Controlled Power Up and Power Down of Multi-Stage Low Drop-Out Regulators

Abstract

Circuits and methods that provide for fast power up and power down times in a multi-stage LDO regulator. In one embodiment, a multi-stage LDO regulator circuit includes, for each stage for which fast power up and/or power down times are desired, at least one transconductance amplifier coupled and configured to compare a primary reference voltage to one of a secondary reference voltage for the stage or an output voltage of the stage, and coupling and configuring the at least one transconductance amplifier to charge and/or discharge an associated capacitor to achieve a desired charge level within a specified time independently of the value of the associated capacitor. In general, the transconductance amplifiers of each stage are configured to charge and/or discharge an associated capacitor in synchronism with a voltage present on the primary reference voltage input.

Claims

1. A power up/down multistage low dropout (LDO) regulator circuit including at least one transconductance amplifier having an output coupled to one or more capacitive nodes of the power up/down multistage LDO regulator circuit and configured to charge and/or discharge the one or more capacitive nodes.

2. The invention of claim 1, wherein the power up/down multistage LDO regulator circuit includes two or more LDO stages each having at least one associated capacitive node.

3. The invention of claim 1, further including: (a) a first voltage source coupled to a first LDO stage having at least one capacitive node coupled to an associated one of the at least one transconductance amplifier; and (b) a second voltage source coupled to a second LDO stage having at least one capacitive node coupled to an associated one of the at least one transconductance amplifier.

4. The invention of claim 3, wherein a first one of the at least one transconductance amplifier includes: (a) a first input connected to the first voltage source; (b) a second input connected to the second voltage source; and (c) an output connected to the first voltage source and to a first capacitive node of the at least one capacitive node of the first LDO stage.

5. The invention of claim 4, wherein the first one of the at least one transconductance amplifier is configured to charge and/or discharge the first capacitive node based in part on a difference between a first voltage applied to the first input and a second voltage applied to the second input.

6. The invention of claim 3, wherein a second one of the at least one transconductance amplifier includes: (a) a first input connected to an output voltage of the first LDO stage; (b) a second input connected to the second voltage source; and (c) an output connected to the output of the first LDO stage and to a second capacitive node of the at least one capacitive node of the first LDO stage.

7. The invention of claim 6, wherein the second one of the at least one transconductance amplifier is configured to charge and/or discharge the second capacitive node based in part on a difference between a first voltage applied to the first input and a second voltage applied to the second input.

8. The invention of claim 1, wherein at least one of the at least one transconductance amplifier is configured to charge and/or discharge the one or more capacitive nodes based in part on a difference in voltage between a first voltage source and a second voltage source.

9. A power up/down multistage low dropout (LDO) regulator circuit including: (a) a first LDO stage having at least one capacitive node; and (b) at least one transconductance amplifier having an output coupled to one of the at least one capacitive node of the first LDO stage and configured to charge and/or discharge the one capacitive node during at least one of a power up time or a power down time.

10. The invention of claim 9, further including: (a) a first voltage source coupled to the first LDO stage; (b) a second LDO stage having at least one capacitive node; and (c) a second voltage source coupled to the second LDO stage.

11. The invention of claim 10, wherein a first one of the at least one transconductance amplifier includes: (a) a first input connected to the first voltage source; (b) a second input connected to the second voltage source; and (c) an output connected to the first voltage source and to a first capacitive node of the at least one capacitive node of the first LDO stage.

12. The invention of claim 11, wherein the first one of the at least one transconductance amplifier is configured to charge and/or discharge the first capacitive node based in part on a difference between a first voltage applied to the first input and a second voltage applied to the second input.

13. The invention of claim 10, wherein a second one of the at least one transconductance amplifier includes: (a) a first input connected to an output voltage of the first LDO stage; (b) a second input connected to the second voltage source; and (c) an output connected to the output of the first LDO stage and to a second capacitive node of the at least one capacitive node of the first LDO stage.

14. The invention of claim 13, wherein the second one of the at least one transconductance amplifier is configured to charge and/or discharge the second capacitive node based in part on a difference between a first voltage applied to the first input and a second voltage applied to the second input.

15. A method of synchronizing a power up/down multistage low dropout (LDO) regulator circuit to a first voltage source, including: (a) providing a power up/down multistage LDO regulator having n LDO stages, where n≥2, at least a first LDO stage having at least one capacitive node; (b) coupling at least one transconductance amplifier to a corresponding one of the at least one capacitive node of the first LDO stage; and (c) charging and/or discharging the at least one capacitive node using the at least one corresponding transconductance amplifier during at least one of a power up time or a power down time.

16. The method of claim 15, further including: (a) coupling the first voltage source to the first LDO stage; (b) providing a second LDO stage having at least one capacitive node; and (c) coupling a second voltage source to the second LDO stage.

17. The method of claim 16, wherein a first one of the at least one transconductance amplifier includes: (a) a first input connected to the first voltage source; (b) a second input connected to a second voltage source; and (c) an output connected to the first voltage source and to a first capacitive node of the at least one capacitive node of the first LDO stage.

18. The method of claim 17, further including charging and/or discharging the first capacitive node using the first one of the at least one transconductance amplifier based in part on a difference between a first voltage applied to the first input and a second voltage applied to the second input.

19. The method of claim 16, wherein a second one of the at least one transconductance amplifier includes: (a) a first input connected to an output voltage of the first LDO stage; (b) a second input connected to the second voltage source; and (c) an output connected to the output of the first LDO stage and to a second capacitive node of the at least one capacitive node of the first LDO stage.

20. The method of claim 19, further including charging and/or discharging the second capacitive node using the second one of the at least one transconductance amplifier based in part on a difference between a first voltage applied to the first input and a second voltage applied to the second input.

Description

DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a block diagram of a prior art electronic circuit powered by an SMPS.

[0022] FIG. 2 is a schematic diagram of a prior art embodiment of a 2-stage LDO regulator.

[0023] FIG. 3 is a graph showing voltages V.sub.REF2, V.sub.REF1, V.sub.LDO1, and V.sub.LDO2 as a function of time for an example modeled embodiment of the multi-stage LDO regulator of FIG. 2.

[0024] FIG. 4 is a block diagram of a first embodiment of a fast power up/down multi-stage LDO regulator circuit.

[0025] FIG. 5A is a symbolic depiction of a typical Gm amplifier.

[0026] FIG. 5B is a graph showing the output I.sub.OUT of the Gm amplifier of FIG. 5A as a function of V.sub.POS—V.sub.NEG.

[0027] FIG. 6 is a graph showing voltages V.sub.REF2, V.sub.REF1, V.sub.LDO1, and V.sub.LDO2 as a function of time for an example modeled embodiment of the multi-stage LDO regulator of FIG. 4.

[0028] FIG. 7 is a block diagram of a second embodiment of a fast power up/down LDO regulator having n stages, where n≥2.

[0029] FIG. 8 is a process flow chart showing one method for providing fast power up and/or power down times in a multi-stage LDO regulator circuit.

[0030] Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0031] The present invention encompasses circuits and methods that provide for fast power up and power down times in a multi-stage low drop-out (LDO) regulator.

[0032] In order to speed up the power up (charging) and power down (discharging) times of a multi-stage LDO regulator, embodiments of the present invention inject an auxiliary current into certain capacitive nodes coupled to the reference voltage sources during power up, and provide an additional sink capability to sink current from those capacitive nodes during power down (discharge). In multi-stage LDO regulators, such embodiments need to be designed with care. For example, the reference voltage sources may be coupled to different capacitor values, or may be coupled to the same capacitor values but have different output impedances, which in either case could lead to very different power up and power down times for the capacitive nodes and thus of the multi-stage LDO regulator as a whole. As another example, a multi-stage LDO regulator tends to be designed as a Class A amplifier, which provides a weak and not well controlled discharge strength for the output voltage side of the regulator. Embodiments of the present invention are therefore designed to adapt the current flowing to or from certain capacitive nodes so as to charge or discharge those nodes within the same time period independently of the capacitor values and independently of the output impedance of the reference voltage sources.

[0033] FIG. 4 is a block diagram of a first embodiment of a fast power up/down multi-stage LDO regulator circuit 400. The circuit 400 is similar in structure to the 2-stage LDO regulator 200 of FIG. 2, and functions in the same manner after power up and before power down. However, to provide for a fast power up and/or power down capability, a transconductance amplifier (also known as a “Gm amplifer”) 402a, 402b, 402c, is coupled to one or more capacitive nodes to be rapidly charged and/or discharged. A Gm amplifier sources or sinks a current proportional to a difference in voltage of its two inputs. In some embodiments, one or both of the inputs to a Gm amplifier may be scaled (e.g., by a resistive divider) to bias the output of the Gm amplifier.

[0034] FIG. 5A is a symbolic depiction of a typical Gm amplifier 500. A positive input is configured to receive a voltage V.sub.POS, and a negative input is configured to receive a voltage V.sub.NEG. The output of the Gm amplifier 500 is a current I.sub.OUT that is proportional to the difference between V.sub.POS and V.sub.NEG. In some embodiments, the Gm amplifier 500 has an ENABLE input configured to receive a control signal that selectively enables or disables the Gm amplifier 500. As a person of ordinary skill will understand, the Gm amplifiers 402a, 402b, 402c shown in FIG. 4 are typically disabled in normal operation to ensure that their outputs do not compete or conflict with the output of any LDO stage.

[0035] FIG. 5B is a graph showing the output I.sub.OUT of the Gm amplifier 500 of FIG. 5A as a function of V.sub.POS−V.sub.NEG (graph line 501). In a range bracketed by the dotted lines 502a, 502b, I.sub.OUT is directly proportional to V.sub.POS−V.sub.NEG. The proportionality factor is called the transconductance, Gm, of the device, and is essentially the slope of graph line 501 between the dotted lines 502a, 502b. As graph line 501 makes clear, a Gm amplifier is able to sink or source current, depending on the polarity of the differential voltage.

[0036] In the example illustrated in FIG. 4, a first, input-side Gm amplifier 402a has a first input (positive, in this example) coupled to V.sub.REF2 and a second input (negative, in this example) coupled to V.sub.REF1. The output of the first Gm amplifier 402a is coupled to the conductor that connects the reference voltage source 202 to the decoupling capacitor C.sub.REF1 and to the control input of LDO1.

[0037] When powering up, the first Gm amplifier 402a is enabled and any difference in the voltages at the inputs of the Gm amplifier 402a will result in generation of a current by the Gm amplifier 402a that is added to the current from the reference voltage source 202 to more rapidly charge the decoupling capacitor C.sub.REF1. More specifically, since V.sub.REF2 will initially be greater than V.sub.REF1 during a power up period (since V.sub.REF1 is derived from V.sub.REF2), the Gm amplifier 402a will proportionally source a current to the decoupling capacitor C.sub.REF1, thus more rapidly charging that capacitor. Because the output of the Gm amplifier 402a is proportional to the difference between its voltage inputs, the source current will dynamically vary as the difference between the two voltages changes over time. Accordingly, as the difference diminishes, the output current also diminishes.

[0038] Conversely, because a Gm amplifier can sink current as well as source current, when powering down, any difference in the voltages at the inputs of the Gm amplifier 402a will result in the Gm amplifier 402a proportionally sinking current from the decoupling capacitor C.sub.REF1, thus discharging that capacitor more rapidly. More specifically, since V.sub.REF2 will be less than V.sub.REF1 during a power down period (since V.sub.REF1 is held higher than V.sub.REF2 by the decoupling capacitor C.sub.REF1), the Gm amplifier 402a will sink current from the decoupling capacitor C.sub.REF1 regardless of the sink capability of the associated reference voltage source 202, thus more rapidly discharging that capacitor. Again, since the output of the Gm amplifier 402a is proportional to the difference between its voltage inputs, the current sinking capability will dynamically vary as the difference between the two voltages changes over time. Accordingly, as the difference diminishes, the output current sinking also diminishes.

[0039] Thus, in both power up and power down periods, the Gm amplifier 402a automatically determines the amount of source or sink current needed to charge or discharge its associated capacitive node independently of the impedance of its associated reference voltage source or the capacitance of its associated decoupling capacitor.

[0040] In the illustrated example, a second, output-side Gm amplifier 402b has a first input (positive, in this example) coupled to V.sub.REF2 and a second input (negative, in this example) coupled to V.sub.LDO1. The output of the Gm amplifier 402b is coupled to the conductor that connects the output of LDO1 to the decoupling capacitor C.sub.LDO1. Accordingly, in response to the difference between V.sub.LDO1 and V.sub.REF2, the Gm amplifier 402b dynamically supplies a proportional auxiliary source current to charge the decoupling capacitor C.sub.LDO1 during a power up period, and a proportional auxiliary current sink to discharge the decoupling capacitor C.sub.LDO1 during a power down period.

[0041] Similarly, in the illustrated example, a third Gm amplifier 402c has a first input (positive, in this example) coupled to V.sub.REF2 and a second input (negative, in this example) coupled to V.sub.LDO2. The output of the Gm amplifier 402c is coupled to the conductor that connects the output of LDO2 to the decoupling capacitor C.sub.LDO2. Accordingly, in response to the difference between V.sub.LDO2 and V.sub.REF2, the Gm amplifier 402c dynamically supplies a proportional auxiliary source current to charge the decoupling capacitor C.sub.LDO2 during a power up period, and a proportional auxiliary current sink to discharge the decoupling capacitor C.sub.LDO1 during a power down period.

[0042] A Gm amplifier may be coupled to every node of a multi-stage LDO regulator that needs assistance during power up and/or power down periods to achieve a desired charge level within a specified time. In some embodiments, if the time for powering down is not a critical specification, then Gm amplifiers need not be added to the output side of one or more constituent LDOs. In some embodiments, if the time for powering down is not a critical specification, then the Gm amplifiers on the control input side of one or more constituent LDOs may be replaced by simpler differential voltage amplifiers. In some embodiments, one or more Gm amplifiers generally would be disabled in “normal” operation, when the multi-stage LDO regulator is not powering up or down. However, in some embodiments, one or more Gm amplifiers may be enabled in “normal” operation.

[0043] FIG. 6 is a graph 600 showing voltages V.sub.REF2, V.sub.REF1, V.sub.LDO1, and V.sub.LDO2 as a function of time for an example modeled embodiment of the multi-stage LDO regulator 400 of FIG. 4. When enabled, the primary or “lead” voltage reference V.sub.REF2 at capacitor C.sub.REF2 in Stage 2 ramps up sharply (˜0.02 sec) from zero volts to a maximum value (˜2.4V in this example), which defines a “charge” period starting at dotted line 602. All of the other voltages V.sub.REF1, V.sub.LDO1, and V.sub.LDO2 closely follow the lead voltage reference V.sub.REF2. Thus, the secondary voltage reference V.sub.REF1 at capacitor C.sub.REF1 in Stage 1 ramps up sharply from zero volts to near its maximum value (˜1.3V in this example) within less than ˜0.03 sec. As a result, the output voltage at capacitor V.sub.LDO1 in Stage 1 quickly ramps up from zero volts to near its maximum value (˜2.8V in this example) in about the same time (less than ˜0.03 sec.), and the output voltage at capacitor V.sub.LDO2 in Stage 2 quickly ramps up from zero volts to near its maximum value (˜2.4V in this example) in about the same time (less than ˜0.03 sec.).

[0044] Similarly, during a “discharge” period starting at dotted line 604, the primary voltage reference V.sub.REF2 at capacitor C.sub.REF2 in Stage 2 decays much more rapidly than the discharge times shown at dotted line 304 in FIG. 3 for the prior art circuit of FIG. 2, and the rates of decay of the voltages V.sub.REF1, V.sub.LDO1, and V.sub.LDO2 closely fall in synchronism with the primary voltage reference V.sub.REF2.

[0045] In a modeled embodiment of the multi-stage LDO regulator 400 of FIG. 4, the graph lines shown in FIG. 6 were essentially the same over a wide range of capacitor values for the capacitor C.sub.REF1 (e.g., from about 10 nF to about 10 μF), indicating that the rate of charge and discharge of the capacitor C.sub.REF1 was independent of its capacitive value and essentially solely determined by the sink/source current provided by or through the Gm amplifier 402a. Similarly, the graph lines shown in FIG. 6 were essentially the same over a wide range of capacitor values for the capacitors C.sub.LDO1 and C.sub.LDO2 (e.g., from about 10 nF to about 10 μF), indicating that the rate of charge and discharge of the capacitors C.sub.LDO1 and C.sub.LDO2 was independent of its capacitive value and essentially solely determined by the sink/source current provided by or through the Gm amplifier 402c.

[0046] The use of Gm amplifiers as described above can be extended to multi-stage LDO regulators having more than two stages. For example, FIG. 7 is a block diagram of a second embodiment of a fast power up/down LDO regulator 700 having n stages, where n≥2. Stages 1 to n−1 in FIG. 7 are essentially the same as Stage 1 in FIG. 4, except that only Stage 1 in FIG. 7 is coupled to V.sub.IN; each intermediate stage 2 through n−1 is powered by a previous LDO stage. The output LDO stage, Stage n, is essentially the same as Stage 2 in FIG. 4 and is powered by stage n−1.

[0047] In the example shown in FIG. 7, a Gm amplifier is coupled to every capacitor that needs assistance during power up and/or power down periods to achieve a desired charge level within a specified time. In some alternative embodiments, if the time for powering down is not a critical specification, then Gm amplifiers need not be added to the output side of one or more constituent LDO stages to charge or discharge the corresponding output-side decoupling capacitors C.sub.LDOn. In some alternative embodiments, if the time for powering down is not a critical specification, then the Gm amplifiers on the control input side of one or more constituent LDO stages may be removed or may be replaced by simpler differential voltage amplifiers.

[0048] The use of a Gm amplifier as described above can be beneficially applied to single stage LDO regulators. For example, an LDO regulator comprising only Stage 2 of FIG. 4 may benefit from adding a Gm amplifier 402c to the output side in order to improve power down discharge times (i.e., faster discharging of the decoupling capacitor C.sub.LDO2).

[0049] Embodiments of the present invention beneficially utilize the proportional current sourcing and sinking capabilities of Gm amplifiers to charge or discharge certain capacitive nodes of a multi-stage LDO regulator in synchronism with a primary reference voltage source. By using Gm amplifiers to compare relative voltages between the primary reference voltage source and a capacitive node, the power up and power down times of the whole system can be made to be dependent only on the start-up time of the primary reference voltage source, regardless of the capacitor values and independent of the output impedance of any reference voltage source. Furthermore, by synchronizing power up and power down of all devices within a multi-stage LDO regulator by use of Gm amplifiers as described above, the relative voltage relationships of such devices are maintained at all times, thus allowing the use of low voltage devices rather than high voltage devices (needed if the relative voltage relationships of such devices can be excessive).

[0050] System Aspects

[0051] Referring back to FIG. 1, it should be appreciated that multi-stage LDO regulator circuits in accordance with the present invention provide improved performance for products that include a switched-mode power supply 102 for providing clean power to noise-sensitive circuitry 110, particularly RF circuitry. In particular, because some or all of the input and/or output capacitors of such a multi-stage LDO are actively charged and/or discharged to achieve a desired charge level within a specified time independently of the value of such capacitors (and independently of the output impedance of the reference voltage sources), such a multi-stage LDO regulator circuit provides fast power up and power down times. Accordingly, such a multi-stage LDO regulator better meets the needs of downstream powered circuitry, which may be specified by a product developer as expecting power within a set amount of time after the LDO regulator is enabled, and as expecting power to be absent (off) a set amount of time after the LDO regulator is disabled.

[0052] Methods

[0053] Another aspect of the invention includes methods for providing fast power up and/or power down times in a multi-stage LDO regulator circuit. For example, FIG. 8 is a process flow chart 800 showing one method for providing fast power up and/or power down times in a multi-stage LDO regulator circuit. The method includes, for each stage of the LDO regulator circuit for which fast power up and/or power down times are desired, coupling at least one transconductance amplifier to compare a primary reference voltage to one of a secondary reference voltage for the stage or an output voltage of the stage, and coupling and configuring the at least one transconductance amplifier to charge and/or discharge an associated capacitor to achieve a desired charge level within a specified time (which is essentially independent of the value of the associated capacitor) [Block 802].

[0054] Additional aspects of the above method may include configuring the at least one transconductance amplifier of each stage to charge and/or discharge the associated capacitor in synchronism with the primary reference voltage.

[0055] Fabrication Technologies & Options

[0056] The term “MOSFET”, as used in this disclosure, includes any field effect transistor (FET) having an insulated gate whose voltage determines the conductivity of the transistor, and encompasses insulated gates having a metal or metal-like, insulator, and/or semiconductor structure. The terms “metal” or “metal-like” include at least one electrically conductive material (such as aluminum, copper, or other metal, or highly doped polysilicon, graphene, or other electrical conductor), “insulator” includes at least one insulating material (such as silicon oxide or other dielectric material), and “semiconductor” includes at least one semiconductor material.

[0057] As used in this disclosure, the term “radio frequency” (RF) refers to a rate of oscillation in the range of about 3 kHz to about 300 GHz. This term also includes the frequencies used in wireless communication systems. An RF frequency may be the frequency of an electromagnetic wave or of an alternating voltage or current in a circuit.

[0058] Various embodiments of the invention can be implemented to meet a wide variety of specifications. Unless otherwise noted above, selection of suitable component values is a matter of design choice. Various embodiments of the invention may be implemented in any suitable integrated circuit (IC) technology (including but not limited to MOSFET structures), or in hybrid or discrete circuit forms. Integrated circuit embodiments may be fabricated using any suitable substrates and processes, including but not limited to standard bulk silicon, silicon-on-insulator (SOI), and silicon-on-sapphire (SOS). Unless otherwise noted above, embodiments of the invention may be implemented in other transistor technologies such as bipolar, BiCMOS, LDMOS, BCD, GaAs HBT, GaN HEMT, GaAs pHEMT, and MESFET technologies. However, embodiments of the invention are particularly useful when fabricated using an SOI or SOS based process, or when fabricated with processes having similar characteristics. Fabrication in CMOS using SOI or SOS processes enables circuits with low power consumption, the ability to withstand high power signals during operation due to FET stacking, good linearity, and high frequency operation (i.e., radio frequencies up to and exceeding 50 GHz). Monolithic IC implementation is particularly useful since parasitic capacitances generally can be kept low (or at a minimum, kept uniform across all units, permitting them to be compensated) by careful design.

[0059] Voltage levels may be adjusted, and/or voltage and/or logic signal polarities reversed, depending on a particular specification and/or implementing technology (e.g., NMOS, PMOS, or CMOS, and enhancement mode or depletion mode transistor devices). Component voltage, current, and power handling capabilities may be adapted as needed, for example, by adjusting device sizes, serially “stacking” components (particularly FETs) to withstand greater voltages, and/or using multiple components in parallel to handle greater currents. Additional circuit components may be added to enhance the capabilities of the disclosed circuits and/or to provide additional functionality without significantly altering the functionality of the disclosed circuits.

[0060] Circuits and devices in accordance with the present invention may be used alone or in combination with other components, circuits, and devices. Embodiments of the present invention may be fabricated as integrated circuits (ICs), which may be encased in IC packages and/or in modules for ease of handling, manufacture, and/or improved performance. In particular, IC embodiments of this invention are often used in modules in which one or more of such ICs are combined with other circuit blocks (e.g., filters, amplifiers, passive components, and possibly additional ICs) into one package. The ICs and/or modules are then typically combined with other components, often on a printed circuit board, to form an end product such as a cellular telephone, laptop computer, or electronic tablet, or to form a higher level module which may be used in a wide variety of products, such as vehicles, test equipment, medical devices, etc. Through various configurations of modules and assemblies, such ICs typically enable a mode of communication, often wireless communication.

CONCLUSION

[0061] A number of embodiments of the invention have been described. It is to be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, some of the steps described above may be order independent, and thus can be performed in an order different from that described. Further, some of the steps described above may be optional. Various activities described with respect to the methods identified above can be executed in repetitive, serial, and/or parallel fashion.

[0062] It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the following claims, and that other embodiments are within the scope of the claims. In particular, the scope of the invention includes any and all feasible combinations of one or more of the processes, machines, manufactures, or compositions of matter set forth in the claims below. (Note that the parenthetical labels for claim elements are for ease of referring to such elements, and do not in themselves indicate a particular required ordering or enumeration of elements; further, such labels may be reused in dependent claims as references to additional elements without being regarded as starting a conflicting labeling sequence).