Abstract:
Methods for synchronizing non-constant frequency switching regulators with a phase locked loop are disclosed. The methods enable non-constant frequency switching regulators to be synchronized with a phase locked loop to achieve constant frequency operation in steady state while retaining the advantages of non-frequency operation to improve transient response and operate over a wider range of duty cycles. In addition, the methods enable multiple non-constant frequency regulators to be synchronized and operated in parallel to deliver higher power levels to the output than a single switching regulator.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to switching voltage regulators. More specifically, the present invention provides circuits and methods for synchronizing non-constant frequency switching regulators with a phase locked loop. 
     BACKGROUND OF THE INVENTION 
     Voltage regulators are an essential component of most electronic devices which operate at a specified DC voltage. Typically, the electronic devices are powered with a source voltage that is fluctuating (i.e., provided by a power supply connected into a wall socket) or at an inappropriate amplitude (i.e., provided by a battery). The purpose of a voltage regulator is to convert the source voltage into the operating DC voltage of the electronic devices. 
     One type of a commonly used voltage regulator is a switching voltage regulator. Switching voltage regulators employ one or more switching elements and an inductor, transformer, or a capacitor as an energy storage element between the source and the load. The switching elements may be, for example, power metaloxide semiconductor field-effect transistor (MOSFET) switches. The switching regulator regulates the voltage across the load by varying the ON-OFF times of the switching elements so that power is transmitted through the switching elements and into the energy storage element in the form of discrete current pulses. The current pulses may be generated by one-shot timers or other circuitry. The energy storage element then converts these current pulses into a steady load current so that the load voltage is regulated. 
     Switching regulators include control circuitry to control the ON-OFF times of the switching elements. The percentage of time that a switching element is ON is referred to as its duty cycle. The duty cycle can be varied in three ways by: (1) fixing the frequency of the pulses and varying the ON or OFF time of each pulse; (2) fixing the ON or OFF time of each pulse and varying the frequency of the pulses; or (3) varying both the ON and OFF times of each pulse and varying the frequency of the pulses (e.g., hysteretic mode control). Examples of prior art constant frequency switching regulators include the LT1307, LTC1625, and LT1074, developed by Linear Technology Corporation, of Milpitas, Calif. Examples of prior art non-constant frequency switching regulators include the MAX1710 (constant on-time), developed by Maxim Integrated Products, Inc., of Sunnyvale, Calif., the CS5120 (constant off-time), developed by ON Semiconductor, of Phoenix, Ariz., and the LT1500, LTC1148, and LTC1778 of Linear Technology Corporation. 
     Constant frequency switching regulators are in general preferred to non-constant frequency switching regulators, since the frequency can be selected to avoid noise-sensitive regions. For example, when using switching regulators in communications equipment such as wireless devices, it is desirable to keep the switching frequency away from the communication frequencies of the communications equipment. Constant frequency operation also enables multiple power converters to be synchronized when it becomes necessary to deliver higher power levels to the output. 
     However, constant frequency switching regulators are in general more complex to design, have a slower transient response, and cannot operate over as wide a range of duty cycles as non-constant frequency switching regulators. Switching regulators must be able to operate efficiently at low duty cycles and over a wide range of input and output voltages to provide the voltages required by modern electronic devices, which may be very low compared to the source voltages. With today&#39;s microprocessors requiring faster transient response and lower operating voltages than previous generations, every effort must be made to improve the transient response and increase the duty cycle range of switching regulators, while meeting cost goals. 
     At present, there are no switching regulators that simultaneously provide the advantages of both constant frequency and non-constant frequency operation. While constant frequency regulators suffer in the transient response and the range of operating duty cycles as compared to non-constant frequency regulators, non-constant frequency regulators may not be able to avoid the sensitive frequencies of the electronic devices and deliver high power levels to the output. 
     Current non-constant frequency regulators such as the MAX1710 and the LTC1778 are able to achieve approximately constant frequency operation through the use of a flexible one shot timer to control the ON-time of one of the switching elements. The one shot timer allows the switching regulators to operate at very low duty cycles and convert high input voltages to low output voltages. However, the switching frequency can still vary significantly due to second order effects in the switching regulator. 
     In view of the foregoing, it would be desirable to provide circuits and methods for achieving constant frequency operation with non-constant frequency switching voltage regulators. 
     It further would be desirable to provide circuits and methods for adjusting the switching frequency of a non-constant frequency switching regulator through the I ON  and V ON  inputs of a one shot timer used to control the duty cycle of the switching regulator. 
     It also would be desirable to provide circuits and methods for synchronizing multiple switching regulators to deliver higher power levels to the output. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide circuits and methods for achieving constant frequency operation with non-constant frequency switching voltage regulators. 
     It is a further object of the present invention to provide circuits and methods for adjusting the switching frequency of a non-constant frequency switching regulator through the I ON  and V ON  inputs of a one shot timer used to control the duty cycle of the switching regulator. 
     It also is an object of the present invention to provide circuits and methods for synchronizing multiple switching regulators to deliver higher power levels to the output. 
     These and other objects of the present invention are accomplished by providing circuits and methods for synchronizing non-constant frequency switching regulators. In a preferred embodiment, non-constant frequency switching regulators are synchronized with a phase locked loop. The phase locked loop controls the duty cycle of the switching transistors in the switching regulator by adjusting the I ON  and V ON  inputs of the one-shot timer used in the switching regulator. The circuits and methods of the present invention are applicable to both synchronous and non-synchronous switching regulators employing current-mode control, voltage-mode control, or a hybrid of current-mode and voltage-mode control. In addition, the circuits and methods of the present invention may be used to synchronize a variety of switching regulators, such as boost (step-up), buck (step-down), or buck-boost switching regulators, with constant ontime, constant off-time, or hysteretic mode control. 
     Advantageously, the present invention enables a non-constant frequency switching regulator to be synchronized with a phase locked loop to achieve constant frequency operation in steady state while having a wider duty cycle range and faster transient response than a constant frequency switching regulator. 
     In addition, the present invention enables multiple regulators to be synchronized and operated in parallel to deliver higher power levels to the output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of the present invention will be 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 schematic diagram of an illustrative prior art step-down, non-constant frequency synchronous switching voltage regulator using current-mode control; 
     FIG. 2 is a schematic diagram of an illustrative prior art one shot timer to be used in accordance with the synchronous switching voltage regulator of FIG. 1; 
     FIG. 3A is a schematic diagram of an exemplary embodiment of the non-constant frequency synchronous switching voltage regulator of FIG. 1 synchronized with a phase locked loop in accordance with the principles of the present invention; 
     FIG. 3B is a schematic diagram of an alternative embodiment of the non-constant frequency synchronous switching voltage regulator of FIG. 1 synchronized with a phase locked loop in accordance with the principles of the present invention; and 
     FIG. 4 is a schematic diagram of two non-constant frequency switching voltage regulators of FIG. 1 synchronized with a phase locked loop in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides methods for synchronizing non-constant frequency switching regulators with a phase locked loop. To provide background for the present invention, the operation of an illustrative and previously known non-constant frequency synchronous switching regulator is described. Then, the methods for synchronizing such previously known regulators with a phase locked loop are disclosed. Referring to FIG. 1, a schematic diagram of an illustrative prior art step-down, non-constant frequency synchronous switching voltage regulator using current-mode control is described. Switching regulator  10  typically is used for DC-to-DC conversion of an unregulated supply voltage V IN , such as a battery, to a regulated output voltage V OUT  for driving a load R L . Although R L  is simply shown as a resistor, it may be, for example, a portable communication device or a computer. Examples of step-down, non-constant frequency synchronous switching regulators using a one shot timer to control the duty cycle of the switching transistors include the LTC1778, the LTC3711, and the LTC3714, sold by Linear Technology Corporation, of Milpitas, Calif., and the MAX1710, sold by Maxim Integrated Products, Inc., of Sunnyvale, Calif. 
     The LTC1778, the LTC3711, and the LTC3714use current-mode control, whereas the MAX1710uses a hybrid of current-mode and voltage-mode control. 
     Switching regulator  10  operates as follows: at the beginning of a cycle, one shot timer  11  generates a pulse that causes driver  12  to turn ON main switching transistor  13  and driver  14  to turn OFF synchronous switching transistor  15 . This results in a voltage of approximately V IN -V OUT  across inductor  16 , causing the current in this inductor to increase. When the one shot timer pulse ends, the output of one shot timer  11  goes low, causing driver  12  to turn OFF main switching transistor  13  and driver  14  to turn ON synchronous switching transistor  15 . As a result, a voltage of −V OUT  is applied across inductor  16 , causing the current in this inductor to decrease. 
     While the inductor current flows through synchronous switch  15 , it creates a voltage equal to the product of the inductor current and the ON-resistance of switch  15 . This voltage is sensed by current amplifier  17  and applied to current comparator  18 . When the sense voltage drops below the control voltage V c , the output of current comparator  18  goes high and initiates another pulse from one shot timer  11 , thereby repeating the cycle. During the time that synchronous switch  15  is off, blanking circuitry  19  disables the output of current comparator  18 . The frequency at which one shot timer  11  operates is referred to as the switching frequency. Inductor  16  and capacitor  24  form a low-pass filter to remove undesirable harmonics of the switching frequency from output voltage V OUT . 
     The control voltage V c  determines the inductor current through the current-mode loop comprising current sense amplifier  17 , current comparator  18 , one shot timer  11 , and drivers  12  and  14  with switches  13  and  15 . The control voltage is determined by the voltage error loop comprised of resistor divider  20 , error amplifier  21 , compensation components  22 , and current comparator  18 . In this type of current-mode regulation, the control voltage V c  corresponds to the inductor current valley. If V OUT  decreases, the resulting voltage drop at the input of error amplifier  21  causes an increase in the control voltage V c  that appears across compensation components  22 . This results in an increase in the average inductor current, causing V OUT  to increase until the negative input to error amplifier  21  matches the reference. Conversely, if V OUT  increases, the control voltage V c  is temporarily reduced, decreasing V OUT  until the negative input to error amplifier  21  again matches the reference. In this way, the control voltage V c  is continuously adjusted such that the output voltage is maintained constant. 
     The use of one shot timer  11  allows switching regulator  10  to turn on main switch  13  for a very small amount of time. Small and constant switch on-times allow switching regulator  10  to operate at very low duty cycles and convert high input voltages to low output voltages. However, a constant on-time requires that the off-time vary with changes in the input and output voltages as well as with load current. Therefore, the switching frequency will also vary. 
     To minimize this variation, one shot timer  11  accepts V IN  and V OUT  as inputs to generate an on-time pulse that is proportional to V OUT  and inversely proportional to V IN . This maintains the switching frequency substantially constant because the on-time changes appropriately as V IN  and V OUT  vary. However, a variety of second order effects such as parasitic resistances and switching losses can cause the required on-time at a particular frequency to deviate from that given by one shot timer  11 . The result is that the switching frequency can still vary significantly. 
     Referring now to FIG. 2, a schematic diagram of an illustrative prior art one shot timer to be used in accordance with the synchronous switching voltage regulator of FIG. 1 is described. The input voltage V IN  of synchronous switching voltage regulator  10  of FIG. 1 is connected to the I ON  input of one shot timer  11 , while the output voltage V OUT  of regulator  10  of FIG. 1 is connected to the V ON  input. In addition to inputs I ON  and V ON , one-shot timer  11  contains input IN and output OUT. 
     One shot timer  11  operates as follows. First, the input voltage V IN  minus the 0.7V from transistor  26  appears across timing resistor  25  (R ON ). Then, the current I ON  through resistor R ON  is transferred to timing capacitor  29  through current mirrors  26 - 27  and  28 . While the input IN to one shot timer  11  is low, the current I ON  flows out through reset switch  32 , causing the output of comparator  31  to be low. 
     When the input IN to one shot timer  11  goes high, latch  33  is set and its Q output goes high. This turns off switch  32  and sets output OUT high. Because switch  32  is OFF, timing capacitor  29  is charged up by the mirrored current from the I ON  input. When the voltage V RAMP  across timing capacitor  29  reaches the output voltage V OUT , the output of comparator  31  goes high, thereby resetting latch  33 . The result is that one shot timer  11  generates an output pulse that is proportional to V OUT  and approximately inversely proportional to V IN . 
     Referring now to FIG. 3A, a schematic diagram of an exemplary embodiment of the non-constant frequency synchronous switching voltage regulator of FIG. 1 synchronized with a phase locked loop in accordance with the principles of the present invention is described. In this circuit, switching voltage regulator  10  of FIG. 1 is synchronized with phase locked loop  34  to control the on-time of one shot timer  11  so that the switching frequency is locked to a reference clock. The result is that a steady state constant switching frequency is achieved. 
     Phase locked loop  34  includes AND gate  37  and data flip flops  35  and  36 . When the flip flop outputs are both zero, a rising edge from CLOCK sets flip flop  36 , and a subsequent rising edge from the OUT output of one shot timer  11  sets flip flop  35 . When both flip flops are set, AND gate  37  causes both flip flops  35  and  36  to reset. The output of flip flop  36  is a square wave having a rising edge corresponding to the rising edge of the clock and whose falling edge corresponds to the rising edge from the OUT output of one shot timer  11 . A loop filter comprising resistors  38  and  39 , and capacitor  40  provides an average DC value from the output of flip flop  36 . 
     The on-time of one shot timer  11  is controlled as follows. When the duty cycle of flip flop  36  is less than 50%, the average DC value provided by flip flop  36  is less than V cc /2, causing amplifier  41  to increase the V ON  input to one shot timer  11 . The on-time of switching regulator  10  is increased as described above in connection with FIG. 2. A longer on-time increases the phase delay between CLOCK and the OUT output of one-shot timer  11  as well as the duty cycle of flip flop  36 . Analogously, when the duty cycle of flip flop  36  is greater than 50%, the output of amplifier  41  is reduced. Consequently, the on-time of switching regulator  10  decreases. The phase delay between the reference clock and the OUT output of one shot timer  11  also decreases. Phase locked loop  34  therefore continuously adjusts the on-time of one shot timer  11  to maintain the duty cycle of flip flop  36  at 50%. This maintains switching regulator  10  at the same frequency as CLOCK with a 180° phase delay. 
     Referring now to FIG. 3B, a schematic diagram of an alternative embodiment of the non-constant frequency synchronous switching voltage regulator of FIG. 1 synchronized with a phase locked loop in accordance with the principles of the present invention is described. In the circuit of FIG. 3B, phase locked loop  34  controls the I ON  input of one shot timer  11  rather than the V ON  input as in the circuit in FIG.  3 A. The polarity of the inputs to amplifier  41  have also been reversed. As a result, when the duty cycle of flip flop  36  is less than 50%, the output of amplifier  41  decreases, causing a corresponding decrease in the voltage at the I ON  input of one shot timer  11 . This increases the on-time of switching regulator  10 . The result is that switching regulator  10  is kept at the same frequency as CLOCK with a 180° phase delay. 
     Although phase locked loop  34  is used to synchronize switching regulator  10  of FIG. 1, it will be understood by one skilled in the art that phase locked loop  34  may be used to synchronize other types of non-constant frequency switching regulator circuits, including synchronous and non-synchronous regulators, such as boost (step-up), buck (step-down), or buck-boost switching regulators, with constant on-time or constant off-time, and using any of several other control techniques. These control techniques include current-mode control with other current sense elements such as a sense resistor or current sense transformer in a variety of locations, voltage-mode control, as well as hybrid control techniques such as sensing based on output capacitor voltage change. Examples of switching regulators using hybrid control techniques include the MAX1710, sold by Maxim Integrated Products, Inc., of Sunnyvale, Calif., and the CS5120, sold by ON Semiconductor, of Phoenix, Ariz. 
     Referring now to FIG. 4, a schematic diagram of two non-constant frequency switching voltage regulators of FIG. 1 synchronized with a phase locked loop in accordance with the principles of the present invention is described. Switching voltage regulators  10 A and  10 B operate independently and are connected in parallel, sharing common input capacitor  23 , common output capacitor  42 , and common current control voltage V c  set by feedback network  43 , error amplifier  44 , and compensation network  45 . The output OUT of one shot timer  11 B of switching regulator  10 B forms the CLOCK input to phase locked loop  46 . Phase locked loop  46  controls the on-time of switching regulator  10 A in the same manner as described above in FIG.  3 A. Because the clock input of flip flop  36  now comes from switching regulator  10 B, switching regulator  10 A and switching regulator  10 B operate at the same constant switching frequency with a 180° phase delay. In steady-state, the system forms a two-phase switching voltage regulator with reduced input and output ripple currents as well as reduced inductor size and capacitance when compared to a single switching voltage regulator. 
     Further, it will be understood by one skilled in the art that phase locked loop  46  may be used to synchronize other types of non-constant frequency switching regulator circuits, including synchronous and non-synchronous regulators, such as boost (step-up), buck (step-down), or buck-boost switching regulators, with constant on-time, constant off-time, or hysteretic control, and using any of several other control techniques. These control techniques include current-mode control with other current sense elements such as a sense resistor or current sense transformer in a variety of locations, voltage-mode control, as well as hybrid control techniques such as sensing based on output capacitor voltage change. 
     In addition, it will be understood by one skilled in the art that phase locked loop  46  may be used to synchronize a plurality of switching regulator circuits to form multiple power converters having two or more phases, multiple inputs and a single output, and multiple outputs with a single input. 
     Although particular embodiments of the present invention have been described above in detail, it will be understood that this description is merely for purposes of illustration. Specific features of the invention are shown in some drawings and not in others, for purposes of convenience only, and any feature may be combined with other features in accordance with the invention. Steps of the described processes may be reordered or combined, and other steps may be included. Further variations will be apparent to one skilled in the art in light of this disclosure and such variations are intended to fall within the scope of the appended claims.