GB2211043A - A current switching arrangement - Google Patents

Abstract

An arrangement wherein current leakage in a switched off condition is limited by a series electronic switch. The electronic switch is controlled by a control signal and the gain of the electronic switch is controlled by a gain control circuitry activated by the control signal.

Description

A CURRENT SWITCHING ARRANGEMENT The present invention relates to current switching and more specifically but not exclusively to switching where there is a requirement for very low current leakage in the switched off-state.

In low power systems, such as radio pagers, it is common practice to switch off the system circuitry when it is not being used.

In radio pager circuits where the receiver is only switched on for short periods to determine whether a transmission is present, only if a transmission signal is detected will the receiver remain on for any length of time. This type of system is thus normally off, the current consumed in this off-state will therefore determine the battery life of the system. It is consequently desirable to ensure the current comsumed in this state is as low as possible.

Previously, arrangements to provide such low power consumption have included switching off all the current sources in the circuit (Figure 1) or placing an electronic switch 20 in series with the supply (Figure 2). In both arrangements, a control signal initiates switching.

In the circuit as shown in Figure 1, switching off is achieved by turning off the current sources. The standby current of the circuit in this off-state is the sum of the following: the leakage through the decoupling capacitor 3; the combined leakage of all the transistors in the circuit; and the current consumed by the current source control circuit.

The arrangement of Figure 2 where the switch is in series with the supply, leakage from the decoupling capacitor 3 and circuit transistors is eliminated. With such an arrangement however the electronic switch has a high standby leakage itself. As the input must be driven by a very small control current and the output must supply the total chip current, the switch it must have a high current gain. Thus, any leakage at the input of the electronic switch is amplified by the gain of the switch to produce a much larger standby leakage current at the output.

The leakage from either arrangement is thus substantially equivalent.

It is an objective of the present invention to provide a current switching arrangement which substantially relieves the level of standby current leakage.

According to the present invention, there is provided a current switching arrangement, for an electrical system, having an electronic switch in series with a current supply, the electronic switch, under the control of a control signal, being adapted to switch the current supply from an ON state to an OFF state and vice versa, and a gain control arrangement for the electronic switch activated by the control signal whereby the gain of the electronic switch is controlled by positive feedback in order that, when the switch is in the ON state, the gain is high whilst when the switch is in the OFF state, the gain is low.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawing, Figure 3, in which a current switching arrangement according to the present invention is illustrated.

The use of a supply series switch provides the greatest potential to eliminate current leakage by the decoupling capacitor 3 and circuit, however the standby, OFF state current must be reduced.

An electronic switch, which uses very little current in the OFF state, is required.

In some low power, battery-operated systems such as radio pagers, an on-off control signal is generated by a microprocessor in accordance with a user time slot regime. The contol signal switches the supply series switch in the user's designated time slot. In the switching arrangement of the present invention, the control signal is also used to control the gain of the switch. Switch gain being switched to a low when the switch is "OFF" and switched to a high when the switch is "ON". The gain control system is by positive feedback and consequently only a small drive capability is required when the switch is OFF and a high drive capability when the switch is ON. The gain control circuit can thus be 'ganged' to the on/off control signal to further minimise the power consumption in the off state.

Consider Figure 3, illustrating a switching arrangement according to the present invention, a node a) is the switch control point. When node a) is low, node b) will source current dependent upon the operational values of resistors R1 - R5. In this node a) low state the current flows through path (1) and the gain from a) - b) is ssnpn. ss2 pnp. IL (Where ss= current amplification factor, I = current and L = Diffusion length). When node a) is taken high, the current flow (which will just be leakage current) will be along path (2). Here the switch gain between nodes a) and b) is nominally 0. Current leakage however will be (3pnp. IL.A typical value for ss is 50, a leakage current reduction by, a factor of approximately 25 is therefore possible.

The resistors R4 and R5 limit the maximum gain of the switch but can be removed to achieve circuit operation from 0.9V so meeting low power operation design criteria. A diode D1 prevents saturation of Transistor T8. Transistor TI j is made larger (has a larger diffusion area) than T10 to ensure sufficient base drive for transistor T8.

In a power ON situation i.e. node a) low, T7 is switched off and consequently all other elements connected thereto.

In a power OFF situation i.e. node a) high, T7 is switched on and T1, T8 and T14 switched off.

It will be appreciated that the polarity of the circuit may be inverted to provide a wide dynamic range switching npn current sink to ground. Two or more of the present switching assemblies may be cascaded to increase the drive capability of the current source (or sink), through connecting the output of the first assembly to the input of a second assembly etc. and removing T1 and R1 from the second circuit.

To reduce the gain of T2, T3 and T6 in the "OFF", the state pnp transistors with the same emitter and base connections as T4 may have their collectors connected to their bases.

The switching arrangement of the present invention may be constructed on a single substrate as an integrated circuit.

The switching arrangements of the present invention can be used with any low supply, battery economised circuit.

It will be appreciated the above arrangement illustrates only one practical embodiment of the present invention and alternative means of varying the gain of the series supply switch under the control of a control signal could be employed.

Furthermore, it should be noted that the control signal need not be "ganged" from the system but may be specifically directed to control the switching function.

Claims (5)

1. A current switching arrangement, for an electrical system having an electronic switch, control means for producing a control signal arranged to switch the current supply between an ON state and an OFF state, and a gain control arrangement activated by the control signal whereby the gain of the electronic switch is controlled by positive feedback in order that, when the switch is in the ON state, the gain is high whilst, when the switch is in the OFF state, the gain is low.

2. A current switching arrangement as claimed in Claim 1 wherein the means signal is a control microprocessor unit.

3. A current switching arrangement as claimed in Claim 1 or 2 wherein the gain control arrangement and the electronic switch are ganged to operate simultaneously on application of the control signal.

4. A current switching arrangement as claimed in any preceding claim wherein the electronic switch and the gain control arrangement are constructed upon a single substrate.

5. A current switching arrangement substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.