PATENT DOCUMENT

Publication Number: US-7772818-B2
Application Number: US-73275607-A
Country: US
Kind Code: B2

Title: Method and apparatus for measuring an average output current of a switching regulator using current-sensing-circuitry

Abstract:
One embodiment of the present invention provides an apparatus that measures the average-output-current produced by a switching regulator within an electronic device. The apparatus includes current-sensing-circuitry coupled to a switching field-effect-transistor (FET) within the switching regulator, wherein the current-sensing-circuitry is configured to bypass a small sense current from the conducting current of the switching-FET according to a sense ratio, wherein the conducting current is controlled by a control signal for the switching regulator. The apparatus also includes a current-to-voltage-converter coupled to the current-sensing-circuitry which is configured to convert the sense current into a sense voltage. The apparatus further includes voltage-averaging-circuitry which is configured to produce an average-sense-voltage from the sense voltage. This sense voltage is coupled to the input of the voltage-average-circuitry through a switch, which is gated by the control signal. The average-output-current of the switching regulator is indicated by the average-sense-voltage.

Claims:
1. An apparatus that measures an average-output-current produced by a switching regulator within an electronic device, comprising:
 current-sensing-circuitry coupled to a switching field-effect-transistor (FET) within the switching regulator, wherein the current-sensing-circuitry is configured to bypass a small sense current from the conducting current of the switching-FET according to a sense ratio, wherein the conducting current is controlled by a control signal for the switching regulator; 
 a current-to-voltage-converter coupled to the current-sensing-circuitry configured to convert the sense current into a sense voltage; and 
 voltage-averaging-circuitry configured to produce an average-sense-voltage from the sense voltage,
 wherein the sense voltage is coupled to the input of the voltage-average-circuitry through a switch; and 
 wherein the switch is gated by the control signal which is directly connected to the switch; 
 
 wherein the average-output-current of the switching regulator is indicated by the average-sense-voltage. 
 
   
   
     2. The apparatus of  claim 1 , wherein the current-sensing-circuitry includes a sense FET coupled in parallel with a low-side switching FET in the switching regulator. 
   
   
     3. The apparatus of  claim 2 , wherein the current-to-voltage converter is configured to provide both the sense FET and the low-side switching FET with the same voltage. 
   
   
     4. The apparatus of  claim 1 , wherein the voltage-averaging-circuitry includes a track-and-hold circuit. 
   
   
     5. The apparatus of  claim 1 , wherein the voltage-average-circuitry is configured to average the sense voltage input when the switch is activated by the control signal and is configured to maintain the average-sense-voltage when the switch is deactivated by the control signal, thereby facilitating tracking of the average-sense-voltage during a period of the control signal. 
   
   
     6. The apparatus of  claim 1 , wherein the average-output-current is used to determine power consumption of the electronic device. 
   
   
     7. The apparatus of  claim 1 , wherein the current-sensing-circuitry, the current-to-voltage-converter, and the voltage-averaging-circuitry are integrated onto an integrated circuit chip. 
   
   
     8. The apparatus of  claim 1 , wherein the current-to-voltage-converter and the voltage-averaging-circuitry are integrated onto an integrated circuit chip. 
   
   
     9. A method for measuring an average-output-current produced by a switching regulator within an electronic device, comprising:
 using current-sensing-circuitry which is coupled to a switching field-effect-transistor (FET) within the switching regulator to bypass a small sense current from the conducting current of the switching-FET according to a sense ratio, wherein the conducting current is controlled by a control signal for the switching regulator; 
 converting the sense current into a sense voltage using a current-to-voltage-converter coupled to the current-sensing-circuitry; and 
 producing an average-sense-voltage from the sense voltage using voltage-averaging-circuitry,
 wherein the sense voltage is coupled to the input of the voltage-average-circuitry through a switch; and 
 wherein the switch is gated by the control signal which is directly connected to the switch; 
 
 wherein the average-output-current of the switching regulator is indicated by the average-sense-voltage. 
 
   
   
     10. The method of  claim 9 , wherein the current-sensing-circuitry includes a sense FET coupled in parallel with a low-side switching FET in the switching regulator. 
   
   
     11. The method of  claim 10 , wherein the current-to-voltage converter is configured to provide both the sense FET and the low-side switching FET with the same voltage. 
   
   
     12. The method of  claim 9 , wherein the voltage-averaging-circuitry includes a track-and-hold circuit. 
   
   
     13. The method of  claim 9 , further comprising:
 averaging the sense voltage input using the voltage-average-circuitry when the switch is activated by the control signal; and 
 maintaining the average-sense-voltage using the voltage-average-circuitry when the switch is deactivated by the control signal, 
 thereby facilitating tracking of the average-sense-voltage during a period of the control signal. 
 
   
   
     14. The method of  claim 9 , further comprising determining a power consumption of the electronic device based on the average-output-current. 
   
   
     15. The method of  claim 9 , wherein the current-sensing circuitry, the current-to-voltage-converter, and the voltage-averaging-circuitry are integrated onto an integrated circuit chip. 
   
   
     16. The method of  claim 9 , wherein the current-to-voltage converter and the voltage-averaging-circuitry are integrated onto an integrated circuit chip.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to techniques for determining the power consumption of an electronic device, such as an integrated circuit (IC) chip. More specifically, the present invention relates to a method and apparatus for determining the power consumption of an electronic device by measuring an average output current generated by a switching regulator that supplies power to the electronic device. 
   2. Related Art 
   Rapid advances in computing technology presently make it possible to perform trillions of operations each second on data sets as large as a trillion bytes. These advances can be largely attributed to an exponential increase in the density and complexity of integrated circuits (ICs). Unfortunately, in conjunction with this increase in computational power, power consumption and heat dissipation of ICs has also increased dramatically. 
   Increasing power consumption and associated heat dissipation creates serious challenges for power management and cooling in computing devices, especially for portable computers. If the real-time power consumption of system components can be measured, the system can provision power to system components more intelligently, and can also adjust cooling mechanisms, for example, by increasing/decreasing fan speed, to more efficiently remove waste heat from the computer system. 
   Modern power supplies within computer systems often utilize switching regulators to provide a substantially constant voltage to drive system components, such as IC chips. A switching regulator typically comprises control logic, a switching circuit, and an “LC tank” circuit. The control logic typically generates two square-wave control signals that are complements of each other. The switching circuit typically includes at least one high-side switching metal-oxide-semiconductor field-effect transistor (MOSFET) and one low-side switching MOSFET, which are coupled in series. The two out-of-phase control signals from the control logic are coupled to the gates of the two switching MOSFETs to drive the two MOSFETs. Because each control signal switches between a high voltage and a low voltage, the two MOSFETs will be turned on and off periodically by the control signals. 
   To convert AC output currents from the MOSFETs into a DC voltage, the MOSFETs are coupled to the LC tank circuit. Specifically, when the high-side MOSFET is turned on and the low-side MOSFET is turned off, the output current flows through the inductor L and capacitor C. This causes energy to be stored within the inductor and the capacitor. A portion of the output current also drives the load. Next, when the high-side MOSFET is turned off and the low-side MOSFET is turned on, the energy stored within the inductor and the capacitor continues to provide a near DC drive current to the load. 
   Note that the output current from the switching regulator to the load can change dynamically during system operation. For example, the output current to a CPU typically increases as the utilization of the CPU increases, whereas the output current to the CPU decreases as the CPU utilization drops. During this time, the voltage on the load remains constant. Consequently, one can monitor the power usage of the load by monitoring the average output current from the switching regulator. 
   One technique to measure the average output current from the switching regulator is to insert a current sensing component, for example, an ammeter, in series with the load. However, this technique can cause significant amount of dissipative loss on the current sensor because the entire output current flows through this current-sensing component. 
   Another technique is to use a component that is already in series with the load to measure the output current. For example, one can obtain the output current by first measuring the voltage across inductor L and then computing the current by dividing the voltage by the resistance of inductor L. Unfortunately, the resistance of inductor L is typically not a constant and is difficult to measure. For example, this resistance can change significantly because of temperature variations. 
   Hence, what is needed is a method and an apparatus for determining the average output current from a switching regulator without the problems described above. 
   SUMMARY 
   One embodiment of the present invention provides an apparatus that measures the average-output-current produced by a switching regulator within an electronic device. The apparatus includes current-sensing-circuitry coupled to a switching field-effect-transistor (FET) within the switching regulator, wherein the current-sensing-circuitry is configured to bypass a small sense current from the conducting current of the switching-FET according to a sense ratio, wherein the conducting current is controlled by a control signal for the switching regulator. The apparatus also includes a current-to-voltage-converter coupled to the current-sensing-circuitry which is configured to convert the sense current into a sense voltage. The apparatus further includes voltage-averaging-circuitry which is configured to produce an average-sense-voltage from the sense voltage. This sense voltage is coupled to the input of the voltage-average-circuitry through a switch, which is gated by the control signal. The average-output-current of the switching regulator is indicated by the average-sense-voltage. 
   In a variation on this embodiment, the current-sensing-circuitry includes a sense FET coupled in parallel with a low-side switching FET in the switching regulator. 
   In a further variation on this embodiment, the current-to-voltage converter is configured to provide both the sense FET and the low-side switching FET with the same voltage. 
   In a variation on this embodiment, the voltage-averaging-circuitry includes a track-and-hold circuit. 
   In a variation on this embodiment, the voltage-average-circuitry is configured to average the sense voltage input when the switch is activated by the control signal and is configured to maintain the average-sense-voltage when the switch is deactivated by the control signal, thereby facilitating tracking of the average-sense-voltage during a period of the control signal. 
   In a variation on this embodiment, the average-output-current is used to determine power consumption of the electronic device. 
   In a variation on this embodiment, the current-to-voltage-converter and the voltage-averaging-circuitry are integrated onto an IC chip. The current-sensing-circuitry can also be integrated onto the IC chip. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1A  presents an exemplary circuit diagram of a typical switching regulator. 
       FIG. 1B  illustrates inductor current as a function of time during switching operation in accordance with an embodiment of the present invention. 
       FIG. 2  illustrates an exemplary circuit diagram for a switching regulator coupled to average-output-current-measurement circuitry in accordance with an embodiment of the present invention. 
       FIG. 3  illustrates an integrated average-current-measurement circuit  300  in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   Overview 
   The present invention determines the real-time power consumption of an integrated circuit (IC) chip by measuring the average output current from the switching regulator that supplies power to the IC chip. Specifically, a current-sensing-circuit is used to bypass a tiny sense current from a main switching-regulator-output-current according to a precise ratio. A current-to-voltage-converter is then used to convert this sense current into a sense voltage, which can be more conveniently manipulated. Next, the sense voltage is averaged using a voltage-averaging-circuit to produce an average readout. In particular, the voltage-averaging operation is synchronized with the switching operation of the switching regulator, thereby allowing the average readout to be an accurate representation of the average output current of the switching regulator. 
   A Typical Switching Regulator 
     FIG. 1A  presents an exemplary circuit diagram of a typical switching regulator  100 . It contains a high-side switching metal-oxide-semiconductor field-effect-transistor MOSFET  102  (“high-side FET” hereafter) and a low-side switching MOSFET  104  (“low-side FET” hereafter), which are both controlled by control logic  106 . In one embodiment of the present invention, MOSFET  102  and  104  are power MOSFETs. 
   Control logic  106  drives the FETs  102  and  104  separately using periodic control signals  108  and  110  (shown as squares waves), wherein the two control signals are complements of each other. Hence, during each control period, the two switching FETs take turns conducting current. 
   More specifically, when high-side FET  102  is turned on by control signal  108  (i.e., when control signal  108  is high and control signal  110  is low), the drain voltage supply “V 1 ” causes a linearly increasing current I L  to flow through inductor  112  and capacitor  114 . This allows energy to be stored in the inductor and the capacitor. Part of the conducting current also drives load  118 . Meanwhile, low-side FET  104  is inactive. 
   When low-side FET  104  is conducting (i.e., when control signal  110  is high and control signal  108  is low), both inductor  112  and capacitor  114  release stored energy to drive load  118 , which causes the inductor current I L  to decease linearly. Note that during each switching cycle, capacitor  114  operates to “smooth out” the ripples in the voltage at node  116  so that load  118  receives a “regulated” voltage. Meanwhile, high-side FET  102  is inactive. 
     FIG. 1B  illustrates inductor current as a function of time during switching operation in accordance with an embodiment of the present invention. Note that inductor current I L  has a sawtooth waveform wherein the shape and amplitude of the sawtooth is determined by the duty cycle of control signals  108  and  110 . Also note that inductor current I L  has an average value I ave  which remains at substantially the same value during the two phases of each switching cycle. Consequently, one can measure the average switching regulator current when either the high-side FET is active or the low-side FET is active. 
   Measuring Average Output Current of the Switching Regulator 
     FIG. 2  illustrates an exemplary circuit diagram for a switching regulator coupled to average-output-current-measurement circuitry in accordance with an embodiment of the present invention. 
   In one embodiment of the present invention, the average-output-current-measurement circuitry includes three subcomponents: a current sensing circuit, a current-to-voltage converter, and a voltage-averaging circuit. We describe each of subcomponents in more detail below. 
   Current Sensing Circuit 
   The embodiment of the present invention illustrated in  FIG. 2  utilizes a current sensing circuit to bypass a tiny sense current from the main current being measured. In this embodiment, a current sensing circuit  202  comprises a sensing FET  204  which is coupled in parallel with a low-side FET  206 . More specifically, the drains and gates of both sensing FET  204  and low-side FET  206  are tied together. Hence, the same control signal that controls low-side FET  206  also controls sensing FET  204 . The source of low-side FET  206  is connected to the ground while the source of sensing FET  204  is used as the output node. Although the sources of the two FETs are not tied together, it is desirable that they have the same voltage. We describe a technique that sets the source voltage of sensing FET  204  to be the same as the source voltage of low-side FET  206  below. 
   Sensing FET  204  conducts a tiny sense current  208 , which is a small fraction of a main switching regulator current  209  conducted by low-side FET  206 . The ratio between main switching regulator current  209  and sense current  208  is proportionate to a predetermined large number. In one embodiment, this ratio is greater than 500. 
   In one embodiment of the present invention, sensing FET  204  and low-side FET  206  are fabricated using the same semiconductor processes and same materials, and therefore the sense ratio can be precisely controlled by the physical design parameters, such as by the gate width to gate length ratio (W/L). Note that if the gate-to-source voltage V GS  and drain-to-source voltage V DS  are substantially the same for these two FETs, the sense ratio equals (W/L) FET 206 /(W/L) FET 204 . Note that both FETs  204  and  206  can be N-type FETs or P-type FETs. Also note that the drain and the source within each FET is interchangeable. 
   Although we describe a current sensing circuit  202  in the context of a simple sensing FET, other techniques can be used. For example, it is possible to bypass a small sense current according to a precise ratio from the main current without using sensing FET  204 . In one embodiment of the present invention, current sensing circuit  202  can be alternatively coupled in parallel with high-side FET  210  to draw a sense circuit when high-side FET  210  is conducting. 
   Note that because sense current  208  is significantly smaller than the main current, sensing circuit  208  consumes very little power and has negligible effect on the switching regulator. However, this current can be difficult to measure because it has an AC behavior and very small amplitude. This problem can be remedied by applying a gain to the sensing current as is described below. 
   Current Gain Stage 
   Sense current  208  in sensing FET  204  is very small and therefore can be difficult to measure. Hence, one embodiment of the present invention provides a gain to the sense current prior to measuring this current. More specifically, this gain can be provided by a current-to-voltage (I-V) converter  212 . Generally, an I-V converter converts a current i to a voltage v according to v=iR, wherein R is a known resistance. As seen in  FIG. 2 , I-V converter  212  comprises: an operational amplifier (op-amp)  214 , a high precision resistor  216  which is coupled to a voltage source “V 2 ”, and a transistor  218  which is coupled between resistor  216  and the output of op-amp  214 . Note that resistor  216  can also be implemented as an active load. 
   Inverting input  220  of op-amp  214  is coupled to the source node of sensing FET  204 . Additionally, non-inverting input  222  of op-amp  214  is connected to the ground, which also brings the voltage at inverting input  220  and the source node of sensing FET  204  to ground (due to a “virtual ground”). Consequently, low-side FET  206  and sensing FET  204  have exactly the same voltages V GS  and V DS , which ensures that the current ratio between the two FETs is based on the predetermined sense ratio. 
   Note that because op-amp  214  and voltage V 2  do not draw current from the inputs, the current flowing through resistor  216  and transistor  218  also equals sense current  208 . Hence, the drain voltage of transistor  218  equals V 2 −I sense R, wherein I sense  represents sense current  208 . Because V 2  and R are both known, this drain voltage can be used as an accurate indicator of sense current  208 . We refer to this drain voltage as sense voltage  224  below. 
   Note that the current gain stage in the present invention is not limited to the specific configuration of I-V converter  212 . Any other circuit that is capable of holding the source node of sensing FET  204  to ground and performing a current-to-voltage conversion can be used in place of I-V converter  212 . 
   Voltage Averaging Circuit 
   Sense voltage  224  is still an AC signal, but is considerably easier to manipulate than sense current  208 . As seen in  FIG. 2 , sense voltage  224  is coupled to a voltage-averaging-circuit  226 , which performs an averaging operation on this input voltage. 
   In one embodiment of the present invention, voltage-averaging-circuit  226  is a track-and-hold (T/H) circuit. This T/H circuit comprises a switch  228  and a capacitor  230 . In one embodiment of the present invention, switch  228  is implemented using a MOSFET  228 . As seen in  FIG. 2 , the gate of MOSFET  228  is coupled to the control signal which controls both low-side FET  206  and sensing FET  204 . Meanwhile, the drain voltage of MOSFET  228  is coupled to sense voltage  224 . 
   The averaging operation in  FIG. 2  proceeds as follows.
         Tracking Phase: When the control signal is high, MOSFET  228  is turned on. This allows sense voltage  224  to charge or discharge capacitor  230 , depending on the previous voltage value at node  232 . The charging/discharging process “smoothes out” the input waveform and results in a near constant voltage  V  on node  232 . Note that this average process is synchronized with the time window when low-side FET  206  is conducting. More specifically, it begins when FET  206  turns on and ends when FET  206  turns off. Consequently, the average voltage  V  at node  232  provides an accurate representation of sense current  208 .   Holding Phase: When the control signal is low, MOSFET  228  is turned off. Because there is no current path for capacitor  230 , capacitor  230  holds the average voltage  V  at node  232  until next control signal period begins.       

   Note that the average voltage  V  is valid during a full control logic period. This is possible because the control signal for the low-side FET is also used to synchronize the current sensing at sensing FET  204 , and the voltage averaging at node  232 . In one embodiment of the present invention, the average voltage  V  at node  232  can be accurately measured by inserting a voltage follower  234  which decouples the capacitor  226  from the voltage measuring mechanism at V out    236 . 
   Because the average output voltage  V  can be measured, an average sense current Ī sense  is obtained according to (V 2 −  V )/R. Consequently, the average current produced by the switching regulator during each control period equals N(V 2 −  V )/R, wherein N is the sense ratio. Furthermore, the power consumed by the load can be obtained by multiplying this average current by the constant voltage on the load. 
   Note that although we describe using a simple MOSFET and a capacitor to perform a timed voltage averaging operation, other circuits that perform a timing controlled voltage averaging function can be used in place of voltage averaging circuit  226 . 
   Integrated Circuit 
   Note that the above described three circuit modules can be integrated into a single IC module. 
   For example,  FIG. 3  illustrates an integrated average-current-measurement circuit  300  in accordance with an embodiment of the present invention. Specifically, average-current-measurement circuit  300  includes sensing FET  302 , I-V converter  304 , and voltage-averaging-circuit  306 , which are coupled together in the same manner as described in  FIG. 2 . IC  300  further comprises an input  308  for receiving the control signal for sensing FET  302  and circuit  306 , and an input  310  which is coupled to the drain of low-side FET of the switching regulator. IC  300  also comprises a single voltage output  312 . 
   In a further embodiment, the low-side FET and the sensing FET can be replaced by an integrated current sensing FET, which comprises a main power FET and a sense FET. In this embodiment, the I-V converter and the voltage-averaging-circuit can be further integrated into an IC chip. This integrated IC has a number of inputs, which include a current input from the sense FET, and a control input from the control logic. The IC provides a single voltage output which represents the average switching regulator current. 
   Note that in both embodiments, the integrated ICs can be disabled by the control signal input. 
   The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.

Metadata:
Filing Date: 20070403
Publication Date: 20100810
Grant Date: 20100810
Priority Date: 20070403
Inventors: SMITH ERIC
Assignee: APPLE INC
CPC Classifications: [{"code": "G01R19/0092", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R19/003", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R19/0092", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R19/003", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 39826390