WO2008050265A2

WO2008050265A2 - High impedance load detection

Info

Publication number: WO2008050265A2
Application number: PCT/IB2007/054244
Authority: WO
Inventors: Brendan Kelly
Original assignee: Nxp B.V.
Priority date: 2006-10-23
Filing date: 2007-10-18
Publication date: 2008-05-02
Also published as: WO2008050265A3

Abstract

The present invention provides for a method of detecting a high impedance load condition and to a related load detector, in which current to the said load (12) and to a reference load (24) by way of main (20) and sense (22) cells respectively of a FET device (18), and wherein it is determined whether the impedance of the said load exceeds a threshold value such that the determination remains within the impedance/resistance domain, and with the threshold value being determined from the value of the impedance of the reference load divided by the ratio of the said main cells to the sense cells of, the FET device and such that the threshold value can then be readily selected by the user through selection of the reference load, which can comprise an external resistor.

Description

DESCRIPTION

HIGH IMPEDANCE LOAD DETECTION

The present invention provides for a method of detecting a high impedance load condition and to a related detecting device.

Circuits and arrangements for determining the flow condition of load current find wide applications in situations where a fault condition within the load is to be identified and indicated.

For example, in protected high side MOSFET devices, current detection/measurement circuits are employed to determine whether the on- state current through the load is at a normal level or a low level. As an example, a low level will commonly arise due to, for example, a blown lamp filament which will lead to a low on-state current.

Such devices find ready application in the automotive field with regard to the manner of driving and monitoring the external lighting system.

Currently existing protected high side devices generally comprise a low current detector system based upon a comparison of on-state voltage drop across the load with an internal reference value.

Disadvantages are however found to exist with such known low current detector systems.

Insofar as such known systems provide a status signal if the monitored voltage drop across the load falls below a threshold value, an internal reference and precision comparator is employed in this regard. Such comparator generally comprises a CMOS comparator and it will be appreciated that typically unthmmed CMOS offset voltages can cause significant operational variations, particularly in low current detection threshold values.

To take account of this, the current detection threshold is generally set at a relatively high value in order to accommodate the full possible range of comparator offset voltages. In recent times there is an increased emphasis on achieving lower voltage drop in load switches, to reduce unwanted dissipation. Therefore practical applications tend to require lower threshold values that are not generally feasible with CMOS comparators.

Also, since the low current detection threshold is set internally within the device, different versions of the device will be required for different applications requiring different threshold values and it is considered that the reference value may not accurately track the performance of, for example, the MOSFET, within the device.

Yet further, internally generated thresholds are somewhat dependent upon the process employed and also to application conditions such as temperature and supply voltage. Furthermore for a given load condition or impedance the outcome of a traditional low load current detection depends directly on the battery supply voltage and therefore thresholds have to be set to take into account a full range of battery voltages, often encompassing a ratio of between three to one and six to one.

Typically a low current detection threshold must be chosen to avoid false or nuisance indications when a good load is driven from a low battery supply, but such a threshold which will be several times smaller than the nominal load current and cannot be used for example o determine if one or two parallel lamps has failed.

Generally, and in accordance with the realisation of the present invention, such known low current detectors are not necessarily well suited to a primary purpose of detecting high load resistance/impedance conditions and which, it is considered, require a threshold that depends upon supply voltage. Other known devices that are based upon current measurement comprise analogue proportional current measurement systems which are arranged to provide an output current which is proportional to the current through the load being monitored.

However, such further known systems can prove disadvantageous insofar as the customer-related application of the device requires implementation of either analogue comparison, or of an analogue-to-digital- converter function. Also, different silicon designs are required if both low current detection and analogue proportional current measurement is required.

The present invention therefore seeks to provide for a method of detecting high impedance load conditions, and a related detecting device, having advantages over known such methods and devices.

It is one particular object of the present invention that a low detection device be provided that can be readily incorporated within protected high side MOSFET devices.

According to one aspect of the present invention there is provided a method of detecting a high impedance load condition, including delivering current to the said load and to a reference load by way of main and sense cells respectively of an FET device, the method further including the step of determining whether the impedance of the said load exceeds a threshold value, wherein the threshold value comprises the impedance of the reference load divided by the ratio of the said main cells to the sense cells of the FET device.

The method according to the present invention proves advantageous insofar as the possible fault detection offered by the invention is directed to a determination of the load impedance value and, in particular, wherein the threshold value can be readily set and determined on the basis of the main/sense cell ratio and the impedance value of the reference load both of which can be readily user-determined. The method is primarily independent of process and bias conditions insofar as it is based upon the operational sensor cells and main cells operating under equivalent conditions.

As mentioned, the load monitoring and impedance determination is then defined in terms of high load resistance/impedance detection with regard to the readily determinable threshold value, rather than merely a low current detection threshold and the associated disadvantages that arise therewith. Further, insofar as the method is based upon the aforementioned main/sense cell ratio, it can readily be employed to identify higher load resistances, i.e. lower load currents as compared with the current art when, for example, voltage drop regulation is employed. Preferably the reference load comprises a resistor which is external to the FET device and which, advantageously, is therefore readily selectable by the user.

In addition to the facilitating of the ease of setting the threshold resistance value, the design selection offered by the selectable external resistor advantageously allows for a single basic device designed to be employed in various different applications requiring various threshold and economies of scale can therefore be achieved by way of the present invention. Advantageously, the method includes the step of employing voltage drop regulation for the main cells of the FET device which, advantageously, can be provided by way of a gate drive reduction system.

Such a reduction system is advantageous in imposing a minimum voltage drop on the FET device and so can therefore serve to restrict the detrimental impact of comparator offset voltage.

Of course, it should be appreciated that the present invention can provide for a method of high side protection including a method of detecting a high impedance load condition such as that defined above.

According to another aspect of the present invention there is provided a high impedance load detector including an FET device having main and sense cells for delivering current to the said load and to a reference load respectively, comparator means for determining whether the impedance of the load exceeds a threshold value, wherein the threshold value is determined by an impedance value of the reference load divided by the ratio of main cells to sense cells within the FET device.

As noted above, in relation to the related method, a detector embodying the present invention as defined above is advantageous insofar as it is defined in terms of a high load resistance/impedance detection threshold rather than, as with the current art, a low current detection threshold. Higher load resistances/impedances can therefore be determined by appropriate threshold values which can be readily selected and user implemented.

Advantageously, the reference load comprises a resistor located externally of the detector. Again, the detection threshold can therefore advantageously can be readily user-selected without requiring any access to, or redesign of, the detector itself. Thus, a single device can readily be used in a variety of applications, requiring a variety of different threshold values.

Yet further, the device can include a voltage drop regulator for the main cells of the FET device and which advantageously serves to impose a minimum voltage drop on the FET device and thereby restrict the impact of comparator offset voltage.

Advantageously, the voltage drop regulator can comprise a gate drive reduction system. According to a yet further feature of the present invention, the detector device can be arranged to be configurable as a proportional current measure.

This advantageously allows adoption of a common silicon design for the realisation of either low current detection, through high impedance detection, or reconfiguration as a proportional current measure system. Advantageously, the device can be arranged to be reconfigured by way of readily selectable metalisation options.

As an alternative, the device can be arranged to be reconfigured by means of an additional selection device.

The selection device can advantageously be arranged to drive a further FET device for achieving the current measure function, or can be configured in a fully-on condition for determination of a high impedance condition.

Of course, it will be appreciated that the present invention can provide for a protected high side MOSFET device including a detector device such as that defined above. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a schematic circuit diagram of a high impedance load detector embodying the present invention; Fig. 2 is a similar schematic circuit diagram of a high impedance load detector according to another embodiment of the present invention; and

Fig. 3 is a further schematic circuit diagram of a high impedance load detector including selective current measurement functionality.

From the discussion above and from the further details below, it will be appreciated that the present invention provides for a method, and related device, for detecting high impedance loads and which represents an alternative to known load current detectors based on the on-state voltage drop and as currently employed, for example, in integrated protected high side switch devices.

The method and related device of the present invention overcome the traditional limitations imposed by the offset voltage of CMOS comparators as employed within such on-state voltage drop devices and so thereby allow for detection at much lower current threshold values, through the adoption of appropriately high load resistance/impedance values.

Importantly, the threshold value employed by the present invention can be process independent, and readily determined externally by the user if required.

A common design can be employed for various applications unlike the traditional on-state voltage drop based low current detectors which exhibit an inbuilt process, and bias-condition dependent threshold and so generally have to be designed and implemented on a per application basis.

As mentioned in particular with the embodiment illustrated in Fig. 3, and as discussed further below, a detector embodying the present invention can advantageously be provided by way of a circuit design sharing similarity with that required to implement a proportional current measure and so, advantageously, a common circuit design can be employed to selectively offer either the function of a proportional current measure, or high impedance load detection as required.

Turning now to Fig. 1 there is illustrated a circuit arrangement 10 for driving a load 12, such as, for example, a car head lamp filament, and in which a high load impedance detector 14 embodying the present invention is provided.

A battery 16 provides driving current for the load 12 and this is delivered through, in the illustrated example, MOSFET device 18. The MOSFET device 18 provides proportional currents through main cells 20 and sense cells 22, and which currents are delivered respectively to the load 12 and to a reference load 24 which, in accordance with a particular advantageous aspect of the present invention comprises a resistor 24 provided externally of the silicon body of the detector device 14. The external resistor 24 is connected to virtual ground and, as mentioned, the ratio between the current through the main load 12 and the current between the external resistor 24 is determined by the ratio of the main cells 20 to the sense cells 22 of the MOSFET device 18. Voltage values 2OA, 22A are derived from the main cells 20, and sense cells 22 and delivered to a comparator 26 provided with an output 28 arranged to signal when a threshold value for the impedance of the load 12 has been met.

With regard to the currents flowing through the load 12 and external resistor 24 at the time the threshold impedance value at the load 12 is met, it can be appreciated that this can be represented by:

I₁₂ = 124 X K where

I₁₂ is the current through the load 12 I₂₄ is the current through the external resistor 24 K = the internal ratio of the main cells to the sense cells with the MOSFET 18. While the threshold varies with battery voltage and again, at the threshold condition when the voltage at the output of the main cells 20 and sense cells 22 is equal, then the above relationship can, in accordance with Ohms law, be represented as:

Where

R-12TH is the load impedance R₂₄ is the impedance of the external resistor 24

Thus, it will be appreciated that the value of the impedance of load 12 that is to be set as the threshold value can readily be determined by R₂₄/K which can be readily user selected through the choice of the external resistor 24. In particular, it will be appreciated that through simple design choice of the external resistor 24, the threshold impedance value for determining that the impedance of the load 12 has increased to an inappropriately high level, can be user-selected for one of a variety of applications, without requiring redesign of the detector 14. Through definition of the threshold value in the above-mentioned manner, it will be appreciated that, in accordance with the invention, the threshold value is now generally unaffected by process parameters, internal gate drive and temperatures etc.

While it might be considered that the offset voltage of the comparator 26 can serve to restrict accuracy, this can readily be addressed in accordance with the embodiment illustrated in Fig. 2.

Here, like features are identified by the same reference numerals as employed in Fig. 1 and insofar as the arrangement can employ a battery 16 for driving, by way of a MOSFET device 18, a load 12 and wherein an external resistor 24 is again provided to receive current from sense cells of the

MOSFET device 18 having main cells delivering current to the load 12. In this embodiment however, the comparator 26 is considered to be a potential source of threshold inaccuracy due to offset values and so a voltage drop regulation arrangement is employed.

As illustrated, a voltage drop regulator provides a constant reference voltage VBLREF relative to the battery 30 which is equivalent to the battery

16which is delivered to a further comparator 32 of the voltage drop regulator 34 and which serves to maintain the voltage drop across the main cells of the

MOSFET device 18 at a level at least equivalent to V_BLREF-

Through the choice of a value for V_BLREF that is significantly larger than the offset voltage of the comparator 26, the effect of that offset voltage is then limited.

Such voltage drop regulation allows for advantageously high impedance thresholds for the load 12 to be employed even though the comparator 26 might comprise a mere standard CMOS comparator. Turning further now to Fig. 3, there is illustrated an embodiment of the present invention comprising a high impedance load detector with voltage drop regulation such as that employed within the embodiment of Fig. 2, but which also includes selective current measuring functionality.

Again, like reference numerals are employed for the features previously found in the embodiments of Figs. 1 and 2 such that, in summary, the embodiment of Fig. 3 comprises a high impedance load detector 14B for driving a load 12 by way of a battery 16 and a MOSFET device 18 again offering main and sense cells, wherein the sense cells provide current for the external resistor 24. The comparator 26 is again employed within the detector along with the voltage drop regulation circuitry 32, and related battery 30, of the Fig. 2 embodiment. However, additionally, the embodiment of Fig. 3 includes selection circuitry 36 located at the output of the comparator 26 and a FET device 38 selectively operable under control of the selection 36 circuitry within the current path leading from the sense cells of the MOSFET device 18 to the external resistor 24. It will be appreciated that the embodiment of Fig. 3 takes advantage of the design similarities between a high impedance load detector of the present invention and a proportional current measure. Primarily the selection circuitry 36 is employed to determine the function of the FET device 38. That is, should the selection circuitry 36 be controlled so as to turn the FET device 38 fully-on, then the current rising within the sense cells of the MOSFET device 18 is simply delivered to the external resistor 24 in a manner similar to that of Fig. 1 and Fig. 2 embodiments such that appropriate choice of the impedance value of resistor 24, the threshold impedance value for load 12 can be set. When functioning as a high impedance load detector, the output signal

28 indicating that the threshold impedance value has been met by the change in impedance of the load 12 is output via the selector device 36.

If, however, the selection device 36 is employed such that the FET device 38 is driven by the output of the comparator 26, then the FET device 38 functions as a proportional current measure. In this manner, it will be appreciated that a slight additional design alteration to, for example, the high impedance load detector of Fig. 2 serves to increase the potential functionality of the detector circuit such that it can function as an impedance load detector proportional current measure as required and without requiring any major redesign nor significant additional cost.

A common silicon design is therefore achieved which, in one embodiment (not illustrated) can provide the required functionality through appropriate metallisation as an alternative to, or in addition to, the selector device 36 which, in general, is arranged to respond to input digital signals. Thus, the embodiment of Fig. 3 proves particularly advantageous for providing a high impedance load detector for use within a protected high side MOSFET device and which through relatively minor redesign and digital selection, can also offer proportional current measurement functionality.

As will therefore be appreciated from the above, the high impedance load detection offered by the present invention allows for a common device to be employed for a variety of applications each requiring different threshold values, and which can be selected through choice of an external resistor of appropriate value.

All embodiments of the invention advantageously eliminate process dependency and tracking errors as exhibited by known voltage-drop type load current detection systems and, with particular regard the embodiment of Fig. 2, the invention can readily benefit from voltage drop regulation so as to reduce the possible otherwise limiting effect of comparator offsets. Such regulation is not possible with current voltage-drop load current detection systems. Also, it will be appreciated that the detection threshold is now directly related to load impedance as compared with known low current detectors in which the determination of load status is affected as much by battery supply voltage as it is by current and load impedance.

With regard to the embodiment of Fig. 3, both potential functions advantageously achieve benefits in the same way from voltage drop regulation and through the efficient and economical use of silicon real estate and development resources.

As will be appreciated, and a particular aspect of the present invention finds use where both a high-side switch, and an on-state fault detection protection system, are required. As an example, in automotive applications, such functions are most frequently required for use with filament lamp loads in order to provide an indication of failed or accidental disconnection of the lamp filament. Further, the invention could find ready use in relation to heater, motor, or solenoid loads so as to provide confirmation that the load is connected and operating normally. Of course, as further examples, the invention may also find more general application, for example, such as in relation to industrial switches for programmable controllers and/or driving loads in a variety of industrial or consumer applications.

While particularly with automotive applications, requirements generally arise for a device that has integrated drive and protection circuitry, it should be appreciated that the invention can find ready use in simpler unprotected devices.

Claims

1. A method of detecting a high impedance load condition, including delivering current to the said load and to a reference load by way of main and sense cells respectively of an FET device, the method further including the step of determining whether the impedance of the said load exceeds a threshold value, wherein the threshold value comprises the impedance of the reference load divided by the ratio of the said main cells to the sense cells of the FET device.

2. A method as claimed in Claim 1 , wherein the reference load comprises a resistor external to the FET device.

3. A method as claimed in Claim 1 or 2, and including a step of employing voltage drop regulation for the main cells of the FET device.

4. A method as claimed in Claim 3, wherein the voltage drop regulation is provided by way of a gate drive reduction system.

5. A method of providing high side protection including a method of detecting a high impedance load condition as claimed in any one or more of Claims 1 to 4.

6. A high impedance load detector including an FET device having main and sense cells for delivering current to the said load and to a reference load respectively, comparator means for determining whether the impedance of the load exceeds a threshold value, wherein the threshold value is determined by an impedance value of the reference load divided by the ratio of main cells to sense cells within the FET device.

7. A detector as claimed in Claim 6, wherein the reference load comprises a resistor located externally of the detector.

8. A detector as claimed in Claim 6 or 7, and including a voltage drop regulator for the main cells of the FET device.

9. A detector as claimed in Claim 8, wherein the voltage drop regulator can comprise a gate drive reduction system.

10. A detector as claimed in any one or more of Claims 6 to 9, and arranged to be configurable as a proportional current measure.

11. A detector as claimed in Claim 10 and arranged to be reconfigured by way of readily selectable metalisation options.

12. A detector as claimed in Claim 10 and arranged to be reconfigured by means of a user operable selector device.

13. A detector as claimed in Claim 12, wherein the selector device is arranged to drive a further FET device for achieving the current measure function or to be configured in a fully-on condition for initiating a high impedance detection condition.

14. A protected high side MOSFET device including a detector device as claimed in any one or more of Claims 6 to 13.