Current source

Abstract: A current source using a bandgap voltage circuit includes a current gain circuit between the output of the bandgap circuit and the current output transistor. On-off control is provided by a switchable bias circuit providing an ON potential to start the bandgap and a clamping circuit opening the feedback loop.

Description: The present invention relates to a current source circuit using a bandgap voltage circuit. Current sources using bandgap voltage circuit are known in the art for example from US Patent 5581174. The present invention provides embodiments which have improved power supply rejection and which allow for turning on and off of the current generator in a simple manner. Further embodiments allow for multiple current outputs. US-A-5,703,937 relates to a reference voltage generator which provides for an optimum control of the output voltage as a function of temperature. US-A-6,087,820 describes a method and circuit for producing output current wherein two currents are added, each having opposing temperature coefficients to produce the output current. The first of these currents is a scale copy of the current produced in a temperature compensated bandgap reference circuit. The second current is derived from a temperature stable voltage produced by the bandgap circuit provided by a positive temperature coefficient resistance. According to the present invention there is provided a current source having a sensing transistor and a bandgap circuit having first and second control transistors and a current mirror, the sensing transistor having a control electrode and a main current path, the main current path being connected to a feedback resistance at a first node, the other end of the feedback resistance being at a reference potential, each of the first and second control transistors having respective control electrodes, respective emitters and respective collectors, the first node being connected to the control electrodes of the first and second control transistors, the emitter of the first control transistor coupled to the reference potential via a first resistance and the emitter of the second control transistor coupled to the emitter of the first control transistor via a second resistance, the current mirror having a diode-connected transistor and a controlled transistor, the diode connected transistor connecting the collector of the first control transistor to a power rail and the controlled transistor connecting the collector of the second control to the power rail, the bandgap circuit being dimensioned to provide a first potential across said feedback resistance, characterised by a current amplifier having an input and an output, the input being connected to the collector of the second control transistor and the output being connected to the control electrode of the sensing transistor. Preferably the first and second control transistors are of a first conductivity and the current mirror transistors are of a second opposite conductivity and wherein the current amplifier has a first amplifying transistor of said second conductivity having a control electrode connected to the collector of the second control transistor and a collector connected to the input of a second current mirror, said second current mirror comprising transistors having said first conductivity coupled to said reference potential. Advantageously said second current mirror has an output connected to a diode-connected transistor of said second conductivity type, said output being further connected to the control electrode of said sensing transistor. Conveniently the controlled transistor of the said current mirror has a first width and the amplifying transistor has a greater width. Preferably the current source further comprises a start up circuit for the bandgap, the start up circuit having a pull-up transistor for pulling said first node up to a second potential having a lesser magnitude than the first potential. Advantageously said pull-up transistor is an emitter follower of said first conductivity, and has a base connected to a voltage source comprising plural series diodes. Conveniently said base is further connected to a switch for selectively shorting said diodes in response to a control signal. Conveniently again said switch is an n FET. Preferably said start up circuit further comprises a clamping transistor connected to the collector of the second transistor for selectively turning off said first amplifying transistor in response to said control signal. Advantageously said clamping transistor is a p FET. Preferably said switch is an n FET, and said start up circuit further comprises a p FET connected to the collector of the second transistor for selectively turning off said first amplifying transistor in response to said control signal, the current source having a control terminal for receiving a first voltage level operable to turn off said current source and a second voltage level operable to start said current source, said control terminal being connected to a control electrode of the p FET and to the gate of the n FET via an inverter. Preferably again the current source has a plurality of second conductivity type output transistors, each having an emitter connected to said power supply rail, a base connected to the control electrode of the sensing transistor, wherein each of said output transistors has a collector providing a respective current output. Advantageously at least one of said output transistors has greater width than another of said output transistors whereby said at least one output transistor provides a higher output current. A preferred but exemplary embodiment of the invention will now be described with reference to the accompanying figure which shows a schematic diagram of a current source in accordance with the invention. The current source of the embodiment consists of a bandgap circuit 1 which has a first NPN bipolar transistor 2 having a base connected in common to that of a second NPN bipolar transistor 7. The first bipolar transistor 2 has a greater effective width that the second transistor 7, for example five times greater. The effect is that for a similar base-emitter potential the first transistor 2 will conduct more current than the second transistor 7. The emitter of the first transistor 2 is connected to the emitter of the second transistor 7 via a resistance 19 and the emitter of the second transistor 7 is connected to a reference potential VEE via a resistance 15. The collectors of the first 2 and second 7 NPN transistors are connected to a positive supply rail Vcc via a current mirror 6 composed of PNP transistors 9,28. The second NPN transistor 7 has its collector connected to the positive supply rail Vcc via a diode-connected PNP transistor 9 which has its base connection in common with a controlled PNP transistor 28 serving to connect the collector of the first NPN transistor 2 to the positive supply rail Vcc. The collector of the first NPN transistor 2 is further connected to the base of a first amplifying PNP transistor 44 which has an emitter connected to the positive supply rail and a collector connected to a second current mirror 3. The second current mirror 3 has a first NPN transistor 46 which is diode-connected, and which has an emitter connected to the reference rail VEE. The base of transistor 46 is connected in common to the base of a controlled NPN transistor 45, with emitter connected to the reference rail VEE and with a collector connected to a diode-connected PNP transistor 47 and the emitter of transistor 47 connected to the positive supply rail. Together the transistors 44-47 form a current amplification circuit. To provide current gain the first amplifying transistor 44 is wider than the controlled transistor 28 of the first current mirror 6, for example twice as wide. In the preferred embodiment transistors 45, 46 and 47 are of the same size as transistor 28. The collector of transistor 45 is also connected to the base of a sensing transistor 10, being a PNP transistor having its emitter connected to the positive supply rail Vcc. The collector of transistor 10 is connected to the reference rail VEE via a feedback resistor 60, the node 50 between the transistor 10 and the resistor 60 being connected to the commoned bases of the first and second NPN transistors 2, 7. In operation the bandgap circuit being connected in a loop including the current amplifier and the feedback resistor provides a constant potential at the node 50. The constant potential at node 50 is produced by virtue of a constant current through the sensing transistor 10 and the base potential of the sensing transistor 10 is thus such as to give rise to this constant current. The base potential is fed to three output PNP transistors 11, 12, 13, each of which has a respective emitter connected to the positive supply rail and a respective collector forming an output node 101, 102, 103. In the embodiments shown transistor 11 and 12 are each twice the width of transistor 10 and transistor 13 is four times the width of transistor 10. As a result output terminals 101 and 102 each produce a magnitude of current double that of the current through transistor 10 whereas the node 103 produces a current four times the magnitude of the current through transistor 10. The current source circuit has a high power supply rejection, defined as the amount of variation of power supply voltage which appears in the output current. The power supply rejection at the output, which depends upon the power supply rejection at node 50 is the ratio of the output resistance of the sensing transistor 10 to the feedback resistance 60 divided by the loop gain of the circuit. Given that the node 50 is in the feedback loop and given the gain of the loop including the current amplifying circuit a theoretical value of power supply rejection of minus 78dB may be achieved in embodiments of the invention. The circuit so far defined is unlikely to be self-starting. To achieve self-starting it is necessary to cause the bandgap circuit 1 to start to conduct. To achieve this an NPN emitter follower transistor 26 has its emitter connected to the commoned bases of the first and second NPN transistors 2 and 7. The collector of the emitter follower 26 is connected to the positive supply rail Vcc and the base is connected to the positive supply rail Vcc via a resistor 61. The base is further connected to the reference rail VEE via the series connection of two diode-connected NPN transistors 4A and 4B. A switch in the form of an N-FET 35 has its main current path connected between the base of emitter follower transistor 26 and the reference supply rail VEE and the FET has a gate connection to the output of a CMOS inverter having a P-type pull-up transistor 36 and an N-type pull-down transistor 37. The gates of the transistors 36 and 27 are connected in common to a control terminal 40 which is also connected to a P-type transistor 41 having its main current path between the positive supply rail Vcc and the collector of the second NPN transistor 2. The operation of the start-up circuitry will now be described:- When the control terminal 40 is at a high potential the gate of the switch 35 is at a low potential and therefore the switch 35 remains non-conducting. In this situation current flows through the resistor 61 to the series connection of the diodes 4A and 4B causing a base potential on the emitter follower 26 of two diode voltages above the reference potential. Hence the emitter of the emitter follower 26 will have a potential of one diode voltage above the reference potential and this value is fed to the commoned gates of the first and second NPN transistors of the bandgap circuit 1, this potential being sufficient to start the bandgap. Once the bandgap loop is operational the potential at the node 50 is higher than one diode potential above the reference rail and as a result the emitter follower 26 plays no part in the normal operating mode. The P-type transistor 41 constitutes a control for turning off the current source. During the start-up condition the high potential at the control terminal 40 maintains the P-transistor 41 off, therefore not affecting operation of the bandgap. When however the potential at the control terminal 40 falls towards the reference level, the P-type transistor 41 turns on and pulls the collector of the second NPN transistor 2 of the bandgap towards the positive supply potential. This in turn causes the current amplifying transistor 44 to turn off and turns off the bandgap loop. At the same time the low potential at control terminal 40 is supplied to the inverter 36, 37 and the N-type switch 35 turns on shorting out the diodes 4A and 4B and reducing the base voltage of the emitter follower 26 to substantially zero. The constant current circuit described produces a constant current output over temperature and supply voltage. It is turned on and off easily and the control circuitry for starting and stopping operation has no substantial effect on operation.

Claims: A current source having a sensing transistor (10) and a bandgap circuit (1) having first (7) and second (2) control transistors and a current mirror (6), the sensing transistor having a control electrode and a main current path, the main current path being connected to a feedback resistance (60) at a first node (50), the other end of the feedback resistance being at a reference potential (V EE ), each of the first and second control transistors having respective control electrodes, respective emitters and respective collectors, the first node (50) being connected to the control electrodes of the first and second control transistors(2,7), the emitter of the first control transistor (7) being coupled to the reference potential via a first resistance (15) and the emitter of the second control transistor coupled to the emitter of the first control transistor via a second resistance (19), the current mirror (6) having a diode-connected transistor (9) and a controlled transistor (28), the diode-connected transistor connecting the collector of the first control transistor (7) to a power rail (V CC ) and the controlled transistor connecting the collector of the second control transistor (2) to the power rail, the bandgap circuit being dimensioned to provide a first potential across said feedback resistance, characterised by a current amplifier (44,46,45,47) having an input and an output, the input being connected to the collector of the second control transistor and the output being connected to the control electrode of the sensing transistor. The current source of claim 1 wherein the first and second control transistors (2,7) are of a first conductivity and the current mirror transistors (9,28) are of a second opposite conductivity and wherein the current amplifier (44-47) has a first amplifying transistor (44) of said second conductivity having a control electrode connected to the collector of the second control transistor (2) and a collector connected to the input of a second current mirror (45,46), said second current mirror comprising transistors having said first conductivity coupled to said reference potential. The current source of claim 2, wherein said second current mirror has an output connected to a diode-connected transistor (47) of said second conductivity, said output being further connected to the control electrode of said sensing transistor. The current source of claim 2 or claim 3 wherein the controlled transistor (28) of said current mirror has a first width and the amplifying transistor (44) has a greater width. The current source of claim 2, 3 or 4 and further comprising a start up circuit for the bandgap circuit, the start up circuit having a pull-up transistor (26) for pulling said first node (50) up to a second potential having a lesser magnitude than the first potential. The current source of claim 5 wherein said pull-up transistor is an emitter follower of said first conductivity, and has a base connected to the power rail through a resistor (61) and to the reference potential through plural series diodes (4A,4B). The current source of claim 6 wherein said base is further connected to a switch for selectively shorting said diodes in response to a control signal. The current source of claim 7 wherein said switch is an n FET. The current source of claim 6 or 7 wherein said start up circuit further comprises a clamping transistor (41) connected to the collector of the second transistor (2) for selectively turning off said first amplifying transistor (44) in response to said control signal. The current source of claim 7, 8 or 9 wherein said clamping transistor (41) is a p FET. The current source of claim 7, wherein said switch is an n FET, wherein said start up circuit further comprises a p FET connected to the collector of the second transistor (2) for selectively turning off said first amplifying transistor (44) in response to said control signal, the current source having a control terminal (40) for receiving a first voltage level operable to turn off said current source and a second voltage level operable to start said current source, said control terminal being connected to a control electrode of the p FET (41) and to the gate of the n FET (35) via an inverter (36, 37). The current source of claim 11 having a plurality of second conductivity output transistors (11, 12, 13), each having an emitter connected to said power supply rail, a base connected to the control electrode of the sensing transistor (10), wherein each of said output transistors has a collector providing a respective current output (101, 102, 103). The current source of claim 12 wherein at least one of said output transistors has greater width than another of said output transistors whereby said at least one output transistor provides a higher output current. The current source of claim 13 as dependant upon claim 11, wherein said first voltage level received by said control terminal (40) is low such that said first amplifying transistor (44) causes said output transistors (11, 12, 13) and the bandgap circuit (1) to be turned off while at the same time said low voltage causes said n Fet (35) and said inverter to turn off said pull-up transistor (26). The current source of claim 14, wherein said first voltage level is such that the clamping transistor (41) is turned on pulling the base of said amplifying transistor (44) to said power rail and thereby turning said amplifying transistor off, wherein since no current can conduct the output transistors (11, 12, 13) are turned off. The current source of claim 14, wherein at said first voltage level said inverter (36, 37) causes the n Fet (35) to turn on thereby shorting out said plural series diodes and pulling the base of said pull-up transistor (26) low to thereby turn off said pull-up transistor.

Inventors: JOHNSON PETER, JOHNSON, PETER
Applicants: SGS THOMSON MICROELECTRONICS, STMICROELECTRONICS LIMITED
Application year: 2001
Main classification: G05F3
IPC codes: G05F3