Abstract:
An integrated circuit has a first external supply terminal and a second external supply terminal for applying an external supply voltage to the circuit. The integrated circuit includes an analog unit supplied by at least one internal supply voltage derived from the external supply voltage, a low-pass filter connected to the first external supply terminal and to the second external supply terminal, and a driver connected between the low-pass filter and the analog unit for supplying the at least one internal supply voltage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductors, and, more particularly, to integrated circuits. 
     BACKGROUND OF THE INVENTION 
     A circuit integrated in a chip of semiconductor material is typically supplied by an external supply voltage applied to corresponding terminals of the circuit. In some cases, the integrated circuit includes an analog unit which requires an extremely precise supply voltage with a very high (theoretically infinite) power-supply noise rejection (or PSNR). For example, an integrated circuit may be considered, in which there is a microprocessor and a phase-locked loop (or PLL) which generates an internal clock signal, derived from a corresponding external clock signal, for the microprocessor. The PLL includes a voltage-controlled oscillator (or VCO) which produces a signal having a frequency which depends on a control signal (a voltage) or, more precisely, on a difference between a supply voltage of the oscillator and the control signal. 
     A problem of this integrated circuit is that the external supply voltage (from which the supply voltage of the oscillator is derived) is subject to interference which is generated, for example, by digital circuits of the microprocessor (for example, during the switching of electronic switches) and which modifies the working frequency of the oscillator. In the example, this generates a skew between the external clock signal and the internal clock signal, with a consequent jitter defined as the difference, in absolute value, between the maximum advance skew and the maximum delay skew. 
     A known approach includes the use of a separate external supply voltage for the analog unit; however, this requires two additional supply terminals and considerably complicates a system in which the integrated circuit is connected. Another known approach provides for the use of a reference unit (for example, of the bandgap-based type) which is formed in the integrated circuit and can provide a supply voltage of very precise value, irrespective of interference and temperature variations. However, this reference unit is very complex (since it is typically formed by many transistors) and thus occupies a considerable space on the chip in which the integrated circuit is formed. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome the above-mentioned drawbacks. 
     In order to achieve this and other objects, an integrated circuit having a first external supply terminal and a second external supply terminal for applying an external supply voltage to the circuit, is provided. The integrated circuit also comprises an analog unit supplied by at least one internal supply voltage derived from the external supply voltage, and includes a low-pass filter connected to the first external supply terminal and to the second external supply terminal. Furthermore, a driver is connected between the low-pass filter and the analog unit for supplying the at least one internal supply voltage. 
     Moreover, a corresponding method of supplying an analog unit formed in an integrated circuit is also provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and the advantages of the integrated circuit according to the present invention will become clear from the following description of a preferred embodiment thereof, given by way of non-limiting example, with reference to the appended drawings, in which: 
     FIG. 1 is a block diagram of the integrated circuit according to the present invention; and 
     FIG. 2 is a schematic view of the oscillator with the respective supply circuit in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference in particular to FIG. 1, an integrated circuit  100  (formed in a chip of semiconductor material), in which there is a microprocessor (μP)  105  and a PLL  110 , is shown. The PLL  110  receives an external clock signal Cke and generates a corresponding internal clock signal Cki which is supplied to the microprocessor  105 . The internal clock signal Cki has, for example, a frequency twice that of the external clock signal Cke and has a duty cycle of precisely 50%. 
     The PLL  110  includes a phase and frequency detector (or PFD)  115  which receives the external clock signal Cke and the internal clock signal Cki. The detector  115  generates two output signals Vu and Vd of constant amplitude and of a duration proportional to a phase difference, i.e., a delay or an advance, respectively, of the external clock signal Cke relative to the internal clock signal Cki. The signals Vu, Vd are applied as inputs to a charge pump  120  and to a low-pass filter  125  (connected in cascade) in order to generate a control signal Vc having an amplitude dependent on the signals Vu, Vd. The control signal Vc is applied to an input of a voltage-controlled oscillator (VCO)  130  to latch its working frequency to the frequency of the external clock signal Cke. An output signal Vo of the oscillator  130  is supplied to a frequency divider  135  (for example, dividing by  2 ), which generates the internal clock signal Cki (supplied to the detector  115  and to the microprocessor  105 ). 
     Of course, the present invention applies to a PLL having a different structure, can be used as a discriminator in a frequency-modulation system, etc. The present invention may also be used in an integrated circuit comprising an operational amplifier or any other analog unit which requires high power-supply noise rejection. 
     With reference now to FIG. 2, the integrated circuit  100  has two external supply terminals  205   g  and  205   v  connected, respectively, to a reference terminal (or ground) and to a positive terminal of a supply +Vdd (the negative terminal of which is connected to the ground terminal), for example, equal to +3.3V. The oscillator  130  is a differential ring oscillator formed in CMOS technology. The oscillator  130  includes  3  inverting cells I 1 , I 2  and I 3 , connected to one another in a ring with an output terminal of the last inverting cell I 3  (which generates the output signal Vo) connected to an input terminal of the first inverting cell I 1 . The oscillator  130  has two internal supply terminals  210   l  and  210   h  to which an internal supply voltage Vi (for example equal to 1.5 V) is applied. Each of the inverting cells I 1 , I 2  and I 3  is connected directly to the internal supply terminal  210   h  and is connected to the internal supply terminal  210   l  by a respective regulating resistor R 1 , R 2  or R 3 . The regulating resistors R 1 -R 3  are typically formed by MOS transistors with the control signal Vc being applied to the gate terminals. These transistors have a resistance (between the source terminal and the drain terminal) which is variably dependent on the control signal Vc. 
     Each inverting cell I 1 -I 3  has a propagation delay so that the oscillator generates a square wave having a frequency equal to twice the propagation delay multiplied by the number of inverting cells I 1 -I 3 . The propagation delay depends on the supply voltage of each inverting cell I 1 -I 3  and can thus be adjusted by varying the resistance of the control resistors R 1 -R 3  (and hence the voltage drop between their terminals) by the control signal Vc. Similar considerations apply if the supply voltage (external and internal) has a different value, if the oscillator is formed by a different number of inverting cells (in any case, an uneven number), is not differential, has a different structure, etc. 
     The oscillator  130  is supplied by a circuit comprising a low-pass filter  215  formed by an n-channel MOS transistor Mn 1 , a capacitor C 1  (with a capacitance, for example, of 100 pF), and a p-channel MOS transistor Mp 1 . The transistor Mn 1  has its source terminal connected to the external supply terminal  205   g , its gate terminal connected to the external supply terminal  205   v , and its drain terminal connected to a terminal of the capacitor C 1 . Similarly, the transistor Mp 1  has its source terminal connected to the external supply terminal  205   v , its gate terminal connected to the external supply terminal  205   g , and its drain terminal connected to the other terminal of the capacitor C 1 . 
     The integrated circuit  100  preferably also has an external control terminal  205   c  connected to the drain terminal of the transistor Mp 1 . An external capacitor Ce (with a capacitance, for example, of a few nF) is connected between the external control terminal  205   c  and the ground terminal. The low-pass filter  215  has a differential output formed by the terminals of the capacitor C 1 . Respective driving elements  220   l  and  220   h  are connected between each output terminal of the low-pass filter  215  and the internal supply terminals  210   l  and  210   h . The driving elements  220   l ,  220   h  operate as impedance separators and supply the current required by the oscillator  130 . 
     In particular, the driving element  220   h  is formed by an n-channel MOS transistor Mn 2  having its drain terminal connected to the external supply terminal  205   v , its gate terminal connected to the drain terminal of the transistor Mp 1 , and its source terminal connected to the internal supply terminal  210   h . Preferably, a capacitor Cn 2  (with a capacitance, for example, of 10 pF) is connected between the internal supply terminal  210   h  and the external supply terminal  205   g . The driving element  220   l  is formed by a p-channel MOS transistor Mp 2 , having its drain terminal connected to the external supply terminal  205   g , its gate terminal connected to the drain terminal of the transistor Mn 1 , and its source terminal connected to the internal supply terminal  210   l . A capacitor Cp 2  is connected between the internal supply terminal  210   l  and the internal supply terminal  210   h.    
     The transistors Mn 1 , Mp 1  operate as resistors so that, in a steady state, the capacitor C 1  is charged to the voltage +Vdd, while the voltage between the terminals of the transistors Mn 1  and Mp 1  (between the source and drain terminals) is zero. The voltage between the terminals of the capacitor C 1  keeps the transistors Mn 2  and Mp 2  conductive with a fixed voltage between the gate terminal and the source terminal. The capacitors Cn 2  and Cp 2  are thus charged, bringing the circuit to an equilibrium condition in which the voltages at the internal supply terminals  210   h  and  210   l  are kept at constant values, for example, of 2.5 V and 1V, respectively. The internal supply voltage Vi between the internal supply terminals  210   l  and  210   h  is consequently also constant, at the value of 1.5V in the example in question. 
     It is now assumed that the external supply voltage +Vdd is subject to interference, represented, for example, by a voltage step of 0.3V (of a limited duration such as 50 ns). This interference is filtered by the low-pass filter  215 . In particular, since the voltage between the terminals of the capacitor C 1  cannot vary instantaneously, the interference is initially distributed to the terminals of the transistors Mn 1  and Mp 1 . The voltage between the terminals of the capacitor C 1  is thus modified with an exponential law, with a time constant which is directly proportional to the capacitance of the capacitor C 1 . The voltage at the gate terminals of the transistors Mn 2 , Mp 2  is thus affected by the interference to a lesser extent, consequently reducing the variation in the internal supply voltage Vi. 
     The cut-off frequency of the low-pass filter  215  is limited at the lower end by the capacitance of the capacitor C 1  which cannot adopt excessively high values. The external capacitor Ce (with a much higher capacitance) substantially reduces the lower cut-off frequency of the low-pass filter  215 , enabling low-frequency interference (for example, below 1 kHz) to also be eliminated. 
     The capacitors Cn 2 , Cp 2  form a further filter for this interference. In particular, the capacitors Cn 2  and Cp 2  shunt the interference towards the external supply terminals  205   g  and  205   v , respectively. The capacitor Cp 2  which is connected to the source terminal of the transistor Mn 2  (and not directly to the external supply terminal  205   v ) is also affected by this interference to a reduced extent. This further filter intervenes, however, purely for interference of relatively high frequency since the capacitors Cn 2 , Cp 2  have a low capacitance (substantially lower than that of the capacitor C 1 ) to not excessively limit the frequency response of the oscillator  130  to the control signal Vc. 
     The integrated circuit of the present invention provides an extremely high power-supply noise rejection. This enables, for example, the skew between the external clock signal and the internal clock signal to be limited, with very low jitter. This result is achieved by a relatively simple circuit which occupies a limited space on the chip of semiconductor material in which the integrated circuit is formed. 
     Similar considerations apply if the oscillator does not have a differential structure (and the driving element  220   l  and the transistor Mn 1  are therefore eliminated), if the low-pass filter is formed in a different manner, if the external capacitor is connected between the drain terminal of the transistor Mn 1  and the supply terminal +Vdd, if other equivalent driving means are provided, etc. The integrated circuit of the present invention may, however, also be formed without an external control terminal, with different values of the capacitances of the capacitors C 1 , Cn 2  and Cp 2 , with the capacitor Cp 2  connected directly to the external supply terminal  205   v , or without the capacitors Cn 2  and Cp 2 . 
     The preferred embodiment described above is also particularly compact, since it uses a small number of simple transistors. Moreover, it enables the oscillator to be switched off very rapidly, simply by making the transistors Mn 1  and Mp 2  non-conductive. Similar considerations apply if the transistors Mn 1 , Mp 2  are replaced by other equivalent resistive elements, if the capacitors C 1 , Cn 2 , Cp 2  are formed by MOS transistors (or other equivalent elements), etc. 
     Naturally, in order to satisfy contingent and specific requirements, a skilled artisan may apply to the above-described integrated circuit many modifications and variations, all of which, however, are included in the scope of protection of the invention as defined by the appended claims.