Abstract:
The low-power relaxation oscillator comprises a first module ( 21 ) having a ramp generator formed by a reference current source ( 31 ) and a storage capacitor ( 32 ) defining a ramp voltage (V ramp1 ), and a voltage comparator (m 1,  m 2 ) for comparing the ramp voltage with a reference voltage, a second module ( 22, 41, 42,  V ramp2 , m 3,  m 4 ) similar to the first module and an asynchronous flip-flop ( 23 ) receiving the output signal of the comparator of the first module at a first input (s) and the output signal of the comparator of the second module at a second input (r). For each module a generator of said reference voltage is configured by adding a reference resistance ( 33, 43 ) between the reference current source and the storage capacitor. Thus, the generation of the reference voltage and the ramp voltage is conducted on the very same current branch. This enables the electrical power consumption of the oscillator to be reduced.

Description:
This application claims priority from European Patent Application No. 08163797.7, filed Sep. 5, 2008, the entire disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates in general to a relaxation oscillator used in particular in radio-frequency applications. More specifically, the invention relates to a low-power relaxation oscillator that can be integrated into a radio identification transponder (RFID). 
     TECHNOLOGICAL BACKGROUND 
     A relaxation oscillator is known from the prior art and its basic diagram is provided in  FIG. 1 . Such a relaxation oscillator comprises two like modules  1 ,  2 , each having a ramp generator  3 ,  4  formed by a reference current source  5 ,  6  and a storage capacitor  7 ,  8  defining a ramp voltage (V ramp1 , V ramp2 ), a reference voltage generator  9  and a voltage comparator  10 ,  11  for comparing the ramp voltage (V ramp1 , V ramp2 ) with the reference voltage (V ref ). An asynchronous RS type flip-flop  12  receives the output signal of the comparator  10  of the first module at a first input (s) and the output signal of the comparator  11  of the second module  2  at a second input (r). Then as a result of the bias of the output terminals q and qb the flip-flop  12  alternately supplies control signals for charging the capacitor  7  of the first module and for charging the storage capacitor  8  of the second module. For this, each ramp generator  3 ,  4  is controlled by means of a switch  13 ,  14 . 
     The operation of such a relaxation oscillator is described in association with  FIG. 2  that shows the ramp voltages (V ramp1 , V ramp2 ) in relation to time. In an initial stage, the switch  13  is open, the reference current source  5  charges the storage capacitor  7  by means of the reference current I ref . At the same stage, the switch  14  is closed so that the storage capacitor  8  is short-circuited and is therefore not charged. When the ramp voltage V ramp1  reaches the level of the reference voltage V ref , the voltage comparator  10  detects this and the signal that it outputs is modified so that the flip-flop  12  changes state. The output signals at terminals q and qb are inverted. Thus, switches  13  and  14  are closed and open respectively. The charge stored on the capacitor  7  is then discharged to Vss by short-circuit, while charging the capacitor  8  begins. Similarly, when the ramp voltage V ramp2  reaches the level of the reference voltage V ref , the voltage comparator  11  detects this and the signal that it outputs is modified so that the flip-flop  12  changes state. The output signals at terminals q and qb are once again inverted and the initial cycle can start again. These two charge phases successively of capacitor  7  and capacitor  8  represent an oscillation period. 
     Another relaxation oscillator of the prior art is described in patent document U.S. Pat. No. 6,720,836. This document describes a relaxation oscillator with current sources coupled to one another. 
     For radio-frequency identification applications involving RFID transponders, the consumption of current sources used in such relaxation oscillators is one of the most critical parameters. The theoretical minimum current consumption for such a relaxation oscillator is given by the formula C*V ref /T=I ref . To minimise current consumption for a set oscillation period, the storage capacitor C and the reference voltage V ref  must be reduced to the full. For this purpose, it is important to note that the reference voltage V ref  corresponds to the oscillation amplitude. 
     The existing solutions have one or more disadvantages listed below and are therefore further removed from the theoretical minimum consumption. 
     Firstly, an oscillator like that shown in  FIG. 1  consumes the equivalent of the theoretical minimum current consumption just with the ramp generator (via the source of reference current I ref ). Therefore, all the other consumer circuits will take such an oscillator further away from the theoretical minimum consumption. These other consumer circuits comprise in particular the reference voltage generator, comparators of the ramp voltage with the reference voltage. 
     An oscillator such as that described in patent document U.S. Pat. No. 6,720,836 has the major disadvantage of having an oscillation amplitude associated with the gate-source voltage V gs  of a CMOS type transistor that can thus not be decreased in order to reduce current consumption. Moreover, such a relaxation oscillator uses a current mirror with a size ratio of 1 to 10 between the transistors of the mirror, and this requires a significant surface over an integrated circuit. 
     In addition, it must also be noted that the voltage amplitude is only in theory equal to V ref . In reality, a phenomenon of overrun occurs in the switching phases as a result of the reaction time of the comparators. This is the case in particular in oscillator solutions using transistors that do not operate fully in saturated mode. 
     Finally, the minimum value for the storage capacitor is determined in particular by the value of the interference capacitor on the upper plate of the storage capacitor. These interference capacitors come from metal connections and other devices connected to the upper plate of the storage capacitor. Previous solutions based in particular on the Vittoz loop connect numerous or extensive devices to this capacitor. The operation of these devices in weak inversion increases the capacitor compared to an operation of these devices in strong inversion. Therefore, it will be important to ensure that a limited storage capacitor is maintained. 
     SUMMARY OF THE INVENTION 
     An aim of the present invention is to address the disadvantages mentioned above by providing a low-power relaxation oscillator, i.e. one with a reduced current consumption as close as possible to the theoretical minimum consumption without compromising the supply voltage and the temperature stability. 
     In this aim, a first aspect of the present invention relates to a low-power relaxation oscillator comprising a first module having a ramp generator formed by a reference current source and a storage capacitor supplied by the reference current source via a reference current branch and defining a ramp voltage, and a voltage comparator for comparing the ramp voltage with a reference voltage, a second module similar to the first module and an asynchronous flip-flop receiving the output signal of the comparator of the first module at a first input and the output signal of the comparator of the second module at a second input and alternately outputting control signals for charging the capacitor of the first module and the capacitor of the second module. This relaxation oscillator is characterised in that for each of the first and second modules a reference voltage generator is configured by adding a reference resistance arranged on said reference current branch in such a manner that the reference voltage is defined by the reference current flowing through the reference resistance. 
     Because of the features of the oscillator according to the invention, the generation of the reference voltage and the storage capacitor are arranged on the very same current branch between the two poles of supply of this oscillator. The arrangement of the reference resistance in the oscillator according to the invention generates a single comparator threshold. The arrangement of the reference resistance in the reference current branch is all the more advantageous because the reference current supplied by the reference current source, in a classic application, is virtually independent of the supply voltage. The reference current passing through the reference resistance and the current charging the storage capacitor then form the very same current. In other words, the reference voltage and the ramp voltage are generated by the same reference current passing through the very same reference current branch. This reduces the electrical power consumption of the oscillator significantly. 
     According to a preferred embodiment, the comparator is configured by a modified current mirror comprising two transistors, of which a first is integrated into the reference current branch between the reference resistance and the storage capacitor and the second is integrated into a comparison current branch supplied by a source of dynamic comparison current. 
     As a result of these particular features of the invention, the minimum operating voltage of the oscillator is relatively low. It is composed of the saturation voltage of the source of reference current, of reference voltage and of gate-source voltage of the first transistor. The saturation voltage is low, much lower than the gate-source voltage of a typical transistor, and the reference voltage can be low. The consumption of the oscillator is minimal and its operating voltage is low. Thus, a low-power oscillator is provided in reality. 
     According to a preferred embodiment, the reference resistance is arranged between the drain and the gate of the first transistor. This first transistor is thus diode-connected but with the reference resistance interposed between its drain and its gate. The gate of the second transistor is connected to the drain of the first transistor. 
     According to an advantageous variant, the transistor present in the comparison current branch is arranged in a cascode configuration with an additional transistor. 
     According to another advantageous embodiment, the reference current source of the first module and that of the second module is the very same reference current source used to alternately supply the first and the second module respectively on the basis of control signals delivered to the output of the asynchronous flip-flop. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Other features and advantages of the present invention will become clearer after reading the following detailed description of the embodiments of the invention given by way of fully non-restrictive example and illustrated by the attached drawings, wherein: 
         FIG. 1  as already described, shows a relaxation oscillator according to the prior art; 
         FIG. 2  as already described, shows the development of the ramp voltages of a basic relaxation oscillator in relation to time; 
         FIG. 3  shows a low-power relaxation oscillator according to a first embodiment of the present invention; 
         FIG. 4  shows a low-power relaxation oscillator according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be described below solely on the basis of non-restrictive examples in association of  FIGS. 3 and 4 .  FIG. 3  shows a low-power relaxation oscillator according to a first embodiment of the invention. The overall structure of the relaxation oscillator according to this first embodiment is still like that presented in association with the base relaxation oscillator according to the prior art. Two like modules  21 ,  22  are evident that each comprise a ramp generator, a generator of a reference (voltage?) and a comparator. The output of each of these two modules is then connected to the input terminals of an asynchronous flip-flop  23 , the outputs of which are relooped onto the input of each of the modules. However, the present invention differs from the oscillator according to the prior art by the configuration of the modules. In particular, an advantageous combination of the ramp generator with the generator of a reference voltage and the comparator is provided in order to reduce the current consumption of the module to a minimum. 
     We will now describe the first module  21  in detail and this description will apply equally to the second module  22 . The ramp generator is still based on the principle of charging a storage capacitor  32  by means of a reference current I ref  from a source of reference current  31 . 
     The generator of a reference voltage  34  is advantageously integrated into the ramp generator by the introduction of a reference resistance  33  in series with the reference current source  31 . Thus, the reference current I ref  passes through the resistance so that the voltage at its terminals becomes a reference voltage V ref . It will be noted that the amplitude of oscillations can be chosen arbitrarily by the choice of reference resistance for this purpose. 
     At the same time, the comparator is formed by two transistors m 1  and m 2 , of which the first transistor m 1  is integrated into the reference current branch, i.e. that having the reference current I ref  flowing through it, so that it does not cause any additional consumption of current, and the second transistor is integrated into a comparison current branch supplied by a source of dynamic comparison current  35 , i.e. in which the comparison current I cmp  is produced and consumed solely during the short instants of comparison. The two transistors m 1  and m 2  thus form a current mirror. More specifically, the current terminals (source and drain) of the first transistor m 1  are connected between the reference resistance  33  and the storage capacitor  32  and the control terminal (gate) is connected between the reference resistance  33  and the reference current source  31 . Transistor m 1  is used in strong inversion. With respect to transistor m 2 , its current terminals are connected between earth, in particular V ss , and the source of dynamic comparison current  35  and its control terminal is connected to the reference current branch between the reference resistance and transistor m 1  (i.e. the drain of this transistor m 1 ). It will be noted that the capacitor  32  is also connected to earth, i.e. at the same reference voltage as a current terminal of transistor m 2 . Transistor m 2  is in saturation mode during comparison when the output of the comparator, defined by the current terminal (drain) connected to the comparison current source, is close to the reference supply voltage V dd  at the beginning of the comparison. The output of the comparator is preferably amplified in an inverter (or in one variant a plurality thereof) that is a low current consumer and is then input into an asynchronous flip-flop  23 . 
     The asynchronous flip-flop is an RS type, for example. This flip-flop  23  thus receives the output of the comparator of the first module  21  at a first input (s) and the output of the comparator of the second module  22  at a second input (r) and outputs control signals to alternately charge the storage capacitor of the first module and the storage capacitor of the second module by means of opposed outputs (q) and (qb) that alternately switch switches  24  and  25 . 
     Such an oscillator has proved very advantageous in that it only requires a reference current source that enables the storage capacitor to be charged and also the reference voltage to be supplied for the comparator. Moreover, the comparator is integrated so that the first transistor m 1 , i.e. that in the reference current branch, does not consume additional current and so that the second transistor m 2 , i.e. that in the dynamic current branch, only consumes current during the transition of the comparator when the ramp voltage reaches the reference voltage. 
     We will now describe the operation of the relaxation oscillator of  FIG. 3  in relation to time. For this, the important voltages to be considered are the ramp voltages V ramp1  and V ramp2  in comparison with the reference voltage V ref , as well as the potential of the nodes V x1 , V y1  with respect to the first module  21  and V x2 , V y2  with respect to the second module  22 . 
     The voltage at the node V x1 =V ramp1 +V gs −V ref , where V gs  is the gate-source voltage of transistor m 1  and V ref  is the reference voltage, i.e. I ref *R. At the start, the switch  24  is closed, short-circuiting the storage capacitor  32  that is then discharged. Transistor m 1  is conductive while transistor m 2  is non-conductive. Thus, the ramp voltage V ramp1  is zero as is the comparison current I cmp . In the same period, the voltage at the node V y1 =V dd . It is understood that the presence of the dynamic comparison current source allows the node V y1  to be kept at a defined voltage (V dd ) without then consuming current outside of the comparison, i.e. during the transition that will be explained below. 
     Then upon instruction from the flip-flop via the control signal q, switch  24  is opened and in parallel switch  25  is closed via the control signal qb. The storage capacitor  32  of the first module  21  is then charged and in parallel the voltage V ramp1  increases and the storage capacitor  42  of the second module  22  is abruptly discharged because of the short-circuiting by switch  25 . The second module  22  is then in the initial state described above for the first module, i.e. V ramp2 =0 and V y2 =V dd . The signal output by the second module  22  to the flip-flop  23  is then inverted. 
     When the ramp voltage V ramp1  reaches the level of the reference voltage V ref , the voltage at the node V x1  becomes equal to the voltage V gs . Transistor m 2  then becomes conductive. As a result of this, the voltage at the node V y1  drops to V ss . The signal output by the comparator of the first module to the flip-flop  23  is thus inverted. Since the signal received at the input terminal (s) of the flip-flop has been inverted, outputs q and qb are themselves inverted so that the switch  24  is closed. The signal received at the input terminal (r) of the flip-flop is then itself inverted. A cycle resembling that just described for the first module starts for the second module and so on. Thus, the development of the ramp voltages over time is similar to that shown in  FIG. 2 . The oscillation period is given by the formula:
 
 T=C*V   ref   /I   ref ;
 
where V ref =R*I ref , the above formula can read as follows:
 
 T=R*C.  
 
     Thus, the oscillation period T is only dependent on the reference resistance R and the storage capacitor C. The temperature coefficient of the capacitor is generally negligible and that of the resistance is relatively low. There therefore results an oscillator of low consumption without thus interfering with the supply voltage and the temperature stability. 
       FIG. 4  shows a low-power relaxation oscillator according to a second embodiment of the present invention. This embodiment is the same as the embodiment shown in  FIG. 3  in every aspect except for the sources of reference current used. In this oscillator according to this second embodiment, an advantageous provision is to only use a single reference current source  31  common to both modules  21  and  22  in order to reduce current consumption even further. For this, switches  26  and  27  are integrated between the common source of reference current  31  and the reference resistance  33  of the first module and the reference resistance  43  of the second module respectively. Inverters  28  and  29  are provided to control these two switches  26  and  27  to ensure that the reference current source supplies the current for charging the capacitor  32 , and  42  in turn, when they are not short-circuited. 
     It is understood that various modifications and/or improvements evident to a person skilled in the art can be applied to the various embodiments of the invention described in the present description without departing from the framework of the invention as defined by the attached claims. In particular, a cascode configuration could be used instead of the transistor (m 2 , m 4 ) present in the comparison current branch of the modules.