Integrated circuit phase-locked loop charge pump

Abstract: A charge pump (2) is supplied to reset in rated conditions the error signal of a phase-locked loop of the type whereby a phase detector (1) periodically supplies this pump (2) with a first and second impulse having emission instants dependent on the phase ratio between phase-locked loop input signals and are allocated to control circuit output increase or decrease respectively by means of ring filter (3a, 3b). The pump features circuits (21, 22, 23, 24, 28) to transform the first and second impulse into a first and second voltage signal of longer duration than maximum impulse duration, the voltage signals having values the difference of which depends on the phase ratio between the loop input signals, and to generate a current signal representative of such difference.

Description: This invention refers to CMOS technology analog integrated circuits and more in particular to a current or charge pump to be used in a phase-locked loop (PLL) constructed as an integrated item in this technology, and to a process for generating charge pumping signals. The charge pump is a loop used to cancel the PLL error signal in rated conditions. The pump is typically connected between the phase comparator (or phase and frequency comparator) and the PLL ring filter and essentially consists of a current generator receiving increase and decrease impulses from the comparator and causing a variation to a filter capacitor proportional to any difference in duration of such impulses. Greater details on phase-locked loops are available in documents "Low-Jitter Process Independent DLL and PLL Based on Self-Biased Technique", by J. G. Maneatis, IEEE Journal of Solid State Circuit, Volume 31, Number 11, November 1996, pages 1723 and following, and "Fully Integrated CMOS Phase-Locked Loop with 15 to 240 MHz Locking Range and ± 50 ps Jitter" by I. Novof et al., ISCCC Digest Technical Papers, pages 112 and following, February 1995. Both documents also describe differential control structures for the PLL oscillator. EP 0674392 A1 describes a phase locked loop in which a phase/frequency detector supplies a charge pump with logic signals 0 or 1, leading to an increase or a decrease of the frequency. The loop circuit is generally designed to shield the circuit of the loop from noise. The charge pump is generally desired to start operating even for small loop input phase errors and its sensitivity is therefore an important requirement. Small phase errors correspond to a very small difference in duration between the two impulses supplied to the pump and therefore a very short duration ring filter control signal. In implementing the pump as an integrated circuit, due to the unavoidably far from ideal circuit component features, it was found that correction introduced in case of very short impulses cannot be forecast and is therefore difficult to control. Furthermore, if the PLL oscillator is controlled in the differential mode - a solution normally adopted to reduce jitter - the effects due to each of the control signals are summed together and increase correction indeterminateness. A further problem is the fact that the filter contains a relatively small capacity capacitor, because of technological reasons dependent on integration processes. It follows that the resistance in the filter must in principle be relatively high for reasons of stability, which can cause system malfunctioning additional to the indeterminateness introduced by the too short duration of filter control signals. This last problem was solved by providing resistor-less filters and supplying the loop oscillators with an additional control generated by a supplementary pump for instance, as described in the two documents mentioned above, particularly the one by Novof et al.. This control signal would however be a short duration impulse signal, which disturbs oscillator operation. According to this invention a charge pump is however supplied, which eliminates the need to supply very short impulses to the ring filter and oscillator. According to the invention, the pump allocated to generating a current signal for controlling at least the ring filter in a phase-locked loop where a phase detector periodically supplies such pump with first and second impulses, whereby the time difference between their emission instants depends on the loop input signal phase ratio, which have a predetermined simultaneous presence and which are arranged to respectively control the increase or decrease of a loop output magnitude, the loop comprising: a logic network for recognizing the predetermined simultaneous presence of such first and second impulse and generating a third impulse representative of such simultaneous predetermined presence; a delay element receiving such third impulse and re-emitting it after a preset period of time longer than the predetermined simultaneous presence and shorter than the repetition period of such first and second impulses; first and second integrator circuits respectively actuated by the first and second impulse and deactivated by such third impulse, and generating output signals whose amplitudes are tied to the duration of such first and second impulses respectively and whose duration depends on such emission instants of said first and second impulses and the duration of the preset period of time, subtraction means connected to the output of the first and second integrator circuit and generating at least a first of said current signals having both amplitude and sign representative of any different amplitude between the output signals of such two integrator circuits to be supplied to phase-locked loop ring filter(s). The invention also concerns a process for generating ring filter control current signals by using the pump according to the invention, the process comprising the following operation: recognizing the predetermined simultaneous presence of such first and second impulse and generating a third impulse representative of such predetermined simultaneous presence; delaying such third impulse for a preset period of time longer than the predetermined simultaneous presence of such first and second impulse and shorter than the period of such first and second impulses; converting such first and second impulses into respective first and second voltage signals whose amplitudes are tied to the emission instants of such first and second impulses and whose durations depend on the emission instants of said first and second impulses and the duration of the preset period of time; obtaining from such first and second voltage signals at least a first current signal representative of their difference, for ring filter control. Reference is made to the enclosed drawing, for greater clarification, where: Fig. 1 is the general diagram of a differential structure phase-locked loop, Fig. 2 is a diagram of conventional pump input/output signals, Fig. 3 is the block diagram of a charge pump covered by the invention, Fig. 4 is the circuit diagram of an element of the charge pump covered by the invention, Fig. 5 is a diagram illustrating the operation of the invention, Fig. 6 is the circuit diagram of another element of the charge pump covered by the invention. With reference to Fig. 1, a phase-locked loop essentially consists of: a phase (or phase and frequency) comparator 1, receiving at a first input 10 the reference signal with which it must lock and at a second input 11 the signal to control, and generating on outputs 12, 13 a pair of voltage impulses (hereafter also referred to as impulses U, D), whose duration averaged over the period expresses the input signal phase difference, a charge pump 2 generating a pair of complementary current signals on outputs 2a, 2b, whose average value is tied to the difference in duration of the two impulses U, D, a pair of ring filters 3a, 3b each associated to one of pump 2 outputs 2a, 2b and receiving the signal present on the corresponding output; the filters are considered as only consisting of capacitors, as resistance functions are simulated by a suitable connection between the pump and the oscillator, oscillator 4 piloted both by ring filter 3a, 3b output signals and by a pair of further complementary signals on wires 2c, 2d from the charge pump to compensate for the lack of resistance in filters 3a, 3b and allocated to generating the loop output signal, a first frequency divider 6 acting on oscillator output signal 5 whose output is brought to the second input 11 of comparator 1. Figure 2 shows the input and output signals of a conventional pump, with reference to one filter 3a or 3b. It is postulated that the signal on wire 11 is delayed with respect to the signal on wire 10, so impulse U on wire 12 precedes the impulse D on wire 13 by a time α equal or equivalent to a phase difference; impulses U and D remain present simultaneously for a time Δ which is the duration of the predetermined simultaneous presence. In typical PLL applications such as frequency synthesizers or dock signal recovery circuits, charge pump 2 must operate correctly at values of α lower than 10 ps and the values of Δ can be of several hundred picoseconds (e.g. 400 ps). The impulsive character of the signal present in pump 2 outputs is thus obvious. Figure 3 gives a functional diagram of the pump. Impulses U, D present on comparator 1 outputs 12, 13 are respectively supplied to a first input (charge input) of a pair of integrator circuits 21, 22 and to the two inputs of an AND gate 23 whose output is connected to a delay element 24 (such as a pair of inverters for instance) suitable to introduce onto the signal supplied by the AND gate a delay t o which must be longer than the period of simultaneous presence of the two impulses U, D and therefore than the duration of the shorter of the two, account being taken that these end at the same instant. This duration is the "minimum duration" mentioned in the presentation of the invention and in the claims. Delay element 24 output 25 is in its turn connected to a second input (discharge input) of the two integrators 21, 22. Integrator outputs 26, 27 are brought respectively to the two inputs of a differential amplifier 28, associated to means for generating a pair of complementary current output signals (still considering the case of a differential output), each proportional to phase difference. For ease of drawing reference, these means are indicated by the current generator symbol inside the amplifier. Amplifier 28 output signals are in their turn supplied to respective current mirror systems 29a, 29b, suitable to supply two different current output signals, these being respectively control signals for filters 3a, 3b and additional control signals for the oscillator. For ease of drawing reference, the means for adjusting common mode voltage of the differential structure part has not been shown. These means are provided on all circuits having this structure and present no innovative characteristic, so their description is unnecessary. An example of their implementation is anyhow described in the article by Novof et al. mentioned above. As can be appreciated from figure 4, CMOS technology circuits 21, 22 include: a first type n transistor T1 making up a current generator and provided with a grounded source electrode, a second type p transistor T2 making up a capacitor and provided with a plate connected to voltage Vdd, a third and a fourth transistor T3, T4 acting as switches, the first brought to conduction by the signal U (or respectively D) present on wire 12 (or respectively 13) and the second brought to conduction by the discharge signal present on wire 25. Figure 5 shows the trend of signals generated by circuits 21, 22 in the same phase difference considered for figure 2. Signals on lines 10 to 13 are therefore the same as those of figure 2. Lines 25 and 26-27 show the behavior of tension on the wires with the name number. The two signals on wires 26, 27 are shown overlapping to better illustrate operation of the invention. Circuit 21 is activated at the arrival of signal U; transistor T3 conducts and the capacitor made up by transistor T2 starts to charge through the current generator made up by transistor T1. Tension present on wire 26 decreases linearly until it reaches a minimum value of V U at the end of the duration of impulse U. With delay α, the same situation occurs for circuit 22, whose capacitor made up by T2 charges, so voltage on wire 27 reaches a value of V D . At the end of interval Δ, the switches made up by T3 of the two circuits open and thus transistors T4 are disabled, and voltages V U , V D remain unchanged until the end of interval t o , when the signal generated by delay element 24 arrives. For all this time, differential amplifier 28 generates a signal proportional to V U - V D (or a pair of complementary signals each proportional to V U - V D in case of a differential output) and therefore to the phase difference used to control the filter. At the end of interval t o , the signal on wire 25 brings transistors T4 to conduction and the capacitors made up by T2 therefore discharge and return to initial voltage Vdd. The above dearly applies, mutatis mutandis , to when impulse D precedes impulse U. Circuits 21, 22 are practically time-voltage converters; the difference between generation instants of signals U and D is in actual fact transformed into voltage difference [V U - V D ] that lasts for a time that can be as long as required (obviously within the limit that it has to be shorter than the period of signals on wires 10 and 11) and is given by the sum of maximum duration of impulses U, D and time t o . Rapid response is only required at the level of circuits 21, 22 that can consist of components T1 to T4 to boost performance in terms of velocity as they have to generate control signals only used locally and from which no great robustness is required. Figure 6 shows the circuit of differential amplifier 28 (inclusive of transistors T5 to T9) and current mirror systems 29a, 29b (inclusive of transistors T10 to T19 and T20 to T29 respectively) each consisting of a pair of mirrors each provided with a pair separation stage. The two mirrors of each pair are allocated to generate current signals on outputs 2a, 2c and 2b, 2d respectively. In particular, for generating the signal on wire 2a, amplifier transistor T8 output voltage is applied to the gate of mirror transistor T10, whose drain current is then transformed into output current through pair separating stage T11 to T13 and transistor T14. Similarly, for generation of current on wire 2c, amplifier transistor T8 output voltage is applied to the gate of mirror transistor T15, whose drain current is then transformed into output current through pair separating stage T16 to T18 and transistor T19. The same layout is repeated for generating current on wires 2b, 2d starting from the transistor T9 voltage. It should be noted that the output current could theoretically be picked up directly by transistors T10, T15 or T20, T25 respectively (which therefore make up the current generators shown inside block 28 of figure 2). These transistors must however have substantially similar characteristics to those of transistors T8, T9, which are small to ensure sufficiently fast response, and transistors T10, T15 or T20, T25 would not feature satisfactory behavior as current generators. This is the reason why it was considered advisable to introduce mirrors T11 to T14, T16 to T19 (T21 to T24, T26 to T29). It is obvious that the above description is only given as a non-limiting example and that variations and modifications are possible without leaving the field covered by the invention. In particular, even though reference was made to a ramp that decreases from Vdd to V D or V U for the differential amplifier piloting signal, the opposite solution can obviously be adopted, with a ramp increasing from the ground value. Circuit modifications to the diagrams of Fig.s 4 and 6 are obvious for technicians.

Claims: Phased-locked loop charge pump for the generation of current signals for the control of a phase-locked loop in which a phase detector (1) periodically supplies said pump (2) with first and second impulses, whereby the time difference between their emission instants depends on input signal phase ratio, which have a predetermined simultaneous presence and which are arranged to respectively control the increase or decrease of a loop output magnitude, which pump comprises a logic network (23) recognizing the predetermined simultaneous presence of such first and second impulse, and generating a third impulse representative of such simultaneous predetermined presence; characterized by a delay element (24) receiving such third impulse and re-emitting it after a preset period of time longer than the predetermined simultaneous presence and shorter than the repetition period of such first and second impulses; first and second integrator circuits (21, 22) respectively actuated by the first and second impulse and deactivated by such third impulse, and generating output signals whose amplitudes are tied to the duration of such first and second impulses respectively and whose duration depends on such emission instants of said first and second impulses and the duration of the preset period of time; subtraction means (28) connected to the outputs of the first and second integrator circuit (21, 22) and generating at least a first of said current signals having both amplitude and sign representative of any different amplitude between the output signals of such two integrator circuits (21, 22), to be supplied to phase-locked loop ring filter(s) (3a, 3b). A pump according to claim 1, characterized in that such subtraction means (28) are suitable to generate a second of said current signals supplied directly to oscillator (4) of the phase-locked loop to simulate the resistance of filter(s) (3a, 3b). A pump according to claim 1 or 2, characterized in that such subtraction means consist of a current output differential amplifier (28). A pump according to claim 3, characterized in that such differential amplifier (28) is associated to at least one current mirror element (29a, 29b) including a first and a second mirror (T10 to T14, T15 to T19; T20 to T24, T25 to T29) for generating the first and the second current signal. A pump according to any of the claims from 1 to 4, characterized in that such differential amplifier (28) is associated to a pair of current mirror elements (29a, 29b) for generating the first and the second current signal and respective complementary signals. A pump according to any of the claims from 1 to 5, characterized in that the integrator circuits (21, 22) include: a current generator (T1); a capacitor (T2); and a pair of switches (T3, T4) the first being closed by the first or respectively the second impulse and the second being dosed by the third impulse. A pump according to claim 6, characterized in that such capacitor (T2) is connected to a supply voltage and that such current generator (T1) is connected to earth. A process for generating charge pumping signals for the control of a phase-locked loop whose control signals which serve to control respectively the increase or decrease of the loop output magnitude are obtained from first and second impulses, emitted periodically by a phase detector (1) at respective instants that depend on phase-locked loop input signal phase ratio, and having a predetermined simultaneous presence, characterized by the operations: recognizing the predetermined simultaneous presence of such first and second impulse and generating a third impulse representative of such predetermined simultaneous presence; delaying such third impulse for a preset period of time longer than the predetermined simultaneous presence of such first and second impulses and shorter than the repetition period of such first and second impulses; converting such first and second impulses into respective first and second voltage signals whose amplitudes are tied to the emission instants of such first and second impulses and whose durations depend on the emission instants of said first and second impulses and the duration of the preset period of time; obtaining from such first and second voltage signals at least a first current signal representative of their difference, serving for the control of phase-locked loop ring filter(s) (3a, 3b). A process according to claim 8, characterized in that such first and second voltage signals are obtained by integrating respectively the first and second impulses for periods of time that start at the instant of emission of the respectively first or second impulse and end at the instant of generation of the third impulse. A process according to claim 8 or 9, characterized in that the first and second voltage signals are converted into at least a first and a second current signal, the first one being supplied to the ring filter(s) (3a, 3b) and the second one being to be sent directly to an oscillator circuit (4) for simulating a filter resistance. A process according to any of claims 8 to 10, characterized in that such first and second voltage signals are converted into first and second current and respective complementary signals.

Inventors: BALISTRERI EMANUELE, BURZIO MARCO, BALISTRERI, EMANUELE, BURZIO, MARCO
Applicants: TELECOM ITALIA SPA, TELECOM ITALIA S.P.A.
Application year: 2000
Main classification: H03L7
IPC codes: H03L7