# Patent

## Title
DC-DC converter switching transistor current measurement technique

## Abstract

A method is described comprising conducting a first current through a switching transistor. The method also comprises conducting a second current through a pair of transistors whose conductive channels are coupled in series with respect to each other and are together coupled in parallel across the switching transistor's conductive channel. The second current is less than and proportional to the first current.

## Background
PRIORITY APPLICATION
This application is a divisional of U.S. application Ser. No. 11/173,760, filed Jun. 30, 2005 now U.S. Pat. No. 8,134,548, which is incorporated herein by reference in its entirety.


BACKGROUND
DC-DC converters typically craft a DC voltage by full wave rectifying and filtering one or more time varying signals. Because of the switching undertaken in the full wave rectification process, significant amounts of current are frequently “switched” back-and-forth at rapid pace by large transistors. It is often helpful to measure the current through these transistors to, for instance, determine whether or not the DC-DC converter is being loaded, monitor any ripple currents resulting from rectification, etc.
Two “straight-forward” techniques are readily known in the art for measuring current: 1) shunt inductance; and, 2) series resistance. Shunt inductance induces a current measurement signal in an inductor by coupling magnetic fields that are produced by the current signal being measured through the inductor. Unfortunately, shunt inductance is not practical for rapidly changing currents because the bandwidth of an inductor is limited (i.e., the inductor will increasingly attenuate the current measurement signal as its frequency increases).
The series resistance technique, which is shown in FIG. 1, does not typically suffer from limited bandwidth issues because a pure resistance does not change its resistive properties as a function of signal frequency. Unfortunately, however, the series resistance technique is also not practical for large currents (such as those drawn by a DC-DC converter's switching transistors) because a large current being driven through a resistance will tend to dissipate large amounts of power (through the relationship P=I2R) which may result in overheating; or, if the power “problem” is handled by using a very small series resistance, inaccuracy results because the signal V=I*R may become too small to measure.



FIGURES
The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:
FIG. 1 shows resistance couples in series with a DC-DC converter switching transistor;
FIG. 2 shows an embodiment of a current measurement circuit coupled to a DC-DC converter switching transistor;
FIG. 3 shows an equivalent circuit for the circuit of FIG. 2;
FIG. 4 shows another circuit that measures DC-DC converter switching transistor current using a similar M1 through M4 structure as shown in FIG. 2;
FIG. 5 shows an extension of the circuit of FIG. 4 capable of providing an indication of the output current from the pair of switching transistors Q1 and Q2;
FIG. 6 shows a computing system whose processor(s) include a DC-DC converter.



DETAILED DESCRIPTION
FIG. 2 shows a circuit directed to measuring the IDS current of transistor Q1 without the use of a series resistance through which the IDS current flows. Here, transistor Q1 may be a large switching transistor whose drain-to-source current (IDS) is rapidly changing with large amplitude. A pair of transistors M1, M2 having their conductive channels coupled in series (where, the conductive channel of a transistor is understood to be the channel between its drain and source nodes). The drain of M1 is coupled to the drain of the switching transistor's conductive channel. The source of M2 is coupled to the source of the switching transistor's conductive channel (through the ground node). A node between the M1 and M2 transistors is coupled to an input of an amplifier (the source node of M5). A second pair of transistors M3 and M4 have their conductive channels coupled in parallel (their drains are tied together and their source nodes are tied to ground) and a node between the second pair of transistors M3, M4 is coupled to a reference input of the amplifier (the source node of M6).
According to the theory of operation of the circuitry of FIG. 2, a current flow of approximately kIDS will flow through transistor M1, where k<1 (e.g., in many applications k is expected to be within a range of 0.0001 to 0.01 inclusive). The kIDS current flow through transistor M1 influences an output signal Io which can be used as an indicator of the current IDS through transistor Q1. In a typical implementation transistors M1 and M2, as well as M3 and M4 are scaled down versions of transistor Q1 (e.g., being designed with identical doping profiles and gate lengths but with different gate widths). Since a large transistor Q1 is often implemented as a parallel connection of many identical small transistors (legs), the smaller transistors M1, M2, M3, and M4 can be implemented using just a few of the same small transistors.
The proportionality factor can be approximately computed from the transistors' widths as 1/k=WQ1*(1/WM1+1/WM2). In some applications a value of k>1 may be desirable. The M1 and M2 structure essentially enables fast and accurate measurement of the IDS current without actually imposing a series resistance through the IDS current path. To ensure proper operation, M1 and M3 may be matched and M2 and M4 may be matched respectively. To improve the accuracy at larger drain-to-source voltages VDS in Q1, especially, when Q1 is operating in saturation, M1 may be chosen to be smaller than M2. To enhance the output signal Io, M1 may be chosen to be larger than M2.
Transistors M5, M6, M7 and M8 form a common gate amplifier having: 1) a reference leg M6, M7 whose current, IBIAS, flows into the resistive network formed by transistors M3 and M4; and, 2) a measurement leg M5, M8 whose current Ix flows into the resistive network formed by transistors M1 and M2.
The principle of operation of the entire circuit is that voltage changes in proportion to kIDS at the source of transistor M5 causes the gate-to-source voltage of transistor M5 to differ with respect to that of transistor M6 such that an output current Io is created having a component that varies in proportion to kIDS. The current IBIAS may be injected using various techniques, such as, e.g., using a reference current circuit or a resistor connected to the gates of M6 and M7.
The principle of operation of the circuit of FIG. 2 is more easily viewed with the equivalent circuit of FIG. 3. Comparing FIGS. 2 and 3, note that: 1) transistor M1 has been replaced by a variable resistance R1 and a voltage source VDS causing a current of kIDS; 2) transistor M2 has been replaced by resistance R2; 3) transistor M3 has been replaced by variable resistance R3; and, 4) transistor M4 has been replaced by resistance R4.
Transistors M2 and M4 are designed to be in the linear mode over the range of operation for the circuit. Therefore, M2 and M4 will behave like resistors (i.e., approximately a linear relationship between its drain-to-source voltage and its drain-to-source current). Moreover, like M2 and M4, transistors M1 and M3 are also ideally designed to remain in linear mode of operation, when transistor Q1 is in linear mode, and to be in saturation mode when Q1 is in saturation mode. In order to help effect this behavior, the gates of transistors M2 and M4 are tied to a fixed voltage (VCC) and the gates of transistors M1 and M3 are tied to the gate of transistor Q1.
With transistors M5 and M6 in saturation mode, and with transistors M7 and M8 forming a current mirror, it can be shown that the output current Io for the circuit of FIG. 3 can be approximated as:

Io≈kIDS(R1/(R1//R2+R5))+((R1//R2)−(R3//R4))/(R1//R2+R5))IBIAS  Eqn. 1

where IDS is the current being measured (i.e., the drain-to-source current of transistor Q1), k is a proportionality constant between the drain-to-source currents of transistors Q1 and M1, and R5 is the common-gate input resistance of transistor M5. Moreover,

R3//R4=(R3R4)/(R3+R4)  Eqn. 2a

R1//R2=(R1R2)/(R1+R2)  Eqn. 2b

which corresponds to the effective resistances of resistors R3 and R4 in parallel and R1 and R2 in parallel, respectively. Since R1=R3 and R2=R4, IBIAS does not add to the output signal Io. The value of R5 depends on the transconductance parameter 13 of M5 and on the bias current IBIAS:

R5=1/sqrt(2βM5(IBIAS−Io))  Eqn. 3

In a typical design R5 may be large compared to R1//R2, e.g., ten times larger, to reduce the power consumption of the common-gate amplifier. In other designs R5 may be comparable to or even smaller than R1//R2 in order to improve linearity. To ensure proper operation the bias current should be larger than the maximum output current: IBIAS>Io. For negative currents Io<0 the bias current may be reduced to a very small value, e.g., by operating M5 and M6 at or slightly above threshold. The presence of M3 also provides for good suppression of noise from the gate node of transistor Q1 since the noise injected through the gate capacitances of M1 and M3 approximately cancels out. Note that although the reference voltage of FIGS. 2 and 3 correspond to a ground node, another fixed voltage could be used (such as a supply node) provided appropriate offset were applied to VCC.

Other Circuits
FIG. 4 provides another circuit design that uses the M1 through M4 structure as a device for measuring IDS, but uses a different common-gate amplifier, formed by M5 through M11, to generate the output signal Io. This amplifier is essentially equivalent to a combination of two replicas of the amplifier M5 through M8 in FIG. 2 and provides improved range and linearity at a reduced bias current.
When the current IDS of Q1 is small, i.e., the output signal is approximately −IBIAS<Io<+IBIAS, both parts of the amplifier, M5 through M8 and M9 through M11 respectively contribute to the output signal Io. When current is large and positive, i.e., Io>+IBIAS, M9 through M11 will increase their contribution to Io due to the nonlinearity of the circuit, whereas M5 through M8 will reduce and eventually cease their contribution to the output signal.
Furthermore, when the current is large and negative, i.e., Io<−IBIAS, M9 through M11 will reduce their contribution to Io and M5 through M8 will take over. Thus, the non-linearities of both halves of the amplifier compensate each other, resulting in improved linearity and range at a smaller bias current. The method for properly choosing the bias current IBIAS and the sizes of M5 through M11 usually involves simple calculations and circuit simulations, which anyone skilled in the art can easily carry out.
FIG. 5 shows a circuit that measures the current through both of switching transistors Q1 and Q2. M13 through M16 are the PMOS equivalent of M1 through M4 respectively, and, M7, M8, M11, M12 are coupled so that they become the PMOS equivalent of M5, M6, M10 and M9 respectively. Arrangement in this manner allows for an output Io that varies with IDS—Q2-IDS—Q1.
Possible Applications of Current Measurement Circuits
It is envisioned that embodiments of the current measurement approaches described herein can be used in “on-chip” DC-DC converters. A DC-DC converter is a device that converts a first fixed voltage into a second fixed voltage. Here, for example, transistor Q1 (and transistors Q1 and Q2 in FIG. 4) can be a large switching transistor in an “on-chip” DC-DC converter (and transistors Q1 and Q2 can be large switching transistors in an “on-chip” DC-DC converter).
The current measurement circuit can be used for various functions such as, to name a few: 1) soft-switching, i.e., to monitor the currents through switching transistors in order to determine the proper time for turn-off; 2) monitoring the ripple current in DC-DC converters; 3) “safe turn off” in “on-chip” DC-DC converters (e.g., turning off the DC-DC converter when the inductor currents flowing through the switching transistors are small enough to prevent voltage overshoot); and, 4) monitoring the load current, e.g., in “on-chip” DC-DC converters used to a CPU (central processing unit) or part of a CPU.
FIG. 6 shows an embodiment of a computing system. The exemplary computing system of FIG. 6 includes: 1) one or more processors 601 having an “on-chip” DC-DC converter 610; 2) a memory control hub (MCH) 602; 3) a system memory 603 (of which different types exist such as DDR RAM, EDO RAM, etc); 4) a cache 604; 5) an I/O control hub (ICH) 605; 6) a graphics processor 606; 6) a display/screen 607 (of which different types exist such as Cathode Ray Tube (CRT), Thin Film Transistor (TFT), Liquid Crystal Display (LCD), DPL, etc.; 8) one or more I/O devices 608.
The one or more processors 601 execute instructions in order to perform whatever software routines the computing system implements. The instructions frequently involve some sort of operation performed upon data. Both data and instructions are stored in system memory 603 and cache 604. Cache 604 is typically designed to have shorter latency times than system memory 603. For example, cache 604 might be integrated onto the same silicon chip(s) as the processor(s) and/or constructed with faster SRAM cells whilst system memory 603 might be constructed with slower DRAM cells.
By tending to store more frequently used instructions and data in the cache 604 as opposed to the system memory 603, the overall performance efficiency of the computing system improves. System memory 603 is deliberately made available to other components within the computing system. For example, the data received from various interfaces to the computing system (e.g., keyboard and mouse, printer port, LAN port, modem port, etc.) or retrieved from an internal storage element of the computing system (e.g., hard disk drive) are often temporarily queued into system memory 603 prior to their being operated upon by the one or more processor(s) 601 in the implementation of a software program.
Similarly, data that a software program determines should be sent from the computing system to an outside entity through one of the computing system interfaces, or stored into an internal storage element, is often temporarily queued in system memory 603 prior to its being transmitted or stored. The ICH 605 is responsible for ensuring that such data is properly passed between the system memory 603 and its appropriate corresponding computing system interface (and internal storage device if the computing system is so designed). The MCH 602 is responsible for managing the various contending requests for system memory 603 access amongst the processor(s) 601, interfaces and internal storage elements that may proximately arise in time with respect to one another.
One or more I/O devices 608 are also implemented in a typical computing system. I/O devices generally are responsible for transferring data to and/or from the computing system (e.g., a networking adapter); or, for large scale non-volatile storage within the computing system (e.g., hard disk drive). ICH 605 has bi-directional point-to-point links between itself and the observed I/O devices 608.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

## Claims

1. A computing system, comprising:
one or more processors formed on a single chip;
a direct-current to direct-current (DC-to-DC) converter circuit coupled to the one or more processors and formed on the single chip, the DC-to-DC converter circuit including
a switching transistor, and
a current measurement circuit, the current measurement circuit having a first pair of transistors with their respective conductive channels coupled in series, a drain of one of the first pair of transistors coupled to a first end of a conductive channel of the switching transistor, and a source of the other of the first pair of transistors coupled to a second end of the conductive channel of the switching transistor;

an amplifier having a measurement leg coupled to a node between the first pair of transistors; and
a second pair of transistors having their respective conductive channels coupled in parallel, a node between the second pair of transistors being coupled to a reference leg of the amplifier.

2. The computing system of claim 1, further comprising:
a memory controller coupled to the one or more processors; and
a graphics processor coupled to the memory controller.

3. The computing system of claim 1, wherein the transistors of the first pair of transistors and the transistors of the second pair of transistors are all approximately the same size.

4. The computing system of claim 1, wherein a transistor from the first pair of transistors and a transistor from the second pair of transistors each have their respective gates coupled to a gate of the switching transistor.

5. The computing system of claim 1, wherein a first transistor from the first pair of transistors and a first transistor from the second pair of transistors each impose substantially the same resistance in the DC-to-DC converter circuit, the resistance being greater than the resistance imposed by a second transistor of the second pair of transistors.

6. The computing system of claim 1, wherein the amplifier comprises a common gate amplifier.

7. The computing system of claim 1, wherein a first transistor of the first pair of transistors is to provide to a second transistor of the first pair of transistors a current that is approximately proportional to the current within the conductive channel of the switching transistor.

8. The computing system of claim 7, wherein a proportionality constant relating the current from the first transistor to the current within the conductive channel of the switching transistor is less than 1.

9. The computing system of claim 8, wherein the proportionality constant is within a range from about 0.0001 to about 0.01.

10. A method to monitor current usage in a computing system, the method comprising:
using one or more processors to monitor current through a switching transistor of a direct-current to direct-current (DC-to-DC) converter circuit, the method including
conducting a first current through the switching transistor, and
conducting a second current through a pair of transistors having serially-coupled conductive channels, the pair of transistors passing the second current through the conductive channels, the pair of transistors coupled in parallel across a conductive channel of the switching transistor, the second current being less than and approximately proportional to the first current.

11. The method of claim 10, further comprising conducting a third current from a reference node of an amplifier to a node between a second pair of transistors that are coupled in parallel with each other, a gate of one transistor of the second pair of transistors being coupled to a gate of the switching transistor.

12. The method of claim 10, wherein the monitoring of the current through the switching transistor is to determine a time to turn-off the computing system.

13. The method of claim 10, wherein the monitoring of the current through the switching transistor is to monitor a ripple current in the DC-to-DC converter circuit.

14. The method of claim 10, wherein the monitoring of the current through the switching transistor is to determine a load current used to power the one or more processors.

15. The method of claim 10, further comprising generating a signal that varies linearly with a difference between the first current and another current flowing through a second switching transistor.

16. An apparatus, comprising:
a direct-current to direct-current (DC-to-DC) converter circuit including
a first switching transistor,
a second switching transistor, and
a current measurement circuit for each of the first switching transistor and the second switching transistor, each current measurement circuit having a pair of transistors with serially-coupled conductive channels, a drain of one of each of the pair of transistors coupled to a first end of a conductive channel of a respective one of the first switching transistor and the second switching transistor, and a source of the other one of each of the pair of transistors coupled to a second end of the conductive channel of each of the respective ones of the first switching transistor and the second switching transistor; and

an amplifier having a measurement leg coupled to a node between each of the pairs of transistors, the amplifier to generate an output that varies according to a differential between the current flowing through the first switching transistor and the second switching transistor.

17. The apparatus of claim 16, wherein each of the transistors in the current measurement circuit have approximately the same doping profile and gate length as the first switching transistor.

18. The apparatus of claim 16, wherein each of the transistors in the current measurement circuit have different gate widths than the first switching transistor.

19. The apparatus of claim 16, wherein a proportionality constant relating a current from the current measurement circuit to a current within a conductive channel of each of the switching transistors, respectively, is related to gate widths of each of the transistors in the current measurement circuit.

20. A computing system, comprising:
one or more processors formed on a single chip;
a direct-current to direct-current (DC-to-DC) converter circuit coupled to the one or more processors and formed on the single chip, the DC-to-DC converter circuit including
a switching transistor, and
a current measurement circuit, the current measurement circuit having a pair of transistors with their respective conductive channels coupled in series, a drain of one of the pair of transistors coupled to a first end of a conductive channel of the switching transistor, and a source of the other of the pair of transistors coupled to a second end of the conductive channel of the switching transistor; and

a common gate amplifier having a measurement leg coupled to a node between the pair of transistors.

21. The computing system of claim 20, further comprising a second pair of transistors having their respective conductive channels coupled in parallel, a node between the second pair of transistors being coupled to a reference leg of the common gate amplifier.

22. A computing system, comprising:
one or more processors formed on a single chip;
a direct-current to direct-current (DC-to-DC) converter circuit coupled to the one or more processors and formed on the single chip, the DC-to-DC converter circuit including
a switching transistor; and
a current measurement circuit, the current measurement circuit having a pair of transistors with their respective conductive channels coupled in series, a drain of a first of the pair of transistors coupled to a first end of a conductive channel of the switching transistor, and a source of a second of the pair of transistors coupled to a second end of the conductive channel of the switching transistor, the first transistor of the pair of transistors being configured to provide to the second transistor of the pair of transistors a current that is approximately proportional to the current within the conductive channel of the switching transistor; and

an amplifier having a measurement leg coupled to a node between the pair of transistors.

23. The computing system of claim 22, further comprising a second pair of transistors having their respective conductive channels coupled in parallel, a node between the second pair of transistors being coupled to a reference leg of the amplifier.