FIELD OF INVENTION

The invention relates to an overshoot protection circuit. More particularly, the invention relates to an overshoot protection circuit suitable for use in a low quiescent current low-dropout voltage regulator.

BACKGROUND OF THE INVENTION

Low-dropout voltage regulator or LDO voltage regulator is one of the fundamental components of a power management unit. A typical low-dropout voltage regulator comprises a reference voltage source, an error amplifier (EA) and series pass transistor element (BJT or MOSFET). Referring to Fig. 1, illustrated is a schematic diagram of a conventional low-dropout voltage regulator. The low-dropout voltage regulator is used to provide a constant and stable output voltage regardless of load impedance, input voltage and temperature variation. This low-dropout voltage regulator is suitable for use in power-budgeted applications, such as battery-powered devices or green-compliant equipment with long stand-by operation, where it helps extend the battery life and reduces the overall power consumption. The low-dropout voltage regulator is also capable of minimizing current consumption down to micro amperes or even to nano-amperes, which is crucial for current consumption of the devices in sleep mode and during zero current loads.

However, a low-dropout voltage regulator with a low quiescent current will result in a low bandwidth error amplifier. This slows down the transient response of the regulated output voltage and causes the voltage to overshoot and undershoot during current load switching from high to low and low to high, respectively, as depicted in Fig. 2. Subsequently, a large overshoot on the regulated output voltage may damage hundreds or even millions of transistors that are tied to this low-dropout voltage regulator. Meanwhile, the effect of voltage undershooting is not critical as voltage overshooting.

There are a few patented technologies from the prior art relating to the overshoot protection system. US8169202B2 discloses a low-dropout regulator capable of preventing damage caused by an overshooting on regulated output voltage, the low- dropout regulator comprises a pass transistor to receive an unregulated power supply voltage to generate a regulated output voltage, a constant overcurrent limiting circuit to limit an output current through the pass transistor to below a predetermined current and a foldback overcurrent limiting circuit to enable the constant overcurrent limiting circuit to further decrease the output current.

Accordingly, it would be desirable to provide an overshoot protection circuit which implements a novel technique for suppressing overshooting on regulated output voltage of a low-dropout voltage regulator while maintaining its low quiescent current requirement. This invention provides such a circuit and method thereof.

SUMMARY OF INVENTION

One object of the invention is to provide an overshoot protection circuit on regulated output voltage of low-dropout voltage regulator. Another object is to maintain a low- dropout voltage regulator at a low quiescent current while providing this overshoot protection circuit to the low-dropout voltage regulator.

The invention provides an overshoot protection circuit comprising an AC coupled circuit to detect an overshooting on regulated output voltage of a main low-dropout voltage regulator; a non-inverting amplifier for voltage signal amplification upon a detection of spike on regulated output voltage, a pull-down transistor to suppress the overshooting on regulated output voltage and a delay circuit to prolong viability of the non-inverting amplifier for having sufficient time to complete the process of overshooting suppression, characterized in that the non-inverting amplifier is operable between an inactive mode and an active mode, where the non-inverting amplifier is maintained in an idle state during the inactive mode and is initiated for voltage signal amplification during the active mode, wherein the non-inverting amplifier is further coupled to a current load sensor to detect current load level of the main low-dropout voltage regulator and to instruct the non-inverting amplifier to switch between the two modes based on the detected current load level of the main low-dropout voltage regulator.

Preferably, the current load sensor mirrors a pass-gate transistor to sense a fraction of current load level driven by main low-dropout voltage regulator.

Preferably, the current load sensor compares the current flow with a value indicative of an overshoot and thereby activating the non-inverting amplifier that is normally maintained at an idle state to amplify the spike signal in the event of an overshoot.

Preferably, the pull-down transistor is providing a sinking current path to ground to suppress overshooting on regulated output voltage when the amplified signal by amplifier exceeds a threshold voltage off pull-down transistor.

In a further aspect of this invention, there is provided a method for suppressing overshooting on a low quiescent low-dropout regulated output voltage with an overshooting protection circuit, the method comprising the steps that are characterized by detecting, by a current load sensor, current load level of the main low-dropout voltage regulator and thereby instructing a non-inverting amplifier to switch between an inactive mode and an active mode based on the sensed current load level; detecting, by an AC coupled circuit, an overshooting voltage signal and transmit to the non-inverting amplifier for signal amplification; and suppressing, by a pull-down transistor, the overshooting on regulated output voltage upon receiving the amplified signal from the non-inverting amplifier.

Preferably, the method further comprises the steps of comparing, by the current load sensor, the current flow with a value indicative of an overshoot and activating, by the current load sensor, the non-inverting amplifier that is normally maintained at an idle state to amplify the spike signal in the event of an overshoot.

Preferably, the method further comprises the steps of disabling the non-inverting amplifier when the current load level falls within certain current value.

One skilled in the art will readily appreciate that the invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments described herein are not intended as limitations on the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawing the preferred embodiments from an inspection of which when considered in connection with the following description, the invention, its construction and operation and many of its advantages would be readily understood and appreciated.

Fig. 1 is a block diagram illustrating a conventional low-dropout voltage regulator.

Fig. 2 is a waveform illustrating undershoot and overshoot voltage when the current load is switching from low to high and from high to low.

Fig. 3 is a block diagram illustrating an overshoot protection circuit according to the present invention.

Fig. 4 is a flow chart illustrating a method for suppressing overshoot based on the overshoot protection circuit.

Fig. 5 is a graph showing comparison of results between a main low-dropout voltage regulator with and without overshoot protection circuit (OPC) during current load switching from high to low.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an overshoot protection circuit for supressing overshoots in a circuity. Although the invention can be presented in a number of different embodiments, and incorporating a variety of different components and technologies, the various embodiments of the invention have some common elements. According to the concept of the invention, an overshoot protection circuit is provided with a non-inverting amplifier that is operable between an inactive mode and an active mode, whereby the non-inverting amplifier is maintained in an idle state during the inactive mode and is initiated for signal amplification during the active mode, where the mode switching of the non-inverting amplifier is initiated by a load current sensor based on a current load level of main low-dropout voltage regulator.

The invention will now be described in greater detail, by way of example, with reference to the drawings.

Referring to Fig. 3, there is provided an overshoot protection circuit 1 comprising a current load sensor 13 for detecting a current load of main low-dropout voltage regulator 2, an AC coupled circuit 14 to transmit a signal to a non-inverting amplifier 11 for signal amplification upon a detection of spike on the regulated output voltage and a pull-down transistor 12 for suppressing the overshooting voltage to a stable voltage upon receiving the amplified signal from the non-inverting amplifier 11. In this particular embodiment, the current load sensor 13 is coupled to the non-inverting amplifier 11 to sense current load level of the main low-dropout voltage regulator and to instruct the non-inverting amplifier 11 to switch between either an inactive mode or an active mode based on the detected current load level. Preferably, the non-inverting amplifier 11 is maintained in an idle state during the inactive mode and is initiated for signal amplification during the active mode.

In one preferred embodiment, the non-inverting amplifier 11 is disabled and is normally maintained at the idle state. The non-inverting amplifier 11 is activated by receiving voltage signal through delay circuit 15 upon initiation by the current load sensor 13. Preferably, the non-inverting amplifier 11 is a single-stage amplifier, a multi-stage amplifier or a transconductance amplifier. By way of example, a non inverting amplifier 11 is used in this application. The non-inverting amplifier 11 provides a positive terminal input that is coupled to the AC coupled circuit 14, a negative terminal input coupled to a resistor divider network, and an output that is connected to the pull-down transistor 12.

Preferably, the current load sensor 13 converts a current sensed to voltage signal with a value indicative of an overshoot and thereby activating the non-inverting amplifier 11 that is normally maintained at an idle state to amplify the spike voltage signal in the event of an overshoot.

In one preferred embodiment, a delay circuit 15 is provided for providing an input that is connected to the resistor divider network in the current load sensor 13 and an output connected to enable input terminal of the non-inverting amplifier 11. Advantageously, the delay circuit 15 delays input signal for the non-inverting amplifier to certain time prolongs viability of the non-inverting amplifier 11 for having sufficient time to complete the process of overshooting suppression. By way of example, the delay circuit 15 is in the form of a gate delay circuit topology, or a resistor and capacitor delay topology, or any combination thereof.

In one particular embodiment, the pull-down transistor 12 provides a sinking current path to ground the main low-dropout voltage regulator 2 to suppress the overshoot voltage to a stable voltage. The pull-down transistor 12 is enabled by non-inverting amplifier 11 when the amplified signal exceeds the threshold voltage of pull-down transistor. Preferably, the pull-down transistor 12 provides a drain connected to the output of main low-dropout voltage regulator 2, a gate connected to the output of the non-inverting amplifier 11, and a source connected to ground.

Fig. 4 illustrates an exemplary embodiment for a method for suppressing overshoot based on the above-mentioned of the overshoot protection circuit 1. At step 100, a current load sensor senses a current load level driven by low-dropout voltage regulator At step 200, a current load sensor 13 detects certain current load by mirroring the current flow at the pass-gate transistor 3 and thereby instructing a non inverting amplifier 11 to switch between an inactive mode and an active mode based on the sensed current load level. The current load sensor 13 compares the current flow with a value indicative of an overshoot. The non-inverting amplifier 11 is maintained in an idle state during the inactive mode at step 800 and the non-inverting amplifier 11 that is normally maintained at an idle state is activated to amplify a spike signal in the event of an overshoot during the active mode at step 300. In one preferred embodiment, a delay circuit 15 receive a signal to activate the non-inverting amplifier 11 when the current flow exceeds a value indicative of an overshoot. At step 400 and 500, an AC coupled circuit 14 transmits a signal to the non-inverting amplifier 11 for signal amplification upon a detection of spike in the current flow. At step 600, a pull down transistor 12 is turned on for suppressing the overshoot voltage to a stable voltage upon receiving the amplified signal from the non-inverting amplifier 11. At step 700, an overshoot protection circuit is put in standby mode after certain delay set up by delay circuit 15 due to low current load detected by the current load sensor 13.

Fig. 5 illustrates a graph showing comparison of results between main low-dropout voltage regulator 2 with and without overshoot protection circuit (OPC) during current load switching from high to low. The present invention suppresses the overshooting on the regulated output voltage to a stable state when the current load starts to decrease drastically.

The present disclosure includes as contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangements of parts may be resorted to without departing from the scope of the invention.