A resettable short-circuit protection circuit can terminate excessive fault currents automatically and quickly. The short-circuit protection circuit is switchable and has a low input impedance during normal operation so that there is not a significant voltage drop across the switching elements of the protection circuit. The short-circuit protection circuit allows a power source internal to a portable device to be safely connected to an external accessory where there exists the possibility that the connection could be shorted at the time power is first supplied to the external accessory or a short develops afterwards. After terminating a short-circuit condition, the protection circuit may be reset by cycling an enable signal. The fault termination and reset timing may be configured by selection of internal resistance and capacitance values.

FIELD OF INVENTION

The invention generally relates to fault protection devices, and more particularly, to a short-circuit protection circuit for protecting power sourcing equipment.

BACKGROUND

It has become relatively commonplace for portable devices, such as cellular phones, personal digital assistants (PDAs), laptop computers and the like, to supply power to certain external accessories, such as speakers, earphones, pointing devices and the like. The external accessories can present short-circuit conditions when attached to the portable device. In the portable devices, accidental short-circuiting of an external power supply output is a problem. Without short-circuit protection, if a power supply output is accidentally shorted to ground, the output voltage decreases and the output current increases significantly. The increased output current caused by the short circuit may damage the power sourcing device and its internal components.

Short-circuit protection circuits for portable devices are known. However, known short-circuit protection circuits are not completely adequate because they may not be able to terminate some short-circuit conditions quickly enough to prevent damage to the portable power sourcing device. Thus, there is a need for an improved short-circuit protection circuit that is suitable for use with portable devices.

SUMMARY

It is an advantage of the present invention to provide a resettable short-circuit protection circuit that can terminate excessive fault currents automatically and quickly. It is a further advantage of the invention to provide a resettable short-circuit protection circuit that has low impedance during normal operation so that there is not a significant voltage drop across the switching elements of the protection circuit.

In accordance with an aspect of the invention, the short-circuit protection circuit allows a power source internal to a portable device to be safely connected to an external accessory where there exists the possibility that the connection could be shorted at the time power is first supplied to the external accessory or a short develops afterwards. After terminating a short-circuit condition, the protection circuit may be reset by cycling an enable signal.

The invention is not limited to the above advantages and aspects. Other advantages and aspects of the invention will be or will become apparent to one with ordinary skill in the art upon examination of the following figures and detailed description. It is intended that all such advantages and aspects be included within this description, be within the scope of the invention, and be protected by the accompanying claims.

DETAILED DESCRIPTION

The following detailed description, which references to and incorporates the drawings, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit but only to exemplify and teach the invention, are shown and described in sufficient detail to enable those skilled in the art to practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art.

FIG. 1is a schematic diagram of a short-circuit protection circuit100in accordance with an exemplary embodiment of the present invention. The short-circuit protection circuit100includes a first p-channel metal-oxide-semiconductor field-effect transistor (MOSFET)102having a drain for receiving power from a power supply (PS)120and a gate (i.e., a control input) connected to a first node107. A second p-channel MOSFET (pMOSFET)112has a source connected to the source of the first pMOSFET102, a drain for outputting power to a load118, and a gate connected to the first node107. A first n-channel MOSFET (nMOSFET)103has its drain connected to the first node107, a source connected to a second node109, and a gate connected to the drain of the second pMOSFET102. A second nMOSFET104has its drain connected to the second node109, a source connected to a ground, and a gate for receiving an enable signal from a means for switching the enable signal, such as a controller.

A first resistor106is connected between the sources of the first and second pMOSFETs102,112and the first node107. A second resistor108is connected between the first node107and the second node109. A capacitor110is connected between the first node107and the second node109. A third resistor116is connected between the gate of the second nMOSFET104and the ground. A fourth resistor114connected between the gate of the first nMOSFET103and the ground.

The first, third and fourth resistors106,116,114can each have a value of 100K ohms, and the second resistor108can have a value of 1 Meg ohms. The capacitor110can have a value of 0.1 μF.

The power supply120is connected to a power source for supplying power to the first pMOSFET102. The power supply120is any suitable device for supplying a regulated voltage, such a switched-mode power supply (SMPS), including a boost converter. The power source is any suitable means for providing electrical power, such as a battery. The output of the power source can be conditioned by passive elements (not shown) such as one or more inductors, resistors and/or capacitors, and/or active elements, such as one or more diodes, zener diodes and the like, as is known to those skilled in the art. The power source provides power at a source voltage Vsourceto the input of the power supply120. Internal resistance of the power supply120causes a voltage drop proportional to the load current Iload, and thus, the power supply may output a voltage Vsourcethat is slightly less than the source voltage Vsource.

The load118receives load current Iloadand load voltage Vloadfrom the output of the short-circuit protection circuit100. The load118is any electrical device attachable to the output of the protection circuit100that can receive power from the protection circuit100. By way of example, the load118can be one or more speakers, a headphone set, microphone, camera, disk drive, memory stick, printer or the like.

The first pMOSFET102, when off, blocks reverse currents flowing into the protection circuit from the load118, thus protecting the power supply120and power source from the reverse currents. The current blocking is provided by the internal diode of the first pMOSFET102. The first pMOSFET102is optional, and it may be omitted from the circuit100entirely, or alternatively, it may be bypassed by short circuit113if its reverse blocking functionality is not desired.

The first pMOSFET102(when included) and the second pMOSFET112provide a low impedance switching path through the protection circuit100from Vsource′to Vload.

The operation of the protection circuit100is now described. Initially, the protection circuit100is in an off-state. When the load118is connected in a normal state (no short-circuit condition present), the enable is driven high, which turns on nMOSFET104. The capacitor110, which was discharged to zero volts during the off-state via the second resistor108, is pulled down to ground. This pulls the gate(s) of pMOSFET102,112to nearly ground for a period determined by the RC time constant of first resistor106and capacitor110. At this point, the pMOSFETs102,112are enhanced, routing power to Vloadand to the gate of nMOSFET103. The nMOSFET103is thus enhanced, shorting the capacitor100and further enhancing the pMOSFETs102,112. This positive feedback continues until the pMOSFETs102,112and the first nMOSFET103are maximally enhanced.

If the protection circuit100transitions from an off-state to an on-state, and a short-circuit condition initially exists at the load, the above-described operational condition is established, except that the first nMOSFET103may not be enhanced due to the short at VIoad. Thus, the pMOSFETs102,112turn on momentarily for a period determined by the RC time constant of the first resistor106and the capacitor110, and then turn off.

If a short circuit occurs during a normal on-state, Vloadis depressed by a value determined by the load current Iloadand the sum of the supply path resistances: the power supply120resistance Zsource, the channel resistance of the first pMOSFET102Rdson, and the channel resistance of the second pMOSFET112Rdson. At the point that the gate voltage of the first nMOSFET103is depressed below its threshold Vgsth, the first nMOSFET103turns off, and subsequently the pMOSFETs102,112turn off in a time determined by the RC time constant of the first resistor106and the capacitor110.

After a short-circuit condition occurs on the load118, the short-circuit protection circuit100is reset by toggling the enable signal for a predetermined period of time based on the RC time constant which is the product of the values of the capacitor110and the second resistor108. When Vloadhas been detected to have been disconnected either at the initial attempt to supply power into an existing load short or due to a temporary short terminating Vload, the enable signal may be driven low for a period determined by the time constant of the second resistor108and the capacitor110, and then driven high again. This will allow the capacitor110to discharge such that the subsequent low-to-high transition on the enable input will cause a “fresh” turn on, as described above for the off-state to on-state transitions.

FIG. 2is a schematic diagram of a short-circuit protection circuit200in accordance with another exemplary embodiment of the present invention. The protection circuit200ofFIG. 2generally performs in the same manner as the protection circuit100ofFIG. 1. However, in this embodiment, the protection circuit200includes a first NPN bipolar junction transistor (BJT)202in place of the first nMOSFET103and a second NPN BJT204in place of the second nMOSFET104. The topology of the passive network of the resistors206,208and capacitor210is the same as that shown inFIG. 1for resistors106,108and capacitor110, but the values of resistors206,208and capacitor210are different than their counterparts ofFIG. 1to adjust for the different performance characteristics of the NPN transistors202,204. A biasing resistor212is connected between the base (i.e., control input) of the first NPN transistor212and the drain of the pMOSFET112. A biasing resistor network213is connected between the enable input, base and emitter of the second NPN transistor204.

The circuits100,200can be implemented using any suitable technology, including discrete components, integrated circuit technology or any combination the foregoing. In an exemplary discrete component implementation, the pMOSFETs102,112can be part no. FDC640P, available from Fairchild Semiconductor Corporation, and the nMOSFETs103,104can be part no. BSS138, also available from Fairchild Semiconductor Corporation.

AlthoughFIGS. 1 and 2depict the power supply120as residing outside of the protection circuits100,200, the power supply120, as well as the power source, can be included within either of the protection circuits100,200.

FIG. 3is a signal trace diagram300illustrating exemplary operation of the short-circuit protection circuit100ofFIG. 1. In this operational scenario, a positive source voltage Vsourceis continuously supplied to the input of the power supply120. The power supply120outputs a positive voltage Vsource′, which is slightly less than the source voltage Vsource. At time t0, the enable signal goes high, causing the protection circuit100to supply Vloadat its output to the load118.

At time t1, the output current Iloadbegins to increase due to a short circuit on the load118. This causes the supply voltage Vsourceand output voltage Vloadto decrease. The load current Iloadcontinues to increase during the short-circuit condition until the output voltage Vloaddrops to the gate-source threshold voltage Vgsthof the nMOSFET103at time t2. At this point, nMOSFET103turns off, which turns off the pMOSFETs102,112. Iloaddrops to zero, and Vsource′returns to its normal value.

At time t3, the enable signal is toggled off for a period of time which is at least the RC constant of the second resistor108and the capacitor110. This resets the circuit100so that it can continue to supply power to the load118after the enable signal returns to high, at time t4.

FIG. 4is a conceptual block diagram of a wireless mobile handset400(e.g., a cellular phone, personal digital assistant (PDA) or the like) that includes a short-circuit protection circuit414, which is either of the short-circuit protection circuits100,200ofFIGS. 1 and 2. The wireless mobile handset400includes at least one antenna401, a controller402having a processor and memory (not shown) and an air interface with radio frequency transceiver424having a transmitter (Tx)408and a receiver (Rx)410. The controller402is generally implemented in one or more digital signal processors (DSPs) and/or application specific integrated circuits (ASICs) and includes power management features. The controller memory stores one or more software programs executed by the controller to perform its functions.

The controller402includes a signal interface for driving the enable input of the short-circuit protection circuit414to selectively enable, disable or reset the protection circuit414. The controller402also includes means for monitoring Vloadto determine its value. Such means can include an A/D converter. The controller402can determine when the enable signal should be toggled in order to reset the protection circuit414by periodically comparing measured and expected values Vloadwith the current state of the enable signal.

Other embodiments and modifications of the invention will readily occur to those of ordinary skill in the art in view of the foregoing teachings. Thus, the above summary and detailed description is illustrative and not restrictive. The invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings. The scope of the invention should, therefore, not be limited to the above summary and detailed description, but should instead be determined by the appended claims along with their full scope of equivalents.