Patentamt JEuropaischesEuropean Patent Office @ Publication number: 0 357 366 Office europeen des brevets A1

© EUROPEAN PATENT APPLICATION

® Application number: 89308704.9 (§) Int. CI.5: H 03 K 17/06 G 05 F 3/26 @ Date of filing: 29.08.89

© Priority: 30.08.88 US 238090 ® Applicant: International Business Machines Corporation Old Orchard Road @ Date of publication of application: Armonk, N.Y. 10504 (US) 07.03.90 Bulletin 90/10 @ Inventor: Larson, Tony Ray 6961 Vereda Sombria @ Designated Contracting States: DE FR GB Tucson Arizona 85746 (US) Tretter, Larry Leeroy 9258 E. 42nd Street Tucson Arizona 85730 (US)

@ Representative: Atchley, Martin John Waldegrave IBM United Kingdom Limited Intellectual Property Department Hursley Park Winchester Hampshire S021 2JN (GB)

(§3) Driver circuit for electronic circuitry. (57) The present invention relates to a switched current mode drive circuit comprising an MOS reference (22; 32) and control element (24; 34) connected between a reference voltage source and a current source; an MOS mirroring transistor (26; 36) connected between the reference voltage source and load means in mirrored configura- tion to the reference transistor; and signal means connected between the control element and the mirroring transistor for controlling drain potential of the reference transistor whereby it follows the drain potential of the mirroring transistor.

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Bundesdruckerei Berlin I EP 0 357 366 A1 2

Description DRIVER CIRCUIT FOR ELECTRONIC CIRCUITRY

The present invention relates to an improved mirroring transistor connected between the ref- driver circuit for electronic circuitry. More particu- erence voltage source and load means in mirrored larly the invention concerns a switched current 5 configuration to the reference transistor; and signal mode driver designed for use in input-output means connected between the control element and channel selection functions associated with an IBM the mirroring transistor for controlling drain potential (Registered Trade Mark) 360/370 channel, capable of the reference transistor whereby it follows the of interconnection with a plurality of peripheral drain potential of the mirroring transistor. devices for receiving selection signals from the 10 In order that the invention may be more readily channel and redriving the signals to the next control understood, an embodiment will now be described unit. with reference to the accompanying drawings in In the use of IBM 360/370 channel systems where which: a plurality of peripheral devices must be connected Figure 1 represents a typical known type of to one or more central processing units (CPU's) it is 15 non-current mode line driver designed in bipo- common to use a control unit associated with each lar technology, peripheral device being attached to the 360/370 Figure 2a represents a switched current channel. Such control units use a module that mode driver in accordance with the invention, performs power supply, logic and switching func- Figure 2b represents an MOS technology tions with respect to the connection lines that are 20 realisation of a switched current mode driver required to initiate and terminate operations bet- such as illustrated in Figure 2a, ween the 360/370 channel interface and the respec- Figure 3a represents an improved output tive control units. In such systems it is usual to resistance current mirror driver circuit in ac- provide an integrated select-out bypass chip that cordance with the invention, performs select-out and select-in control for a single 25 Figure 3b is a graph of the electrical output IBM system 360 and system 370 1/0 interface. The characteristics of the circuit of Figure 3a, control unit attached to the 360/370 channel inter- Figure 4a is a schematic of another improved face uses a select-out signal to detect the start of a current mirror driver circuit, and selection sequence from the channel. Unlike the Figure 4b is yet another improved driver other interface lines, the select-out signal not only is 30 circuit similar to that illustrated in Figure 4a. received by each control unit, but also passes Figure 1 shows a typical known type of line driver through logic and is redriven to the next control unit implementation. This basic circuit has been used attached to the channel. The integrated select-out extensively in bipolar versions of I/O drivers of the bypass function provides an electrical bypass for the type generally described herein. select-out/select-in signal when the control unit is 35 In the driver circuit illustrated in Figure 1, a +6 volt powered off. Besides the select out bypass function, supply voltage is provided through a current limiting the internal select-out bypass design contains the resistor 11 to the collector of an NPN transistor 12 driver and receiver used when the control unit is having its emitter connected to a 95 ohm load powered up to receive and redrive the select-out resistor at the output. The driver input is connected signal to the next control unit. 40 to the base of the transistor and a 750 ohm resistor Driver requirements for use in IBM 360/370 14 is provided between the input and supply voltage channels to drive the select-in and select-out lines terminals. require capability to drive 3.9 volts into a 95-ohm The configuration may be implemented in MOS cable and be short circuit protected. Modern technology simply by replacing the NPN device with systems also require short circuit detection (RAS) 45 a MOSFET (MOS Field Effect Transistor). However and a high impedance mode for testing purposes. the large current limiting resistor and the 6 volt Bipolar driver circuits have been used in the past in power supply requirements are very undesirable. typical line driver implementations. However the Many newer technologies do not allow operation of usual 6-volt power supply requirement for bipolar drivers at 6 volts, and systems are less expensive if circuits is undesirable. Moreover, the large current 50 all circuits use a single power supply, typically 5 volts limiting resistors associated with such circuits result with a 10% tolerance. The 35 ohm current limiting in excessive power dissipation. Consequently, a resistor dissipates a significant amount of power, need exists to utilise MOS (metal oxide silicon) especially when the driver is short circuited. It is circuitry for such line driver implementations in order often added as an external resistor, increasing both to operate at the lower power supply voltages, 55 the module pin count and the production costs. typically 5 volts, available with more recently de- A basic Switched Current Mode Driver (SCMD) signed systems, and to reduce power dissipation. configuration, shown in Figure 2a, would partially The object of the present invention is to provide overcome these problems. When switched ON, a an improved driver circuit for electronic circuitry. current Ion from a current source is forced through The invention relates to a switched current mode 60 the load. The voltage developed at the output is then driver comprising an MOS reference transistor and Vout = Ion X Rload. control element connected between a reference For Ion > 42.2 ma and Rload > 92.6 ohms, the voltage source and a current source; an MOS voltage developed exceeds the required 3.9 volts.

2 3 EP 0 357 366 A1

When the SCMD is switched OFF, the load pulls the forcing all nodes of the mirroring devices to the line to a logic zero level, as does the classic driver of same potential. This is commonly done by "stacking" Figure 1. mirroring devices. Stacked mirroring devices, how- The power dissipated by a SCMD under short ever, require higher power supply voltages to bias circuit conditions is only 5 the additional devices. Thus, prior art stacked Power = Ion X Vdd. mirroring devices only improve the output imped- For Ion = 42 ma and Vdd = 5.0 volts, the power ance when the output potential is significantly lower dissipated is 0.21 watts. Under similar conditions, than Vdd and it has been found that the 3.9 volt the circuit of Figure 1, dissipates requirement of the typical SCMD cannot be met with Power = Vdd2/35 = 1.02 Watts 10 this type of current mirroring device. Thus, the SCMD dissipates about one-fifth the A solution to the problem is. to add a feedback power of a conventional bipolar driver under short circuit that controls the drain of the reference circuit conditions. The SCMD requires no compli- device, as illustrated in Figure 3a. In Figure 3a a cated protection circuit to shut it down during short current mirroring circuit is illustrated wherein device circuits. Unlike many other drivers, the SCMD 15 32 serves as a reference, device 34 is a control returns to normal operation as soon as a short element and device 36 is the mirroring device. As circuit is removed, without requiring a reactivation shown, all devices 32, 34, 36 are P channel sequence such as cycling its input. MOSFETS. Vdd is connected to the source of device A SCMD may be provided by a current mirroring 32, and its drain is connected to the source of device device and a . Figure 2b shows such a driver 20 34. The drain of device 34 is connected to a current configuration implemented in MOS technology. In reference 33, and to the gate of device 32. The gate Figure 2b, the Switched Current Mode Driver utilises of device 32 is also connected to the gate of device a supply voltage Vdd connected to the source of a P 36, with the source of device 36 being connected to channel reference device 22, having its gate and Vdd and its drain being connected to load impedance drain connected to a current reference 23 and a 25 37, as illustrated. The circuit otherwise includes a switch 24 respectively. A current mirroring device, conventional operational 38 having its typically another P channel device 26, is provided, output connected to the gate of device 34. An again having its source connected to the supply inverting input of the OP AMP 38 is connected to the voltage Vdd, its gate connected to the drain of device drain of device 32 and the non-inverting input is 22, and its drain connected to an output or load 30 connected to the drain of mirroring device 36. impedance 27. The gates as well as the sources of the two When the switch 24 is open, the reference current mirroring devices (32 and 36) are tied together, flows through device 22 and is mirrored in device 26 forcing their Vgs potentials to track each other. The forcing the output high. When the switch 24 is OP AMP 38 and the device 34 force the drain of closed, it bypasses device 22 so no current is 35 device 32 to follow the potential at Vout which is also reflected in device 26. With no current in device 26, the drain voltage of device 36. Therefore, the gates, the load resistor 27 pulls the output low. Choice of sources, and drains of the two mirroring devices 32 appropriate sizes for device 22 and device 26 allows and 36 are at the same potential. Vgate must adjust them to remain saturated while the source-to-drain itself so that the reference current flows through voltage of device 26 is made arbitrarily small. 40 device 32. Since device 36 sees the same terminal Theoretically, any supply voltage greater than the voltages as device 32 does, it mirrors the reference required output voltage across resistor 27 is suffi- current "perfectly", whether or not the devices are cient to power the circuit. This allows the switched operating in saturation. As Vout approaches Vdd, the current mode driver to operate from a typical 5 volt, source-to-drain voltage of the mirroring devices 100/o supply and meet the 3.9 volt logic one level. 45 becomes very small, forcing the devices out of The precision of current mirroring devices in any saturation and into the linear region. The mirroring technology can be high, due to devices continue to operate with the devices in the good tracking of parameters across carefully de- linear region by lowering the potential at the gates of signed devices. It depends primarily on parameter devices 32 and 36. This, in turn, increases the gate matching rather than on the absolute values. MOS 50 drive of the mirroring devices, thereby maintaining technology is especially well suited for current . constant current. This range of ideal operation is mirroring device designs because of extremely low shown in Figure 3b as region "B". gate current. The only parameter of concern not If device 33 were an ideal current source, Vgate cancelled by tracking is the output conductance, would continue dropping as Vout were brought Gds, which may, however, be reduced to an 55 arbitrarily close to Vdd. There is, of course, a limit to arbitrarily small value by device design. Therefore, how low Vgate can drop and still sustain current the theoretical accuracy of a SCMD implemented in through the non-ideal device 33 current source. MOS technology is limited only by the accuracy of When this limit, typically 1 volt, is reached, a further the reference current and the degree of matching of increase in Vout will not produce a corresponding the devices. 60 drop in Vgate. Instead, the reference current begins In MOS technology, the output impedance of a to decrease, which in turn decreases the mirrored SCMD is increased as device lengths are increased. output current, Iout. This is shown as region "A" in Unfortunately, area and capacitance considerations Figure 3b and can be made arbitrarily small by limit the length that may be used in practice. Current making device 33 a more perfect current source errors may be eliminated, even in short devices, by 65 (e.g. connect it to a negative power supply instead of

3 5 EP 0 357 366 A1 6 ground). Even without a perfect Iref source, this claimed in claim 1 wherein the signal means improved mirroring device allows operation much comprises another MOS transistor (42) having closer to the Vdd rail than the other circuits. its source connected to the drain of said Device 34 cannot force the drain of device 32 to mirroring transistor, and its gate and drain drop below the potential of Vgate. If Vout falls below 5 connected to the gate of said control transistor VGATEthe feed back circuit effectively short circuits and to the bias means respectively. the gates of the mirroring devices to the drain of device 32 through device 34. This mode of operation is shown as region "C" in Figure 3b. The characteris- tics of the mirroring device in this region are nearly 10 identical to those of a simple mirroring device built with equivalent device sizes. The normal mode of operation for this improved current mirroring device, when implementing a SCMD is in region "B". Because the near ideal 15 characteristics of region "B" extend very close to Vdd and allow devices forming the SCMD to operate in the linear region. Lower power supply voltages and much smaller devices may be used in this configuration than in previous configurations. 20 A "battery" that drops the same voltage as the gate-to-source voltage, Vgs, of device 34 may replace the operational amplifier of Figure 3a as shown in Figure 4a where like elements are represented by like primed numerals. This "battery" 25 is easily implemented as a scaled device biased by a small constant current source as shown in Figure 4b. As long as the Vgs voltage of device 42 is equal to that of device 34", the mirror devices 32" and 36" will see the same potential at all nodes and the circuit 30 will behave just like the one in Figure 3a. The advantages of this circuit are its smaller area, reduced complexity and wider bandwidth due to the removal of the operational amplifier.

Claims

1. A switched current mode driver circuit comprising 40 an MOS reference transistor (22; 32) and control element (24; 34) connected between a reference voltage source and a current source; an MOS mirroring transistor (26; 36) connected between the reference voltage source and load 45 means in mirrored configuration to said ref- erence transistor; and signal means connected between said control element and said mirroring transistor for con- trolling drain potential of said reference transis- 50 tor whereby it follows the drain potential of said mirroring transistor. 2. The switched current mode driver circuit as claimed in claim 1 wherein said signal means comprises operation amplifier means (38) con- 55 nected in feedback configuration from the drain of said mirroring transistor and the drain of said reference transistor to the gate of said control element. 3. The switched current mode driver circuit as 60 claimed in claim 1 wherein the signal means comprises a battery (40; 42) connected bet- ween the drain of said mirroring transistor and the gate of said control transistor. 4. The switched current mode driver circuit as 65

4 EP 0 357 366 A1

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Fug* 4ij European Patent Application Number 3 EUROPEAN SEARCH REPORT Office

EP 89 30 8704

DOCUMENTS CONSIDERED TO BE RELEVANT Citation of with Relevant CLASSIFICATION OF Category document indication, where appropriate, THE of relevant passages to claim APPLICATION ant. CI. 5) EP-A-0 189 894 (OMRONTATEISI 1,2 H 03 K 17/06 CA) G 05 F 3/26 * Page 86, line 3 - page 88, line 1; figure 55 *

US-A-4 532 481 (MEZA et. al . ) 1,2 * Column 2, line 13 - column 3, line 24; figures 2,3 *

PATENT ABSTRACTS OF JAPAN, vol. 3, no. 155 (E-160), 19th December 1979, page 128 E 160; & JP-A-54 136 261 (NIPPON DENKI K.K. ) 23-10-1979 * Abstract *

WO-A-8 907 792 (ANALOG DEVICES INC.) * Page 4, line 5 - page 9, line 6; figure 3 *

TECHNICAL FIELDS SEARCHED (Int. C1.5)

H 03 K G 05 F H 03 F

The present search report has been drawn up for all claims Place of search Date of completion of the search Examiner THE HAGUE 04-12-1989 CANTARELLI R.J.H.

CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document