# Patent

## Title
Integrated current sensor

## Abstract

A current sensor includes coupled inductors that generate an output current responsive to a detected current. The coupled inductor is implemented in an integrated circuit. An integrator circuit generates a sensed voltage responsive to the output current.

## Background
TECHNICAL FIELD OF THE INVENTION
The present invention relates to current sensors, and more particularly, to an improved current sensor having low loss, small size, low cost and high accuracy.
BACKGROUND OF THE INVENTION
Within various circuit implementations, such as power supplies, there is often the need to detect a current provided at a particular point within a circuit to use as feedback for controlling other parts of the circuit. Various solutions are presently used to sense currents within electronic circuits but each of these suffer from various shortcomings. A first approach, illustrated in FIG. 1, utilizes a resistor 102 connected across the inputs of an operational amplifier 104 to provide a voltage VSENSE that may be used to determine a current 106. A low value resistor 102 may be used in the range of 10 milliohms. The problem with this approach is the high loss provided by the circuit. This may be overcome by reducing the resistor 102 to reduce the loss, however, this also reduces the signal VSENSE that may be detected. The resistor 102 is not integrated and this type of circuit may be used to sense current within direct current (DC) applications.
Referring now to FIG. 2, there is illustrated a further prior art system utilizing a hall effect device 202 connected across the inputs of an operational amplifier 204. The hall effect device 202 generates a voltage across the inputs of the operational amplifier 204 responsive to the current 206 to provide the output signal VSENSE. While this approach has a low loss, the use of the hall effect device 202 causes the circuit to have a higher cost, and the accuracy and noise issues are greater within the hall device as the hall voltage is a small value. This circuit may also be used to detect current in a direct current (DC) system.
Referring now to FIG. 3, there is illustrated the use of a magneto resistive sensor. The magneto resistive sensor consists of a magneto resistive element 302 connected across the inputs of operational amplifier 304 to detect the current 306. The magneto resistive element 302 has the property that the resistance of the element changes with respect to the magnetic field caused by the current 306. This circuit requires the use of special technology which raises the cost of the device. Additionally, accuracy issues arise even though the current may be sensed with very low loss.
Referring now to FIG. 4, an alternative prior art method for detecting current is through the use of a current transformer 402 is illustrated. The current transformer has a primary side 404 with a single loop and a secondary side 406 with multiple loops. A load resistance 408 is in parallel with the secondary side 406 of the transformer 402. The current transformer 402 is used to detect the current 410. The transformer 402 creates an output current equal to Ip/n with Ip being the detected current and n being the turns ratio of the transformer 402. The resistance of the transformer is reflected to the primary side with the ratio 1/n2. A current transformer will only work within alternating current (AC) circuits. While current transformers work well for detecting currents, they are large and have a medium loss level associated therewith. Thus, some method for detecting a current within a power electronic circuit that overcome the shortcomings of these prior art methods would be greatly desirable.
Another method for measuring currents involves the use of a Rogowski coil. The voltage induced in a Rogowski coil is very small and easily disturbed when measured current is less than, for example, 100 Amps. A Rogowski current transducer has a number of advantages over the current transformer illustrated in FIG. 4. For example, it is linear, has no core saturation effects and has wide band width, wide measurement range and a simple structure. This Rogowski coil comprises a toroidal winding placed around a conductor being measured. It consists of a wire wound on a non-magnetic core. The coil is effectively a mutual inductor coupled to the inductor being measured where the output from the winding is an EMF proportional to the rate of change of current.
SUMMARY OF THE INVENTION
The present invention disclosed and claimed herein, in one aspect thereof, comprises a current sensor. The current sensor includes coupled inductors for generating an output current responsive to a detected current. The coupled inductor is implemented in an integrated circuited. An integrator circuit generates a sensed voltage responsive to the output current.



BRIEF DESCRIPTION OF THE DRAWINGS 
For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying Drawings in which:
FIG. 1 illustrates a prior art current sensor;
FIG. 2 illustrates a further prior art current sensor;
FIG. 3 illustrates yet another prior art current sensor;
FIG. 4 illustrates a further prior art current sensor;
FIG. 5a illustrates a coil in close proximity with a large current carrying wire according to the present disclosure;
FIG. 5b illustrates a perspective cut away view of the integrated circuit, including a coupled coil and wire;
FIG. 6 is a cross-sectional view of a first embodiment an integrated current sensor package;
FIG. 7 is a top view of the first embodiment of the integrated current sensor package;
FIG. 8 is a simulation of the integrated current sensor illustrated in FIGS. 6 and 7;
FIG. 9 is a cross-sectional view of an alternative embodiment of the integrated current sensor package;
FIG. 10 is a top view of the alternative embodiment of the integrated current sensor package of FIG. 9;
FIG. 11 is a simulation of the alternative embodiment of the integrated current sensor package illustrated in FIGS. 9 and 10;
FIG. 12 is a schematic diagram of the integrated current sensor;
FIG. 13 is a schematic diagram illustrating the integrated current sensor within a switched power supply circuit;
FIG. 14 is a timing diagram illustrating operation of the switched power supply circuit of FIG. 13;
FIG. 15 illustrates a further method for controlling the reset switch of the integrated current sensor;
FIG. 16 is a top view of the further embodiment of the integrated current sensor package;
FIG. 17 is a cross-sectional view of the embodiment of the integrated current sensor package in FIG. 16; and
FIG. 18 is a further cross-sectional view of the embodiment of the integrated current sensor package in FIG. 16.



DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 5a, there is illustrated a coil 502 in close proximity with a large current carrying wire 504 such that the coil 502 and current carrying wire 504 act as coupled inductors. The coupled inductors, along with on-chip electronics which will be discussed herein below, allow the creation of the VSENSE signal which is proportional to the input current Ip in a manner that has very low loss, is very small and is a low cost implementation. This provides a better solution than all of the implementations described with respect to FIGS. 1-4. The current provided through the current carrying wire may be up to 10 amps. The coil 502 is placed very near the current carrying wire 504 in order to create the inductive coupling between the wire 504 and coil 502. The wire 504 only overlaps only one side of the coil 502 such that the winding are all going the same way and the magnetic flux will add together. This causes an induced current in the other side of the coil 502 that is not overlapped by the wire 504.
Referring now to FIG. 5b, there is illustrated a perspective cut away view of the coil 502 and wire 504 illustrated in FIG. 5a. In this configuration one of the coupled inductors is placed within a silicon dioxide layer 604 on top of a die 606 of an integrated circuit chip. The coil 502 consists of metal runs within the M5 layer of the silicon dioxide layer 604. The wire 504 would rest on the silicon dioxide layer 604 in close enough proximity to the coil 502 such that the current passing through wire 504 would induce another current within the portion of the coil 502 over which the wire 504 was not located.
There are multiple ways for implementing the coupled inductor configuration illustrated in FIG. 1 within a chip package. The first of these comprises an on-chip solution with bumping copper as illustrated in FIG. 6. Flip chip bump houses can deposit a copper wire 602 on top of a silicon dioxide layer 604 of a die 606. The copper wire 602 may comprise 15 μm of copper. The coil 502 is embedded within the M5 layer of the silicon dioxide layer 604.
Referring now to FIG. 7, there is illustrated a top view of the package configuration. The copper wire 602 is placed upon the silicon dioxide layer 604 of the die 606 (not shown). The coil 502 is placed within the M5 layer of the silicon dioxide 604 parallel to the wire 502. Bond wires 702 connect the copper wire 602 above the die 606 with external outputs. The bond wires 702 support a maximum current of 1-2 amps, thus many bond wires are required to be connected to the copper wire 602 for higher currents. Additional bond wires 704 connect the silicon dioxide layer 604 of the die 606 to external output pins 706 of the chip. Using the above-described package configuration, a 10 amp sensor may be constructed.
Referring now to FIG. 8, there is provided a simulation of the inductive coil package illustrated in FIGS. 6 and 7. The coil 802 on the primary side comprises a 500 pH coil. The coil 804 on the secondary side comprises a 2 μH coil. Connected to a first side of the 500 pH coil 802 is a 1.5 milli-ohm resistor in series with a 0.5 milli-ohm resistor 810 which comprises the parasitic capacitance of the coil 802. Connected to one output side of the 2 μH coil 804 is a 20 kiliohm resistor 812 which comprises the parasitic capacitance of the coil 804. The 0.5 milli-ohm transistor 810 comprises the resistance provided by the bond wires and package. Since the copper wire 602 is not too thick and lies very close to the coil 502 of the chip, coupling coefficients between the copper wire 602 and the coil 502 are very good, assuming there is a distance of approximately two micrometers from the M5 layer to the copper wire 602.
Referring now to FIG. 9, there is illustrated an alternative configuration wherein a package lead frame and flip chip configuration are used. A custom package lead frame may be designed as follows. The die 902 is placed upside down with the silicon dioxide layer 904 suspended a short distance above a large copper slug 906. The copper slug 906 may have a large cross sectional area for low loss. In this embodiment the slug 906 has a 200×200 μm cross section. The die 902 is suspended above the copper slug 906 on solder balls 908. The solder balls 908 rest on top of a lead frame 910. When heat is applied to the circuit, the solder bumps 908 reflow causing the silicon dioxide layer 904 to rest directly upon the copper slug 906. In this design, the die chip 902 would be bumped and then flipped.
Referring now to FIG. 10, there is illustrated a top view with the silicon dioxide layer 904 resting on top of the copper slug 906. Bond wires 1002 may then be connected to the silicon dioxide layer 904. This design has a very low resistance.
Referring now to FIG. 11, there is illustrated the simulation of the embodiment illustrated in FIGS. 9 and 10. In this simulation wherein a 200×200 μm copper slug 906 is utilized that is 3 μm away from the coil 502, the primary side consists of a 520 pH coil 1102 in series with a 0.5 milli-ohm resistor 1104. The secondary side consists of 2 μH coil 1106 in series with a 20 kili-ohm resistor 1108. The coupling coefficient is reduced due to the lower current density in the slug.
Referring now to FIG. 16 there is illustrated a bottom view of a further configuration wherein a lead on chip configuration is used. The lead frame 1602 is connected to the die 1604 by bond wires 1606. The wire 1608 in connected to the die 1604 by tape. The wire 1608, a current carrying conductor, is coupled to a coil in the die 1604.
Referring now to FIG. 17, there is illustrated a cross sectional view of FIG. 16 along line B-B. The die 1604 is connected to the wire 1608 via the tape 1702 as described previously. The lead frame 1602 connects to the die 1604 via bond wires 1606. The tape 1702 is approximately 75 μm thick. The entire structure is contained within a mold compound 1704. Referring now to FIG. 18, there is illustrated a cross sectional view of FIG. 16 along line A-A.
Referring now to FIG. 12, there is illustrated a schematic diagram of the electronic circuit necessary for recreating the VSENSE signal when detecting the current Ip using the coupled indicator as illustrated in FIG. 5. The coupled inductor 1202 comprises either of the configuration packages described hereinabove or, alternatively, may comprise a different undescribed configuration package that places the coil in close proximity with the wire to inductively couple them together. The primary side is modeled by inductor 1204 in series with resistor 1206. The secondary side is modeled by inductor 1208 which is connected to a resistor 1210. The resistor 1210 is then connected to ground. The other side of inductor 1208 outputs the induced current In which is connected to the negative input of an operational amplifier 1212. The positive input of operational amplifier 1212 is connected to ground.
The current through the secondary is dominated by the resistive loss of resistor 1210 and is the derivative of the primary current. An integrator circuit 1218 is used to integrate the induced current In. The integrator circuit 1218 consists of the operational amplifier 1212, a capacitor 1214 connected between the output of operational amplifier 1212 and the negative input of operational amplifier 1212 and a reset switch 1216 connected between the output of operational amplifier 1212 and the negative input of operational amplifier 1212 in parallel with capacitor 1214. Thus, the current In may be determined according to the equation:




  
    I
    n
  
  =
  
    
      Lm
      
        R
        1
      
    
    ⁢
    
      
        d
        ip
      
      
        d
        t
      
    
  




By integrating on the capacitor 1214 an output voltage VSENSE is attained according to the following equation:





  
    V
    SENSE
  
  =
  
    
      
        1
        C
      
      ⁢
      
        ∫
        
          
            I
            n
          
          ⁢
          
            d
            t
          
        
      
    
    =
    
      
        
          L
          m
        
        
          
            R
            1
          
          ⁢
          C
        
      
      ·
      ip
    
  




In this case, Lm, the mutual inductance is well controlled, but can vary from part to part due to assembly variations. The capacitance C will vary from part to part and probably can be controlled to +/−5% accuracy. The capacitor 1214 will not have any appreciable temperature coefficient. R1 is dominated by the metal resistance of the coil and will vary from part to part. It is equal to the value of the resistor 1210 and also has a large temperature coefficient.

In order to obtain overall accuracy for the capacitance C which varies from part to part, a factory calibration using a one time programmable (OTP) memory 1220 can be used. In a preferred embodiment, a low cost 32 bit OTP memory may be utilized. The OTP memory 1220 provides a control variable to a programmable gain amplifier 1222. The first gain stage 1223, consisting of programmable amplifier 1222, programmable resistance 1224 and the OTP memory 1220, compensates for part to part variations of the circuit. The OTP memory 1220 is programmed at the factory based upon measurements made there. The programmable gain amplifier 1222 has its negative input connected to the output of the operational amplifier 1212. A programmable resistance 1224 is connected between the output of the programmable amplifier 1222 and ground. The positive input of programmable amplifier 1222 is connected to the programmable resistance 1224. The value of the programmable resistance 1224, and thus the gain of the first gain stage 1223, is controlled by the values provided from the OTP memory 1220.
A second gain stage 1226 compensates for differences in the resistance caused by temperature variations in the device. A temperature sensor 1228 and A-D converter 1230 are used to generate a digital temperature value to compensate for the coil resistance temperature coefficient. The temperature sensor 1228 detects the temperature and generates an analog representation of the temperature. The ADC1230 converts the analog signal into a digital signal. The digital temperature value is provided via a control bus 1231 to control logic 1232. In one embodiment the control logic 1231 may consist of a look-up table. The control table would include various control values associated with particular temperature values. Alternative embodiments may include a microprocessor programmed to control the output according to various temperature levels or other types of digital logic. The control logic 1232 provides a control value to the programmable gain amplifier 1234 and programmable resistance 1236. The negative input of programmable amplifier 1234 is connected to the output of programmable amplifier 1222. The programmable resistor 1236 is connected between the output of programmable amplifier 1234 and ground. The positive input of programmable amplifier 1234 is connected to the programmable resistance 1236. The particular value of the programmable resistance 1236 , and thus the gain of the second gain stage 1226, is controlled via the output from the control logic 1232. The output of programmable amplifier 1234 provides the compensated VSENSE signal. The code provided by the control logic 1232 is updated during the phase in which the operational amplifier 1212 is reset responsive to a reset signal applied to switch 1216 the reset signal is applied while the sensed current ip is zero.
The current sensor is designed to be used in, for example, a switched power supply. When the current ip is equal to zero, a reset signal may be applied to switch 1216 to reset the capacitor 1214, and the logic value applied to amplifier 1234 via control logic 1232 is updated responsive to the presently sensed temperature from temperature sensor 1228. Referring now to FIG. 13, there is provided one example of how to apply the reset signal to a current sensor 1302 within a buck converter circuit. In this case, the buck converter circuit control signal φ1 is applied to a transistor 1304 having its drain/source path connected between 12 volts and node 1306. A second transistor 1308 has its drain/source path connected between node 1306 and node 1310. Transistor 1308 is controlled by a second control signal φ2. The current sensor 1302 is connected between node 1310 and ground to detect current ip and provide a control signal VSENSE. An inductor 1312 is connected between node 1306 and node 1314. A capacitor 1316 is connected between node 1314 and ground. A load 1318 is also connected between node 1314 and ground. In one embodiment, the reset signal to switch 1216 of the current sensor 1302 may be configured to be the control signal φ2.
As illustrated in FIG. 14, the current ip is zero when signal φ1 goes low and when signal φ2 goes high at, for example, time t1. Integrator 1218 is reset during phase two when signal φ2 goes high and the current sensor would accept signal φ2 as an input to drive the reset signal to switch 1216, since the current ip is zero during this time. As can be seen each time the signal φ2 goes high, the current ip is zero enabling the reset signal to be applied to the integrator circuit 1218.
Referring now to FIG. 15, there is illustrated an alternative embodiment wherein the reset signal to the reset switch 1216 is generated responsive to a one shot circuit consisting of negative glitch detect circuit 1502 and one shot circuit 1504. When the current ip goes low as illustrated, for example, at t1 in FIG. 14, the negative glitch detect circuit 1502 will detect the negative edge of current ip. In response to this detection, the negative glitch detect circuit 1502 generates a pulse to the one shot circuit 1504. The one shot circuit 1504 then generates the reset signal to the reset switch 1216 responsive to the pulse from the negative glitch detect circuit 1502. Other methods for detecting when the sensed current ip goes to zero may also be utilized for generating the reset signal to reset switch 1216. The examples illustrated in FIGS. 13-15 are merely provided as examples of some embodiments thereof.
Although the preferred embodiment has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the scope of the invention as defined by the appended claims.

## Claims

1. A current sensor, comprising:
coupled inductors including a first and second inductor for generating an output current responsive to a current in the first inductor, the coupled first and second inductors respectively implemented in an integrated circuit package and an integrated circuit die disposed in the integrated circuit package;
an integrator circuit for generating a sensed voltage responsive to the output current;
a first compensation circuit for compensating the sensed voltage for manufacturing variations in the integrator circuit;
a second compensation circuit for compensating the sensed voltage responsive to a sensed temperature; and
wherein the first compensation circuit includes,
a first programmable amplifier for compensating the sensed voltage responsive to a first control value, the first control value configuring the first programmable amplifier to compensate for the manufacturing variations in the integrator circuit; and
a memory for storing the first control value.

2. The current sensor of claim 1, wherein the integrator circuit further comprises:
an operational amplifier for generating the sensed voltage responsive to the output current;
a capacitor connected between an input of the operational amplifier and the output of the operational amplifier; and
a switch connected between the input of the operation amplifier and the output of the operational amplifier for resetting the integrator circuit responsive to a reset signal.

3. The current sensor of claim 1, wherein the second compensation circuit further comprises:
a second programmable amplifier for compensating the sensed voltage responsive to a second control value, the second control value configuring the second programmable amplifier to compensate for temperature differences;
a temperature sensor for generating a temperature signal; and
control logic responsive to the temperature signal for generating the second control value.

4. The current sensor of claim 3, wherein the control logic further comprises a lookup table, the lookup table containing a plurality of second control values, each of the second control values associated with a particular temperature signal.

5. The current sensor of claim 3 further including an analog to digital converter for generating a digital temperature signal from an analog signal from the temperature sensor.

6. The current sensor of claim 1, wherein the integrator circuit is reset responsive to a reset control signal.

7. The current sensor of claim 1 wherein the first inductor is connected to external package connections.

8. The current sensor of claim 1 wherein the integrated circuit die and the first inductor are enclosed in a mold compound forming a part of the integrated circuit package.

9. A current sensor, comprising:
coupled inductors for generating an output current responsive to a detected current, the coupled inductors respectively implemented in an integrated circuit and an integrated circuit package;
an integrator circuit for generating a sensed voltage responsive to the output current, wherein the integrator circuit is reset responsive to a reset control signal;
a first compensation circuit for compensating the sensed voltage for manufacturing variations in the integrator circuit; and
a second compensation circuit for compensating the sensed voltage responsive to a sensed temperature; and wherein
the first compensation circuit includes,
a first programmable amplifier for compensating the sensed voltage responsive to a first control value, the first control value configuring the first programmable amplifier to compensate for the manufacturing variations in the integrator circuit; and
a memory for storing the first control value.

10. The current sensor of claim 9, wherein the integrator circuit further comprises:
an operational amplifier for generating the sensed voltage responsive to the output current;
a capacitor connected between an input of the operational amplifier and the output of the operational amplifier; and
a switch connected between the input of the operation amplifier and the output of the operational amplifier for resetting the integrator circuit responsive to the reset control signal.

11. The current sensor of claim 9, wherein the second compensation circuit further comprises:
a second programmable amplifier for compensating the sensed voltage responsive to a second control value, the second control value configuring the second programmable amplifier to compensate for temperature differences;
a temperature sensor for generating a temperature signal; and
control logic responsive to the temperature signal for generating the second control value.

12. The current sensor of claim 11, wherein the control logic further comprises a lookup table, the lookup table containing a plurality of second control values, each of the second control value associated with a particular temperature signal.

13. The current sensor of claim 11 further including an analog to digital converter for generating a digital temperature signal from an analog signal from the temperature sensor.

14. A current sensor, comprising:
coupled inductors for generating an output current responsive to a detected current, the coupled inductors respectively implemented in an integrated circuit and an integrated circuit package;
an integrator circuit for generating a sensed voltage responsive to the output current, the integrator circuit further including,
an operational amplifier for generating the sensed voltage responsive to the output current;
a capacitor connected between an input of the operational amplifier and the output of the operational amplifier;
a switch connected between the input of the operation amplifier and the output of the operational amplifier for resetting the integrator circuit responsive to a reset signal;

a first compensation circuit for compensating the sensed voltage for manufacturing variations in the integrator circuit, the first compensation circuit further including,
a first programmable amplifier for compensating the sensed voltage responsive to a first control value, the first control value configuring the first programmable amplifier; and
a memory for storing the first control; and

a second compensation circuit for compensating the sensed voltage responsive to a sensed temperature, the second compensation circuit further including,
a second programmable amplifier for compensating the sensed voltage responsive to a second control value, the second control value configuring the second programmable amplifier to compensate for temperature differences;
a temperature sensor for generating a temperature signal; and
control logic responsive to the temperature signal for generating the second control value.

15. The current sensor of claim 14, wherein the control logic further comprises a lookup table, the lookup table containing a plurality of second control values, each of the second control value associated with a particular temperature signal.

16. The current sensor of claim 14 further including an analog to digital converter for generating a digital temperature signal from an analog signal from the temperature sensor.

17. The current sensor of claim 14, wherein the integrator circuit is reset in response to a reset control signal.

18. The current sensor of claim 17, wherein the reset control signal comprises a switching control signal for a switched power supply.

19. The current sensor of claim 17, further including:
a negative glitch detect circuit for detecting a negative edge of the detected current and generating a third control signal responsive thereto; and
a one shot circuit for generating the reset control signal responsive to the third control signal.