A LEVEL SHIFTER FOR A POWER SUPPLY, FOR EXAMPLE IN A TELEVISION APPARATUS

Abstract: A level shifter (600) receives a variable input voltage (V NINI ), such as a beam current representative voltage of a cathode ray tube in a television receiver, and produces an output voltage (V NIN ) that changes when the input voltage changes, such that the output voltage (V NIN ) is level shifted relative to the input voltage (V NINI ). The output voltage (V NIN ) is coupled to an input terminal (+) of an error amplifier (610) of a power supply voltage regulator. The level shifter (600) includes a current mirror arrangement responsive to the input voltage. A band gap type voltage source (699) determines the amount of level shifting that is produced by the level shifter (600).

Description: The invention relates to a level shifter as used for example in an input stage of a power supply regulator of a television apparatus, in particular to a circuit arrangement that combines an input signal with a reference signal to produce an output signal that is level shifted relative to that of the input signal. In a power supply arrangement of, for example a television receiver, see for example DE-A-3028589, a voltage representative of a DC, regulated supply voltage B+ is coupled to, for example, an inverting input terminal of a B+ voltage regulator. A reference voltage is coupled to, for example, a noninverting input terminal of the regulator. The feed-back voltage that is representative, for example, of regulated supply voltage B+ is compared, in the input stage, with the reference voltage to generate an output voltage that is coupled to a controllable arrangement. The controllable arrangement regulates voltage B+ at a level that is determined by the reference voltage. Voltage B+ may be used, for example, to energize a deflection circuit output stage for a cathode ray tube (CRT). In a similar prior art cirucit, a voltage representative of, for example, a level of the beam current in the CRT is summed with the reference voltage, such that the sum of both, instead of the reference voltage alone, is applied to the noninverting input terminal of the input stage. A variation of the beam current representative voltage varies voltage B+ so as to maintain constant the raster width in the CRT when the beam current changes. The summation of the constant reference voltage and the variable beam current representative voltage is done, in such prior art circuit, by coupling the beam current representative voltage via a zener diode to the noninverting input terminal. The zener diode develops between its anode and cathode electrodes the reference voltage. Thus, the reference voltage that level shifts the beam current representative voltage is series coupled with the beam current representative voltage. A disadvantage of such zener diode level shifting is that a change in the zener voltage due to temperature, ageing or an inherent noise in the zener diode, causes the level to change. It may be desirable to sum the beam current representative voltage with a reference voltage to generate a sum voltage that is applied to the noninverting input terminal of the input stage without using such zener diode as the level shifter. In accordance with the invention, there is provided a power supply that includes a level shifter that level shifts an input voltage that varies an output voltage of said power supply in response to variation of the input voltage, comprising a source of supply voltage; a controllable conductive element coupled to said supply voltage for generating therefrom said output voltage; means, including a comparator having one input coupled to receive a signal indicative of said output voltage and having an output signal coupled to a control terminal of said conductive element for varying, in accordance with the output signal of said comparator, the conduction of said conductive element to control said output voltage; characterized in that said level shifter has the form of a current mirror arrangement including a first transistor responsive to a current in a first circuit branch for generating a current in a second circuit branch which includes a first main current conducting electrode of said first transistor, that is the current mirror of said current in said first circuit branch, said first main current conducting electrode being coupled at a junction terminal to a source of predetermined current; a source of said input voltage coupled to an input terminal of said first transistor for varying said current in said first main current conducting electrode to produce a current that varies in accordance with said input voltage and with the difference between said current in said first main current conducting electrode and said current in said second circuit branch; a first resistance coupled to said junction terminal for conducting said differences related current to develop a voltage thereacross that varies in accordance with said input voltage, and a source of temperature compensated first voltage coupled via said first resistance to said junction terminal such that said voltage across said resistance is combined with said first voltage for developing a temperature compensated second voltage at said junction terminal that varies in accordance with said input voltage and that is level shifted in accordance with said temperature compensated first voltage, said temperature compensated, level shifted, second voltage being coupled to another input of said comparator for varying said output signal of said comparator in accordance therewith to control said output voltage. In an embodiment of the invention, the reference voltage is produced by a voltage source of, for example, the well-known bandgap type. Thus, the beam current representative voltage is summed with and level shifted by the reference voltage that is produced by the bandgap type source. Advantageously, the bandgap type source is less susceptible than the zener diode to temperature changes, component ageing or noise. In the drawings: FIGURE 1 illustrates a simplified schematic diagram of a power supply regulator circuit that includes a level shifter, embodying an aspect of the invention; FIGURE 2 illustrates a detailed schematic diagram of the level shifter of FIGURE 1; and FIGURES 3, 4, 5 and 6 illustrate level shifters embodying different aspects of the invention, respectively. FIGURE 1 illustrates a simplified schematic diagram of a power supply of a television receiver, not shown in the FIGURES, that includes a regulator 100 that is an integrated circuit that regulates a supply voltage B+. Voltage B+ may be used, for example, to energize a horizontal deflection circuit or output stage 99 of the television receiver. A voltage V+, representative of voltage B+, is obtained from output stage 99. Voltage V+ is coupled to a voltage divider 605 that includes series coupled resistors 601, 604 and 602. Resistor 604 includes a wiper k for developing at wiper k a voltage that is representative of, for example, voltage B+. The voltage at wiper k, that is adjustable by varying the position of wiper k, is coupled to an inverting input terminal 608 of an error amplifier 610 via a resistor 607. A small voltage that is proportional to the beam current in the CRT of the receiver is coupled from a tertiary winding of transformer T to a terminal 611 to form a voltage V NINI that is indicative of the beam current. Voltage V NINI that varies when a variation of the beam current occurs, is coupled via a level shifter 600, embodying an aspect of the invention, to a noninverting input terminal 609 of error amplifier 610 to produce an input voltage V NIN . Level shifter 600 establishes a fixed offset voltage between terminals 611 and 609 that is determined by a voltage V BG . Voltage V BG is generated in a bandgap type voltage source 699. Bandgap type voltage source 699 advantageously maintains voltage V BG constant such that voltage V BG is affected significantly less by component aging or tolerance than would have occurred had a zener diode been used. As explained later on the feed back arrangement of regulator 100 causes voltage B+ to be such that voltage V IN becomes equal to voltage V NIN . An integrating filter 612 is coupled between inverting input terminal 608 and an output terminal 618 of amplifier 610 to provide the loop filter of regulator 100. A filtered error voltage V 0 , developed at terminal 618 is coupled to a first input terminal of an adder 613. A horizontal rate sawtooth generator 98 develops a horizontal rate signal, having an upramping portion, which is added to error voltage V 0 in adder 613. The sum signal that is also upramping, is applied to an inverting input terminal (614) of a comparator 615 functioning as a pulse width modulator. When, during its upramping portion, the output of adder 613 becomes more positive than a constant DC voltage V REF , that is coupled to a noninverting input terminal of comparator 615, a negative going transition at an output terminal 615a of comparator 615 is coupled via a buffer amplifier 616 to a control terminal 617a of a switch 617b of a switch mode power supply output stage 617 to turn on switch 617b of output stage 617. An input terminal 617c of output stage 617 is coupled to unregulated voltage V UR . Regulated voltage B+ is developed at an output terminal 617d of output stage 617. The duration, during each horizontal period, H, in which switch 617b conducts is determined by the level of error voltage V 0 of error amplifier 610. Thus, regulated voltage B+ is determined by voltage V NIN . As indicated before, voltage V NIN is produced by level shifter 600, embodying an aspect of the invention, that is described now in detail. FIGURE 2 illustrates a schematic diagram of level shifter 600 of FIGURE 1 and of error amplifier 610. Similar numbers in FIGURES 1 and 2 represent similar items or functions. Level shifter 600 of FIGURE 2 is temperature compensated over a wide range of ambient operating temperatures, such as between 0°C and 70°C, to produce voltage V NIN that is substantially unaffected by a change in the temperature within such range. A temperature compensated current control arrangement 650 generates a control voltage V BR on a rail signal line 900. Rail signal line 900 is coupled to the base electrode of each of transistors Q142, Q725, Q727, Q736 and Q737. The emitter electrodes of the above-mentioned transistors are coupled through corresponding resistors to a fixed DC voltage V cc . Current control arrangement 650 controls voltage V BR in such a way that the collector current in each of the above-mentioned transistor stays substantially constant when the temperature changes. An example of an arrangement that is similar to current control arrangement 650 is described in detail in U.S. Patent No. 3,886,435, in the name of S. A. Steckler, entitled VBE VOLTAGE SOURCE TEMPERATURE COMPENSATION NETWORK. Level shifter 600 includes transistors Q736 and Q737. The emitter currents in transistors Q736 and Q737 are controlled by resistors R728 and R69, respectively, having the same value, so as to cause the respective collector currents of transistors Q736 and Q737, that are temperature compensated, to be equal. The collector of transistor Q737 is coupled to a current mirror arrangement that includes transistors Q733, Q734 and Q735. The collector of transistor Q737 is coupled to the collector of transistor Q734. The emitter of transistor Q735 is coupled to each of the bases of transistors Q733 and Q734. Transistor Q735 provides the base current drive to each of transistors Q733 and Q734. The emitter of transistor Q734 is coupled to ground via a resistor R732. The P-N junction of transistor Q734 between the base and emitter electrodes of transistor Q734, provides temperature compensation that compensates for a temperature related variation of the base-emitter voltage of transistor Q733. The emitter of transistor Q733 is coupled through resistor R731 to terminal 611, where voltage V NINI of FIGURE 1 is developed. The value of resistor R731 is equal to that of resistor R732. Voltage V NINI is prevented from exceeding predetermined limits in either polarity by a diode network 675. The collector of transistor Q733 is coupled to the collector of transistor Q736 at a junction terminal 733A. Assume that voltage V NINI is zero. In this case, the current mirror arrangement of transistors Q733, Q734 and Q735, produces a collector current i Q733 in transistor Q733 that is equal to the collector current i Q734 in transistor Q734 because the base current of transistor Q735 is negligible. As explained before, when voltage V NINI is zero, collector current i Q736 in transistor Q736 is equal to collector current i Q737 in transistor Q737 over a wide temperature range. Also, when voltage V NINI is zero, each of collector current i Q734 and its mirror the collector current i Q733 is equal to current i Q737 over such wide temperature range. It follows that current i Q733 is also equal to current i Q736 . Bandgap type voltage source 699 supplies temperature compensated reference voltage V BG that is coupled via a resistor R729 to terminal 733A. Because, as described before, when voltage V NINI is zero, current i Q733 is supplied entirely by current i Q736 , and because the impedance at terminal 733A, that is contributed by the collectors of transistors Q733 and Q736 is high, a current i R729 in resistor R729 is zero; therefore, voltage V NIN at terminal 733A is equal to voltage v BG . Thus, in accordance with an aspect of the invention, when voltage V NINI is zero, voltage V NIN is level shifted by an amount that is equal to voltage V BG . When voltage V NINI at terminal 611 is different from zero, currents i Q736 and i Q737 will not be equal. The difference current between currents i Q733 and i Q736 will cause a voltage to develop across resistor R729 that, in turn, will cause a corresponding change in voltage V NIN at terminal 733A. Because transistor Q733 is coupled, relative to voltage V NINI , as a common base amplifier, and because resistors R731 and R729 are, illustratively, equal, the gain, or the ratio between voltage V NIN and voltage V NINI , is one, resulting in an amplifier having a unity gain. In carrying out another aspect of the invention, voltage V NIN , that is level shifted relative to voltage V NINI by an amount that is equal to voltage V BG , follows variations of voltage V NINI that occur in a range between positive and negative values. Voltage V BG is temperature compensated and has a tolerance range that is narrow relative to, for example, a zener diode. Furthermore, component aging affects voltage V BG substantially less than it affects, for example, the breakdown voltage of a zener diode. Moreover, the level shifting caused by level shifter 600 is, advantageously, less susceptible to temperature, aging and noise when compared with that produced by a corresponding level shifter in the prior art that utilizes a zener diode interposed between a beam current input terminal and a noninverting input terminal of a differential amplifier to perform such level shifting. Should a temperature change cause a corresponding change in current i Q736 , for example, that, as indicated before, would be relatively small, transistors Q737, Q733, Q734 and Q735 will cause a proportional change in current i Q733 to occur that will prevent even such small change in temperature from affecting the difference current between currents i Q736 and i Q733 . Therefore, when voltage V NINI is zero, voltage V NIN is, advantageously, not affected by collector currents i Q736 and i Q737 , but is entirely determined by voltage V BG that is temperature compensated. It should be understood that temperature compensation may be adequate even when voltage V NINI is significantly different from zero. If temperature compensation, in this case, is inadequate, a further improvement in temperature compensation may be obtained by coupling the terminal of, for example, resistor R732, that in FIGURE 2 is grounded, to a voltage that is different from zero and that is related to, for example, voltage V NINI . Advantageously, voltage V BG , as explained before, is maintained at tight tolerances, is temperature compensated and is substantially unaffected by components aging. Therefore, advantageously, no factory temperature burn-in process is required prior to the installment of regulator 100 of FIGURE 1 in the television receiver. Furthermore, voltage divider 605 that includes resistors 601, 604 and 602 is required to compensate, advantageously, only for a narrower tolerance range than in prior art circuits in which a zener diode is used for performing the level shifting function of level shifter 600 of FIGURE 2. Voltage V IN is coupled to the base of a transistor Q721. The clamping operation of a pair of transistors Q145 and Q146 prevents voltage V IN from being above voltage V BG or from being below voltage V BG by more than a predetermined magnitude. Voltage V IN is coupled to inverting input terminal 608 and voltage V NIN is coupled to noninverting input terminal 609 of error amplifier 610. Amplifier 610 includes a current source formed by a transistor Q142 that provides the combined emitter currents of a transistor Q148 and of a transistor Q149, coupled as a differential amplifier. The bases of transistors Q148 and Q149 are coupled to the emitters of transistors Q721 and Q723 respectively. Transistors Q721 and Q723 operate as emitter followers to couple voltages V IN and V NIN to the bases of transistors Q148 and Q149, respectively. A current mirror arrangement 610b that is coupled to the collectors of transistors Q148 and Q149 causes a current i₆₁₀, coupled to integrating filter 612 of FIGURE 1, to be equal to the difference between the collector currents in transistors Q148 and Q153. Consequently, current i₆₁₀ that is coupled to filter 612 of FIGURE 1, is proportional to the difference between voltages V IN and V NIN . The proportionality factor is determined by the gain of error amplifier 610. FIGURES 3, 4, 5 and 6 illustrate level shifters 600a, 600b, 600c and 600d, respectively, embodying other aspects of the invention, respectively. In FIGURES 2-6, numbers and symbols of similar items or functions are similar except that they include the letters a, b, c and d, in FIGURES 3, 4, 5 and 6, respectively. In FIGURE 3, resistors R731 and R732, that are used in the circuit of FIGURE 2, were eliminated. Without resistors R731 and R732, the gain of amplifier 600a of FIGURE 3 is, advantageously, higher than unity. In FIGURE 3, a diode D cma and a temperature compensated current source I 1a cause the collector currents in transistors Q737a and Q736a to be, for example, equal. Similarly, transistor Q734a causes the collector current in transistor Q733a to be, for example, equal to that in each of transistors Q737a and Q736a when voltage V NINI is zero. In FIGURE 4, voltage V NINIb is applied differentially between the emitters of transistors Q733b and Q734b. Such arrangement provides, advantageously, an improved common mode rejection. In FIGURE 5 the type, N-P-N or P-N-P, of the corresponding transistors is opposite than that in FIGURE 2 so that voltage V NINIc may, if desired, be referenced to voltage V cc , instead of to ground. In FIGURE 6, the input impedance to voltage V NINId is, advantageously, higher than to voltage V NINI of FIGURE 2 because of the usage of a transistor Q750 that is coupled as an emitter follower.

Claims: A power supply that includes a level shifter (600) that level shifts an input voltage (V NINI ) that varies an output voltage (B+) of said power supply in response to variation of the input voltage, comprising : a source (V UR ) of supply voltage; a controllable conductive element (617b) coupled to said supply voltage for generating therefrom said output voltage (B+); means, including a comparator (610) having one input (-) coupled to receive a signal (V IN ) indicative of said output voltage (B+) and having an output signal (V₀) coupled to a control terminal (617a) of said conductive element (617b) for varying, in accordance with the output signal (V₀) of said comparator, the conduction of said conductive element (617b) to control said output voltage; characterized in that : said level shifter has the form of a current mirror arrangement (Q733,734,735) including a first transistor (Q733) responsive to a current (i Q737 ) in a first circuit branch for generating a current (i Q736 ) in a second circuit branch which includes a first main current conducting electrode (at 733a) of said first transistor (Q733), that is the current mirror of said current (i Q737 ) in said first circuit branch, said first main current conducting electrode (collector in Fig.2) being coupled at a junction terminal (733A) to a source (Q736) of predetermined current (100µA in Figure 2); a source (611) of said input voltage (V NINI ) coupled to an input terminal (emitter in Fig.2) of said first transistor (Q733) for varying said current in said first main current conducting electrode (collector in Fig.2) to produce a current (i R729 ) that varies in accordance with said input voltage (V NINI ) and with the difference between said current (i Q733 ) in said first main current conducting electrode (collector) and said current (i Q736 ) in said second circuit branch; a first resistance (R729) coupled to said junction terminal (733A) for conducting said differences related current (i R729 ) to develop a voltage thereacross that varies in accordance with said input voltage (V NINI ); and a source (699) of temperature compensated first voltage (V BG ) coupled via said first resistance (R729) to said junction terminal (733A) such that said voltage across said resistance (R729) is combined with said first voltage (V BG ) for developing a temperature compensated second voltage (V NIN ) at said junction terminal (733A) that varies in accordance with said input voltage (V NINI ) and that is level shifted in accordance with said temperature compensated first voltage (V BG ), said temperature compensated, level shifted, second voltage (V NIN ) being coupled to another input (+) of said comparator (610) for varying said output signal (V₀) of said comparator (610) in accordance therewith to control said output voltage (B+). A power supply according to Claim 1 characterized in that said second circuit branch comprises a second transistor (Q736) having a main current electrode that is coupled to said junction terminal (733A) and wherein said first circuit branch comprises a third transistor (Q737) that generates, in a main current conducting electrode thereof, said current (i Q737 ) that is coupled to said current mirror arrangement (Q733,734,735) such that said current mirror arrangement generates said current in said first main current conducting electrode of said first transistor (Q733) at a level that is in accordance with said current in said third transistor (Q737). A power supply according to Claim 2 characterized by a temperature compensated means (650) coupled to corresponding control electrodes of each of said second (Q736) and third (Q737) transistors for generating said corresponding currents in said first and second circuit branches that remain substantially unchanged over said wide range of temperatures. A power supply according to Claim 2 or 3 characterized in that said current mirror arrangement further includes a fourth transistor (Q735) that is coupled to a control electrode of said first transistor (Q733) and that is coupled in a path of said current in said third transistor (Q737) to develop a voltage at said control electrode of said first transistor (Q733) that causes said current in said first transistor (Q733) to be the current mirror of said current in said fourth transistor (Q735). A power supply according to Claim 4 characterized in that said input voltage (V NINI ) is coupled between corresponding main current conducting electrodes of each of said first (Q733) and fourth (Q735) transistors for varying, in accordance with said input voltage, said currents in each of said first (Q733) and fourth (Q735) transistors. A power supply according to any of Claim 2 - 5 characterized by a second resistance (R731), via which said input voltage (V NINI ) is coupled to an emitter electrode of said first transistor (Q733). A power supply according to Claim 6 characterized in that said first transistor (Q733) forms an amplifier having a gain that is determined by a ratio between said second (R731) and first (R729) resistances. A power supply according to Claim 6 characterized in that said second resistance (R731) has a value substantially less than said first resistance (R729) such that said gain is higher than unity. A power supply according to Claim 6 characterized by a fourth transistor (Q750) responsive to said input voltage (V NINId ) and coupled as an emitter follower such that an emitter electrode of said fourth transistor (Q750) is coupled to said emitter electrode of said first transistor (W 733d ). A power supply according to any of claims 2 - 9 characterized in that an impedance that is formed at said junction terminal (733A) by said first (Q733) and second (Q736) transistors is substantially higher than said first resistance (R729). A power supply according to any preceding Claim characterized in that said source of said first voltage (V BG ) comprises a bandgap type voltage source (699). A power supply according to any preceding Claim characterized in that said current mirror arrangement (Q733,734,735) maintains a ratio between said current in said first main current conducting electrode and said current in said second circuit branch unchanged over a wide range of temperatures. A power supply according to any preceding Claim characterized in that said first voltage (V BG ) is developed at a first terminal of said first resistance (R729) and said second voltage (V NIN ) is developed at a second terminal of said first resistance (R729) remote from said first terminal and wherein said second voltage (V NIN ) comprises an algebraic sum of said first voltage (V BG ) and said voltage across said first resistance (R729).

Inventors: CRAFT, JACK, LOW, MICHAEL LOUIE, YORKANIS, BERNARD JOSEPH
Applicants: RCA THOMSON LICENSING CORPORATION
Application year: 1987
Main classification: H04N3
IPC codes: H04N3, H04N5, G05F3