IES65854B2 - Power supply control circuit - Google Patents

Abstract

A power supply control circuit comprises a field effect transistor FET connected between a battery BAT and a microprocessor uP to be powered, and a bistable circuit 11 comprising cross-coupled transistors Q1 and Q2 connected to the power supply and to the switching device. When power is turned on by closing switch SW the circuit 11 assumes a state wherein transistor Q2 is on and transistor Q1 is off so that the current drawn by the transistor Q2 through a resistor R4 provides a voltage drop across the source-drain of the FET sufficient to turn the latter on to allow power to be supplied to the microprocessor. However, upon certain predetermined condition(s) being met the microprocessor turns transistor Q1 on by a signal at its base thereby turning transistor Q2 off so that the FET turns off to remove power from the microprocessor.

Description

POWER SUPPLY CONTROL CIRCUIT The present invention relates to a power supply control circuit, for example for a microprocessor-based product.

Microprocessor-based products, in particular battery-powered products, require minimum power consumption to ensure low running costs and maintenance of such products. It is known for electronic circuits including a microprocessor to switch into a low power consumption standby mode.

However, such microprocessors and their associated circuitry still draw significant current while in standby mode and present a drain in battery power.

It is an object of the present invention to provide a power supply control circuit which mitigates the above problem.

According to the present invention there is provided a power supply control circuit comprising an electronic switching device connected between a power supply and a further circuit to be powered, and a bistable circuit connected to the power supply and to the switching device^ the bistable circuit assuming a first state when power is turned on in which state the switching device is made conductive to allow power to be supplied to the further circuit, and the bistable circuit assuming a second state upon certain condition(s) being met in which state the switching device is made non-conductive to remove power from the further circuit.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic diagram of a power supply control circuit for a microprocessor-based product.

Referring now to the drawing, a microprocessor control circuit generally indicated at 10 comprises a D.C. power supply, such as a battery BAT, connected through a switch SW to a latch circuit 11. The latch circuit 11 includes a pair of transistors Q1 and Q2 which are cross-coupled by the -2S 6 £854 base of each transistor being connected to the collector of the other transistor via a respective current limiting resistor R1 or R2. » The collector of Q1 is connected to the switch SW through a parallel h combination of a resistor R3 and a capacitor Cl and the collector of Q2 is connected to the switch SW through a resistor R4.

The circuitry is switched on by closing the switch SW. The capacitor Cl initially provides a short circuit across the resistor R3 and so the transistor Q2 turns on before the transistor Ql. Capacitor Cl then charges up to reduce the quiescent current required to keep transistor Q2 turned on.

Switching transistor Q2 on pulls its collector low, tying the base of transistor Ql low, thus keeping the transistor Ql turned off.

The resistor R4 is connected across the gate and source of a MOSFET transistor FET. When the transistor Q2 is turned on, current flows through the resistor R4 causing a voltage drop across the resistor which exceeds the gate/source voltage Vgs required to turn the FET on.

The drain of the FET is connected through voltage and current limiter circuitry 12 to the microprocessor uP. The circuitry 12 includes a zener diode ZD1 and capacitor C2 to limit voltage surges and an emitter follower circuit including a resistor R5 and a transistor Q3 to limit current surges when the latch circuitry 11 changes state.

Thus, when the switch SW is initially closed the FET is switched on to provide power from die battery to the microprocessor during normal operation of the product controlled by the microprocessor. The FET also supplies power to other circuit components of the product of which the microprocessor forms part. The microprocessor remains on until certain predetermined condition(s) are met which determine that the product can or should be turned off to reduce power consumption. For example, one such condition may be that the product has been left switched for a certain period without being used. Alternatively, a condition may be that the product has been in use for too long and may be dangerous. -3If any such condition is detected, the microprocessor drives an output pin 3 O/P high. The O/P pin is connected to the base of the transistor Q1 and thus forces the transistor Q1 to turn on. This causes the collector of x transistor Q1 and the base of transistor Q2 to be pulled low and so transistor Q2 is turned off. Turning transistor Q2 off stops current flowing through R4 and so the FET is switched off, causing power to be removed from the microprocessor and any circuitry connected through the FET to the battery BAT.

It will be seen that once transistor Q1 has been switched on, the state of the output pin O/P does not matter as the transistor Q1 will be held on by the voltage at the collector of the transistor Q2 which goes high when the latter turns off. The resistors Rl to R4 are thus chosen to be as high as possible to reduce both the current drawn by the latch circuit 11 during normal operation when transistor Q2 is turned on, and during the reduced power state when transistor Q1 is turned on.

Claims (5)

1. f

1. A power supply control circuit comprising an electronic switching device connected between a power supply and a further circuit to be * powered, and a bistable circuit connected to the power supply and to the switching device, the bistable circuit assuming a first state when power is turned on in which state the switching device is made conductive to allow power to be supplied to the further circuit, and the bistable circuit assuming a second state upon certain predetermined condition(s) being met in which state the switching device is made non-conductive to remove power from the further circuit.

2. A power supply control circuit as claimed in claim 1, wherein the further circuit comprises a microprocessor and the bistable circuit assumes the second state in response to a signal from the microprocessor when a said condition is met.

3. A power supply control circuit as claimed in claim 1 or 2, wherein the switching device has current input and output terminals and a control terminal, wherein an impedance is connected across one of said input and output terminals and the control terminal so as to make the switching device conductive when the voltage drop across the impedance exceeds a threshold value, and wherein in the first and second states the bistable circuit draws respectively sufficient and insufficient current through the impedance to establish a voltage drop thereacross which exceeds the threshold value.

4. A power supply control circuit as claimed in claim 3, wherein the bistable circuit comprises cross-coupled second and third electronic switching devices arranged so that when one of the second and third switching devices is conductive the other is non-conductive and vice versa, wherein the second switching device is connected in series with die impedance, and wherein the second switching device is made conductive when power is turned on. -5

5. A power supply control circuit as claimed in claim 4, wherein the first switching device is a field effect transistor and the bistable circuit comprises cross-coupled transistors.