# Patent

## Title
Voltage regulator

## Abstract

A method and apparatus are provided. The apparatus includes a plurality of devices forming a positive feedback loop for driving a regulated output voltage towards a reference voltage. Device ratios of at least two of the plurality of devices are set such that the positive feedback loop is stable.

## Background
BACKGROUND
1. Technical Field
The present application related to the regulation of output voltage and in particular, but not exclusively, to circuits used for such regulation.
2. Description of the Related Art
Voltage regulators may be used to keep a supply voltage stable in the presence of varying load conditions. These may be implemented on an on-chip environment however due to stability and transient response requirements of the on-chip environment an off-chip capacitor is often implemented as part of the voltage regulator. An off-chip capacitor adds to a cost of manufacture as well as prevents a fully on-chip implementation of a system.
BRIEF SUMMARY
According to a first aspect, there is provided an apparatus comprising: a plurality of devices forming a positive feedback loop for driving a regulated output voltage towards a reference voltage; wherein device ratios of at least two of the plurality of devices are set such that the positive feedback loop is stable.
The plurality of devices may comprise a sensing element configured to sense a change in the regulated output voltage. The plurality of devices may comprise a control element configured to generate a control signal in response to an indication of the sensed change in the regulated output voltage. The control signal may drive the regulated output voltage towards the reference voltage.
A relationship between the device ratio of the sensing element and the device ratio of the control element may be such that the loop is stable.
The plurality of devices may comprise a current mirror configured to provide the indication of the change to the control element. The current mirror may be configured to provide the indication of the change by adjusting a current provided to the control element in response to the sensing element sensing a change in the regulated output voltage.
The current mirror may comprise a first and second device and a relationship between a device ratio of the first device and a device ratio of the second device may be such that the positive feedback loop is stable.
The plurality of devices may be transistors and a device ratio may correspond to a gate width to length ratio of a device. The relationship between the device ratios providing a stable loop gain may provide a loop gain of the positive feedback loop to be less than one. The apparatus may be a voltage regulator.
According to a second aspect, there is provided a method comprising: driving a regulated output voltage towards a reference voltage by a positive feedback loop formed by a plurality of devices; wherein device ratios of at least two of the plurality of devices are set such that the positive feedback loop is stable.
The method may further comprise: sensing by a sensing element a change in the regulated output voltage. The method may further comprise: generating by a control element a control signal in response to an indication of the sensed change in the regulated output voltage.
Driving the regulated output voltage towards the reference voltage may comprise driving the regulated output voltage towards the reference voltage by the control signal. A relationship between the device ratio of the sensing element and the device ratio of the control element may be such that the loop is stable.
The plurality of devices may comprise a current mirror and the method may further comprise: providing the indication of the change to the control element by the current mirror.
The method may further comprise: providing the indication of the change by adjusting a current provided to the control element in response to the sensing element sensing a change in the regulated output voltage.
The current mirror may comprise a first and second device and a relationship between a device ratio of the first device and a device ratio of the second device may be such that the positive feedback loop is stable.
The plurality of devices may be transistors and a device ratio may correspond to a gate width to length ratio of a device. The relationship between the device ratios providing a stable loop gain may provide a loop gain of the positive feedback loop to be less than one.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Embodiments of the present application will now be described with reference to the following figures in which:
FIGS. 1 and 2 show schematic diagrams of a voltage regulator circuit according to embodiments;
FIG. 3 is a flow diagram depicting the method step associated with some embodiments;
FIG. 4 is a circuit diagram of a voltage regulator according to one embodiment;
FIG. 5A is a circuit diagram of a voltage regulator according to a further embodiment;
FIG. 5B shows a circuit diagram of a voltage regulator according to yet another alternative embodiment;
FIG. 6 is a circuit diagram of a reference voltage node according to one embodiment;
FIG. 7 is a circuit diagram of a reference voltage node according to a further embodiment; and
FIGS. 8 to 12 are signal diagrams showing a comparison of the behavior of an embodiment with other voltage regulators.



DETAILED DESCRIPTION
Voltage regulators implemented on an integrated circuit, often called on-chip regulators, usually require a capacitor, for example, on the order of a micro farad, to be connected externally to the integrated circuit, often called an off-chip component. This off-chip capacitor is necessary to support the stability and improve a transient performance of the on-chip voltage regulator. The requirement of the off-chip capacitor may add to the cost of the voltage regulator and prevents fully on chip implementation.
Designs and techniques omitting the off-chip capacitor may require the output capacitance to be very small (for example, few hundred picofarads) which may cause poor transient performance. These designs may not be suited for many applications of the on-chip voltage regulator.
Embodiments of the present application may provide a voltage regulator suitable for on-chip implementation without the need for an off-chip capacitor. In some embodiments, a stability of the voltage regulator may be dependent on a ratio of devices used to implement the regulator rather than a capacitor. In some embodiments an NMOS pass element and a positive feedback circuit may be implemented to provide a transient response suitable for the voltage regulator.
FIG. 1 shows an example of a voltage regulator according to one inventive embodiment.
FIG. 1 comprises an unregulated voltage input Vin 101 and a voltage output Vout 104. The voltage input Vin 101 may be regulated by a voltage regulator 100 in order to provide the regulated voltage output Vout 104 to the rest of the integrated circuit. Vout 104 is provided across a load RL 103. RL may represent the load provided by the integrated circuit. It will be appreciated that the load RL 103 may be variable depending on the operation of the integrated circuit. The current IRL may therefore vary over time and it is desired to hold Vout constant with variations in RL and IRL.
A pass element 102, in this case an NMOS field effect transistor, is provided with its drain terminal connected to the voltage input Vin 101, source terminal connected to the voltage output Vout 104 and gate terminal connected to the output of a block A 106.
The pass element 102 may be configured to pass a current IRL 109 to the load RL 103 of the integrated circuit.
The voltage regulator 100 may further include a reference voltage Vref 105 and a supply voltage 107 both coupled as inputs to the block A 106. The block A may also have an input coupled to Vout 104.
The voltage regulator 100 may control the gate terminal voltage 108 of the pass element 102 in order to control Vout 104 to correspond to Vref 105. It will be appreciated that in some embodiments, Vout 104 is controlled to be substantially equal to Vref 105 however in other embodiments, Vout 104 may be controlled to be a multiple or factor of Vref 105 or have an offset with respect to Vref 105.
The value of Vout 104 may be dependent on the values of IRL and RL as follows:

Vout=IRL×RL  (i)

RL may be variable and the voltage regulator 100 may be operable to adjust the current IRL passed by the pass element in order to drive Vout 104 to correspond with Vref 105. In order to do this block A may control the voltage of the gate terminal 108 to adjust the gate source voltage Vgs of the pass element 102 which in turn controls the current IRL 109.
Block A may operate as a positive feedback loop. For example, a decrease in an internal current in block A may cause a decrease in Vout which causes the voltage of the gate terminal 108 to be adjusted to decrease the current IRL 109. In other words a decrease in the current of a sensing element in block A will result in a decrease in the current IRL 109. Similarly, an increase in the current of the sensing element in block A (due to for example a decrease in the load RL), will result in an increase of the current IRL 109.
In embodiments, the positive feedback loop may have a gain less than or in the vicinity of 1 to provide an unconditionally stable loop.
It will be appreciated that the value of the loop gain may be chosen to be close enough to 1 that the loop is unconditionally stable. For example the gain may be in the vicinity of 0.6 to 0.9.
FIG. 2 shows an embodiment of the components of block A.
FIG. 2 comprises the reference voltage Vref 105, voltage Vin 101, pass element 102, output voltage Vout 104 and load RL 103 and shows the current IRL 109 as described in relation to FIG. 1.
FIG. 2 further shows a sensing element 201, current mirror 202 and control element 203. The control element 203 may be coupled to Vref 105, to the gate terminal 108 of the pass element 102 and to the current mirror 202. A control current 205 may be provided to the control element 203 from the current mirror 202.
The sensing element 201 may be coupled to Vout 104, to the connection between the control element 203 and the gate terminal 108 and to the current mirror 202. A sensing current 204 between the current mirror 202 and the sensing element 201 may be set by the sensing element 201.
In operation, the sensing element 201 may sense a change in Vout 104 which may adjust the sensing current 204. The current mirror 202 will adjust the control current 205 to mirror the adjusted sensing current 204 and provide the control current 205 to the control element 203. The adjusted control current 205 will cause the control element 203 to adjust the gate terminal 108 voltage of the pass element 102. The adjusted gate terminal voltage 108 will drive Vout towards the reference voltage Vref 105.
FIG. 3 shows an example of the method steps carried out in accordance with the embodiment of FIG. 2.
At step 301 a change in the regulated voltage output is sensed. The sensed change in the regulated output voltage may cause a current through the sensing device to change as shown in step 302.
At step 303, a control current is adjusted by mirroring the sensing current. While, in the foregoing, the control current adjustment is described as being carried out by a current mirror, it will be appreciated that the adjustment may be carried out by any suitable circuitry.
The method then proceeds to step 304 where the gate terminal voltage of the pass element 102 is adjusted in response to the adjusted control current 205.
At step 305, the regulated voltage 104 is adjusted in response to the gate terminal voltage 108 to drive the regulated voltage back towards the reference voltage. With this positive feedback loop, the level of Vout is kept constant.
FIG. 4 shows the circuitry of block A 106 according to a first embodiment.
It will be appreciated that the embodiment of FIG. 4 is one example of the specific devices for use in the embodiments of FIGS. 1 to 3. Like reference numerals indicate like features.
In the embodiment of FIG. 4, the sensing element 201 may comprise a first n-channel MOSFET MN1 and the control element 203 may comprise a second n-channel MOSFET MN2. The current mirror 202 may comprise a first p-channel MOSFET MP1 401 and a second p-channel MOSFET MP2 402.
A source terminal of MN1 201 may be coupled to the regulated voltage output Vout 104. A drain terminal of MN1 201 may be coupled to a drain terminal of MP1 401. A gate terminal of MN1 201 may be coupled to the gate terminal 108 of the pass element 102 and to a gate terminal of MN2 203.
The gate terminal of MN2 203 may further be coupled to a drain terminal MP2 402. A source terminal MN2 203 may be coupled to the reference voltage Vref 105. The drain terminal of MN2 203 may be further coupled to a drain terminal of MP2 402.
With MP1 and MP2 forming the current mirror 202, the drain terminal of MP1 401 may further be coupled to a gate terminal of MP1 401. A source terminal of MP1 401 and a source terminal MP2 402 may both be coupled to the supply voltage 107. The respective gate terminals MP1 401 and MP2 402 may be coupled together.
The embodiment of FIG. 4 may further include an on-chip capacitor 403 coupled across the load RL 103. A first terminal of the on-chip capacitor 403 may be coupled to the source terminal of MN1 201 and Vout 104. A second terminal of the capacitor 403 may be coupled to ground.
The sensing current 204 may flow from the drain terminal of MP1 401 to the drain terminal of MN1 201. The control current 205 may flow from the drain terminal of MP2 402 to the drain terminal of MN2 203.
The sensing element 201, current mirror 202 and control element 203 act as a positive feedback loop providing the control voltage at the gate terminal of the pass element MP 102.
In operation, a change in Vout 104 is a change in the voltage at the source terminal of the sensing element MN1 201. The change in the source terminal voltage of MN1 201 additionally changes the gate source voltage VGS of the sensing element MN1 201 which causes a change in the sensing current 204 being passed through the sensing element 201.
The current being passed through MP1 401 is the sensing current 204 and a source gate voltage VSG of MP1 will be adjusted in response thereto. The gate voltage of MP1 sets the gate voltage of MP2 and thus the source gate voltage VSG changes accordingly. The change in VSG of MP2 causes the control current 205 being passed through MP2 to change.
In this manner the sensing current 204 is mirrored in the control current 205 by the current mirror. It will be appreciated that the configuration of the current mirror 202 in the embodiment of FIG. 4 is by way of example only and other configurations of a current mirror or circuitry having the necessary functionality may be realized without departing from the scope of the invention.
The control current 205, set by the current mirror 202, is passed through the control element MN2 203. A change in the control current 205 causes the gate source voltage VGS of MN2 to be adjusted. It will be appreciated that with the drain terminal of MN2 coupled to the gate terminal of MN2, the transistor MN2 will be in saturation mode. As the source terminal of MN2 is coupled to the reference voltage Vref 105, the voltage at the gate terminal of MN2 is changed in response to the change in VGS and the control current 205.
The reference voltage Vref 105 plus the VGS of MN2 203 is equal to the VGS of the control element MN1 102 plus the regulated output voltage Vout 104 which can be shown by the following equation:

VREF+VGSMN2=VGSMN1+VOUT  (ii)

It will be appreciated that while VGSMN2 and VGSMN1 are not exactly matched in size, their sizes are close enough to approximate VGSMN2≈VGSMN1 in this equation. In some embodiments, the difference in size between these transistors causes load regulation and is a trade-off for stability. For example if sizes of MN1 and MN2 are very close then VREF and VOUT will be very close to each other but the stability will be poor.
Equation (ii) may then this simplify to:

VREF=VOUT  (iii)

In the embodiment of FIG. 4, the reference voltage VREF 105 is provided as a node with low impedance, for example a buffered node. Examples of such a low impedance node are discussed in relation with FIGS. 5A and 6.
The sensing element 201, current mirror 202 and control element 203 may form a positive feedback loop. This positive feedback loop may further be designed having device ratios that bring the loop gain close to 1. The stability of the voltage regulator may be provided by having this positive feedback loop having device ratios providing a gain of close to 1. The voltage regulator of these embodiments therefore does not require an external capacitor to ensure stability.
The voltage regulator 100 may include an on-chip capacitor CL 403. The capacitor CL 403 provides immediate additional current in the event of any sudden transient current requirements. However CL 403 is preferably implemented on-chip and may be in the range of a few hundred picofarads to a few nano farads depending on the application. The circuit arrangement permits the capacitor 403 to be very small.
As described above, the voltage regulator of some embodiments may comprise a positive feedback loop having a sensing element and a control element. The sensing element may sense a change in an output voltage, provide this information to the control element and the control element may regulate or control the output voltage to correspond to a reference voltage. The control element may counteract any change in the output voltage so that the output voltage is regulated to the reference voltage.
The stability of the positive feedback loop may be provided by adjusting the device ratio of the sensing element and the control element to control a loop gain. The ratios may be designed or chosen so that the loop is stable. An external or off-chip capacitor may not be required and the voltage regulator may be implemented on-chip.
The gain of the positive feedback loop may be determined by the trans-conductance and impedance of the devices in the loop. The determination of loop gain and the selection of device ratios will be discussed in relation to the embodiment of FIG. 4.
The positive feedback loop of FIG. 4 comprises four devices: the sensing element MN1 201, the two current mirror devices MP1 401 and MP2 402, and the control element MN2 203.
If VREF is considered as perfect voltage source (in other words having zero impedance) then the loop gain A of the positive feedback loop can be given by:

A=(gMP2×gMN1)/[(gMP1×gMN2)(1+(gMP×RL))]  (iv)

where:

gMP2 is the trans-conductance of MP2 402;
gMN1 is the trans-conductance of the sensing element MN1 201;
gMP1 is the trans-conductance of MP1 401;
gMN2 is the trans-conductance of the control element MN2 203;
gMP is the trans-conductance of the pass element MP 102;
RL is the value of the load RL 103; and
the output impedances are ignored.
The trans-conductance of a device may be dependent on ratio of the device gate width (W) to gate length (L). If the W/L ratio of MP1 401 and MP2 402 are the same:

gMP1=gMP2  (v)

and for a wide output current range (up to few hundreds of mA):

gMP×RL>>1  (vi)

Taking (v) and (vi), the equation (iv) simplifies to:

A=gMN1/gMN2  (vii)

or the ratio of the trans-conductance of the sensing element 201 to the control element 203.

For the positive feedback loop to be stable, it is helpful to have a gain less than 1; namely, A<1, and the gMN2 will be greater than gMN1 to provide this. It will be appreciated that the trans-conductance of a device may be related to (at least in part) a gate width (W) to gate length (L) ratio of the device.
In some embodiments, the W/L of the sensing element MN1 201 to W/L of the control element MN2 may be slightly smaller than the W/L of MP1 to the W/L of MP2. In some embodiments this relationship may be selected so that gMN2>gMN1 and that the positive feedback loop is stable with A<1.
In other words the devices may be designed so the device ratios correspond to:

(W/L)MN1/(W/L)MN2=m*(W/L)MP1/(W/L)MP2  (viii)

where:

(W/L)MN1 is the gate width to gate length ratio of MN1;
(W/L)MN2 is the gate width to gate length ratio of MN2;
(W/L)MP1 is the gate width to gate length ratio of MP1;
(W/L)MP2 is the gate width to gate length ratio of MP2; and
m<1 provides a stable positive feedback loop.
In some embodiments, the pass element MP 102 has a much larger (for example in the region of few hundred times) W/L ratio than the sensing element MN1 201.
The larger W/L ratio of the pass element MP 102 may result in a greater change in the current of the pass element MP 102 for a change in VGS than in the sensing element 201.
It will be appreciated that in the foregoing VREF has been assumed to be an ideal node with no impedance. In some embodiments, VREF may be provided as a buffered node in order to be a low impedance node.
If impedance (Rs) at VREF node is considered then equation (vii) modifies to:

A=(gMN1/gMN2)+gMN1*RS  (ix)

For a stable system the gain A<1 and thus Rs should be sufficiently small to guarantee this.
In one example this requirement can be met if this node VREF is driven by a voltage buffer circuit. In another example a modified source follower may be used. It will however be appreciated that other techniques ensuring low impedance may be used to implement VREF. Examples of implementations of the reference node will be discussed in relation to FIGS. 7 and 8.
Equations (iv) to (ix) consider the devices of the current mirror to have equivalent W/L ratios and the W/L ratios of the sensing device and the control device are set to for a stable loop gain. In other embodiments, the sensing and control devices may be considered to have equivalent sizes (for example W/L ratios) and the W/L ratios of the current mirror devices may be set for a stable loop gain. For example MP2 may be of a smaller size than MP1 in order to provide a loop gain of less than but close to 1.
Embodiments of the present application may provide on-chip voltage regulator 100 stability without an off-chip capacitor. Some embodiments may also provide a low output impedance at the regulated voltage node VOUT 104.
The output impedance of node Vout 104 may be a measure of the voltage regulation and the output impedance at Vout 104 may be related to the gain A of the positive feedback loop as described below. The device ratios may be selected for a value of A that provides stability as well as good voltage regulation.
The impedance at the regulated node Vout 104 to the first order may be given by:

RVOUT=[(1/RL)+{gMP/(1−A)}]−1  (x)

where:

RVOUT is the output impedance of the regulated node Vout 104;
RL is the load;
gMP is the trans-conductance of the pass element MP 102; and
A is the positive loop gain.
As can be seen from (x) as A tends towards 1, the voltage regulation improves as the RVOUT decreases. There is therefore a trade-off between stability and voltage regulation since as A approaches 1 regulation becomes better while stability becomes poor.
In some embodiments, the device ratios (for example the gate width and lengths of the sensing and control elements) may be selected so that A is in the range of:

0.6<A<0.9  (xi)

This may provide a compromise between the voltage regulation and the stability. In some embodiments MN1 201 and MN2 203 may be the same type of devices and proper layout can ensure a very precise setting of the loop gain across PVT variation.
FIG. 5A shows a further embodiment of the circuitry of block A 106 of FIG. 1. It will be appreciated that while the circuitry of block A 106 in the embodiment of FIG. 5A, this embodiment may provide an alternative example of a positive feedback loop and the device ratios of devices in this embodiment may be set to provide a gain A for stability similarly to that of the embodiment of FIG. 4.
In the embodiment of FIG. 5A, the reference load 105 need not be a low impedance note from the other figures.
It will be appreciated that like reference numerals in FIG. 5A depict like features.
The block 106 in FIG. 5A comprises a first MOSFET M1 501, a second MOSFET M2 502, a third MOSFET M3 503 and a fourth MOSFET M4 504. M1 501 may be an n-channel MOSFET, while M2, M3 and M4 may be p-channel MOSFETs.
A source terminal of M1 501 may be coupled to the output voltage VOUT 104 and a drain terminal of M1 501 may be coupled to a drain terminal of M3 503. A gate terminal of M1 501 may be coupled to a gate terminal of the pass element MP 102 as well as to a source terminal of M2 502.
Respective source terminals of M3 503 and M4 504 may be coupled to the supply voltage 107. Respective gate terminals of M3 503 and M4 504 may be coupled together. In addition, the gate and drain terminals of M3 503 may be coupled together.
The source terminal of M2 502 may further be coupled to the drain terminal of M4 504. A drain terminal of M2 502 made be coupled to ground with a gate terminal of M2 502 coupled to the reference voltage VREF 105. It will be appreciated that the configuration of M1 501 may be similar to that of the sensing element 201 in other embodiments. Additionally it will be appreciated that M3 503 and M4 504 may provide a current mirror.
FIG. 5A provides an alternative configuration for the sensing element, control element and current mirror of FIG. 4 and it will be appreciated that this embodiment works similarly to that of FIG. 4.
FIG. 5B shows yet another alternative embodiment for the implementation of block A. In FIG. 5B, another example is provided in which block A is connected to provide a positive feedback circuit. In this example, there is a translinear cross-quad circuit structure as shown in FIG. 5B. The current mirror for devices 503, 504 is provided from an additional device 506 which is coupled to a current source 507. The transistor 506 provides a current mirror signal to drive transistors 504, 503 rather than the arrangement shown in FIG. 5A. In addition, the transistors 502, 501 are cross-coupled, having the gate of transistor 501 coupled to the drain of transistor 502 and the gate of transistor 502 coupled to the drain of transistor 501. One significant feature of the circuit of FIG. 5B is that the loop gain is deliberately set to be less than one (1). This is done by ensuring that the transistors in the loop are not exactly matched and that the ratios have a loop gain resulting in less than one (1).
It will be appreciated that in the example circuitry of FIGS. 4 and 5, a sensing element senses a change in the regulated output voltage and feeds this information back to a control element in a positive feedback loop. The control element may control a pass element to adjust the regulated output voltage towards a reference voltage. The gain of the positive feedback loop may be adjusted by adjusting the W/L ratios of the devices in the positive feedback loop. In some embodiments the ratio of the W/L of the control element to the W/L of the sensing element may be controlled to provide a loop gain A<1. The loop gain may be selected to be less than 1 for stability but close to 1 for voltage regulation. In some embodiments the loop gain may be in the range of 0.6<A<0.9.
It will be appreciated that while the equations determining loop gain have been laid out with specific reference to the embodiment of FIG. 4, similar equations may be applied to the embodiment of FIG. 5A.
As described above, in some embodiments, the reference voltage node VREF 105 may be implemented as a low voltage node. It will be appreciated that in some embodiments, for example the embodiment of FIG. 6, the VREF 105 need not be low impedance and the specific implementation of VREF is optional.
FIGS. 6 and 7 show two examples of the implementation of VREF 105 as a low impedance node.
In these examples, the voltage regulator has been implemented for following specifications in CMOS055 technology: Input voltage range: 3.3V+/−10%; Output voltage range: 1.2V+/−100 mV; Maximum load current: 200 mA; Minimum CL: 5 nF. It will however be appreciated that these examples may be applicable to other or additional specifications.
FIG. 6 shows a first example of the reference voltage node 105.
FIG. 6 comprises block A 106, pass element 102, load 103 and capacitor CL 403. Block A 106 may be coupled to a first source voltage VI_1 107. The pass element 102 may be coupled to a third voltage VI_3 101. Block A 106 may further be coupled to a voltage reference node VREF 105 via line 701.
While the circuitry of block A 106 has been depicted as being in line with that of the embodiment of FIG. 4, it will be appreciated that the circuitry of block A may be implemented according to a different embodiment, for example the embodiment of FIG. 5A or FIG. 5B.
VREF 105 may comprise a bandgap voltage reference circuit 604, an amplifier 603, an n-channel MOSFET 605 and a capacitor 606. VREF may be coupled to a second supply voltage VI_2 602.
The bandgap voltage reference circuit 604 may be coupled to the second supply voltage VI_2 602 and to ground. The bandgap voltage reference circuit 604 may provide a voltage reference VR to the inverting input of the amplifier 603.
A positive power supply terminal of the amplifier 603 may be coupled to the second voltage source VI_2 602 and a negative power supply terminal of the amplifier 603 may be coupled to ground. The non-inverting input of the amplifier 603 may be coupled to a drain terminal of the n-channel MOSFET 605. An output of the amplifier 603 may be coupled to a gate terminal of the n-channel MOSFET 605 and a source terminal of the n-channel MOSFET 605 may be coupled to ground.
The capacitor 606 may be coupled across the drain terminal and source terminal of the n-channel MOSFET 605. VREF 105 may be coupled to block A 106 at the drain terminal of the n-channel MOSFET 605 which is depicted at 601.
In the embodiment of FIG. 6, VREF 105 may be implemented as a voltage buffer circuit. The capacitor 606 may provide a compensation capacitance to support the stability of voltage buffer. In this embodiment, the capacitor may be in the order of tens of picofarads.
In operation, the bandgap voltage reference circuit 604 may receive the second supply voltage VI_2 602 and buffer it against changes in temperature. This voltage reference VR may be provided to the amplifier 603 which receives a feedback voltage VREF provided to block A 106. The difference between the voltage reference form the bandgap voltage reference circuit 604 and the feedback voltage VREF 601 is used to drive VREF 601 to VR. This may be carried out by controlling the gate terminal of the n-channel MOSFET 605 to adjust the drain terminal voltage of the MOSFET 605. In this manner a reference voltage node may have low impedance.
In this example, the voltage at the VI_1 107 may be sufficiently higher (for example greater than 1V) than the voltage at VOUT 104. This may be in order to bias the transistors in block A. The voltage provided to the pass element VI_3 101 and the voltage provided to the reference node VI_2 602 may be marginally higher than the output voltage VOUT 104 (for example greater than 200 mV higher). This slightly higher voltage may account for the voltage drop across devices.
FIG. 7 shows a second embodiment of a low impedance voltage reference node.
FIG. 7 comprises block A 106, pass element 102, load 103 and capacitor 403. It will be appreciated that while the circuitry of block A 106 has been depicted as being that of the example FIG. 4, other circuitry may be used. Block A 106 may be coupled to a voltage reference node 105 at coupling 701.
The reference node 105 comprises a second voltage supply VI_2 702, a bandgap voltage reference 704, an amplifier 723, a first capacitor 705, a first transistor T1 706, a second transistor T2 707, a third transistor T3 708, a fourth transistor T4 712, a fifth transistor T5 713, a sixth transistor T6 714 and a second capacitor 715.
T1 706 and T4 712 may be p-channel MOSFETs while T2 707, T3 708, T5 713 and T6 714 may be n-channel MOSFETs.
The bandgap voltage reference circuit 704 may be coupled to the second voltage supply VI_2 702 and to ground. The bandgap voltage reference circuit 704 may provide a reference voltage VR to an inverting input of the amplifier 703. A positive voltage supply of the amplifier may be coupled to the second voltage supply VI_2 702 and a negative voltage supply of the amplifier may be coupled to ground.
An inverting input of the amplifier 703 may be coupled to a source terminal of T1 706 and an output of the amplifier 703 may be coupled to a gate terminal of T1 706 via gate voltage VGATE 709. The first capacitor 705 may be coupled between the output of the amplifier 703 and ground.
A drain terminal of T1 706 may be coupled to a drain terminal of T2 707. A source terminal of T2 707 may be coupled to ground and a gate terminal of T2 707 may be coupled to a bias signal Vbias1 711. The drain terminal of T2 707 may be further couple to a gate terminal of T3 708. A source terminal of T3 708 may be coupled to ground and a drain terminal of T3 708 may be coupled to the source terminal of T1 706.
VGATE 709 may be further provided to a gate terminal of T4 712. The second capacitor 715 may be coupled between a source terminal of T4 712 and ground. A drain terminal of T4 712 may be coupled to a drain terminal of T5 713 and to a gate terminal of T6 714. A gate terminal of T5 713 may be coupled to the bias signal Vbias1 710. A source terminal of T5 713 may be coupled to ground. A drain terminal of T6 714 may be coupled to the source terminal of T4 712 and a source terminal of T6 714 may be coupled to ground.
The source terminal of T4 and drain terminal of T6 714 may be coupled to the block A 106 and provide a reference voltage VREF on line 701.
In the embodiment of FIG. 7, the VREF node 105 is realized through a super source follower (SSF) comprising the transistors T4 712, T5 713 and T6 715. This SSF may be biased through a replica super source follower comprising T1 706, T2 707 and T3 708. The first and second capacitors 705 and 715 may be provided as compensation capacitors
In this embodiment, the compensation capacitance required to achieve a low impedance value for the voltage node VREF 105 may be small. In some examples the capacitance of this embodiment may be 50% smaller than other implementations of the node. A smaller capacitance may lead to a smaller required area.
It will be appreciated that the implementations of FIGS. 6 and 7 are example implementations only and other implementations may be used in embodiments. For example, a low impedance VREF 105 may also be realized through several other techniques such as various variants of source followers, flipped source followers, various variants of voltage buffers etc.
FIGS. 8 to 12 shows simulation results of a comparison between an example of a voltage regulator according to an embodiment of the present application and an ideal voltage source, and PMOS based regulator.
In the comparison, the ideal voltage source has the following characteristics:

    
    
        10 nH boding wire inductance
        100 mOhm bonding wire resistance
        5 nF on-chip capacitance
    
    


The PMOS based conventional voltage regulator has the following characteristics:

    
    
        10 nH boding wire inductance
        100 mOhm bonding wire resistance
        2 uF off-chip capacitor
        5 nF on-chip capacitance
    
    


The example of an on-chip voltage regulator according to an inventive embodiment has the following characteristic:

    
    
        5 nF on-chip capacitance
    
    


FIGS. 8 to 12 plot the computer simulated regulated output voltage against time and the load current against time for the three different voltage regulators.
FIG. 8 shows the behavior of the regulators in the comparison when the load current rises from 10 mA to 200 mA in 10 ns. In the graph of FIG. 8, the load current is shown rising from approximately 0 mA to 200 mA over a short timeframe, in the range of less than 10 ns. After the load current reaches 200 mA, it remains stable for the remainder of the graph in FIG. 8. Shown above the load current in FIG. 8 is the output voltage as simulated for the three different types of circuits, a conventional PMOS voltage source of the type used in the prior art, an ideal voltage source, and the inventive on-chip voltage regulator of the type described and shown herein within respect to FIGS. 1-7. As can be seen, the inventive on-chip voltage regulator briefly dips from 1.2V to just less than 1V at the instant of the rise and then, at less than 50 ns, has stabilized at 1.1V and remains stable at 1.1V for the entire operation.
FIG. 9 shows the behavior of the regulators in the comparison when the load current drops from 200 mA to 10 mA in 10 ns. As can be seen in the graph of FIG. 9, when the load current drops from 200 mA to approximately 0 mA in 10 ns, the three different voltage sources respond differently. The ideal voltage source rises quickly, as does the conventional prior art voltage source. However, the inventive on-chip voltage regulator has a slight rise and then quickly stabilizes at 1.2V.
FIG. 10 shows the behavior of the regulators in the comparison with a 150 MHz load current. As can be seen, when the current fluctuates with a frequency of about 150 MHz, the ideal voltage source and the conventional prior art voltage source take significant time to settle out and begin to match the frequency of the load current. The on-chip voltage regulator as described herein is able to quickly match the frequency changes in the load current, and keep the output voltage relatively stable at approximately 1.1V with deviations of less than a few percent from the target regulated voltage of 1.1V.
FIG. 11 shows the behavior of the regulators in the comparison with a 150 MHz load current on a smaller scale.
FIG. 12 shows the behavior of the regulators in the comparison when the load current rises from 0 mA to 200 mA in 2 μs.
In some embodiments, a voltage regulator 106 may be provided without a replica bias architecture. The voltage regulator may implement a positive feedback loop for the sensing and control of the regulated output voltage. A negative feedback loop may therefore be avoided in the regulator. The voltage regulation in some embodiments may be controller by the positive feedback loop. Some embodiment may provide a lower voltage headroom requirement than other implementation of a voltage regulator. This may be due in some embodiments to no replica NMOS being implemented in the feedback circuit. In some embodiments stability may be ensured by making positive feedback circuit's loop gain<1. In these embodiments stability may be dependent on device ratios and not on a capacitor value. In some embodiments, an off-chip capacitor may be negated.
The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.
These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.

## Claims

1. An apparatus, comprising:
a plurality of devices forming a positive feedback loop for driving a regulated output voltage towards a reference voltage, the plurality of devices including:
a sensing element configured to sense a change in the regulated output voltage, and
a control element configured to generate a control signal in response to an indication of the sensed change in the regulated output voltage; and

a ratio of a transductance of the sensing element to a transductance of the control element being set to provide that the positive feedback loop is stable.

2. The apparatus of claim 1 wherein the control signal drives the regulated output voltage towards the reference voltage.

3. The apparatus of claim 1 wherein the plurality of devices comprises a current mirror configured to provide the indication of the sensed change to the control element.

4. The apparatus of claim 3 wherein the current mirror is configured to provide the indication of the sensed change by adjusting a current provided to the control element in response to the sensing element sensing a change in the regulated output voltage.

5. The apparatus of claim 3 wherein the current mirror comprises a first and second device and a relationship between a device ratio of the first device and a device ratio of the second device is such that the positive feedback loop is stable.

6. The apparatus of claim 1 wherein the plurality of devices are transistors and a device ratio corresponds to a gate width to length ratio of a device.

7. The apparatus of claim 1 wherein the ratios of the transconductance providing a stable loop gain provides a loop gain of the positive feedback loop to be less than one.

8. The apparatus of claim 1 wherein the apparatus is a voltage regulator.

9. A method, comprising:
driving a regulated output voltage towards a reference voltage by a positive feedback loop formed by a plurality of devices, the plurality of devices including a sensing element and a control element;
sensing by the sensing element a change in the regulated output voltage;
generating by the control element a control signal in response to an indication of the sensed change in the regulated output voltage; and
setting a ratios of transductance of the sensing element and the control element to provide that the positive feedback loop is stable.

10. The method of claim 9 wherein driving the regulated output voltage towards the reference voltage comprises driving the regulated output voltage towards the reference voltage by the control signal.

11. The method of claim 9 wherein the plurality of devices comprises a current mirror and the method further comprises:
providing the indication of the change to the control element by the current mirror.

12. The method of claim 11, further comprising:
providing the indication of the change by adjusting a current provided to the control element in response to the sensing element sensing a change in the regulated output voltage.

13. The method of claim 11 wherein the current mirror comprises a first and second device and a relationship between a device ratio of the first device and a device ratio of the second device is such that the positive feedback loop is stable.

14. The method of claim 9 wherein the plurality of devices are transistors and a device ratio corresponds to a gate width to length ratio of a device.

15. The apparatus of claim 9 wherein the ratios of the transconductance providing a stable loop gain provides a loop gain of the positive feedback loop to be less than one.