset when the supply voltage Vdd falls below a predetermined minimum value. From the above description it is clear that the circuit 1 in accordance with the present invention solves the technical problem by allowing driving of the comparator 2 as well as the blocks 11 and 12 when they are off when the circuit has generated the power on reset circuit signal. In this manner there is null consumption of the circuit under rated operating conditions. A rapid and automatic emergence from the off state is ensured after the supply voltage drop which causes immediate restarting of the initialization phase. Variations and modifications can be made to the circuit 1 of the present invention within the scope defined by the following claims.

Claims: Electronic power on reset circuit (1) of the type comprising a comparator (2) having at least two inputs and one output (A) for receiving on one input (+) a first reference signal coming from a generator block (11) and on the other input a second signal proportional to a supply voltage (Vdd) emitted by a divider block (12) and for producing at the output (A) an initialization signal (INTPOR) and at least a first (I1) and a second inverter (I2) connecting said output (A) of the comparator (2) to a third enablement input (10) of the comparator (2) as well as to a respective turn off enablement inputs (15,13) of said generator block (11) and said divider block (12), said generator block (11) having an output (Y) connected to said one input (+) of the comparator (2) in such a way to form with said pair of inverters (I1, I2) a feedback path of said comparator (2), and said divider block (12) having an output (G) connected to said other input (-) of the comparator (2) in such a way to form with said pair of inverters (I1, I2) a further feedback path of said comparator (2) characterized in that the first inverter (I1) comprises a complementary pair of first and second transistors (M12, M13), the first transistor (M12) being in diode configuration and being connected to a power supply reference (Vdd) through a first supplemental transistor (M266) and directly to an output (U1) of the first inverter (I1), said first supplemental transistor (M266) having its gate terminal (G66) connected to the gate terminal (G13) of the second transistor (M13) and to the output (A) of the comparator (2), said second transistor (M13) being inserted between the output (U1) of the first inverter (I1) and a ground reference (GND), said output (U1) of the first inverter (I1) being further connected to the ground reference (GND) through a high-value resistor (R28) and producing a negated enablement signal (EN-N) and in that the second inverter (I2) comprises a complementary pair of third and fourth transistors (M271, M268), the fourth transistor (M268) being in diode configuration and connected to said ground reference (GND) through a second supplemental transistor (M269) and directly to an output (U2) of the second inverter (I2), said second supplemental transistor (M269) having its gate terminal (G69) connected to the gate terminal (G71) of the first transistor (M271) and to the output (U1) of the first inverter (I1), said third transistor (M271) being inserted between the power supply reference (Vdd) and the output (U2) of the second inverter (I2). Circuit in accordance with claim 1 and characterized in that said generator block (11) comprises:a first (6) and a second (7) current mirrors forming a biasing network (5) between said supply voltage (Vdd) and a reference voltage (GND) with each mirror (6,7) comprising and being driven by a respective turn off transistor (M260,M258) having its control terminal connected downstream of said inverter pair (I1, I2). Circuit in accordance with claim 1 and characterized in that said divider block (12) comprises a resistive divider (8) and an inverter pair (I8,I9) inserted between the output (Y) of the generator block (11) and one end of the resistive divider (8). Circuit in accordance with claim 1 and characterized in that said comparator (2) comprises a differential cell having an output node (E) coupled to said output through inverters (I21,I24) placed in series with there being inserted between said output node (E) and a reference voltage (GND) an turn off enablement transistor (M256) driven by a signal (EN-N) emitted by the first of said inverter pairs (I1,I2).

Inventors: BRANCHETTI MAURIZIO, GHEZZI STEFANO, MACCARRONE MARCO, BRANCHETTI, MAURIZIO, GHEZZI, STEFANO, MACCARRONE, MARCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H03K17
IPC codes: H03K17