Current sense circuit having multiple pilot and reference transistors

• US 5,867,014 A
• Filed: 11/20/1997
• Issued: 02/02/1999
• Est. Priority Date: 11/20/1997
• Status: Expired due to Term

• Current Assignee: Fairchild Semiconductor Corporation (ON Semiconductor Corporation)
• Original Assignee: Impala Linear Corp (ON Semiconductor Corporation)
• Inventors: D'Angelo, Kevin P., Wrathall, Robert S.
• Primary Examiner(s): NGUYEN, MATTHEW VAN
• Application Number:
  US08/975,324
• Time in Patent Office: 439 Days
• Field of Search: 323/312, 323/315, 323/316, 330/257, 330/288
• US Class Current: 323/316
• CPC Class Codes: G01R 19/0092 measuring current only all subgroups of G01R19/00 take precedence
• CPC Class Codes: G01R 19/16571 comparing AC or DC current with one threshold, e.g. load current, over-current, surge current or fault current G01R19/16514, G01R19/16519, G01R19/16528, G01R19/16533, G01R19/1659 take precedence; measuring currents by using elements sensitive to the magnetic field generated G01R15/14; measuring earth resistance G01R27/18; testing for leakage or short circuits in electrical apparatus G01R31/52
• CPC Class Codes: G05F 3/242 with compensation for device parameters, e.g. channel width modulation, threshold voltage, processing, or external variations, e.g. temperature, loading, supply voltage
• CPC Class Codes: H03K 17/0822 in field-effect transistor switches

Abstract

A current sense circuit utilizes multiple resistive reference switches connected in electrical series to reduce the level of required reference current (Iref), while maintaining the integrity of tracking current (Iout) through a resistive power switch. Typically, all of the reference switches are MOS transistors connected in electrical series. The first embodiment includes establishing a ratio (n) of series reference transistors to series pilot transistors, n>1. In another embodiment, the series connection of reference switches is in parallel with a single reference resistor and is identical to a series connection of a number (NP) of pilot switches. In a third embodiment, the techniques of the first two embodiments are combined (i.e., n>1 and NP>1). The current sense circuit is utilized to monitor output current through a power switch from a circuit load. Each of the pilot and reference switches is fabricated such that conductivity through the power switch would be greater than conductivity through an isolated pilot or reference switch by a scaling factor. In the preferred embodiment, a scaling factor (SF) is established by fabricating the pilot and reference switches to have gate widths that are smaller than the gate width of the power switch, such that Iref=Iout/(SF×n×NP).

Claims

1. A current sense circuit comprising:

a voltage source having first and second terminals;

a load connected to said first terminal of said voltage source;

power switching circuitry for regulating conduction through said load, said power switching circuitry including a resistive power switch connected to establish a first current path between said load and said second terminal of said voltage source;

current pilot circuitry having at least one resistive pilot switch and a pilot resistance connected in electrical series to establish a second current path between said load and said second terminal, said first and second current paths having electrical characteristics such that sense current through said second current path is at a generally fixed fraction of load current through said first current flow path;

reference circuitry including current means for establishing a reference current and including at least two resistive reference switches in electrical series to form a third current path between said current means and said second terminal of said voltage source, said reference circuitry further including a resistive path in electrical parallel with said third current path, said reference switches each having electrical characteristics that are substantially related to electrical characteristics of said pilot switch; and

means for controlling said power switching circuitry in response to said sense current.

2. The current sense circuit of claim 1 wherein said resistive path of said reference circuitry has a resistance that is substantially equal to a multiple (n) of said pilot resistance, where n is equal to the ratio of reference switches to pilot switches.

3. The current sense circuit of claim 2 wherein said pilot and reference switches are MOS transistors having gates connected to a common switch control signal, said reference resistance path being a plurality of resistors, each having substantially the same resistance as said pilot resistance.

4. The current sense circuit of claim 1 wherein said current pilot circuitry includes a number of pilot switches that is proportionally related to the number of said reference switches.

5. The current sense circuit of claim 4 wherein said resistive path of said reference circuitry is a single resistor in parallel with said series connection of said reference switches.

6. The current sense circuit of claim 4 wherein said resistive path of said reference circuitry includes a separate reference resistor in parallel with each of said reference switches.

7. The current sense circuit of claim 1 wherein said pilot switch is a first MOS transistor having a first gate geometry, said power switch being a second MOS transistor having a second gate geometry substantially larger than said first gate geometry by a scaling factor SF, thereby determining said generally fixed fraction of sense current to load current.

8. The current sense circuit of claim 7 wherein reference switches are third MOS transistors having said first gate geometry, each of said first and second MOS transistors having a gate connected to a single switch control signal.

9. The current sense circuit of claim 1 wherein said means for controlling said power switching circuitry includes a differential amplifier having first and second inputs, said first input being connected to a junction of said pilot switch and said pilot resistance, said second input being connected to a junction of said current means and said third current path.

10. A current sense circuit comprising:

a circuit load;

a power transistor having a first source-drain conduction path for conducting current from said circuit load, said power transistor having a first geometry;

a pilot transistor having a second source-drain conduction path for conducting current from said circuit load, said pilot transistor having a second geometry distinguishable from said first geometry such that a current through said pilot transistor is scaled relative to current through said power transistor;

a pilot resistance connected at a first differential input node to said pilot transistor on a side of said second source-drain conduction path opposite to said circuit load;

a current generator having reference output of a reference current;

a plurality of reference transistors having third source-drain conduction paths connected in electrical series to form a path from a second differential input node at said reference output of said current generator;

at least one reference resistor in parallel with said third source-drain conduction path of at least one of said reference transistors; and

a differential amplifier having first and second inputs connected to said first and second differential input nodes, respectively and having an output that is responsive to electrical characteristics at said first and second inputs.

11. The current sense circuit of claim 10 wherein said power, pilot and reference transistors have gate terminals connected to a single source of a switch control signal.

12. The current sense circuit of claim 10 wherein said power transistor and said pilot resistance are connected to electrical ground, said load circuit and said current generator being connected to a voltage source.

13. The current sense circuit of claim 10 wherein said first geometry is distinguishable from said second geometry with respect to transistor gate widths, said power transistor having a gate width that is greater than a gate width of said pilot transistor by a scaling factor (SF), a current level (Iref) at said second differential input node thereby being at least as great as a current level (Iout) through said power transistor divided by a product of said scaling factor and a ratio (n) of reference transistors to pilot switches, such that Iref=Iout/(SF×

n).

14. The current sense circuit of claim 13 further comprising a plurality of said pilot transistors having second source-drain conduction paths connected in series, said current level at said second differential input node thereby being generally equal to said current level through said power transistor divided by a product of said scaling factor, said ratio (n) and the number of pilot transistors (NP), such that Iref=Iout/(SF×

n×

NP).

15. A current sense circuit comprising:

a voltage supply having first and second terminals;

a circuit load connected to said first terminal, said circuit load having an output current (Iout);

a power transistor having a main conduction path from said circuit load to said second terminal and having a gate terminal connected to a source of a control signal, said power transistor having a first gate width;

a plurality (NP) of pilot transistors having main conduction paths that are connected in series from said circuit load, each pilot transistor having a second gate width less than said first gate width by a first scaling factor (SF);

a pilot resistance connected at a first node between said pilot transistors and said second terminal;

a current generator connected to said first terminal for generating a reference current (Iref) at a second node;

a plurality of reference transistors having main conduction paths that are connected in series from said second node to said second terminal and in parallel with a reference resistance, wherein the number of reference transistors is a multiple (n) of NP where n is equal to or greater than one, said reference and pilot transistors having gates connected to said source of said control signal; and

differential means for determining a voltage difference between voltages developed at said first and second nodes.

16. The current sense circuit of claim 15 wherein said power, pilot and reference transistors are connected such that Iref is generally equal to Iout/(SF×

NP×

n).

17. The current sense circuit of claim 15 wherein said differential means has an output connected to control said source of said control signal.

18. The current sense circuit of claim 17 wherein said differential means is a differential amplifier.