Sensed switch current control

FIELD OF INVENTION

0001The present embodiments relate to sensed switching.

BACKGROUND

0002Sensed switches are often used to control the operation of loads instead of powered switches. Powered switches are disposed serially with a load to directly control current delivered to the load. In contrast, sensed switches control the load current indirectly. The state of the switch, open or closed, is instead sensed by applying a low current signal and measuring the voltage. The opportunity to use a low current, voltage measurement leads to reduced wiring harness complexity, weight, and costs. In complex electrical systems with numerous switch-controlled loads, such as automobile vehicles, the cost savings may be considerable.

0003Determining the state of a sensed switch typically involves applying a test current and a voltage comparison. For example, a voltage level dictated by the state of the switch is compared with a threshold voltage. The voltage level is ideally not dependent on the voltage drop across the switch contacts. But unfortunately, the switch contacts oxidize over time due to humidity and contamination, increasing the resistance presented by the switch itself. The increased resistance and test current result in an increase in the sensed voltage, thereby increasing the risk of incorrect operation (i.e., detection). Switch contact oxidation may be especially challenging in connection with normally open switches, i.e., switches with contacts that close upon application of an external force.

0004The oxidation challenge presented by sensed switches is not applicable to the powered switch approach. In powered switches, the current levels are high enough to burn off any oxidation of the switch contacts. Because the current levels may be much lower with sensed switches, a wetting current is used to remove the oxidation from the switch contacts. The wetting current is typically a temporary current level of the current that flows through the switch contacts when the switch transitions from open to closed. The temporary current level is sufficient to remove the oxidation. A circuit used to detect the state of the switch may also be configured to control the application of the wetting current.

BRIEF DESCRIPTION OF THE DRAWINGS

0005The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the various embodiments. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.

0006<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of an exemplary circuit configured to control current provided to an external sensed switch in accordance with one embodiment.

0007<figref idref="DRAWINGS">FIG. 2</figref> is a schematic circuit diagram that depicts current control circuitry of the circuit of <figref idref="DRAWINGS">FIG. 1</figref> in greater detail.

0008<figref idref="DRAWINGS">FIG. 3</figref> is a timing diagram to show a number of signal levels during operation of the circuit of <figref idref="DRAWINGS">FIG. 1</figref> in accordance with one embodiment.

0009<figref idref="DRAWINGS">FIG. 4</figref> is a process flow diagram of an exemplary method of wetting current control in accordance with one embodiment.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

0010Embodiments of methods, devices, and circuits for controlling current delivered to a sensed switch are described. The current delivered to the sensed switch may include a wetting current. The wetting current may be delivered to a normally open sensed switch. The circuit may be configured to consume no current or power while the switch is open. The wetting current may be applied for a predetermined period of time after the sensed switch is closed. The level of current may then be switched (e.g., reduced) to a sustaining current or other (e.g., lower) level of current after the predetermined time period. The methods may be implemented by a device, unit, or other circuit coupled to a comparator directed to detecting and/or sensing the state of the switch.

0011Without quiescent power (or current) drain, the current control may be provided in a manner independent of system state. For example, the electrical system in which the sensed switch current control is provided may be in a shutdown or other low power state. A low power state may be useful in applications in which the power source is a battery, such as a vehicle battery. Thus, the current control is provided in a manner that does not drain the battery while waiting for closure of the sensed switch. In vehicular examples, the battery is not drained while the engine of the vehicle is not running.

0012The lack of quiescent current drain may be achieved as a result of an arrangement of passive (e.g., resistive) circuit elements that does not include a current path to ground while the sensed switch is open. The passive circuit elements are arranged in a circuit with a number of control switches configured to switch between current paths for the current. The switching provided by the circuit arrangement establishes multiple current levels to be provided to the sensed switch despite not being arranged in a current regulating loop or other circuit that uses quiescent current to control a switching network. One of the multiple current levels may be selected using the circuit arrangement, including an initial or default current level suitable for use as a wetting current for the sensed switch. The passive or resistive nature of the circuit arrangement may reduce susceptibility to damage and electrical overstress (EOS) induced by external transients, such as transients arising from electrostatic discharge (ESD) events.

0013Closure of the sensed switch provides power to the current control circuit. For instance, closure of the sensed switch may awaken a microprocessor, logic circuit, or other controller used to provide a control signal to a number of switches for selection of one of the multiple current levels. Such switch control and associated current to control switching occurs after the controller wakes upon closure of the sensed switch.

0014<figref idref="DRAWINGS">FIG. 1</figref> depicts an electrical system <b>10</b> in which a sensed (or load control) switch <b>12</b> is provided to control the operation of a load <b>14</b>. The load <b>14</b> may be a motor, lamp, or any other type of load. The load <b>14</b> may be configured for direct current (DC) or alternating current (AC) operation. In this embodiment, the load <b>14</b> is powered by a DC power source V+ that also provides power for the sensed switch <b>12</b>. In other cases, different voltage sources are used. For example, the power source for the load <b>14</b> may be a high voltage AC power source, and the power source for the sensed switch <b>12</b> may be a low voltage DC power source, which may or may not be derived from or otherwise related to the high voltage AC power source.

0015In some cases, the electrical system <b>10</b> is a vehicular electrical system. The DC power source V+ may be a 12 Volt vehicular battery. In these and other cases, the load <b>14</b> is one of a number of loads controlled by respective sensed switches <b>12</b>. The nature and characteristics of the electrical system <b>10</b> may vary considerably.

0016The state of the sensed switch <b>12</b> determines whether power is delivered to the load <b>14</b>. As a sensed switch, the sensed switch <b>12</b> is not disposed in the current path of the power delivered to the load <b>14</b>. In some cases, the sensed switch <b>12</b> is a push-button switch or other normally open sensed switch. For example, in vehicular embodiments, the sensed switch <b>12</b> may be a push-button switch, such as a power window push-button switch, directly actuated by an operator or other occupant of the vehicle. The sensed switch <b>12</b> may be actuated in a variety of other ways. For example, the sensed switch <b>12</b> may be actuated through the opening of a vehicle door or other indirect actuation mechanism.

0017In the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>, the sensed switch <b>12</b> is configured as a switch to ground. As a switch to ground, the closure of the sensed switch <b>12</b> establishes a connection to ground. The connection to ground lowers a voltage level, which is sensed to control the delivery of power to the load <b>14</b>. In other embodiments, the sensed switch <b>12</b> is configured as a switch to battery or other voltage source. The sensed switch <b>12</b> may be configured to establish a connection to any reference voltage.

0018A control module or unit <b>16</b> senses the connection to ground at an evaluation node <b>17</b> of the control unit <b>16</b>. The sensed switch <b>12</b> is coupled to the node <b>17</b>. During operation, a wetting current Iwet for the sensed switch <b>12</b> flows through the node <b>17</b>. In the switch to ground example of <figref idref="DRAWINGS">FIG. 1</figref>, the connection to ground lowers the voltage level at the node <b>17</b>. The control unit <b>16</b> is configured to detect the lowered voltage level at the node <b>17</b> and, thus, the state of the sensed switch <b>12</b>.

0019The control unit <b>16</b> controls the delivery of power to the load <b>14</b> in accordance with the state of the sensed switch <b>12</b>. The control unit <b>16</b> may thus be referred to or configured as a switch detection unit or circuit. In the switch to ground example of <figref idref="DRAWINGS">FIG. 1</figref>, the control unit <b>16</b> allows power to reach the load <b>14</b> upon detecting the lowered voltage level at the node <b>17</b>. To this end, the control unit <b>16</b> includes a power transistor <b>18</b> that acts as a switch to allow current to flow through the load <b>14</b>. Activation of the power transistor <b>18</b> is controlled by the remainder of the control unit <b>16</b>.

0020The power transistor <b>18</b> is disposed in the current path of the load <b>14</b> rather than the sensed switch <b>12</b>. In this example, the power transistor <b>18</b> is a discrete power field effect transistor (FET) device. In other examples, the power FET device is part of an integrated circuit. Other types of transistor devices may be used, such as bipolar junction transistor devices. Other types of switches may be used, including relays.

0021The control unit <b>16</b> may be one of several units in the electrical system <b>10</b>. Multiple loads <b>14</b> may be controlled by each control unit <b>16</b>. Some of the components of the control unit <b>16</b> may be replicated, with a respective instance of the component being provided for each load <b>14</b>. For example, the control unit <b>16</b> may include multiple power transistors <b>18</b>, one for each load <b>14</b>.

0022In the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>, the control unit <b>16</b> includes a control circuit <b>20</b> and a wetting circuit <b>22</b>. The control circuit <b>20</b> and the wetting circuit <b>22</b> may be disposed on respective integrated circuit (IC) chips. In some cases, the control circuit <b>20</b> (and/or the wetting circuit <b>22</b>) includes multiple IC chips. The control circuit <b>20</b>, the wetting circuit <b>22</b>, the FET device <b>18</b>, and any other components of the control unit <b>16</b> may be mounted on a common circuit board, and/or disposed in a common housing, and/or otherwise integrated in any other manner or to any other desired extent. The sensed switch <b>12</b> and the load <b>14</b> may be external to the board or housing of the control unit <b>16</b>, or be otherwise disposed remotely from the control unit <b>16</b>. Wiring <b>24</b> may be used to establish a connection between the control unit <b>16</b> and the sensed switch <b>12</b>, and wiring <b>26</b> may be used to establish a connection between the control unit <b>16</b> and the load <b>14</b>. For example, a wiring harness may be used to carry the wiring <b>24</b>, <b>26</b> from a door, dashboard, or other panel or portion of a vehicular interior, to another location in the vehicle at which the control unit <b>16</b> is located, such as an electronics cabinet under the dashboard. The manner in which the sensed switch <b>12</b> and the load <b>14</b> are connected to the control unit <b>16</b> may vary. For example, components in addition to the wiring harness may be used, including, for instance, fuses.

0023The length of the wiring <b>24</b> may be sufficiently extensive to present significant parasitic capacitance and resistance, which are indicated schematically as an external capacitor <b>28</b> and an external resistor <b>30</b>. In some cases, the capacitor <b>28</b> and/or the resistor <b>30</b> are additionally or alternatively representative of one or more discrete components disposed in series with the wiring <b>24</b>. For example, a series resistor may be included for purposes of electrostatic discharge (ESD) protection.

0024The wiring <b>24</b> couples the sensed switch <b>12</b> to a pin <b>32</b> of the wetting circuit <b>22</b>. The pin <b>32</b> may be one of a set of pins of the IC chip in which the detection circuit is integrated. The packaging of the wetting circuit <b>22</b> may vary. The nature of the pin <b>32</b> may thus be configured as a post, solder bump, or other connection point of the packaging of the wetting circuit <b>22</b>. In the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>, the pin <b>32</b> corresponds with the node <b>17</b>. The pin <b>32</b> and the node <b>17</b> may thus be disposed at the same voltage level. In other cases, the node <b>17</b> and the pin <b>32</b> may not constitute a common node. For example, a series resistor may be disposed between the node <b>17</b> and the pin <b>32</b>.

0025The control circuit <b>20</b> may be or include a microcontroller or other controller <b>34</b> or other logic circuit configured to implement a number of logic functions. The functions may include controlling the power transistor <b>18</b>. To that end, a control signal may be generated by the controller <b>34</b> and provided to a gate or other control terminal of the power transistor <b>18</b>. The functions may also include analysis of the state of the sensed switch <b>12</b>. The state of the sensed switch <b>12</b> is used to determine whether to generate the control signal. The functions still further include wetting current control or other current selection. The controller <b>34</b> implements the control of the wetting current function as described below. Each function may be implemented by a separate logic block, software or firmware module, or other component of the controller <b>34</b> and/or the control circuit <b>20</b>. The logic blocks or other components of the controller <b>34</b> directed to implementing these functions may be integrated to any desired extent. For example, a single routine may be implemented by the controller <b>34</b> to provide all of the functions. Additional functions may also be provided by the controller <b>34</b> and/or the control circuit <b>20</b>. For example, the control circuit <b>20</b> may be configured to provide diagnostic testing for the sensed switch <b>12</b>.

0026The wetting current is provided to the sensed switch <b>12</b> to burn off the oxidation on the contacts of the sensed switch <b>12</b>. The term wetting current is used herein to refer to either a current and/or voltage level sufficient to remove the oxidation. The aspects of the wetting current to be controlled include, for instance, the duration of the wetting current. In other cases, the duration and/or other aspects of the wetting current are determined passively (e.g., via a charging or discharging capacitor).

0027The controller <b>34</b> may be or include a general microprocessor or an application-specific microprocessor, such as an application-specific integrated circuit (ASIC). In other embodiments, a field-programmable gate array (FPGA) or other controller may be used as the controller <b>34</b>. The controller <b>34</b> and/or the control circuit <b>20</b> may include any combination of firmware and general-purpose memory to store instructions to be executed during operation.

0028The control circuit <b>20</b> and the wetting circuit <b>22</b> are responsive to the state of the sensed switch <b>12</b>. In this example, the control circuit <b>20</b> includes a comparator <b>36</b>. In other cases, the comparator <b>36</b> is part of, or integrated with, the wetting circuit <b>22</b> or another circuit. The comparator <b>36</b> has an input terminal <b>38</b> at the node <b>17</b> to detect the voltage level at the node <b>17</b>, which is representative of the state of the sensed switch <b>12</b>. The comparator <b>36</b> accordingly provides an output indicative of the state of the sensed switch <b>12</b> based on the voltage level at the input terminal <b>38</b> (and the node <b>17</b>). In this example, the input terminal <b>38</b> is one of a pair of input terminals, the other being an input terminal <b>40</b> that receives a threshold voltage, VT. Either one of the input terminals <b>38</b>, <b>40</b> may be configured as an inverting input terminal, while the other is configured as a non-inverting input terminal. During operation, the voltage level at the node <b>17</b> is compared to the level of the threshold voltage. The control circuit <b>20</b> (e.g., the controller <b>34</b>) is communicatively coupled to an output of the comparator <b>36</b>. The control circuit <b>20</b> receives the output of the comparator <b>36</b> to determine whether to activate the power transistor <b>18</b> and provide current control signaling (e.g., wetting current control signaling).

0029The controller <b>34</b> may be configured to enter a shutdown or other low power mode while waiting for closure of the sensed switch <b>12</b>. The controller <b>34</b> may be activated or awakened from the low power mode by the output from the comparator <b>36</b>. For example, the comparator <b>36</b> may be configured to provide a logic “1” signal or other high voltage level upon closure of the sensed switch <b>12</b>. The controller <b>34</b> may be configured to awaken when the output of the comparator <b>36</b> changes from a logic “0” signal or other low voltage level to the high voltage level.

0030The configuration of the comparator <b>36</b> may vary. For example, the comparator <b>36</b> may be or include various types of analog-to-digital converters and/or amplifier circuits, including, for instance, operational amplifier (op-amp) circuits.

0031The threshold voltage may be provided by threshold circuitry. The threshold circuitry may be coupled to the power source V+ to establish the threshold voltage. In some cases, the threshold circuitry is or includes a voltage divider arrangement. Other types of circuit arrangements may be used. For example, the threshold circuitry may include one or more active devices.

0032The control circuit <b>20</b> and the wetting circuit <b>22</b> may be integrated to any desired extent. For example, the comparator <b>36</b> may be integrated with the control circuit <b>20</b>. In some cases, a microcontroller, such as a mixed signal FPGA, may include both an analog-to-digital converter to act as the comparator <b>36</b> and one or more logic blocks to implement the logic functionality of the controller <b>34</b> and/or other component of the control circuit <b>20</b>, such as a drive switch <b>42</b> (<figref idref="DRAWINGS">FIG. 2</figref>) or other switch or logic.

0033The wetting circuit <b>22</b> includes current control or selection circuitry <b>44</b> configured to selectively provide multiple levels of current to the sensed switch <b>12</b>. The current is provided via the evaluation node <b>17</b> upon closure of the sensed switch <b>12</b>. In some cases, the multiple current levels include a wetting current level (e.g., 15-20 milliamps), and one or more lower current levels (e.g., 1-2 milliamps) sufficient to sustain the closure of the switch. A sustaining, or sealing current or fret current, may be applied after a predetermined time period (e.g., 20 milliseconds) since the closure of the sensed switch <b>12</b>. Changing from the wetting current to the sustaining current lowers power dissipation, which may be useful in conserving the battery or other power source of the system <b>10</b>. The number of different current levels may vary. For example, two or more different sustaining current levels may be provided.

0034The wetting circuit <b>22</b> and/or the current control circuitry <b>44</b> may be configured to provide other functions in addition to current control. For instance, the current control circuitry <b>44</b> may support the detection of the state of the sensed switch <b>12</b>. The current control circuitry <b>44</b> may be configured as or include pull-up circuitry or pull-down circuitry. In the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>, the current control circuitry <b>44</b> includes an arrangement of passive circuit elements, some of which provide pull-up functionality. The pull-up (or pull-down) circuitry establishes the voltage level at the node <b>17</b>. In this example, the pull-up circuitry pulls up the voltage level at the node <b>17</b> when the sensed switch <b>12</b> is open. The voltage level of the node <b>17</b> accordingly does not float when the sensed switch <b>12</b> is open. The comparator <b>36</b> may thus reliably detect the state of the sensed switch <b>12</b>.

0035The current control circuitry <b>44</b> is disposed between the evaluation node <b>17</b> and a node or pin <b>46</b> for the voltage source V+. When the sensed switch <b>12</b> is closed, current flows from the voltage source node <b>46</b> and through the evaluation node <b>17</b> to reach the sensed switch <b>12</b>. The voltage source node <b>46</b> may or may not correspond with a pin or other outward facing node of the wetting circuit <b>22</b>. The voltage source node <b>46</b> is depicted in <figref idref="DRAWINGS">FIG. 1</figref> as a pair of pins for ease in illustration. Only a single pin may be used to connect the current control circuitry <b>44</b> to the voltage source V+. In some cases, the voltage source node <b>46</b> may correspond with an internal node of the current control circuitry <b>44</b>, rather than a pin or other outward-facing node.

0036The current control circuitry <b>44</b> includes control switches <b>48</b>, <b>50</b> disposed between the voltage source node <b>46</b> and the evaluation node <b>17</b> to switch between multiple current paths for the current flowing through the sensed switch <b>12</b>. In this example, the control switches <b>48</b>, <b>50</b> switch between a pair of current paths Iwet and Isustain associated with a wetting current level and a sustaining current level, respectively. The wetting current passes through the control switch <b>48</b> when flowing along the current path Iwet. The control switch <b>50</b> is coupled to a control terminal <b>52</b> of the control switch <b>48</b> to deactivate the control switch <b>48</b> and allow the sustaining current to flow through the current path Isustain.

0037The control switch <b>50</b> is configured to change the voltage level at the control terminal <b>52</b> of the control switch <b>48</b>. In the example of <figref idref="DRAWINGS">FIG. 1</figref>, closure of the control switch <b>50</b> applies a high voltage (e.g., the voltage at the voltage source node <b>46</b>) to the control terminal <b>52</b>. In other cases, closure of the control switch <b>50</b> may pull the voltage at the control terminal <b>52</b> to a low voltage (e.g., ground) or another voltage level. In either case, the voltage change deactivates the control switch <b>48</b> to switch between the wetting current path and the sustaining current path (and, thus, the corresponding current levels). In other words, the closure of the control switch <b>50</b> causes control switch <b>52</b> to open.

0038The control switch <b>50</b> is responsive to a control signal generated by the control circuit <b>20</b> and/or the controller <b>34</b>. The control signal may be provided to select (e.g., decrease) the current that flows through the sensed switch <b>12</b>. For instance, the current may be lowered from the level of the wetting current to the level of the sustaining current. The control signal is generated in response to a change in the output of the comparator <b>36</b>.

0039The control signal may be directly applied to the control switch <b>50</b> as shown in <figref idref="DRAWINGS">FIG. 1</figref>. Alternatively, the control switch <b>50</b> is indirectly responsive to the control signal. In the example of <figref idref="DRAWINGS">FIG. 2</figref>, the control signal is generated by the controller <b>34</b> and provided to a control terminal of the drive switch <b>42</b>. The drive switch <b>42</b> may be disposed between the control switch <b>50</b> and a reference voltage (e.g., ground). In other cases, the control signal may be provided to a switch or other element of the wetting circuit <b>22</b>.

0040The current control circuitry <b>44</b> includes an arrangement of passive circuit elements configured to establish the current levels for the respective current paths. In the example of <figref idref="DRAWINGS">FIG. 1</figref>, wetting and sustaining current levels are established for current flow along the current paths Iwet and Isustain, respectively. The arrangement of passive circuit elements is disposed between the voltage source node <b>46</b> and the evaluation node <b>17</b>. Use of the passive circuit elements allows the current control circuitry <b>44</b> to be configured such that no current path to ground is present in the arrangement of the passive circuit elements when the sensed switch <b>12</b> is open.

0041In the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>, the circuit arrangement includes passive elements <b>60</b>, <b>62</b>, each of which includes a respective resistance. Each resistance may include one or more resistors or other resistive elements. The resistances may be provided as discrete or integrated components. Each passive element <b>60</b>, <b>62</b>, <b>64</b> may include a single circuit element or a network or set of multiple circuit elements. For example, one or more of the passive elements <b>60</b>, <b>62</b>, <b>64</b> may include a resistive element and a diode element.

0042The passive element <b>60</b> is disposed along the current path Iwet to establish the wetting current level. The resistance of the passive element <b>60</b> may establish the resistance presented by the current path Iwet, thereby setting the wetting current level. The wetting current level may be established by the passive element <b>60</b> in combination with one or more other resistive elements, either internal or external to the current control circuitry <b>44</b>. For example, in some cases, the external resistance <b>30</b> may provide sufficient series resistance along the current path Iwet to affect the wetting current level. Current flowing through the current path Iwet from the voltage source node <b>46</b> to the sensed switch <b>12</b> passes through the control switch <b>48</b>. The current then passes through the passive element <b>60</b> before reaching the evaluation node <b>17</b>. The passive element <b>60</b> may be in series with the conduction path of the control switch <b>48</b>. In this example, the current path Iwet does not include any active circuit elements between the conduction path of the control switch <b>48</b> and the evaluation node <b>17</b>. The passive element <b>60</b> may be the only circuit element between the conduction path of the control switch <b>48</b> and the evaluation node <b>17</b>. In other embodiments, other circuit elements may be present between the control switch <b>48</b> and the evaluation node <b>17</b>.

0043The passive element <b>62</b> is disposed along the current path Isustain to establish the sustaining current level. The resistance of the passive element <b>62</b> may establish the resistance presented by the current path Isustain, thereby setting the sustaining current level. The resistance of the passive element <b>62</b> may be higher than the resistance of the passive element <b>60</b>. The higher resistance of the passive element <b>62</b> may accordingly lead to a lower current level for the current path Isustain relative to the current level for the current path Iwet. The sustaining current level may be established by the passive element <b>62</b> in combination with one or more other resistive elements, either internal or external to the current control circuitry <b>44</b>. For example, in some cases, the external resistance <b>30</b> may provide sufficient series resistance along the current path Isustain to affect the sustaining current level. Alternatively or additionally, a circuit element of the passive element <b>64</b> may affect the sustaining current level.

0044The passive element <b>62</b> may support or provide a sensing or trigger function in addition to establishing the sustaining current level. The passive element <b>62</b> is disposed between the evaluation node <b>17</b> and the control terminal <b>52</b> of the control switch <b>48</b>. The passive element <b>62</b> may thus couple the voltage level at the evaluation node <b>17</b> to the control terminal <b>52</b>. With that coupling, the passive element <b>62</b> may trigger activation of the control switch <b>48</b>. The activation allows the current to flow along the current path Iwet. The activation is thus based on the voltage level at the evaluation node <b>17</b>. For instance, in the example of <figref idref="DRAWINGS">FIG. 1</figref>, upon closure of the sensed switch <b>12</b>, the voltage level at the evaluation node <b>17</b> drops to a level near ground. The passive element <b>62</b> may act as a coupling, trigger or sense element to accordingly lower the voltage level at the control terminal <b>52</b>. If the control switch <b>48</b> is activated by a low voltage (e.g., near or below ground, as in a p-channel FET device), then the control switch <b>48</b> closes to allow current to flow through the current path Iwet. In other cases, the arrangement of passive circuit elements may be configured to provide a high voltage (e.g., above a positive threshold voltage, as in an n-channel FET device) for activation of the control switch <b>48</b>.

0045The current path Isustain does not include any active circuit elements between the control terminal <b>52</b> of the control switch <b>48</b> and the evaluation node <b>17</b>. The passive element <b>62</b> may be the only circuit element between the control terminal <b>52</b> of the control switch <b>48</b> and the evaluation node <b>17</b>. In other embodiments, other circuit elements may be present between the control terminal <b>52</b> and the evaluation node <b>17</b>.

0046The passive element <b>64</b> is disposed between the voltage source node <b>46</b> and the control terminal <b>52</b> of the control switch <b>48</b> to enable the activation and deactivation of the control switch <b>48</b>. In some cases, the passive element <b>64</b> includes a resistive element to ensure that the control switch <b>48</b> remains off (or open) while the sensed switch <b>12</b> is open. For example, placing a sufficiently large resistive element between the voltage source node <b>46</b> and the control terminal <b>52</b> may establish that a change in the voltage level at the evaluation node <b>17</b> is the only way to activate (or close) the control switch <b>48</b>. The passive element <b>64</b> may accordingly help prevent or reduce leakage current through the control switch <b>48</b>. Alternatively or additionally, the passive element <b>64</b> includes a diode element, such as a zener diode <b>78</b>, as described in connection with the example of <figref idref="DRAWINGS">FIG. 2</figref>. The diode element may be used to help protect the control switch <b>48</b> by clamping or limiting the voltage at the control terminal <b>52</b>.

0047During operation, with the sensed switch <b>12</b> open, the evaluation node <b>17</b> is pulled high (e.g., up toward or near the voltage level at the voltage source node <b>46</b>) by the passive elements <b>62</b> and <b>64</b>. The voltage level at the control terminal <b>52</b> of the control switch <b>48</b> is also pulled high (e.g., up toward or near the voltage level at the voltage source node <b>46</b>). In this example, the control switches <b>48</b>, <b>50</b> are configured to be activated (or closed) by a low voltage (e.g., near or below ground) at the control terminals <b>52</b>, <b>58</b>, respectively. As a result, the control switch <b>48</b> is off (or open) when the voltage level at the control terminal <b>52</b> is pulled high. The control switch <b>50</b> is also off (or open) because the output of the comparator <b>36</b> is indicative of the open state of the sensed switch <b>12</b> and the control signal provided to the control switch <b>50</b> is also indicative of the open state. For example, when comparator <b>36</b> indicates that the sensed switch <b>12</b> is open, controller <b>34</b> may apply a high voltage to the control terminal <b>58</b> of the control switch <b>50</b>, turning the control switch <b>50</b> off. As a result, the voltage level at the control terminal <b>52</b> of the control switch <b>48</b> is solely established via the passive element <b>62</b>.

0048Once the sensed switch <b>12</b> closes, the voltage level at the evaluation node <b>17</b> is pulled down (e.g., to or near ground) in this example. As the current flows, the passive elements <b>62</b>, <b>64</b> (and the external resistance <b>30</b>) form a voltage divider to establish the voltage level at the control terminal <b>52</b>. The relative resistances of the passive elements <b>62</b>, <b>64</b> are such that the voltage level at the control terminal <b>52</b> is pulled sufficiently down to activate (or close) the control switch <b>48</b>. The current path Iwet to the sensed switch <b>12</b> is thus established. At this point, the wetting current flows from the voltage source node <b>46</b>, and through the control switch <b>48</b> and the passive element <b>60</b>. The resistance presented by the passive element <b>60</b> determines the level of the wetting current.

0049The output of the comparator <b>36</b> also changes (e.g., goes high) upon closure of the sensed switch <b>12</b>. The controller <b>34</b> is responsive to the output in multiple ways. First, the controller <b>34</b> activates the power transistor <b>18</b> to power the load <b>14</b> upon closure of the sensed switch <b>12</b>. After a predetermined time period has elapsed (e.g., 20-30 milliseconds) from closure of the sensed switch <b>12</b>, the controller <b>34</b> also generates the control signal to change the current path in the current control circuitry <b>44</b>. In some cases, the controller <b>34</b> changes the voltage level of the control signal from a low to high voltage level (e.g., from a logic low level at or near ground to a logic high level at or near 5 Volts), but other voltage changes may be used. The control signal activates the control switch <b>50</b>, which pulls the voltage level of the control terminal <b>52</b> of the control switch <b>48</b> up to or toward the voltage source V+. The higher voltage level causes current to flow along the current path Isustain through the passive element <b>62</b>. The higher voltage level also deactivates (or opens) the control switch <b>48</b>, such that current no longer flows through the passive element <b>60</b>. The difference in the resistance levels presented by the passive elements <b>60</b>, <b>62</b> thus leads to a change (e.g., decrease) in the current level for the sensed switch <b>12</b>. For example, the passive element <b>62</b> may have a resistance about 5-10 (e.g., <b>6</b>) times larger than the resistance of the passive element <b>60</b>, thereby establishing a sustaining current level 5-10 times lower than the wetting current level. Other resistance ratios may be used.

0050The passive circuit elements of the current control circuitry <b>44</b> establish the multiple current levels without being disposed in a current regulating loop or other current source arrangement. As a result, the multiple current levels may be provided without the quiescent current that typically occurs with such arrangements. In contrast, the network of passive (e.g., resistive) circuit elements and other components of the current control circuitry <b>44</b> may provide the multiple current levels without any quiescent current.

0051Various aspects of the control unit <b>16</b> or the wetting circuit <b>22</b> may be replicated to support multiple sensed switches <b>12</b>. Each wetting circuit <b>22</b> may be configured to handle multiple sensed switches <b>12</b>. Each wetting circuit <b>22</b> may thus have multiple comparators <b>36</b>. Respective current control circuitry <b>44</b> may be provided for each sensed switch <b>12</b>. Alternatively or additionally, each control unit <b>16</b> includes multiple detection circuits <b>22</b>. A single microprocessor or other control circuit <b>20</b> may be coupled to the multiple comparators <b>36</b> and/or multiple detection circuits <b>22</b>. The control circuit <b>20</b> may thus be configured to handle the detection and testing of multiple wetting current paths and/or multiple detection circuits <b>22</b>. Alternatively, one or more aspects of the control circuit <b>20</b> are separately provided for each wetting circuit <b>22</b> and/or for each sensed switch <b>12</b>.

0052The mechanism for transition between the states of the sensed switch <b>12</b> may vary. For example, the sensed switch <b>12</b> may be a momentary push button switch or a toggle push button switch. Any type of switch may be used for the sensed switch <b>12</b>, including non-push-button switches.

0053<figref idref="DRAWINGS">FIG. 2</figref> depicts the current control circuitry <b>44</b> of the wetting circuit <b>22</b> in greater detail. In this example, each of the control switches <b>48</b>, <b>50</b> is or includes a p-channel field effect transistor (FET) device. The p-channel FET devices may be configured for activation when the voltage on the respective control terminal <b>52</b>, <b>58</b> is near, at, or below ground. The control terminals <b>52</b>, <b>58</b> correspond with respective gate terminals of the FET devices. The source and drain terminals of the FET devices <b>48</b>, <b>50</b> are connected along the current paths Iwet and Isustain, respectively. In this example, a source terminal of each FET device <b>48</b>, <b>50</b> is connected to the voltage source node <b>46</b>. In other cases, one or more passive elements may be disposed between the FET device <b>48</b> or <b>50</b> and the voltage source node <b>46</b>. The drain terminal of the FET device <b>48</b> is connected in series with the passive element <b>60</b>. The drain terminal of the FET device <b>50</b> is connected to the gate terminal of the FET device <b>48</b>. During operation, the wetting current flows through the source and drain terminals of the FET device <b>48</b>, and the sustaining current flows through the source and drain terminals of the FET device <b>50</b>.

0054In some cases, other types of transistor devices or switches may be used in or as the control switches <b>48</b>, <b>50</b>. For example, one or both of the control switches <b>48</b>, <b>50</b> may be configured as bipolar junction transistor (BJT) devices or relays. In BJT examples, the base terminals of the BJT devices may be connected to receive a control current level for activation and deactivation. Additional and/or alternative connections to the BJT devices may be used. The connections may include one or more resistances to limit the base current to appropriate levels.

0055In the embodiment of <figref idref="DRAWINGS">FIG. 2</figref>, the passive element <b>60</b> along the current path Iwet is or includes a single resistor <b>70</b>, although multiple resistors may be used in an alternate embodiment. The passive element <b>62</b> along the current path Isustain is or includes a single resistor <b>72</b>, although multiple resistors may be used in an alternate embodiment. In one example with a voltage source V+ of about 12 Volts (e.g., a vehicle battery), the resistors <b>70</b>, <b>72</b> may have resistances of about 1 kilohm and about 6 kilohms, respectively. The wetting and sustaining currents may thus differ by a factor of 6. A wide variety of resistance levels, current levels, and differences therein may be used. The resistors <b>70</b>, <b>72</b> may be configured as discrete and/or integrated components. For example, the resistor <b>70</b> may be formed in the same substrate as the FET device <b>48</b>.

0056Resistors <b>74</b> and <b>76</b> are connected to the gate terminal <b>58</b> of the control switch <b>50</b>. The resistor <b>74</b> is disposed between the voltage source node <b>46</b> and the gate terminal <b>58</b> of the FET device <b>50</b>. The resistor <b>76</b> is disposed between the gate terminal <b>58</b> and the drive switch <b>42</b>. In the example of <figref idref="DRAWINGS">FIG. 2</figref>, activation (or closing) of the drive switch <b>42</b> pulls the gate terminal <b>58</b> to ground via the resistor <b>76</b>. Conversely, when the drive switch <b>42</b> is not activated (or is open), the resistor <b>74</b> acts as a passive pull-up for the FET device <b>50</b>. The resistor <b>74</b> pulls the voltage at the gate terminal <b>58</b> up toward the voltage level of the voltage source node <b>46</b> unless the drive switch <b>42</b> is activated. The resistor <b>74</b> thus ensures that the control switch <b>50</b> remains deactivated until otherwise directed by the control signal and the drive switch <b>42</b>.

0057In the embodiment of <figref idref="DRAWINGS">FIG. 2</figref>, the passive element <b>64</b> includes a zener diode <b>78</b> connected to the gate terminal <b>52</b> of the FET device <b>48</b>. The zener diode <b>78</b> may be connected in parallel with the resistor <b>74</b> between the voltage source node <b>46</b> and the gate terminal <b>52</b> of the FET device <b>48</b>. The zener diode <b>78</b> protects the FET device <b>48</b> by limiting the voltage applied to the control terminal <b>52</b>. A zener diode <b>80</b> may be connected to the gate terminal <b>58</b> of the FET device <b>50</b> for the same reason. The zener diodes <b>78</b>, <b>80</b> may not be included in other embodiments, such as those having lower voltage source levels and/or FET devices with wider tolerances.

0058The passive element <b>64</b> also includes a resistor <b>82</b>. The resistor <b>82</b> is connected in parallel with the zener diode <b>78</b> between the voltage source node <b>46</b> and the gate terminal <b>52</b> of the FET device <b>48</b>. In the example of <figref idref="DRAWINGS">FIG. 2</figref>, the resistor <b>82</b> acts as a passive pull-up, ensuring that the FET device <b>48</b> remains deactivated unless the sensed switch <b>12</b> is closed.

0059The resistor <b>82</b> may have a large resistance relative to the other resistors in the current control circuitry <b>44</b>. The large resistance may also establish an appropriate voltage level for the gate terminal <b>52</b> when the sensed switch <b>12</b> is closed. With current flowing through the evaluation node <b>17</b> upon closure of the sensed switch <b>12</b>, the resistors <b>72</b>, <b>82</b> form a voltage divider. The voltage level at the gate terminal <b>52</b> is then brought low (e.g., near ground) if the resistance presented by the resistor <b>72</b> is much lower than the resistance of the resistor <b>82</b>. In some examples, the resistor <b>82</b> may have a resistance of about 10 megaOhms. A wide range of other resistances may be used.

0060As shown in <figref idref="DRAWINGS">FIG. 2</figref>, the circuit arrangement of the current control circuitry <b>44</b> does not present a current path to ground when the sensed switch <b>12</b> is open. The resistors <b>70</b> and <b>72</b> are disposed along the current paths Iwet and Isustain, respectively, without any active elements between the FET devices <b>48</b>, <b>50</b>, and the evaluation node <b>17</b>. The resistors <b>70</b> and <b>72</b> are thus used to establish the wetting and sustaining current levels without having to resort to current regulating loops or other current source arrangements in which a path to ground is present throughout operation. The circuit arrangement may vary from the example of <figref idref="DRAWINGS">FIG. 2</figref> and still avoid a quiescent current path to ground. For example, the current control circuitry <b>44</b> may have a differently configured level shifter, such as one in which the control signal is provided directly, rather than indirectly, to the level shifter. The components of the level shifter may also differ in examples in which a BJT device is used for the control switch <b>48</b>.

0061<figref idref="DRAWINGS">FIG. 3</figref> shows a set of signal waveforms to depict the operation of the current control circuitry <b>44</b>. A waveform <b>300</b> depicts the voltage at the evaluation node. At time t<b>1</b>, the sensed switch is closed, and the voltage drops from a high level (e.g., near the voltage source level) to a low level (e.g., near ground). At that point, the voltage at the gate terminal of the control switch <b>48</b> (<figref idref="DRAWINGS">FIGS. 1 and 2</figref>) also drops from a high level (e.g., near the voltage source level) to a low level (e.g., near ground), as shown in waveform <b>302</b>. The current flowing through the evaluation node and the sensed switch accordingly rises to the wetting current level, as shown in waveform <b>304</b>.

0062After a predetermined time from time t<b>1</b> (e.g., 20 milliseconds or some other time period), the controller <b>34</b> (<figref idref="DRAWINGS">FIG. 1</figref>) generates a control signal to lower the current to the sustaining current level. The control signal may be provided as a logic signal waveform <b>306</b> that rises from a low level (e.g., at or near ground) to a higher level (e.g., at or near 5 Volts). The change occurs at time t<b>2</b> in this example. In response to the control signal, the voltage level at the gate terminal of the control switch <b>50</b> (<figref idref="DRAWINGS">FIGS. 1 and 2</figref>) drops from a high level (e.g., near the voltage source level) to a low level (e.g., near ground), as shown in waveform <b>308</b>. The control switch <b>50</b> is activated, thereby returning the voltage level at the gate terminal of the other control switch <b>48</b> to the higher level, as shown in waveform <b>302</b>. The control switch <b>48</b> is deactivated, changing the current path to the sensed switch. As a result, the current level drops to the sustaining current level as shown in waveform <b>304</b>. The sustaining current is provided until the sensed switch closes and the voltage level at the evaluation node is accordingly pulled back up to the high level at time t<b>3</b> as shown in waveform <b>300</b>.

0063<figref idref="DRAWINGS">FIG. 4</figref> shows an exemplary method <b>400</b> for current control or selection. The method may be implemented by the control circuits or controllers described above. In some cases, another processor or controller may be used to implement the method either in conjunction with the above-described controllers or separately therefrom. The method <b>400</b> includes a sequence of acts or steps, only the salient of which are depicted for convenience in illustration. Additional, fewer, or alternative acts may be included. For example, the method <b>400</b> may include providing currents at additional current levels. The ordering of the acts may vary in other embodiments. For example, the order in which the different current levels is provided may vary.

0064The method <b>400</b> is described in connection with a switch to ground sensed switch arrangement (e.g., the arrangement of <figref idref="DRAWINGS">FIGS. 1 and 2</figref> in which the sensed switch <b>12</b> is coupled to ground). The method <b>400</b> may alternatively be applied in connection with a switch to battery arrangement (e.g., an embodiment in which a sensed switch is coupled to a positive voltage reference).

0065The method <b>400</b> may be applied to selectively control the application of wetting current to a normally open sensed switch (also referred to as a “load control switch”). With the sensed switch normally open, oxidation of the switch contacts may occur over time. The contacts of the sensed switch may thus benefit from the application of the wetting current each time that the sensed switch is closed. The method is directed to selecting between the wetting current and one or more other (e.g., reduced) current levels. Unlike other current source techniques for selectively providing the wetting current, the current control or selection may be provided without current drain when the sensed switch is open.

0066The method <b>400</b> may begin with, or include, act <b>402</b> in which the sensed switch is closed. The closure is sensed in act <b>404</b> with a passive element connected to an evaluation node for the sensed switch. The passive element may include a resistor or resistance as described above. The sensing of the closure activates a control switch in act <b>406</b> to deliver current (e.g., wetting current) in act <b>408</b> through the sensed switch and along a current path in which the control switch is disposed (e.g., the wetting current path). The current is delivered at a level established by another passive element disposed along the current path in series with the control switch.

0067The closure of the sensed switch is also (e.g., simultaneously) detected in act <b>410</b> with a comparator. The comparator is connected to the evaluation node to detect the closure as described above. The detection of the closure causes the output of the comparator to change. The change in the output may awaken or activate a controller or control circuit in act <b>412</b>. Once awakened or otherwise active, the controller may initiate a timer in act <b>414</b> determinative of the time period during which the current is provided at the first level (e.g., wetting current level). The timer may be digitally implemented or implemented via analog circuitry (e.g., with a capacitor). The timer may be used to determine when to switch the current to a second current path. The control signal determination may be based on alternative or additional factors. For instance, a variety of non-time-based factors may be used, such as factors relating to the condition of the power source.

0068In the embodiment of <figref idref="DRAWINGS">FIG. 4</figref>, expiration of the timer causes the controller to generate a control signal in act <b>416</b> to change the level of current provided to the sensed switch. For example, the control signal may lead to discontinuing the wetting current and initiating a sustaining current. In some cases, the control signal is provided to close (or turn off) a drive switch in act <b>418</b>.

0069The control signal causes a second control switch to be activated in act <b>420</b>, thereby causing the first control switch to be deactivated as described above. The deactivation and activation of the first and second switches causes the current path to change a second current path (e.g., the sustaining current path) as described above. A second passive circuit element is disposed along the second current path to establish a second (e.g., lower) current level for the current. In some cases, the second passive circuit element corresponds with the passive element used to sense the closure of the sensed switch (e.g., in act <b>404</b>).

0070The above-described devices and methods provide a temporary wetting current (or other current level) without an active circuit to select and/or initiate the current level. The wetting current is provided without wasting power in the active circuit while waiting for closure of the sensed switch.

0071In a first aspect, a circuit for a sensed switch includes an evaluation node through which current flows from a voltage source node to the sensed switch when the sensed switch is closed, and first and second control switches disposed between the voltage source node and the evaluation node to switch between first and second current paths for the current. The current passes through the first control switch when flowing along the first current path. The second control switch is coupled to a control terminal of the first control switch to deactivate the first control switch and allow the current to flow through the second current path. The circuit further includes a plurality of passive circuit elements configured to establish first and second current levels for the current when flowing along the first and second current paths, respectively, and disposed between the voltage source node and the evaluation node in a circuit arrangement in which no current path to ground is present when the sensed switch is open.

0072In a second aspect, a circuit for a sensed switch includes a comparator configured to detect a state of the sensed switch and having an input terminal at an evaluation node having a voltage level representative of the state of the sensed switch and through which wetting and sustaining currents flow from a voltage source node to the sensed switch when the sensed switch is closed. The circuit further includes first and second control switches disposed between the voltage source node and the evaluation node to switch between first and second current paths for the current. The wetting current passes through the first control switch when flowing along the first current path. The second control switch is coupled to a control terminal of the first control switch to deactivate the first control switch and allow the sustaining current to flow through the second current path. The circuit further includes a first resistance disposed along the first current path to establish a first current level for the wetting current when flowing along the first current path, and a second resistance disposed along the second current path and configured to establish a second current level for the sustaining current when flowing along the second current path. The second element is disposed between the evaluation node and the control terminal of the first control switch to trigger activation of the first control switch to allow the wetting current to flow along the first current path based on the voltage level of the evaluation node.

0073In a third aspect, a method of providing wetting current for a sensed switch includes sensing closure of the sensed switch to activate a first control switch to deliver current through the sensed switch and along a first current path at a first current level, activating a controller in response to an output of the comparator indicative of the closure of the sensed switch, and generating, with the controller, a control signal to switch the current to a second current path. Generating the control signal includes activating a second control switch to deactivate the first control switch. A plurality of passive circuit elements are configured to establish the first and second current levels and disposed between a voltage source node and an evaluation node at which a comparator detects the closure of the sensed switch in a circuit arrangement in which no current path to ground is present when the sensed switch is open.

0074Although described in connection with sensed switches for use in vehicles, the disclosed embodiments are not limited to any particular type or application of sensed switches. The sensed switches may be used to control any type of load. The sensed switches are thus not limited to motors (or DC motors), lamps, or other types of loads commonly present on vehicles. The sensed switches are thus also not limited to uses involving 12-Volt batteries or other batteries.

0075The disclosed embodiments are also compatible with a variety of different sensed switch environments. The wetting current diagnostics may be provided regardless of the external resistance and/or capacitance presented by the wiring harness and/or other components or aspects of the system in which the sensed switch is disposed. The disclosed embodiments may utilize a voltage threshold established for the comparator of the detection unit to avoid any requirements for customization to a specific switch environment.

0076Although described in connection with single-pole, single-throw switches, the disclosed embodiments are not limited to any particular type of switch. The number of poles may vary. The number of connection options may also vary. For example, the disclosed embodiments may be configured for use with double-throw or triple-throw switches.

0077While the wetting current diagnostics are useful for normally open switches, the disclosed embodiments may be used in connection with normally closed switches and/or other types of switches. The extent to which wetting current is useful for the sensed switch may vary.

0078The present invention is defined by the following claims and their equivalents, and nothing in this section should be taken as a limitation on those claims. Further aspects and advantages of the invention are discussed above in conjunction with the preferred embodiments and may be later claimed independently or in combination.

0079While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications may be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.