Abstract:
A buck power converter creates a desired output voltage from a greater input voltage with higher efficiency than linear regulators or charge pumps. For compact-size and cost sensitive products, the use of the buck power converter is hindered mainly because of lack of physical space and increases in the cost of the passive components like the inductor and capacitor. Techniques are presented to reduce the sizes of the passive components so that they can be integrated on-chip or in-package or on board. A signal converter in the buck power converter determines the duty cycle of a switching control signal. The switching control signal would ordinarily have driven a power switching circuit that provides current to the inductor in the buck power converter. The signal converter outputs a modified (multiphase) switching control signal that includes multiple separated on-periods that taken together approximate the duty cycle of the switching control signal while maintaining the same control loop frequency. The multiphase switching signal drives the power switching circuit to provide current to the inductor during each of the multiple separated on-periods so that the output voltage ripple decreases by a factor of the number of phases in the modified switching signal. In this way, if the ripple amplitude is kept same, the sizes of the passive components can be reduced by the factor of the number of phases in the modified switching control signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    This disclosure relates to power converters. In particular, this disclosure relates to a step down (buck) power converter with inductor based switching, suitable for use with a memory device (e.g., a flash memory device) or other device. 
         [0003]    2. Related Art 
         [0004]    Continual development and rapid improvement in semiconductor manufacturing techniques have led to extremely high density memory devices. The memory devices are available in a wide range of types, speeds, and functionality. Memory devices often take the forms, as examples, of flash memory cards and flash memory drives. Today, capacities for memory devices have reached 64 gigabytes or more for portable memory devices such as Universal Serial Bus (USB) flash drives, and one terabyte or more for solid state disk drives. Memory devices form a critical part of the data storage subsystem for digital cameras, digital media players, home computers, and an entire range of other host devices. 
         [0005]    One important characteristic of a memory device is its power consumption. In an age when many host devices are powered by limited capacity batteries, every fraction of a watt in power saving translates into extended battery life and extended functionality between recharges for the host device. While the memory device is in operation, a power converter provides the power supply to the memory device. A buck power converter typically has much higher efficiency than other types of power converters. This is one reason that buck converters are frequently preferred over linear regulators and charge pump regulators. 
         [0006]    However, memory devices present significant technical challenges to the use of a buck regulator. As one example, the form factor of the circuit board in a memory device is often very small. As a result, it is difficult to find space for large off-chip components like the inductor or capacitor used in a buck regulator. Furthermore, the components add extra cost to the memory device, and cost margins for memory devices are already very small. 
         [0007]    The sizes of the inductor and capacitor are inversely proportional to the switching frequency of the control loop in the buck regulator. According, in the past, very high switching frequencies on the order of tens or hundreds of MHz or higher were used. Unfortunately, high switching frequencies increase design complexity and cost, while reducing the overall power efficiency. Moreover, the bandwidth of the components of a buck regulator is generally preferred to be significantly higher (e.g., 10 times or higher) than the switching frequency. This is often a difficult condition to meet, and commonly imposes significant restrictions on the maximum possible switching frequency. 
         [0008]    One technique for addressing the technical challenges associated with buck converters is to use a multiphase approach with multiple control loops. Each phase requires its own distinct inductor and switching power transistor pairs. The control loops are driven 180 degrees out of phase. Separate pulse width modulated (PWM) signals drive the distinct power transistor pairs. In other words, the conventional multiphase approach requires multiple inductors equal in number to the number of phases. The convention approach also requires multiple power transistor pairs. As noted above, it is difficult and financially prohibitive to provide these extra components, particularly in a small, inexpensive memory device. 
       SUMMARY 
       [0009]    A buck power converter creates a desired output voltage from a greater input voltage, without requiring multiple inductors or capacitors. The buck power converter has a higher efficiency than linear regulators or charge pumps. The buck power converter uses techniques that reduce the sizes of the inductor and capacitor so that they can be integrated on-chip or in-package or on board. 
         [0010]    A signal converter in the buck power converter determines the duty cycle of a switching control signal. The signal converter outputs a modified (multiphase) switching control signal that includes multiple separated on-periods that taken together approximate the duty cycle of the switching control signal while maintaining the same control loop frequency. The multiphase switching signal drives the power switching circuit to provide current to the inductor and capacitor during each of the multiple separated on-periods. The output voltage ripple decreases by a factor of the number of phases in the modified switching signal. In this way, when the ripple amplitude is kept same, the sizes of the passive components may be reduced by the factor of the number of phases in the modified switching control signal. 
         [0011]    Other features and advantages of the inventions will become apparent upon examination of the following figures, detailed description, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The system may be better understood with reference to the following drawings and description. In the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0013]      FIG. 1  illustrates a buck power converter. 
           [0014]      FIG. 2  shows a power switching circuit. 
           [0015]      FIG. 3  shows a power switching circuit. 
           [0016]      FIG. 4  shows a signal diagram including a modified (multiphase) switching signal. 
           [0017]      FIG. 5  shows a signal diagram including a modified (multiphase) switching signal. 
           [0018]      FIG. 6  shows a signal diagram including a modified (multiphase) switching signal. 
           [0019]      FIG. 7  shows a signal diagram including a modified (multiphase) switching signal. 
           [0020]      FIG. 8  shows one example of logic for determining the duration of two phases in a modified switching signal. 
           [0021]      FIG. 9  shows a flow diagram of logic for generating a specific nominal output voltage V o . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The discussion below makes reference to host devices and memory devices. A host device may be a wired or wireless device, may be portable or relatively stationary, and may run from DC (e.g., battery power), AC power, or another power source. A host device may be a consumer electronic device such as a personal computer, a mobile phone handset, a game device, a personal digital assistant (PDA), an email/text messaging device, a digital camera, a digital media/content player, a GPS navigation device, a satellite signal (e.g., television signal) receiver, or cable signal (e.g., television signal) receiver. In some cases, a host device accepts or interfaces to a memory device that includes the power converter. Examples of memory devices include memory cards, flash drives, and solid state disk drives. For example, a music/video player may accept a memory card that incorporates the power converter described below, or a personal computer may interface to a solid state disk drive that includes the power converter below. The power converter may be used in other devices, including in the host device itself. 
         [0023]      FIG. 1  illustrates a step-down (buck) switching power converter  100  (“power converter  100 ”). The power converter  100  includes a control unit  102 , a signal converter  104  in communication with the control unit  102 , and a switching circuit  106  driven by the signal converter  104 . The power switching circuit  106  provides current to an inductor L and a capacitor C as described in more detail below. The power converter  100  provides power to a load (represented in  FIG. 1  having a certain load resistance R L ) at a specific nominal output voltage V o . 
         [0024]    The power converter  100  produces V o  from a reference input voltage V in  provided on the voltage reference input  108 . To do so, the control unit  102  generates a switching control signal (e.g., a Pulse Width Modulated (PWM) signal) on the switching control signal output  110 . The control unit  102 , consistent with buck converter design principles, sets the duty cycle of the switching control signal according to the desired reduction in V in  needed to obtain V o . As one example, assuming V in =3.0V and V o =1.2V, then the duty cycle, D, that the control unit  102  implements is approximately 0.4. In addition, the control unit  102  may adjust the duty cycle of the switching control signal based on a feedback voltage V fb  obtained, for example, from V o  by the voltage divider  112 . The control unit  102  adjusts the switching control signal based on a comparison of the feedback voltage on the feedback input  114  to a reference voltage V ref  provided on the reference input  116 . A control clock signal CLK 1 , provided on the control clock input  118 , provides a reference clock for the control unit  102 . 
         [0025]    However, rather than directly drive a power switching circuit with the PWM signal as in existing buck converters, the PWM signal is first processed by the signal converter  104 . The signal converter  104  implements a multiphase conversion that produces a modified switching signal, MPhase, on the modified switching signal output  120 . As will be explained in more detail below, the signal converter  104  accepts a conversion clock signal CLK 2  on the conversion clock signal input  122 . The conversion clock signal is faster than the reference clock (e.g., by a factor of 10, although other factors may be implemented). The signal converter  104  may generate the modified switching signal based on the conversion clock signal as described in more detail below. 
         [0026]    In summary, the power converter  100  provides the output voltage V o  by generating a modified switching signal that includes multiple separated on-periods that taken together approximate the duty cycle, D. To that end, the control unit  102  outputs the switching control signal characterized by a particular duty cycle (e.g., 0.4). The signal converter  104  receives the switching control signal and determines the duty cycle. The signal converter  104  also outputs a modified switching signal that includes the multiple separated on-periods that taken together approximate the duty cycle. The modified switching signal drives the power switching circuit  106 . The inductor L and the load capacitor C are connected to the power switching circuit. The power switching circuit  106  provides current to the inductor L during each of the multiple separated on-periods, and the capacitor C charges through the inductor L. 
         [0027]    The switching control signal may be a single phase switching signal intended to directly drive a power switching circuit, such as the power switching circuit  106 . However, the signal converter  104  instead drives the power switching circuit with the modified switching signal. As shown in  FIG. 2 , the power switching circuit  106  may include a complementary transistor pair  202  that provides current to the inductor during each of the multiple separated on-periods. As another example,  FIG. 3  shows that the power switching circuit  106  may include a transistor  302  and diode  304  in a configuration that provides the current to the inductor. 
         [0028]    In one implementation, the signal converter  104  determines the duty cycle of the switching control signal as a clock count of the conversion clock signal. For example, the signal converter  104  may determine (e.g., using a counter) that the duty cycle causes the switching control signal to be asserted for a duration of approximately 4 conversion clocks out of every 10 conversion clocks. The signal converter  104  may then create the multiple separated on-periods in the modified switching signal to extend for, in sum, the clock count. The modified switching signal may include the multiple separated on-periods in the same or different period as the switching control signal. In other words, the signal converter  104  may delay the output of the modified switching signal, e.g., by one or more periods of the switching control signal. 
         [0029]    The duty cycles determines an active period and an inactive period of the switching control signal. In one implementation, the signal converter  104  generates the multiple separated on-periods of the modified switching signal to include one or more evenly or unevenly separated on-periods during the active period, and one or more evenly or unevenly spaced separated on-periods during the inactive period. The on-periods during the active period and inactive period taken together approximate the duty cycle. In general, the on-periods may be generated during either or both of the active and inactive periods. 
         [0030]      FIG. 4  shows a signal diagram including a modified switching signal. In particular,  FIG. 4  shows the control clock  402 , conversion clock  404 , and a switching control signal  406 . In addition,  FIG. 4  shows the inductor current  408  assuming the power switching circuit  106  were directly driven by the switching control signal  406 , the modified switching signal  410 , and the multiphase inductor current  412  that results from the modified switching signal  410  driving the power switching circuit  106 . 
         [0031]    The control clock  402  has a period, T. Similarly, the conversion clock  404  has a period that is typically less than T, e.g., T/10. Thus the conversion clock  404  is faster than the control clock  402  by a preselected factor (e.g., 10). The preselected factor may vary widely, and may be chosen due to the availability of various clocks in the device in which the power converter  100  is implemented, may be chosen to keep operation of the power converter  100  within the bandwidth limitations of the circuitry (e.g., a feedback error amplifier and control loop) in the control unit  102 , may be chosen to keep the amount of ripple current in the inductor to less than a specific amount, or may be chosen based on other factors or combinations of factors. As one example, the control clock  402  may have a frequency in the range of approximately 1 MHz to approximately 4 MHz, and the conversion clock  404  may have a frequency in the range of approximately 5 MHz to approximately 40 MHz. Other frequencies may be employed depending on the implementation. 
         [0032]    In the example shown in  FIG. 4 , the switching control signal  406  has a duty cycle of approximately 0.4. The duty cycle establishes, in the switching control signal  406 , an active period  414  and an inactive period  416 . The active period  414  extends for approximately 4 conversion clocks, while the inactive period extends for approximately 6 conversion clocks. 
         [0033]    If the switching control signal  406  were used to drive the power switching circuit  106 , a relatively large inductor ripple current  408  would result. The inductor current increases during the active period  414  as current flows into the inductor, and decreases during the inactive period  416  as current is no longer provided to the inductor. The amount of ripple in the inductor current  408  is illustrated in  FIG. 4  as ΔI (PWM)  418 . 
         [0034]    The signal converter  104  produces the modified switching signal  410 , which results in reduced inductor ripple current. Specifically,  FIG. 4  shows that the modified switching signal  410  includes multiple separated on-periods  420  and  422 . Each of the on-periods  420  and  422  extend for approximately two conversion clocks, and each may be considered a separate phase, though in the same modified switching signal  410 . Taken together, the on-periods approximate the duty cycle of the switching control signal  406  of approximately four conversion clocks. 
         [0035]    The modified switching signal  410  drives the power switching circuit  106 . As a result, the multiphase inductor current  412  evidences reduced ripple. In particular, the power converter  100  provides current to the inductor for approximately the same amount of time overall, but in multiple separated shorter durations. The reduced amount of ripple in the multiphase inductor current  412  is illustrated in  FIG. 4  as ΔI (MPhase)  424 . Accordingly, the output voltage ripple also decreases, e.g., by a factor of the number of phases in the modified switching signal. One beneficial result is that the sizes of the passive components may be reduced commensurate with the number of phases in the modified switching signal, while adhering to the same ripple current and voltage design requirements for the power converter  100 . In other words, the decrease in ripple current and voltage resulting from the multi-phase implementation permits a change to smaller passive components, which in turn causes a counterbalancing increase in the ripple current and voltage while still meeting the design specification parameters. 
         [0036]      FIG. 4  shows that there is a delay in the output of the modified switching signal  410 . In particular, the modified switching signal  410  lags one period behind the switching control signal  406 . The signal converter  104  may instead output the modified switching signal  410  in the same period as the switching control signal  406 , or may delay for additional periods. Whether or not there is a delay, may depend on the implementation of the signal converter  104 . For example, the counter in the signal converter  104  may analyze the switching control signal  406  over its first period to determine the number of conversion clocks during which the switching control signal  406  is active, and thus determine the duty cycle of the switching control signal  406 . Once the duty cycle is known, the signal converter  104  may then in subsequent periods output the multiple separated on-periods in the modified switching signal  410  to extend, in sum, over the duty cycle in terms of the conversion clock or other time reference. 
         [0037]      FIG. 5  continues the example started in  FIG. 4 . In particular,  FIG. 5  shows the inductor voltage signal  502  that results from the inductor current  408  (i.e., from the switching control signal  406 , without using the modified switching signal). The amount of voltage ripple caused by the inductor current  408  is illustrated in  FIG. 5  as ΔV (PWM)  504 .  FIG. 5  also shows the inductor voltage signal  506  that results from the inductor current  412  (i.e., from using the modified switching signal  410  to drive the power switching circuit  106 ). The amount of voltage ripple caused by the inductor current  412  is illustrated in  FIG. 5  as ΔV (MPhase)  508  and is reduced by a factor of the number of phases in the modified switching signal  410 . 
         [0038]      FIG. 6  illustrates an alternative in which the modified switching signal  602  includes the four on-periods  604 ,  606 ,  608 , and  610  that taken together approximate the duty cycle of the switching control signal  406 . In effect, the modified switching signal  602  includes four phases in the same signal for driving the power switching circuit  106 . 
         [0039]    Separating the duty cycle into four distinct on-periods results in the inductor current  612 . Specifically, the modified switching signal  602  provides current to the inductor for approximately the same period of time as the switching control signal  406 , but spread over four separated times. The resulting multiphase inductor current  612  evidences correspondingly reduced ripple because the limited on-times prevent the inductor current from building to levels that would have ordinarily been reached if the switching control signal  406  were used. The amount of ripple in the multiphase inductor current  612  is illustrated in  FIG. 6  as ΔI (MPhase)  614 . A corresponding reduction in the multiphase inductor voltage  616  is illustrated in  FIG. 6  as ΔV (MPhase)  618 . 
         [0040]      FIG. 7  shows an example in which the modified switching signal  702  is separated into eight on-times labeled A through H. In total, the on-times approximate the total on-time resulting from the duty cycle in the switching control signal  406 .  FIG. 7  shows the corresponding reduction in the multiphase inductor current  704 . 
         [0041]    The example in  FIG. 7  illustrates that signal converter  104  may begin to output on-times (e.g., the on-time labeled A) in the modified switching signal  702  as soon as it detects that the switching control signal  406  is asserted. The signal converter  104  may count the duration of the switching control signal  406  (e.g., in terms of the number of conversion clocks). In the meantime, the signal converter  104  continues to output additional on-times. Each on-time may have a specific duration (e.g., half of one conversion clock cycle). 
         [0042]    The switching control signal  406  may count the number of on-times that it has output, and when the switching control signal is de-asserted, continue to output on-times until the duty cycle of the switching control signal  406  is approximated by the total number of on-times (and, e.g., in terms of the conversion clock  404 ). Thus, in  FIG. 7 , the signal converter  104  continues to provide on-time E, F, G, and H. The spacing between on-time pulses may be even or uneven. If the spacing is kept generally even, then extra ripple can be avoided because there will not be extended periods of time when no current is being supplied to the inductor. In  FIG. 7 , the signal converter  104  outputs evenly spaced pulses while the switching control signal  406  is asserted (as the signal converter  104  determines the duty cycle), and also outputs evenly spaced pulses while the signal converter  104  outputs the remaining pulses to cover the total on-time for the duty cycle of the switching control signal  406 . 
         [0043]    In one implementation, the counter logic in the signal converter  104  may perform the following processing: 1) when the switching control signal  406  is asserted, the signal converter  104  asserts the modified switching signal  702  during the on-period of the conversion clock, and de-asserts the modified switching signal  702  during the off-period of the conversion clock; 2) the signal converter  104  counts the number, N, of off-periods of the modified switching signal  702 ; 3) then, when the switching control signal  406  is de-asserted, the signal converter  104  asserts, in an evenly distributed manner, the modified switching signal  702  N times during the de-asserted time of the switching control signal  406 . 
         [0044]    Note that the improvements to the inductor sizing may permit additional implementation options for the inductor. For example, the inductor L may be implemented as a board or package trace inductor (e.g., by using a circuit board trace to form the inductor). Alternatively, the inductor may be integrated on-chip or in-package. For example, the power converter  100  may provide the power supply for a flash memory device and may be integrated into the same package or chip as the flash memory device. 
         [0045]      FIG. 8  shows one example of logic  800  for determining the duration of two phases in the modified switching signal (e.g., the modified switching signal  410 ). A counter  802  determines the duty cycle of the switching control signal by counting, in terms of the number of conversion clocks, the duration the asserted portion of the switching control signal. In the example shown in  FIG. 8 , the clock count is five (5). To determine the duration of on-times for the modified switching signal in two phases, the shift down register shifts the count right one place to divide by two (and drop the remainder). The result is two (2) conversion clock counts for the duration of one of the on-times. In addition, the subtractor  806  subtracts the result of the shift register from the clock count to obtain three (3), the duration for the second on-time in the modified switching signal. 
         [0046]      FIG. 9  shows a flow diagram of logic  900  for generating a specific nominal output voltage V o . The controller  100  receives a switching control signal characterized by a duty cycle, D ( 902 ) and determines the duty cycle, D ( 904 ). The switching control signal may be a single phase switching signal, for example, that would ordinarily drive a power switching circuit to provide current to an inductor and capacitor. 
         [0047]    The controller  100  will output a modified switching signal that includes a predetermined number of phases ( 906 ). As examples, the modified switching signal may include 2, 4, or more phases. For each phase, the signal converter  104  will generate an on-time of a specific duration. In other words, the signal converter  104  outputs multiple separated on-periods in modified switching signal. The multiple separated on-periods, taken together, approximate the duty cycle D. 
         [0048]    The power converter  100  drives a power switching circuit  106  with the modified switching signal. As a result, the power converter  100  provides current to an inductor and capacitor during each of the multiple separated on-periods. A desired output voltage V o  results. 
         [0049]    As noted above, a control clock signal may be provided to the control unit  102  for generating the switching control signal. Also, a conversion clock signal that is faster than the control clock signal may be provided to the signal converter  104  for generating the modified switching signal. The duty cycle may be determined as a clock count of the conversion clock signal, and the multiple separated on-periods may be created to match the clock count. 
         [0050]    The power converter  100  may output all or part of the modified switching control signal in the same period or different period (e.g., delayed by one period or more) as the switching control signal. In some implementations, the duty cycles determines an active period and an inactive period of the switching control signal. Outputting the modified switching signal may then include outputting evenly or unevenly spaced separated on-periods during the active period, and outputting evenly or unevenly spaced separated on-periods during the inactive period. The on-periods during the active period and inactive period taken together approximate the duty cycle. 
         [0051]    The design parameters for any particular implementation may vary widely. The design parameters include the inductor, L, and capacitor, C, values, the desired output voltage, the reference input voltage, the source clock frequency, the conversion clock frequency, and other parameters. One specific implementation example is: L=100 nH, C=100 nF, reference clock=1 MHz, conversion clock=50 MHz, reference input voltage=3.3V, and an output voltage programmable from 0.95V to 1.3V. 
         [0052]    The methods, control unit  102 , signal converter  104 , switching circuit  106 , and other logic described above may be implemented in many different ways in many different combinations of hardware, software or both hardware and software. For example, all or parts of the power converter  100  may be circuitry in a controller, a microprocessor, or an application specific integrated circuit (ASIC), or may be implemented with discrete logic or components, or a combination of other types of circuitry. Parts of the logic (e.g., counting logic) may be implemented as instructions for execution by a processor, controller, or other processing device and may be stored in a machine-readable or computer-readable medium such as a compact disc read only memory (CDROM), magnetic or optical disk, flash memory, random access memory (RAM) or read only memory (ROM), erasable programmable read only memory (EPROM) or other machine-readable medium. The instructions may be included in firmware that a controller executes. For example, the firmware may be operational firmware for a memory device. The controller may execute the instructions to generate an operating voltage V o  for any desired part of the memory device. 
         [0053]    While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.