Programmable-Output Voltage Regulator

The present invention relates to a programmable-output voltage regulator.

Voltage regulators are devices for automatically regulating and maintaining output voltage within predetermined limits in spite of variations in quantities such as supply voltage and load current. For this purpose, known regulators roughly comprise a power element between the supply and load, and a regulating loop consisting of a differential amplifier input-connected to a stable reference voltage and to the regulator output, and output-connected to a gain stage for so driving the power element as to withstand any variation in output voltage. Known regulators frequently also present a shorting protection circuit for detecting the load current and intervening in the event this exceeds a predetermined value. The output voltage is normally picked up by the differential amplifier via a resistive divider.

Recently, a demand has arisen for what are known as "programmable-output" voltage regulators, that is, capable of providing for at least two different output values. These are particularly requested for so-called "smart cards," the standards governing which demand that the rate at which the output voltage varies during transient states falls within a predetermined range, regardless of the type of load or output voltage.

Such a condition is particularly hard to meet when the load is of a capacitive type, in which case, the regulator is called upon to supply a high current during transient states in which the output voltage increases from the steady-state to a higher value, and to absorb the load current during transient states in which the output voltage switches from the higher to a lower value, while at the same time ensuring the above condition is met.

Known voltage regulators fail to cater for the above requirements, owing to the fact that, at the beginning of the output voltage rising transient state, a short-circuit condition is produced by the output capacitor, thus resulting in undesired operation of the shorting protection circuit; whereas, during the output voltage decreasing transient state, voltage decreasing depends on the condition of the load, by virtue of the discharge current of the capacitor being absorbed by the resistive part of the load. In either case, therefore, the voltage variation caused by switching between two stable steady-state values is uncontrolled, and may assume levels that are unacceptable by present standards.

It is an object of the present invention to provide an externally-programmable-output voltage regulator capable of maintaining within predetermined limits the rate at which the output voltage varies during transient states between two steady-state values.

In short, according to the present invention, the rate at which the output voltage varies is controlled precisely, so as to assume a predetermined design value. Said rate preferably equals the slew rate of the regulating loop, which is achieved by unbalancing the differential stage. For preventing intervention of the shorting protection circuit, this is turned off for a predetermined length of time during the rising transient state, whereas, during the falling transient state, a PNP transistor stage is turned on for absorbing the discharge current of the capacitive load.

A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawing, which shows a simplified circuit diagram of the voltage regulator according to the present invention.

FIG. 1 is a circuit diagram of a programmable-output voltage regulator according to the present invention.

Number 1 in the accompanying drawing indicates a voltage regulator comprising, in known manner, an NPN-type transistor 2 defining a power element between input terminal 3 and output terminal 4 of the regulator; a resistive divider 5 between output terminal 4 and a ground (reference potential) line 6; and a regulating loop 7 including a differential amplifier 8 and an NPN type gain transistor 9.

More specifically, transistor 2 presents the collector terminal connected to input 3, the emitter terminal connected to output 4, and the base terminal connected to the collector of transistor 9, which in turn presents the emitter terminal connected to ground line 6, and the base terminal connected to the output of amplifier 8. Output terminal 4, at which an output voltage Vo is present, is connected to a load 35 here including a resistor 36 and a capacitor 37. Amplifier 8 presents an inverting input connected to a reference voltage source (not shown) supplying reference potential V.sub.R and a non-inverting input connected to node 10 defined by two resistors 11, 12 forming divider 5. A frequency compensating capacitor 13 is provided between the collector and base of transistor 9, and a current source 15 (generating current I.sub.p) between the collector and base of transistor 2.

Regulator 1 also comprises, in known manner, a shorting protection circuit 17 in turn comprising a current sensor formed by a resistor 18 between input terminal 3 and the collector of transistor 2; a detecting stage 19 for detecting short circuiting on the basis of the voltage across resistor 18 (and therefore proportional to the current through resistor 18); and an actuating element formed by an NPN type transistor 20 with the base terminal connected to the output of detecting stage 19, the collector terminal connected to the output of amplifier 8, and the emitter terminal connected to ground line 6. Detecting stage 19 may operate, for example, as a straightforward comparator, for supplying an output control signal to transistor 20 when the voltage drop across the resistor, as determined by measuring the voltage at the two terminals of resistor 18 to determine if it exceeds a predetermined threshold.

Regulator 1 also comprises a further input terminal 22 supplied with a logic control signal V.sub.C the value of which depends on the (higher or lower) steady-state value of output voltage V.sub.o. Terminal 22 is connected to the input of a delay circuit 23, which is activated in response to each leading edge of signal V.sub.C, and supplies an output control pulse of duration .tau. to the gate terminal of a MOS transistor 24 defining a controlled switch and formed, for example, by a VDMOS (Vertical Diffused Metal Oxide Semiconductor). Duration .tau. is appropriately selected at the design stage, and substantially corresponds to the rise time (up ramp) of the transient output voltage. Transistor 24 is also connected by the drain terminal to the base of transistor 20, and by the source terminal to ground line 6.

Input terminal 22 is also connected to the gate terminal of a further MOS type transistor 25 defining a further controlled switch and having the source terminal connected to ground line 6, and the drain terminal connected to node 10 via resistor 26.

Regulator 1 also comprises an equivalent PNP transistor element 28 formed by a PNP transistor 29 and an NPN transistor 30. More specifically, transistor 29 presents the emitter terminal connected to output terminal 4, the base terminal connected to that of transistor 2, and the collector terminal connected to the base terminal of transistor 30, which in turn presents the collector terminal connected to output terminal 4, and the emitter terminal connected to ground line 6.

Regulator 1 of FIG. 1 operates as follows. When control signal V.sub.C presents a low logic level, transistor 25 remains off to give a regulated output voltage V.sub.o1 of:

where R.sub.1, R.sub.2 are the respective resistances of resistors 11 and 12.

In the steady-state condition, equivalent transistor 28 is off; capacitor 37 is charged to voltage V.sub.o1 ; differential amplifier 8 is balanced; regulator 1 supplies output current I.sub.o =V.sub.o1 /R.sub.o where R.sub.o is the resistance of resistor 36; the output of delay circuit 23 is low; transistor 24 is off; and protection circuit 17 is on. Regulator 1 thus provides normally for maintaining a stable output voltage, by virtue of any change in V.sub.o resulting in a similar change in the voltage at node 10 and, consequently, in operation of regulating loop 7 (which increases or decreases the base current of transistor 2) so as to restore the output voltage to the programmed V.sub.o1 value.

When signal V.sub.C switches from LOW to HIGH, this results in a transient output voltage to a higher value, and in temporary turn-off of protection circuit 17. That is, when V.sub.C switches to high, transistor 25 is turned on; resistor 26 is connected parallel to resistor 12 of divider 5; the resulting negative voltage peak at node 10 totally unbalances amplifier 8, thus reducing the output current of amplifier 8 and the current through transistor 9, and producing a corresponding increase in the drive current from source 15 to the base of transistor 2 and, consequently, in output current I.sub.o ; and output voltage V.sub.o increases until, by virtue of regulating loop 7, the voltage at node 10 is restored to V.sub.R, amplifier 8 is again balanced, and the output voltage reaches the new steady-state value V.sub.02 of:

where R.sub.p is the resistance of resistor 26, and the saturation resistance of MOS transistor 25 is considered negligible in relation to R.sub.p. Naturally, different R.sub.p values result in different output voltages.

At the same time, the rising edge of signal V.sub.C turns on delay circuit 23, the output of which switches to turn on MOS transistor 24; and transistor 24 short circuits the base-emitter junction of transistor 20, so that transistor 20 remains off regardless of the output of detecting circuit 19, and protection stage 17 is turned off.

In the above transient phase, the peak current supplied by the regulator shown may be even much higher than the steady-state value. In fact, in the case of a voltage regulator of the general type described, if SR is the slew rate obtainable; .delta.V.sub.o /.delta.t is the change in output voltage per unit of time; I.sub.op is the bias current of the differential stage; and C is the capacitance of capacitor 13:

(see, for example, "Analysis and Design of Analog Integrated Circuits" by P. R. Gray and R. G. Meyer; John Wiley & Sons, 1977; page 541-544, Par. 9.6.1, where I.sub.x =l.sub.op.

Disregarding for the time being resistance R.sub.o of load resistor 36, if C.sub.o is the capacitance of load capacitor 37, the mean current I.sub.o supplied by regulator 1 to capacitor 36, for producing a voltage excursion .delta.V.sub.o in relation to the steady-state value within a time interval .delta.t, equals:

and the maximum current I.sub.omax equals:

Substituting (3) in (5), this gives:

and, bearing in mind also load resistor 36:

For ensuring a predetermined voltage ramp, therefore, supply current I.sub.o may be even much higher than steady-state load current V.sub.o /R.sub.o. For example, assuming C.sub.o =1 .mu.F; C=20pF; V.sub.o =25 V; I.sub.op =20 .mu.A; and R.sub.o =250 .OMEGA.; the transient state current requirement (C.sub.o I.sub.op /C=1 A) is greater than the steady-state current (in this case, 0.1 A).

In the case of known regulators of the type described, output current is normally limited by the current gain of the output stage of the regulator (including transistor 2 and source 15) and by operation of protection circuit 17. More specifically, current supply depends substantially on the area of power transistor 2 and on the current I.sub.p supplied by source 15 for driving transistor 2, and as such may be modified at the design stage so that it is sufficiently high or at least greater than the maximum value achievable according to equation (7). The second limitation (operation of circuit 17) is normally dictated at the design stage by thermal and/or reliability factors, and as such can only be modified at the expense of endangering the reliability of the regulator. As already stated, therefore, according to the present invention, circuit 17 is turned off over the up ramp of the output voltage, thus preventing current supply from being limited by the protection circuit, and so enabling it to reach the maximum value achievable on the basis of current I.sub.p.

Over the up ramp, therefore, the load is supplied with current the value of which is so controlled as to produce an increase in voltage regardless of the load and equal to the slew rate of the regulator, i.e., I.sub.op /C.

When signal V.sub.C switches from HIGH to LOW, this again unbalances amplifier 8, and produces a transient state wherein output voltage V.sub.o switches from V.sub.o2 to V.sub.o1. That is, when signal V.sub.C switches to low, MOS transistor 25 is turned off so that resistor 26 is no longer connected in parallel to resistor 12; the resistance between node 10 and ground increases to produce a positive voltage peak at node 10; the non-inverting input of amplifier 8 presents a more positive potential as compared with the inverting input; the drive current of transistor 9 increases, so that transistor 9 absorbs all of source current I and turns off power transistor 2; and capacitor 37 begins discharging into the load (resistor 36). At this phase, the collector voltage of transistor 9 is reduced, and, by virtue of this occurring faster than the change in output voltage (and consequently in the emitter voltage of transistor 2), equivalent PNP transistor 28 is eventually turned on, so as to absorb the load current and enable discharge at a rate independent of resistor 36. More specifically, the down ramp of the output voltage is limited solely by the bias current of amplifier 8, and equals the slew rate, again equal to I.sub.op /C. For preventing speed from being limited over the down ramp, the equivalent PNP transistor must obviously also be so designed as to guarantee a maximum current as per equation (7).

In this phase, the delay circuit remains inoperative, by virtue of the output current being inverted and so failing anyway to turn on protection circuit 17.

The advantages of the regulator according to the present invention will be clear from the foregoing description. Firstly, both the up and down ramps are accurately controllable regardless of the load connected to the output, by virtue of the quantities by which the ramp is defined (i.e., the bias current of the differential stage and the capacitance of the compensating capacitor) being appropriately selectable at the design stage and producible to a high degree of accuracy. Secondly, such sizing provides for obtaining symmetrical up and down ramps using components of limited area, even in the case of equivalent transistor 28, by virtue of the configuration shown enabling the same maximum current to be obtained with a smaller area as compared with the area required by a single PNP type transistor.

Finally, the solution described is straightforward in design, and provides for trouble-free implementation, in both discrete and integrated manner, using proved manufacturing techniques.

To those skilled in the art it will be clear that changes may be made to the regulator as described and illustrated herein without, however, departing from the scope of the present invention. For example, the change in output voltage may be achieved differently; equivalent transistor 28 may consist of a single device; and, if symmetrical up and down ramps are not required, current supply over the down ramp may be limited by appropriately sizing equivalent transistor 28.