Multi-phase switching regulator

[0001] This application claims the benefit of provisional patent application No. 60/333,404 to Schiff et al., filed Nov. 26, 2001.

[0002] 1. Field of the Invention

[0003] This invention relates to the field of switching voltage regulators, and particularly to multi-phase switching voltage regulators.

[0004] 2. Description of the Related Art

[0005] Switching voltage regulators provide a predetermined and substantially constant output voltage from a source voltage that may be fluctuating, or that may be at an inappropriate amplitude for the load. Such regulators typically employ one or more switching elements. The switching elements may be, for example, field-effect transistor (FET) switches. Control circuitry regulates the current supplied to the load by varying the ON-OFF times of the switching elements (i.e., the regulator's duty cycle, which is the percentage of time that a switch is ON during a cycle of operation). Inductors and capacitors are typically used to convert the switched current pulses into a steady flow of load current. Switching regulators are frequently used in portable battery-powered electronic products, as they provide high operating efficiency and long battery life with little heat generation.

[0006] A switching voltage regulator may be arranged to operate using one or more different regulation modes. For example, for voltage-mode regulation, a feedback signal V_fb which varies with the regulator's output voltage is fed to a comparator along with a periodic sawtooth waveform; the comparator's output controls the duty cycle of the switching elements. For current-mode regulation, a voltage V_i is generated which is proportional to the current in the output inductor. V_i and V_fb are provided as inputs to a comparator, the output of which controls the duty cycle of the switching elements.

[0007] Several methods have been employed to obtain inductor current information for use in a switching regulator's control loop. Typically, a small voltage is sensed, either continuously or periodically sampled, to determine the current. One approach is described in U.S. Pat. No. 5,982,160 to Walters et al., in which an R-C network is connected across the output inductor, with the small resulting signal sent to the controller. Another approach is described in U.S. Pat. No. 5,847554 to Wilcox et al., which samples the voltage drop across the regulator's MOSFET switches when they are on.

[0008] Advanced microprocessors require power supplies that provide a low voltage (e.g., <2 volts) at high current. One way in which high currents are provided is with a multi-phase switching regulator, in which the components of a number (N) of single-phase regulators are repeated to produce N output currents, which are summed together to provide the total output current. However, implementing any of the regulators cited above as a multi-phase regulator requires summing and/or sampling data in the controller IC, which can introduce unwanted complexity and processing delays into the IC design.

[0009] Another switching regulator issue concerns maintaining the output voltage within specified limits in response to load transients. One approach to this problem is known as adaptive voltage positioning (AVP), in which the output voltage is positioned within the specified range so that it best withstands a load transient. This is conventionally accomplished by establishing a droop impedance for the output, by inserting information about the desired output current droop into the output voltage feedback loop as an error signal. However, this approach can result in sluggish and imprecise control on the output voltage for dynamic changes in the load.

[0010] A multi-phase switching voltage regulator is presented which overcomes the problems noted above, enabling precise multiple-loop control which is particularly well suited to AVP power supplies.

[0011] The invention provides a total current circuit, which makes use of an amplifier circuit to sum together individual phase currents to generate a signal representing the total instantaneous output current in the inductors I_out. For an N-phase switching regulator, N current sensing elementswhich can be the output inductors themselves or current sense resistors connected in series with the inductorscarry respective phase currents. A summing circuit, preferably made from N resistors each having a resistance R_p, is connected to sum the voltages present at the switch node sides of the sensing elements, and to provide the summed voltages at a summing node. An amplifier circuit is connected to the summing node at a first input and to the regulator's output terminal at a second input, and produces an output voltage V_cs. When V_out is subtracted from V_cs,the result is proportional to I_out.

[0012] The amplifier circuit preferably comprises an operational amplifier, with a feedback resistor having a resistance R_cs, and a filter capacitor having a capacitance C_cs connected across the feedback resistor. The op amp is arranged such that, when the time constant R_csC_cs is made substantially equal to the inductors' time constant L/R₁, the output voltage V_cs of the op amp is given by: $V_{\mathrm{cs}}=V_{\mathrm{out}}-\frac{R_l{R}_{\mathrm{cs}}}{R_p}{I}_{\mathrm{out}},$

[0013] where R₁ is the inductors' resistance and V_out is the regulator's output voltage. V_cs is thus proportional to total instantaneous inductor output current I_out.

[0014] The invention also provides a unique means for direct insertion of total inductor output current information into a switching regulator's voltage-mode control loop, to provide AVP for the output voltage. A desired output voltage is specified by providing a reference voltage V_ref, and a current circuitpreferably the total current circuit described aboveproduces an output voltage V_droop which varies with the the regulator's total filtered instantaneous output current I_out(fltr). A summation circuit produces an output voltage V_set which is given by:

[0015] V_set and V_out are presented to the inputs of a voltage control error amplifier circuit, which produces an output V_err that varies with the difference between V_set and V_out. A control circuit operates the regulator's switches in response to V_err. The control circuit, total current circuit, summation circuit, and voltage control error amplifier circuit are arranged such that the regulator's output has a desired droop impedance R_o, to provide AVP of V_out as a function of the total filtered instantaneous inductor output current I_out(fltr). This serves to directly insert instantaneous inductor output current information into the reference voltage input of the regulator's voltage control error amplifier, which enables fast, accurate control of output voltage for dynamic changes in load.

[0016] Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.

[0017] FIG. 1 is a schematic/block diagram of a total current circuit for a multi-phase switching regulator per the present invention.

[0018] FIG. 2 is a schematic/block diagram of a preferred embodiment of a total current circuit for a multi-phase switching regulator per the present invention.

[0019] FIG. 3 is a schematic/block diagram of a voltage-mode control loop for a multi-phase switching regulator per the present invention.

[0020] FIG. 4 is a schematic/block diagram of a preferred embodiment of a voltage-mode control loop for a multi-phase switching regulator per the present invention.

[0021] FIG. 5 is a schematic/block diagram of a multi-phase switching regulator which includes a total current circuit and a voltage-mode control loop per the present invention.

[0022] FIG. 6 is a schematic/block diagram of a multi-phase switching regulator which shows one possible implementation of the regulator's control circuitry.

[0023] A schematic/block diagram illustrating the principles of a total current circuit for a multi-phase switching regulator in accordance with the present invention is shown in FIG. 1. The number of phases is identified herein as N; e.g., for the exemplary regulator shown in FIG. 1, N2. For each phase, the regulator includes a switching element (10,12) made up of one or more switching transistors, with each switching element connected to the switch node side of a respective output inductor (14, 16). The opposite, output sides of the inductors are connected together at a common output terminal 18. The N switching elements are operated by a controller 20, which operates the elements so as to provide an output voltage V_out at output terminal 18 and an total inductor output current I_out to a load 22. Load 22 normally includes a parallel filter capacitor (not shown) to reduce the ripple voltage component of the output voltage V_out. Each inductor carries a respective phase current I_px (x1, 2, . . . ), which are summed together at output terminal 18 to provide total inductor output current I_out.

[0024] The present total current circuit includes N current sensing elements. The current sensing elements may be the output inductors themselves, or current sense resistors (23,24) connected in series between each inductor and output terminal 18. Each current sensing element carries a respective phase current I_px. A summing circuit 25 is connected to sum the N voltages present at the switch node sides of the circuit's current sensing elements: when the sensing elements are the inductors, circuit 25 is connected to the inductors' switch node sides; when sense resistors are used, circuit 25 is connected to the resistors' inductor sides. The summed voltages are provided at a summing node 26.

[0025] The present total current circuit also includes an amplifier circuit 28, which is connected to summing node 26 at a first input and to the regulator's output voltage V_out. Amplifier circuit 28 is arranged to sum together the individual phase currents to generate an output voltage (V_cs) which directly represents the total instantaneous inductor output current I_out. Output voltage V_cs can then be used, for example, as an input to controller 20 to provide current-mode control of the regulator, or as an input to a current limiting circuit.

[0026] Note that, though a two-phase switching regulator is shown in FIG. 1, the invention can be extended to any number of phases.

[0027] A preferred embodiment of a total current circuit for a multi-phase switching regulator in accordance with the present invention is shown in FIG. 2. Summing circuit 25 is preferably implemented with N resistors, each of which has a resistance R_p and is connected to the switch node side of a respective current sensing element at one end and to summing node 26 at its other end.

[0028] Amplifier circuit 28 preferably comprises an operational amplifier A1, having its inverting input connected to summing node 26 and its non-inverting input connected to V_out. A feedback resistor having a resistance R_cs is connected between the A1's output and inverting input; R_cs sets the A1's gain, and the load line required by the system being powered. A filter capacitor having a capacitance C_cs is preferably connected across R_cs. When so arranged, the output voltage V_cs produced by A1 is approximately given by: $\begin{array}{cc}{V}_{\mathrm{cs}}=V_{\mathrm{out}}-\frac{R_{\mathrm{cs}}{R}_l\left(1+s\frac{L}{R_l}\right)}{R_p\left(1+{\mathrm{sC}}_{\mathrm{cs}}{R}_{\mathrm{cs}}\right)}{I}_{\mathrm{out}},& \left[\mathrm{Eq}.1\right]\end{array}$

[0029] where R₁ and L are the resistance and inductance values, respectively, for output inductors 14 and 16. Equation 1 presumes that R₁ and L are approximately equal for all output inductors and that all summing resistors have a resistance approximately equal to R_p; those skilled in the art can calculate V_cs precisely for non-equal values.

[0030] As shown in equation 1, if V_out is subtracted from V_cs, the result is proportional to the regulator's total instantaneous inductor output current I_out, with the proportionality varying with frequency. If the time constant C_csR_cs is made substantially equal to the time constant L/R₁ of each output inductor (again presuming that R₁ and L are approximately equal for all output inductors), the equation for output voltage V_cs simplifies to: $\begin{array}{cc}{V}_{\mathrm{cs}}=V_{\mathrm{out}}-\frac{R_l{R}_{\mathrm{cs}}}{R_p}{I}_{\mathrm{out}}.& \left[\mathrm{Eq}.2\right]\end{array}$

[0031] The V_out term can be removed from equation 2 with the use of a summation circuit 30, which receives V_cs at one input, V_out at a second input and produces an output V_droop given by: $\begin{array}{cc}{V}_{\mathrm{droop}}=V_{\mathrm{cs}}-V_{\mathrm{out}}=-\frac{R_l{R}_{\mathrm{cs}}}{R_p}{I}_{\mathrm{out}}=-R_o{I}_{\mathrm{out}},& \left[\mathrm{Eq}.3\right]\end{array}$

[0032] where R_o is the regulator's output or droop impedance, given by: $\begin{array}{cc}{R}_o=\frac{R_l{R}_{\mathrm{cs}}}{R_p}& \left[\mathrm{Eq}.4\right]\end{array}$

[0033] Thus, when the C_csR_cs and L/R₁ time constants are made substantially equal, V_droop is directly proportional to total inductor output current I_out. The amplitude of V_droop can be set by properly selecting the value of R_cs, which is also used to establish the regulator's droop impedance R_o. Signals V_cs and/or V_droop can be used, for example, as a feedback signal provided to controller 20 for one or more of the regulator's control loops, or for monitoring the output current with respect to a current limit. V_cs and/or V_droop could also be used as a feedback signal in a switching regulator which employs AVP. Note, however, that the present total current circuit can be used with any single or multi-phase switching regulator, with or without AVP. The V_droop signal can be used to implement AVP, giving the regulator a finite output impedance.

[0034] When so arranged, the present total current circuit offers high speed and accurate continuous sensing of total inductor output current. Sensing the total inductor current at the output is an inherently more accurate method than other current sensing methods such as peak current detection in the high side MOSFET or sampling the current across a sense element such as the low side MOSFET.

[0035] Note that if current sense resistors (23,24) are used as the current sensing elements instead of the output inductors (14,16), the need for filter capacitance C_cs is eliminated.

[0036] The resistance R₁ of the output inductors is due to their copper windings. As such, R₁ has an associated temperature dependence, represented herein with a temperature coefficient t_c. To compensate for this and thereby improve the accuracy of the total current circuit, feedback resistance R_cs can be selected to have a temperature coefficient which is approximately equal to t_c. Making the temperature coefficient of R_cs equal to t_c also provides continuous time constant tracking; i.e., time constant C_csR_cs remains substantially equal to time constant L/R₁ over temperature. This provides the invention an advantage over prior art designs, in that a single resistor and capacitor are used to provide C_csR_csL/R₁, whereas in the prior art, such as U.S. Pat. No. 5,982,160 discussed above, individual R-C networks are required across each inductor.

[0037] Amplifier circuit 28 can be realized with, for example, a current feedback or voltage feedback op amp, a transconductance amplifier, or any number of other implementations. Amplifier circuit 28 preferably has a monotonic relationship between its output voltage and the difference between its two inputs. The amplifier also preferably has a low offset input voltage to provide good accuracy, and a high gain (if it is an op amp) or a constant gain (if it is a transconductance amplifier).

[0038] The basic principles of a voltage-mode control loop for use in a switching voltage regulator which employs AVP is shown in FIG. 3. For each phase, the regulator includes a switching element (100,102) made up of one or more switching transistors, with each switching element connected to the switch node side of a respective output inductor (104, 106). The opposite, output sides of the inductors are connected together at a common output terminal 108. The switching elements are operated by a control circuit 110, which operates the elements so as to provide an output voltage V_out at output terminal 108 and an total inductor output current I_out to a load 112.

[0039] As noted above, AVP is conventionally accomplished by inserting total inductor output current information into the output voltage feedback loop as an error signal. However, this approach can result in sluggish and imprecise control of the output voltage for dynamic changes in the load. The invention overcomes this by inserting total inductor output current information directly into the reference voltage input of the regulator's voltage control error amplifier 114. Amplifier 114 receives a voltage V_set at one input and output voltage V_out at a second input, and is arranged to provide a feedback signal V_err to control circuit 110 to make V_outV_set. The present voltage-mode control loop includes a summation circuit 116, which receives a voltage V_ref representing the desired output voltage at one input, and a voltage V_tc representing the regulator's total inductor output current at a second input, and produces voltage V_setV_refV_tc at its output. By so doing, total current signal V_tc offsets reference voltage V_ref to reposition output voltage V_out based on the total inductor output current I_out, thereby providing AVP. This approach provides faster feedback of the total inductor output current information into the voltage-mode control loop, when compared with prior art methods which insert the total inductor output current information into the output voltage feedback as an error signal.

[0040] FIG. 4 is a diagram of a preferred embodiment of a voltage-mode control loop for a multi-phase switching regulator per the present invention. Voltage control error amplifier circuit 114 preferably comprises an operational amplifier A2, which receives voltage V_set from summation circuit 116 at its non-inverting input, and a voltage which represents V_out at its inverting input. A loop compensation network 118 is connected between A2's output and its inverting input. Since the reference voltage provided to summation circuit 116 is often provided by a digital-to-analog converter (DAC), V_ref is identified as V_dac in FIG. 4.

[0041] When the circuit generating voltage V_tc is arranged such that V_tcI_outR_o, where R_o is the droop impedance, then the regulated output voltage V_out produced by the regulator of FIG. 4 is given by:

[0042] where I_out(fltr) is the total inductor current without its ripple component, i.e. the filtered total inductor current. Thus, the desired output voltage V_dac is offset by the output droop impedance (R_o) times the filtered output current (I_out(fltr)), to provide AVP of output voltage V_out.

[0043] A2's inverting input may be connected to V_out directly for a transconductance amplifier, or through a series resistance R_b for an operational amplifieras shown in FIG. 4. For the preferred embodiment, the resistor R_b and a current source 120 which causes a current I_fb to flow through R_b sets the no-load offset voltage from the reference voltage (V_dac) in AVP converters, such that the regulated output voltage V_out is given by:

[0044] Loop compensation network 118 may optionally include a capacitor having a capacitance C_b connected across resistance R_b to provide lead compensation. The loop compensation network 118 may be set up such that the regulator's output impedance, including the parallel filter capacitor of the load, is substantially equal to the droop resistance R_o over a wide frequency range.

[0045] The novel voltage-mode control loop preferably receives voltage V_tc from the total current circuit described above. This is illustrated with reference to the multi-phase switching regulator shown in FIG. 5. For each phase, the regulator includes a switching element (200, 202) made up of one or more switching transistors, with each switching element connected to the switch node side of a respective output inductor (204,206). The opposite sides of the inductors are connected together at a common output terminal 208. The switching elements are operated by a control circuit 210, which operates the elements so as to provide an output voltage V_out at output terminal 208 and an total inductor output current I_out to a load 212.

[0046] Here, a total current circuit in accordance with the present invention produces voltage V_droop, which is proportional to the regulator's total inductor output current I_out. Summing circuit 25 sums the voltages at the switch node sides of output inductors 204 and 206 at a summing node 26. The summed voltages are fed to amplifier circuit 28, preferably implemented with op amp A1 and feedback resistance and capacitance R_cs and C_cs, respectively. Op amp A1 also receives output voltage V_out at an input, and produces voltage V_cs at its output. Subtractor circuit 30 subtracts V_out from V_cs to produce V_droop.

[0047] V_droop is provided to the summation circuit 30 of a voltage-mode control loop per the present invention, along with reference voltage V_ref which represents the desired output voltage. V_droop thus offsets V_ref, with voltage V_set resulting. V_set is provided to voltage control error amplifier 114, which is preferably implemented with op amp A2, loop compensation network 118, current source 120, resistance R_b, and capacitance C_b. The output of A2 is provided to control circuit 210 to provide the regulated output voltage with AVP.

[0048] When so arranged, and if time constant C_csR_cs is made substantially equal to the output inductors' time constant L/R₁ (where L and R₁ are the inductance and resistance, respectively, of each output inductor), the regulated output voltage V_out is given by:

[0049] where droop impedance R_o is given by: $R_o=\frac{R_l{R}_{\mathrm{cs}}}{R_p}.$

[0050] This arrangement provides a novel method for creating a signal (V_droop) proportional to total filtered inductor output current (I_out(fltr)), as well as a unique method of inserting the output current information into the voltage control loop for providing AVP of the output.

[0051] As noted above, though FIG. 5 depicts a two-phase regulator, the invention is intended to encompass all single and multi-phase converters, with or without AVP.

[0052] One possible implementation of a control circuit 210, suitable for use with the present invention to form a multi-phase switching regulator, is shown in FIG. 6. The output inductors, total current circuit, and voltage-mode control loop circuit are as shown in FIG. 5. The control circuit includes a pair of gate driver circuits 300, 302 which provide the signals that operate the switching elements (200,202). Additional control circuitry processes the feedback signal produced by voltage control error amplifier A2 and drives the gate driver circuits. The regulator shown in FIG. 6 provides multiple feedback loop control with AVP for a two-phase converter; note that this configuration can be extended to any number of phases.

[0053] While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.