Electrical circuit for selecting a desired power source

A circuit that allows selection of a power source among a plurality of power sources is disclosed. In an embodiment, either one or both voltage sources Vaux and Vapp may be available in a system. If both sources are available, then the circuit enables the system to use source Vaux. However, if only one of the two sources is available, then the circuit enables the system to use that available source.

FIELD OF THE INVENTION

The present invention relates generally to electrical circuits and, more specifically, to a circuit that enables selection of a desired power source.

BACKGROUND OF THE INVENTION

In various applications, there is a need to select a desired power source, e.g., a voltage source, among various voltage sources. Many approaches use expensive, larger, and specialized chips that would monitor and control multiple voltage sources of different values. These chips also need additional support components such as a separate source to power up the chips, and may cause a center point of failure within the circuit using the chips because if a chip or the source supporting the chip does not function properly, then it renders the whole circuit inoperable. Therefore, it is desirable that mechanisms be provided to solve the above deficiencies and related problems.

SUMMARY OF THE INVENTION

The present invention is related to a circuit that allows selection of a power source such as a voltage source, a current source, etc., among a plurality of power sources. In an embodiment, either one or both voltage sources Vauxand Vappmay be available in a system. If both sources are available, then the circuit enables the system to use source Vaux. However, if only one of the two sources is available, then the circuit enables the system to use that available source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the invention.

Embodiments of the invention use Field Effect Transistors (FETs) that operate differently, depending on whether the transistor is a P-channel or an N-channel. A FET having a P-channel or an N-channel is commonly referred to as a PFET or an NFET, respectively. For illustration purposes, the voltage at the gate of a FET with respect to its source is referred to as voltage VGS, and the voltage at the drain of the FET with respect to its source is referred to as voltage VDS. The threshold voltage to turn on a FET is referred to as voltage VT, which is negative for a PFET and positive for an NFET. When a FET is turned on, the amount of current flows between the source and the drain depends on the relationship between |VDS| and |VGS−VT|, and the current flows from the drain to the source in an NFET, and from the source to the drain in a PFET. Those skilled in the art will recognize that the direction of the current flows varies depending on points of references, whether the current is represented by protons, electrons, etc.

Exemplary family members of FETs include Junction Field-Effect Transistors (JFETs) and Insulated-Gate Field-Effect Transistors (IGFET), which are commonly referred to as Metal-Oxide-Semiconductor FET (MOSFET). The invention is not limited to a particular type of FET, and embodiments of the invention may use equivalences of FETs.

Embodiments of the Invention, Positive Voltage Sources

FIG. 1shows a circuit100upon which embodiments of the invention may be implemented. Transistor X1is an NFET, while transistors X2and X3are PFETs. Depending on various situations, circuit100selects either power or voltage source Vauxor Vappto be available as voltage Vout. In an embodiment, sources Vauxand Vappare of the same positive voltage level of 3.3V, voltage threshold VTis −0.8V for PFET X2and X3and 0.8V for NFET X1. That is, if VGSfor PFETs X2and X3are less than or equal to −0.8V, then PFETs X2and X3are on, and are off otherwise. Conversely, if VGSfor NFET X1is greater than or equal to 0.8V, then NFET X1is on, and is off otherwise.

However, the invention is not limited to a particular level of threshold VT, source Vaux, or source Vapp. Various levels working with transistors are within the scope of embodiments of the invention. For example, threshold VTmay be at 0.65V, 0.7V, −0.65V, −0.7V, etc., and Vauxand/or Vappmay range from −12V to +12V, etc. Nevertheless, when sources Vauxand Vappare negative, then PFETs X2and X3are replaced by NFETs, and NFET X1is replaced by a PFET. Further, a current source and/or a device operating with a voltage source that can power on/off the transistors are within the scope of embodiments of the invention.

When Both Sources VAUXand VAPPare Available

At the same time, voltage Vauxat gate G22passes through resistor R1and results in about 3.3V at gate G12of NFET X1. As a result, VGSof X1is at 3.3V, which is greater than VTof X1at 0.8V, and NFET X1is therefore on. Because NFET X1is on, the voltage at drain D16of NFET X1is pulled to the voltage at its source S14, which is ground or 0V. Further, NFET X1being on serves as a current path for the current provided by voltage source Vapp. That is, NFET X1allows this current to flow through drain D16and source S14to ground.

Additionally, the current at gate G22of PFET X2flows through drain D36and source S34of PFET X3to result in a voltage 3.3V at source S34of PEET X3. Because voltage at gate G32and at source S34of X3is at 0V and 3.3V, respectively, VGSof X3is −3.3V, which is less than VTof PFET X3at −0.8V, and which confirms that PFET X3is on, enabling current to flow between drain D36and source S34. Voltage Voutis in fact voltage at source S34, which, as discussed, is provided by voltage Vaux.

When Only Source VAUXis Available

When only source Vauxis available, voltage at gate G22is at 3.3V and at drain D26is 0V, resulting in 0V for VGSof PFET X2, which turns PFET X2off because turning on PFET X2requires a VGSof less than or equal to −0.8V. Similar to the above situation when PFET X2is off, voltage Vauxat gate G22passes through resistor R1and results in about 3.3V at gate G12of NFET X1. As a result, VGSof X1is at 3.3V, which is greater than VTof X1at 0.8V, and NFET X1is therefore on. Because NFET X1is on, the voltage at drain D16of NFET X1is pulled to the voltage at its source S14, which is ground or 0V. Further, the current at gate G22of PFET X2flows through drain D36and source S34of PFET X3to result in a voltage 3.3V at source S34and drain D36of PFET X3. Because voltage at gate G32and at source S34of X3is at 0V and 3.3V, respectively, VGSof X3is −3.3V, which is less than VTat −0.8V of PFET X3, and which confirms that PFET X3is on, enabling current to flow between drain D36and source S34. Voltage Voutis in fact voltage at source S34, which, as discussed, is provided by voltage Vaux.

When Only Source VAPPis Available

When only source Vappis available, source Vauxis not available, resulting in 0V at gate G22, and 3.3V at drain D26. Therefore VGSof PFET X2is −3.3V, which turns it on and thus allows current to flow between drain D26and source S24. Voltage at source S24is thus pulled to voltage at drain D26, i.e., voltage of Vapp, and is thus at 3.3V. In fact, voltage at source S24is the same as at source S34and is also Vout.

Voltage at gate G22at 0V results in 0V at gate G12, and also results in 0V for VGSof NFET X1, which turns NFET X1off.

Voltage Vappof 3.3V at drain D26results in 3.3V at drain D16, which is the same as at gate G32. Because voltage at gate G32and at source S34is both at 3.3V, resulting in 0V for VGSof X3, which turns X3off.

The above analysis is consistent with the fact that voltage Voutis at 3.3 V and is provided by source Vapp.

Summary of Operation of Circuit100

FIG. 2shows a table200summarizing the operation of circuit100. Line210indicates that when both Vauxand Vappare available, Voutis at 3.3V and is provided by source Vaux. Line220indicates that if only Vauxis available, then Voutis at 3.3V and is provided by source Vaux. Line230indicates that if only Vappis available, then Voutis at 3.3V and is provided by source Vapp. Line240indicates that if both Vauxand Vappare not available, then Voutis at 0V.

Resistors and Capacitors

Resistor R1, together with capacitor C1, constitutes an RC circuit that eliminates conflict or race conditions between sources Vauxand Vappwhen both sources are available. This RC circuit controls the rate of time it takes for the gate of PFET X1to be charged on. In the embodiment ofFIG. 1, the selected value for this RC circuit slows down the activation of the gate of PFET X1by 1/10*6 second. As a result, if Vapparrives at the RC circuit before Vauxdoes, then the RC circuit allows time to shutdown voltage Vappand thus allows Vaux, instead of Vapp, to be selected as Vout.

Resistor R2provides a path for current to sink from gate G22. Resistor R3limits the amount of current being sunk through PFET X1when X1is on. In theFIG. 1embodiment, the current limit is 0.67A (=3.3V/5K) or 2.1 mW.

Resistors R1, R2, and R3are at 20K OHM, 5K OHM, and 5K OHM, respectively, and capacitor C1is at 0.1 micro Fahrad. Selecting a value for resistors R1, R2, and R3is based on the resistor's power dissipation, the ability of the source to provide for the current drawn by the resistors, etc. Those skilled in the art will recognize that the lower value or the lower resistance of a resistor results in increased power loss through the resistor. As indicated above resistor R3sinks 2.1 mW, and a resistor rated to 62 mW, which is much higher than 2.1 mW, was selected for R3.

Additional Applications for Circuit100

Embodiments of the invention are applicable when one or a plurality of circuits100is added to another circuit100so that the resulting circuit may select a desired voltage source from a plurality of voltage sources. There are various ways to add a circuit100to an existing circuit having multiple circuits100. Embodiments of the invention are applicable in those various ways wherein a circuit100receives two voltage sources as direct inputs. For example, the output source of a first circuit100and another source may be direct inputs to a second circuit100. Alternatively, two output sources of two circuits100are direct inputs to a third circuit100, etc. The invention is not limited to how a circuit100is added to another circuit having circuits100. InFIG. 1, sources Vauxand Vappare direct inputs to circuit100.

FIG. 3shows a first circuit300using circuits100(1) and100(2), in accordance with an embodiment. Circuit100(1) and circuit100(2) operate in accordance with the described-above explanation. That is, circuit100(1) selects appropriate voltage sources V1and V2to provide voltage Vout1. Circuit100(2) selects appropriate voltage sources Vout1and V3to provide voltage Vout2.

FIG. 4shows a second circuit400using circuits100, in accordance with an embodiment. Circuit400is similar to circuit300with the addition of circuit100(3) that appropriately selects voltage sources Vout2and V4to provide voltage Vout3.

FIG. 5shows a third circuit500using circuits100, in accordance with an embodiment. Circuit500includes the same number of circuits100(1),100(2), and100(3) as in circuit400, but have different arrangements. Circuit100(1) selects voltage sources V1and V2to provide voltage Vout1. Circuit100(2) selects voltage sources V3and V4to provide voltage Vout2, and circuit100(3) selects voltage sources Vout1and Vout2to provide voltage Vout3.

Using the above example, one or a combination of one or a plurality of circuits100,300,400, and500may be used to build a circuit that can select a desired voltage source among various sources.

Embodiments of the Invention, Negative Voltage Sources

In the embodiment ofFIG. 1, voltage sources Vauxand Vappare positive. However, embodiments of the invention are also applicable when Vauxand Vappare negative. In such a case, PFETs X2and X3are replaced by NFETs, e.g., X2′ and X3′, respectively, and NFET X1is replaced by a PFET, e.g., X1′. For illustration purposes, negative input voltage sources Vauxand Vappare referred to as V′auxand V′app, and negative output Voutis referred to as V′out.FIG. 6shows a circuit600using negative input voltage sources as described, in accordance with an embodiment.

When Both Input Sources V′AUXand V′APPare Available

When both sources V′auxand V′appare available, the voltage at gate G22′and at source S24′ of NFET X2′ is −3.3V (Vaux) and −3.3V (Vout), respectively. Consequently, VGSof X2′ is at 0V, which is less than VTof X2′ at 0.8V, and NFET X2′ is therefore off.

At the same time, voltage V′auxat gate G22′ passes through resistor R1and results in about −3.3V at gate G12′ of PFET X1′. As a result, VGSof X1′ is at −3.3V, which is lesser than VTof X1′ at −0.8V, and PFET X1′ is therefore on. Because PFET X1′ is on, the voltage at drain D16′ of PFET X1′ is pulled to the voltage at its source S14′, which is ground or 0V. Further, PFET X1′ being on serves as a current path for the current provided by voltage source V′app. That is, PFET X1′ allows this current to flow between drain D16′, source S14′, and ground.

Additionally, the current at gate G22′ of NFET X2′ flows through drain D36′ and source S34′ of NFET X3′ to result in a voltage −3.3V at source S34′ of NFET X3′. Because voltage at gate G32′ and at source S34′ of X3is at 0V and −3.3V, respectively, VGSof X3′ is 3.3V, which is greater than VTof NFET X3′ at 0.8V, and which confirms that NFET X3′ is on, enabling current to flow between drain D36′ and source S34′. Voltage V′outis in fact voltage at source S34′, which, as discussed, is provided by voltage V′aux.

When Only Source V′AUXis Available

When only source V′auxis available, voltage at gate G22′ is at −3.3V and at drain D26′ is 0V, resulting in 0V for VGSof NFET X2′, which turns NFET X2′ off because turning on NFET X2′ requires a VGSof more than or equal to 0.8V. Similar to the above situation when NFET X2′ is off, voltage V′auxat gate G22′ passes through resistor R1and results in about −3.3V at gate G12′ of PFET X1′. As a result, VGSof X1′ is at −3.3V, which is less than VTof X1′ at −0.8V, and PFET X1′ is therefore on. Because PFET X1′ is on, the voltage at drain D16′ of PFET X1′ is pulled to the voltage at its source S14′, which is ground or 0V. Further, the current at gate G22′ of NFET X2′ flows through drain D36′ and source S34′ of NFET X3′ to result in a voltage −3.3V at source S34′ and drain D36′ of NFET X3′. Because voltage at gate G32′ and at source S34′ of X3′ is at 0V and −3.3V, respectively, VGSof X3′ is 3.3V, which is greater than VTat 0.8V of NFET X3′, and which confirms that NFET X3′ is on, enabling current to flow between drain D36′ and source S34′. Voltage V′outis in fact voltage at source S34′, which, as discussed, is provided by voltage V′aux.

When Only Source V′APPis Available

When only source V′appis available, source V′auxis not available, resulting in 0V at gate G22′, and −3.3V at drain S36′. Therefore VGSof NFET X2′ is 3.3V, which turns it on and thus allows current to flow between drain D26′ and source S24′. Voltage at source S24′ is thus pulled to voltage at drain D26′, i.e., voltage Of V′app, and is thus at −3.3V. In fact, voltage at source S24′ is the same as at source D34′ and is also V′out.

Voltage at gate G22′ at 0V results in 0V at gate G12′, and also results in 0V for VGSof PFET X1′, which turns PFET X1′ off.

Voltage V′appof −3.3V at drain D26′ results in −3.3V at drain D16′, which is the same as at gate G32′. Because voltage at gate G32′ and at source S34′ is both at −3.3V, resulting in 0V for VGSof X3′, which turns X3′ off.

The above analysis is consistent with the fact that voltage V′outis at −3.3V and is provided by source V′app.

Similar to positive input voltage sources, various combinations of circuits600that receive negative input voltage sources may be built to create a circuit that selects a negative input source among many negative sources.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof. However, it will be evident that various modifications and changes may be made without departing from the broader spirit and scope of the invention. For example, resistive elements and capacitive elements may work in place of resistors and capacitors, respectively; switches may work in place of transistors, etc. Accordingly, the specification and drawings are to be regarded as illustrative rather than as restrictive.