Switching circuit

• US 5,164,659 A
• Filed: 08/29/1991
• Issued: 11/17/1992
• Est. Priority Date: 08/29/1991
• Status: Expired due to Term

• Current Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Su, Stephen, Schultz, Warren
• Primary Examiner(s): Wong, Peter S.
• Application Number:
  US07/751,853
• Time in Patent Office: 446 Days
• Field of Search: 323/282, 323/283, 323/284, 323/285, 323/351, 340/652, 340/660-664, 361/18, 361/86-88, 361/91
• US Class Current: 323/351
• CPC Class Codes: H02H 3/12 responsive to underload or no-load for motors H02H7/0827
• CPC Class Codes: H02M 3/156 with automatic control of output voltage or current, e.g. switching regulators
• CPC Class Codes: H03K 17/063 in field-effect transistor switches
• CPC Class Codes: H03K 17/0822 in field-effect transistor switches
• CPC Class Codes: H03K 17/18 Modifications for indicating state of switch
• CPC Class Codes: H03K 17/6874 in a symmetrical configuration

Abstract

A low impedance load is switched between a power supply source and ground through a series connected power MOSFET the conduction of which is controlled by a switching circuit. The switching circuit includes a linear loop that regulates the switch voltage to a minimum value to produce current flow through the load and the MOSFET. In addition, the switching circuit includes a current supply placed in parallel to the MOSFET which is set to supply a minimum predetermined current that is representative of the value of current flow through the load under an open load condition. If the load current falls below this set current value an open load status flag is provided by the switching circuit as the loop is no longer in regulation.

Claims

1. A circuit for operatively switching utilization means to a power supply source wherein the utilization means includes a controlled switching device series connected to load means such when the switching device is operative the load means is coupled to the power supply source, comprising:

a current source responsive to an input signal applied to an input of the circuit for providing a current of a predetermined magnitude, said current source being coupled to the interconnection point between the switching device and the load means; and

a regulating loop responsive to said input signal for both rendering the switching device operative and for regulating the potential at said interconnection point to a predetermined voltage when the current flowing through the load means exceeds the magnitude of said current provided by said current source and non-regulating said potential when said current through the load means is less than said current of said current source.

2. The circuit of claim 1 including said current source being coupled in parallel to said regulating loop;

and circuit means responsive to the magnitude of said current flowing through the load means decreasing below said predetermined magnitude of said current of said current source during the presence of said input signal for providing an output signal indicating the same.

3. The circuit of claim 2 wherein:

the switching device is a MOSFET having first and second electrodes and a control electrode, said first and second electrodes being in series connection with the load means; and

said regulating loop includes a comparator operational amplifier which is enabled by said input signal and having inverting and non-inverting inputs and an output, said output being coupled to said control electrode of said MOSFET, said inverting input being coupled to a reference voltage and said non-inverting input being coupled to said interconnection point between the switching device and the load means.

4. A switching circuit for operatively connecting and disconnecting a load to a power supply source, comprising:

switching means series connected to the load for electrically connecting the load to the power supply source when rendered operative to produce current flow through the load;

a current source responsive to an input signal for providing a current of a predetermined magnitude when the current flowing through the load exceeds said predetermined magnitude as the load is electrically connected to the power supply source, said current source being coupled to the interconnection between said switching means and the load; and

a linear regulating loop responsive to said input signal for both rendering said switching means operative and for regulating the magnitude of voltage developed at said interconnection between said switching means and the load to a predetermined value when the load current exceeds said current of said current source.

5. The circuit of claim 4 wherein said regulating loop is out of regulation as said load current becomes less than said current of said current source and said current source current can no longer be sustained to render said switching means non-operative.

6. The circuit of claim 5 wherein said switching means includes a transistor having first and second electrodes and a control electrode, said first and second electrodes being coupled

in parallel to said current source and in series with the load;

and said regulating loop includes an operational amplifier which is enabled by said input signal and having an output coupled to said control electrode of said transistor and first and second inputs, said first input being coupled to said interconnection between said switching means and the load and said second input being coupled to a first reference voltage.

7. The circuit of claim 6 wherein said transistor is a power MOSFET having a drain, source and gate being said first, second and control electrodes respectively, said drain being connected to said load, said gate being coupled to the output of said operational amplifier;

and a first resistor coupled between said source and circuit ground reference.

8. The circuit of claim 7 including circuit means responsive to said load current becoming less than said current of said current source for providing an output signal indicative of the same.

9. The circuit of claim 8 including:

a MOSFET having drain, source and gate electrodes, said drain electrode being coupled to said drain of said power MOSFET, said source electrode being coupled to said first input of said operational amplifier and said gate electrode being coupled to a terminal to which said input signal is applied; and

a second resistor coupled between said source electrode of said MOSFET and circuit ground reference.

10. The circuit of claim 9 wherein said current source includes:

an additional power MOSFET having a drain, source and gate, said drain being coupled to the power supply source the latter being an AC voltage supply, said gate being coupled to said output of said operational amplifier; and

a third resistor coupled between circuit ground reference and said source of said additional power MOSFET.

11. The circuit of claim 10 including:

an additional MOSFET having drain, source and gate electrodes, said drain electrode being coupled to said drain of said additional power MOSFET, said gate being coupled to said terminal to which said input signal is applied;

a fourth resistor coupled between said source electrode of said additional MOSFET and said circuit ground reference, wherein said second and fourth resistors respectively correspond to said current source; and

said operational amplifier having a third input coupled to said source electrode of said additional MOSFET.

12. The circuit of claim 11 wherein said circuit means includes:

a first comparator having first and second inputs and an output, said first input being coupled to a second reference voltage, said second input being coupled to said source electrode of said MOSFET;

a second comparator having first and second inputs and an output, said first input being coupled to a third reference voltage, said second input being coupled to said source electrode of said additional MOSFET; and

a logic gate having a first, second and third inputs respectively coupled to said outputs of said first and second comparators and said terminal to which said input signal is applied, and having an output at which said output signal is produced.

13. The circuit of claim 6 including circuit means responsive to said load current becoming less than said current of said current source for providing an output signal indicative of the same.

14. The circuit of claim 13 wherein said transistor is a power MOSFET having a drain, source and gate corresponding to said first, second and control electrodes respectively, said drain being coupled to the power supply source, said source being coupled to an output terminal to which the load is connected;

and said current source being coupled between said drain and source of said power MOSFET.

15. The circuit of claim 14 wherein said circuit means includes:

a resistive divider coupled between said second input of said operational amplifier and said power supply source;

a comparator having first and second inputs and an output, said first input being coupled to said first input of said operational amplifier, said second input being coupled to said resistive divider; and

a logic gate having first and second inputs and an output, said first input being coupled to said output of said comparator, said second input receiving said input signal and said output being coupled to said output of the circuit.

16. A method for sensing the open load status of a load driven by a switching circuit the latter of which includes a transistor the main electrodes of which are series connected to the load and which electrically couples the load to a power supply when switched to a conducting state to provide load current through the load, comprising the steps of:

rendering the transistor conducting responsive to an input signal being applied to the circuit;

regulating the voltage at the series connection between the transistor and the load when the load current exceeds a predetermined magnitude while fully rendering the transistor conductive when the voltage developed at the series interconnection between the transistor and the load exceeds the value of the regulated voltage; and

supplying a predetermined current having said predetermined magnitude which is indicative of the load being open.