Open-loop line driver control method and apparatus

According to an embodiment, a circuit includes an amplifier and an open-loop control system. The amplifier has an output stage for amplifying a signal, a power supply for driving a supply voltage of the output stage to different voltage levels responsive to being modulated and a pulse width modulator for modulating the power supply responsive to a mask input. The open-loop control system includes a mask generator and a detector. The mask generator is configured to generate the mask input as a function of the envelope of the signal. The detector is configured to detect discontinuities in the mask input and compensate for the discontinuities.

BACKGROUND

Certain communication technologies such as xDSL have a high peak-to-average power ratio (PAR) which is the ratio of the peak signal power to the average power of the signal, where “x” specifies a particular variant of DSL (digital subscriber line). The term xDSL refers to DSL technologies such as ADSL (asymmetric DSL), HDSL (high bit rate DSL), IDSL (ISDN DSL), SDSL (symmetric DSL), VDSL (very high speed DSL), etc. These and other types of xDSL systems are generically referred to herein as “DSL” systems. A high PAR requirement places severe constraints on the line driver circuitry of DSL equipment. The line driver is typically used to amplify a signal and drive the amplified signal onto a line. Many types of conventional line driver circuits include class-A or class-AB amplifiers. However, class-A and -AB amplifiers are not well suited for high PAR applications such as DSL because theses classes of amplifiers remain fully powered even when not transmitting at peak power, thereby wasting significant energy.

Class-H amplifiers can be used in DSL equipment to reduce power consumption. A class-H amplifier can be constructed by adjusting the supply voltage of a conventional class-AB amplifier using a dynamic DC-DC converter in response to amplitude fluctuations in the signal to be transmitted. The power draw of a class-H amplifier is ideally reduced by the crest factor of the signal, where the crest factor is the square root of the PAR. Using a class-H amplifier as a line driver for high PAR applications potentially yields substantial power savings, especially for DSL technologies having a very high crest factor. In a practical realization, although the gain is reduced by necessary tolerance ranges in the voltage tracking, it is possible in principle to reduce the power consumption to 50% to 70% relative to a conventional class-AB amplifier.

In conventional systems, the output voltage of the DC-DC converter coupled to a class-H amplifier is typically set by means of a pulse width modulator (PWM). The duty cycle of the PWM determines the level of supply voltage applied to the class-H amplifier. The duty cycle of the PWM is a function of a mask input to the PWM. The profile of the mask depends on the envelope of the input signal of the amplifier. Since the output voltage of the DC-DC converter cannot follow the signal profile at every desired speed, the amplifier supply voltage, as early as before the occurrence of a signal peak, must be ramped up from a specific base level to the corresponding peak voltage in a controlled manner with a finite edge. Otherwise, the signal being amplified will be distorted. In addition, after the occurrence of the signal peak, the amplifier supply voltage should be ramped down back to the base level or some other suitable voltage level to save power. The profile of mask input to the PWM should thus correspond to changes in the signal being amplified so that the amplifier supply voltage can be properly ramped up and down to prevent clipping of the output signal while maximizing efficiency.

The DC-DC converter that generates the amplifier supply voltage has two operating modes: a continuous mode and a discontinuous mode. In the continuous mode, a current always flows through a coil inductor of the converter during the entire switching cycle, and the ratio of output voltage to input voltage of the DC-DC converter depends to a first approximation on only the duty ratio D of the PWM as given by:

Vout=-Vi⁢⁢n·D1-D(1)
If the load current falls below a certain limit value, the coil current decreases to zero during certain portions of the switching cycle. In this so-called discontinuous mode, the output voltage is no longer dependent only on the duty ratio of the PWM, but it is also dependent on the inductance L of the coil, the PWM frequency 1/Ts and the load resistance R of the class-H stage as given by:

The mask input to the PWM can be described as a superposition of temporally offset and scaled ramp functions and can be generated in various ways. Upon each change from a flat level to a rising or falling edge, from a rising edge to a falling edge, or vice versa, a discontinuity point forms in the profile of the mask. Each discontinuity causes the DC-DC converter to affect a transient oscillation in accordance with its ramp response. Moreover, the DC-DC converter is ideally supplied with an uncontrolled input voltage in order to avoid an additional loss of efficiency as a result of a further DC-DC conversion. Ultimately, operation in the discontinuous mode is greatly dependent on the tolerance of the coil inductance L of the DC-DC converter. For these reasons, typically a closed-loop controller feeds back the output of the DC-DC converter to control the input of the PWM. The closed-loop controller compares the output voltage of the DC-DC converter with the value of the mask and correspondingly adapts the mask input to the PWM so that the output voltage of the converter follows the mask within acceptable tolerances.

Implementing a class-H amplifier by means of a closed-loop control system has several disadvantages. For example, the closed-loop control system requires feedback. Since the specific application involves the implementation of closed-loop control and PWM generation in the data pump portion of the line card, a further line is needed per channel for returning the output voltage or an equivalent measurement signal as a feedback signal. These extra feedback lines are in addition to the line required to communicate the PWM signal to the line driver. All of these additional lines significantly increase the routing outlay on the line card. Also, the closed-loop controller requires additional circuitry outlay in the data pump. For example, converting the amplifier voltage into a usable digital feedback signal requires additional A/D circuitry in the data pump or conversion into an additional PWM signal in the line driver. Also, a counter is typically needed in the data pump to evaluate the duty ratio. Each of these extra circuits requires additional outlay.

The closed-loop controller for a class-H amplifier also never exactly hits a predetermined target value. Particularly in the case of flat voltage levels, it should be expected that periodic control patterns will be established since the controller jitters about the target value. These control patterns are also superimposed on the supply voltage and forwarded to the signal output of the line driver in a manner attenuated by the power supply rejection ratio (PSRR). This causes disturbance frequencies in the signal being amplified which are difficult to predict and may lie in the useful band. In order to reduce these disturbance frequencies to a minimum, a comparatively high resolution of the PWM duty ratio is required. This necessitates a counter having a very high clock rate, making the implementation of the closed-loop controller even more difficult.

SUMMARY

According to an embodiment, a circuit includes an amplifier and an open-loop control system. The amplifier has an output stage for amplifying a signal, a power supply for driving a supply voltage of the output stage to different voltage levels responsive to being modulated and a pulse width modulator for modulating the power supply responsive to a mask input. The open-loop control system includes a mask generator and a detector. The mask generator is configured to generate the mask input as a function of the envelope of the signal. The detector is configured to detect discontinuities in the mask input and compensate for the discontinuities.

DETAILED DESCRIPTION

FIG. 1illustrates an embodiment of a transmitter circuit100including an amplifier110and an open-loop control system120. The amplifier110includes a power supply112, an output stage114and a pulse width modulator (PWM)116. The transmitter circuit100is well-suited for high PAR applications such as DSL. In one embodiment, the amplifier110is a class-H amplifier and the power supply112is a buck-boost DC-DC voltage converter. The power supply112provides the supply voltage (VDD/SS—DYN) for the output stage114of the amplifier110. In one embodiment, the output stage114is a class-AB output stage. The supply voltage output by the power supply112is modulated so that the supply voltage applied to the output stage114depends on or corresponds to the envelope of a signal of interest x(n). This way, energy consumption can be significantly reduced without clipping or otherwise distorting the signal during amplification. The open-loop control system120controls operation of the power supply112as a function of the envelope of the signal of interest without having to observe the power supply output voltage, reducing the overall complexity of the transmitter circuit100. Because the control system120is open-loop, it is operated in a well-defined and predictable manner to ensure that the amplifier110is adequately powered during normal operation.

To that end, the power supply112preferably operates exclusively in the continuous mode where a current flows through a coil inductor of the power supply112during the entire switching cycle. In addition, the supply voltage (VSUPP—CONST) provided to the power supply112is preferably kept generally constant. Also, the open-loop control system120includes a mask generator122and a discontinuity detector124for controlling operation of the power supply112. In more detail, the PWM116of the amplifier110outputs a signal (MOD) that modulates the power supply112responsive to a mask input m′(n) to the PWM116. The mask m(n) is generated by the mask generator122as a function of the envelope of the signal of interest x(n). As such, the mask m(n) has a profile that corresponds to the envelope of the signal of interest x(n). The discontinuity detector124identifies discontinuities in the mask m(n) and compensates for the discontinuities to form the PWM mask input m′(n).

The term ‘discontinuity’ as used herein means any type of transition in the mask m(n) that causes ringing or transient oscillations in the power supply output. Transient oscillations in the power supply output caused by discontinuities in the mask m(n) adversely affect operation of the output stage114, particularly when the transient oscillations cause clipping or distortion of the amplified signal y(t). The mask can consist of at least three different regions: flat areas, rising edges and falling edges. Thus, at least six different types of discontinuities can occur in the mask m(n): flat→rising; rising→flat; flat→falling; falling→flat; rising→falling and falling→rising.

FIG. 2illustrates a discontinuity resulting from a falling-to-flat transition in the mask m(n) which causes a corresponding transient oscillation in the power supply output (VDD/SS—DYN). The falling transition in the mask m(n) corresponds to a decrease in the envelope of the signal of interest x(n). The resulting transient oscillation in the power supply output can cause clipping or distortion of the amplified signal y(t) depending on the magnitude and duration of the transient oscillation.

FIG. 3illustrates an exemplary mask profile and the corresponding effect the mask m(n) has on the power supply output (VDD/SS—DYN) when the open-loop control system120is inactive. Discontinuities in the mask m(n) that are not compensated for cause problematic overshoot (300) and undershoot conditions (302) in the power supply output as shown inFIG. 3. Overshoots in the voltage supplied to the output stage114of the amplifier110waste energy and reduce efficiency. Undershoots also result in inefficiency and may cause clipping or distortion of the signal being amplified because the amplifier supply voltage may not be sufficiently high. The power supply output should generally track the mask profile with little or no overshoot and undershoot so that the amplifier110is optimally powered at all times.

Compensating for discontinuities in the mask m(n) enables the PWM116to reliably modulate the power supply112so that the voltage supplied to the output stage114of the amplifier110generally tracks the original mask m(n), ensuring reliable and efficient operation of the amplifier110. The open-loop control system120does not detect and compensate for discontinuities in the mask m(n) based on a feedback signal from the amplifier output. Instead, the mask m(n) is compensated before being input to the PWM116. In one embodiment, the discontinuity detector124filters the mask m(n) to compensate for discontinuities and the filtered mask m′(n) is then input to the PWM116. In another embodiment, the detector124generates negative compensation pulses. According to this embodiment, the compensation pulses cause counteracting transient oscillations in the power supply output which cancel the transient oscillations caused by discontinuities in the mask m(n). The compensation pulses are superimposed on the mask m(n) to form a compensated mask m′(n) which is then input to the PWM116for modulating the power supply112.

FIG. 4illustrates an embodiment of a compensation pulse (CP) generated in response to the falling-to-flat transition in the mask m(n) shown inFIG. 2. The compensation pulse causes a counteracting transient oscillation in the power supply output (VDD/SS—DYN) which cancels the transient oscillation caused by the discontinuity in the mask.

FIG. 5shows the modified mask m′(n) after the compensation pulse shown inFIG. 4is superimposed on the original mask m(n). The modified mask m′(n) is then input to the PWM116, which in turns modulates the power supply112. The transient oscillation in the power supply output caused by the discontinuity in the mask is effectively canceled by the counteracting transient oscillation caused by the compensation pulse. The result is that the amplifier110is reliably powered by the power supply112even when the envelope of the signal of interest x(n) fluctuates as shown inFIG. 6.

FIG. 6illustrates the exemplary mask profile shown inFIG. 3after discontinuity compensation. As shown inFIG. 6, the power supply output (VDD/SS—DYN) has virtually no overshoots or undershoots when the open-loop control system120is active even though there are discontinuities in the original mask m(n). Instead, the supply voltage output by the power supply112closely tracks the modified mask m′(n) input to the PWM116, enabling the amplifier110to efficiently amplify the signal of interest x(n) without clipping or otherwise distorting the signal. A delay element140is included in the transmitter circuit100for delaying the signal of interest x(n) so that the open-loop control system120has sufficient time to modulate the power supply112and the amplifier110has sufficient time to respond. A digital-to-analog converter (D/A)150converts the delayed signal x(n) to an analog signal {circumflex over (x)}(t) which is then input to the output stage114of the amplifier110for amplification. Alternatively, the delay can be realized in the analog domain. In one embodiment, the position of the delay element140and the D/A converter150in the signal path is switched so that the delay element140receives the analog output of the D/A converter150, thus delaying the signal of interest in the analog domain. Broadly, the signal of interest can be intentionally delayed at any desirable point in either the digital or analog domain. The amplified signal output ŷ(t) by the transmitter circuit100is driven onto a communication line such as a DSL line, e.g. via a hybrid circuit (not shown).

FIG. 7illustrates an embodiment of the discontinuity detector124of the open-loop control system120. According to this embodiment, the detector124detects discontinuities in the original mask m(n), generates corresponding compensation pulses CP(ti) which counteract transient oscillations in the power supply output (VDD/SS—DYN) caused by the discontinuities and superimposes the compensation pulses on the original mask m(n) to form a modified mask m′(n) that is input to the PWM116. For the superposition principle to be optimally employed, the power supply112preferably operates linearly to a first approximation. Accordingly, the gradient of the rising and falling edges of the mask m(n) is selected by the mask generator122to be smaller than the technological circuit maximum. As a result, nonlinear elements in the system behavior are reduced, and moreover, dependencies on component tolerances can be largely ignored.

In more detail, the discontinuity detector124includes discontinuity detection logic200for detecting discontinuities in the mask m(n). In an embodiment, the discontinuity detection logic200takes the second derivative of the mask m(n) with respect to time to detect discontinuities in the mask. The output of the detection logic200is a series of Dirac pulses individually scaled by a scaling factor A(pi) based on the assumption that the amplitude of the oscillation generated by the discontinuity p at time i is directly proportional to the change in the gradient at the discontinuity point as given by:

A⁡(pi)=-ⅆ2⁢yⅆt2⁢|t=ti(4)
The negative sign in equation (4) ensures that the transient oscillation in the power supply output (VDD/SS—DYN) caused by the compensation pulse CP(ti) counteracts the transient oscillation caused by the discontinuity piin the mask m(n).

A folding (e.g. convolution) operation is then performed on the scaling factor A(pi) by a compensation pulse prototype filter202. In an embodiment, a rectangular signal function rect() is input to the filter202which outputs rectangular compensation pulse prototypes having a fixed width (Tp) based on rising and falling transitions in the mask m(n) as given by:

C⁢⁢P⁢⁢P⁡(pi)=rect⁡(ni-n0⁡(pi,y⁡(ni))Tp)(5)
where n represents the sample index, i the discontinuity index, pithe discontinuity type (e.g., flat→rising, rising→flat, etc.), no is the delay from the discontinuity to the start of the pulse and Tp is the pulse width.

The output of the compensation pulse prototype filter202is input to a scaling block204that scales the amplitude of the filter output by a base scaling factor B. In one embodiment, the base scaling factor B is a function of the pulse type pi. Furthermore, the base scaling factor B can be a function of the present voltage level at the discontinuity point. In another embodiment, the base scaling factor B applied to each compensation pulse is a constant factor determined empirically. In each embodiment, the scaling block204outputs the compensation pulses CP(ti). A signal combiner206superimposes the compensation pulses on the original mask m(n) to form the modified mask m′(n) which is input to the PWM116.

Compensation pulse CP(ti) is generated for the ithdiscontinuity of type piat the instant tias given by:

C⁢⁢P⁡(ti)=B·A⁡(pi)·rect⁡(ti-t0Tp)(6)
As such, each compensation pulse CP(ti) generated by the discontinuity detector124is a scaled and temporally shifted generally rectangular signal having an amplitude corresponding to the gradient of a rising/falling transition in the original mask m(n). The temporal duration Tp of the compensation pulses CP(ti) is preferably freely selectable within a certain framework, where a change in Tp yields a corresponding change in the base scaling factor B. The duration of each compensation pulse CP(ti) is preferably very short. In one embodiment, the duration of the compensation pulses CP(ti) is a multiple of the PWM period Ts.

The compensation effect provided by the open-loop control system120can be improved by positioning the compensation pulses CP(ti) with a certain temporal delay to with respect to each discontinuity point in the mask m(n). In one embodiment, each compensation pulse CP(ti) is delayed approximately 0.5 μs to 1 μs with respect to the corresponding discontinuity. In addition, the base scaling factor B and the temporal offset to employed by the open-loop control system120can be a function of discontinuity type pi. Alternatively, a uniform base scaling factor B and uniform temporal offset t0can be used regardless of discontinuity type.

FIG. 8illustrates another embodiment of the discontinuity detector124of the open-loop control system120. According to this embodiment, the detector124is a pre-filtering block400for filtering the mask m(n) to compensate for discontinuities. The detector124can be implemented as a single pre-filtering block or a plurality of sub-filters. In either case, the filtered mask m′(n) output is then input to the PWM132.

The term ‘circuit’ as used herein can include a single chip, multiple chips or a combination of one or more chips and software. As such, the amplifier110and the open-loop control system120can be fabricated on the same chip or different chips. For example, the amplifier110can be included in a line driver chip. The open-loop control system120and the D/A converter150can be included in a digital front-end (DFE) chip and/or an analog front-end (AFE) chip. The delay element140, if implemented as an analog block, can be part of the line driver chip. Otherwise, the delay element140can be included in the DFE chip or the AFE chip. Similarly, the mask generator122and the discontinuity detector124can be included in the line driver chip, the DFE chip or the AFE chip. The DFE and AFE chips can be combined in a single package, or the AFE chip and the line driver chip can be combined in a single package. Alternatively, the DFE, AFE and line driver can be included in a single chip.