GB2198559A - Voltage reference circuit for a vehicle ignition system - Google Patents

Abstract

A voltage reference circuit includes a differential amplifier (A1) provided with resistive feedback (R2, R3). The amplifier inputs are provided with reference voltages by respective transistors (T1, T2) each coupled to a current source (S1, S2). The resistive feedback compensates the loading effect of the amplifier and ensures a fixed reference output voltage condition. The circuit may be used in an ignition control system (Fig. 2) as part of a low voltage winding current control responsive to engine speed and load, for determining dwell angle and spark timing. <IMAGE>

Description

IMPROVEMENTS IN VEHICLE IGNITION SYSTEMS This invention relates to electronic ignition systems for petrol engine vehicles, and in particular to a voltage reference circuit for use in such systems.

According to one aspect of the invention there is provided an ignition control system for a petrol engine, the system including a coil having a low voltage winding and a high voltage winding, means for sensing engine speed, means for sensing engine load, means for controlling current through the low voltage winding of the coil so as to determine, in response to the speed and load, dwell angle and spark timing, and means for controlling the current through the low voltage winding whereby, when said current exceeds a predetermined value, that current is switched off to initiate the spark.

According to another aspect of the invention there is provided a voltage reference circuit, including a differential amplifier, voltage reference sources one for each amplifier input, a feedback loop from the amplifier output to the inputs so as to define the amplifier gain, and feedback means coupled between the amplifier and the reference sources whereby the amplifier output is held at a fixed voltage relative to a supply voltage.

Embodiments of the invention would now be described with reference to the accompanying drawings in which: Fig. 1 shows a voltage reference circuit, and Fig. 2 shows an ignition control system incorporating the voltage reference circuit of Fig. 1.

Referring to Fig. 1, the circuit includes a differential amplifier Al the inputs of which are held each at a respective reference voltage Vl, V2. Each reference voltage is provided by a field effect transistor (Tl, T2) coupled to a current source (S1, S2). The gate of each transistor is connected to its drain. The inverting input of the amplifier Al is connected to its respective reference voltage source via a resistor R1, whilst the non-inverting input is directly coupled to its corresponding reference source. In use the current through each transistor Tl, T2 is sufficient to saturate that transistor. The transistor Tl and T2 are similar so that their electrical characteristics are substantially identical.

The gain of the amplifier Al is determined by the resistor R2 coupled between the amplifier output and the inverting input. A further resistor R3 couples the amplifier output to the non-inverting input. The amplifier Al is thus connected as an operational amplifier (op-amp).

Typically, the values of the resistors R1, R2 and R3 are chosen such that R1=R2 and R3=4 R1 The output voltage of the amplifiers, relative to the supply voltage, is determined by the ratio of the current generated by the current sources S1 and S2. As the two transistors are both saturated the source-drain current of each is proportional to the square of its effective base drive. If we assume for example a current ratio I1:I2 of 4:1 then the ratio of effective gate drive is 2:1. Thus the voltage V1 and V2 at the inputs of the amplifier are given by V1 = VDD - VT - VGD V2 = VDD - VT - 2VGD where VT is the transistor threshold voltage and VGD is the gate-drain voltage.

The negative feedback provided by the R1 and R3 provides the amplifier with a gain of -l (VlVouT) with respect to V2.

i.e. VOUT ~ V2 2 1 Hence we derive V OUT VDD VT Thus the output voltage is at a fixed reference value with respect to the supply voltage.

The feedback current flowing through the amplifier is compensated by the resistor R2. This resistor has four times the value of Rl and R3 and ensures that the feedback current I3 and I4 are maintained in the ratio 4:1. If we assume an ideal operational amplifier we have 14 = OUT )/ V2)/R2 13 = OUT )/ V2)/R3 i.e. I3 = 4 Il The corresponding model equations for the above are 41 = Il + I3 I = I2 + I4 i.e. we have Il = 4I2.

The feedback resistor R2 thus compensates the loading effect of the operational amplifier circuit and ensures a fixed reference output voltage condition.

In the circuit shown in Fig. 1 transistors Tl and T2 are p-channel transistors. It will be appreciated that by reversing the power supply and using n-channel transistors a fixed reference voltage relative to the ground rail may be generated.

The circuit of Fig. '1 has particular application as a reference voltage source in vehicle electronic circuitry, e.g. electronic ignition systems. By providing fixed reference points such circuits are then able to compensate for variations in transistor threshold voltages arising from minor process differences between batches of semiconductor wafers. Thus, the circuit of Fig. 1 is used here, not to define a reference point directly, but to ensure that hysteresis levels for comparator circuits are fixed and independent of process variations.

Fig. 2 shows an ignition control circuit in schematic form. Timing signals for the circuit are provided by a sensor 21 disposed adjacent a toothed wheel 22 mounted, e.g., on the engine camshaft. Typically the sensor 21 is a magnetic sensor. A further sensor 23 responds to the pressure pertaining in the venturi section of the inlet magnified so as to provide a measure of engine load. Typically the sensor 23 comprises an electrical LC oscillator in which the frequency is determined by a pressure responsive variable capacitor. The outputs of the sensors 21 and 23 are fed to a processor or control circuit 28 which, in response to the sensor inputs, switches current from the vehicle battery B1 through the primary winding 29 of the ignition coil. The coil current is switched to provide correct timing and dwell angle according to the engine condition.The pressure sensor frequency is mixed with the output of a reference oscillator 27. The processor 28 determines the corresponding beat frequency to provide a measure of the engine load. This, together with the engine speed, determines the switching of the coil current.

The system is supplied with power from a regulated power supply 26, typically between 4 and 6 volts output, coupled between the processor 28 and the vehicle battery B1.

The coil current is coupled to earth via a low value resistor R4 the terminals of which are coupled to a voltage comparator circuit 30. Typically the value of the resistor R4 is about 0.025 ohms. The voltage across this resistor, corresponding to the coil current, is compared with a reference voltage derived by the comparator from reference source 31 incorporated in the comparator. This voltage reference circuit 31 may be of the type shown in Fig. 1. When the resistor voltage drops, and hence the coil current, exceeds a predetermined value set by the voltage of the reference source 31, the processor switches off the coil current thus initiating the spark at an earlier time than the correct time. In normal running of the system this early firing of the spark does not occur.

It is in fact a failure condition wherein the coil current is limited to protect the processor switching circuitry from overload. Such a condition may occur if current is allowed to flow through the coil for an excessive time, e.g. as a result of failure of the sensor 21 from which timing information is obtained.

Typically the comparator circuit 30 switches to a high condition when the voltage across the resistor R4 reaches 0.12 volts corresponding to a coil current of about 5 amps. This high output causes the comparator to switch off the coil current. The comparator output returns to its low condition when the voltage across R4 has fallen to between 55 and 75% of its maximum value, the precise amount of hysteresis being determined by the reference circuit 31.

The circuits 30, 31, 26 and 27 together with processor 28 may be provided in the form of a single integrated circuit.

The circuit of Fig. 1 is not of course limited to vehicle applications but can be used in a variety of integrated circuits to provide a voltage reference function.

Claims (7)

CLAIMS:-

1. A voltage reference circuit, including a differential amplifier, voltage reference sources one for each amplifier input, a feedback loop from the amplifier output to the inputs so as to define the amplifier gain, and feedback means coupled between the amplifier and the reference sources whereby the amplifier output is held at a fixed voltage relative to a supply voltage.

2. A circuit as claimed in claim 1, wherein each said voltage reference source comprises a field effect transistor having its source coupled to its gate and to a current source.

3. A circuit as claimed in claim 1 or 2, wherein the feedback for the amplifier output to the inputs is resistive.

4. A voltage reference circuit substantially as described herein with reference to and as shown in Fig. 1 of the accompanying drawings.

5. A vehicle ignition control system provided with a voltage reference circuit as claimed in any one of claims 1 to 4.

6. An ignition control system for a petrol engine, the system including a coil having a low voltage winding and a high voltage winding, means for sensing engine speed, means for sensing engine load, means for controlling current through the low voltage winding of the coil so as to determine, in response to the speed and load, dwell angle and spark timing, and means for controlling the current through the low voltage winding whereby, when said current exceeds a predetermined value, that current is switched off to initiate the spark.

7. An ignition control system substantially as described herein with reference to and as shown in Fig. 2 of the accompanying drawings.