Abstract:
Switching regulator circuits and methods are provided for regulating output voltage that include an adjustable minimum peak inductor current level for Burst Mode in current-mode DC-DC regulators. Minimum peak inductor current level control is achieved during Burst Mode by allowing external control for adjusting the burst threshold level. A single pin can be used to distinguish between forced continuous and Burst Mode as well as set the burst threshold level during Burst Mode, or an alternate method of setting a variable burst threshold level can be implemented in which two pins are used, one for selecting the mode of operation and the other for setting the burst threshold level during Burst Mode. The above principles can be applied to both step-up and step-down regulator circuit configurations, as well as both synchronous switching regulators and non-synchronous switching regulators.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to voltage regulators. More particularly, this invention relates to circuits and methods that provide the ability to adjust the minimum peak inductor current level for Burst Mode™ (hereinafter, “Burst Mode”) in current-mode DC-DC regulators (i.e. regulators that respond to measurements of the output current). 
     Voltage regulators are power supply circuits that use a closed loop design to provide a predetermined and substantially constant output voltage, even while using an input voltage source which may be poorly specified or fluctuating. Furthermore, many electronic products use voltage regulators to convert an input voltage into a regulated output voltage that may be higher or lower than the input voltage. Accordingly, voltage regulators function as both a voltage converter in addition to a voltage stabilizer. 
     There are two major types of regulators: linear regulators and switching regulators. In a typical linear regulator, the output voltage is regulated by adjusting a passive element (e.g., a variable resistor) to control a continuous flow of current from the voltage source to the load. 
     Switching regulators, on the other hand, are essentially DC-DC converters that operate by switching current ON and OFF to control the output voltage. Switching voltage regulators typically employ one or more switching devices, along with an inductor and a capacitor in order to store and transfer energy to a load. These regulators are able to regulate the voltage being supplied to the load by turning the switching element(s) ON and OFF, thereby controlling the amount of power being transmitted through the inductor in the form of discrete current pulses. The inductor and the capacitor convert the supplied current pulses into a steady load current so that the load voltage is regulated. Ultimately, regulation of the output voltage is achieved through adjustment of the switch ON-OFF timings based on feedback signals indicative of the output voltage and load current. 
     Switching regulators that operate in current-mode are particularly desirable. They provide good line and load transient signal rejection, and possess inherent current-limiting capabilities during fault conditions (e.g., output short circuits). Current-mode switching regulators typically monitor the inductor current and compare it with a peak inductor current level to determine when it is appropriate to turn OFF the main switching element, thereby eliminating the supply of excess current. 
     Normally, current-mode switching regulator circuits include the following: a logic section, an output switch or switches controlled by the logic section, an oscillator for providing periodic timing signals to turn ON the main switch, a current amplifier that relays a sense voltage that is dependent on the inductor current, an error amplifier that adjusts its output voltage depending on load conditions, and a current comparator producing a signal which causes the logic section to turn OFF the main switch when the sense voltage increases above the voltage emerging from the error amplifier. 
     A particular type of regulator which often operates in current-mode as described above is the synchronous switching regulator. These regulators have a main switching element and a synchronous switching element which are driven out of phase with respect to each other in order to supply current at a regulated voltage to a load. Synchronous switching regulators differ from non-synchronous switching regulators in that a diode is replaced with a synchronous switching element, and the result, typically, is decreased power loss in the switching regulator. 
     A major benefit of switching regulators, such as synchronous switching regulators, is that they typically exhibit greater efficiency (where efficiency is defined as the ratio of the power provided by the regulator to the power provided to the regulator) than can be found in linear regulators, thereby leading to significant reductions in unwanted heat dissipation. As a result, many switching regulators can eliminate the use of a heat sink that an equivalent linear design would require. 
     In particular, synchronous switching regulators that employ MOSFET (metal-oxide semiconductor field-effect transistor) switches are widely used in portable battery-powered electronic products and products in which only limited heat generation can be tolerated. Because these voltage regulators exhibit higher efficiency, they provide relatively long battery life with little heat generation. For this reason, these regulators are often employed in systems such as cellular telephones, cordless telephones, personal pagers, laptop computers, and wireless modems. 
     The efficiency of switching regulators, however, is not always maximized and varies proportionally to the size of the load. It is a function of output current and typically decreases when the switching regulator is providing small amounts of current to the load. This occurs because even as the load decreases, a fixed amount of power is dissipated in the drive circuitry irrespective of the load size. 
     The above described loss of efficiency at lighter loads is common in switching regulators that operate in a forced continuous mode of operation. In forced continuous mode, the efficiency loss at lighter loads for switching regulators becomes greater because the main switch is periodically turned ON and OFF regardless of operating conditions. Therefore, these regulators may become inefficient for smaller loads because of the energy, in the form of gate charge, that is required to constantly turn the main switch and synchronous switch ON and OFF regardless of load conditions. 
     An effective alternative to operating in forced continuous mode is to allow the regulator to enter Burst Mode operation. When operating in this mode, the regulator may omit switching cycles when the load is light, thereby reducing transistor gate charge losses. This is possible because, when operating in Burst Mode, the switching transistor(s) and other unneeded portions of the regulator circuit are made to remain OFF as the load current drops below a specified value. This technique is therefore used to reduce switching losses in a switching regulator and increase the operating efficiency at low output current levels. 
     Available regulators capable of operating in Burst Mode use essentially the same circuitry as described above for typical switching regulators, with the addition of a burst comparator and circuitry that provides a burst threshold level. This additional circuitry may be used to shut down the majority of a regulator circuit under specified conditions in order to reduce power consumption. Examples of regulators that use Burst Mode and forced continuous mode are Linear Technology Corp.&#39;s LTC1435 and LTC1735 series products. 
     A disadvantage of prior Burst Mode type regulators results from the inability to externally control the burst threshold level, which sets the minimum peak inductor current level (the level below which the circuit enters into Burst Mode). For higher burst threshold levels, the efficiency at light loads is higher at the expense of higher output voltage ripple (an undesirable characteristic). For lower burst threshold levels, the output voltage ripple is lower at the expense of slightly reduced efficiency for light loads. Accordingly, because the burst threshold level, which sets the minimum peak inductor current level, is fixed internally in current regulators that operate in Burst Mode, the output voltage ripple and efficiency of a regulator cannot be tailored to fit the requirements of different applications. 
     In view of the foregoing, it would be desirable to provide a circuit and method for varying the peak inductor current level for Burst Mode in current-mode DC-DC converters to optimize efficiency of a regulator, and to allow the voltage ripple of the regulated output voltage to be adjusted over a continuous range of values. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a technique for varying the burst threshold level, thereby also allowing the selection of a desired minimum peak inductor current level, for current-mode switching regulators operating in Burst Mode. 
     In accordance with this and other objects of the present invention, switching voltage regulator circuitry with an externally adjustable peak inductor current level is described. The circuitry can operate in either step-up (boost) or step-down (buck) configurations, with minor modifications, in any regulator that utilizes Burst Mode operation. 
     One embodiment of the current invention improves on prior regulators by incorporating a single pin that is used for selecting the mode of regulator operation (either forced continuous mode or Burst Mode) as well as setting the burst threshold level during Burst Mode operation. 
     Another embodiment of the present invention provides an alternate method of implementing a variable peak inductor current level through the use of two pins. One pin is used for selecting between forced continuous mode and Burst Mode, and another pin for setting the burst threshold level during Burst Mode operation. 
     The above embodiments preferably alleviate the problems associated with using a fixed minimum peak inductor current level for current control. Accordingly, the present invention allows the output voltage ripple and efficiency of a switching regulator to be adjusted to fit different application requirements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages of the present invention will become apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
     FIG. 1 is a circuit diagram of a conventional step-down switching voltage regulator. 
     FIG. 2 is a circuit diagram of an embodiment of a step-down switching voltage regulator with an adjustable minimum peak inductor current level for Burst Mode in accordance with the principles of the present invention. 
     FIG. 3 is a circuit diagram of an alternate embodiment of a step-down switching voltage regulator with an adjustable minimum peak inductor current level for Burst Mode in accordance with the principles of the present invention. 
     FIG. 4 is a circuit diagram of an embodiment of a step-up switching voltage regulator with an adjustable minimum peak inductor current level for Burst Mode in accordance with the principles of the present invention. 
     FIG. 5 is a circuit diagram of an alternate embodiment of a step-up switching voltage regulator with an adjustable minimum peak inductor current level for Burst Mode in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention relates to switching voltage regulators with an externally adjustable minimum peak inductor current level for Burst Mode in current-mode DC-DC regulators. 
     In the past, typical switching voltage regulators used a fixed burst threshold level, thereby causing the minimum peak inductor current level to be set at a particular value. This is disadvantageous because the output voltage ripple and efficiency of a regulator cannot be customized to fit different application requirements. 
     A typical prior current-mode step-down synchronous switching voltage regulator  100  employing Burst Mode operation that operates with a fixed burst threshold level is shown in FIG. 1, and operates as follows. 
     Voltage regulator  100  of FIG. 1 uses oscillator  111  (together with the other components described below that comprise control circuitry  110 ), or any other suitable device which is capable of providing the circuit with switch timing (i.e., by generating a narrow pulse at a constant frequency). Regulator  100  also contains logic  112 , which may be a pulse-width modulator (PWM) circuit or any other suitable circuit capable of driving switch drivers  113  and  114 , thereby controlling the duty cycle of main power switch  121  (i.e., the amount of time switch  121  is ON compared to the period of an ON/OFF cycle). At the beginning of each cycle, the oscillator pulse originating from oscillator  111  propagates through logic  112 , causing main switch driver  113  to turn ON main switch  121  and also causing driver  114  to turn OFF synchronous switch  122 . This in turn creates a voltage differential of approximately Vin-Vout across inductor  131 . As a result, the current through inductor  131  increases linearly, and a greater amount of current is supplied to the load (modeled by resistor  134 ). It should be noted that although MOSFETs are used for the switching elements in this invention, any other type of suitable switching element may be used without departing from the principles of the present invention. 
     When main switch  121  is ON, the inductor current flows through sense resistor  123 , developing a sense voltage across it equaling the product of the inductor current and the value of sense resistor  123 . This voltage is then amplified by current amplifier  115  and compared to the buffered Ith voltage, which is controlled by error amplifier  117  (as explained below). When the amplified sense voltage increases above the buffered Ith voltage, current comparator  118  trips and signals logic  112  to cause drivers  113  and  114  to simultaneously turn main switch  121  OFF and synchronous switch  122  ON, respectively. This changes the voltage across inductor  131  to approximately −Vout, causing the inductor current to decrease until the next oscillator pulse once again turns main switch  121  ON and synchronous switch  122  OFF. 
     The voltage at Ith, which controls the peak inductor current at which current comparator  118  trips and causes main switch  121  to be shut OFF, is adjusted by error amplifier  117 . The error amplifier  117  senses the output voltage of the regulator and adjusts the Ith voltage accordingly. The Ith signal is then stabilized by compensation circuit  141 , comprising a resistor and a capacitor, and coupled to the input of current comparator  118  through voltage buffer  142 . 
     Error amplifier  117  operates by comparing a feedback signal voltage Vfb obtained from resistor divider  132  to a reference voltage Vref. When a large load step is applied to voltage regulator  100 , the load current drawn from regulator  100  increases. This causes a reduction in the value of Vfb relative to Vref, resulting in error amplifier  117  raising the voltage at Ith. This results in the average inductor current ultimately being raised to a point where it matches the new load current. 
     Conversely, when the demand for load current decreases, there is an increase in Vfb relative to Vref. This causes error amplifier  117  to reduce the voltage at Ith. As a result, although main switch  121  continues to switch ON at the same frequency (i.e., at the beginning of each cycle), its duty cycle is decreased, and therefore, it conducts less current as the load current decreases. This process continues and Ith is reduced until the average inductor current decreases enough so that it is equal to the new load current. 
     Moreover, the regulator of FIG. 1 enables the selection of either forced continuous or Burst Mode operation through the use of an external pin. Forced continuous operation reduces noise and RF interference and minimizes output voltage ripple while Burst Mode operation provides higher efficiency by reducing transistor gate charge losses at light loads. 
     When operating in forced continuous mode, the inductor current is allowed to become negative and the circuit behaves in the manner described above. When the main switch  121  shuts OFF, the synchronous switch  122  turns ON, and remains ON, until the next oscillator pulse triggers the main switch to return to the ON state. 
     Alternatively, main switch  121  can be made to remain OFF when the regulator is operating in Burst Mode and certain conditions are met. In Burst Mode, main switch  121  and synchronous switch  122  operate intermittently at light loads in order to increase efficiency by minimizing switching losses. Additionally, in Burst Mode, the current reversal comparator  116  is enabled and the inductor current is not allowed to become negative. Current reversal comparator  116  monitors the current flowing through synchronous switch  122  and signals logic  112  to shut OFF synchronous switch  122  when the inductor current becomes negative. 
     During Burst Mode operation, transistors  151  and  152  of burst clamp circuitry  150  clamp the minimum value of the buffered Ith voltage at the burst threshold level, thereby setting the minimum peak inductor current level, Iburst. The voltage on Ith is then monitored by burst comparator  119  to determine when sleep mode is enabled and disabled. When main switch  121  turns ON, the inductor current has to increase to Iburst before it can shut OFF. Then, as the output load current drops, the peak inductor current decreases to keep the output voltage in regulation. When the load current demanded drops to a current less than Iburst minus one half of the ripple current in the inductor, however, the burst threshold forces the peak inductor current to remain equal to Iburst. Since the average inductor current is greater than the output load current, the voltage on Ith will be reduced by error amplifier  117  until the burst comparator  119  trips. When burst comparator  117  trips, sleep mode is enabled and both power switches  121  and  122 , along with most of the remaining circuitry, are shut OFF to minimize power consumption. At this point, the load current is solely supplied by output capacitor  133 . When the output voltage drops, causing the voltage at Ith to increase by the amount of the hysteresis in burst comparator  119 , sleep mode is de-asserted and normal operation is resumed. 
     One limitation of the circuitry in FIG. 1 is the inability to change the burst threshold level. It is therefore not possible to vary the minimum peak inductor current level of regulator  100  when it is operating in Burst Mode in order to customize the sleep period between pulses or the output voltage ripple as may be necessary to achieve a certain level of efficiency for a specific application. 
     FIG. 2 shows one embodiment of a step-down current-mode switching voltage regulator with an adjustable minimum peak inductor current level for Burst Mode operation according to the principles of the present invention. Operating under forced continuous, FIG. 2 functions as follows. 
     Oscillator  211  provides the switch timing mechanism, turning main switch  221  ON and synchronous switch  222  OFF at the beginning of each cycle. A sense voltage resulting from the inductor current flowing through resistor  223  is amplified by current amplifier  215 , the output of which is used as one of the inputs to current comparator  218 . Error amplifier  217  compares a feedback voltage with a reference voltage to raise or lower the Ith voltage as necessary, providing the second input to current comparator  218  which serves to signal the main switch  221  to be turned OFF when the inductor current increases greater than the current level set by Ith. 
     Moreover, the aforementioned limitation of conventional regulators operating in Burst Mode is overcome by the present invention in the following manner. The regulator in FIG. 2 preferably incorporates a single external pin for the purpose of selecting the mode of operation (either Burst Mode or forced continuous mode) as well as setting the burst threshold level during Burst Mode operation. 
     In FIG. 2, the FCONT signal is used to instruct logic  212  as to whether switching regulator  200  should operate in forced continuous mode or Burst Mode. In one embodiment, when FCONT is a logic HIGH, the mode of operation is set to forced continuous. Conversely, when FCONT is a logic LOW, the regulator enters Burst Mode. In addition, as shown in FIG. 2, Vburst is used to set the burst threshold level, thereby also controlling the minimum peak inductor current level. 
     Mode selection circuitry  260 , comprising mode comparator  261  having a MODE selection input and a threshold voltage input, a transmission gate consisting of n-channel transistor  264  and p-channel transistor  265 , inverters  262  and  263  and additional n-channel transistor  266 , provides the FCONT and Vburst signals to logic  212  and burst clamp circuitry  250 , respectively. When the voltage at MODE is above the threshold voltage Vth, the output of mode comparator  261  is LOW. This causes the output of inverter  262  to make the signal at FCONT a logic HIGH, causing the regulator to operate in forced continuous mode. In addition, the LOW output of mode comparator  261  also shuts OFF n-channel transistor  264  and p-channel transistor  265 , and the gate of n-channel transistor  266  is driven HIGH to bring the burst threshold level to 0 Volts. 
     Alternatively, when the voltage at MODE is less than Vth, the output of mode comparator  261  becomes HIGH. In this case, the output of inverter  262  makes the signal at FCONT a logic LOW, thereby bringing the regulator into Burst Mode. In this state, n-channel transistor  264  and p-channel transistor  265  of the transmission gate are turned ON and n-channel transistor  266  is turned OFF. Because the transmission gate is turned ON, Vburst becomes substantially equivalent to the voltage being applied at MODE. Accordingly, during Burst Mode, the voltage at MODE may be varied to adjust the burst threshold level as desired. Moreover, through varying the burst threshold level, it is possible to adjust the minimum peak inductor current level Iburst as necessary. This gives the user the ability to set the output voltage ripple and efficiency of regulator  200  during light loads to satisfy applications with varying requirements. 
     In order to achieve a compromise between low output voltage ripple and efficiency, it is also possible to produce pulse skipping behavior while in Burst Mode. This can be accomplished by connecting the MODE pin to ground. This sets the burst threshold level at 0 Volts, and sets Iburst to 0 Amperes. In this condition, the peak inductor current is limited by the minimum on-time of the current comparator. If the load demand is less than the average of the minimum on-time inductor current, switching cycles will be skipped to keep the output voltage in regulation. 
     Another embodiment of the present invention is shown in FIG. 3, wherein two pins are used to implement a variable burst threshold level for Burst Mode. The regulator shown in FIG. 3 functions similarly to that shown in FIG. 2, except for the portion of the circuit responsible for controlling whether regulator  300  operates in forced continuous mode or Burst Mode, as well as setting the voltage threshold level during Burst Mode operation. 
     When a voltage above the threshold voltage Vth is applied at MODE to mode comparator  361  of mode selection circuitry  360 , the output of mode comparator  361  is LOW. This causes the output of inverter  362  (i.e., the signal at FCONT) to be HIGH, thereby causing the regulator to operate in forced continuous mode. Alternatively, if the voltage applied at MODE is less than Vth, the signal at FCONT is LOW and the regulator enters Burst Mode. 
     The second pin in FIG. 3 provides a connection to burst clamp circuitry  350  for setting the burst threshold level. In this manner, when the signal applied at MODE causes the regulator to operate in Burst Mode, the burst threshold level can be adjusted in order to manipulate the minimum peak inductor current level, Iburst. Once again, this provides a substantial improvement over the prior art, allowing the user to tailor the output voltage ripple and efficiency of regulator  300  to meet the requirements of a specific application. 
     It should be noted that, although FIG.  2  and FIG. 3 each show step-down synchronous switching regulator embodiments of the present invention, applicants&#39; invention is not limited in this regard. The benefits of the present invention are equally applicable to other types of regulators, such as step-up synchronous switching regulators, step-up and step-down non-synchronous switching regulators, or any other suitable type of regulator. 
     FIG. 4 is a circuit diagram of an exemplary embodiment of a step-up switching voltage regulator with an adjustable minimum peak inductor current level for Burst Mode in accordance with the principles of the present invention. FIG. 4 operates by using primarily the same basic elements (and rearranging their parts) that are present in the step-down regulator shown in FIG.  2 . It also utilizes diode  422  to prevent capacitor  432  from discharging to ground. The step-up regulator shown in FIG. 4 functions as described below. 
     When circuit conditions cause main switch  421  to close, the input voltage is applied across inductor  423 . During this charge phase, current begins to flow through inductor  423 . Additionally, diode  422  prevents capacitor  432  from discharging to ground, and capacitor  432  is responsible for supplying the load with current. 
     Once switch  421  opens, capacitor  432  is charged by the energy stored in inductor  423 . At this time, an extra current starts flowing through the load, thereby causing the output voltage to rise (energy is also supplied directly from the input source as long as diode  422  is forward biased). After some time period, switch  421  is once again closed. This cycle repeats itself, maintaining the required output voltage level and supplying the required current to the load as necessary. 
     The remainder of the circuit components in FIG. 4 behave as previously explained for the comparable circuit components found in FIG.  2 . Current comparator  416  compares the outputs from current amplifier  414  and error amplifier  415  in order to determine when main switch  421  should be turned OFF. 
     Furthermore, the MODE selection input determines whether regulator  400  operates in forced continuous mode or Burst Mode, and also sets the burst threshold level when Burst Mode is selected. Mode comparator  461 , a transmission gate comprising transistors  464  and  465 , transistor  466 , and inverters  462  and  463  comprise the mode selection circuitry  460  that provides the signals at FCONT and Vburst, which respectively determine the mode of operation of regulator  400  and (when appropriate) the burst threshold level. As shown in FIG. 4, mode comparator  461  compares the MODE selection input with a threshold voltage, Vth. When the voltage at MODE is above Vth, the output of mode comparator  461  is LOW. This, in turn, causes the output of inverter  462  to make the signal at FCONT a logic HIGH, causing the regulator to operate in forced continuous mode. In addition, the LOW output of mode comparator  461  also shuts OFF n-channel transistor  464  and p-channel transistor  465 , and the gate of n-channel transistor  466  is driven HIGH to bring the burst threshold level to 0 Volts. 
     Alternatively, when the voltage at the MODE selection input is less than Vth, the output of mode comparator  461  becomes HIGH. In this case, the output of inverter  462  makes the signal at FCONT a logic LOW, thereby bringing the regulator into Burst Mode. In this state, the transmission gate comprising n-channel transistor  464  and p-channel transistor  465  is turned ON (both transistors are ON) and n-channel transistor  466  is turned OFF. As a result, Vburst becomes substantially equivalent to the voltage being applied at the MODE selection input, and therefore, the voltage at MODE may be varied during Burst Mode to adjust the burst threshold level as desired. In this manner, it is possible to adjust the minimum peak inductor current level, Iburst, as necessary in order to tailor the output voltage ripple and efficiency of regulator  400  as desired. 
     Another embodiment of a step-up switching voltage regulator with an adjustable minimum peak inductor current level for Burst Mode is shown in FIG.  5 . The majority of the circuit shown in FIG. 5 operates similarly to the circuit shown in FIG.  4  and described above. Moreover, in accordance with the principles of the present invention, FIG. 5 shows two pins which are used for the selection of the regulator operating mode and setting the burst threshold level. 
     The operation of regulator  500 , in either forced continuous mode or Burst Mode, is selected by supplying a voltage at the MODE selection input. If a voltage above the threshold voltage Vth is applied at the MODE selection input, the output of mode comparator  561  is LOW, and therefore, the output of inverter  562  (i.e., the signal at FCONT) is HIGH. This causes regulator  500  to operate in forced continuous mode. Alternatively, if the voltage applied at the MODE selection input is less than Vth, the signal at FCONT is LOW and the regulator enters Burst Mode. 
     The second pin (i.e., the Vburst selection input) shown in FIG. 5 provides a user with the ability to set the burst threshold level. When the signal applied at the MODE selection input causes the regulator to operate in Burst Mode, the burst threshold level may be adjusted through the use of the Vburst selection input. In this manner, it is possible to control the minimum peak inductor current level, Iburst. 
     It should be noted that although FIGS. 2-5 each show a particular embodiment of a regulator in accordance with the principles of the present invention, the present invention is not limited in any way in this respect. The principles of the present invention may be applied to any type of regulator (e.g., step-up synchronous switching voltage regulator), and the benefits of doing so should be understood by one skilled in the art. Moreover, it should known by one skilled in the art that the circuitry used to set the burst threshold level in accordance with the principles of the present invention, as described above, is only exemplary and applicants&#39; invention is not limited in this manner.