Abstract:
A method of monitoring supply voltage includes providing a single reference voltage, providing a single ratioed supply voltage, comparing the reference voltage to the ratioed supply voltage to provide an output signal, wherein the output signal comprises a first logic value in first and second operating conditions, and a second logic value in a third operating condition, wherein the first, second, and third operating conditions are determined by two crossing points of the reference voltage and ratioed supply voltage characteristics. The first and second operating conditions can represent undervoltage and overvoltage conditions, and the third operating condition can represent a normal operating condition. The reference voltage can be provided by a bandgap reference circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to power-on reset and voltage supply monitoring integrated circuits. 
         [0003]    2. Description of the Related Art 
         [0004]    Virtually every digital and mixed-signal (digital/analog) electronic and electrical system faces the problem of the initial indetermination of its state, when the power is ramped from zero volts to its nominal value. A common solution that has spurred a plethora of prior art is the adoption of a so-called power-on reset (P.O.R.) circuit, that can monitor the level of one or more supply lines, and issue a digital “reset” signal to initialize the digital circuitry and in particular any finite state machines (FSMs) present inside the system. Solutions have been devised that rely on the simplest voltage divider and Schmitt trigger approach, cited as prior art by e.g. Yukawa (U.S. Pat. No. 5,136,181); often complemented by a more complicated but digital-like, supply-referred CMOS solution such as found in Yukawa, or Ikezaki (U.S. Pat. No. 6,144,237). Other approaches recognize the limitations of a CMOS-only implementation, susceptible to V-threshold shifts and referred to the power rail; and those less temperature-stable while more process-dependent. Those solutions adopt self-compensating circuits such as variants of the bandgap reference block, where the temperature dependence of a p-n junction&#39;s ΔV (ΔV BE  for the bipolar transistor case, or simple ΔV D  for the diode case) which is approximately −2 mV/° C. due to inherent physical properties of silicon, gets compensated by an Ohmic drop I·R of a PTAT (Proportional To Absolute Temperature) current flowing into a resistor featuring minimal, or often even advantageous, sheet resistivity p dependence on temperature. Being based on inherent semiconductor properties, this approach also is less susceptible to process shifts, which—in case they affect the resistors, for example, in the ΔV D +R·I=V constant  mesh voltage balance outlined above—can easily be trimmed by laser, fuses, E 2 PROM-based techniques at the inception of the circuit&#39;s product life in the factory. Such general approach to define a suitable and most invariant reference voltage against which the power supply can be compared has been either modified or cited as prior art by e.g. Zhou (U.S. Pat. No. 6,847,240) and Morigami (U.S. Pat. No. 5,070,295, using diode/resistor combinations); or also by Tang et al. (US Patent Publication No. 2009/0102522), Chung (U.S. Pat. No. 5,959,477), and Hou (US Patent Publication No. 2008/0079467), which use a more classic configuration of the bipolar-based bandgap cell. One advantage of such idea is the fact that a bandgap-reference integrated circuit is very commonplace in a number of precision circuits, such as A-to-D and D-to-A data converters where the analog signal has respectively to be weighted, and synthesized, with respect to a stable, accurate, and precise voltage reference (see e.g. cited Zanchi patents regarding voltage reference systems); or data acquisition front-ends equipped with a variable-gain amplifier (VGA) where the relative strength of the input signal is object of comparison against a full-scale level to be maintained constant. 
         [0005]    The purpose of the POR circuit is to signal when the lower supply rail has reached a suitable operational voltage, that allows the digital logic to work reliably as expected. With the exception of Ikezaki, where the POR output is not issued for V supply  close to 0, all the other prior arts known to the inventor are either issuing a POR signal when the V supply  is less than a certain threshold; or issuing a finite time-pulse when the POR circuit determines a threshold of “acceptable voltage” has been exceeded. When the implementation itself is not conducive to such voltage characteristic, such as in Hou and as outlined by Zhou with regards to previous prior art, such anomaly is removed by means of additional “gating” circuitry (Hou) or by modifying the electrical configuration of the circuit (Zhou). 
         [0006]    What is desired is a simple POR circuit and method of operation that can provide indications of undervoltage, overvoltage, and normal operating conditions that is suitable for use on integrated circuits. 
       SUMMARY OF THE INVENTION 
       [0007]    According to the present invention, a method of monitoring supply undervoltage and overvoltage includes providing a single reference voltage, providing a single ratioed supply voltage, comparing the reference voltage to the ratioed supply voltage to provide an output signal, wherein the output signal comprises a first logic value in an undervoltage and an overvoltage condition, and a second logic value in a normal operating condition. The reference voltage can be a bandgap voltage. The undervoltage condition is a condition wherein the reference voltage is less than the ratioed supply voltage for voltages up to a first voltage crossing of the reference voltage and the ratioed supply voltage characteristics. The first voltage crossing occurs during a transitional phase of the static characteristic of the reference voltage. The overvoltage condition is a condition wherein the reference voltage is less than the ratioed supply voltage for voltages greater than a second voltage crossing of the reference voltage and the ratioed supply voltage characteristics. The second voltage crossing occurs during either a transitional phase or an invariant phase of the reference voltage characteristic against the supply voltage being monitored. The normal operating condition is a condition wherein the reference voltage is instead greater than the ratioed supply voltage, i.e. for voltages between a first voltage crossing and a second voltage crossing of the reference voltage and the ratioed supply voltage characteristics. 
         [0008]    The method of the present invention is performed without a clock. The reference voltage is reference voltage that can be shared with at least one other component in an integrated circuit and is an unswitched reference voltage. Comparing is performed by a single comparator having a first input directly coupled to the ratioed supply voltage and a second input directly coupled to the reference voltage. Comparing can include hysteresis. Additional delay and/or output signal processing can be performed. 
         [0009]    In a circuit embodiment, the reference voltage can be provided by a mixed bipolar and MOS bandgap reference circuit, or by a bipolar bandgap reference circuit having substrate PNP devices. Other bandgap or reference circuits may also be used. 
         [0010]    The circuit and method of the present invention may be used on any mixed-signal (analog and digital) circuits with a stable bandgap reference and sensitivity to both undervoltage (for reasons of functionality) and overvoltage (for reasons of reliability). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  contains two plots of an inverted POR signal with respect to supply voltage as is known in the prior art; 
           [0012]      FIG. 2  is a plot of another inverted POR signal with respect to supply voltage according to the present invention; 
           [0013]      FIG. 3  is a plot of the startup voltage characteristics for two prior art voltage monitoring circuits; 
           [0014]      FIG. 4  is a plot of the static bandgap voltage versus supply voltage as is known in the prior art; 
           [0015]      FIG. 5  is a plot of the threshold voltage characteristic intersecting the bandgap voltage characteristic at two distinct threshold voltages according to the present invention; 
           [0016]      FIG. 6  is a schematic of a power-on reset circuit suitable for indicating the undervoltage, overvoltage, and normal operating voltage conditions based upon the two distinct threshold voltages of  FIG. 5  according to the present invention; 
           [0017]      FIG. 7  is a schematic of a first transistor-level implementation of the circuit of  FIG. 6 ; and 
           [0018]      FIG. 8  is a schematic of a second transistor-level implementation of the circuit of  FIG. 6  with improved rejection to supply variations. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The purpose of the present invention is to leverage the most stable POR configuration so far existent in the state of the art, i.e. the bandgap-based power-on reset circuit, to not only provide a signal that identifies when a suitable level for the V supply  has been reached; but to provide, in addition, a means to signal when a suitable level for the V supply  has been exceeded. In such paradigm, the digital logic that controls the rest of the I.C. or S.o.C., not only can be reset, initialized, and further on activated when V supply  is high enough; but it can be configured to enter a protective (and self-protective) mode of operation when V supply  is excessive, such as in case of power overshoot, surge due to AC-line instability, sudden shorts with a higher voltage rail, or load-dump transient for voltage-regulated subsystems (such as driven by a Low Drop Out regulator, LDO). Referring now to  FIG. 1 , the usual output signal characteristics  102  or  104  (to Ikezaki) of a prior-art POR circuit is then modified to the output signal characteristic  202  in  FIG. 2  of another POR circuit according to the teachings of the present invention. 
         [0020]    Referring now to  FIG. 2 , the output signal characteristic  202  according to the present invention is shown. V THR  is chosen so as V supply &gt;V THR  guarantees an operationally reliable voltage for the circuit; V THR ″ is chosen so as V supply &lt;V THR ″ does not constitute a reliability concern for the circuit at hand. The overvoltage protection is desirable for example to prevent the I.C.&#39;s overheating (in most instances, a rise in V supply  determines a super-linear rise in I supply which ends up destroying the part for long V supply  transients, especially in BICMOS or bipolar circuits subject to thermal runaway and 2 nd  order breakdown), which is particularly true in power stages subject to tight SOA (Safe Operating Area) requirements. But in the most aggressive digital technology implementations, sub-100 nm CMOS for example, the degree of TDDB (Time-Dependent Dielectric Breakdown) and in general of GOI (Gate-Oxide Integrity) susceptibility is greatly exacerbated, to the point where the life-time specification of the product can be significantly shortened for even short intervals of supply overvoltage, corresponding to power-rail failure episodes. The case is worse for continuous-time constant overvoltage; but process specifications de-rate their devices&#39; lifetimes against the sum of time intervals in which the circuit has been exposed to overstress, clearly indicating the need for a circuit that could detect even brief, temporary events. 
         [0021]    For aerospace application, where the lifetime of the part is much longer than for commercial applications, as well as in military applications where supply overvoltages can occur as a consequence of deliberate attacks, this sort of protection is extremely valuable. Moreover, for the case of subsystems internally supplied by down-conversion voltage regulators (like single-supply parts using dual- or triple-gate technology, e.g. 3.3V/1.8V gate oxide thicknesses), radiation events such as heavy-ion hits can easily upset internal nodes to the regulator, causing substantial destabilization of the LDO or other equivalent regulation loop, that can cause the output voltage to rail to the undesirable higher voltage for extended periods of time. 
         [0022]    Therefore, unlike most state of the art available, the invention not only issues a POR signal upon an under/overvoltage event, but maintains the  POR  signal active for the whole duration of the events, and provides a warning signal for some time after the cessation of the offending event for added security. Notice how, also in the previous figure,  POR  has been plotted; in fact, when the supply is zero or very low,  POR  can only be zero—which to a fully-powered external system still reads “POR condition detected” correctly. 
         [0023]    Notice too that supply voltage levels considered suitable for normal circuits could instead engender overvoltage stress for voltage-multiplied circuits such as charge pumps and bootstrap blocks, whose sensitive nodes could be locally equipped with the POR/monitoring circuit to be disclosed in the invention. 
         [0024]    The earlier prior art for overvoltage/undervoltage detection circuitry such as Cantor (U.S. Pat. No. 3,789,242) does not rely on a precise and stable bandgap voltage reference, but only on voltage comparisons and diode thresholds: which is very unreliable in integrated monolithic implementations. The same principle already exploited in the POR regulation in prior art can instead be conveniently extended to determine the additional “malfunction zone” outlined in the previous figure. As described respectively in Hou and Zhou, the start-up characteristics  302  and  304  of a conventional bipolar or diode-based bandgap reference against time are shown in  FIG. 3 . 
         [0025]    Since the curves  302  and  304  shown in  FIG. 3  have been simulated or measured by their respective inventors against a supply voltage ramp, linear with time, in reality such curves can be proven to be proportional to the static voltage “transfer function” V DD →V BG  of the bandgap cell as is shown in curve  402  of  FIG. 4 . 
         [0026]      FIG. 4  can be complemented with the corresponding V threshold  level  502 , as shown in  FIG. 5 , that will be proportional to V DD  via a temperature-independent and process-independent method of adjustment such as a simple resistive division, which determines V THR  and V THR   ″ : 
         [0027]    Notice that separate straight lines can be adopted to intercept the V BG  (V DD ) characteristic at the desired V THR , V THR   ″  voltages; and—although such technique can accommodate non-monotonicity of the transfer curve such as the one in Zhou—different R-ladders and comparators will be needed. Instead, a most advantageous case of monotonic V BG  (V DD ) has been devised and simulated: in such case, one single comparator produces the desired  POR  output, which does not require additional digital logic, minimizing the number of radiation-prone devices needed for a minimal realization of the invention. 
         [0028]    Referring to  FIG. 6 , a circuit  600  is shown that is capable of providing a power-on reset signal at node  608  that provides a logic zero in an undervoltage and overvoltage condition, and a logic one in a normal operating condition using the two distinct threshold voltages according to the present invention. The circuit includes a single bandgap circuit  602 , a resistor divider  604 , a single comparator  606 , and an output node  608 . The bandgap circuit  602  can be shared with other components on the same integrated circuit. Circuit  600  is not clocked or switched as is described in further detail below. 
         [0029]    After the comparator, some form of edge-activated monostable circuit  612  having an input  610  coupled to node  608  and an output  614  can be adopted to prolong the  POR  signal for a suitable time, for example transforming a quick violation of the allowed supply voltage “window” into a pulse of a time duration sufficient to enable digital processing. Notice however how the bandgap  602  does not require to be switched (as in Tang et al.); nor coupled to topologically similar post-processing stages (Chung); nor windowed through other circuits (Hou, Zhou). The constant characteristic of the V BG  level for higher V DD  levels guarantees the existence of a second intercept point V THR   ″ ; while the first intercept point derives from the “S-shaped” nature of the curve. The closest prior art to the proposed implementation is by Lyles et al. (US Patent Publication No. 2010/0225300), which indeed adopts the same bandgap/comparator arrangement and POR output convention, but it is switched, unlike the present invention. Although it also makes use of a single comparator, such solution requires to switch the bandgap between two states: that is, prevents the bandgap cell from being used for other blocks of the circuitry, requiring de facto a dedicated bandgap for the POR alone. The necessity of switching the V BG  characteristic “ON” and “OFF” is not presently used in the invention; since the intersection of the V BG  (V DD ) and the V threshold  (V DD ) curves automatically satisfies the requirements for under-voltage and over-voltage protection. 
         [0030]    A transistor-level realization of the circuit  600  corresponding to the block diagram presented in the previous  FIG. 6  is provided in the next schematic  700  of  FIG. 7 . The circuit  700  of  FIG. 7  includes PMOS devices  702  and  704 , NPN bipolar devices  706  and  708 , and resistors  710 ,  712 ,  714 , and  716 . Resistor  714  can be made variable to provide optional hysteresis. Comparator  718  has a positive input coupled to the bases of transistors  706  and  708 , and a negative input coupled to the output of the voltage divider at node  715 . Optional inverters  720  and  722  can be interposed between the output of comparator  718  and the inverted POR signal at node  724 . The NPN bipolar nature of the bandgap cell can of course be identically substituted by usage of PNP, diodes, or—as also advocated by Lyles—by a Zener diode circuit (provided the slope of the characteristic, now no longer “S-shaped” but rather a “saturation” curve, is compatible with the slope of the voltage divider generating V threshold ). 
         [0031]    Two-stage bandgaps add resiliency against the comparator&#39;s kick-back phenomenon; V DD -independent bandgaps offer better “S-shape” turn-on characteristics and provide improved PSRR as compared to the previous embodiment, which—in a context of POR applications having glitching supply—can be important. A detail of a PNP-diode schematic is proposed in the final implementation, shown in circuit  800  of  FIG. 8 . 
         [0032]    Circuit  800  includes a bandgap reference portion including resistors  802 ,  804 ,  806 , substrate PNP transistors  808  and  810 , and an amplifier including PMOS transistors  816  and  818 , NMOS transistors  812  and  814 , current source  813 . A voltage divider includes resistors  824  and  826 . Resistor  824  can be made variable to provide hysteresis, using the output of comparator  828  as a control signal. Comparator  828  has a positive input coupled to the drain of transistor  812 , and to a negative input coupled to the center tap of the resistor divider at node  820 . Inverters  830  and  832  can be interposed between the output of comparator  828  and the output signal node  834  if desired to prolong the duration of the  POR  signal. 
         [0033]    It should be emphasized that the definition of the boundaries of the undervoltage, normal operation, and overvoltage regions of the power-on reset and monitoring circuit, since they rely on the voltage thresholds that are defined by the crossing of the bandgap static characteristic and the ratioed supply characteristic, can be set with limited precision. In particular, unless the static bandgap curve versus supply can be controlled, the slope of the ratioed supply characteristic constitutes the only degree of freedom that can be exploited to define the thresholds: therefore, the two thresholds are inherently linked and cannot be set independently. Although the correctness of the positioning of the three regions is ensured by construction, and the functionality of both the undervoltage power-on reset and the overvoltage monitor does not require a high degree of precision, more flexibility in the definition of the threshold voltages would be desirable. The compact circuit disclosed in the present invention can be modified to accommodate these features without increase in power or area, and with moderate complexity. Unlike a combination of the bandgap reference and a multiplicity of comparators and resistor dividers with combinatorial digital logic interpreting the output—which could be realized trivially by implementing a number of prior art solutions—not only the slope of the ratioed supply characteristic curve can be modified, in fact; but the bandgap reference voltage can be ratioed in turn, producing a replica of the non-linear bandgap voltage versus supply characteristic scaled in amplitude, simply by way of a controlled resistive partition. The comparator input does not typically draw any current, hence a simple variable potentiometer (e.g. a digitally controlled switched-resistor ladder in an integrated realization) can be used to add a further degree of freedom to the invention, allowing both thresholds to be adjusted. 
         [0034]    Note that the non-linear, saturating nature of the bandgap curve allows for better tunability of the thresholds on the high side (overvoltage detection) than on the low side (undervoltage). This descends from the very nature of the bandgap circuit, and is compatible with the desired functionality of the monitor circuit. Indeed, both a Brokaw-cell and a Widlar-cell based bandgap will provide zero output for low supply voltages (initial dead-zone); until the supply exceeds the combination of BJT&#39;s VBE and resistor drops sufficient for the self-regulating loop of the circuit to become operational and stabilize (intermediate rising output); until finally the supply is high enough to ensure the wanted behavior of the bandgap circuit, whose output is constant over supply (final output voltage plateau). The output voltage is the sum of a PTAT (Proportional To Absolute Temperature) voltage drop over a resistor, and a CTAT (Complementary To Absolute Temperature) voltage drop across a p-n semiconductor junction, hence independent from absolute temperature to a first order, and independent from the supply owing to the voltage drops being referred to ground. Though the rising output zone of the characteristic could theoretically be adjusted by tuning the resistance inside the bandgap cell, it is typically more important for the electrical performance of the circuit at large to optimize the output voltage flatness against temperature (which relies on the same circuit parameters): hence a fine-tuning of the bandgap characteristic for sake of a POR operation should be effected by modifying the output directly, rather than altering the Kirchhoff&#39;s Voltage Laws balance within the core cell. 
         [0035]    In conclusion, this invention combines the best prior art circuit approaches in a novel manner to provide a temperature, and process corner, stable voltage threshold to generate a POR condition for undervoltages; and recognizes that the same circuit can naturally provide an overvoltage protection, by issuing a POR signal that the digital logic can interpret to shut off the most sensitive areas of the I.C. under control, and prevent in so doing either immediate damage, or long-term life-time reduction. 
         [0036]    It will be apparent to those skilled in the art that various modifications and variations can be made to the invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.