Load pole stabilized voltage regulator

Abstract: A voltage regulator with load pole stabilization is disclosed. An error amplifier has a non-inverting input receiving a reference voltage and an inverting input receiving a feedback voltage from the output of the voltage regulator. A gain stage has an input connected to the output of the error amplifier and an output connected to a pass transistor that provides current to a load. A variable impedance device such as a FET transistor configured as a variable resistor is connected between the input and output of the gain stage to provide variable zero to cancel the varying pole when the output current drawn by the load fluctuates. Consequently, the disclosed voltage regulator has high stability without a significant increase in power dissipation.

Description: The present invention relates to electronic circuits used as voltage regulators and more specifically to circuits and methods for stabilizing a voltage regulator. The problem addressed by this invention is encountered in voltage regulation circuits. Voltage regulators are inherently medium to high gain circuits, typically greater than 50db, with low bandwidth. With this high gain and low bandwidth, stability is often achieved by setting a dominate pole with a load capacitor. However, achieving stability over a wide range of load currents with a low value load capacitor (∼0.1uF) is difficult because the load pole formed by the load capacitor and load resistor can vary by more than three decades of frequency and be as high as tens of kHz requiring the circuit to have a very broad bandwidth of greater than 3 MHz. These broad bandwidth circuits, however, are incompatible with the power IC fabrication process used to manufacture voltage regulators. A prior art solution to the stabilization problem is illustrated in Figure 1. The voltage regulator 2 in Figure 1 converts an unregulated V CC voltage, 12 volts in this example, into a regulated voltage V REG , 5 volts in this example. An amplifier 6, resistor 22, and capacitor 12 are configured as an integrator amplifier to set the dominant pole of the system. Resistor 10 is added to provide a zero to cancel the pole of the load (load pole). The integrator amplifier drives a pass transistor 8 that provides current to the load. A feedback network including resistors 14 and 16 form a voltage divider circuit which is used to scale the output voltage such that the output voltage can be fed back to the inverting input of an error amplifier 4. The resistor 18 and capacitor 20 are not part of the voltage regulator 2 but rather are the schematic representation of the typical load on the voltage regulator circuit. In this prior art example, the zero associated with the voltage regulator 2 can be calculated as: f zero = 1 2Π RC where R = resistance of the resistor 10 and C = capacitance of the capacitor 12; and the pole associated with the pull down resistors and load can be calculated as: f pole = 1 2Π R L C L where R L = resistance of the load = R14 and R16 in parallel with R18. C L = is the capacitance of C20 which is typically around .1 microfarad. As can be seen from the above equation, the pole associated with the prior art circuit is load ( R L ) dependent and can vary from 16 Hz to 32 kHz for an R14 + R16 equal to 100 kilo-ohms and R18 ranging from 50 ohms to 1 mega-ohm. As will be appreciated by persons skilled in the art, the wide variation of the pole frequency is difficult to stabilize and may result in uncontrollable oscillation of the voltage regulator. A prior art solution to this problem is to change the pull down resistors R14 + R16 from 500 kilo-ohms to around 500 ohms which changes the pole frequency to a range of 3.2 kHz to 32 kHz, which is a frequency spread of 1 decade instead of 3 decades. However, the power dissipated by the pull down resistor R18 increases, as shown below: power = (12v-5v) (I load + I pull down ) = (7v) (100mA) + (7v) (10mA) Consequently, the 500 ohm resistor adds 70 milli-watts of power dissipation in the chip which is approximately a 10% increase in power dissipation for the added stability. Therefore, it is desirable to provide a voltage regulator with load pole stabilization without significantly increasing power dissipation. The present invention provides this and other advantages as will be illustrated by the following description and accompanying figures. EP-A-0 766 164, which is prior art to this application only for the purposes of novelty under Article 54 and 56 of the European Patent Convention, discloses a voltage regulator with load pole stabilization including an error amplifier, an integrator which includes a switched capacitor, a pass transistor and a feedback circuit. The switched capacitor is controlled by a voltage controlled oscillator. US-A-4 349 775 discloses a temperature compensated voltage regulator for a photovoltaic battery charging system. The voltage regulator comprises an operational amplifier which includes a linearly temperature dependent current source coupled to one of its input terminals. According to the present invention there is provided a voltage regulator, having an associated zero in its frequency response, comprising: an error amplifier having a first input for receiving a reference voltage, a second input, and an output; a gain stage having an output and an input connected to the output of the error amplifier; a compensation capacitor connected to the gain stage; an output stage having an input connected to the output of the gain stage; and having an output; and a feedback path connected between the second input of the error amplifier and the output of the output stage, characterised in that said voltage regulator further comprises: a transistor connected in series between the input and output of the gain stage and having a control input; a sensing circuit having an input connected to the output of the gain stage and an output connected to the control input of said transistor, said sensing circuit sensing the voltage at the output of the gain stage and varying the voltage at the control input of said transistor in proportion to said sensed voltage, wherein said transistor is a variable impedance device operable together with said compensation capacitor to vary the frequency of the associated zero of the voltage regulator response as the output current of the voltage regulator varies. According to the present invention there is also provided a method for stabilizing a regulating voltage from a voltage regulator having an associated load pole in its frequency response, said voltage regulator including a gain stage having an output and an input and a compensation capacitor, characterised in that the method comprises the steps of: sensing a voltage at the output of said gain stage; generating a signal that varies in proportion to said sensed voltage; and driving a control input of a transistor connected in series between the input and output of said gain stage, said transistor being a variable impedance device, with the generated signal to vary the impedance of the transistor, whereby the frequency of the associated zero of the voltage regulator response varies as a function of the load current of the voltage regulator to cancel the load pole of the voltage regulator. When the output current drawn by the load fluctuates according to the load condition thereby varying the load pole, the FET transistor varies the zero of the voltage regulator to cancel the varying load pole. Consequently, the voltage regulator according to the present invention has high stability without a significant increase in power dissipation. Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a voltage regulator according to the prior art. Figure 2 is a schematic diagram of a voltage regulator according to the present invention. Figure 3 is a detailed schematic diagram of the load pole stabilized voltage regulator of Figure 2 according to the present invention. A load pole stabilized voltage regulator 3 according to the principles of the present invention is illustrated in Figure 2. The load pole stabilized voltage regulator 3 is similar to the regulator 2 of Figure 1 except that the resistor 10 is replaced with a variable impedance device 7 having an input 9 connected to the output of the gain amplifier 6. In operation, when the output current drawn by the load fluctuates according to the load condition, the load pole frequency also varies. However, the variable impedance device 7 varies the zero of the voltage regulator in a corresponding manner to cancel the varying load pole. For example, when the current drawn by the load increases, the pole frequency also increases and the regulator 3 becomes unstable. The increased load current causes the amplifier 6 to decrease its output voltage and thereby allows more current to pass through the pass transistor 8. In turn, the variable impedance device 7 receiving the decreased voltage through the input 9 decreases its resistance. The decreased resistance of the variable impedance device 7 increases the zero of the regulator 7 to cancel the increasing load pole frequency as will be explained in greater detail with reference to Figure 3. It is important to note, however, that while the compensation capacitor and variable impedance device 7 are shown as being connected between the input and output of the amplifier 6, the capacitor and variable impedance device can be connected anywhere in the voltage regulator so long as it provides frequency compensation ( e . g ., compensated to ground or pole splitting). For example, while the input 9 of the variable impedance device 7 is shown as being indirectly connected to the output of the regulator 3, the input 7 can also be directly connected to the output of the regulator. Also, while the regulator 3 as shown in Figure 2 includes both the error amplifier 4 and the gain stage 6, persons of ordinary skill in the art will appreciate that the regulator can be designed with only the error amplifier 4 without the gain stage 6. For example, the output of the error amplifier 4 can be connected directly to the input of the output stage 8 and the resistor 10 and the compensation capacitor 12 can be connected between the output of the error amplifier 4 and the inserting input of the error amplifier 4. Illustrated in Figure 3 is a voltage regulator 30 according to the present invention. An output 32 of the voltage regulator 30 provides output current to a load 34 which is represented as a resistor 36 and a capacitor 38 connected in parallel with each other. A feedback network 40 connected between the output 32 and ground is shown as a voltage divider including series connected resistors 42 and 44 and outputting a divided voltage. In the embodiment shown, the resistance ratio between the resistors 42 and 44 is 4:1. Thus, in a steady load condition the divided output voltage is approximately 1 volt assuming a regulating voltage V REG of 5 volts. The output of the feedback network 40 is connected to an inverting input 48 of an error amplifier 46 through a feedback path 50. A non-inverting input 52 of the error amplifier 46 is connected to a reference voltage V REF , 1.25 volts in this example. The non-inverting and inverting inputs 52, 48 are respectively connected to the bases of a pair of differentially connected pnp transistors 54, 56. The emitters of the pnp transistors 54, 56 are connected to a current source 58 and the collectors are connected to a current mirror circuit comprising a pair of npn transistors 60. 62. Accordingly, the current flowing through the npn transistor 60 is mirrored to the npn transistor 62. The output 64 of the error amplifier 46 is connected to an input 66 of a gain stage 67. The gain stage 67 includes a cascade connected pnp transistors 68, 72 and a resistor 70 connected between the base of the npn transistor 72 and ground. The gain stage 67 is a negative gain amplifier where the higher input voltage results in lower output voltage at an output 74. The output 74 of the gain stage 67 is connected to an input of an output stage 76. In the embodiment shown, the output stage 76 is implemented as a pass element such as a PMOS transistor 78 having a source connected to a supply voltage V CC and a gate connected to the output 74 of the gain stage 67. The drain of the PMOS transistor 78 is connected to the feedback network 40 and the output 32 of the voltage regulator 30. An operation of the voltage regulator 30 will now be explained with an example where the load 34 starts to draw more current from the output 32. The increased current draw by the load 34 lowers the current flowing through the feedback network 40 and its output voltage decreases. The decreased output voltage from the feedback network 40 is fed back to the inverting input 48 of the error amplifier 46 through the feedback path 50. In response, the pnp transistor 56 turns on harder and conducts more current. The extra current provided by the transistor 56 flows through the output 64. Because the constant current flowing through the transistor 60 is mirrored to the transistor 62, the npn transistor 68 of the gain stage 67 receives the extra current through its input 66. Consequently, the transistor 68 draws more current and the voltage drop across the resistor 70 increases. The increase in voltage at the base of the transistor 72 pulls down the voltage at the output 74 of the gain stage 67. Thus, the gain stage 67 is a negative gain amplifier where the increases in the input voltage results in decreases in the output voltage. The pass transistor 78 receives the lower voltage from the gain stage output 74 at its gate and allows more current to pass through, thereby increasing the voltage at the output 32. The voltage at the output 32 increases until it reaches the regulating voltage V REG . To achieve stability in the voltage regulator 30, a variable impedance device such as a PMOS FET transistor R eff and a compensation capacitor C comp are connected in series between the output 74 and the input 66 of the gain stage 67. The compensation capacitor C comp , together with the PMOS transistor R eff , which is configured as a variable resistor, vary the zero of the voltage regulator to track the varying pole of the load as will be explained below. A sensing circuit 80 includes a PMOS transistor 82 having its gate connected to the output 74 of the gain stage 67 and its source connected to the supply voltage V CC . The drain of the PMOS transistor 82 is connected to a current mirror comprised of two npn transistors 84, 86 having their emitters connected to ground. The collector of the transistor 86 receives current from a current source 88 and is connected to the gate input of the FET transistor R eff . The sensing circuit 80 senses the voltage at the output 74 of the gain stage 67 and varies the gate to source voltage of the FET transistor R eff and thereby changing the resistance across the source and drain of the FET transistor R eff . Specifically, the PMOS transistor 82 senses the voltage being applied to its gate and varies the current being provided to the transistors 84, 86. The size ratio of the transistors 78 and 82 as shown is approximately 100:1 so that the transistor 82 dissipates very little power. The transistor 84 mirrors the current flowing therethrough to the npn transistor 86 and the voltage at the gate of the FET transistor R eff is inversely proportional to the load current drawn by the load 34. In the example given above where the current drawn by the load 34 increases, the load resistance represented by the resistor 36 decreases. Since the pole frequency is inversely proportional to the load resistance, the load pole frequency increases and as a result, the voltage regulator becomes unstable. To stabilize the regulator, the gain stage 67 together with the sensing circuit 80 increases the gate to source voltage V GS of the FET transistor R eff . The FET transistor R eff is configured as a variable resistor whose resistance is inversely proportional to the gate to source voltage V GS minus the threshold voltage V T . Thus, the resistance across the drain and source of the FET transistor R eff decreases. The decreased resistance of the FET transistor R eff increases the zero of the voltage regulator 30 to track the increasing pole frequency of the load 34 when more current is demanded by the load 34. Conversely, when the current drawn by the load 34 decreases, the load pole frequency decreases and the zero of the voltage regulator 30 decreases to cancel the decreasing pole frequency of the load 34. Thus, the voltage regulator according to the present invention has high stability without a significant increase in power dissipation. While the word "connected" is used throughout the specification for clarity, it is intended to have the same meaning as "coupled." Accordingly, "connected" should be interpreted as meaning either a direct connection or an indirect connection. For example, the gate input of the FET transistor R eff is coupled or indirectly connected to the output 32 through the sensing circuit 80 and the PMOS transistor 78.

Claims: A voltage regulator, having an associated zero in its frequency response, comprising: an error amplifier (4;46) having a first input for receiving a reference voltage (Vref), a second input, and an output; a gain stage (6;67) having an output and an input connected to the output of the error amplifier; a compensation capacitor (12,Ccomp) connected to the gain stage; an output stage (8;76) having an input connected to the output of the gain stage; and having an output; and a feedback path (3;50) connected between the second input of the error amplifier and the output of the output stage, characterised in that said voltage regulator further comprises: a transistor (7;Reff) connected in series between the input and output of the gain stage (6;7) and having a control input; a sensing circuit (80) having an input connected to the output of the gain stage (6;67) and an output connected to the control input of said transistor (7;Reff), said sensing circuit sensing the voltage at the output of the gain stage (6;67) and varying the voltage at the control input of said transistor in proportion to said sensed voltage, wherein said transistor is a variable impedance device operable together with said compensation capacitor to vary the frequency of the associated zero of the voltage regulator response as the output current of the voltage regulator varies. The voltage regulator of claim 1 wherein the sensing circuit (80) comprises: a sensing transistor (82) connected to the output of the gain stage (6;67) and a current mirror (84,86) connected to the sensing transistor (82) and the input of the variable impedance device (7;Reff). The voltage regulator of claim 1 or 2, wherein the variable impedance device (7;Reff) and the compensation capacitor (12;Ccomp) are connected in series. The voltage regulator of any preceding claim, further comprising a feedback network (40) connected between the output stage and the feedback path. The voltage regulator of claim 4 wherein the feedback network (40) includes a voltage divider (42,44). The voltage regulator of any preceding claim wherein said error amplifier (4;46) generates an error signal indicative of a difference between said reference voltage (Vref) and said output of the output stage. The voltage regulator of claims 2 to 6 wherein said sensing transistor (82) senses the current drawn from the voltage regulator. The voltage regulator of any preceding claim wherein the transistor (7;Reff), the compensation capacitor (12,Ccomp) and the gain stage (6;67) together form an integrator amplifier. The voltage regulator of any preceding claim, wherein the transistor is an FET. A method for stabilizing a regulating voltage from a voltage regulator having an associated load pole in its frequency response, said voltage regulator including a gain stage having an output and an input and a compensation capacitor, characterised in that the method comprises the steps of: sensing a voltage at the output of said gain stage; generating a signal that varies in proportion to said sensed voltage; and driving a control input of a transistor connected in series between the input and output of said gain stage, said transistor being a variable impedance device, with the generated signal to vary the impedance of the transistor, whereby the frequency of the associated zero of the voltage regulator response varies as a function of the load current of the voltage regulator to cancel the load pole of the voltage regulator. The method according to claim 10, wherein the said transistor is an FET.

Inventors: EDWARDS WILLIAM E, EDWARDS, WILLIAM E.
Applicants: ST MICROELECTRONICS INC, STMICROELECTRONICS, INC.
Application year: 1998
Main classification: G05F1
IPC codes: G05F1, G05F3, H03F3