Distributed power delivery scheme for on-die voltage scaling

Abstract

Device comprising: a processor chip with multiple power domains, each of which is to be supplied by a low-dropout (LDO) voltage regulation circuit, where each LDO voltage regulation circuit comprises: (i) at least one power gate transistor for providing different supply modes including an on mode, an off mode and a variable voltage output mode, and (ii) a voltage regulation control circuit to be enabled during the variable voltage mode and disabled during the on and off modes.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to integrated circuits and, more particularly, to the delivery of variable voltages by an on-die voltage regulator. BACKGROUND

[0002] Computer devices can spend a large portion of their time idle. Therefore, power savings during idle mode can be crucial for reducing power consumption.

[0003] Some processors may allow core voltages to be adjusted depending on activity. For example, some central processing units (CPUs) may have the ability to generate a voltage identification (VID) signal. The VID signal can indicate to the power supply the amount of voltage required by the CPUs. A common way to provide this variable voltage may be through the use of an external voltage regulator (VR). However, an external VR may be more costly than a fixed voltage regulator and may require more physical board area. Additionally, an external VR may be slow to adjust its power output. As a result, external VRs may not be well suited to supporting dynamic power savings of CPUs over small time increments.

[0004] Several on-die power-saving techniques exist today without the use of an external VR. These techniques include clock scaling, clock gating, and power gating. Clock scaling can refer to scaling the clock frequency according to the workload to conserve dynamic active power. Clock gating can refer to maintaining the states of certain processor logic blocks when the logic blocks are not processing data to eliminate switching power consumption. Although clock scaling and / or clock gating can reduce dynamic power consumption, an external VR may still be required to change the supply voltage and reduce power dissipation.

[0005] Power gating can refer to turning off power to specific processor logic blocks that are not currently in use in order to reduce a processor's overall power dissipation. Power gating can act as an on / off control via supply voltages. Ideally, power-gated logic blocks can consume no power at all. As such, power gating can be well-suited to placing logic blocks into standby or sleep mode. However, due to the inherent latencies associated with entering or exiting power-gated states, power gating may not be tolerable under normal operating conditions.

[0006] There is a need to save an even greater amount of power, especially in circumstances where one or more power domains of a processor cannot be completely turned off, but also do not process time-sensitive data. Because external VRs can be expensive and inefficient, it may also be advantageous to have a fine-grained on-die power delivery mechanism that addresses the following needs: delivering variable voltage levels without using an external VR; adjusting Vcc based on the process corner to meet product requirements; operating different logic blocks at different clock frequencies at different voltages with a common input voltage; and generating variable voltage levels from a common input voltage to reduce the number of VR platform rails. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Embodiments of the present disclosure are described by way of example only, but not limitation, shown in the accompanying drawings, in which like references indicate like elements, and in which: Fig. 1 is a block diagram of an example of a high speed low dropout (HS-LDO) voltage regulation circuit in accordance with various embodiments; Fig. 2 is a block diagram illustrating the structural relationship among various components of the HS-LDO circuit in accordance with various embodiments; Fig. 3 is a block diagram of an exemplary N-stage pre-driver unit and an exemplary P-stage driver unit coupled to a PGT unit according to various embodiments; Fig. 4 is a flow diagram illustrating a portion of an example operation of the HS-LDO circuit according to various embodiments. Fig. 5 is a block diagram illustrating two ways to integrate an HS-LDO circuit into a power domain of a processor according to various embodiments. Fig. 6 is a block diagram illustrating an example computer system suitable for use in practicing various embodiments of the present invention. DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0008] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration, embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of embodiments according to the present invention is defined by the appended claims and their equivalent interpretations.

[0009] Various operations may in turn be described as multiple discrete operations in a manner that may assist in understanding embodiments of the present invention; however, the order of description should not be construed to imply that these operations are order dependent.

[0010] The terms "coupled" and "connected," and their derivatives, may be used. It should be understood that these terms are not synonymous. Rather, in certain embodiments, "connected" may be used to indicate that two or more elements are in direct physical or electrical contact with each other. "Coupled" may mean that two or more elements are in direct physical or electrical contact. However, "coupled" may also mean that two or more elements are not in direct contact with each other, but still cooperate or interact with each other.

[0011] For the purpose of description, an expression in the form "A / B" or in the form "A and / or B" means (A), (B) or (A and B). For the purpose of description, an expression in the form "at least one of A, B, and C" means (A), (B), (C), (A and B), (A and C), (B and C) or (A, B and C). For the purpose of description, an expression in the form "(A)B" means (B) or (AB), ie, A is an optional element.

[0012] The description may use the terms "in one embodiment" or "in embodiments," which may refer to one or more of the same or different embodiments. Furthermore, the terms "comprising," "including," "having," and the like, as used with respect to embodiments of the present invention, are synonymous.

[0013] The description may use various terms such as "operational amplifier," "gate," "transistor," "resistor," "PMOS," and "NMOS," etc., to represent various components used in different embodiments. It will be appreciated that these components may be implemented in various ways and / or replaced with components of similar functionality. For example, a "resistor" or an "operational amplifier" may be implemented with multiple resistors and / or transistors. Therefore, the terms used throughout this disclosure are for the purpose of illustration only and should not be construed as limitations.

[0014] A computing device's processor may have several power domains. Each power domain may have its own phase-locked loop (PLL) to control its clock frequency, and it may independently manage its own power consumption. Power gating, along with clock gating and clock scaling, can reduce a processor's power consumption. Typically, a power gate (PGT) unit may be connected to a logic block within a power domain, and a power domain may contain multiple PGT units. Electronic Design Automation (EDA) tool vendors may provide schematics for various standard power gate (PGT) units, also known as PGT cells, in their EDA tools. The PGT units may be automatically mapped to their respective logic blocks through a process called Automated Placement and Route (APR).

[0015] A regular PGT unit can support two modes of operation, a fully on mode where a logic block connected to the PGT unit can operate at normal voltage, and a fully off mode where the logic block can be completely turned off, i.e., receives approximately 0 volts from the PGT unit.

[0016] According to various embodiments, an HS-LDO voltage regulation circuit may be coupled to the PGT units of a power domain within a processor to provide the PGT units with a third mode of operation: variable voltage mode. In this variable voltage mode, the output voltage of the PGT units may change depending on the processor's payload. For example, the processor may issue a VID signal indicating to the HS-LDO circuit that a lower voltage is desired to conserve power when a power domain is not processing time-sensitive data but cannot be completely turned off. As the processor's load increases, the HS-LDO circuit may route all PGT units within the power domain to provide a higher voltage to meet the processor's demand.

[0017] In various embodiments, the PGT units may operate under "fully on" or "fully off" modes, in which the logic blocks connected to the PGT units operate either normally at full voltage or off and receiving 0 volts from the PGT units when the HS-LDO circuit is disabled. In various embodiments, the PGT units may operate under variable voltage mode when the HS-LDO circuit is enabled. In variable voltage mode, the HS-LDO circuit may route all PGT units within the power domain to provide power output based on a VID signal or other voltage regulation signal to their respective logic blocks.

[0018] In various embodiments, the logic block connected to the PGT unit may receive a voltage signal that is lower than the fully on voltage under normal operating conditions when the HS-LDO circuit is enabled and the PGT units operate in variable voltage mode.

[0019] However, the logic blocks can still be kept in active operation, compared to standby or sleep states, where the logic blocks merely receive their state information but do not actively process any information. Therefore, the HS-LDO circuit may not exhibit the entry-exit latencies associated with some other power-saving approaches.

[0020] Fig. Figure 1 is a block diagram of an exemplary HS-LDO voltage regulation circuit according to various embodiments. In embodiments, as illustrated, an HS-LDO circuit 100 may include a centralized low-speed loop (LSL) 110 and various other components.

[0021] In various embodiments, the centralized LSL 110 may include an operational amplifier (op-amp) 105. The op-amp 105 may receive a reference voltage Vref 101 at one of its input terminals. The op-amp 105 may generate a setpoint voltage signal Vset 102 at its output terminal. The output terminal of the op-amp 105 may be further coupled to a second input terminal of the op-amp 105 via one or more components, which may include an N-stage replica 103 and a P-stage replica 104. The coupling between the output terminal and the second input terminal of the op-amp 105 may form a feedback loop. The op-amp 105 may produce the output Vset 102 approximately equal to the input Vref 101, similar to a unity-gain isolation amplifier.The details of the N-stage replica 103 and the P-stage replica 104 are provided in later sections of this disclosure.

[0022] In various embodiments, the Vref 101 may be generated by an external reference voltage generator or by a component of the HS-LDO circuit 100 included in Fig. 1. The reference voltage can be generated by a bandgap reference generator, a resistive ladder, or some other known devices. Vref 101 can be generated dynamically based on a VID signal from the processor or based on another voltage control signal.

[0023] In various embodiments, the op-amp 105 may include an operational transconductance amplifier (OTA). Using an OTA in the HS-LDO circuit 100 may provide greater space savings and better stability for the HS-LDO circuit 100. In various embodiments, the op-amp 105 may include additional input / output terminals that may be Fig. 1 will not be shown.

[0024] In various embodiments, the HS-LDO circuit 100 may further include multiple N-stage pre-driver units 130 and multiple P-stage driver units 140 for supplying Vset 102 to multiple PGT units 150. The details of the N-stage pre-driver units 130 and the P-stage driver units 140 are provided in later sections of this disclosure.

[0025] Fig. 2 is a block diagram illustrating the structural relationship among various components of the HS-LDO circuit 100 according to various embodiments. In various embodiments, as illustrated, a voltage reference generator 210 may generate Vref 101. Vref 101 may be provided to the op-amp 105. The output terminal of the op-amp 105 may be coupled to multiple N-stage pre-driver units 130. Each N-stage pre-driver unit 130 may be coupled to multiple P-stage driver units 140. Each P-stage amplifier 140 may be further coupled to multiple PGT units 150. The HS-LDO circuit 100 may have a tree-like structure, with each stage of components fanning out in turn from the op-amp 105.This structure can help separate and isolate the LSL 110 from the remaining portions of the HS-LDO circuit 100, and it can ensure that variations in the remaining portions of the HS-LDO circuit 100, including transient changes to load 170 of the PGT units 150, do not destabilize the LSL 110 and Vset 102. Furthermore, with the tree-like structure, more PGT units 150 can be coupled to the LSL 110, improving the scalability of the HS-LDO circuit 100.

[0026] With further reference to Fig. 1, in various embodiments, each PGT unit 150 may provide Vout 160 to a logic block connected to the PGT unit. The logic block is shown in Fig. 1 as the load 170. In various embodiments, each PGT unit 150 and its associated P-stage driver unit 140 may form a high-speed loop (HSL) 120. More details on the HSL 120 are provided in later sections of this disclosure. While a conventional low-dropout (LDO) voltage regulator circuit may provide variable voltages to the load 170, decoupling the HSL 120 from the LSL 110 may assist the HS-LDO circuit 100 in maintaining the fast current / voltage transient steps of the load 170 within a specified tolerance band. Additionally, a conventional LDO circuit may require a larger amount of on-die capacitance due to its compensation requirement.

[0027] In various embodiments, the HS-LDO circuit 100 may also include various filters 131 placed between and coupled to the N-stage pre-driver units 130 and the P-stage driver units 140. The filters 131 may further isolate the LSL 110 from the remaining portions of the HS-LDO circuit 100. For example, in various embodiments, Vset 102 may be a low-frequency signal. Therefore, the filters 131 may be used to cut any frequency, including the packet resonant frequency, typically around 300 MHz, to eliminate any parasitic packet noise. The filters 131 may also prevent feedback of noise disturbances experienced at the output stage due to unpredictable variations at the load 170. In various embodiments, the filters 131 may include RC filters and / or AC compensators.

[0028] Fig. 3 is a block diagram of an exemplary N-stage pre-driver unit and an exemplary P-stage driver unit coupled to a PGT unit according to various embodiments. As illustrated, in various embodiments, an N-stage pre-driver unit 330 may include a current source 331, a first bias resistor 333, a second bias resistor 334, a first N-type transistor 335, and a second N-type transistor 336. The two N-type transistors 335 and 336 may be N-type MOSFETs or NMOSes. As illustrated, a signal Vset 332 may represent the setpoint voltage signal provided by the op-amp 105 in Fig. 1, which may be approximately equal to Vref 101. In various embodiments, Vset 332 may be coupled to the gate terminal of N-type transistor 335 and serve as the DC bias setpoint for N-stage pre-driver unit 330. In various embodiments, N-type transistor 335 may operate in a manner similar to a gate amplifier since the Vset 332 signal may be AC ​​ground. In various embodiments, N-stage pre-driver unit 330 may produce an output 337 during steady-state operation that is approximately equal to Vset 332 less a threshold voltage Vth of N-type transistor 335. In various embodiments, resistors 333 and 334 may be selected accordingly to adjust the bias voltage such that N-stage pre-driver unit output 337 may be generated.

[0029] In various embodiments, a P-stage driver unit coupled to a PGT unit 340 may include a current sink 341, a first bias resistor 343, a second bias resistor 344, a first P-type transistor 346, and a PGT unit 350. The P-type transistor 346 may be a P-type MOSFET or a PMOS. As illustrated, the output 337 of the corresponding N-stage pre-driver unit may be coupled to the gate terminal of the P-type transistor 346 and serve as a DC bias setpoint for the PGT unit 350. In various embodiments, the P-type transistor 346 may operate in a manner similar to a common-gate amplifier, since the output 337 of the N-stage pre-driver 330 may be AC ​​ground. The PGT unit 350 can provide a Vout 360 to a load 370.In various embodiments, resistors 343 and 344 may be selected accordingly to adjust the bias voltage so that PGT 350 can operate and supply a DC current to load 370.

[0030] In various embodiments, during steady-state operation, the PGT unit 350 may generate Vout 360 that is approximately equal to the output 337 of the N-stage pre-driver unit plus a threshold voltage Vth of the P-type transistor 346. As previously described, the N-stage pre-driver unit 330 may shift Vset 332 downward based on Vth of the N-type transistor 335, and the P-stage driver unit coupled to a PGT unit 340 may shift the output 337 of the N-stage pre-driver unit downward based on Vth of the P-type transistor 346. Subsequently, the PGT unit 350 may generate Vout 360 that is approximately equal to Vset 332 and subsequently approximately equal to Vref 101, as long as the Vth of the N-type transistor 335 is selected to be approximately equal to the Vth of the P-type transistor 346.In various embodiments, balancing Vth of the N-type transistor 335 and the P-type transistor 346 may help maintain the full operating range of the HS-LDO circuit 100.

[0031] In various embodiments, the threshold voltage of the N-type and P-type transistors may be around 350 mV. In various embodiments, Vout 360 may operate in a range of 350 mV to 970 mV, or at any voltage as indicated by a voltage regulation signal, such as a VID signal.

[0032] In various embodiments, the PGT unit 350 with the associated P-stage driver unit may form the HSL 120. If the load 370 to the PGT unit 350 produces sharp spikes within a short time frame, such as within 1 ns, this may cause a sudden voltage drop on Vout 360. This sudden voltage change may be referred to as an AC voltage drop. The P-type transistor 346 may detect this drop and cause the current across resistors 343 and 344 to increase, thereby increasing the voltage across the gate and source terminals of the PGT unit 350, which in turn may output a higher current to compensate for the voltage drop. If the load 370 to the PGT unit 350 reduces sharply within a short time frame, this may cause a spike in Vout 360.The P-type transistor 346 can detect this drop and cause the current through resistors 343 and 344 to increase, thereby increasing the voltage between the gate and source terminals of the PGT unit 350, which in turn can reduce the output current to compensate for the voltage drop. In various embodiments, decoupling the HSL 120 from the LSL 110 can improve the stability of the HS-LDO circuit 100.

[0033] Although Fig. 3 illustrates only a single N-stage pre-driver unit and a single P-stage driver unit coupled to a PGT unit in various embodiments, a single N-stage pre-driver unit 330 may be coupled to hundreds of P-stage driver units. Similarly, a single P-stage driver unit may be coupled to hundreds of PGT units 350. Each PGT unit 350 may be coupled to its own P-type transistor 346. However, the PGT units 350 coupled to the same P-stage driver unit may share bias resistors 343 and 344. This may reduce the complexity and area requirements of the HS-LDO circuit 100 because individual bias circuitry is not required for each PGT unit 350.

[0034] In various embodiments, current source 331 and current sink 341 may be selected based on resistors 333, 334, 343, and 344 and the desired bandwidth of HSL 120. Resistors 333, 334 and resistors 343, 344 may or may not be identical. In various embodiments, N-type transistors 335 and 336 may or may not be structurally identical. In various embodiments, N-type transistors 335 and 336 may or may not have the same physical dimension as P-type transistor 346.

[0035] With further reference to Fig. 1, in various embodiments, the N-stage replica 103 may be a replica of all the N-stage pre-driver units 330, and the P-stage replica 104 may be a replica of all the P-stage driver units, excluding the PGT units 350 and the load 370. The replicas 103 and 104 in the LSL 110 may further improve the accuracy of the feedback loop to the op-amp 105. In various embodiments, if the load 370 does not require precise delivery of voltage levels, the LSL 110 may also be coupled to the load 370 without passing through the N-stage pre-driver unit and the P-stage driver unit.

[0036] Fig. 4 is a flowchart illustrating a portion of an example operation of the HS-LDO circuit according to various embodiments. In various embodiments, the HS-LDO circuit 100 may wait for a control signal at block 410, as illustrated. The control signal may or may not be the VID signal from the processor. The HS-LDO circuit 100 may receive the control signal at block 420. The control signal may indicate to the HS-LDO circuit 100 that the PGT units 150 connected to the HS-LDO circuit 100 should operate in the fully on or fully off mode, as indicated at blocks 430 and 440. In these two modes of operation, additional selection logic (not shown) may disable the HS-LDO circuit 100 to allow the PGT units 150 to operate in switched modes and act as on / off circuits to the load 170 connected to the PGT units 150.Alternatively, the control signal may indicate to HS-LDO circuit 100 that the PGT units 150 connected to HS-LDO circuit 100 should operate in variable voltage mode, as indicated in block 450. In this mode, additional selection logic may enable HS-LDO circuit 100, thereby causing PGT units 150 to output a voltage specified by the control signal. In various embodiments, HS-LDO circuit 100 may continue to wait for the next control signal in block 410 once the control signal has been processed.

[0037] In various embodiments, the selection logic can sequentially turn on / off all PGT units 150 within a power domain in the blocks 430, 440, and 450 in the manner of a daisy chain. This can mitigate the steepness of the current required by the load 170 as it is turned on / off, and it can prevent supply voltage disturbances when thousands of PGT units 150 within a power domain are turned on / off simultaneously.

[0038] Fig. 5 is a block diagram illustrating two ways to integrate an HS-LDO circuit into a power domain of a processor according to various embodiments. In the concentrated power gate approach, as illustrated in Fig. 5(a), the PGT devices can be placed within the bump pitch located on a PGT overlayer 511 and a PGT underlayer 513 of the power domain 510. Standard power gate devices can typically be much smaller than the bump pitch. Therefore, the HS-LDO circuit components can be added to the bump pitch by inserting the free areas within the bump pitch.

[0039] In the distributed power gating scheme, as illustrated in Fig. 5(b), multiple PGT units 521 may be output throughout the power domain 520. Standard PGT units, such as those provided by EDA vendors, may have a low fill factor, making it possible to insert various components of the HS-LDO circuitry into the PGT units 521, such as the selection logic used to enable / disable the PGT units 521, as previously illustrated. In addition, a front-end 522 containing the LSL 110 and other components of the HS-LDO circuitry may also be added to the power domain 520 to be shared by all PGT units 521. In various embodiments, the process of adding the HS-LDO circuitry to the PGT units may be assisted by the APR process.In various embodiments, the additional area required to implement the HS-LDO circuit in either the lumped or distributed method may be negligible. In various embodiments, the HS-LDO circuit may increase the chip area by less than 3%.

[0040] Fig.6 illustrates an example computer system suitable for use in practicing various embodiments of the present invention. As shown, computer system 600 may include multiple processors or processor cores 602 and system memory 604. For the purposes of this application, including the claims, the terms "processor" and "processor cores" may be considered synonymous unless the context clearly requires otherwise. The processors 602 (or other elements of the computer system 600) may include, as previously illustrated, one or more power domains, one or more HS-LDO circuits, and one or more PGT units.

[0041] Additionally, computer system 600 may include mass storage devices 606 (such as floppy disks, hard disks, compact disc read-only memory (CD-ROM), and so on), input / output devices 608 (such as keyboards, cursor controls, and so on), and communication interfaces 610 (such as network cards, modems, and so on). These elements may be coupled to one another via system bus 612, which represents one or more buses. In the case of multiple buses, these are bridged by one or more bus bridges (not shown).

[0042] Each of these elements may perform its conventional functions known in the art. In particular, system memory 604 and mass storage 606 may be used to store a working copy and a permanent copy of the programming instructions that implement one or more operating systems, drivers, applications, and so forth, collectively referred to herein as 622.

[0043] The permanent copy of the programming instructions can be placed into the permanent storage 606 at the factory or in the field, for example, through a distribution medium (not shown), such as a compact disc (CD), or through the communication interface 610 (from a distribution server (not shown)). That is, one or more distribution media containing an implementation of the agent program can be used to distribute the agent and program various computing devices.

[0044] The remaining composition of these elements 602-612 is known and will therefore not be described further.

[0045] While specific embodiments have been described, it will be apparent to those skilled in the art that a wide variety of other and / or similar implementations may be made in place of the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover all adaptations or variations of the embodiments discussed herein. Therefore, it is expressly understood that the embodiments of the present invention are limited only by the claims and their equivalent interpretations.

Claims

[1] Device comprising: a processor chip with multiple power domains, each of which is to be supplied by a low-dropout (LDO) voltage regulation circuit, where each LDO voltage regulation circuit comprises: (i) at least one power gate transistor for providing different supply modes including an on mode, an off mode and a variable voltage output mode, and (ii) a voltage regulation control circuit to be enabled during the variable voltage mode and disabled during the on and off modes. [2] The apparatus of claim 1, wherein the voltage regulation control circuit comprises at least one analog control loop for regulating the output voltage from the at least one power gate transistor during the variable voltage mode, wherein the analog control loop is to be disabled during the on and off modes. [3] The device of claim 1, further comprising: a selection logic for each LDO voltage regulation circuit to disable the voltage regulation control circuit for the on and off modes. [4] The device according to any one of claims 1 to 3, wherein the voltage regulation circuit further comprises for each LDO voltage regulation circuit: (i) an operational amplifier (OP-AMP) having a first input terminal configured to receive a reference voltage, a second input terminal coupled to an output terminal of the OP-AMP to form a feedback loop; (ii) a pre-driver unit coupled to the output terminal of the OP-AMP; and for causing it to generate an output voltage based on the reference voltage during the variable voltage mode. [5] The device of claim 4, wherein the OP-AMP is an operational transconductance amplifier. [6] The apparatus of claim 4, wherein the OP-AMP is configured to receive the reference voltage from a bandgap reference voltage generator configured to generate the reference voltage based on a voltage identification (VID) signal. [7] The device of claim 4, wherein the pre-driver unit comprises an N-type transistor and the driver unit comprises a P-type transistor. [8] The device of claim 4, wherein the pre-driver unit and the driver unit further comprise one or more operating point setting resistors. [9] The device of claim 4, further comprising: a filter connected between the pre-driver and driver units. [10] The device of any one of claims 4 to 9, wherein the feedback loop is a first feedback loop, and corresponding driver units of a plurality of driver units in the voltage regulation circuit are coupled to corresponding power gate units of a plurality of power gate units in the voltage regulation circuit to form a plurality of second feedback loops, the first feedback loop being decoupled from the plurality of second feedback loops. [11] Computer system comprising: a processor chip; an external power supply to provide power to the processor chip; and an I / O device coupled to the processor chip to provide a user with access to the computer system, the processor chip comprising two or more logic blocks with separate power domains, each power domain being powered by an associated at least one power gate unit having a supply input coupled to the external power supply, a control input, and a supply output for providing an internal power supply to a corresponding power domain; wherein each power domain comprises a low-dropout (LDO) control circuit coupled to the control input of its associated at least one power gate unit and a selection logic, wherein the LDO control circuit is configured to control the at least one power gate unit to provide a variable voltage output in a variable voltage output mode, wherein the selection logic is configured to deactivate the LDO control circuit to save power for a switching mode, wherein the at least one power gate transistor is either (i) is switched on to couple the external power supply with the internal power supply for an on mode, or (ii) is switched off to decouple the external power supply from the internal power supply for an off mode. [12] The computer system of claim 11, wherein the LDO control circuit comprises a low-speed loop circuit and a high-speed loop circuit. [13] The computer system of claim 11 or 12, wherein the at least one power gate unit comprises a P-type transistor device. [14] A computer system according to any one of claims 11 to 13, wherein each power domain comprises a plurality of power gate units controlled by a common LDO control circuit. [15] A computer system according to any one of claims 11 to 14, wherein the selection logic sequentially turns on / off power gate units in a power domain in a chained manner.

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