Abstract:
A switching voltage regulator circuit includes, in part, a latch, a pair of switches, a sensing circuit, an amplifier, a digital control block, and a comparator. The switches are responsive to the latch, and the sensing circuit is responsive to a current flowing through the switch that is on. The amplifier is responsive to a reference voltage signal and a voltage feedback signal to generate a first intermediate voltage signal. The digital control block receives the reference voltage signal and the voltage feedback signal and generates a second intermediate voltage signal operative to cause the difference between the voltage feedback signal and the reference voltage signal to be less than a predefined value. The first and second intermediate voltages define a threshold value. The comparator is adapted to receive the output of the sensing circuit and the threshold value and to change the state of the latch in response.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application claims benefit under 35 USC 119(e) of U.S. Provisional Application No. 60/870,567, filed on Dec. 18, 2006, entitled “Hybrid DC-DC Switching Regulator Circuit,” the content of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to electronic circuits, and more particularly to power supply integrated circuits. 
         [0003]    In integrated circuits (IC), there is often a need to generate a regulated DC voltage from a unregulated DC supply voltage. One known circuit for achieving this is commonly referred to as pulse width modulated (PWM) regulator. Such a regulator often uses a feedback loop to maintain its output voltage equal to a reference voltage and to maintain stability in the loop. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    A switching voltage regulator circuit, in accordance with one embodiment of the present invention includes, in part, a latch, a pair of switches, a sensing circuit, an amplifier, a digital control block, and a comparator. The switches are responsive to the latch, and the sensing circuit is responsive to a current flowing through the switch that is on. The amplifier is responsive to a reference voltage signal and a voltage feedback signal to generate a first intermediate voltage signal. The digital control block is adapted to receive the reference voltage signal and the voltage feedback signal and to generate, in response, a second intermediate voltage signal operative to cause the difference between the voltage feedback signal and the reference voltage signal to be less than a predefined value. The first and second intermediate voltages define a-threshold value. The comparator is adapted to receive the output of the sensing circuit and the threshold value and to change the state of the latch in response. 
         [0005]    In one embodiment, the feedback voltage is generated by dividing the output voltage. In another embodiment, the feedback voltage is the output voltage. In one embodiment, the first and second switches are MOS transistors. In another embodiment, the first and second switches are bipolar transistors. 
         [0006]    In one embodiment, the digital control block further includes, in part, an analog-to-digital converter, a digital control engine responsive to the analog-to-digital converter and adapted to cause the difference between the voltage feedback signal and the first reference voltage signal to be less than a predefined value, and a digital-to-analog converter responsive to the digital control block. In one embodiment, the voltage regulator includes a memory, and a clock and timing signal generation block. In one embodiment, the digital control block generates a biasing signal. 
         [0007]    A switching voltage regulator circuit, in accordance with another embodiment of the present invention includes, in part, a digital control block and N voltage regulation channels. The digital control block receives a first reference voltage and selectively receives one of N feedback voltages, and generates a first intermediate voltage signal operative to cause the difference between the selected feedback voltage and the reference voltage to be less than a predefined value. Each voltage regulation channel includes, in part, a latch, first and second associated switches responsive to an output of the latch; a sensing circuit associated with the first and second switches and responsive to a current flow through the associated switches, an associated amplifier responsive to a reference voltage signal and an associated voltage feedback signal to generate an associated second intermediate voltage signal, and a comparator adapted to receive the output of the associated sensing circuit and further to receive a threshold value defined by the first intermediate voltage signal and the associated second intermediate voltage signal to change the state of the latch in response. 
         [0008]    In one embodiment, each feedback voltage is generated by dividing an associated output voltage. In another embodiment, each feedback voltage represents the associated output voltage. In one embodiment, the first and second switches in each channel are MOS transistors. In another embodiment, the first and second switches in each channel are bipolar transistors. In one embodiment, the digital control block comprises an analog-to-digital converter, a digital control engine responsive to said analog-to-digital converter and adapted to cause the difference between the feedback voltage signal and the first reference voltage to be less than a predefined value, and a digital-to-analog converter responsive to said digital control block. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of a hybrid DC-DC switching regulator circuit, in accordance with one embodiment of the invention. 
           [0010]      FIG. 2  is a block diagram of the digital control block of  FIG. 1 , in accordance with one embodiment of the present invention. 
           [0011]      FIG. 3A  illustrates the short-term transient response of the output voltage of the hybrid DC-DC switching regulator of  FIG. 1 . 
           [0012]      FIG. 3B  illustrates the long-term transient response of the output voltage of the hybrid DC-DC switching regulator of  FIG. 1 . 
           [0013]      FIG. 4  is a block diagram of a hybrid DC-DC switching regulator circuit, in accordance with another embodiment of the invention. 
           [0014]      FIG. 5  is a schematic diagram of a multi-channel DC-DC switching regulator circuit, in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  is a block diagram of a hybrid DC-DC switching regulator circuit  100 , in accordance with one embodiment of the invention. DC-DC switching regulator circuit  100 , hereinafter alternatively referred to as regulator  100 , includes a control loop  160  and a digital control block  202 . The short term loop transient response of regulator  100  is determined by the control loop  160 , and the DC accuracy of regulator  100  is provided by a relatively slower feedback loop built around DCB  202 . Although the following description is provided with reference to a buck regulator, it is understood that any other switching regulator topology, such as boost, buck-boost, or otherwise may be used. 
         [0016]    The switching cycle begins when oscillator  106  sets the SR latch  118 . The output signal of SR Latch  118  is buffered by one or more drivers disposed in driver block  120  and applied to the low side switch (LSS)  122  and the high side switch (HSS)  124 . Switch  124  is shown as being a PMOS transistor  124 , and switch  122  is shown as being an NMOS transistor  122 . In other embodiments, switches  122  and  124  may be bipolar NPN and PNP transistors, or both switches may be NMOS or NPN transistors. When the SR latch  118  is set, transistor  122  is turned off and transistor  124  is turned on. The current through transistor  124  is equal to the current I ind  flowing through inductor  116 . The resistance of resistors  112  and  114  are selected to be large enough so that the average value of the current through inductor  116  is nearly equal to the load current I L  flowing through load resistor  110 . 
         [0017]    The voltage developed across sense resistor  126  is proportional to the inductor current I ind . Current sense amplifier  128  senses the current flowing through resistor  126  and generates a voltage in response. To avoid sub-harmonic oscillations when the duty-cycle is larger than 50%, adder  150  adds a slope compensation signal  134  to the output signal A of current sense amplifier  128  and delivers the resulting signal to the positive input terminal of comparator  130 . The voltage signal applied to the positive input terminal of comparator  130  is compared to a threshold voltage applied to the negative input terminal of comparator  130 . As the inductor current builds up, at some point the voltage at the positive input of comparator  130  becomes larger than this threshold voltage and the comparator  130  trips, causing SR latch  118  to be reset. When SR latch  118  is reset, transistor  124  is turned off and transistor  122  is turned on. The cycle repeats itself when the next clock pulse from oscillator  106  sets latch  118 . This method of control is known as peak current control. Other types of current mode control methods may be employed in the current loop, such as constant ON time control, constant OFF time control, etc. 
         [0018]    The threshold voltage level applied to the negative input terminal of comparator  130  is supplied, in part, by amplifier  104 , and as a function of the difference between reference voltage V REF  ( 132 ) and the feedback voltage V FB . Amplifier  104  is a low-gain high-bandwidth amplifier (LGHBA) which together with current sense amplifier  126 , comparator  130 , latch  118 , driver  120  and switches  122  and  124  form a low gain, high bandwidth loop  160  which is responsible for the fast transient response of regulator  100 . 
         [0019]      FIG. 2  is a block diagram of DCB  202 , in accordance with one embodiment of the present invention. Referring concurrently to  FIGS. 1 and 2 , N-Bit Analog-to-Digital Converter (ADC)  306  is shown as having differential inputs and a sampling rate of f S . In other embodiments, described below, ADC  306  may have a single-ended input. ADC  306  samples the voltage difference between reference voltage VREF and feedback voltage VFB and converts this difference to a corresponding N-bit wide digital code word at its output. 
         [0020]    The Digital Control Engine (DCE)  302  receives the N-bit wide digital code word from ADC  306  and processes it according to a control algorithm to provide an M-bit wide digital code word that is supplied to Digital-to-Analog Converter (DAC)  308 . The algorithm implemented by DCE  302  may be a digital filter algorithm mimicking the behavior of a high-gain low-bandwidth amplifier, such as an integrator, or may be a non-linear function adapted to bring the feedback voltage V FB  close to reference voltage V REF  such that the difference between voltages V FB  and V REF  is less than a predefined value. DAC  308  uses the M-bit word to bring the output voltage VOUT back into regulation using the slower time constants of the Digital Feedback Loop (DFL). The resolution of ADC  306 , i.e., N, is typically selected so as to be less than the DAC  308  resolution, i.e., M, to avoid limit cycling of the output voltage. DAC  308  generates an analog voltage signal at its output in response to the M-bit wide digital code word it receives at its input. The voltage generated by DAC  308  is added by adder  154  to the output voltage signal of amplifier  104  and establishes the threshold voltage level applied to the negative input terminal of comparator  130 . Signal CTRL generated by DCE  302  is optionally used to control the operations of one or more blocks of the voltage regulation of the present invention. For example, signal CTRL may be used to set the bias currents/voltages to optimize the performance of the various analog blocks disposed in control loop  160  of the present invention to account for environment parameters, external component values and operating conditions. In one embodiment (not shown), signal CTRL is used to optimize the operating condition of amplifier  104 . 
         [0021]    Memory  310  supplies information to DCE  302 . Although not shown, in one embodiment, memory  310  includes a non-volatile (NVM) and a volatile Memory (VM). The NVM may be used to store such data as, e.g., calibration information, loop parameters, external component values and parameters for the programmable features of the regulator that are desired to be retained in case of a power loss. VM may be used as a scratch pad by the DCE  302  and may also store run-time status information. The Clock &amp; Timing Generator  304  generates the timing signals for the ADC  306 , DCE  302 , DAC  308 , and memory  310 . 
         [0022]    Referring to  FIG. 1 , in order to describe the operation of the control loop  160  and external components  150 , assume that the load current I L  changes from a low level I L1  to a higher level I L2  in a time interval Δt that is small compared to the response time TDSL of loop  160  (fast load transient). Further assume that resistor  114  has a resistance of 0, and resistor  112  is an open circuit. Further assume that the voltage V 1  supplied at the output terminal OUT of DCB  202  remains relatively constant within time intervals in the order of T DSL . This is a valid assumption, since the response time T DDCB  of the DCB  202  is selected to be larger than that of loop  160  so as to provide a relatively slow response and a high gain. This output load transient event is illustrated in  FIG. 3   a . A voltage ripple which is expected to be present on the output voltage of a switching regulator is omitted for clarity. 
         [0023]    When a large load current transient is applied to the output, it causes on the output voltage (i) a voltage spike induced by the Equivalent Series Inductance ESL (not shown) of the output capacitor  108 , (ii) an offset voltage induced by the Equivalent Series Resistance ESR (not shown) of the output capacitor  108  and (iii) a voltage droop caused by the loop response time. The effects of L ESL  and R ESR  can be kept relatively small by proper selection of external components and by following proper layout techniques. As an example, a load current step of 0 to 100 mA in 100 ns would cause a peak output voltage deviation of 1 mV due to 1 nH of ESL. The contribution of ESR to the transient output voltage deviation is also relatively small. As an example, a load current step of 0 to 100 mA would cause a peak output voltage deviation of 1 mV due to 10 mΩ of ESR. The voltage droop is caused by the non-zero loop response time T DSL  Assuming that ΔI L  is the difference between I L2  and I L1 , the following approximation can be written about the droop rate: 
         [0000]        d ( V OUT)/ dt=ΔI   L   /C   OUT   (1) 
         [0024]    During the period T DSL  of loop  160 , the load current is supplied by C OUT . At the end of T DSL , the maximum output voltage deviation from the initial regulation value of VOUT L1  may be written as: 
         [0000]      Δ V OUT MAX   =ΔI   L   *T   DSL   /C   OUT   (2) 
         [0025]    After the expiration of T DSL , sub-loop  160  catches up with the droop and brings the output voltage to VOUT L2     —     TR , as shown by the following expression. 
         [0000]      Δ V OUT TR   =V OUT L1   −V OUT L2     —     TR   ≅ΔI   L   *R   SNS   *A   CSAMP   /A   LGHBA   (3) 
         [0026]    In expression (3), A LGHBA  is the voltage gain of amplifier  104 , A CSAMP  is the voltage gain of current sense amplifier  128 , and R SNS  is the resistance of resistor  126 . ΔVOUT TR  represents the transient load regulation characteristic of regulator  100 . In expression (3), slope compensation signal  134  is omitted since its effect is relatively small at large current levels. 
         [0027]    The following are exemplary numerical values of a few parameters associated with regulator  100 : 
       I L1 =50 mA 
     I L2 =500 mA 
     A LGHBA =2 
     A CSAMP =4 
       [0028]    R SNS =50 mΩ 
       T DSL =150 ns 
     C OUT =1 μF 
       [0029]    d(VOUT)/dt=450 mV/μs 
       ΔVOUT MAX =67.5 mV 
     ΔVOUT TR =45 mV 
       [0030]    The above example shows that the loop  160  is able to catch the output voltage after a droop of 67.5 mV in response to a fast-load transient from 50 mA to 500 mA. If the load transient duration is longer than the response time, the loop  160  will be able to keep up with the changing load current demand at the output and consequently ΔVOUT MAX  will be equal to ΔVOUT TR . 
         [0031]    After these initial events and following the load transient, DCB  202 , which has a response time of T DDCB , brings the output voltage back to DC regulation as illustrated in  FIG. 3B . This is accomplished by DAC  308  ( FIG. 2 ) updating the voltage at the output node of the DCB  202  at a rate of f U  updates per second (T U =1/f U  in  FIG. 3B ). The output will be brought back to within ΔVOUT of VOUT L1  after a time period of T DDCB  by the slower digital feedback loop that includes DCB  202 . Assuming that the equivalent DC gain of DCB  202  from the input of ADC  306  to the output of DAC  308  is A DCB , ΔVOUT may be calculated from the expression below: 
         [0000]      Δ V OUT=Δ I   L   *R   SNS   *A   CSAMP /( A   LGHBA   *A   DCB )  (4) 
         [0032]    As an example assume that A DCB  is equal to 50, accordingly: 
         [0000]      Δ V OUT=(45 mV)/(50 V/V)=0.9 mV 
         [0033]    If smoother transitions are desired at the output between DAC updates, a smoothing circuit (not shown) can be coupled to the output of DAC  308  output. For example, an RC low pass filter may be used to provide the smoothing function. The resulting output voltage waveform when such a smoothing circuit is used is shown in  FIG. 4B  as dotted lines  420 . In one embodiment, ADC  306  has a single-ended input and may sample the signals REF and FB signals at different times, store them in MEM  310 , and compute the difference in digital domain. In another embodiment, the difference between the values of signals REF and FB may be determined by an analog signal conditioning circuit. The output of the signal conditioning circuit is then applied to the single-ended ADC  306 . 
         [0034]      FIG. 4  is a block diagram of an regulator  400 , in accordance with another embodiment of the present invention. In regulator  400 , DCB  302  samples the output voltage V OUT  directly and without using a voltage divider. 
         [0035]      FIG. 5  is a schematic diagram of a regulator  500 , in accordance with another embodiment of the present invention. As shown in  FIG. 5 , in regulator  500 , DCB  202  is shown as controlling two output voltages V OUT1  and V OUT2 , respectively at output terminals OUT 1  and OUT 2  using a time domain multiplexing scheme. Multiplexer (MUX)  504  selects the error signal from either FB 1  or FB 2  and supplies the selected signal to DCB  202 . DCB  202  supplies its output signal OUT to one of the sample-and-hold (SAH) blocks  506   a  and  506   b . In other words, if signal FB 1  from terminal  124   a  is selected by Mux  504 , output signal OUT of DCB  202  is supplied to SAH  506   a . If, on the other hand, mux  504  selects signal FB 2  from terminal  124   b , output signal OUT of DCB  202  is supplied to SAH  506   b . The select signal Sel of MUX  504  is supplied by DCB  202  via signal line  508 . Signals  510  and  512  are used to control the sampling and holding functions of the sample-and-hold blocks  506   a  and  506   b . Additionally, the ADC, DAC, DCE in the DCB, can be further utilized by other purposes when they are needed to process Hybrid DC-DC converter data, such as diagnostics, supervisory functions, and communications. Although not shown, the time multiplexing of the DCB may be extended to more than two voltage regulation channels. 
         [0036]    By assigning the responsibilities of DC performance and of transient performance onto two separate feedback paths, the present invention makes it possible to independently optimize the performance of each path, resulting in a Hybrid DC-DC Switching Regulator that is fast and accurate. 
         [0037]    The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. The invention is not limited by the type of amplifier, pulse-width generator, feedback circuit, configuration matrix, switch, etc. The invention is not limited by the type of integrated circuit in which the present invention may be disposed. Nor is the invention limited to any specific type of process technology, e.g., CMOS, Bipolar, or BICMOS that may be used to manufacture the present invention. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.