Error-compensated charge pump circuit, and method for producing an error-compensated output current from a charge pump circuit

A charge pump circuit has a charge pump having at least two switched current sources arranged in series. The difference between the currents produced in the two switched current sources contributes to the output current from the charge pump. At least one of the two current sources is controlled by means of a control signal. A control circuit produces the control signal for the at least one controlled current source such that the currents produced in the two current sources are regulated to the same value.

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the priority date of German application DE 10 2004 019 652.4, filed on Apr. 22, 2004, the contents of which are herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to an error-compensated charge pump circuit and to a method for producing an error-compensated output current from a charge pump circuit. The invention also relates to a PLL (phase locked loop) circuit having an error-compensated charge pump circuit.

BACKGROUND OF THE INVENTION

Phase locked loops, also called PLL circuits, are used in many technical fields of application. Inter alia, they are used for frequency multiplication, frequency division and as frequency synthesizers.

FIG. 1shows a schematic illustration of the design of an ordinary PLL. This comprises a phase/frequency detector PFD which compares a reference frequency signal frefwith a frequency signal fout/N, which is obtained by an N-fold frequency divider N_DIV from the output signal foutfrom a voltage-controlled oscillator VCO, for phase and frequency differences. The phase/frequency detector PFD outputs switching signals INCR and DECR which are characteristic of the phase and frequency difference between frefand fout/N. These switching signals INCR, DECR are supplied to a charge pump CP which charges and discharges a loop filter CL and thereby prescribes the voltage V0at the input of the voltage-controlled oscillator VCO.

In the steady state, fout=N·fref, and the two signals frefand fout/N are in phase with one another.

The notional design of the charge pump CP, which is shown inFIG. 1, is known. The charge pump CP comprises two switched constant current sources1,2that are arranged in series between VDD and VSS. The constant current source1produces a current Iup, and the constant current source2produces a current Idown. The constant current source1can be connected to the loop filter CL by means of a switch S1, and charges the loop filter when the switch is in the closed position. Correspondingly, the constant current source2(current sink) can be connected to the loop filter CL by means of a switch S2, and the loop filter CL is discharged with the current Idownwhen S2is in the closed position and S1is in the open position. The output current from the charge pump CP, which current charges/discharges the loop filter CL, is thus formed by the difference between the two switched currents Iupand Idown. The switches S1, S2are in the form of MOS control transistors that are actuated by the switching signals INCR and DECR.

The switching signals INCR and DECR are binary signals. For the period in which the switching signal INCR (DECR) assumes the logic value 1 (“high”), the loop filter CL is charged with the current Iup(discharged with the current Idown). The charge pump CP thus takes pulses of the switching signals INCR and DECR and produces proportional current pulses Iupand Idown.

For the performance of a PLL, it is crucial for the current pulses to be produced with a high level of accuracy. The currents Iupand Idownare intended to be of the same magnitude. This is not always the case in practice, however. The reasons for this are: a mismatch in the speed of p-channel and n-channel transistors in the current sources, a mismatch in the drain/source voltages UDSof p-channel and n-channel transistors in the current sources, and differences in the current sources1,2in the charge pump CP when p-channel and n-channel switching transistors S1, S2are actuated in the same way.

In the steady state of the PLL, the following applies:
∫(Iup−Idown)dt=0.  (1)

If the currents Iupand Idownhave different absolute values, it is necessary to compensate for this in the control loop operation in line with equation (1) through different duty cycles for the switching signals INCR and DECR. This is detrimental to the control accuracy of the PLL and causes systematic control jitter to occur.

The specification U.S. Pat. No. 6,611,160 describes a differential charge pump that comprises two charge pump circuits. The charge pump has complementary transistors that insulate the charge pump from switching noise that occurs in the switching transistors.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.

The invention is directed to an error-compensated charge pump circuit which, when used in a phase locked loop, allows a reduction in the systematic control jitter. The invention is also directed to a method for producing an error-compensated output current from a charge pump circuit which allows control operation in a phase locked loop with reduced systematic control jitter.

According to one aspect of the invention, the inventive charge pump circuit has a charge pump which has at least two current sources which are arranged in series and are each switched by an associated switching signal, where the difference between the currents produced in the two switched current sources contributes to the output current from the charge pump. In accordance with the invention, at least one of the two current sources is a current source which can be controlled by means of a control signal. The charge pump circuit also comprises a control circuit for producing the control signal for the at least one controlled current source, wherein the control circuit regulates the currents produced in the two switched current sources to the same value.

One idea on which the invention is based is to compensate for the different magnitudes of the currents Iupand Idownproduced by the constant current sources by virtue of at least one of the two current sources being a controllable current source. This allows the absolute values of the charging and discharging currents provided by the charge pump circuit to be of the same magnitude. The result of this is that in the steady state of the control loop it is possible to use an identical or substantially identical duty cycle for the switching signals which switch the two current sources, which means that the control jitter in a PLL with an inventive charge pump is significantly reduced.

Whereas constant currents Iupand Idownwith different absolute values in a PLL based on the prior art give rise to PLL control operation with different duty cycles, PLL control operation with ideally identical duty cycles for the switching signals is made possible in a PLL with the charge pump circuit of the present invention through regulation of the level of the discharging and/or charging currents.

In order to be able to set the charging and discharging currents obtained at the output of the charge pump (which corresponds to the output of the charge pump circuit) to (absolute-value) equality using the at least one controllable current source, the control circuit for producing the control signal is actuated in one embodiment by the output of the charge pump and by at least one of the switching signals.

One exemplary embodiment of the invention is characterized in that the control circuit has a control circuit charge pump, whose design is substantially identical to that of the charge pump, and has a differential amplifier which outputs the control signal. In this case, one input of the differential amplifier is connected to a node between the series-connected current sources of the charge pump, and the other input of the differential amplifier is connected to a node between the series-connected current sources of the control circuit charge pump.

The substantially identical circuit design for the charge pump and for the control circuit charge pump in one example makes it possible to compensate for systematic errors, such as the mismatch in the drain/source voltage Udsof the p-channel and n-channel transistors in the current sources of the charge pump or of the control circuit charge pump. The effect achieved by actuating the differential amplifier using the potentials between the series-connected current sources of the charge pump and control circuit charge pump is that in the steady state the operating point of the control circuit charge pump is simulated by the operating point of the charge pump.

In another embodiment of the invention the charge pump circuit is characterized in that the two switched current sources are controllable current sources and are actuated by the control signal. This means that the symmetry of the charge pump in relation to the production of the charging current and discharging current continues to be provided.

In one embodiment of the invention, the two current sources in the control circuit charge pump are jointly actuated by a single switching signal. This ensures that when the charge pump circuit is used in a PLL in the steady state the switching signals ideally have identical duty cycles.

A PLL circuit in line with the invention preferably comprises a current-controlled oscillator, with not only a first inventive charge pump circuit but also a further inventive charge pump circuit contributing to the control current for the current-controlled oscillator.

DETAILED DESCRIPTION OF THE INVENTION

Identical or similar circuit elements to those inFIG. 1are denoted by the same reference symbols inFIG. 2. To avoid repetition, reference is made to the description relating toFIG. 1.

In accordance with one embodiment of the invention, the PLL has two charge pump circuits CP1and CP2. The charge pump circuit CP1comprises a charge pump CP_PROP and a control circuit CP_CON1. The charge pump circuit CP2has a charge pump CP_INT and a control circuit CP_CON2.

The charge pump circuit CP1is connected to the input of a current-controlled oscillator CCO by means of an electrical connection10. In addition, the input of the CCO is connected to the output of the charge pump circuit CP2by means of an electrical connection11and a voltage-to-current converter VTOI. A loop filter CL is charged/discharged by the output current from the charge pump circuit CP2.

The charge pump circuit CP1represents the proportional path in the phase locked loop and affects the phase control of the PLL. The charge pump circuit CP2, the loop filter CL and the voltage-to-current converter VTOI represent the integral path of the control loop and effect the frequency control of the PLL. It should be pointed out that instead of a CCO it is also possible to provide a VCO after suitable current-to-voltage conversion in the electrical connection10. In this case, the voltage-to-current converter VTOI is dispensed with (or the VTOI and the CCO form the VCO).

The text below gives a more detailed description of the design of the charge pump circuit CP1.

The charge pump CP_PROP differs from the charge pump CP shown inFIG. 1by virtue of the constant current source1(which produces the constant current Iup) having a controllable current source3connected in parallel with it. Similarly, the constant current source (current sink)2, which produces the constant current Idown, has a controllable current source4connected in parallel with it. The controllable current sources3,4are voltage-controlled and are actuated by a common actuation signal5. The controllable current source3produces a current Ic1that is added to the constant current Iup. The controllable current source4produces a current IC2that is added to the current Idownfrom the constant current source2.

The electrical connection10is connected to a node K1between the two switches S1and S2. In this respect, the parallel circuit comprising the current sources1and3determines the charging current Iup+IC1and the parallel circuit comprising the current sources2,4determines the discharging current Idown+IC2, which both flow via the electrical connection10.

The two controllable current sources3,4may be produced with different input transistors (p-channel and n-channel transistors), for example, which means that the actuating response is different. Another option (not shown) is for the controllable current sources3,4to be actuated by means of two separate actuation signals5(inverted and non-inverted).

It should be pointed out that, unlike inFIG. 1, the switch S1is actuated by the switching signal DECR and the switch S2is actuated by the switching signal INCR.

In terms of the circuit formed by the current sources1,2,3,4and switches S1, S2, the control circuit CP_CON1, which provides the actuation signal5for the charge pump CP_PROP, has an identical design to the charge pump CP_PROP. Unlike the circuit design for the charge pump CP_PROP, however, the two switches S1and S2are actuated by the same switching signal (INCR). In addition, the node K1′ between the switches S1and S2is used not to actuate the CCO but rather to drive the non-inverting input of a differential amplifier6. The node K1′ is also connected to VSS via a capacitor CR.

The inverting input of the differential amplifier6is connected to the node K1of the charge pump CP_PROP by means of the electrical connection10.

It should be noted that in one embodiment the current sources1,2,3,4and switching transistors S1, S2in the charge pump CP_PROP and in the control circuit CP_CON1are on the same chip and are produced using the same technologies.

The mode of actuation of the charge pump circuit CP1is as follows: the differential amplifier6compares the voltage UAat the operating point (node K1) of the charge pump CP_PROP with the voltage U′Aat the operating point (node K1′) of the charge pump1,2,3,4, S1, S2in the control circuit CP_CON1. The compensating currents IC1and IC2generated in the controllable current sources3,4in the control circuit CP_CON1are mirrored in the charge pump CP_PROP and are added to the constant currents Iupand Idown(which in practice are only approximately constant currents, since they have a certain dependency on the level of the operating point voltage UA), which results in compensation for the systematic error (absolute-value discrepancy between Iupand Idown). When IC=IC1−IC2, the time profile for the voltage U′A(t) is as follows:

The transient condition is as follows:
U′A(t)−UA(t)=const fort→∞.(3)

In the steady state, the operating points U′Aand UAthus differ by a constant value const, the result of which is that the actuation signal5has the correct level for current compensation.

For the voltage UA(t) at the operating point (node K1) of the charge pump CP_INT (in the charge pump circuit CP2), the following applies:

The operating point UAof the charge pump CP_PROP in the steady state is determined by the transfer function of the current-controlled oscillator CCO and is generally different from the operating point UAof the charge pump CP_INT in the steady state.

In equations (2) and (4), con1and con2denote constants that influence the transient period and can be set in suitable fashion by applying a charge (precharging) to capacitors (e.g. CR) before the transient process (t=0).

In the steady state, the following applies:
∫(Iup−Idown+Ic)dt=0 with the conditionDECR=INCR(duty cycle).  (5)

On account of the total identity of the circuits CP_PROP and CP_CON1in relation to the charge pump, the accuracy of the current compensation expressed by equation (5) is now determined only by the inevitable differences (mismatch) between identical transistors (in the case of operation with almost the same operating points UAand U′A). In other words the inevitable transistor mismatch between transistors of substantially identical design—in the charge pumps in CP_PROP and CP_CON1, but no longer by a mismatch in UDSin p-channel and n-channel transistors in the charge pumps or as a result of differences between the constant current sources1,2when the switching transistors S1and S2are actuated in the same way. It should further be pointed out that the inventive regulation of the compensating current ICof the present invention also corrects parasitic switching currents that arise as a result of the occurrence of parasitic charges during switching operations in the switching transistors S1, S2.

The design of the charge pump circuit CP2corresponds essentially to the design of the charge pump circuit CP1, which is why a detailed description is not given, in order to avoid repetition. The only difference in relation to the circuit design is that in the charge pump CP_INT the switch S1is actuated by the switching signal INCR and the switch S2is actuated by the switching signal DECR, and that in the control circuit CP_CON2the non-inverting input of the differential amplifier6is connected to the node K1(operating point UA) and the inverting input is connected to the node K1′ (operating point U′A) of the control circuit charge pump1,2,3,4, S1, S2. The whole circuit shown inFIG. 2can be produced on a single chip.

The different actuation of the current-controlled oscillator CCO by the charge pump circuits CP1and CP2—the charge pump circuit CP1actuates the input of the CCO directly, while the charge pump circuit CP2actuates the CCO using the loop filter CL and the voltage-to-current converter VTOI—means that the magnitudes of UA, U′Ain the control circuits CP_CON1and CP_CON2and the magnitudes IC1, IC2and ICin the charge pumps CP_PROP and CP_INT are generally different. Hence, different compensating currents ICare generally produced in the charge pump circuits CP1and CP2, with the actuation signals5in the charge pump circuit CP1and in the charge pump circuit CP2generally also assuming different values (in the steady state). To avoid unnecessarily complex mathematical representation, the variability of the variables IC1, IC2, IC, UAand U′Ain the charge pump circuits CP1and CP2is not taken into account in the notation (e.g., through appropriate indexes).

One identical feature of the charge pump circuits CP1and CP2, however, is the joint actuation by means of the switching signals DECR and INCR with the condition DECR=INCR (identical duty cycles) in the steady state.