DIGITAL DOUBLE-LOOP OUTPUT VOLTAGE REGULATION

[0001] This patent application is a continuation-in-part (CIP) of application Ser. No. 11/605,045, filed Nov. 27, 2006, which was a continuation of application Ser. No. 11/084,766, filed Mar. 18, 2005, now issued as U.S. Pat. No. 7,141,956 on Nov. 28, 2006.

[0002] 1. Field of the Invention

[0003] The present invention relates to voltage regulator circuits, and more particularly to digital control over a switched mode voltage regulator using dual feedback loops for improved regulation.

[0004] 2. Description of Related Art

[0005] Switched mode voltage regulators are known in the art to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode voltage regulator provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. A buck converter is one particular type of switched mode voltage regulator that includes two power switches that are typically provided by MOSFET transistors. A filter capacitor coupled in parallel with the load reduces ripple of the output current. A pulse width modulation (PWM) control circuit is used to control the gating of the power switches in an alternating manner to control the flow of current in the output inductor. The PWM control circuit uses feedback signals reflecting the output voltage and/or current level to adjust the duty cycle applied to the power switches in response to changing load conditions.

[0006] Conventional PWM control circuits are constructed using analog circuit components, such as operational amplifiers and comparators. But, it is desirable to use digital circuitry instead of the analog circuit components since digital circuitry takes up less physical space and draws less power. A conventional digital PWM control circuit includes a subtractor that produces an error signal representing the difference between a signal to be controlled (e.g., output voltage (V_o)) and a reference voltage. An analog-to-digital converter (ADC) converts the error signal into a digital signal. The digital error signal is provided to a loop compensation filter having a transfer function H(z) that provides stability for the voltage regulator feedback loop. A digital pulse width modulator (DPWM) then produces a proportional pulse width modulated signal that is used to control the power switches of the voltage regulator.

[0007] In order to keep the complexity of the PWM control circuit low, it is desirable to hold the number of bits of the digital signal to a small number. At the same time, however, the number of bits of the digital signal needs to be sufficiently high to provide enough resolution to secure precise control of the output value. If the output voltage needs to be programmable through a large range, it is even more difficult to maintain a small DC error on the subtractor and therefore set point accuracy errors will increase. While the circuit can be made accurate over a wide range by providing adjustable gain and offset, this comes with additional cost and complexity. Moreover, the ADC needs to be very fast to respond to changing load conditions and enable fast transient response of the feedback loop. Current microprocessors exhibit supply current slew rates of up to 20 A/μs, and future microprocessors are expected to reach slew rates greater than 350 A/μs, thereby demanding extremely fast response by the voltage regulator. Very often, fast response time and DC precision are contradictory requirements. The bit size of the digital signal also affects the complexity of the digital circuitry that implements the transfer function H(z) and hence the associated cost.

[0008] Thus, it would be advantageous to provide a system and method for digitally controlling a switched mode voltage regulator that overcomes these and other drawbacks of the prior art. More specifically, it would be advantageous to provide a double-loop output voltage control circuit for controlling a switched mode voltage regulator using digital circuitry having better repeatability and accuracy.

[0009] The present invention provides a switched mode voltage regulator having a digital control system. Generally, the voltage regulator comprises at least one power switch adapted to convey power between respective input and output terminals of the voltage regulator and a digital controller adapted to control operation of the power switches responsive to an output of the voltage regulator. The digital controller further comprises dual digital control loops in which a first control loop provides high speed with lower regulation accuracy and a second control loop has high accuracy with lower speed. Thus, the invention provides the advantages of both high speed and high accuracy.

[0010] More particularly, the first digital control loop includes a first analog-to-digital converter providing a first digital error signal representing a difference between a first output measurement of the voltage regulator and a reference value, a first digital filter providing a digital control output based on the first digital error signal, and a digital pulse width modulator providing a control signal to the power switches. The control signal has a pulse width corresponding to the digital control output. The second digital control loop includes a second analog-to-digital converter providing a second output measurement of the voltage regulator. The second digital control loop provides a second digital error signal representing a difference between the second output measurement and the reference value. The second analog-to-digital converter has greater resolution than the first analog-to-digital converter. The second digital error signal is applied to the first digital control loop to thereby improve accuracy of the first output measurement.

[0011] In an embodiment of the invention, the first digital control loop further comprises a first analog-to-digital converter providing a first digital measurement of the voltage regulator output, a digital filter providing a digital control output based on a first digital error signal and a second digital error signal, and a digital pulse width modulator providing a control signal to the at least one power switch based on the digital control output. The first digital error signal comprises a difference between the first digital measurement and a digital reference value. The second digital error signal comprises a sum of the first digital error signal and a time varying portion of the digital reference value.

[0012] The second digital control loop includes a second analog-to-digital converter providing a second digital measurement of the voltage regulator output. The second analog-to-digital converter has greater resolution than the first analog-to-digital converter. The second digital control loop provides the digital reference value based on a desired output voltage setpoint. The second digital control loop provides the time varying portion of the digital reference value based on a difference between the second digital measurement and the output voltage setpoint.

[0013] More particularly, the digital filter comprises proportional, integral and derivative arithmetic units. The first digital error signal is provided to the proportional and derivative arithmetic units. The second digital error signal is provided to the integral arithmetic unit.

[0014] A more complete understanding of the system and method for digitally controlling a switched mode voltage regulator will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.

[0015] FIG. 1 depicts a switched mode voltage regulator having a conventional digital control circuit;

[0016] FIG. 2 depicts a switched mode voltage regulator having a digital control circuit with a second analog control loop;

[0017] FIG. 3 depicts a switched mode voltage regulator having a digital double-loop control circuit in accordance with a first embodiment of the invention;

[0018] FIG. 4 depicts an exemplary digital filter for use in the digital double-loop control circuit of FIG. 3;

[0019] FIG. 5 depicts a switched mode voltage regulator having a digital double-loop control circuit in accordance with a second embodiment of the invention;

[0020] FIG. 6 depicts an exemplary digital filter for use in the digital double-loop control circuit of FIG. 5;

[0021] FIG. 7 depicts a switched mode voltage regulator having a digital double-loop control circuit in accordance with a third embodiment of the invention; and

[0022] FIG. 8 depicts an exemplary digital controller with setpoint modulation for use with respect to the digital double-loop control circuit of FIG. 7.

[0023] The present invention provides a digital double-loop output voltage control circuit for controlling a switched mode voltage regulator. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more figures.

[0024] FIG. 1 depicts a switched mode voltage regulator 10 having a conventional digital control circuit. The voltage regulator 10 comprises a buck converter topology to convert an input DC voltage V_in to an output DC voltage V_o applied to a resistive load 20 (R_load). The voltage regulator 10 includes a pair of power switches 12, 14 provided by MOSFET devices. The drain terminal of the high side power switch 12 is coupled to the input voltage V_in, the source terminal of the low side power switch 14 is connected to ground, and the source terminal of power switch 12 and the drain terminal of power switch 14 are coupled together to define a phase node. An output inductor 16 is coupled in series between the phase node and the terminal providing the output voltage V_o, and a capacitor 18 is coupled in parallel with the resistive load R_load. Respective drivers 22, 24 alternatingly drive the gate terminals of the power switches 12, 14. In turn, digital control circuit 30 (described below) controls operation of the drivers 22, 24. The opening and closing of the power switches 12, 14 provides an intermediate voltage having a generally rectangular waveform at the phase node, and the filter formed by the output inductor 16 and capacitor 18 converts the rectangular waveform into a substantially DC output voltage V_o.

[0025] The digital control circuit 30 receives a feedback signal from the output portion of the voltage regulator 10. As shown in FIG. 1, the feedback signal corresponds to the output voltage V_o, though it should be appreciated that the feedback signal could alternatively (or additionally) correspond to the output current drawn by the resistive load R_load or a combination thereof. The feedback path may further include a voltage divider provided by resistors 26, 28 to reduce the detected output voltage V_o to a representative voltage level. The digital control circuit 30 provides a pulse width modulated waveform having a duty cycle controlled to regulate the output voltage V_o (or output current) at a desired level. Even though the exemplary voltage regulator 10 is illustrated as having a buck converter topology, it should be understood that the use of feedback loop control of the voltage regulator 10 using the digital control circuit 30 is equally applicable to other known voltage regulator topologies, such as boost and buck-boost converters in isolated or non-isolated configurations.

[0026] More particularly, the digital control circuit 30 includes subtractor 32, analog-to-digital converter (ADC) 34, digital filter 36, and digital pulse width modulator (DPWM) 38. The subtractor 32 receives as inputs the feedback signal (i.e., output voltage V_o) and a voltage reference (Ref) and provides an analog voltage error signal (Ref-V_o). The ADC 34 produces a digital representation of the voltage error signal. The digital filter 36 has a transfer function H(z) that transforms the voltage error signal to a digital output provided to the DPWM 38, which converts the digital output into a waveform having a proportional pulse width. As discussed above, the pulse-modulated waveform produced by the DPWM 38 is coupled to the gate terminals of the power switches 12, 14 through respective drivers 22, 24. The digital filter 36 may further comprise an infinite impulse response (IIR) filter having filter coefficients that may be selectively modified through a suitable input to thereby alter the performance characteristics of the digital filter. As discussed above, a drawback of the conventional digital control circuit 30 is that the subtractor 32 has limited static accuracy.

[0027] To improve the output voltage set point accuracy of the digital control circuit 30, a second analog control loop 40 may be added, as shown in FIG. 2. The second control loop includes an amplifier 46 and an integrator 48. As with the first control loop, the second control loop 40 receives a feedback signal from the output portion of the voltage regulator 10 that corresponds to the output voltage V_o. The feedback path may further include a voltage divider provided by resistors 42, 44 to reduce the detected output voltage V_o to a representative voltage level. The feedback signal is provided to the inverting input terminal of the amplifier 46, and the non-inverting input terminal of the amplifier is coupled to a reference voltage. The amplifier 46 is selected to have lower bandwidth than the subtractor 32, thereby allowing greater accuracy with lower speed. The output of the amplifier 46 is provided to the integrator 48, which in turn provides an adjusting voltage to the subtractor 32 of the first loop through a suitable resistor. The integrator 48 assures that the error signal of the second control loop remains at zero during steady state operation. The first control loop provides fast transient response, while the second control loop provides high DC accuracy under steady state conditions.

[0028] Referring now to FIG. 3, a switched mode voltage regulator having a digital double-loop control circuit is illustrated in accordance with a first embodiment of the invention. The digital control circuit includes a serial interface 52 that permits bidirectional data communication with a host system to receive data to control operation of the digital control circuit, and hence the voltage regulator, and to send status information back to the host system. A digital-to-analog converter 56 is coupled to the serial interface 52. A digital reference value provided from the host system via the serial interface 52 (or retained in memory within the serial interface 52) is converted by the digital-to-analog converter 56 to a reference voltage, that is in turn provided to the subtractor 32 for comparison to the representation of the output voltage V_O. This way, the host system can define the reference voltage, and thereby control the output voltage V_O. The serial interface 52 also communicates filter coefficient values to the digital filter 36 from the host system to thereby control the characteristics of the digital filter 36. In these respects, the digital control circuit includes a first control loop that is substantially the same as the circuit described above with respect to FIG. 1.

[0029] A second digital control loop is provided by analog-to-digital converter 58 and a digital filter circuit 70. The analog-to-digital converter 58 receives a feedback signal corresponding to the output voltage V_o, reduced to a representative voltage level by voltage divider provided by resistors 62, 66. The analog-to-digital converter 58 is coupled to the serial interface 52 through a monitoring circuit 54. This way, the analog-to-digital converter 58 provides an accurate digital measurement of the output voltage, and this information may be communicated back to the host system through the monitoring circuit and the serial interface 52. In a preferred embodiment of the invention, the digital-to-analog converter 56 has a much lower resolution than the monitoring analog-to-digital converter 58. The resolution of the digital-to-analog converter 56 is selected to correspond to the specific supply voltage requirements of different loads R_load. The analog-to-digital converter 34 has a small conversion range, but needs to be fast. Since there is always some residual ripple voltage present at the output of the regulator and the analog-to-digital converter 34 needs to have a fast response time, the ripple voltage cannot be filtered out since this would slow down the conversion process. The ripple therefore yields to an additional error signal in the first loop. The monitoring analog-to-digital converter 58 can run with a rather low sampling rate, but it should be accurate. To increase accuracy, the monitoring analog-to-digital converter 58 will include an anti-aliasing filter on its input which also will reduce the ripple voltage seen on the output of the regulator. This analog-to-digital converter 58 will therefore measure the true average value of the output and therefore has inherently better accuracy than the analog-to-digital converter 34.

[0030] The digital filter circuit 70 further includes a digital comparator 76, a digital filter 74, and a variable resistor 72. The digital comparator 76 receives at a first input the digital reference value provided by the host system and at a second input the digital measurement of the output voltage V_O, and produces a digital error value. The digital error value passes through the digital filter 74 and controls the setting of the variable resistor 72. The variable resistor 72 is part of the voltage divider defined by resistors 28 and 64. Accordingly, the representation of the output voltage V_O provided to the subtractor 32 may be adjusted by controlling the setting of the variable resistor 72.

[0031] FIG. 4 illustrates an embodiment of the digital filter circuit 70 in greater detail. As discussed above, the digital reference value will usually have less resolution than the monitoring output of the analog-to-digital converter 58. In the embodiment of the FIG. 4, the reference signal has nine-bit resolution and the monitoring output has twelve-bit resolution. A digital comparator 82 is shown as having two twelve-bit inputs. The reference signal is multiplied by eight (i.e., by adding three trailing 0 bits) to scale it to the same width as the monitoring output. The digital comparator 82 compares the values and generates two outputs (i.e., A>B, and A<B). The two signals control an up/down counter 84 that acts as an integrator. Thus, the counter is incremented when the reference signal exceeds the monitoring output (A>B), and the counter is decremented when the monitoring output exceeds the reference signal (A<B). The counter 84 is selected such that it does not over-roll (i.e., the count does not go below zero and stops when it has reached its maximum). As shown in FIG. 4, the counter 84 has four-bit resolution with a range from zero to fifteen.

[0032] A variable resistor is formed from field effect transistors 86₁-86₄, each having a source terminal coupled to ground and respective drain terminals coupled to resistors 88₂-88₅. Resistors 88₁ and 92₁-92₄ are coupled together in series and between successive ones of the transistors 86₁-86₄. The gate terminals of the transistors 86₁-86₄ are coupled to respective bits of the four-bit output of the counter 84. By activating individual ones of the field effect transistors 86₁-86₄, and thereby coupling associated ones of the resistors in parallel, the effective resistance of the variable resistor is changed. The values of the resistors may be selected such that the output voltage changes (e.g., from −2% to +2%) when the counter changes from zero to fifteen.

[0033] The counter 84 is clocked by a signal having a frequency that is substantially lower than the PWM frequency of the first digital control loop. In an embodiment of the invention, the counter 84 is clocked by a signal having a frequency ranging from 100 to 1000 times lower than the PWM frequency. Accordingly, the second digital control loop is substantially slower than the first digital control loop, yet provides higher accuracy in view of the larger resolution of the monitoring analog-to-digital converter 58.

[0034] Since the digital comparator 82 and the counter 84 are simple digital circuits, it is relatively easy to implement these circuits within a single digital control circuit containing both digital control loops. A drawback of this embodiment is that the digital filter 74 still acts on an analog circuit, i.e., the variable resistor 72. Thus, the digital correction value is converted back into an analog signal before acting upon the first digital control loop. It would therefore be further advantageous to have a control circuit that can be implemented using entirely digital circuitry.

[0035] Referring now to FIG. 5, a switched mode voltage regulator having a digital double-loop control circuit is illustrated in accordance with a second embodiment of the invention. This embodiment differs from the preceding embodiment by including a digital filter circuit 100 having a digital comparator 102, a digital filter 104, and adder 106. As in the preceding embodiment, the digital comparator 102 compares the digital reference value provided by the host system (or retained in memory) with the digital measurement of the output voltage V_O, and produces a digital error value. The digital error value passes through the digital filter 104 and provides a digital value to the adder 106. The adder combines the digital reference value with the filtered digital value to produce an adjusted digital reference value. The adjusted digital reference value is provided to digital-to-analog converter 56, which converts the digital reference value to a reference voltage that is in turn provided to the subtractor 32 for comparison to the representation of the output voltage V_O. Thus, the digital filter 104 modifies the reference value directly instead of using the resistor divider of the first control loop.

[0036] Since the reference digital-to-analog converter 56 has lower resolution than the monitoring analog-to-digital converter 58, the adjusted digital reference value may fall between discrete points of the digital-to-analog converter, which is exacerbated by the fact that the second digital control loop runs at a much lower frequency. Accordingly, in an embodiment of the invention, the digital filter circuit 100 is adapted to virtually increase the resolution of the reference digital-to-analog converter 56. Moreover, the digital filter circuit 100 takes advantage of the fact that the first digital control loop has a low pass filter characteristic. In particular, if the digital reference value can be switched up and down by one count fast enough, then the first digital control loop will average the switching reference value and present an average reference value at the output of the reference digital-to-analog converter 56.

[0037] More specifically, FIG. 6 shows the digital filter circuit 100 of FIG. 5 in greater detail. The digital filter circuit includes a phase accumulator that provides dithering of the digital reference value. The digital filter circuit is further illustrated as including counter 112, adders 114, 116, 120, and phase converter 118. As in the embodiment of FIG. 4, digital comparator 102 compares the monitored and referenced values and generates two outputs (i.e., A>B, and A<B). The two signals control an up/down counter 112 that acts as an integrator. Thus, the counter is incremented when the reference signal exceeds the monitoring output (A>B), and the counter is decremented when the monitoring output exceeds the reference signal (A<B). The counter 112 generates a six-bit digital error value that is divided such that the most significant two bits are provided to the adder 114 and the least significant four bits are provided to the adder 120. These least significant bits are considered to be the fractional part of the correction signal and are dithered over time by the phase register 118, which stores a continuous sum of the four-bit error values. The adder 120, which combined with the phase register 118 provides a phase accumulator in which the lower four bits of the error value are added to the phase value, is in turn fed back to the phase register. Whenever the adder 120 overflows, it produces a carry bit that is provided to adder 116. By adding the carry from the digital error value produced by adder 114, the adder 116 results in dithering of the fractional part of the digital error value E(5:0).

[0038] By way of example, the average value of the dithered reference can be set in increments ranging from 0, 1/16, 2/16 . . . 15/16, . . . 3 14/16, 3 15/16, etc. Thus, the resolution of the digital-to-analog converter 56 resolution can be programmed in fractional amounts to permit controlling of the output voltage of the first loop in a more accurate way without requiring a digital-to-analog converter having high resolution.

[0039] Referring now to FIG. 7, a switched mode voltage regulator having a digital double-loop control circuit is illustrated in accordance with a third embodiment of the invention. This embodiment differs from the preceding embodiments by converting the output voltage V_O directly into a digital value, instead of the error of the output voltage V_O. The output voltage V_O is applied directly to the analog-to-digital converter 34, which provides a digital value to a subtractor 132. As in the preceding embodiment, the digital comparator 102 receives at a first input the digital reference value provided by the host system and at a second input the digital measurement of the output voltage V_O, and produces a digital error value. The digital error value passes through the digital filter 104 and provides a digital value to the adder 106. The adder 106 combines the digital reference value with the filtered digital value to produce an adjusted digital reference value. The adjusted digital reference value is provided to the subtractor 132, which subtracts the digital value of the output voltage V_O from the adjusted digital reference value.

[0040] As with the preceding embodiment, the digital filter 104 modifies the reference value directly instead of using a resistor divider in the first control loop. But, a drawback with this arrangement is that a time varying reference value as generated by the second feedback phase accumulator adds noise into the main feedback loop. This negatively affects the output voltage ripple and noise of the switched mode voltage regulator.

[0041] FIG. 8 illustrates a digital controller for use in a switched mode voltage regulator that minimizes the noise in the main feedback loop. Both the main loop and secondary loop are split so that the reference value includes a time invariant portion (i.e., that changes slowly) and a time varying portion that contains modulation to provide fractional least significant bit (LSB) set point accuracies. In the main loop, the digital filter is implemented as a classical PID filter with distinct proportional 144, integral 146, and derivative 142 arithmetic units having outputs combined by adder 148. As in FIG. 7, the adjusted digital reference value is provided to the subtractor 132, which subtracts the digital value of the output voltage V_O from the adjusted digital reference value to yield a first error signal (VERR1[7:0]). The first error signal is provided directly to the derivative and proportional arithmetic units 142, 144, and is added to the time varying portion of the reference value (Carry) by adder 152 to yield a second error signal (VERR2[7:0]). The second error signal is provided to the integral arithmetic unit 146. The combined outputs of the proportional, integral, and derivative arithmetic units 144, 146, 144 are provided to the digital pulse width modulator (DPWM) 38.

[0042] As in FIG. 7, the secondary loop includes a digital filter that includes a phase accumulator that provides dithering of the digital reference value. The digital filter includes a counter 162, adder 174, and phase converter 172. The counter 162 and phase converter 172 are driven by a common clock. The digital comparator 102 compares the monitored and referenced voltage values and generates two outputs (i.e., A>B, and A<B). The two signals control the up/down counter 162 that acts as an integrator. Thus, the counter 162 is incremented when the reference signal exceeds the monitoring output (A>B), and the counter is decremented when the monitoring output exceeds the reference signal (A<B). The counter 162 generates a six-bit digital error value (E[5:0]) that is divided such that the most significant two bits (E[5:4]) are provided to the adder 106 and the least significant four bits (E[3:0]) are provided to the adder 174. These least significant bits are considered to be the fractional part of the correction signal and are dithered over time by the phase register 172, which stores a continuous sum of the four-bit error values. The adder 174, which combined with the phase register 172 provides a phase accumulator 170 in which the lower four bits of the error value are added to the phase value, is in turn fed back to the phase register. Whenever the adder 174 overflows, it produces a carry bit that is provided to adder 152. By adding the carry from the digital error value produced by adder 174, the adder 152 results in dithering of the fractional part of the digital error value E(5:0).

[0043] The integral arithmetic unit 146 of the filter sets the output voltage V_O average value. The proportional and derivative arithmetic units 144, 142 assure good transient response. By providing the time varying error value only to the integral part of the digital filter, the invention achieves two objectives. First, the measurement of the output voltage V_O in the main loop will settle to the average of the time varying reference value, which is a fractional part of the least significant bit (LSB) of the output of the analog-to-digital converter 34 (VO[7:0]). This allows the average output voltage V_O to be set with smaller steps than the analog-to-digital converter 34 would normally allow. Second, the time varying portion of the reference value is only presented to the integral arithmetic unit 146 of the digital filter. Since an integrator provides a low pass filter, the variation of the reference value is highly attenuated by the integral arithmetic unit 146. This keeps the added noise of the control loop to a minimum.

[0044] Having thus described a preferred embodiment of a system and method for digitally controlling a switched mode voltage regulator, it should be apparent to those skilled in the art that certain advantages of the system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.