Abstract:
The monitoring of multiple supply voltages of an integrated circuit is done using a single external capacitor connected to a pin of the integrated circuit. Part of the multiple supply voltages are externally generated and part are internally generated. The internally generated supply voltages may include different voltages with different signs. A logic signal indicating that all the supply voltages have reached pre-established values before enabling functioning of the integrated circuit is generated after an initial soft start phase of the turn-on process.

Description:
FIELD OF THE INVENTION 
     The present invention relates to integrated digital circuits, and, more particularly, to a procedure for monitoring all the power supplies within an integrated circuit (IC). This monitoring is a function referred to as power monitoring. 
     BACKGROUND OF THE INVENTION 
     Integrated systems tend to have several subsystems and power sections integrated on a single chip. These devices commonly use different positive power supplies, and often use negative power supplies to power in the most efficient manner the different chip sections or the various subsystems integrated on the same chip. 
     Sometimes one or more supply voltage sources are externally provided, while others are implemented internally by as many voltage regulators as there are externally generated supply voltages. In these monolithic integrated complex systems, it is very important to monitor the level of all the different supply voltages and to generate a so-called NPOR signal stating their compliance or noncompliance with their respective design values. 
     For an integrated system with one or more regulators generating as many supply voltages within the integrated circuit, it may be crucial to verify that all the supply voltages that are either fed to the IC or generated internally have reached pre-established values before enabling functioning of the integrated circuit. 
     Conventionally, this verification is acknowledged at the end of the charging process via a capacitor charged with a relatively low constant current to prevent the functional circuits from producing false switchings because of an insufficient supply voltage at the turn-on of the IC. Because the capacitor must be charged within a few milliseconds, it must have a relatively high capacitive value. This capacitor is commonly an external capacitor connected to a dedicated pin of the IC. 
     The assertion that all the supplies of the functional circuits of the integrated circuit have reached their pre-established values is done conventionally by forcing the NPOR signal to a high logic level. On the other hand, many electronic systems implement a turn-on method that is commonly referred to as soft start to avoid an abrupt application of the full supply voltages to the IC. Even for this well known soft start function, a relatively slow charging external capacitor having a large capacitance is the common way of implementation. 
     It is evident the burden implied by the use of distinct external capacitors, one for the soft start function and the other for the power monitor function. Each capacitor respectively requires a dedicated pin on the IC. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing background, an object of the present invention is to provide a method and a power monitor circuit for generating a NPOR signal representing the state of the power supplies while using a single external capacitor. This external capacitor is charged with a constant current from a suitable current generator integrated in the device that is used to implement a soft start function of the IC. 
     This and other objects, advantages and features are provided by a method comprising disabling of a first discharge path of the external capacitor when the presence of the externally provided supply voltage(s) is acknowledged allowing the start of the external capacitor charge ramp towards a certain regulated voltage. 
     At least a first replica of the charge ramp is generated, and optionally an inverted second replica of the charge ramp is generated. The replicated voltage ramps are respectively supplied to the regulators that generate positive supply voltages and to the regulators that generate negative supply voltages, respectively for activating the soft start. 
     The method further includes monitoring a signal of assertion that the values of all the externally provided and internally generated supply voltages are correct. Continuation or interruption is conditioned for charging the external capacitor towards the regulated charge voltage for overcoming a first pre-established level of the charge voltage on the external capacitor. This coincides with the end of the soft start procedure by keeping disabled or enabled a second discharge path of the external capacitor until the assertion signal is raised. 
     The end of the turn-on procedure is established by charging the capacitor to a second pre-established voltage which causes a transition from low to high of the NPOR signal. The NPOR signal will remain high as long the signal of assertion is absent for a period of time greater than a pre-established minimum time interval. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sequence of diagrams depicting the turn-on procedure for a positive verification of the internally generated supply voltages according to the present invention; 
     FIG. 2 is a sequence of diagrams depicting the turn-on procedure for a negative verification of the internally generated supply voltages according to the present invention; and 
     FIG. 3 is a basic block diagram for implementing the method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The system of the present invention has been successfully tested in an integrated circuit (IC) having a plurality of regulators therein. In a sample device, a first supply voltage 12V and a second supply voltage 5V are applied to respective pins from external sources. A third supply voltage 3.3V is generated internally and powers a regulator generating a fourth regulated supply voltage 2.5V and a regulator generating a fifth negative supply voltage −5V. 
     The basic diagram of a circuit according to the present invention is depicted in FIG.  3 . FIGS. 1 and 2 illustrate a sequence of diagrams (corresponding to an algorithm) depicting the turn-on procedures for two different situations that may occur at the turn-on of the IC. 
     Referring to the diagram of FIG. 3, both the soft start function and the power monitor function are implemented according to the present invention via a single external capacitor C. The use of a single external capacitor requires only one pin of the IC. The capacitor C is charged by the current generator I towards a certain voltage, which is indicated as Vreg. At turn-on, the transistor M 1  is biased in a conduction state so that the external capacitor C maintains in a discharged state. 
     Upon the rising of a first logic signal  5 / 12 ok indicating application of the externally provided supply voltages to the device, the block PULSE GEN sets the flip-flop FF 1 . The output of FF 1  is coupled to the control node of the transistor M 1 , causing it to turn-off. The PULSE GEN block, besides setting the flip-flop FF 1 , also resets the flip-flop FF 2 . The output Q of the flip-flop FF 2  is coupled to an input of an AND gate. The output of the AND gate is coupled to the control terminal of a second transistor M 2  for keeping it in an off state. The second transistor M 2  is connected in parallel to the external capacitor C. 
     The correct application of the externally provided supplies (5V and 12V) to the IC is acknowledged at the instant t 0 . This is when the external capacitor C starts charging towards Vreg with a constant current established by the charge current generator I. The voltage on the external capacitor C is applied as an input to the block RAMP GEN. The block RAMP GEN generates a direct replica and, eventually, a second replica of the charge ramp of the voltage on the external capacitor C whose level is shifted and whose sign is inverted. 
     The two generated ramps are referred to as RAMP_RISE and RAMP_FALL, and implement the soft start of the internal regulators. The initial soft start interval ends when a certain pre-established voltage of an intermediate value is reached on the external capacitor C. This pre-established voltage is typically the band-gap voltage Vbg. 
     In the illustrated example, the generator RAMP GEN generates a first ramp RAMP_RISE that is substantially a replica of the voltage ramp developing on the external capacitor C. A value of this first ramp extends from zero up to the value of the band-gap voltage Vbg. The generator RAMP GEN also generates a second ramp RAMP_FALL falling from a shifted level of 2*Vbg down to the Vbg level. 
     The soft start interval ends when the voltage on the capacitor C attains the value Vbg. This event is sensed by the comparator COMP at the instant t 1 . The output of the comparator COMP sets the second flip-flop FF 2  with a minimum delay (t 1 +delay). At this point, the power monitor algorithm of the invention discriminates between two distinct situations that may occur at the instant t 1  or, more precisely, at the instant t 1  plus a minimum switching delay of the comparator COMP which establishes the end of the soft start. 
     A second signal or flag DC_ok is produced in a conventional manner in the IC to assert or deny the condition that all the externally provided supply voltages (5V and 12V) and the internally generated supply voltages (3.3V, 2.5V and −5V) have a correct value. This second assertion signal DC_ok is applied to a first input of a NAND gate. The second input of the NAND gate is coupled to the output Q of the second flip-flop FF 2 . The output of the NAND gate is coupled to a second input of an inverting AND gate whose output controls the second transistor M 2 . 
     If at the instant t 1  plus the switching delay of the comparator COMP, and if the signal DC_ok is high confirming the correct positioning on the respective design values of all the supplies in the IC, the transistor M 2  is kept in a cut-off state. This allows the charge ramp of the capacitor C to continue until a second pre-established level is eventually reached. The second pre-established level is commonly a multiple of Vbg, such as 2*Vbg in the illustrated example. 
     The second threshold is sufficiently high to cause the triggering of the Schmitt trigger ST whose output turns on the transistor M 1  by resetting the first flip-flop FF 1 . The transistor M 1  immediately discharges the external capacitor C which ends the turn-on process, and the enablement of the power monitor function by forcing the NPOR output to a high logic level. 
     The block DEGLITCH has the function of masking undue transitions of the NPOR signal during the normal operation of the IC that may occur because of spurious switchings of the DC_ok signal. The signal DC_ok may have an infinitesimal duration which may be caused by accidental transients, noise coming from the main power supply, etc. 
     The output of the NAND gate is applied to an input of the block DEGLITCH which discriminates the duration of eventual switchings from the high to low state of the signal DC_ok. This filters out spurious switchings having a duration shorter than a minimum re-established time interval. 
     The two different situations that may occur during a turn-on process of the IC are shown in FIGS. 1 and 2. By accidental reasons, it may happen that at the instant t 1 , the signal DC_ok has not yet been asserted, i.e., has not yet switched from a low to a high level. In this case, the transistor M 2 , which discharges the external capacitor C, is forced in a conducting state because of the switching of the comparator COMP. Thus, the external capacitor C is kept discharged until the signal DC_ok switches to a high logic level at the instant t 3 . This causes the turning-off of the transistor M 2  and starts anew the charge ramp voltage for the external capacitor C. 
     As may be observed, the stated objective is fully accomplished, with the soft start and power monitor requirements being satisfied by the system disclosed herein which uses a single external capacitor C.