Technique for determining a load current

The present invention is generally directed to a technique for determining a current and, more specifically, to a technique for determining a load current.

In various applications, it is desirable to be able to precisely determine the amount of current that is flowing through a load. For example, in motor vehicles, it is frequently desirable to determine a load current that is flowing through an oxygen sensor heater, as an associated oxygen sensor needs to be maintained at a desired temperature in order to optimally perform. Typically, a resistance value of the heater may be determined from a battery voltage level and a current level flowing through the heater. The temperature of the heater can then be determined by utilizing the resistance value and a known temperature coefficient for the heater. The temperature of the heater is then controlled, typically by pulse width modulating a switch or driver to adjust an average current and power of the heater.

With reference to FIG. 1 , a system 100 that includes a microcontroller 10 has implemented a discrete sense resistor R 1 to measure a current drawn by a heater (load) H. As is shown, the resistor R 1 is coupled between a source of a field-effect transistor (FET) M 1 and ground and a drain of the FET M 1 is coupled to a low-side of the load H, whose high-side is coupled to a positive terminal of battery VBAT. The voltage signal across the resistor R 1 is differentially coupled to an amplifier AV 1 , via a circuit network that includes resistors R 2 and R 3 and capacitor C 1 . The amplifier AV 1 amplifies the voltage dropped across the resistor R 1 to a range that can be accurately sampled by an analog-to-digital converter (ADC) located within the microcontroller 10 .

An output of the amplifier AV 1 provides a voltage that is measured by the ADC. The microcontroller 10 provides a pulse width modulated (PWM) signal to gate drive circuit 12 responsive to the voltage signal provided by the amplifier AV 1 . The gate drive circuit 12 provides a control signal to a gate of the FET M 1 responsive to the PWM signal. One drawback of this design is that the requirements on the resistor R 1 are fairly stringent, due to the high currents involved, when driving an oxygen sensor heater. For example, the resistor R 1 is typically designed to handle currents between about six to ten Amperes. In order to meet power and overall voltage drop requirements, the value of the resistor R 1 has typically been selected to have a relatively small value, e.g., 25 milliOhms. In such an application, the absolute value of the resistor R 1 must generally be tightly controlled, which, in turn, causes the resistor R 1 to be relatively expensive.

Another drawback with the above-described design is that a relatively large number of input/output (I/O) pins are required to interface the discrete devices. This tends to increase the system circuit board size and complexity, which also results in additional cost. Furthermore, using the above-described approach, a temperature of the discrete FET M 1 cannot be readily sensed, as it is not integrated and, thus, complete fault protection of the discrete FET M 1 cannot be achieved.

What is needed is an integrated driver for a heater that can be readily integrated, allows for improved fault handling capability and that provides relatively accurate thermal and current sensing capability. It would also be desirable if the integrated driver reduced I/O pin count over prior designs.

One embodiment of the present invention is directed to an integrated driver that includes a first transistor, a first amplifier, a second transistor, a third transistor, a second amplifier and a fourth transistor. The first transistor includes a first terminal, a second terminal and a control terminal, with the first terminal of the first transistor coupled to one side of a load, e.g., an oxygen sensor heater. The first amplifier includes a first input, a second input and an output, with the first input of the first amplifier coupled to the first terminal of the first transistor. The second transistor includes a first terminal, a second terminal and a control terminal. The control terminal of the second transistor is coupled to the output of the first amplifier. The first terminal of the second transistor is coupled to one side of a power source, via a first impedance.

The third transistor includes a first terminal, a second terminal and a control terminal, with the first terminal of the third transistor coupled to the second terminal of the second transistor and the second input of the first amplifier. The control terminals of the first and third transistors are coupled together and the first and third transistors conduct responsive to a control signal provided on the control terminals of the first and third transistors. The second amplifier includes a first input, a second input and an output, with the first input of the second amplifier being coupled to the first terminal of the second transistor.

The fourth transistor includes a first terminal, a second terminal and a control terminal, with the output of the second amplifier being coupled to the control terminal of the fourth transistor and the second input of the second amplifier being coupled to the second terminal of the fourth transistor. The second terminal of the fourth transistor is also coupled to one side of the power source, via a second impedance, and the first terminal of the fourth transistor provides an output current that is indicative of a load current through the load.

According to one aspect of the present invention, the load is an oxygen sensor heater. According to a different aspect of the present invention, an impedance of the third transistor is about five-hundred times an impedance of the first transistor. According to a different aspect of the present invention, the first transistor is a double-diffused metal-oxide semiconductor (DMOS) driver. According to this aspect of the present invention, the DMOS driver may have an impedance of about fifty milliOhms. According to another embodiment of the present invention, the first transistor has an on-impedance of about fifty milliOhms and the third transistor has an on-impedance of about twenty-five Ohms. According to a different aspect of the present invention, an impedance of the second impedance is about ten times an impedance of the first impedance.

These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 depicts a circuit for driving a load and measuring a load current, according to the prior art;

FIG. 2 depicts an integrated driver with improved load current and sense capability, according to the present invention; and

FIG. 3 depicts another integrated driver with improved load current and sense capability for multiple loads.

According to various embodiments of the present invention, an integrated driver is disclosed that may provide accurate thermal and current sensing capabilities. The integrated driver provides a reduced I/O pin count over prior designs and allows for improved fault handling capability. As used herein, the term “transistor” broadly includes field-effect transistors (FETs) and bipolar junction transistors (BJTs), among other transistors. For simplicity, the discussion herein is primarily directed to FETs.

With reference to FIG. 2 , a system 200 , configured according to one embodiment of the present invention, includes an integrated driver 250 , which is coupled to a microcontroller 10 , an external resistor RX 1 and an oxygen sensor heater H. The microcontroller 10 provides a pulse width modulated (PWM) control signal to the driver 250 to control a temperature of the heater H. As is shown, the integrated driver 250 includes a transistor M 1 , e.g., a double-diffused metal-oxide semiconductor (DMOS) driver, whose control terminal (gate) receives a control signal from gate drive circuit 202 . An input of the gate drive circuit 202 is coupled to an output of the microcontroller 10 . The output of the gate drive circuit 202 is also coupled to a control terminal (gate) of a transistor M 2 , e.g., a DMOS driver, whose second terminal (source) is coupled to ground.

A first terminal (drain) of the transistor M 1 is coupled to a low side of the load H and a second terminal (source) of the transistor M 1 is coupled to ground. The drain of the transistor M 1 is also coupled to a non-inverting input of an operational amplifier OA 1 . An output of the amplifier OA 1 is coupled to a control terminal (gate) of a transistor M 3 , e.g., a DMOS driver. A second terminal (source) of the transistor M 3 is coupled to a first terminal (drain) of the transistor M 2 and to an inverting input of the amplifier OA 1 . As such, a voltage developed at the drain of the transistor M 1 is provided at the drain of the transistor M 2 . One side of a resistor R 1 is coupled to a positive power supply VDD and a second side of the resistor R 1 is coupled to a first terminal (drain) of the transistor M 3 . The drain of the transistor M 3 is also coupled to a non-inverting input of an operational amplifier OA 2 .

An output of the operational amplifier OA 2 is coupled to a control terminal (gate) of a transistor M 4 , e.g., a P-channel DMOS driver. A second terminal (source) of the transistor M 4 is coupled to the power supply VDD, via a variable resistor R 2 . An inverting input of the operational amplifier OA 2 is also coupled to the second terminal of the transistor M 4 . A first terminal (drain) of the transistor M 4 is coupled off-chip to the external resistor RX 1 , which is utilized to convert current ID 4 (conducted by the transistor M 4 ) into a voltage, which is then coupled to an analog-to-digital converter (ADC) input of the microcontroller 10 . Accordingly, the microcontroller 10 can monitor the voltage across the resistor RX 1 to determine a temperature of the heater H and, thus, control the gate drive circuit 202 to achieve a desired temperature for the heater H. Assuming that the transistor M 1 has an impedance of about fifty milliOhms, the transistor M 2 may, for example, be selected to have an impedance of about twenty-five Ohms, which is five-hundred times greater than the impedance of the transistor M 1 . That is, the transistor M 2 may be selected to be about five-hundred times smaller in area than the size of the transistor M 1 .

As mentioned above, the amplifier OA 1 is utilized to force the drain voltage of the transistors M 1 and M 2 to be substantially the same using the transistor M 3 . As the gates of the transistors M 1 and M 2 are common and the sources of transistors M 1 and M 2 are tied to ground, the transistors M 1 and M 2 have the same gate-to-source voltage and the same drain-to-source voltage. As such, a current ID 3 , conducted by the transistor M 3 , is one-five-hundredths of a load current ID 1 , conducted by transistor M 1 . The current ID 3 feeds into resistor R 1 to setup a voltage V 1 at the non-inverting input of the amplifier OA 2 . The amplifier OA 2 is utilized to cause voltage V 2 to equal the voltage V 1 through the transistor M 4 . The current ID 4 carried by the transistor M 4 represents one-five-thousandths of the load current ID 1 , when the value of the resistor R 2 is selected to be ten times larger than the value of the resistor R 1 .

It should be appreciated that the value of the resistors R 1 and R 2 may be of the same implant type. The resistor R 2 may be made adjustable utilizing, for example, a fusible link trim network, in order to trim out second order effects, such as mismatches between the transistors M 1 and M 2 and the resistors R 1 and R 2 . As is shown, the current ID 4 is output from the driver 250 on a pin labeled O 2 SEN. As noted above, the resistor RX 1 , which may be a precision resistor, is utilized to convert the output current ID 4 into a voltage that is in the range required by the ADC of the microcontroller 10 . For example, the value of the resistor RX 1 may be selected to be about 3.48 kOhms. In the case described above, the voltage on the O 2 SEN pin is related to the load current ID 1 by the following equation.

VO 2 SEN=ID 1*(3480/(500*10))= ID 1*0.696

It should be appreciated that the design of FIG. 2 eliminates the need for expensive discrete resistors and eliminates the load current sense resistor from the load current path, which allows the transistor M 1 to have a higher on-resistance than discrete drivers utilized in the prior art approach, while still meeting the same total on-resistance of the system. This allows the up-integrated transistor M 1 to have a smaller area, thus, resulting in cost savings. Another advantage of an integrated driver, constructed according to the present invention, is that the O 2 SEN pin can be shared between multiple outputs. In this design, two or more O2 sensor heater driver circuits may be up-integrated onto the same IC. In this embodiment, both circuits provide output current on the same O 2 SEN pin under digital control such that both outputs are not providing a current feedback signal simultaneously.

It should be appreciated that this feature further reduces the I/O complexity of the integrated driver. Thus, following this approach, the number of I/O pins is greatly reduced. For example, in a two-channel implementation, the number of pins goes from twelve pins in the prior approach to four pins, which reduces board size and complexity, as well as improves the reliability of the system. Finally, up-integrating the O 2 SEN heater driver allows for significantly better fault handling capability, with accurate thermal and current sensing capability for the DMOS driver.

With reference to FIG. 3 , an integrated driver 350 that is capable of sensing the load current of two oxygen sensors is depicted. The integrated driver 350 is similar to the integrated driver 250 of FIG. 2 , with the exception that two sense/drive circuits 210 have been integrated within the driver 350 . Also, each of the circuits 210 includes a transistor M 5 located between a gate of the transistor M 3 and an output of the amplifier OA 1 . The transistor M 5 , working in conjunction with inverter G 1 and transistor M 6 , allows the microcontroller 10 to dictate which of the load currents are to be measured at a given time.

The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.