Abstract:
This document discusses, among other things, apparatus and methods for a detection circuit. In an example, the detection circuit can include a voltage divider configured to receive a first supply voltage from an external device coupled to the detection circuit, first and second transistors configured to receive a control voltage from the voltage divider and to couple an output to ground when the control voltage exceeds a first threshold, and a bias circuit configured to bias the first transistor to set the first threshold.

Description:
CLAIM OF PRIORITY 
     This patent application claims the benefit of priority, under 35 U.S.C. Section 119(e), to Maher, U.S. Provisional Patent Application Ser. No. 61/449,876, entitled “OVER-VOLTAGE TOLERANT LEVEL DETECTION CIRCUIT,” filed on Mar. 7, 2011, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     As mobile electronic devices have become more common and the functionality of the devices have expanded, designers have implemented features that provide a better user experience. Automatic recognition of an attached accessory device is one of many such features. However, to implement these features, a detection circuit can be required that consumes significant power or can be susceptible to accessories that expose the circuit to substantially higher voltages than the mobile device can tolerate. 
     OVERVIEW 
     This document discusses, among other things, apparatus and methods for a detection circuit. In an example, the detection circuit can include a voltage divider configured to receive a first supply voltage from an external device coupled to the detection circuit, first and second transistors configured to receive a control voltage from the voltage divider and to couple an output to ground when the control voltage exceeds a first threshold, and a bias circuit configured to bias the first transistor to set the first threshold. 
     This section is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1  illustrates generally an example detection circuit according to the present subject matter. 
         FIG. 2  illustrates generally a plot of the output of a detection circuit, such as the example shown in  FIG. 1 . 
         FIG. 3  illustrates generally transient waveforms of a detection circuit, such as the example shown in  FIG. 1 . 
         FIG. 4  illustrates generally a relationship of the setpoint resistance to the input voltage that will trigger transition of the output of a detection circuit, such as the example circuit of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices, including portable electronic devices, can include ports for accommodating accessory devices that extend or enhance the electronic devices. Such ports can include, but are not limited to, universal serial bus (USB) ports, for example. The ports can include a supply terminal to supply to, or receive power from, the accessory device. Some devices can include ports that allow connection of a diverse range of accessory devices. Some electronic devices include detection circuits to identify an accessory or an operating condition of an accessory, such as an external voltage coupled to a supply line or other terminal of the accessory port. The detection circuit can be active, and consume power, even when an accessory device is not present. For portable electronic devices that can have a limited power supply, such a circuit can reduce the useable interval of the device between charging of the limited power supply or replacement of the limited power supply. A detection circuit can include more expensive, high voltage components to accommodate the diverse array of accessory devices that can be coupled to the accessory port. 
       FIG. 1  illustrates generally an example detection circuit  100 . In an example, a detection circuit  100  can include an output  105  configured to provide an indication that a device is coupled to an input  101 . In certain examples, the detection circuit  100  can be employed with a mobile electronic system, such as a cell phone or personal media player. In some examples, the input  101  can include a portion of a communication connector, such as a universal serial bus (USB) connector. In certain examples, the input  101  can include a power supply terminal of a communication connector, such as a VBUS terminal of a USB connector. In an example, the output  105  can be indicative of a minimum threshold voltage present on the input  101 . 
     The detection circuit  100  can include a voltage divider  102 . The voltage divider  102  can include a tap that can be coupled to a control node of first and second transistors  103 ,  104 . The first and second transistors  103 ,  104  can be coupled in series between the output  105  and a reference voltage, such as ground. In an example, the output  105  can be coupled to a pull-up resistor  106 . 
     The detection circuit  100  can include bias circuitry  107 . The bias circuitry  107  can include a setpoint resistance  108  and a bias switch  109 , such as a transistor. The detection circuit  100  can detect a voltage level applied to the input  101  that is up to about four times higher than the gate oxide rating of a particular process, such as the process by which the first and second transistors  103 ,  104  are produced, without using high voltage components. The detection circuit  100  can be configured to use an external supply voltage coupled to the input  101  for power such that when an accessory is not connected to the input, the detection circuit  100  consumes substantially no power. 
     In certain examples, as a voltage applied at the input  101  rises, a scaled voltage can be applied to the control nodes of the first and second transistors  103 ,  104 . The scaled voltage can cause the first and second transistors  103 ,  104  to conduct and pull the output  105  to a low logic level, thus providing an indication that a device is coupled to the input  101 , for example. In certain examples, the detection circuit  100  can include additional logic, such as a buffer or an inverter, to isolate the detection circuit  100  from other circuits coupled to the output  105 , or to provide a different logic level. 
     In an example, as the voltage at the input  101  rises, conduction through the first transistor  103  can be delayed using a bias voltage received at a pre-charge node  111  common to both the first and second transistors  103 ,  104 . For example, the pre-charge node  111  can be biased to a voltage VDD−Vt Q3 , where Vt Q3  is the threshold voltage of the bias switch  109  of the bias circuitry  107 . As the second transistor  104  conducts, the voltage at the source can fall according to a function of the setpoint resistance  108 . From a steady state viewpoint, the setpoint resistance  108  can allow non-linear adjustment of the threshold. In an example, the setpoint resistance  108  can be adjustable. In certain examples, a higher setpoint resistance can correspond to a lower threshold. Therefore, in certain examples, the output  105  of the detection circuit  100  can indicate not only that a device is coupled to the port, but that a voltage at the input  101  is at least at or above a minimum threshold voltage level. 
     In an example, a capacitor  110  can be used to filter out high frequency noise as well as control the edge rates of the voltage applied to the control nodes of the first and second transistors  103 ,  104 . In certain examples, other passive or active devices can be used to provide edge rate control. In certain examples, an integrated circuit can include the first and second transistors  103 ,  104 , the bias circuitry  107 , and the pull-up resistance  106 . In certain examples, the setpoint resistance  108  can include a semiconductor resistor. In certain examples, the integrated circuit can further include the edge rate control devices, such as the capacitor  110 . 
       FIG. 2  illustrates generally a plot of the output of an example detection circuit, such as the example detection circuit  100  illustrated in  FIG. 1 . The output of the circuit is represented on the vertical axis and the input of the circuit is represented by the horizontal axis. In an example, as the voltage at the input  101  increases from zero, a control voltage, such as a control voltage for the first and second transistors  102 ,  103 , rises. As the control voltage exceeds a threshold, the first transistor  103  couples the output  105  to ground. As the voltage at the input  101  falls, the control voltage triggers a high impedance state of the first transistor  103 . The high impedance state of the first transistor  103  decouples the output  105  from ground and the pull-up resistor  106  pulls the output  105  to a high logic state. 
       FIG. 3  illustrates generally transient waveforms of an example detection circuit, such as the example detection circuit  100  of  FIG. 1 , where the output includes an inverter. A voltage applied to the input can form a triangular waveform  301  as it transitions from 0 to about 5 volts and back to 0 volts. The output waveform  302  can go from a logic low level to a logic high level as the input voltage rises past about 3 volts. The output logic level can remain at a high level until the input voltage goes below about 2.5 volts. A second triangular waveform  303  can illustrate the scaled voltage of the input applied to the control nodes of the first and second transistors. Notice that the circuit provides a hysteretic behavior of the output in relation to the input voltage. For example, the output of the circuit transitions from low logic to high logic at a first input voltage and transitions from high logic to low logic at a second input voltage. In certain examples, the second input voltage is less than the first input voltage. 
       FIG. 4  illustrates generally a relationship of the setpoint resistance to the input voltage that will trigger a transition of the output of a circuit, such as the example circuit of  FIG. 1 . 
     Additional Notes 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     The above description is intended to be illustrative, and not restrictive. In other examples, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.