[19J
`
`United States Patent
`
`
`Nelson
`
`[11] Patent Number:
`[ 45] Date of Patent:
`
`4,823,070
`Apr. 18, 1989
`
`[54] SWITCHING VOLTAGE REGULATOR
`CIRCUIT
`
`OTHER PUBLICATIONS
`Data sheet, "Switching De-to-DC Microconverter­
`s-LSH 6300 Series", date unknown.
`San Jose, Calif.
`[75] Inventor: Carl T. Nelson,
`Data sheet, Unitrod UC 1846 Current Mode PWM
`Controller integrated circuit, 12/83.
`[73] Assignee: Linear Technology
`Corporation,
`Documents relating to the Linear Technology Corpo­
`Milpitas, Calif.
`ration LT-1070 integrated circuit.
`R. Salce
`Peckman
`
`[21] Appl. No.: 82,989
`
`[22] Filed:
`
`Aug. 3, 1987
`
`-Patrick
`Primary Examiner
`-Kristine
`
`Assistant Examiner
`
`
`Attorney, Agent, or Firm-Laurence S. Rogers
`
`
`
`
`
`Related U.S. Application Data
`
`[57]
`ABSTRACT
`An integrated circuit for use in implementing a switch­
`ing voltage regulator, the integrated circuit including a
`Continuation of Ser. No. 932,158, Nov. 18, 1986, aban­
`power switching transistor, driver circuitry and control
`circuitry, which is operable in a normal feedback mode
`or an isolated flyback mode. The integrated circuit
`
`Int. Cl.4 .............................................. G05F 1/563
`
`U.S. Cl ..................................... 323/285; 323/299;
`includes shutdown circuitry for placing the regulator in
`363/21
`a micro-power sleep mode, and can be packaged in a
`..................... 363/20, 21, 97, 131;
`five-pin conventional power transistor package. The
`Field of Search
`323/282, 284,285,299,267
`terminals of the integrated circuit regulator perform
`multiple functions. A compensation terminal is used for
`frequency compensation, current limiting, soft-start
`References Cited
`
`operation and shutdown. A feedback terminal is used as
`U.S. PATENT DOCUMENTS
`a feedback input when the integrated circuit is in feed­
`4,146,832 3/1979 McConnell ......................... 323/285
`
`
`
`back mode, and as a logic pin to program the regulator
`
`
`
`4,209,826 6/1980 Priegnitz ............................... 363/21
`for isolated flyback operation. The feedback terminal is
`
`
`4,253,137 2/1981 Rao ....................................... 363/21
`also used to trim the flyback reference voltage.
`
`
`4,425,612 1/1984 Bahler et al ........................... 363/21
`
`
`
`4,641,229 2/1987 Easter ................................... 363/21
`
`
`
`4,652,808 3/1987 Mostyn et al. .................. 323/299 X
`
`
`81 Claims, 8 Drawing Sheets
`
`
`
`doned.
`
`[63]
`
`[51]
`[52]
`
`[58]
`
`[56]
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`

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`Apr. 18, 1989
`U.S. Patent
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`Sheet 1 of 8
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`U.S. Patent Apr. 18, 1989
`
`Sheet 7 of 8 4,823,070
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`1
`
`4,823,070 2
`
`SWITCHING VOLTAGE REGULATOR CIRCUIT
`
`only after the switch driving the primary winding is
`opened. This configuration, known as flyback opera­
`tion, allows multiple regulated output voltages and
`This is a continuation of application Ser. No. 932,158,
`requires only one steering diode and one capacitor for
`filed Nov. 18, 1986, entitled "SWITCHING VOLT- 5
`each output.
`AGE REGULATOR CIRCUIT", now abandoned.
`In either of the above transformer configurations, the
`isolation· provided by the transformer between input
`BACKGROUND OF THE INVENTION
`and output circuits is limited by the need to regulate the
`The present invention relates to an integrated circuit
`output voltage by sensing the output voltage of the
`switching voltage regulator circuit having multi-func- 10
`regulator circuit and providing a feedback voltage sig­
`tion terminals. More particularly, the present invention
`nal to the control circuitry of the circuit. The output
`relates to an integrated circuit for use in implementing a
`circuit driven by the secondary winding of the trans­
`switching voltage regulator circuit, the integrated cir­
`former thus remains electrically connected to the input
`circuit driving the primary winding. Voltage regulator
`cuit requiring only five terminals, operable in both feed­
`back and isolated flyback mode, and including a power 15
`configurations which sense output voltage of the circuit
`switching element, a driver network, and control cir­
`for use as a feedback signal are referred to herein as
`cuitry which sets the duty cycle of the switching ele­
`normal feedback mode regulators. Another configura­
`ment.
`tion, known as an isolated flyback mode regulator, al­
`The function of a voltage regulator is to provide a
`lows a transformer secondary winding to be totally
`predetermined and substantially constant output volt- 20
`isolated from the input circuit connected to the primary
`age level from an unregulated input voltage. Two types
`winding by regulating the peak voltage developed
`of voltage regulators are commonly used today: linear
`across the primary winding when the secondary wind­
`regulators and switching regulators.
`ing provides current to the output circuit.
`A linear regulator controls output voltage by control­
`Switching regulators, although more flexible than
`ling the voltage drop across a power transistor which is 25
`linear regulators in circuit applications, are typically
`connected in series with a load. The power transistor is
`more complex than linear regulators. Although several
`integrated circuits in the past have been commercially
`operated in its linear region and conducts current con­
`available for implementing the control, driver and
`tinuously.
`power switch functions of switching regulators, switch­
`A switching regulator controls output voltage by
`using a power transistor as a switch to provide a pulsed 30
`ing regulators utilizing such integrated circuits have
`flow of current to a network of inductive and capacitive
`required substantial engineering expertise and numerous
`energy storage elements which smooth the switched
`discrete components to make them operational. Also,
`current pulses into a continuous and regulated output
`integrated circuits heretofore available typically re­
`voltage. The power transistor is operated either in a
`quired 8-14 terminals for connection to external dis­
`cutoff or saturated state at a duty cycle required by the 35
`crete components, and could not be configured into a
`voltage differential between the input and output volt­
`very low current (shutdown) mode. This quantity of
`ages. Varying the duty cycle varies the regulated output
`terminals prevented such integrated circuits from being
`voltage of the switching regulator.
`packaged in low-cost power transistor packages such as
`The duty cycle of a switching regulator is controlled
`the conventional 5-pin T0-3 type metal can or the T0-
`by monitoring output voltage or current through the 40
`220 type molded plastic packages, and thus limited the
`switch. The latter type of switching regulator is known
`power handling capability of the integrated circuit.
`as a current-mode switching regulator, and is easier to
`Further, heretofore available integrated circuits for use
`frequency stabilize and has better response to transients
`in implementing switching voltage regulators have not
`than does a switching regulator in which the duty cycle
`been capable of use both in normal feedback mode
`45
`is controlled directly by output voltage.
`switching regulator circuits and isolated flyback mode
`Switching regulators have at least two advantages
`regulator circuits.
`over linear regulators. First, switching regulators typi­
`In view of the foregoing, it would be desirable to be
`cally operate with greater efficiency than linear regula­
`able to provide a switching voltage regulator circuit
`tors, a particularly important factor in high current
`which is simple to implement and which is capable of
`regulators. Second, switching regulators are more ver- 50
`versatile and efficient operation.
`satile than linear regulators. Switching regulators can
`It would further be desirable to be able to provide a
`provide output voltages which are less than, greater
`switching regulator circuit, having a very low current
`than, or of opposite polarity to the input voltage, de­
`sleep mode which can be implemented as an integrated
`pending on the mode of operation of the switching
`circuit which includes the power switch, and which can
`regulator, whereas linear regulators can only provide 55
`be packaged in a conventional T0-3 or T0-220 power
`output voltages which are less than the input voltage.
`transistor package.
`Further, switching regulators can be configured to
`It would also be desirable to be able to provide an
`drive current through the primary winding of a trans­
`integrated circuit which can be utilized to implement
`former, the secondary winding of which simultaneously
`both normal feedback mode and isolated flyback mode
`provides current to the load. The transformer provides 60
`regulator circuit topologies.
`current gain, the amount of which is determined by the
`SUMMARY OF THE INVENTION
`turns ratio of the transformer. Multiple outputs are
`possible, each output typically requiring two steering
`It is therefore an object of the present invention to
`diodes, an inductor and a capacitor. Alternatively, the
`provide an integrated circuit for use in implementing a
`transformer may be configured such that current pro- 65
`switching voltage regulator circuit, the integrated cir­
`vided to the primary winding of the transformer by the
`cuit being simple to implement and capable of efficient
`regulator switch is stored as energy in the primary
`operation in numerous switching regulator configura­
`winding and is transferred to the secondary winding
`tions.
`
`HPE 1026-0010
`
`

`

`4,823,070
`
`5
`
`3
`It is a further object of the present invention to pro­
`vide an integrated circuit capable of implementing a
`switching voltage regulator circuit, and capable of op­
`erating in both normal feedback mode and isolated
`flyback mode voltage regulator configurations.
`It is yet a further object of the present invention to
`provide an integrated circuit, for use in implementing a
`switching regulator, which includes control circuitry,
`driver circuitry and the power switch, which can be
`packaged in conventional 5-pin T0-3 or T0-220 power
`packages, and which is capable of operating in a very
`low current sleep mode.
`These and other objects of the present invention are
`accomplished by a novel switching regulator circuit 15
`which can be packaged as an integrated circuit requir­
`ing only five external terminals for connection to dis­
`crete external components. The low number of termi­
`nals is achieved by assigning several functions to indi­
`vidual terminals. One terminal is used for soft starting, 20
`frequency compensation, switch current limiting and
`shutdown. Another terminal is used to receive a feed­
`back signal when the integrated circuit is operated in a
`normal feedback mode switching voltage regulator
`circuit, and alternatively to place the integrated circuit 25
`into an isolated flyback mode and to vary the flyback
`reference voltage in an isolated flyback voltage regula­
`tor circuit.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The above and other objects and advantages of the
`invention will be apparent upon consideration of the
`• following detailed description, taken in conjunction
`with the accompanying drawings, in which like refer-
`ence characters refer to like parts throughout, and in
`which:
`FIG. 1 is a block diagram of a five-terminal current­
`mode switching voltage regulator integrated circuit of
`the present invention;
`FIG. 2 is a schematic diagram of the switching volt­
`age regulator integrated circuit of FIG. 1 connected in
`a normal feedback mode boost regulator configuration
`and including a soft-start circuit, a frequency compensa-
`tion circuit, and an external current limiting circuit;
`FIG. 3 is a schematic diagram of the switching volt­
`age regulator integrated circuit of FIG. 1 connected in
`an isolated flyback mode switching regulator configura­
`tion;
`FIG. 4 is a schematic diagram of a preferred embodi­
`ment of shutdown circuit 122 and reference voltage
`generator 124, as well as reference 120 and regulator
`102, of the switching voltage regulator integrated cir­
`cuit of FIG. 1;
`FIG. 5 is a schematic diagram of a preferred embodi­
`ment of error amplifier 118 and its interconnection with
`mode select circuit 126 of the switching voltage regula­
`tor integrated circuit of FIG. 1;
`FIG. 6 is a schematic diagram of a preferred embodi- 60
`ment of switches 528, 530 and 532 and mode select
`circuit 126 of FIGS. 1 and 5;
`FIG. 7 is a schematic diagram of a preferred embodi­
`ment of comparator 116 of the switching voltage regu-
`lator integrated circuit of FIG. 1; and
`FIG. 8 is a schematic diagram of a preferred embodi­
`ment of variable zener diode 130 of the switching volt­
`age regulator of FIG. 1.
`
`35
`
`40
`
`45
`
`4
`DETAILED DESCRIPTION OF THE
`INVENTION
`FIG. 1 shows a five-terminal integrated circuit 100 of
`the present invention capable of implementing a cur­
`rent-mode switching voltage regulator circuit, and ca­
`pable of being packaged in a conventional 5-pin power
`package. Five terminals are shown, labeled as VIN
`(input supply), Vsw (output), FB (feedback), V c(com-
`10 pensation) and GND (ground).
`Terminal VIN provides a connecting point for input
`voltage, and is used to supply power to the internal
`circuitry of integrated circuit 100. Terminal V sw is the
`output terminal of circuit 100. It provides a connecting
`point between power switch 110 of regulator 100 and
`external components configured to implement a number
`of switching regulator topologies, to convert the pulsed
`current flowing through switch 110 into a regulated
`output voltage. Further, when regulator 100 is operated
`in an isolated flyback mode, as discussed further herein,
`terminal V sw provides a flyback reference voltage
`point which is held to a peak voltage level which ex­
`ceeds the voltage at terminal VIN by a predetermined
`amount.
`Terminal FB provides three functions. First it serves
`as an input for feedback voltage when integrated circuit
`100 is operated in a feedback mode. Second, terminal
`FB acts as a logic pin for programming integrated cir-
`30 cuit 100 for normal feedback or isolated flyback opera­
`tion. As further discussed below, integrated circuit 100
`is converted from normal feedback operation to isolated
`flyback operation when a current exceeding a predeter-
`mined threshold level is conducted out of terminal FB
`by connecting terminal FB to ground through a resis­
`tor. Third, terminal FB is used to establish the relative
`value (to voltage at VIN) of the fly back reference volt­
`age at terminal V sw. The different functions of terminal
`FB, and their implementation, are discussed in greater
`detail below.
`Terminal V cprovides access to a point in the internal
`circuitry of integrated circuit 100 to provide several
`functions. First, a frequency compensation circuit may
`
`response of integrated circuit 100. Second, a current
`
`be connected to terminal V c to control the closed loop
`limit circuit may be connected to terminal V c to limit
`circuit may be connected to terminal V c to ensure that
`
`the peak current through switch 110. Third, a soft-start
`
`the width of the initial current pulse flowing through
`50 switch 110 starts from zero and builds up to a proper
`level gradually when regulator 100 is first powered up,
`thereby avoiding sudden current surges upon start-up of
`the circuitry. Fourth, a shutdown circuit may be con­
`nected to terminal V c for placing regulator 100 into an
`55 inactive sleep mode in which the current drawn by
`regulator 110 is reduced to a very low value. These
`different functions, and their implementation, are de­
`scribed in greater detail below.
`Referring now to the circuitry internal to switching
`regulator circuit 100, connected to terminal VIN is a
`linear voltage regulator 102 which regulates the supply
`voltage applied to terminal VIN to provide a substan­
`tially constant voltage for use by the internal circuitry
`of regulator 100. Voltage regulator 102 may be substan-
`tially any conventional voltage regulator circuit which
`provides a regulated output voltage of about 2.3V (this
`voltage is not critical, and may be varied as desired).
`Voltage regulator 102 is discussed in more detail below.
`
`65
`
`HPE 1026-0011
`
`

`

`4,823,070
`5
`6
`Conventional oscillator 104 is connected to the set
`below with reference to FIG. 4, shutdown circuit 122
`input of conventional set/reset flip-flop 106 to provide
`provides a shutdown signal to regulator 102 and refer­
`flip-flop 106 with a digital clocking signal. The output
`ence generator 120 when the voltage at terminal V c is
`of flip-flop 106 is connected to driver circuitry 108,
`externally pulled down below the 0.15V reference volt­
`which in tum is connected to switch 110. Substantially 5
`age provided by reference generator 124.
`any conventional driver circuitry may be used to pro­
`The on/off duty cycle of switch 110 is determined by
`vide sufficient base drive current to switch transistor
`the output of comparator 116, which is connected to the
`110. Alternatively, a driver circuit may be used of the
`reset input of flip-flop 106. The output state of compara­
`type disclosed in co-pending patent application Ser. No.
`tor 116 at any time depends on the instantaneous values
`932,014 filed Nov. 18, 1986, entitled "Adaptive Transis- 10
`of the voltages at its two inputs. When integrated circuit
`tor Drive Circuit", filed in the name of Carl T. Nelson,
`100 is operated in its normal feedback mode, as de­
`the disclosure of which is incorporated herein by refer­
`scribed below, a voltage proportional to the regulated
`ence.
`output voltage is applied to terminal FB. Typically the
`The digital clocking signal provided by oscillator
`voltage applied to terminal FB is set by a voltage di­
`104, which preferably has a frequency of approximately 15
`vider resistor network comprising two resistors con­
`40 kHz, is used to tum on switch 110 via flip-flop 106
`nected in series between the regulated output of the
`and driver circuitry 108. Switch 110 is a power transis-·
`voltage regulator circuit and ground. Terminal FB is
`tor having a base connected to driver circuitry 108, a
`connected between the two resistors, and the ratio of
`collector connected to terminal V sw and an emitter
`the resistance values of the two resistors determines the
`connected to one end of sense resistor 112, the other end 20
`proportional relationship of the feedback voltage ap­
`of which is connected to terminal GND.
`plied to terminal FB to the regulated output voltage.
`Flip-flop 106 supplies a signal to driver circuitry 108
`The ratio is chosen such that the voltage applied to
`in response to the clock signal provided by oscillator
`terminal FB equals the output voltage of reference gen­
`104. The signal provided by flip-flop 106 in response to
`erator 120 when the regulated output voltage is at a
`the clock signal causes driver circuitry 108 to tum on 25
`desired value. Error amplifier 118 produces a voltage
`switch 110. When regulator 100 is configured in a
`output which changes in proportion to any difference in
`switching regulator with external components as de­
`voltage between the voltage at terminal FB and the
`scribed below, the current flows between terminal V sw
`reference voltage provided by reference generator 120.
`and terminal GND as a consequence of the turning on
`If the voltage at terminal FB exceeds the reference
`of switch 110 and through sense resistor 112, which 30
`voltage, the output voltage of error amplifier 118 drops
`causes a voltage to be generated across sense resistor
`proportionally, and if the voltage at terminal FB falls
`112. Sense resistor 112 in FIG. 1 has a value of approxi­
`below the reference voltage, the output voltage of error
`mately 0.02 ohms, although other values may be used.
`amplifier 118 increases proportionally.
`The inputs of a conventional common base differential
`This voltage output is applied to input V of compara­
`amplifier 114, preferably having a differential voltage 35
`tor 116. The voltage output of amplifier 114, which is
`gain of approximately 6, are connected across sense
`proportional in magnitude to the current through
`resistor 112. The output of amplifier 114 is connected to
`switch 110, is applied to input I of comparator 116. As
`one input (I) of comparator circuit 116 (the details of
`long as the voltage at input V remains higher than the
`which are further discussed with reference to FIG. 7).
`voltage at input I, comparator 116 remains in an output
`Amplifier 114 detects the voltage generated across 40
`state which causes flip-flop 106 to remain set and to
`sense resistor 112 when power transistor 110 conducts,
`thereby maintain the on condition of switch 110 which
`and responsively provides an amplified signal to input I
`was initiated by oscillator 104. On the other hand, if the
`of comparator 116. The second input (V) of comparator
`voltage at input V becomes lower than the voltage at
`116 is connected to the output of an error amplifier 118
`input I, comparator 116 changes its output state to cause
`(the details of which also are discussed below with 45
`flip-flop 106 to reset, thereby causing driver circuitry
`reference to FIG. 5). The inverting input of error ampli­
`108 to tum off switch 110.
`fier 118 is connected to terminal FB. The noninverting
`During normal feedback operation, therefore, switch
`input of error amplifier 118 is connected to an internal
`110 is turned off when switch current reaches a prede­
`reference voltage generator 120. Reference voltage
`termined level set by the output of error amplifier 118.
`generator 120 is a temperature compensated band-gap 50
`If the regulated output voltage rises above a predeter­
`reference voltage circuit (e.g., a Brokaw Cell) having a
`mined steady-state value set by the voltage divider net­
`voltage output of approximately 1.24V. Error amplifier
`work and reference voltage generator 120, the duty
`118 has a differential voltage gain of approximately
`cycle of switch 110 is shortened, because the voltage at
`800-1000, and provides a maximum output voltage of
`input V drops as a result of the voltage differential at the
`approximately 2.0V.
`inputs of error amplifier 118. The voltage at input I
`Error amplifier 118 detects the difference in voltage
`reflecting the switch current crosses the lowered
`between the voltage at terminal FB and the reference
`threshold value set by the voltage at input V earlier in
`voltage provided by reference generator 120, and re­
`the switch cycle than during steady-state operation.
`sponsively provides an error signal to the V input of
`The shortened duty cycle causes the regulated output
`comparator 116. The output of error amplifier 118 is 60
`voltage to drop until it reaches its previous steadystate
`also connected to terminal V c and to one input of shut­
`value. If the regulated voltage falls below the predeter­
`down circuit 122, a second input of which is connected
`mined steady-state value, the duty cycle of switch 110 is
`to reference voltage generator 124. Reference voltage
`lengthened because error amplifier 118 causes the volt­
`generator 124, the details of which are further discussed
`herein, preferably provides a reference voltage of ap- 65
`age at input V to increase above its steady-state value
`proximately 0.15V. The output of shutdown circuit 122
`such that the voltage at input I crosses the threshold
`is connected to regulator 102 and reference voltage
`value set by the voltage at input V later in the switch
`generator 120. As will be explained in greater detail
`cycle than during steady-state operation. The length-
`
`55
`
`HPE 1026-0012
`
`

`

`4,823,070
`8
`7
`ened duty cycle causes the regulated output voltage to
`tween terminals VIN and V sw, and consequently that
`developed across the primary winding of the trans-
`increase until it reaches its previous steady-state value.
`former, to a value equal to the breakdown voltage of
`The voltage V c at input V of comparator 116 varies
`variable zener diode 130.
`between 0.9 and 2.0 volts during normal feedback oper-
`ation. For a voltage at input V below 0.9V, the duty 5
`On each switch cycle, if the voltage at terminal V sw
`rises to a value which exceeds the voltage at terminal
`cycle of switch 110 is zero. Above 0.9V, and up to 2.0V,
`switch 110 closes (turns on) at the beginning of each VIN by more than the breakdown voltage of variable
`cycle of oscillator 104 and opens (turns off) when the
`zener diode 130, a voltage differential is established at
`the inputs of flyback error amplifier 128 which causes
`switch current (collector current through transistor
`110) reaches a trip level set by the voltage at input V of 10 the voltage output of flyback error amplifier 128 to
`decrease. This in tum lowers the switch current trip
`comparator 116. The switch current trip level increases
`from zero, when input V is at a voltage approximately
`level voltage at input V of comparator 116. Conse-
`equal to 0.9V, to approximately 9.0A when the voltage
`quently, the duty cycle of switch 110 is shortened in
`at input V reaches its maximum value of 2.0V. Because
`response to an increase in the voltage at terminal V sw
`this voltage appears at terminal Ve, the peak current 15 above the reference voltage set by the voltage at termi-
`nal VIN and the breakdown voltage of variable zener
`through switch 110 can be limited by externally clamp-
`diode 130. Conversely, if the voltage at terminal V sw
`ing the voltage of terminal V c to a set value below the
`internal clamp value of 2.0V. External current limiting
`does not reach a value equal to the sum of the voltage at
`is but one of several functions of terminal V c. Other
`terminal VIN and the breakdown voltage of variable
`possible functions include frequency compensation, soft 20 zener diode 130, the voltage at the output of flyback
`starting, and total regulator shutdown into a micro-
`error amplifier 128 increases, which in tum raises the
`power sleep mode. The implementation of these func-
`switch current trip level voltage at input V of compara-
`tions will be further discussed below.
`tor 116. The duty cycle of switch 110 is thereby in-
`creased until the voltage at terminal V sw during the
`Terminal FB also serves multiple purposes. During
`normal feedback operation of integrated circuit 100, 25 open condition of switch 110 exceeds the voltage at
`terminal FB acts as the input point for feedback voltage VIN by the breakdown voltage of variable zener diode
`from the voltage regulator output, as previously dis-
`130. During the period when switch 110 is closed, the
`cussed. Terminal FB further acts as a logic pin for pro-
`voltage at the output of flyback error amplifier 128 is
`held substantially constant by a resistance/capacitance
`gramrning regulator 100 for feedback or fully-isolated
`flyback operation. Terminal FB is connected to the 30 network externally connected to terminal V c, as de-
`input of mode select circuitry 126. Mode select circuitry
`scribed more fully below. In this manner, integrated
`126 has an output connected to error amplifier 118 and
`circuit 100, when connected in an isolated flyback regu-
`lator circuit, maintains the peak voltage across the pri-
`to flyback error amplifier 128. Flyback error amplifier
`128 has two inputs, one connected to terminal VIN, and
`mary winding of a transformer connected between ter-
`the other connected to the anode of a variable zener 35 minals VIN and V swat the breakdown voltage of vari-
`diode 130. The cathode of variable zener diode 130 is
`able zener diode 138, and thereby regulates the output
`connected to terminal V sw. The output offlyback error
`voltage of the isolated flyback regulator circuit.
`amplifier 128 is connected to the V input of comparator
`Variable zener diode 130 has a minimum breakdown
`-
`voltage of approximately 16V. The actual value of the
`116.
`By connecting terminal FB to ground through an 40 breakdown voltage is dependent on the value of the
`external resistor connecting terminal FB to the ground,
`external resistor, current having a value determined by
`as will be further discussed below. Terminal FB thus
`the resistance value of the external resistor is drawn out
`provides a third function in that it permits the regulated
`of terminal FB. As a result of the flow of this current,
`flyback voltage to be trimmed by varying the value of
`mode select circuitry 126 disables error amplifier 118 to
`effectively remove it from the circuit, and enables fly- 45 the resistor connected thereto.
`FIGS. 2 and 3 show illustrative application circuits in
`back error amplifier 128 to effectively connect its out-
`which integrated circuit 100 is operated in its normal
`put to the V input of comparator 116, thereby placing
`regulator 100 into its isolated flyback mode of opera-
`feedback mode (FIG. 2) and in its isolated flyback mode
`(FIG. 3).
`tion. A preferred embodiment of mode select circuitry
`126 is discussed below.
`50 Referring first to FIG. 2, a typical implementation of
`In an isolated flyback regulator circuit, discussed in
`a boost regulator using integrated circuit 100 in its nor-
`more detail below with reference to FIG. 3, terminal
`mal feedback mode and connected to discrete external
`components is shown. The boost regulator provides a
`V swis connected to one end of the primary winding of
`a transformer, the other end of which is connected to
`regulated output voltage V oUTwhich is higher than the
`terminal VIN· When switch 110 is closed, current is 55 voltage applied at terminal VIN·
`drawn through the inductive pr