Triangular-wave voltage generator and corresponding class-D amplifier circuit

Abstract

A triangular-voltage generator has an input terminal that receives a power supply voltage and an output terminal that supplies a triangular-wave voltage having a repetition period. An operational amplifier in an integrator configuration has a first input, a second input and an output coupled to the output terminal. The second input receives a reference voltage as a function of the power supply voltage. The first input is selectively and alternately connected to the input terminal during a first half-period of the repetition period and to a reference terminal during a second half-period of the repetition period.

Claims

1. A triangular-voltage generator, comprising: an input terminal configured to receive a first power supply voltage; an output terminal configured to supply a triangular-wave voltage having a repetition period; an operational amplifier in an integrator configuration having: a first input configured to be selectively and alternately connected to said input terminal during a first half-period of said repetition period, via a first resistor element, and to a reference terminal during a second half-period of said repetition period, via a second resistor element; a second input configured to receive a reference voltage; and an output coupled to said output terminal.

2. The generator according to claim 1, wherein the operational amplifier in the integrator configuration includes an integration capacitor connected between said output of said operational amplifier and said first input of said operational amplifier, said integration capacitor configured to be charged by a charging current during said first half-period and discharged by a discharge current during said second half-period.

3. The generator according to claim 2, wherein an amplitude of said triangular-wave voltage is proportional to an amplitude of said first power supply voltage and inversely proportional to a product of a capacitance of said integration capacitor and a resistance of said second resistor element.

4. The generator according to claim 1, further comprising: a first switching element connected between said first input and said first resistor element and controlled to close during said first half-period and to open during said second half-period; and a second switching element connected between said first input and said second resistor element and controlled to open during said first half-period and to close during said second semi-period.

5. The generator according to claim 2, wherein the operational amplifier in the integrator configuration further comprises an integration resistor connected between said output and said first input of said operational amplifier, said integration resistor having a resistance value that is greater than a ratio between said repetition period and the capacitance of said integration capacitor.

6. The generator according to claim 1, further comprising a resistive divider coupled to said input terminal and configured to generate said reference voltage as a division, of a division factor, of said first power supply voltage.

7. The generator according to claim 6, in which the following expression is true:
R.sub.i1=(k1).Math.R.sub.i2 where R.sub.i1 is a resistance of said first resistor element, R.sub.i2 is a resistance of said second resistor element, and k is said division factor.

8. The generator according to claim 6, wherein the division factor of said resistive divider is variable by a variability factor that defines a corresponding variation of said reference voltage.

9. The generator according to claim 8, wherein said resistive divider includes a first divider resistor connected between said input terminal and a dividing node that is coupled to said second input of said operational amplifier, and a second divider resistor connected between said dividing node and said reference terminal; and wherein a resistance of said second divider resistor is a variable resistance such as to define said variability factor of said division factor.

10. The generator according to claim 8, wherein variation of said division factor is determines a variation of a mean value and a substantial invariability of the amplitude of said triangular wave.

11. The generator according to claim 10, wherein said division factor is equal to or greater than 1.5.

12. The generator according to claim 8, further comprising a control circuit configured to measure a mean value of the triangular-wave voltage and determine the variability factor of said division factor as a function of the measured mean value.

13. A Class-D amplifier circuit, comprising: at least one signal input configured to receive an input signal; at least one output configured to provide an amplified output signal; an output switching circuit powered between a first power supply voltage and a reference voltage, said output switching circuit configured to provide said amplified output signal; a feedback circuit in an integrator configuration that connected between said signal output and said signal input; a comparator circuit configured to control said output switching circuit, said comparator circuit having a first comparison input coupled to said feedback circuit and a second comparison input connected to receive a triangular-wave voltage having a repetition period; and a triangular-voltage generator comprising: an input terminal configured to receive the first power supply voltage; an output terminal configured to supply the triangular-wave voltage; a first operational amplifier in an integrator configuration having: a first input configured to be selectively and alternately connected to said input terminal during a first half-period of said repetition period, via a first resistor element, and to a reference terminal during a second half-period of said repetition period, via a second resistor element; a second input configured to receive a reference voltage; and an output coupled to said output terminal.

14. The circuit according to claim 13, wherein said feedback circuit includes: a second operational amplifier having an input connected to said signal input and an output connected to the first comparison input of the comparator circuit; a respective integration resistor connected to form a closed feedback loop between said signal output and said input of the second operational amplifier; and a respective integration capacitor connected between said input and said output of said second operational amplifier; wherein a gain bandwidth product of said amplifier circuit is proportional to an amplitude of said first power supply voltage and inversely proportional to a product of a capacitance of said respective integration capacitor and a resistance of said respective integration resistor.

15. The circuit according to claim 13, wherein said comparator circuit is powered by said first power supply voltage.

16. The circuit according to claim 13, wherein said comparator circuit is powered by a dedicated power supply voltage with an amplitude less than the amplitude of said first power supply voltage; and wherein said triangular-voltage generator further includes a resistive divider circuit coupled to said input terminal and configured to generate said reference voltage as a division of said first power supply voltage with a division factor having a variable value such as to maintain a mean value of said triangular-wave voltage about a value equal to a half-amplitude of said dedicated power supply voltage.

17. A triangular-voltage generator, comprising: an operational amplifier having a first input, a second input and an output that produces a triangular wave signal; a first resistor and first switch connected in series between the first input and a supply voltage node; a second resistor and second switch connected in series between the first input and a ground voltage node; a third resistor and capacitor connected in parallel between the output and the first input; a voltage divider coupled between the supply voltage node and ground voltage node and configured to generate a divided voltage applied to the second input of the operational amplifier; wherein the first switch is actuated during a first half-period of said repetition period and the second switch is actuated during a second half-period of said repetition period.

18. The generator of claim 17, wherein said voltage divider includes a variable resistance.

19. The generator of claim 18, further comprising a control circuit configured to control setting of the variable resistance in response to a mean voltage of the triangular wave signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is further described below with reference to preferred embodiments of same, which are provided purely as non-limiting examples, and to the attached drawings, in which:

(2) FIG. 1 shows a circuit diagram of a closed-loop class-D amplifier circuit,

(3) FIG. 2 is a graph of a loop gain as a function of frequency in the circuit in FIG. 1,

(4) FIG. 3 is a circuit diagram of a triangular-voltage generator of a known type,

(5) FIG. 4 is a graph of a triangular voltage generated by the generator in FIG. 3,

(6) FIG. 5 is a circuit diagram of a triangular-voltage generator according to a first embodiment of the present solution,

(7) FIG. 6 is a circuit diagram of a triangular-voltage generator, according to a second embodiment of the present solution,

(8) FIG. 7 is a graph showing the gain performance of the generator in FIG. 6, and

(9) FIG. 8 shows another variant embodiment of the triangular-voltage generator according to the present solution.

DETAILED DESCRIPTION

(10) FIG. 5 shows a triangular-voltage generator, in this case indicated using reference sign 20, according to a first embodiment of the present solution. This triangular-voltage generator 20 may, for example, be used in the class-D amplifier circuit 1 described previously and shown in FIG. 1, to provide the triangular voltage V.sub.TRI to the input of the comparator circuit 5, enabling the PWM modulation of the output signal generated.

(11) The triangular-voltage generator 20 has an input terminal IN receiving the high power supply voltage V and an output terminal OUT that supplies the triangular voltage V.sub.TRI.

(12) As described above, the output terminal OUT is designed to be coupled electrically to the comparator circuit 5 of the amplifier circuit 1, in particular to the related first comparison input. In particular, this generator 20 is applied advantageously where said comparator circuit 5 is powered by the high power supply voltage V.sub.HV.

(13) In detail, the triangular-voltage generator 20 includes an operational amplifier 21, in integrator configuration, having a first input (inverting), a second input (non-inverting), and an output that supplies the triangular voltage V.sub.TRI at the output terminal OUT. An integration resistor R.sub.F and an integration capacitor C are connected in parallel between the output and the first input of the aforementioned operational amplifier 21 (the following equation being true: R.sub.F>>T.sub.S/C, where T.sub.S again represents the repetition period of the triangular voltage V.sub.TRI).

(14) The first input of the operational amplifier 21 is selectively couplable to the input terminal IN of the voltage generator 20, via a first switch 23 and a first input resistor R.sub.i1. The first input resistor R.sub.i1 and the first switch 23 are connected in series between the aforementioned input terminal IN and the first input of the operational amplifier 21. The first switch 23 is controlled by a first square-wave control signal S.sub.c1 with a period equal to the repetition period T.sub.S and a duty cycle equal to T.sub.S/2.

(15) Said first input of the operational amplifier 21 is also selectively couplable to a reference terminal (ground, gnd) of the voltage generator 20 by means of a second switch 24 and a second input resistor R.sub.i2. The second input resistor R.sub.i2 and the second switch 24 are connected in series between the aforementioned reference terminal (ground, gnd) and the first input of the operational amplifier 21. The second switch 24 is controlled by a second square-wave control signal S.sub.c2 with a period equal to the repetition period T.sub.S and a duty cycle equal to T.sub.S/2. In particular, the first and second control signals S.sub.c1, S.sub.c2 are in antiphase, i.e. the first control signal S.sub.c1 is active in a first half-period T.sub.S/2, while the second control signal S.sub.c2 is active in the second half-period T.sub.S/2 of the repetition period T.sub.S.

(16) The first input of the operational amplifier 21 is therefore selectively and alternately connected to the input terminal IN (in a first half-period T.sub.S/2 of the repetition period T.sub.S), or to the reference terminal (in a second half-period T.sub.S/2 of the same repetition period T.sub.S).

(17) The voltage generator 20 also includes a divider circuit 26 formed by first and second divider resistors R.sub.A, R.sub.B connected in series between the input terminal IN and the reference terminal (ground, gnd), together forming an internal dividing node N.sub.p.

(18) In particular, the second input of the operational amplifier 21 is connected to the aforementioned internal dividing node N.sub.p and receives a reference voltage V.sub.REF.

(19) It is evident that the aforementioned reference voltage V.sub.REF is given by the following expression:
V.sub.REF=V.sub.HV/k,
where k is the division factor of the divider circuit 26, which is in turn given by:
k=(R.sub.A+R.sub.B)/R.sub.B.

(20) Moreover, the reference voltage V.sub.REF is the mean value V.sub.TRI.sub._.sub.MEAN of the triangular voltage V.sub.TRI.

(21) When in use, the first and second switches 23, 24 are alternately commanded to open/close in the first and second semi-periods T.sub.s/2 of the repetition period T.sub.s, such that in a first half-period T.sub.s/2 a charging current is supplied at the first input of the operational amplifier 21 via the first input resistor R.sub.i1 (the first switch 23 is closed and the second switch 24 is open), and in the second half-period T.sub.s/2 a discharge current is extracted from said first input of the operational amplifier 21 via the second resistor R.sub.i2 (the first switch 23 is open and the second switch 24 is closed).

(22) In steady state, the following equality relation is satisfied between the charging and discharge currents, both of which are equal to an integration current I:

(23) $\frac{V_{\mathrm{HV}}\left(1-1/k\right)}{R_{i1}}=\frac{V_{\mathrm{HV}}}{k{R}_{i2}},$
from which the following can be derived:
R.sub.i1=(k1).Math.R.sub.i2.

(24) If this relation is satisfied, it is easy to demonstrate that the amplitude of the triangular voltage V.sub.TRI is given by:

(25) $V_{\mathrm{TRI}}=\frac{V_{\mathrm{HV}}}{k}\frac{1}{2{\mathrm{fsR}}_{i2}C},$
and is therefore proportional to the amplitude of the high power supply voltage V.sub.HV and inversely proportional to the product R.sub.i2C.

(26) It is therefore sufficient to ensure that the resistance and capacitance values R.sub.i2 and C are matched to the respective resistance and capacitance values R.sub.2 and C.sub.1 of the product R.sub.2C.sub.1 in the expression of the gain bandwidth product GBWP of the amplifier circuit 1 (see discussion above), such that the amplitude V.sub.TRI satisfies the aforementioned requirements, making the gain bandwidth product GBWP of the amplifier circuit 1 substantially independent of the process and temperature spreads.

(27) Furthermore, the mean value V.sub.TRI.sub._.sub.MEAN of the triangular voltage V.sub.TRI is equal to V.sub.HV/k. With k=2, this mean value is perfectly centered in relation to the dynamic (0-V.sub.HV) of the comparator circuit 5 of the amplifier circuit 1

(28) A triangular-voltage generator according to a second embodiment of the present solution, is described below with reference to FIG. 6, and is also indicated using reference sign 20. This embodiment is advantageously used where the comparator circuit 5 of the amplifier circuit 1 is powered by the dedicated power supply voltage V.sub.DD, having a different value, in particular a lesser value, than the high power supply voltage V.sub.HV.

(29) The triangular-voltage generator 20 of FIG. 6 differs from the solution described above with FIG. 5 in that the division factor k is variable and obtained for example by introducing a variable resistance for the second divider resistor R.sub.B.

(30) In this embodiment, it is in fact necessary for the triangular voltage V.sub.TRI to adapt to the input dynamic of the comparator circuit 5, limited to the range 0-V.sub.DD. The variation in the division factor k (in this case by varying the resistance of the second divider resistor R.sub.B) makes it possible to maintain the mean value of the triangular voltage V.sub.TRI.sub._.sub.MEAN (again given by V.sub.HV/k) around the value V.sub.DD/2, regardless of the amplitude value of the dedicated power supply voltage V.sub.DD.

(31) The variation of the division factor k nonetheless results in a modification of the amplitude of the triangular voltage V.sub.TRI (not just of the mean value of same). Advantageously, the configuration described determines that the effect (or sensitivity) of variation on the mean value of the triangular voltage V.sub.TRI.sub._.sub.MEAN is large, such as to be adaptable to the spread of the dedicated power supply voltage value V.sub.DD, and simultaneously to determine that the effect of variation on the amplitude of said triangular voltage V.sub.TRI is minimal (or that the amplitude of the triangular voltage V.sub.TRI is substantially insensitive to the variation in the division factor k).

(32) In particular, modifying the division factor k by a factor k changes the reference voltage V.sub.REF accordingly by a factor V.sub.REF.

(33) It can be shown that, as a result of such modification, the following expressions are true, identifying the corresponding modification of the mean value V.sub.TRI.sub._.sub.MEAN and of the amplitude V.sub.TRI of the triangular voltage:

(34) $V_{\mathrm{TRI\_MEAN}}=1+\frac{1}{2}{R}_F\left(\frac{1}{R_{1i}}+\frac{1}{R_{i2}}\right)V_{\mathrm{REF}},\mathrm{and}$$V_{\mathrm{TRI}}=\frac{1}{4f_S{R}_{i2}C}\frac{k-2}{k-1}{V}_{\mathrm{REF}}.$

(35) Bearing in mind that, as discussed above, the amplitude of the triangular voltage V.sub.TRI is in this case given by:

(36) $V_{\mathrm{TRI}}=\frac{V_{\mathrm{HV}}}{k}\frac{1}{2f_S{R}_{i2}C},$
and that the mean value of said triangular voltage is equal to V.sub.TRI.sub._.sub.MEAN=V.sub.HV/k, the aforementioned expressions can be rewritten as follows:

(37) $V_{\mathrm{TRI\_MEAN}}=1+\frac{1}{2}\frac{R_F}{R_{i2}}\frac{k}{k-1}{V}_{\mathrm{REF}}=G_{\mathrm{MEAN}}.Math.V_{\mathrm{REF}}\mathrm{and}$$V_{\mathrm{TRI}}=\frac{1}{2}\frac{V_{\mathrm{TRI}}}{V_{\mathrm{TRI\_MEAN}}}\frac{k-2}{k-1}{V}_{\mathrm{REF}}=G_{\mathrm{AMP}}.Math.V_{\mathrm{REF}},$
in which G.sub.MEAN represents the gain associated with the mean value V.sub.TRI.sub._.sub.MEAN and G.sub.AMP represents the gain associated with the amplitude of said triangular voltage V.sub.TRI, as a function of the division factor k.

(38) In particular, given that the amplitude V.sub.TRI and the mean value V.sub.TRI.sub._.sub.MEAN of the triangular voltage can have a fixed value, once the dedicated power supply voltage V.sub.DD is known, the aforementioned variations V.sub.TRI.sub._.sub.MEAN and V.sub.TRI then depend only on the division factor k.

(39) As shown in FIG. 7, advantageously, for division factor values k that are greater than a given threshold, for example 1.5 (k1.5), the evolution of the amplitude gain, G.sub.AMP, is substantially constant in relation to the division factor k, while the evolution of the mean-value gain, G.sub.MEAN, varies widely in relation to said division factor k.

(40) As sought after, the solution described therefore makes it possible, by modifying the division factor k (in this example by varying the resistance of the second divider resistor R.sub.B), to desirably vary the mean value of the triangular voltage V.sub.TRI.sub._.sub.MEAN, without substantially modifying the amplitude of said triangular voltage V.sub.TRI.

(41) Said amplitude can therefore be appropriately dimensioned to satisfy the requirements for obtaining a gain bandwidth product GBWP that is substantially constant, while exploiting the variability of the mean value of the triangular voltage V.sub.TRI.sub._.sub.MEAN to adapt to the variable dynamic of the comparator circuit 5 of the amplifier circuit 1.

(42) It can be shown that the variation (trimming) of the resistance value of the second divider resistor R.sub.B can also help to reduce or eliminate the offset of the amplifier circuit 1, improving precision in the generation of the triangular voltage V.sub.TRI.

(43) The advantages of the solution proposed are clear from the above description.

(44) In any case, it is emphasized that the triangular-voltage generator 20 described above makes it possible to: provide a substantially constant gain bandwidth product GBWP, regardless of process and/or temperature variations, thereby maximizing the performance of a related class-D amplifier circuit, for example in terms of PSRR and THD, limit noise generation, since the only noise contributors are associated with resistor elements (R.sub.A, R.sub.B, R.sub.i1, R.sub.i2), in the absence of transistors and current mirrors, limit offset generation, given that only the operational amplifier 21 contributes to offset (in the absence of transistors and current mirrors) and that the variability in the division factor k can be used to further limit said offset, improve linearity, given that the charging and discharge currents are determined by resistors, which provide better linearity than transistors, for example, and improve sensitivity, given that the division ratio k can be varied to adapt to any possible power supply voltage of the comparator circuit.

(45) It is evident that modifications and variations may be made to the subject matter described and illustrated without thereby moving outside the scope of protection of the present invention, as defined in the attached claims.

(46) In particular, the variation of the division factor k in the second embodiment could applied be achieved similarly by varying the resistance of the first divider resistor R.sub.A of the divider circuit 26.

(47) Furthermore, as illustrated in FIG. 8, a closed-loop control circuit 30 may be advantageously implemented, again in the second embodiment described, to measure the mean value of the triangular voltage V.sub.TRI.sub._.sub.MEAN and, on the basis of said measurement regarding the current value of the dedicated power supply voltage V.sub.DD, to appropriately adjust the resistance R.sub.B, in order to implement dynamic control (in real time) of the mean value V.sub.TRI.sub._.sub.MEAN of the triangular voltage V.sub.TRI.sub._.sub.MEAN without however changing the amplitude of same.