CAPACITIVE REGULATION

FIELD OF THE INVENTION [0001] This invention pertains generally to the field of voltage regulation circuits and, more particularly, to their wake-up behavior.

BACKGROUND

[0002] Voltage regulators are commonly found as peripheral elements on integrated circuits needing well regulated voltage levels for their operations. For example, a typical NAND memory chip has internal active and standby power supply regulators. During the transition from a standby mode to an active mode, although power supply level (the regulators' output) is initialized to desired level, active regulators tend to suffer from long wake-up times. High RC time constants, associated with the regulator's feedback loop, delays loop response during wake-up. Furthermore, high load currents can cause the power supply's output level to droop. This droop can be significantly high. The impact can be seen as a wrong data transfer, which becomes more evident at double data rate (DDR) speeds.

SUMMARY OF THE INVENTION

[0003] According to a general aspect of the invention, a method of operating a voltage regulator circuit to provide an output voltage level on an output node is presented. The regulator circuit includes a pass transistor connected between a supply level and the output node of the voltage regulator circuit and has a gate connected to an output node of an op-amp having first and second inputs, where the first input of the op-amp is connected to receive a reference voltage. The method includes operating the voltage regulator in a standby mode and subsequently operating the voltage regulator circuit in an active mode. In the standby mode the op-amp's output node is connected to receive he supply level. In the active mode the op-amp's output node is not connected to receive the supply level, the output node of the voltage regulator circuit is connected to ground through a resistive voltage divider, and the second input of the op-amp is connected to a node of the resistive voltage divider. In transitioning from the standby mode to the active mode, the connection of the second input of the op-amp to the node of the resistive voltage divider is delayed relative to disconnecting of the op-amp's output node from the supply level and the connecting of the voltage regulator circuit's output node to ground through the resistive voltage divider.

[0004] In other aspects, voltage regulation circuitry provides an output voltage level on an output node. The voltage regulation circuitry includes an op-amp and a pass transistor. The op-amp has first and second inputs, where the first input is connected to receive a reference voltage, and an output of the op-amp is connectable to a supply level through a second switch. The pass transistor is connected between the supply level and the output node of the voltage regulator circuitry and having a gate connected to the output node of the op-amp. First and second resistances are connected in series between the voltage regulator circuitry's output node and, through a third switch, ground, wherein the second input of the op-amp is connectable to a node between the first and second resistances through a fourth switch. First and second capacitors are connected in series between the op-amp's output node and the second input of the op-amp and having an intermediate node between the first and second capacitors connected to the output node of the voltage regulator circuitry. When operating in a standby mode, the first switch and second switches are on and the third and fourth switches are off; when operating in an active mode, the first and second switches are off and the third and fourth switches are on, and, when transitioning from the standby mode to the active mode, the turning on of the fourth switch is delayed relative to the turning off of the first and second switches and the turning on of the third switch.

[0005] Various aspects, advantages, features and embodiments of the present invention are included in the following description of exemplary examples thereof, whose description should be taken in conjunction with the accompanying drawings. All patents, patent applications, articles, other publications, documents and things referenced herein are hereby incorporated herein by this reference in their entirety for all purposes. To the extent of any inconsistency or conflict in the definition or use of terms between any of the incorporated publications, documents or things and the present application, those of the present application shall prevail. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 shows an example of a voltage regulator circuit.

[0007] Figure 2 is a set of waveforms showing the wake-up behavior of the circuit of Figure 1. [0008] Figure 3 shows an exemplary embodiment of a voltage regulator circuit.

[0009] Figure 4 is a set of waveforms showing the wake-up behavior of the circuit of Figure 3.

[0010] Figure 5 is an equivalent circuit for Figure 3 when under capacitive regulation.

[0011] Figure 6 is a version of Figure 4 for illustrating the behavior when under capacitive regulation.

[0012] Figure 7 is a block level representation of Figure 5.

DETAILED DESCRIPTION

[0013] The following considers techniques for improving the wake-up response of voltage regulators. More specifically, the regulator's feedback loop response is improved by pushing the active regulator in capacitive feed-back mode only during wake -up. This speeds up the loop response, hence improves wake-up time.

[0014] For background, Figure 1 illustrates an example of a regulator circuit and Figure 2 is a set of waveforms to shows its wake-up behavior. In Figure 1, a pass device M _P 21 is connected between the supply level V _EXT the regulator's output node OUT that can be applied to a load, here represented by C _L 31. The gate of M _P 21 is connected to output node PPG of an op-amp. The op-amp is here implemented as having its first input connected to the gate of a transistor Ml 11 and its second input connected to the gate of a transistor M2 13, where Ml 11 and M2 13 are both connected to ground though a current source 19 and respectively connected to the supply level through PMOS 15 and PMOS 17, where the gates of both of these PMOS are connected to a node between Mi l l and PMOS 15. The output PPG is then taken from a node between M2 13 and PMOS 17. Although this particular implementation of the op-amp is used in all of the following discussion, other standard implementations and variations can be used. [0015] The gate of M2 13 is connected to a reference level V _REF , such as would be supplied from a band-gap circuit, for example, and the node MON for the gate of Ml 11 is also connectable (through switch SWl 41) to the reference level or (through switch Msw 47) to an intermediate node MONB of a resistive divider formed of Rl 23 and R2 25 connected in series between OUT and (through a switch SW3 45) to ground. A coupling capacitor Cc 27 and a lead capacitor C _LE A _D 29 are used to increase the stability of the regulator when turned on and are connected in series between PPG and MON, with their intermediate node connected at OUT. PPG is also connectable (through switch SW2 43) to the supply level. Figure 1 also shows the parasitic capacitances C _M ON 53 and C _M ON _B 51, where C _M ON represents device parasitic capacitance and routing capacitance and C _M O _NB represents device parasitic capacitance, the parasitic capacitance of resistors Rl 23 and R2 25, and routing capacitance.

[0016] In the exemplary embodiment, SWl 41, SW 3 45, and M _sw 47 are implemented as NMOS devices and SW2 43 is implemented as a PMOS device. SW2 43, SW 3 45, and Msw 47 have their gates connected to receive the enable signal EN for the regulator, while SWl 41 receives the inverse of EN at its gate. Here, EN is the control signal that determines whether the chip is working in standby mode or active mode. Consequently, when EN is low, SWl 41 and SW2 43 are on, SW3 45 and Msw 47 are off; and EN is high, SWl 41 and SW2 43 are off, SW3 45 and M _sw 47 are on. The regulator is enabled by EN going high. The regulator here is taken to be an example of an active regulator capable of driving the load when active. For example, a NAND memory chip typically has standby mode and active modes. Active regulators work in the active mode whereas a standby regulator is used in the standby mode to maintain voltage levels, but need not drive the active circuit. The outputs of the regulators can be connected and, depending upon the mode of the chip, either the standby or the active regulator will drive the output. Consequently, only one of the active regulator and standby regulator need to be on at any time, depending on the mode. While still able to maintain a steady supply in standby situations, the standby regulator itself consumes less power and it can deliver less power to the load, whereas the active regulator consumes more power and is capable of driving a high current load. In the exemplary embodiment, the switch SWl 41 connects the gate of Ml 11 to V _REF during standby; more generally, as the actively regulator is turned off, this input of the op-amp could be any value while off, but connecting it to the reference level allows for faster settling.

[0017] Figure 2 shows a corresponding set of control signals for Figure 1. Before turning on the regulator, the regulator EN=0 and the device is in standby mode, with PPG connected to V _EXT and MON connected to V _REF - The regulator output is initialized to V _DD , where V _DD is the final regulation voltage, by a standby regulator (not shown in Figures 1 or 3) connectable to OUT. The MONB node is the same as V _DD due to there being no current in the resistive path. After the regulator is on (EN=1), a wake-up process begins as MONB node discharges to V _REF - The MONB node has two discharge paths: through R2 25 to ground and through Msw 47 to C _LE A _D 29. The Pv2 to ground current path will help in discharging, while Msw to C _LE A _D is an additional current path that is created due to different initial voltages at the MONB and MON nodes. This will make MON node rising above V _REF during the wake period, as shown at the bump. Since the potential at MON node is more than V _REF , the current in M2 13 will be less the current in Ml 11, slowing the discharge of the PPG node and causing wake-up time to increase. If the load draws current is before the regulator is settled, the OUT droop further increases. Because of this, internal circuits using this supply may fail to respond, resulting in error in data transfer.

[0018] One way that wake-up time can be reduced by increasing the tail current of the op-amp; however, this increases the power consumption of the circuit. Another way to deal with the problem is to have a separate discharge path for the high capacitive node at the gate of the pass device Mp 21; but again, this takes extra power and the duration of discharge has to be precisely controlled across process variations. If the discharge is higher than required, an unwanted overshoot will be seen at the regulator output.

[0019] Figure 3 presents an exemplary embodiment of a voltage regulator circuit incorporating aspects to improve wake-up response without these drawbacks. Figure 3 incorporates the same elements as Figure 1 , but also incorporates a delay element T _D 61. Rather than Msw 47 having its gate connected to receive EN directly, it instead receives a delayed version ENd that the delay element T _D 61 generates from EN. The delay can be implemented by a RC network, where the resistor used in the RC network can be the same as the resistor used in the voltage divider of the regulator.

[0020] Figure 4 illustrates the control signals for Figure 3, where the control signals for Figure 1 (as shown in Figure 2) are overlaid in broken lines for comparison. When EN goes high and the active regulator is turned on, the connection of MONB to MON through M _S w 47 is delayed by delaying the control signal ENd. While M _S w 47is off, C _LE A _D 29 completes the feedback path: i.e., the regulator is in capacitive regulation mode. Due to the initial voltages at both the terminals of C _LE A _D , output node would be regulated to V _DD - The wake -up time in capacitive regulation will be faster since any decrease in the OUT node (due to the resistive discharge path and output current) will be entirely reflected in the MON node. The MON node will not overshoot V _REF as in Figure 1 and PPG will discharge faster. The delay T _D is preferably higher than the time required for the MONB node to discharge to V _REF SO that the MON node will be unaffected by the discharge of MONB. For example, the MONB node can take, say, five times (Rl || R2)*C _M O _NB to discharge to VREF. By using the same type of resistor in the delay element as for Rl and R2, any variation in the MONB discharge time will be tracked by the delay element. The lower value of the delay is then5*(Rl || R2)*C _M O _NB , but there is no maximum limit since a higher delay will not cause any problem due to the capacitive regulation. After the loop is settled, the MON node will be same as V _REF due to high loop gain, forcing the output of the regulator to settle at V _DD itself. By taking the final regulation voltage of capacitive and resistive regulation to be the same, no extra time is taken during the transition from capacitive regulation to resistive regulation.

[0021] To further consider the settling output level during the capacitive regulation phase, Figure 5 considers Figure 3 when EN has gone high between to and ti in Figure 4, but the delayed version of ENd is still low. At this point, SWl 41, SW2 43, and Msw 47 are off and SW3 45 is on and Figure 3 is equivalent to Figure 5. The OUT and MON nodes are connected through C _LE A _D 29. As discussed above, C _M ON 53 is not an actual added circuit element, but only the parasitic capacitance seen at the MON node and which is usually negligible compared to CLEAD (CMON « CLEAD). The final output settling voltage of the capacitive regulation and the resistive regulation is preferably the same. A detailed derivation for the settling voltage of capacitive regulation is given with reference to Figure 6, which again shows the control signals of Figure 3 corresponds to Figure 4 with the broken lines (that corresponded to Figure 2) removed and a time t _p indicating when the pass device M _P 21 turns on. The input-output relation of feedback network is derived first and then the final settling voltage is calculated.

[0022] Referring to Figure 6, at any time t, V(OUT) = V ₀ UT and V(MON) = VMON- At t = t ₀ the regulator is enabled (EN goes high) and at t = t _l Msw is turned on (ENd goes high). During standby (t≤ t ₀ ), V ₀ (t≤ t ₀ ) = V _DD and V _M ON ≤ ) = VREF-

The wake-up period is t ₀ < t < t _x . For t ₀ < t < t , regulator is in open loop (M _p is

OFF) and both CLEAD and CMON discharge and the voltage level changes.

[0023] At t = t , VOUT = VOUT_P and VMON = VMON Referring to the boxed feedback network of Figure 5 :

AV M„ _n ON _r = AV, OUT-„ „

LEAD ~"~ ^MON

V —V c LEAD _ Y _ Y c LEAD

MON P ^y ' OUT P- „ , „ ' KEF ' DD- c LEAD + ^T c ^par c ^ LEAD + ^T c MON

At t = t _p , the PMOS pass device M _P 21 turns on, the loop is closed and the regulator takes some time to settle. Again, by capacitor division, for t > t _p : V MON =

(Equation 2) fiVour ^a w _here p = _R ^C LEA _R D _{AND A} = ^ MON _P P _ _{V ' <} O_ UT P · ^C LEAD

LEAD ⁺ ^MON LEAD ^{+ L} MON

The input and output of the feedback network are related by a feed-back factor (β) and offset (a), as represented by Figure 7 that is a block level representation of Figure 5. This gives:

γ - _σ γ - _a

V _OUT = ^~ ψ ⁸⁵⁵ — (when A is large) (Equation 3)

- + β P

A

[0024] Putting the value of a and from Equation 2 into Equation 3 gives:

c LEAD + ^Τ c ^MON

Using Equation 1 , this becomes

c LEAD + ^Τ c ^MON

Consequently, the final regulation voltage of capacitive regulation is V _DD , which is same as when under resistive regulation.

[0025] Consequently, by employing the aspects described with respect to the embodiment of Figure 3, settling time during wake-up is reduced by capacitive regulation without taking any extra power. As this technique works in a closed loop, it has less process dependency for the final settling voltage and will also reduce unwanted overshoot at the output.

[0026] The foregoing detailed description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.