GB2218544A

GB2218544A - Bandgap startup circuit

Info

Publication number: GB2218544A
Application number: GB8811398A
Authority: GB
Inventors: Paul Anthony Denny
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1988-05-13
Filing date: 1988-05-13
Publication date: 1989-11-15
Anticipated expiration: 2008-05-13
Also published as: GB2218544B; GB8811398D0

Abstract

A bandgap reference circuit has first and second bipolar transistors 2, 4 of similar type but with different current density emitter paths, the first transistor 2 providing a base-emitter voltage to the base of the second transistor 4 to fix its operating point. The collectors of the first and second transistors are coupled to third and fourth transistors 6, 8, respectively, the third and fourth transistors being connected in current mirror configuration to form an active load which forms part of an output load of the circuit across which an output reference voltage is developed. The output reference voltage is sensed by a transistor 24 which controls a current source 20, 26 coupled to the first transistor 2 whereby, should the circuit go towards a collapsed condition, the reduced output reference voltage is sensed, causing increased flow of current from the current source to the first and second transistors, whereby to maintain the circuit in an operative condition. <IMAGE>

Description

BANDGAP STARTUP CIRCUIT This invention relates to a circuit providing a voltage reference known as a bandgap reference circuit.

Such bandgap reference circuits are well known and are normally employed to provide a reference voltage equal to the bandgap voltage of Si, 1.23V, in bipolar integrated circuits on a Silicon substrate. The theory of such circuits is described on pp.

429,431 of "Basic Integrated Circuit Engineering" Hamilton & Howard, McGraw-Hill, 1975, and a typical circuit is shown in Figure 1 as comprising a transistor 2 connected in mirror image configuration to a transistor 4 which has its collector-base path short-circuited to provide an accurate bias for transistor 2. An active load transistor pair 6, 8 is connected as collector loads for transistors 2, 4, and resistors 10, 12 are connected in the collector-emitter path of transistor 4. Normally, a silicon transistor has a base emitter voltage with a negative temperature differential.By arranging for transistors 2, 4 to have different emitter current densities, a differential base-emitter voltage, aVbe is developed which increases positively with temperature, and by arranging the circuit component values suitably, the voltage Ye provided at the collector or transistor 4 is fixed (proportional to Vbe) and temperature invariant.

A problem with such circuits is that they exhibit bistable behaviour, being stable not only at the bandgap reference voltage but also in a collapsed condition in which transistors 2 and 4 are placed in a cut-off condition with no current flowing. This occurs because normally transistor 4 is held in an operating state by reason of negative feedback from the voltage developed across emitter resistor 12. Such negative feedback balances a positive feedback effect in the loop created by transistors 4, 2, 6 and 8 whereby a change say in the base-emitter current in transistor 4 will create changes of current flow in transistors 2, 6 and 8 tending to amplify the change of current in transistor 4.Thus should output transistor 4 the temporarily grounded to a cut-off condition with no current flowing, this condition will be reinforced by the positive feedback effect to hold the circuit permanently in a collapsed condition.

This problem has been sought to be overcome by connecting a capacitor 14 to the collector of transistor 2, which charges up upon start to hold transistor 2 in an operative condition. However, this solution is only partially effective if the output of the circuit is grounded, the capacitor will discharge and the circuit will revert to the collapsed condition.

Another solution which has been proposed is the circuit of Figure 2, wherein similar parts to those of Ffigure 1 are identified by similar reference numerals. In this circuit a current source is provided comprising a transistor 20 biased to a stable operating point by a fixedly biased transistor 22. Such a current source is known as a peaking current source which for low values of voltage across the collector emitter path of transistor 20 provides a high current but for higher voltages provides a much lower current, as indicated in figure 2A. The source is biased to operate at a low current position as indicated so that should the voltage across the collector-emitter path of transistor 2 reduce significantly, the current source will provide an increased current flow to rectify the situation.

Nevertheless if the output of the voltage reference circuit is held at ground, the entire circuit will collapse including the current source.

It is therefore an object of the present invention to provide a bandgap reference circuit which will reliably prevent the circuit entering a collapsed condition.

The present invention provides a bandgap reference circuit including first and second bipolar transistors of similar type but having different current density emitter paths, the first transistor providing a base-emitter voltage to the base of the second transistor to fix the operating point of the second transistor, and the collectors of the first and second transistors being coupled to third and fourth transistors, respectively, the third and fourth transistors being connected in current mirror configuration to form an active load which forms part of an output load of the circuit across which an output reference voltage is developed, a voltage sensing means coupled to sense the output reference voltage and to control a current source in dependence thereon, the current source being coupled to the first and second transistors whereby should the circuit go towards a collapsed condition the reduced output reference voltage is sensed by the voltage sensing means to increase the flow of current from the current source to the first and second transistors whereby to maintain the circuit in an operative condition.

This in accordance with the invention, should the output load be shorted to ground, tending to pull the circuit to a collapsed condition, this will immediately be sensed by the voltage sensing means to hold the current source in a condition in which a large current flow is provided to the circuit to prevent entry into the collapsed condition so that when the shorting condition is removed the circuit will revert to its stable operating condition.

As preferred the voltage sensing means and current source are combined in a long-tailed transistor pair, having the current source in the tail, in which one transistor is responsive to the output reference voltage and the other transistor is responsive to a bias circuit voltage of the current source.

The current source may be any suitable constant current source but as preferred it is of the peaking type illustrated in figure 2, but operating at or near the current peak condition.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings, wherein: Figure 1 is circuit diagram of a known type of bandgap reference voltage circuit; Figure 2 is a circuit diagram of a modification of figure 1 with a circuit for ensuring start up of the reference voltage circuit; and, Figure 3 is a circuit diagram of the bandgap reference voltage circuit of figure 1 but incorporating the present invention.

Referring now to figure 3, similar parts to those shown in figures 1 and 2 have the sense reference numerals. In accordance with the invention, a pair of transistors 24, 26 are provided connected in differential configuration with a common emitter load comprising a current source 20, 22 of the peaking current type described in figure 2 but with an operating point fixed at or close to the peak current flow as indicated in figures 2A. The base of transistor 26 is coupled to the junction between biasing resistors 28, 30 in order to provide a fixed bias to transistor 26. the base of transistor 24 is coupled to the junction (a) between transistor 10 and resistor 8 in order to sense the output reference voltage of the circuit.

Thus in operation, upon start up of the circuit, the base of transistor 26 will immediately be provided with a positive bias to switch on transistor 26 to provide current flow to transistor 2 to ensure that the bandgap reference circuit will enter its stable operating mode and not stay in a collapsed state. In the stable condition the relatively high output reference voltage will switch on transistor 24, thereby switching off transistor 26 so that there is no current loading on the bandgap circuit.

If output load resistor 8 is accidentally shorted to ground, then the voltage at the base of transistor 24 is reduced rapidly so that transistor 26 is switched to a conductive condition so as to provide current flow at (b) to transistor 2 and prevent the circuit from entering a stable collapsed condition. When the short is removed from load resistor 8, the circuit will enter its stable operating condition, assisted by current flow from transistor 26.

Thus if the bandgap is on then node a) is higher than node b) and the startup circuit draws no current from the bandgap. If however the bandgap collapses, node a) is lower than node b) and the circuit will pull current from the bandgap in such a manner as to restart the bandgap, and will continue to do so until the bandgap voltage is restored. Features of this circuit are the provision of a current source - which in this case is of the peaking variety to ensure that excessive current is not drawn from supply at high voltages although a simple mirror would also suffice, and a differential pair connected to this current source with one input connected to bandgap and the other connected to a Vbe. The collector of the transistor whose base is connected to a Vbe must be connected to a node in the bandgap which will cause the bandgap to start when current is extracted.

Any bandgap voltage reference circuit may be started by using this technique.

Claims

CLAIMS:

1. A bandgap reference circuit including first and second bipolar transistors of similar type but having different current density emitter paths, the first transistor providing a base-emitter voltage to the base of the second transistor to fix the operating point of the second transistor, and the collectors of the first and second transistors being coupled to third and fourth transistors, respectively, the third and fourth transistors being connected in current mirror configuration to form an active load which forms part of an output load of the circuit across which an output reference voltage is developed, a voltage sensing means coupled to sense the output reference voltage and to control a current source in dependence thereon, the current source being coupled to the first and second transistors whereby should the circuit go towards a collapsed condition the reduced output reference voltage is sensed by the voltage sensing means to increase the flow of current from the current source to the first and second transistors whereby to maintain the circuit in an operative condition.

2. A circuit according to claim 1 wherein the first transistor has a first resistance connected to its emitter and a second resistance of a larger value connected to its collector, the second resistance forming part of said output load.

3. A circuit according to claim 1 or 2 wherein the voltage sensing means comprises a fifth transistor having its base connected to said output load, the transistor being arranged to cause the current source to provide current flow to said first and second transistors upon sensing a reduced output reference voltage.

4. A circuit according to any preceding claim wherein the fifth transistor is coupled with a sixth transistor in differential pair configuration with said current source being connected in the common emitter path of the fifth and sixth transistors, the collector of the sixth transistor being connected to said first and second transistors and the base of the sixth transistor being connected to a reference voltage having a value less than the output reference voltage.

5. A circuit according to claim 4 wherein the collector of the sixth transistor is connected to the collector of the second transistor.

6. A circuit according to any preceding claim wherein the current source comprises a seventh transistor biased to an operating point by the base-emitter junction of an eighth transistor.

7. A circuit according to claim 6 as dependent on claim 4 or 5 wherein the seventh transistor is positioned in said common emitter path and the base-emitter junction of said eighth transistor is connected to the base of the sixth transistor.

8. A bandgap reference circuit substantially as described with reference to figure 3 of the accompanying drawings.