Method and apparatus for voltage regulation within an integrated circuit

[0002] One or more aspects of the present invention relate generally to voltage regulation within an integrated circuit and, more particularly, to regulation of switch circuit gate voltage within a programmable logic device.

[0003] Programmable logic devices (PLDs) exist as a well-known type of integrated circuit (IC) that may be programmed by a user to perform specified logic functions. There are different types of programmable logic devices, such as programmable logic arrays (PLAs) and complex programmable logic devices (CPLDs). One type of programmable logic devices, known as a field programmable gate array (FPGA), is very popular because of a superior combination of capacity, flexibility, time-to-market, and cost.

[0004] An FPGA typically includes an array of configurable logic blocks (CLBs) surrounded by a ring of programmable input/output blocks (IOBs). The CLBs and IOBs are interconnected by a programmable interconnect structure. The CLBs, IOBs, and interconnect structure are typically programmed by loading a stream of configuration data (bitstream) into internal configuration memory cells that define how the CLBs, IOBS, and interconnect structure are configured. The configuration bitstream may be read from an external memory, conventionally an external integrated circuit memory EEPROM, EPROM, PROM, and the like, though other types of memory may be used. The collective states of the individual memory cells then determine the function of the FPGA.

[0005] The programmable interconnect structure typically includes switch circuits (also known as switch boxes) for interconnecting the various logic blocks within an FPGA. Switch circuits generally include pass transistors for forming programmable connections between input/output lines of logic blocks in response to a gate voltage. A voltage regulator provides and regulates the gate voltage that drives the gates of the pass transistors. As is well known in the art, the speed of propagation of a signal through such a switch circuit improves with higher gate voltage applied to the gates of the pass transistors.

[0006] One method employed by others to provide relatively high gate voltage to pass transistors in a switch circuit is to clamp the gate voltage to an internal supply source, V_cc, when the internal supply source rises above a target gate voltage. However, known voltage regulators are susceptible to one or more of intrinsic voltage offsets caused by process variations and differences in physical layout of the voltage regulator components, though such physical layout may be intended to be symmetric. One or more of these intrinsic voltage offsets may cause the voltage regulator to become unstable thereby producing oscillations in the output voltage, for example.

[0007] Accordingly, it would be both desirable and useful to provide a method and apparatus for voltage regulation within an IC that is less susceptible to one or more intrinsic voltage offsets.

[0008] Method and apparatus for voltage regulation within an integrated circuit is described. In an embodiment in accordance with one or more aspects of the invention, a voltage regulator receives a first reference voltage and provides a regulated voltage. A comparator includes a first input to receive a second reference voltage and a second input to receive the regulated voltage. The comparator includes an offset voltage. The comparator provides a control signal indicative of whether the difference between the second reference voltage and the regulated voltage is greater than the offset voltage. A voltage clamp circuit clamps the regulated voltage to the second reference voltage in response to the control signal.

[0009] In another embodiment in accordance with one or more aspects of the invention, a comparator compares a first reference voltage with a second reference voltage. The comparator provides a control signal indicative of which of the first reference signal and the second reference signal is greater. A multiplexer provides either the first reference voltage or the second reference voltage as output in response to the control signal. A regulator receives the output of the multiplexer and provides a regulated voltage.

[0010] Accompanying drawing(s) show exemplary embodiment(s) in accordance with one or more aspects of the invention; however, the accompanying drawing(s) should not be taken to limit the invention to the embodiment(s) shown, but are for explanation and understanding only.

[0011] FIG. 1 depicts a block diagram showing an exemplary portion of a programmable logic device in which one or more aspects of the invention are useful;

[0012] FIG. 2 depicts a block diagram of an exemplary embodiment of a voltage regulator in accordance with one or more aspects of the invention; and

[0013] FIG. 3 depicts a block diagram of another exemplary embodiment of a voltage regulator in accordance with one or more aspects of the invention.

[0014] Method and apparatus for voltage regulation within an integrated circuit (IC) is described. One or more aspects in accordance with the invention are described in terms of gate voltage regulation of pass transistors within a programmable logic device (PLD). While specific reference is made to regulating gate voltage of pass transistors, those skilled in the art will appreciate that one or more aspects of the invention may be used to regulate other voltages used for various applications within an IC device.

[0015] FIG. 1 depicts a block diagram showing a portion of an exemplary PLD 100. PLD 100 is illustratively shown as including logic blocks 102A through 102D (collectively referred to as logic blocks 102), and switch circuits 104. Logic blocks 102 comprise CLBs, IOBs, or like type well-known circuits. Switch circuits 104 comprise one or more pass transistors, memory cells, and multiplexer circuits, as is well known in the art. Logic blocks 102 are programmably connectable by configuring switch circuits 104 in a well-known manner. An exemplary embodiment of switch circuit 104 is illustratively shown as including a pass transistor 106 and a memory cell 108. Memory cell 108 is coupled to the gate of pass transistor 106 for activating or deactivating pass transistor 106. Pass transistor 106 comprises, for example, an NMOS or a PMOS transistor. Memory cell 108 comprises, for example, SRAM, EPROM, EEPROM, flash memory, antifuse pull-up or pull-down circuits, or any other type of well-known programmable memory cell.

[0016] If pass transistor 106 is activated, line L2 is coupled to line L1, and thus logic block 102D is coupled to logic block 102A. Otherwise, when pass transistor 106 is deactivated, line L2 is not coupled to line L1. Memory cell 108 drives the gate of pass transistor 106 with a gate voltage V_ggfor activation/deactivation. Memory cell 108 receives gate voltage V_ggfrom a voltage regulator 110. Voltage regulator 110 is coupled to a voltage source 112, which produces a reference voltage V_ref. Reference voltage V_refis a target gate voltage, or a fraction of a target gate voltage, for pass transistor 106 and is regulated by voltage regulator 110 to provide gate voltage V_gg.

[0017] FIG. 2 depicts a block diagram of an exemplary embodiment of a voltage regulation circuit 200 in accordance with one or more aspects of the invention. Voltage regulation circuit 200 may be used as voltage regulator 110 shown in FIG. 1 and is described in this context. Voltage regulation circuit 200 includes a reference voltage terminal V_ref203, a gate voltage terminal V_gg208, a supply voltage terminal V_cc210, a voltage regulator 202, a clamp circuit 204, and a comparator 206. Voltage regulator 202 and comparator 206 are circuits known to one of ordinary skill in the art. In one embodiment, clamp circuit 204 is a PMOS transistor, as shown in FIG. 2, whose gate is coupled to CTL 213, and whose source and drain are respectively coupled to Vcc 210 and Vgg 208. However, other embodiments may be used for or in clamp circuit 204, including an NMOS transistor instead of a PMOS transistor or another clamp circuit known to one of ordinary skill in the art. Reference voltage terminal V_ref203 is provided a reference voltage V_ref; supply voltage terminal V_cc210 is provided a supply voltage V_cc; and gate voltage terminal V_gg208 provides a gate voltage V_gg. Reference voltage V_refis a target voltage level, or a fraction of a target voltage level, for gate voltage V_gg.

[0018] Inputs of voltage regulator 202 are respectively coupled to reference voltage terminal V_ref203 and gate voltage terminal V_gg208. An output of voltage regulator 202 is coupled to gate voltage terminal V_gg208. Voltage regulator 202 operates in a well-known manner. Voltage regulator 202 produces gate voltage V_ggresponsive to reference voltage V_ref. When the level of gate voltage V_ggdrops below the level of reference voltage V_ref(or a fraction thereof), regulator 202 increases the level of gate voltage V_gg.

[0019] Inputs of comparator 206 are respectively coupled to supply voltage terminal V_cc210 and gate voltage terminal V_gg208. Comparator 206 includes a control terminal CTL 213. Comparator 206 produces a control signal CTL at control terminal CTL 213 responsive to supply voltage V_ccand gate voltage V_gg. Comparator 206 includes a built-in offset voltage V_offset, which affects the trip point of comparator 206. The trip point of comparator 206 is the point at which the difference between supply voltage V_ccand gate voltage V_ggcauses a change of state of control signal CTL. Instead of a trip point of zero, the trip point is set to V_offset, which can be a positive or a negative offset voltage. That is, comparator 206 drives control signal CTL to a first state if the difference between supply voltage V_ccand gate voltage V_ggis greater than offset voltage V_offset(V_cc−V_gg>V_offset). Comparator drives control signal CTL to a second state if the difference between supply voltage V_ccand gate voltage V_ggis less than offset voltage V_offset(V_cc−V_gg<V_offset).

[0020] As described in more detail below, magnitude of offset voltage V_offsetis selected to be greater than an intrinsic offset voltage of comparator 206. In an embodiment, offset voltage V_offsetis a fixed parameter. For example, an offset can be built into comparator 206 by intentionally mismatching the sizes of transistors of comparator 206 that are coupled to the input terminals of comparator 206. Alternatively, offset voltage V_offsetmay be programmably adjusted during operation of voltage regulation circuit 200 by programmably selecting a different amount of mismatch between the sizes of transistors of comparator 206 that are coupled to input terminals of comparator 206.

[0021] Inputs of clamp circuit 204 are respectively coupled to control terminal CTL 213 and supply voltage terminal V_cc210. An output of clamp circuit 204 is coupled to gate voltage terminal V_gg208. If activated, clamp circuit 204 causes gate voltage V_ggto follow supply voltage V_cc. Activation of clamp circuit 204 is responsive to control signal CTL.

[0022] In operation, the voltage level of reference voltage V_refis selected to be a target voltage level (or some fraction of a target voltage level) for gate voltage V_gg. Voltage regulation circuit 200 has two modes of operation. In a first mode, supply voltage V_ccis less than a sum of gate voltage V_ggand offset voltage V_offset(i.e., V_cc<V_gg+V_offset). In a second mode, supply voltage V_ccis greater than a sum of gate voltage V_ggand offset voltage V_offset(i.e., V_cc>V_gg+V_offset). Stated differently, the difference between supply voltage V_ccand gate voltage V_ggis compared with offset voltage V_offset. In the first mode (V_cc<V_gg+V_offset), the difference is less than offset voltage V_offset. In the second mode (V_cc>V_gg+V_offset), the difference is greater than offset voltage V_offset.

[0023] In the first mode (V_cc<V_gg+V_offset), voltage regulation circuit 200 causes gate voltage V_ggto follow reference voltage V_ref. Thus, as long as supply voltage V_ccremains below the target voltage level for gate voltage V_ggplus offset voltage V_offset, voltage regulation circuit 200 will cause gate voltage V_ggto follow reference voltage V_ref.

[0024] In the second mode (V_cc>V_gg+V_offset), voltage regulation circuit 200 causes gate voltage V_ggto instead follow supply voltage V_cc, which is now above the target voltage level for gate voltage V_gg. In particular, supply voltage V_ccis above the target voltage level for gate voltage V_ggby an amount equal to offset voltage V_offset. Thus, as long as supply voltage V_ccremains above the target voltage level for gate voltage V_ggby an amount equal to offset voltage V_offset, voltage regulation circuit 200 will cause gate voltage V_ggto follow supply voltage V_ccinstead of reference voltage V_ref. This allows voltage regulation circuit 200 to produce as high as possible gate voltage V_gg.

[0025] Moreover, comparator 206 compares supply voltage V_ccwith a sum of gate voltage V_ggand offset voltage V_offset. If supply voltage V_ccis less than the sum of gate voltage V_ggand offset voltage V_offset, then comparator 206 drives control signal CTL to an inactive state (e.g., logically low in an active high embodiment). If control signal CTL 213 is in an inactive state, clamp circuit 204 is not active and does not clamp gate voltage V_ggto the voltage level of supply voltage V_cc. Voltage regulator 202 thus causes gate voltage V_ggto follow reference voltage V_ref. That is, if gate voltage V_ggfalls below reference voltage V_ref(or some fraction thereof), voltage regulator 202 increases gate voltage V_gg.

[0026] If supply voltage V_ccis greater than gate voltage V_ggby an amount equal to V_offset, then comparator 206 drives control signal CTL 213 to an active state (e.g., logically high in an active high embodiment). If control signal CTL 213 is in the active state, clamp circuit 204 is active and clamps gate voltage V_ggto the voltage level of supply voltage V_cc. In this case, supply voltage V_ccis greater than reference voltage V_refby definition. Since gate voltage V_ggis higher than reference voltage V_ref, voltage regulator 202 does not actively regulate gate voltage V_gg.

[0027] Offset voltage V_offsetallows voltage regulation circuit 200 to be less susceptible to an intrinsic offset within comparator 206 caused by, for example, random process variations. For example, random process variations during fabrication of comparator 206 may cause an intrinsic offset approximately between plus and minus five millivolts (±5 mV) to affect the trip point. Without a built-in offset voltage V_offset, a slightly negative intrinsic offset within comparator 206 can cause voltage regulation circuit 200 to become unstable. Specifically, an uncompensated intrinsic offset voltage results in both clamp circuit 204 and voltage regulator 202 being active at the same time, which could result in undesirable oscillations in gate voltage V_gg. That is, voltage regulator 202 will begin over-regulate to compensate for current drawn by clamp circuit 204. If claim circuit 204 deactivates, voltage regulator 202 will continue to over-regulate for some time, resulting in oscillations of gate voltage V_gg.

[0028] By building in offset voltage V_offsetto comparator 206, voltage regulation circuit 200 will maintain stability. For example, in an embodiment, offset voltage V_offsetis a positive voltage greater than the expected value of the intrinsic offset of comparator 206 (e.g., 50 mV). When clamp circuit 204 is actively clamping gate voltage V_ggto the level of supply voltage V_cc, a drop in supply voltage V_ccbelow the sum of gate voltage V_ggand offset voltage V_offsetwill cause clamp circuit 204 to be deactivated. Regulator circuit 202 also remains inactive until such time as gate voltage V_ggdrops below reference voltage V_ref(or some fraction thereof). In this manner, a situation where both regulator 202 and clamp circuit 204 are active at the same time may be avoided (i.e., when V_gg>V_cc).

[0029] Offset voltage V_offsetmay be built into comparator 206 to affect the trip point. In the above example, offset voltage V_offsetis positive. As an alternative, offset voltage V_offsetmay be negative. In each embodiment, comparator 206 is comparing offset voltage V_offsetwith the difference between supply voltage V_ccand gate voltage V_gg.

[0030] FIG. 3 depicts a block diagram of another exemplary embodiment of a voltage regulation apparatus 300 in accordance with one or more aspects of the invention. Voltage regulation apparatus 300 may be used as voltage regulator 110 shown in FIG. 1. Voltage regulation apparatus 300 comprises a reference voltage terminal V_ref303, a supply voltage terminal V_cc305, a gate voltage terminal V_gg307, a voltage regulator 306, a multiplexer 304, and a comparator 302. Reference voltage terminal V_ref303 is provided a reference voltage V_ref; supply voltage terminal V_cc305 is provided a supply voltage V_cc; and gate voltage terminal V_gg307 provides a gate voltage V_gg. Reference voltage V_refis a target voltage level, or a fraction of a target voltage level, for gate voltage V_gg.

[0031] Inputs of comparator 302 are respectively coupled to reference voltage terminal V_ref303 and supply voltage terminal V_cc305. Comparator 302 includes a control terminal CTL 313. Comparator 302 produces a control signal CTL on control terminal CTL 313 responsive to reference voltage V_refand supply voltage V_cc. Control signal CTL is in a first state if V_refis greater than V_cc. Control signal CTL is in a second state if V_refis less than V_cc.

[0032] Inputs of multiplexer 304 are respectively coupled to reference voltage terminal V_ref303 and supply voltage terminal V_cc305. A control terminal of multiplexer 304 is coupled to control terminal CTL 313. Multiplexer 304 includes an output terminal V_newref314. Multiplexer 304 produces a new reference voltage V_newrefon output terminal V_newref314 responsive to control signal CTL.

[0033] Inputs of voltage regulator 306 are respectively coupled to output terminal V_newref314 and gate voltage terminal V_gg307. An output of voltage regulator 306 is coupled to gate voltage terminal V_gg307. Voltage regulator 306 produces a gate voltage V_ggresponsive to new reference voltage V_newref.

[0034] In operation, the level of reference voltage V_refis selected to be the target voltage level for gate voltage V_gg. Voltage regulation apparatus 300 has two modes of operation. In a first mode, supply voltage V_ccis less than reference voltage V_ref(i.e., V_cc<V_ref). In a second mode, supply voltage V_ccis greater than reference voltage V_ref(i.e., V_cc>V_ref). In the first mode (V_cc<V_ref), voltage regulation apparatus 300 causes gate voltage V_ggto follow reference voltage V_ref, which is the target voltage level for gate voltage V_gg. Thus, if supply voltage V_ccremains below the target voltage level for gate voltage V_gg, voltage regulation apparatus 300 will cause gate voltage V_ggto follow reference voltage V_ref.

[0035] In the second mode (V_cc>V_ref), voltage regulation apparatus 300 causes gate voltage V_ggto instead follow supply voltage V_cc, which is now above the target voltage level for gate voltage V_gg. Thus, if supply voltage V_cc, remains above the target voltage level for gate voltage V_gg, voltage regulation apparatus 300 will cause gate voltage V_ggto follow supply voltage V_ccinstead of reference voltage V_ref. This allows voltage regulation apparatus 300 to produce as high as possible gate voltage V_gg.

[0036] More specifically, comparator 302 compares reference voltage V_refwith supply voltage V_cc. When supply voltage V_ccis greater than reference voltage V_ref, comparator 302 drives control signal CTL to cause multiplexer 304 to select supply voltage V_cc. When supply voltage V_ccis less than reference voltage V_ref, comparator 302 drives control signal CTL to cause multiplexer 304 to select reference voltage V_ref. If multiplexer 304 selects supply voltage V_cc, new reference voltage V_newref314 equals supply voltage V_cc. Voltage regulator 306 then causes gate voltage V_ggto follow supply voltage V_cc. When multiplexer 304 selects reference voltage V_ref, new reference voltage V_newref314 equals reference voltage V_ref. Voltage regulator 306 then causes gate voltage V_ggto follow reference voltage V_ref. In this manner, voltage regulation apparatus 300 does not require an additional clamp circuit. Voltage regulation apparatus 300 eliminates the problem caused by the interaction of a regulator and a clamp circuit attempting to control voltage level on a single node.

[0037] In addition, although voltage regulation circuit 200 of FIG. 2 solves the problem of large oscillations in gate voltage V_ggdue to voltage regulator 202 and clamp circuit 204 being active at the same time, voltage regulation circuit 200 causes small oscillations in gate voltage V_gg. Specifically, the intentional offset voltage V_offsetbuilt into comparator 206 will prevent clamp circuit 204 from keeping gate voltage V_ggequal to supply voltage V_cc. If gate voltage V_ggis less than the difference between supply voltage V_ccand offset voltage V_offset, clamp circuit 204 activates and gate voltage V_ggwill approach supply voltage V_ccvery rapidly. However, gate voltage V_ggwill not equal supply voltage V_ccfor long, since clamp circuit 204 deactivates after gate voltage V_ggis greater than the difference between supply voltage V_ccand offset voltage V_offset. Clamp circuit 204 continues to activate and deactivate, causing gate voltage V_ggto oscillate approximately between supply voltage V_ccand the difference between supply voltage V_ccand offset voltage V_offset. A circuit receiving gate voltage V_ggcan function property with these small oscillations as compared to the large oscillations produced if clamp circuit 204 and voltage regulator 202 are both active at the same time.

[0038] Voltage regulation apparatus 300 of FIG. 3, however, avoids producing even small oscillations in gate voltage V_gg. Specifically, intrinsic voltage offsets within comparator 302 or voltage regulator 306 will not produce oscillations in gate voltage V_gg. Rather, such intrinsic voltage offsets will merely shift the final voltage level of gate voltage V_ggby a small amount.

[0039] While the foregoing describes exemplary embodiment(s) in accordance with one or more aspects of the present invention, other and further embodiment(s) in accordance with the one or more aspects of the present invention may be devised without departing from the scope thereof, which is determined by the claim(s) that follow and equivalents thereof. Claim(s) listing steps do not imply any order of the steps.