Voltage reference circuit

DETAILED DESCRIPTION The present invention resides in one aspect in using a bandgap reference circuit to generate the reference voltage for a voltage detection circuit.
As is well known in the art, a bandgap reference circuit includes an operational amplifier having a plus input and a minus input.
An output signal of the operational amplifier is supplied to the gate of a p-channel output transistor which has its source connected to an upper power supply voltage rail and its drain connected to supply a feedback current to first and second resistive chains.
The first resistive chain includes a first resistor and a second resistor connected in series with a first diode-connected bipolar transistor.
The second resistive chain comprises a single resistor connected in series with a second diode-connected bipolar transistor.
The plus input of the operational amplifier receives its input from a node intermediate the first and second resistors of the first resistive chain.
The minus input of the operational amplifier receives its input from a node intermediate the resistor of the second resistive chain and the emitter of the second bipolar transistor.
The collectors of the bipolar transistors are connected to the lower supply rail, which will normally be at ground.
The reference voltage generated by the bandgap circuit is derived from the reference level at an output node at the junction of the first and second resistive chains.
Operation of the bandgap reference circuit is well known to a person skilled in the art and is therefore only discussed briefly herein.
The first bipolar transistor is designed to have an emitter area which is several times larger than the emitter area of the second bipolar transistor.
The base emitter voltage Vbe across the bipolar transistors varies linearly between 0.
8 V and 0.
4 V when the temperature varies from minus 55.
degree.
C.
to 150.
degree.
C.
As the emitter area of the first bipolar transistor is larger than the emitter area of the second bipolar transistor but the current through it is the same, the first bipolar transistor has a lower base emitter voltage across it