On-chip power supply interface with load-independent current demand
An on-chip interface circuit located between the chip's power supply pin and its internal circuitry senses the current load of the internal circuitry and provides a supplemental current sink so that total current demand seen at the power supply pin is substantially constant despite the internal circuitry's variable load. Sensing of the internal load is done by a sensor stage with two parallel branches, each branch having a resistor, a sense transistor, and a current mirror device, which together produce a voltage drop as a control voltage output which relates the internal load to a constant reference current. The supplemental current sink is in the form of a transistor operating below saturation in its linear region and whose gate is coupled to receive the control voltage output of the sensor stage.
1. An interface circuit for use between a power supply pin and internal circuitry with a variable load, the interface circuit providing a substantially load-independent current demand at the power supply pin, the interface circuit comprising:
sensing means, coupled between the power supply pin at an interface input and the internal circuitry at an interface output, for sensing a current load of the internal circuitry and producing a control voltage corresponding to said current load; and
current sinking means, responsive to said control voltage, for sinking extra current in inverse relation to said current load of the internal circuitry, such that total current demand by both the internal circuitry and the extra current sinking means is at least substantially constant up to saturation of said current sinking means.
2. The interface circuit as in claim 1 , further comprising means for establishing a constant reference current for use by said load sensing means.
3. The interface circuit as in claim 2 , wherein said reference current means and said load sensing means both have current mirror devices with the gates of the current mirror devices coupled together such that a constant current corresponding to said reference current is established through a current path in said load sensing means.
4. The interface circuit as in claim 1 , wherein said load sensing means comprises first and second branches connected in parallel between the interface input and ground, each branch having a resistor, a sense transistor and a current mirror device, the current mirror devices in the two branches establishing corresponding current flow in the branches through said mirror devices, said sense transistors in the two branches having a common gate connection between the sense transistor and mirror device of the first branch, said resistor in the first branch providing substantially larger resistance than said resistor in the second branch, the interface output being coupled between the resistor and sense transistor of the second branch such that current flow through the resistor in the second branch provides a measure of the current load of the internal circuitry in the form of a voltage drop, the voltage drop being provided as a control voltage that is output between the sense transistor and current mirror device of the second branch.
5. The interface circuit as in claim 1 , wherein the means for sinking extra current comprises a transistor connected between the interface output and ground, the transistor having a gate coupled to receive the control voltage from the load sensing means, the transistor operating below saturation in a substantially linear region of an operating characteristic of the transistor.
6. An interface circuit for use between a power supply pin and internal circuitry with a variable load, the interface circuit providing a substantially load-independent current demand at the power supply pin, the interface circuit comprising:
a reference current stage having a constant current source and a current mirror device in series establishing a constant reference current therethrough;
a load sensing stage coupled between the power supply pin at an interface input and the internal circuitry at an interface output, the load sensing stage including first and second branches connected in parallel between the interface input and ground, each branch having a resistor, a sense transistor and a current mirror device, the current mirror devices in the two branches and the current mirror device in the reference current stage having the gates of the current mirror devices coupled together to establish current flow through the mirror devices of the two branches which correspond to the constant reference current, the sense transistors of the two branches having a common gate connection between the sense transistor and current mirror device of the first branch, said resistor of the first branch providing substantially larger resistance than said resistor in the second branch, the interface output being coupled between the resistor and sense transistor of the second branch such that current flow through the resistor in the second branch provides a measure of the current load of the internal circuitry in the form of a voltage drop, the voltage drop being provided as a control voltage that is output from the load sensing stage between the sense transistor and current mirror device of the second branch; and
a supplemental load stage including a transistor connected between the interface output and ground, the transistor having a gate coupled to receive the control voltage output from the load sensing stage, the transistor operating below saturation in a substantially linear region of an operating characteristic of the transistor such that the transistor sinks extra current in inverse relation to the current load of the internal circuitry.
7. A method of providing a substantially load-independent current demand at a power supply pin of an integrated circuit notwithstanding a variable load by internal circuitry of the integrated circuit, the method comprising:
sensing, by an interface circuit provided between the power supply pin and the internal circuitry, a current load of the internal circuitry and producing a control voltage corresponding to the sensed current load; and
sinking, by a supplemental load device of the interface circuit responsive to the control voltage, extra current in inverse relation to the current load of the internal circuitry, such that total current demand by both the internal circuitry and the supplemental load device is substantially constant up to a saturation level of the supplemental load device.
8. The method of claim 7 , wherein sensing the current load and producing a corresponding control voltage involves providing a voltage drop from an interface input coupled to the power supply pin across an output impedance of the interface circuit that is coupled to the internal circuitry, the voltage drop across the output impedance being referenced against a voltage drop across a proportional resistor for a constant reference current.
9. The method of claim 8 , wherein the referencing of the voltage drop is accomplished by a load sensing stage of the interface circuit comprising first and second parallel branches connected between the interface input and ground, each branch having a resistor, a sense transistor and a current mirror device, the current mirror devices establishing corresponding current flow in the branches through said mirror devices, the sense transistors having a common gate connection between the sense transistor and mirror device of the first branch, the resistor in the first branch being the proportional resistor that has a substantially larger resistance than the resistor in the second branch, the resistor in the second branch being the output impedance with the interface output coupled between the resistor and sense transistor of the second branch, and wherein the voltage drop across the output impedance is provided as the control voltage between the sense transistor and current mirror device of the second branch.
10. The method of claim 7 , wherein the supplemental load device that sinks the extra current is a transistor connected between an interface output coupled to the internal circuitry and ground, and having a gate coupled to receive the control voltage, the transistor operating below saturation in a substantially linear region of an operating characteristic of the transistor.
11. An integrated circuit comprising:
at least one power supply pin;
internal circuitry drawing current through the power supply pin; and
an interface circuit electrically coupled between the power supply pin and the internal circuitry and providing a supplemental current load such that total current demand by both the internal circuitry and interface circuit is substantially constant at the power supply pin notwithstanding any current load variation by the internal circuitry.
12. The integrated circuit as in claim 11 , wherein the interface circuit includes means for sensing a current load of said internal circuitry and producing a control voltage corresponding thereto and means responsive to the control voltage for sinking extra current in inverse relation to the sensed current load of the internal circuitry.