Circuits and methods for synchronizing non-constant frequency switching regulators with a phase locked loop

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.

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 metal-oxide 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, CA. Examples of prior art non-constant frequency switching regulators include the MAX1710 (constant on-time), developed by Maxim Integrated Products, Inc., of Sunnyvale, CA, the CS5120 (constant off-time), developed by ON Semiconductor, of Phoenix, AZ, 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'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.

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 on-time, 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.

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: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a schematic diagram of an illustrative prior art step-down, non-constant frequency synchronous switching voltage regulator using current-mode control;</li><li>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;</li><li>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;</li><li>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</li><li>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.</li></ul>

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_<b>IN</b><b>,</b> such as a battery, to a regulated output voltage V_<b>OUT</b> for driving a load R_<b>L</b>. Although R_<b>L</b> 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, California, and the MAX1710, sold by Maxim Integrated Products, Inc., of Sunnyvale, California. The LTC1778, the LTC3711, and the LTC3714 use current-mode control, whereas the MAX1710 uses 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_<b>IN</b>-V_<b>OUT</b> 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_<b>OUT</b> 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_<b>C</b>, 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_<b>OUT</b>.

The control voltage V_<b>C</b> 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_<b>C</b> corresponds to the inductor current valley. If V_<b>OUT</b> decreases, the resulting voltage drop at the input of error amplifier 21 causes an increase in the control voltage V_<b>C</b> that appears across compensation components 22. This results in an increase in the average inductor current, causing V_<b>OUT</b> to increase until the negative input to error amplifier 21 matches the reference. Conversely, if V_<b>OUT</b> increases, the control voltage V_<b>C</b> is temporarily reduced, decreasing V_<b>OUT</b> until the negative input to error amplifier 21 again matches the reference. In this way, the control voltage V_<b>C</b> 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_<b>IN</b> and V_<b>OUT</b> as inputs to generate an on-time pulse that is proportional to V_<b>OUT</b> and inversely proportional to V_<b>IN</b>. This maintains the switching frequency substantially constant because the on-time changes appropriately as V_<b>IN</b> and V_<b>OUT</b> 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_<b>IN</b> of synchronous switching voltage regulator 10 of FIG. 1 is connected to the I_<b>ON</b> input of one shot timer 11, while the output voltage V_<b>OUT</b> of regulator 10 of FIG. 1 is connected to the V_<b>ON</b> input. In addition to inputs I_<b>ON</b> and V_<b>ON</b><b>,</b> one-shot timer 11 contains input IN and output OUT.

One shot timer 11 operates as follows. First, the input voltage V_<b>IN</b> minus the 0.7V from transistor 26 appears across timing resistor 25 (R_<b>ON</b>). Then, the current I_<b>ON</b> through resistor R_<b>ON</b> 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_<b>ON</b> 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_<b>ON</b> input. When the voltage V_<b>RAMP</b> across timing capacitor 29 reaches the output voltage V_<b>OUT</b>, 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_<b>OUT</b> and approximately inversely proportional to V_<b>IN</b>.

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_<b>cc</b>/2, causing amplifier 41 to increase the V_<b>ON</b> 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_<b>ON</b> input of one shot timer 11 rather than the V_<b>ON</b> input as in the circuit in FIG. 3A. 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_<b>ON</b> 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, California, and the CS5120, sold by ON Semiconductor, of Phoenix, AZ.

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 10A and 10B 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 11B of switching regulator 10B forms the CLOCK input to phase locked loop 46. Phase locked loop 46 controls the on-time of switching regulator 10A in the same manner as described above in FIG. 3A. Because the clock input of flip flop 36 now comes from switching regulator 10B, switching regulator 10A and switching regulator 10B 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.