Abstract:
A power supply controller for electronic circuits, supplying a power supply voltage (VDC) and preventing operation of the said circuit by using a RESET signal when the said power supply voltage is less than a first predetermined threshold. The controller includes a first comparator (C 2 ) comparing a voltage proportional to the power supply voltage with a reference voltage and activating the reset signal when the voltage proportional to the power supply voltage is less than the reference voltage. A bandgap module supplies a principal reference voltage (VBGAP). Preliminary reference devices immediately supply a preliminary reference voltage (V 09 ), less than the principal reference voltage. Control circuits receive the preliminary reference voltage (V 09 ) and the principal reference voltage (VBGAP) and automatically activate the RESET signal for as long as the principal reference voltage (VBGAP) has not reached a second predetermined threshold.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of the invention is that of electronic circuits. More precisely, the invention concerns the electrical power supply for such circuits and in particular the voltage of the supplied power.  
         BACKGROUND OF THE INVENTION  
         [0002]    Micro-electronic circuits are designed to operate at a nominal power supply voltage (for example VDC=5 V). They can certainly operate over a predetermined range of voltages around this nominal value, however their correct operation cannot be guaranteed outside this range. For example, a memory supplied with a too low voltage (for example VDC MIN=2.55 V) may exhibit random behaviour, leading to undesired read/write operations.  
           [0003]    It is therefore necessary to monitor the supply voltage to the circuit and only permit it to operate when the supply voltage is within the desired range (for example VDC&gt;2.55 V). In this way the circuit will only operate when it is actually capable of operating without causing an error. Below the nominal supply voltage the circuit is forced into RESET mode.  
           [0004]    This is the role of the power supply controller.  
           [0005]    As is well known, in order to know precisely the supply voltage VDC supplied to a circuit, a part of this voltage VDC is generally compared at the output of a voltage reference module in accordance with the principle illustrated in FIG. 1.  
           [0006]    The bandgap module  11  is based on pnp transistors which output a precise reference voltage Vref (for example Vref=1.25 V). A voltage divider  12 , formed from two resistors R 1  and R 2  (for example each of 100 kΩ) outputs a fraction of VDC (in the described example: VDC/2). These two voltages are sent to a comparator  13  which triggers a RESET command whenever VDC/2&lt;Vref. A RESET is therefore obtained whenever VDC/2&lt;2.5 V.  
           [0007]    A problem with this technique is that while it is effective for slow voltage rises, it does not work for rapid voltage rises. In fact, in this case, the output of the bandgap module can remain at 0 V even though the supply voltage VDC has already reached 2 V, for example. The comparator  13  triggers the RESET command even though the supply voltage has still not reached the desired nominal value.  
           [0008]    In order to alleviate this problem, we considered adding to the controller a RC circuit  21  for the supply voltage VDC as shown in FIG. 2. This RC circuit forces a RESET command for rapid rises while waiting for the device in FIG. 1 to trigger.  
           [0009]    However, this technique is not 100% reliable. Depending on the slope of the voltage rise, the supply voltage VDC and the temperature and/or the technology used, the RC circuit  21  may trigger the RESET command while the part of the detector shown in FIG. 1 is no longer operating.  
           [0010]    In addition, this technique is not suitable for circuits using thin film technology (0.35 μm for example) which does not support a supply voltage greater than a predetermined threshold (for example 4 V).  
           [0011]    Therefore, if we consider a circuit that must be able to operate with a battery voltage (VBAT) of between 2.5 and 5.5 V, while the technology cannot support more than 4 V, an internal regulator is connected (from a bandgap module), which will supply the remainder of the circuit with 3 V. However, all the circuits connected to VBAT must then be built with transistors that can withstand 5.5 V. It must therefore use a thicker oxide layer (for example 0.6 μm) which is less efficient. This applies in particular to the bandgap module and the 3 V regulator.  
           [0012]    This implies that the bandgap module only operates from a minimum voltage of 2.4 V, for example, while with 3 V transistors, it would operate from 1.6 V. With such a device, in the case of a slow rise in voltage to 2.3 V, the RC circuit  21  triggers the RESET command while the bandgap module is still at 0 V, since it receives a lower voltage (2.3 V) than its minimum operating voltage (2.4 V).  
           [0013]    Again the comparator  12  triggers the RESET command (since it sees VDC/2=1.15 V&gt;0 V), while the supply voltage VDC has not risen to 2.55 V.  
           [0014]    In addition, using this technique, it is not possible to control voltages of less than 2.4 V since the bandgap module no longer operates.  
           [0015]    The invention has the particular objective of alleviating the problems of current technology.  
           [0016]    More precisely, one objective is to provide a power supply controller for electronic circuits that operates efficiently and reliably to prevent the operation of a circuit until the supply voltage has reached a threshold value, under all conditions, and in particular whether the voltage rises are slow or rapid.  
           [0017]    Another objective of the invention is to provide such a power supply controller that is simple, inexpensive and easy to build and implement. In particular, it is an objective of the invention to provide such a controller, all of whose constituent parts can be built using the same technology, in particular thin film technology.  
           [0018]    The invention also has the objective of providing such a power supply controller that can work at low voltages compared with earlier technology (and for example less than 2.4 V).  
         SUMMARY OF THE INVENTION  
         [0019]    These objectives, as well as others that will be revealed later, are achieved by using a power supply controller for an electronic circuit providing a supply voltage (VDC) and preventing the operation of the said circuit by using a RESET signal when the said supply voltage is below a first predetermined threshold, the said controller comprising a first comparator (C 2 ) comparing a voltage proportional to the said supply voltage with a reference voltage and activating the said RESET signal when the said voltage proportional to the said supply voltage is lower than the said reference voltage and a bandgap module that provides the principal reference voltage (VBGAP).  
           [0020]    According to the invention, the controller includes preliminary reference devices, immediately supplying a preliminary reference voltage (V 09 ), that is lower than the said principal reference voltage, but fairly accurate, and control circuits receiving the said preliminary reference voltage (V 09 ) and the principal reference voltage (VBGAP) and automatically activating the said RESET signal for as long as the said principal reference voltage (VBGAP) has not reached a second predetermined threshold.  
           [0021]    Thus, as long as the principal reference voltage is not available and whether the voltage rise is slow or rapid, means are available to ensure that the RESET command is triggered and that therefore there will be no random operation of the circuit due to the supply voltage being too low.  
           [0022]    It is preferable that the said means of control should include means for selecting a reference voltage (VREF) for the said first comparator (C 2 ) between the said preliminary reference voltage (V 09 ) and the said principal reference voltage (VBGAP), the said preliminary reference voltage (V 09 ) being selected for as long as the said principal reference voltage (VBGAP) has not reached the second predetermined threshold.  
           [0023]    The power supply controller also includes a regulation module providing a regulated supply voltage (VDC) to the said circuit which takes advantage of the said reference voltage (VREF).  
           [0024]    In other words, the reference voltage is chosen from the two available references and switched to the principal reference as soon as possible.  
           [0025]    According to an advantageous aspect of the invention, the said regulated supply voltage (VDC) also supplies the said bandgap module. This permits the latter to be built using thinner technology and to operate at lower voltages.  
           [0026]    According to another advantageous characteristic of the invention, the said control circuits issue a command to the said regulation device, controlling the means of amplification of the said regulated supply voltage (VDC) at a third predetermined threshold, until the said principal reference voltage reaches the said first predetermined threshold.  
           [0027]    The gain is therefore temporarily increased while using the preliminary reference voltage to take account of the fact that it is lower and nevertheless obtain an acceptable supply voltage.  
           [0028]    In accordance with a preferable mode of operation, the said preliminary reference devices include a transistor connected as a diode.  
           [0029]    Advantageously, the said control devices include a second comparator (C 1 ) supplied from the said preliminary (V 09 ) and principal (VBGAP) reference voltages and supplying an inverter (INV) driving a transistor (TP 1 ) provided to force the said RESET signal.  
           [0030]    Preferably, the said means of selection include two transistors, receiving respectively the said preliminary (V 09 ) and principal (VBGAP) reference voltages and driven respectively by the output of the said second comparator (C 1 ) and the output of the said inverter (INV).  
           [0031]    According to an advantageous characteristic, the said regulation devices include an amplifier (AOP) supplying the said regulated supply voltage (VDC) and supplied partly from the said reference voltage (VREF) and partly from a voltage divider to which the said regulated supply voltage (VDC) is fed back.  
           [0032]    Preferably, the said limiting devices include a transistor connected so as to short circuit part of the said voltage divider.  
           [0033]    The invention also concerns electronic components and electronic devices including, or working with, at least one power supply controller as described above. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    Other characteristics and advantages of the invention will be more clearly seen on reading the following description of a preferable method of implementation of the invention given as a simple but not exhaustive example for illustration purposes and the attached drawings, including:  
         [0035]    [0035]FIGS. 1 and 2, already referred to in the introduction, showing two known techniques for monitoring the power supply with and without a RC circuit respectively;  
         [0036]    [0036]FIG. 3 is a functional diagram of a power supply controller according to the invention;  
         [0037]    [0037]FIG. 4 shows in greater detail a method of constructing the control system in FIG. 3;  
         [0038]    [0038]FIGS. 5A and 7B show various simulations of operation of the controller in FIG. 4, respectively:  
         [0039]    FIGS.  5 A and  5 B: rapid voltage rise;  
         [0040]    FIGS.  6 A and  6 B: slow voltage rise;  
         [0041]    FIGS.  7 A and  7 B: voltage drop. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    [0042]FIG. 3 shows the general principle of the invention in the form of a simplified functional diagram.  
         [0043]    As explained in the introduction, the principal purpose of a power supply controller is to prevent the circuit supplied from starting to work before the supply voltage has reached a predetermined sufficient value. For as long as this is not the case, the controller sends a RESET command  31  to the circuit which forces it into the reset mode.  
         [0044]    The controller according to the invention still includes a bandgap module  32 , whose construction is well known, provided to supply a principal stable reference voltage VBGAP. Control circuits  33  direct this to a comparator C 2   34 , which also receives a voltage  35  representative of the supply voltage to the circuit. For as long as the latter is less than the reference voltage, the comparator  34  supplies a RESET command  31 .  
         [0045]    In order to alleviate the problems with the earlier technology, an immediate reference  36  is also provided that instantly provides a preliminary reference voltage (V 09 ) which need only be fairly accurate but which is available instantaneously, before the bandgap module becomes active.  
         [0046]    The control circuits  33  are designed to select as the reference voltage VREF, either the voltage V 09  or the voltage VBGAP, depending on whether the latter is or is not sufficient, for example according to the rule:  
         [0047]    VREF=MAX (V 09 , VBGAP).  
         [0048]    Therefore, during the voltage rise stage, VBGAP is temporarily replaced with another voltage that is immediately available.  
         [0049]    The control circuits  33  operate directly ( 37 ) on the RESET command  31  until the bandgap module starts.  
         [0050]    The controller also includes regulation circuits  38  that deliver the power supply voltage VDC (for example 3 V) to the circuit. These circuits  38  take into account the voltage VREF to adjust the power supply.  
         [0051]    According to a special aspect of the invention, the bandgap module  32  is supplied by the voltage VDC (and not, classically, by the battery voltage VBAT), which permits it to operate at lower voltages and with thinner transistors.  
         [0052]    In addition, the control module  33  delivers to the regulator circuits  38  a gain adjustment command  39  which acts on the circuits  30  for amplifying the regulated voltage for as long as VBGAP has not reached its minimum operating value.  
         [0053]    A special method of implementation of this power supply controller, illustrated in FIG. 4 is now presented.  
         [0054]    The immediate reference V 09  is obtained by using a transistor  41  as a diode and connected to the battery (VBAT, of the order of 2.5 V to 5.5 V), via a resistor  42 , for example 1 MΩ. This transistor then supplies a voltage V 09  lower than the bandgap voltage, of the order of 0.9 V. This voltage can vary between 0.6 V and 1 V depending on the technology, the power supply voltage and especially the temperature.  
         [0055]    This preliminary reference V 09  is present from the start, whatever the voltage rise time. It will serve as a preliminary reference to the regulation circuits  43  to supply a voltage VDC of 3 V.  
         [0056]    The regulation circuits  43  are similar to a multiplier circuit with a factor of 2.4 (3 V/1.25 V=2.4). With the preliminary reference VREF at 0.9 V, it therefore generates 2.16 V. It includes an operational amplifier  431 , whose output supplies the voltage VDC via a transistor  432 . This output is fed back to the input negatively via a resistor  433  of 910 kΩ.  
         [0057]    The output voltage VDC also supplies the bandgap module  44 . It is therefore not supplied directly by the battery VBAT but, like the rest of the circuit, by the voltage VDC. It therefore only sees a maximum voltage of 3 V. Consequently, it can be built using thin oxide transistors that are more efficient.  
         [0058]    The minimum operating voltage of this new bandgap module is therefore 1.6 V instead of 2.4 V.  
         [0059]    A comparator C 1   45  compares the output of the bandgap module (VBGAP) and the preliminary reference voltage V 09 . When the principal reference VBGAP exceeds the preliminary reference V 09 , that is when VDC is greater than 1.6 V and there has been sufficient time for the bandgap module to initialize, the reference voltage VREF is VBGAP. Before that it is V 09 .  
         [0060]    To achieve this, the output of the comparator  45  acts on the two pass transistors  46  and  47  in the following way:  
         [0061]    directly on transistor  46 , which outputs V 09 ;  
         [0062]    via an inverter  48  to transistor  47 , which outputs VBGAP.  
         [0063]    As soon as VBGAP is used as the reference, the regulation circuits  43  regulate at 2.4 times VBGAP and stabilize at around 3 V.  
         [0064]    According to the invention, VREF is therefore initially equal to V 09 , then to VBGAP, as soon as this exceeds V 09 , or:  
         [0065]    VREF=MAX (V 09 , VBGAP).  
         [0066]    VREF feeds a comparator C 2   49  which compares VREF to part of VDC supplied from a voltage divider comprising two resistors  410  and  411  of 100 kΩ and 96 kΩ respectively.  
         [0067]    The output of comparator C 2   49  acts as the RESET command  412 .  
         [0068]    Therefore the comparator  49  only issues the RESET command  412  when VDC/2 is greater than VREF, or VDC&gt;2.55 V.  
         [0069]    An additional safety device blocks the RESET command  412  via a transistor  413  controlled by the inverse (PASS) of the output of the comparator  45 . Therefore, for as long as the comparator  45  has not switched to the voltage VBGAP, the RESET is forced.  
         [0070]    When VREF=VBGAP, comparator  49  is triggered, to issue or not the RESET  412  depending on the level of the supply voltage VDC and therefore very precisely.  
         [0071]    According to a special aspect of the invention, means have been provided for altering the multiplication factor of the regulating circuits  43 . To do this, for as long as VREF=V 09 , the resistor  435  is short circuited when the output of the comparator  45  is active, that is when VREF=V 09 .  
         [0072]    Therefore it is possible to generate a voltage close to 3 V as soon as possible, so as to be certain that the bandgap module starts even if V 09  is only 0.6 V (if not, that is in the absence of transistor  414 , 0.6 V×2.4=1.44 V: the bandgap module might not start with such a supply voltage).  
         [0073]    The general operation of the controller is therefore as follows: as the supply voltage VBAT rises, the voltage V 09  is present from the start. The regulator  43  is used as a reference and generates about 3 V, the gain being controlled by the transistor  414 . This voltage permits the bandgap module  44  to start. During this time, the RESET  412  is forced to VDC.  
         [0074]    When the bandgap module operates (i.e. VBGAP&gt;V 09 ), the regulator  43  then uses VBGAP as a reference and generates 3 V. The comparator C 1   45  triggers the RESET and it is the comparator C 2   49  that controls the latter and will only trigger it when VDC&gt;2.55 V.  
         [0075]    [0075]FIGS. 5A and 5B show a simulation of the various voltages VBAT, V 09 , VBGAP, VDC, RESET (FIG. 5A) and REF, PASS (FIG. 5B), in the case of a rapid voltage rise of 10 μs. (operating even at 100 ns), with VBGAP=1.16 V instead of 1.25 V, and therefore a regulation factor of 2.6 instead of 2.4.  
         [0076]    It can be seen that V 09  is present almost immediately. The regulator tries to generate VDC at about 3 V as soon as VBAT is sufficient for it to operate. The bandgap module is then powered by VDC and takes a little time before starting.  
         [0077]    When VBGAP&gt;V 09 , PASS permits the regulator to use VBGAP as a reference instead of V 09  and VDC will then be regulated accurately from the bandgap module.  
         [0078]    For as long as VBGAP&lt;V 09 , the RESET is blocked to VDC. Therefore the whole circuit supplied from VDC remains subject to RESET. In the case of a rapid voltage rise, VDC&gt;2.55 V when the bandgap exceeds V 09  and the RESET is triggered as soon as PASS switches.  
         [0079]    Then, VBGAP=1.16 for as long as VDC&gt;1.6 V.  
         [0080]    The supply voltages can then be checked up to 1.6 v by altering the ratio R 1 /R 2  of the resistors  410  and  411 .  
         [0081]    [0081]FIGS. 6A and 6B show a simulation of a slow rise in the supply voltage, with duration of 1 ms. V 09  is again present from the start. The regulator tries to generate about 3 V but VBAT is still insufficient for that. Therefore VDC=VBAT for as long as VBAT&lt;3 V. As soon as VDC is sufficient (about 1.5 V), the bandgap module  44  operates. When VBGAP&gt;V 09 , the comparator C 1  switches, as well as PASS, and the new reference for the regulator becomes VBGAP.  
         [0082]    The RESET is therefore no longer controlled by the comparator C 1 . However, the comparator C 2  compares VDC/2 and VREF (=VBGAP) and keeps the circuit under RESET for as long as VDC remains less than 2.55 V.  
         [0083]    Then, when VBAT increases to above 3 V, the regulator  43  fulfils its role and supplies VDC=3 V precisely.  
         [0084]    In the same way, it is possible to control voltage drops to the minimum VDC at which the bandgap module can operate, that is 1.6 V, as shown in FIGS. 6A and 6B, which shows a voltage drop with detection at 2.55 V.  
         [0085]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.