United States Patent (19)
`Wilcox et al.
`
`54) HIGH EFFICIENCY SWITCHING
`REGULATOR WITH ADAPTIVE DRIVE
`OUTPUT CIRCUIT
`75 Inventors: Milton E. Wilcox, Saratoga; Robert C.
`Dobkin, Monte Sereno; Carl T. Nelson,
`San Jose, all of Calif.
`73) Assignee: Linear Technology Corporation,
`Milpitas, Calif.
`
`(21). Appl. No.: 786,500
`22 Filed:
`Jan. 21, 1997
`Related U.S. Application Data
`63 Continuation of Ser. No. 454,595, May 30, 1995, aban
`doned.
`(51) Int. C. ... H03K 171693
`52 U.S. Cl. .......................... 327,1403; 327/170; 327/389;
`327/404; 3271541; 327/543
`58) Field of Search ................................... 327/170,380,
`327/381,389, 391, 403, 404, 405, 538,
`540, 541, 543: 326/87; 323/313, 315, 316
`
`56
`
`References Cited
`U.S. PATENT DOCUMENTS
`8/1986 Konishi .................................... 326/87
`4,604,731
`4,611,135 9/1986 Nakayama et al. .
`... 327/404
`4,616,142 10/1986 Upadhyay et al. .
`... .321/.405
`5,099,192 3/1992 Thayer et al. ..
`... 323/315
`5,194,765 3/1993 Dunlop et al. ............................ 326/87
`5,254,883 10/1993 Horowitz et al.
`... 327,541
`5,457,407 10/1995 Shu et al................................... 326/87
`5,475,332 12/1995 Ishimoto .....
`... 327/404
`5,483,188
`1/1996 Frodsham ................................ 327/380
`FOREIGN PATENT DOCUMENTS
`5-206329 8/1993 Japan ....................................... 326/87
`
`
`
`US005731731A
`Patent Number:
`11
`45 Date of Patent:
`
`5,731,731
`Mar. 24, 1998
`
`7-222438 8/1995 Japan.
`
`OTHER PUBLICATIONS
`"Maxim Max717-Max721 Palmtop Computer and Flash
`Memory Power-Supply Regulators," Maxim Integrated
`Products, Inc., Data Sheet Rev. O, Sunnyvale, California,
`pp. 4-67 to 4-77, published Oct. 1992.
`"Battery Management and DC-DC Converter Circuit Col
`lection: Power-Supply Applications Guide for Portable
`Equipment,” Maxim Integrated Products, Inc., pp. 7-9 and
`11, published 1994.
`
`Primary Examiner Terry Cunningham
`Attorney, Agent, or Firm-Fish & Neave; Robert W. Morris;
`Douglas A. Cardwell
`ABSTRACT
`57
`Switching regulator circuits and methods are provided in
`which the output circuit is adaptable to maintain high
`efficiency over various load current levels. The regulator
`circuits generate one or more control signals in response to
`the load current and selectively route a switch driver control
`signal to one or more switches in the output circuit. The
`Switches differ in their size, such that the most efficient
`Switch can be used at a particular load current level. At low
`load current levels, the driver control signal is routed to
`output circuitry with smaller switch devices, which incur
`Smaller driver current losses for a given frequency of
`operation, thereby increasing the regulator efficiency. At
`high load current levels, the driver control signal is routed to
`large switch devices, which incur greater driver current
`losses for a given frequency of operation, but which have a
`lower impedance. The regulator thus maintains high effi
`ciency over a wide range of load currents while operating at
`a constant frequency.
`
`22 Claims, 5 Drawing Sheets
`
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`U.S. Patent
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`Mar. 24, 1998
`
`Sheet 1 of 5
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`5,731,731
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`H
`
`F.G. 1
`PRIOR ART
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`U.S. Patent
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`Mar. 24, 1998
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`Sheet 3 of 5
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`5,731,731
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`U.S. Patent
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`Mar. 24, 1998
`
`Sheet 4 of 5
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`5,731,731
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`U.S. Patent
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`Mar. 24, 1998
`
`Sheet S of 5
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`5,731,731
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`5,731,731
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`1.
`HGH EFFICIENCY SWITCHENG
`REGULATOR WITH ADAPTIVE DRIVE
`OUTPUT CIRCUIT
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`This application is a file-wrapper-continuation of U.S.
`patent application Ser. No. 08/454,595, filed May 30, 1995,
`now abandoned.
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`operation accounts for the reduced amounts of average
`power dissipation in Switching regulators.
`The above-described difference in efficiency can be more
`apparent when there is a large input-output voltage differ
`ential across the regulator. For example, it is not unusual for
`linear regulators to have efficiencies of less than 25 percent,
`while switching regulators, performing equivalent functions,
`operate at efficiencies of greater than 75 percent.
`Because of their improved efficiency over linear
`regulators, switching regulators, and particularly fixed fre
`quency regulators, are typically employed in battery
`operated communication systems such as cellular
`telephones, cordless telephones, pagers, personal
`communicators, and wireless modems. In Such Systems,
`when the Switching regulator is supplying close to the rated
`output current (e.g., during transmission), the efficiency of
`the overall circuit can be high. However, the efficiency is
`generally a function of output current and typically
`decreases at low output current. This reduction in efficiency
`at low output current can become significant in battery
`operated systems where maximizing battery lifetime is
`desirable.
`The relationship between efficiency and output current is
`reflected in the tradeoffs in selecting the power MOSFET
`switches used in conventional switching regulators. Two
`significant components of operating losses in Switching
`regulators are the power dissipated by the switch and switch
`driver current losses. Large power MOSFETs have a lower
`channel resistance, and hence dissipate less power than
`smaller MOSFETs for a given current. However, because of
`their larger gate area, large MOSFETs have a higher gate
`charge and result in greater switch driver current losses than
`smaller MOSFETs, for a given frequency of operation.
`While switch driver current losses are typically less signifi
`cant than dissipative losses at high output currents, Switch
`driver current losses lead to significant inefficiencies at low
`output currents. Reducing the Switching frequency will
`decrease switch driver current losses, but varying frequency
`is an undesirable method for some applications. Such as
`audio circuits, as discussed above.
`In view of the foregoing, it would be desirable to provide
`a high-efficiency switching regulator circuit.
`It would also be desirable to provide a circuit and method
`for maintaining high efficiency over broad current ranges,
`including low output currents, in a switching regulator.
`It would be further desirable to provide a circuit and
`method for maintaining high efficiency over broad current
`ranges, including low output currents, in a Switching regu
`lator that operates at a constant frequency.
`SUMMARY OF THE INVENTION
`It is therefore an object of the present invention to provide
`a high-efficiency switching regulator circuit.
`It is also an object of the present invention to provide a
`circuit and method for maintaining high efficiency over
`broad current ranges, including low output currents, in a
`switching regulator.
`It is also an object of the present invention to provide a
`circuit and method for maintaining high efficiency over
`broad current ranges, including low output currents, in a
`switching regulator circuit that operates at a constant fre
`quency.
`In accordance with these and other objects of the
`invention, there is provided a switching regulator circuit and
`method which includes an adaptable output circuit to main
`
`BACKGROUND OF THE ENVENTION
`The present invention relates to Switching regulator cir
`cuits. More particularly, the present invention relates to
`circuits and methods for maintaining high efficiency over
`broad current ranges in switching regulators that operate at
`a substantially constant frequency.
`The purpose of a voltage regulator is to provide a prede
`termined and substantially constant output voltage to a load
`from a voltage source which may be poorly-specified or
`fluctuating. Generally, there are two different types of regu
`lators: linear regulators and Switching regulators.
`A linear regulator employs a pass element (e.g., a power
`transistor) coupled in series with a load and controls the
`voltage drop across the pass element in order to regulate the
`voltage which appears at the load. In contrast, a Switching
`regulator employs a switch including a switching element
`(e.g., a power transistor) coupled either in series or parallel
`with the load. The switching regulator controls the timing of
`the turning ON and turning OFF of the switching element
`(i.e., the duty cycle) in order to regulate the flow of power
`to the load. Typical switching regulators employ inductive
`energy storage elements to convert Switched current pulses
`into a steady load current. Thus, power in a Switching
`regulator is transmitted across the switch in discrete current
`pulses, whereas power in a linear regulator is transmitted
`across the pass element as a steady flow of current.
`In order to generate a stream of current pulses, Switching
`regulators typically include control circuitry to turn the
`switch ON and OFF. The switch duty cycle, which controls
`the flow of power to the load, can be varied by a variety of
`methods. For example, the duty cycle can be varied by either
`(1) fixing the pulse stream frequency and varying the ON and
`OFF times of each pulse (i.e., pulse-width modulation, or
`PWM), or (2) fixing either the ON or OFF time of each pulse
`and varying the pulse stream frequency.
`Fixed-frequency PWM is often the more desirable method
`because a varying frequency method may result in frequen
`cies in the audible range, especially at sufficiently low duty
`cycles. This may result in audio voltage noise affecting the
`performance of audio circuits powered by the switching
`regulator. Another problem of a variable switching fre
`quency is that the harmonics of the switching frequency may
`interfere with intermediate frequency (IF) or radio frequency
`55
`(RF) communication circuits.
`Whichever method is used to control the duty cycle,
`switching regulators are generally more efficient than linear
`regulators (where efficiency is the ratio of the power pro
`vided by the regulator to the power provided to it). In linear
`regulators, the pass element is operated in its linear region
`where the pass element conducts current continuously. This
`results in the continuous dissipation of power by the pass
`transistor. In switching regulators, to the contrary, the Switch
`is either OFF (where little power is dissipated by the Switch)
`or ON and in a low impedance state (where a small amount
`of power is dissipated by the switch). This difference in
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`tain high efficiency over various load current levels. The
`regulator circuits generate one or more control signals in
`response to the load current and selectively route a driver
`control signal to one or more switches in the output circuit.
`The switches differ in their size, such that the most efficient
`switch can be used at a particular load current level. At low
`load current levels, the driver control signal is routed to
`output circuitry with smaller switch devices, which have a
`higher impedance, but which incur Smaller driver current
`losses for a given frequency of operation, thereby increasing
`the regulator efficiency. At high load current levels, the
`driver control signal is routed to large switch devices, which
`incur greater driver current losses for a given frequency of
`operation, but which have a lower impedance.
`The circuit and method of the present invention can be
`used with various types of power transistor Switches, such as
`synchronous and non-synchronous switches. Additionally,
`the circuit and method can be used with switches in various
`types of Switching regulator configurations, including volt
`age step-down, voltage step-up, polarity-inverting, and fly
`back configurations.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The above and other objects and advantages of the present
`invention will be apparent upon consideration of the fol
`lowing 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 block diagram of a prior art
`non-synchronous step-down switching regulator circuit
`employing conventional PWM and hysteretic control cir
`cuits;
`FIG. 2 is a general illustration of the inductor current
`waveforms of the prior art high-efficiency control circuit of
`FIG. 1 in PWM and hysteretic modes of operation;
`FIG. 3 is a schematic block diagram of a first embodiment
`of a high-efficiency switching regulator circuit and method
`in accordance with the principles of the present invention, in
`a non-synchronous step-down configuration;
`FIG. 4 is a schematic block diagram of a second embodi
`ment of a high-efficiency switching regulator circuit and
`method in accordance with the principles of the present
`invention, in a synchronous step-down configuration; and
`FIG. 5 is a schematic diagram of a third embodiment of
`a high-efficiency switching regulator circuit and method in
`accordance with the principles of the present invention,
`incorporating a flyback boost mode and incorporating an
`adaptive output circuit including non-synchronous and syn
`chronous switches.
`DETALED DESCRIPTION OF THE
`NVENTION
`FIG. 1 shows a simplified schematic diagram of a known
`non-synchronous step-down switching regulator circuit in
`which a hysteretic control circuit is used to improve effi
`ciency at low load current levels. For further details regard
`ing hysteretic control, see the LTC1147-3.3/LTC1147-5
`datasheet in the 1994 Linear Databook, published by Linear
`Technology Corporation, Milpitas, Calif., the disclosure of
`which is incorporated herein by reference in its entirety. The
`datasheet describes the LTC1147-3.3 and LTC1147-5 High
`Efficiency Step-Down Switching Regulator Controllers,
`commercially available from Linear Technology
`Corporation, Milpitas, Calif.
`Referring to FIG. 1, switching regulator circuit 100 pro
`vides a regulated DC output voltage V (e.g., 5 volts) at
`
`4
`terminal 102 for driving load 103 which, while simply
`shown as a resistor, may be, for example, a portable com
`munication device or other battery-operated system. Regu
`lator 100 operates from an unregulated supply voltage V
`coupled to terminal 104 (e.g., a 12 volt battery).
`Regulator 100 includes PWM control circuit 120, hyster
`etic control circuit 140, control selector 150, and output
`circuit 130. Control selector 150 determines whether PWM
`circuit 120 or hysteretic circuit 140 controls output circuit
`130. Output circuit 130 provides current at a regulated
`voltage to terminal 102.
`Output circuit 130 includes capacitor 131, driver 132.
`MOSFET 134, diode 136, resistor 137, inductor 138, and
`capacitor 139. MOSFET 134, which may be p-channel or
`n-channel, and diode 136 each operate as Switching ele
`ments. MOSFET 134 is coupled in series with diode 136
`between supply voltage V and ground to form a non
`synchronous switch circuit. As used herein, the term "non
`synchronous switch circuit” refers to a switch including one
`switching transistor and one diode, wherein the transistor is
`driven by a drive signal and the diode responds passively,
`such that the transistor and diode switch ON and OFF out of
`phase with each other to supply current to a load.
`MOSFET 134 and diode 136 alternately supply current to
`output capacitor 139 through output inductor 138. Inductor
`138 and capacitor 139 Smooth the alternating supply of
`current to supply a regulated voltage to load 103. Resistor
`137, which is coupled in series between inductor 138 and
`terminal 102, is a small sense resistor used to generate a
`voltage signal proportional to the current I flowing through
`inductor 138. In order to supply the alternating current,
`MOSFET 134 is driven by driver 132, which is controlled by
`PWM control circuit 120 or hysteretic control circuit 140.
`PWM control circuit 120 is a current mode pulse-width
`modulator (PWM) circuit which controls the duty cycle of
`driver 132 at high average load currents to regulate the
`current through inductor 138 such that output voltage Vor
`is equal to the desired regulator voltage. Oscillator 122
`causes control logic 121 to provide an ON pulse at a constant
`frequency to driver 132. Each ON pulse causes driver 132 to
`turn MOSFET 134 ON, thereby turning diode 136 OFF, and
`causing inductor current I, to increase.
`Control logic 121 turns MOSFET 134 OFF when com
`parator 129 indicates that current I has reached the level set
`by differential amplifier 128. Current generates a voltage
`across sense resistor 137 that trips comparator 129 when the
`voltage exceeds controllable offset voltage 123. Offset volt
`age 123 is linearly proportional to the voltage V at the
`output terminal of differential amplifier 128. When control
`logic 121 provides an OFF pulse to turn MOSFET 134 OFF.
`It flows through diode 136 in conventional fashion, but
`ramps down.
`Differential amplifier 128 compares feedback voltage
`V, obtained by dividing output voltage Vor across
`resistors 125 and 126, to voltage reference 127 which is
`indicative of the desired output voltage. When V.
`decreases slightly due to an increase in load current, voltage
`V increases, thereby increasing offset voltage 123. This in
`turn causes comparator 129 and control logic 121 to increase
`the peak current through inductor 138 during each switch
`cycle to the level demanded by the load.
`As discussed above, the power loss resulting from gate
`charge current is a function of MOSFET gate charge and
`switching frequency, and becomes significant at low output
`currents. To provide high-efficiency at low load currents
`(e.g., less than 20 percent of the maximum rated output),
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`regulator 100 employs hysteretic control circuit 140 for
`controlling the switching of MOSFET 134 instead of PWM
`control circuit 120.
`At low load currents, control selector 150 allows hyster
`etic control circuit 140 to control driver 132. Hysteretic
`control circuit 140 turns MOSFET 134 ON less frequently
`in this state of operation under conditions where the output
`voltage V can be maintained substantially at the regu
`lated voltage by output capacitor 139 for a substantial period
`of time. This hysteretic control method is discussed in more
`detail in the LTC1147-3.3/LTC1147-5 data sheet previously
`mentioned and incorporated by reference. This feature of the
`prior art reduces the losses in the regulator circuit because
`MOSFET 134 switches on at a much lower frequency, thus
`drawing less gate charge current. Changing between PWM
`15
`and hysteretic control modes can be accomplished manually
`or automatically. For example, the LTC1147 switches modes
`automatically by limiting the minimum output current in
`PWM mode and detecting an increase in voltage at the
`output.
`In the above-described state of operation (i.e., "hysteretic
`mode”), the load 103 is supported substantially by output
`capacitor 139. Hysteretic control circuit 140 monitors the
`feedback voltage V
`When V falls by a hysteresis amount, control circuit
`140 causes driver circuit 132 to turn MOSFET 134 ON only
`as required to recharge the output capacitor 139 by the
`hysteresis amount. If the load current remains sufficiently
`low, capacitor 139 will recharge by the hysteresis amount
`after only one 0N cycle of MOSFET 134, after which
`MOSFET 134 is turned OFF.
`Thus, during light loads, regulator 100 is adapted to turn
`MOSFET 134 ON only briefly to recharge the output
`capacitor 139. Therefore, Vour oscillates between upper
`and lower thresholds. Circuit 140 adjusts the rate at which
`MOSFET 134 turns ON to recharge output capacitor 139 in
`response to the load current, thus maintaining high efficien
`cies even at low output currents.
`FIG. 2 is a general illustration of the current I flowing
`through inductor 138 during both PWM and hysteretic
`modes of operation. Referring to FIG. 2, trace 201 shows
`that I ramps up and down as MOSFET 134 is switched ON
`and OFF respectively over each switch cycle in PWM mode.
`Trace 202 shows I, in hysteretic mode. As trace 202 shows,
`I, increases during periodt, during which MOSFET 134 is
`switched ON to recharge output capacitor 139. During
`period t MOSFET 134 is turned OFF, diode 136 turns ON,
`and quickly ramps down. During periodt, the load current
`slowly discharges capacitor 139 until it has discharged by
`the predetermined hysteresis value. As the load current
`decreases, V decreases at a lower rate, lengthening period
`ta. Thus, the frequency at which MOSFET 134 is turned ON
`decreases and the gate charge current decreases proportion
`ately.
`The variation in the switching frequency of MOSFET 134
`in hysteretic mode can be undesirable in certain applica
`tions. One disadvantage appears in audio circuit
`applications, where the switching frequency may decrease
`into the audio frequency range while operating at sufficiently
`low load currents. This may result in audio output voltage
`noise which may contaminate audio circuits powered by the
`switching regulator.
`An additional disadvantage of the variable-frequency
`hysteretic mode of switching regulator 100 is that inductor
`138 itself may produce audible noise which may be objec
`tionable to a user of the device employing the regulator
`circuit.
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`A further disadvantage of variable-frequency switching
`regulators in communication circuits is thatharmonics of the
`switching frequency may interfere with the IF or RF fre
`quency communications circuits. The high frequency har
`monics of the switching frequency may drift into frequency
`ranges reserved for IF or RF signals.
`An alternative known method of maximizing efficiency of
`switching regulators is to provide two or more complete
`switching regulators, each of which is most efficient at a
`particular output current, and control circuitry which
`switches the output between the regulators. This approach,
`with manual switching between the regulators, is suggested
`by Battery Management and DC-DC Converter Circuit
`Collection, published by Maxim Integrated Products, Inc.,
`Sunnyvale, Calif., page 9, (1994). A disadvantage of this
`second alternative approach is that it requires two complete
`regulators and is therefore more expensive and cumbersome
`than a single regulator.
`The deficiencies of the switching regulator circuits
`described above are overcome by the switching regulator
`circuits and methods of the present invention. FIG.3 shows
`a constant frequency switching regulator circuit 300 which,
`in accordance with the principles of the present invention.
`includes an adaptable output circuit for high efficiency over
`broad current ranges.
`Referring to FIG. 3, switching regulator 300 also includes
`PWM control circuit 120 of FIG. 1. The PWM control
`circuit may be of the type described above in connection
`with FIG. 1 or it may be implemented utilizing other known
`PWM control circuits. As shown in FIG. 3, PWM control
`circuit 120 is substantially the same as PWM control circuit
`120 of FIG.1. The only exceptions relate to connections for
`external circuitry to monitor signals (e.g., in FIG. 1, hyster
`etic control circuit 140 has connections to PWM circuit 120,
`while in FIG. 3, output control circuit 340 is connected to
`PWM circuit 120 instead.) Circuit 300 also includes adapt
`able output circuit 330, output control circuit 340, and
`control routing circuit 350. As described in greater detail
`below, output control circuit 340 and control routing circuit
`350 cause output circuit 330 to operate with small MOSFET
`335 rather than large MOSFET 134 at low load current
`levels. This feature of the present invention reduces the
`switch driverpower consumption because MOSFET 335 has
`a smaller gate charge than MOSFET 134.
`In accordance with the present invention, switching regu
`lator 300 adapts output circuit 330 to operate at low load
`current levels as follows. Output control circuit 340 includes
`comparator 344 and voltage reference 347 which supplies a
`reference voltage V. As described above with reference
`to FIG. 1, output V of differential amplifier 128 decreases
`in response to decreasing load current, and therefore serves
`as an indicator of load current. The voltage at the output of
`comparator 344 goes high when the load current decreases
`to the point that Vo falls below Vez. Veer is preferably
`chosen to be indicative of the level of V and, hence, the
`load current at which large MOSFET 134 is no longer
`required so that small MOSFET 335 can supply the neces
`sary load current.
`Output circuit 330 includes the following components of
`output circuit 130 of FIG. 1: capacitor 131, switch driver
`132, MOSFET 134, diode 136, capacitor 139, inductor 138,
`and resistor 137. Output circuit 330 also includes small
`switch driver 333 and small MOSFET 335, which form a
`second Switch circuit. Driver 333 and MOSFET 335 are
`preferably smaller than driver 132 and MOSFET 134
`respectively. In response to a low voltage at the output of
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`comparator 344, control routing circuit 350 routes the con
`trol signal from PWM circuit 120 to driver 132, which
`operates MOSFET 134. In response to a high voltage at the
`output of comparator 344 control routing circuit 350
`removes the control signal from driver 132 and instead
`routes it to driver 333.
`Each switch driver may include an idle state which turns
`OFF its respective MOSFET when control routing circuit
`350 removes the control logic signal from its input, as is well
`known to those of ordinary skill in the art. Thus, at low load
`currents, when the voltage at the output of comparator 344
`is high, the control signal from PWM control circuit 120
`controls the switching of MOSFET 335 to supply the
`necessary current to inductor 138. Also, when the voltage at
`the output of comparator 344 is high, large MOSFET 134 is
`not operated, thereby eliminating the switch driver current
`losses associated with charging its gate.
`Thus, in accordance with the present invention, circuit
`300 maintains high efficiency over broad current ranges
`while operating at a constant frequency. In a typical
`application, V may be set at a level equivalent to the
`voltage of V which corresponds to a load current approxi
`mately one-tenth the maximum rated output. In that case,
`regulator 300 switches from large MOSFET 134 to small
`MOSFET 335 at one tenth the maximum rated output
`current. Therefore, small MOSFET 335 is typically one
`tenth the size of large MOSFET 134. Thus, efficiency similar
`to that of prior art regulators, when operating at higher load
`currents, is extended over another decade of load current
`range by the adaptable output circuit of the present inven
`tion.
`It will be apparent to those of ordinary skill in the art that
`although the present invention has been discussed above
`with respect to FIG. 3, wherein the regulator includes
`non-synchronous switch circuits, the present invention may
`also be used in a regulator incorporating a synchronous
`switch circuit as well. As used herein, the term "synchronous
`switch circuit” refers to a switch including at least two
`switching transistors that are driven out of phase with
`respect to each other to supply current at a regulated voltage
`to a load. FIG. 4 shows a schematic diagram of a high
`efficiency switching regulator with adaptable output circuit
`of the present invention in a synchronously switched step
`down configuration.
`Referring to FIG. 4, Switching regulator circuit 400
`includes PWM control circuit 120, output control circuit
`340, and control routing circuit 350, which all operate in a
`manner substantially similar to the like-numbered compo
`nents of regulator 300 of FIG. 3. Circuit 400 also includes
`50
`synchronously switched output circuit 430 which replaces
`output circuit 330 of FIG. 3. As described in greater detail
`below, output control circuit 340 and control routing circuit
`350 cause output circuit 430 to operate with either a large
`pair or a small pair of synchronously switched switching
`transistors.
`Referring to FIG. 4, output circuit 430 includes the
`following components of output circuit 330 of FIG. 3: top
`MOSFETS 134 and 335, drivers 132 and 333, capacitor 131,
`inductor 138, resistor 137, and capacitor 139. Output circuit
`430 also includes inductor 438, drivers 432 and 433, and
`bottom MOSFETS 434 and 435, wherein MOSFET 434 is
`preferably larger than MOSFET 435, and preferably by the
`same ratio as MOSFETs 134 and 335. MOSFET pair 134
`and 434 and MOSFET pair 335 and 435 each form indi
`vidual synchronous switch circuits. In response to a high
`voltage from the PWM circuit 120 control signal, either
`
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`driver 132 turns ON MOSFET 134 to increase the current
`through inductor 138, or driver 333 turns ON MOSFET 335
`to increase the current through inductor 438. Bottom MOS
`FETs 434 and 435 remain OFF in either case. In response to
`a low voltage from the PWM circuit 120 control signal,
`either driver 432 turns ON bottom MOSFET 434 to decrease
`the current through inductor 138, or driver 433 turns ON
`bottom MOSFET 435 to decrease the current through induc
`tor 438. Top MOSFETs 134 and 335 are kept OFF in both
`circumstances. In accordance with the principles of the
`present invention, output control circuit 340 causes control
`routing circuit 350 to route the drive signal from PWM
`circuit 120 to either large MOSFET drivers 132 and 432 or
`small MOSFET drivers 333 and 433 in response to the
`sensed load current, as previously described with reference
`to circuit 300 in FIG. 3.
`The addition of inductor 438 provides several benefits.
`First, inductor 438, because it is used at low load current, is
`typically larger than inductor 138. This reduces the peak
`currentflowing through MOSFETs 335 and 435, and thereby
`reduces dissipative losses. Second, coupling MOSFETs 335
`and 435 to inductor 438, rather than inductor 138, reduces
`the voltage swing on the drains of large MOSFETs 134 and
`434 when MOSFETs 335 and 435 are operated, thereby
`reducing losses caused by capacitive currents flowing in
`MOSFETs 134 and 434.
`It will be apparent to those of ordinary skill in the art that
`although the present invention has been discussed with
`reference to FIGS. 3 and 4, wherein the regulator either
`selects between two non-synchronous switch circuits or
`selects between two synchronous switch circuits, the present
`invention can be used to adapt the size and types of switches
`in more complex regulator output circuits. It will also be
`apparent to those of ordinary skill in the art that PWM
`control circuit 120 is typically designed such that V
`includes a slope compensation component, thus making it a
`potentially inaccurate indicator of load current. FIG. 5
`shows a schematic diagram of a high-efficiency switching
`regulator circuit of the present invention incorporating a
`flyback boost mode, an adaptive output circuit including
`non-synchronous and synchronous switch circuits, and a
`more complex output circuit control.
`Referring to FIG. 5, regulator 500 includes PWM control
`circuit 120, which operates in a manner substantially similar
`to PWM control circuit 120 of FIGS. 3 and 4. However,
`PWM control circuit 120 of FIG.S also includes additional
`external connections to monitor the voltage across resistor
`137. Regulator 500 also includes output circuit 530, output
`control circuit 540, switch logic 550, and flyback override
`circuit 560. As discussed in greater detail below, output
`control circuit 540, control circuit 120, and flyback override
`circuit 560 provide feedbackinformation to switch lo