Low drop-out voltage regulator

<u>FIELD OF THE INVENTION</u>

0001The present invention relates to a low drop-out voltage regulator and in particular to a low drop-out voltage regulator having a fast response time.

<u>BACKGROUND OF THE INVENTION</u>

0002Low drop-out (LDO) voltage regulators are used when a steady voltage level is required that is lower than the supply voltage level. It is necessary for such regulators to be able to provide a steady voltage level at the same time as providing the required current to a load.

0003A P-channel MOS transistor (PMOS) is generally used in LDO voltage regulators as the pass device connected between the supply voltage and the load connected to the output of the LDO circuit. This PMOS is then controlled by control circuitry to perform the role of providing the required voltage level, for whatever current is required by the load.

0004Depending on the type of load, the current required by the load may vary. A problem occurs in some known LDO circuits when the load current is required to vary rapidly. This is because the PMOS pass device is generally a relatively slow device, having a slow response to changes in the control signal provided at its gate terminal. This slow response results in the output voltage of the LDO circuit fluctuating, which is undesirable as this generates noise, and causes problems at high frequencies.

0005In order to minimize the voltage fluctuations at the output of known LDO voltage regulators, an output capacitor is often provided. However, the output capacitor is required to be relatively large in order to adequately minimize voltage fluctuations, for example in the range of 0,5 µF to 10 µF depending on the scale of current variations. The necessity to provide such a large capacitor is disadvantageous as an additional discrete component is required that adds to the cost of manufacturing the device.

0006<patcit id="pcit0001" dnum="US6333623B"><text>US Patent 6,333,623</text></patcit> discloses a low drop-out voltage regulator having an output stage with a pass device and a discharge device. The pass device and the discharge device are both controlled through a single feed back loop.

0007European Patent Application <patcit id="pcit0002" dnum="EP1365302A"><text>1,365,302</text></patcit> discloses a low drop-out regulator configured to provide a high output current with a fast response.

<u>SUMMARY OF THE INVENTION</u>

0008Embodiments of the present invention aim to at least partially address some of the above-mentioned problems.

0009According to a first aspect of the present invention, there is provided a low drop-out DC voltage regulator for regulating a voltage from a DC supply comprising: a pass device controllable to maintain a required voltage at an output of the regulator and arranged to provide a first current from the DC supply, at least part of said first current being provided to a load connected to the output of the regulator; and current regulating means connected to said pass device and to the output of the regulator, said current regulating means arranged to conduct a second current controlled such that the first current through said pass device remains constant irrespective of variations in a load current to said load.

0010According to one embodiment of the present invention, resistance means are provided connected to the pass device and arranged to receive at least part of the first current, the current regulating means being controlled based on a voltage drop across the resistance means.

0011According to a further aspect of the present invention, there is provided a method of regulating a voltage at the output of a low drop-out DC voltage regulator comprising: controlling a pass device to maintain a required voltage at the output of the regulator, the pass device providing a first current from the DC supply, at least part of the first current being provided to a load connected to the output of the regulator; and controlling a current regulating means connected to said pass device to conduct a second current controlled such that the first current through said pass device remains constant irrespective of a load current to said load.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

0012The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figures 1, 2</figref> and <figref idref="f0002">3</figref> illustrate LDO circuits according to first, second and third embodiments of the present invention respectively.</li></ul>

<u>DETAILED DESCRIPTION OF THE INVENTION</u>

0013<figref idref="f0001">Figure 1</figref> illustrates a first embodiment of a low drop-out (LDO) voltage regulating circuit 100. LDO circuit 100 comprises a P-channel MOS transistor (PMOS) 102 having its source terminal connected to an input voltage V_IN on line 104 and its drain terminal connected to a first terminal of a shunt resistor R_SHUNT. The second terminal of the shunt resistor is connected to the output line 106 of the LDO circuit 100. A pass current I_PASS flows through PMOS 102 and through the shunt resistor. The output voltage V_OUT of the LDO circuit on line 106 in this first embodiment is equal to V_IN minus the voltage between the drain and source of PMOS 102, minus the voltage drop across the shunt resistor.

0014A comparator 108 provides a control signal to the gate terminal of PMOS 102. Comparator 108 receives a feedback voltage V_f. Two resistors R1 and R2 are connected in series between the output line 106 and a ground node. A node 109 between resistors R1 and R2 provides the feedback voltage V_f. A reference voltage V_REF is also provided to comparator 108 on line 110, this voltage indicating the required output voltage V_OUT. V_REF could be a fixed voltage if the same output voltage is always required, or could be variable to allow the output voltage V_OUT of the LDO circuit 100 to be varied during use.

0015V_REF and V_f are provided to the gate terminals of transistors 112, 114 respectively of comparator 108. Transistors 112, 114 are N-channel MOS transistors having their source terminals connected to ground via a current source 119. Drain terminals of transistors 112, 114 are connected to respective drain terminals of further transistors 116, 118. Transistors 116, 118 are P-channel MOS transistors having their source terminals connected to line 104. The gates of transistors 116, 118 are connected together and to a node between the drain terminals of transistors 114, 118. The gate terminal of PMOS 102 is connected to the node between the drain terminals of transistors 112, 116.

0016According to this first embodiment, an N-channel MOS transistor (NMOS) 120 is connected between the output line 106 and ground that conducts a current I_A. The drain terminal of NMOS 120 is connected to the output line 106 and the source terminal of NMOS 120 is connected to ground. A comparator 121 comprises four transistors 122, 124, 126, 128, for providing a control voltage to the gate terminal of NMOS 120. Comparator 121 compares the voltage drop across the shunt resistor R_SHUNT with a reference voltage V_A and varies the control signal to NMOS 120 such that the voltage across the shunt resistor is relatively constant, and equal to V_A. A voltage source 130 providing voltage V_A is connected between the first terminal of the shunt resistor and the gate terminal of transistor 122. The gate terminal of transistor 124 is connected to the output line 106, and thus to the second terminal of the shunt resistor. Transistors 122, 124 are P-channel MOS transistors having their source terminals connected together and to a common current source 132, and their drain terminals connected to the drain terminals of transistors 126, 128 respectively. Transistors 126, 128 are N-channel MOS transistors having their source terminals connected together and to a ground node. Furthermore, the gate terminals of transistors 126, 128 are connected together and to the node between the drain terminals of transistors 124, 128. The node 129 between the drain terminals of transistors 122, 126 is connected to the gate terminal of NMOS 120.

0017In operation, comparator 108 provides a control signal to the gate terminal of PMOS 102 controlling PMOS 102 such that the feedback voltage V_f equals the reference voltage V_REF, resulting in the required output voltage V_OUT. At the same time, comparator 121 provides a control signal to the gate terminal of NMOS 120 such that the voltage drop across R_SHUNT is equal to V_A' thus ensuring that the current through R_SHUNT and thus also through PMOS 102, remains relatively constant. When the load current changes rapidly, for example in a step from 2 mA to 10 mA, the voltage across R_SHUNT will suddenly increase above V_A. This will in turn cause transistor 124 of comparator 121 to conduct more than transistor 122, causing the voltage at the drain terminals of transistors 122, 126 to decrease and thus providing a lower voltage at the gate terminal of NMOS 120. The current I_A through NMOS 120 will thus drop, and more of the pass current I_PASS through PMOS 102 will be provided to the load at the output line 106. This effect will continue until the required load current has been satisfied, and the voltage across the shunt resistor has returned to V_A. NMOS 120 being a relatively fast device compared to PMOS 102, an increase in load current can therefore be compensated much more quickly than if PMOS 102 alone responded. Likewise, a rapid reduction in load current will result in an increased voltage V_OUT at the output of the LDO circuit, which can be quickly compensated by control of NMOS 120 such that more current I_A is conducted to ground.

0018According to the embodiment of <figref idref="f0001">Figure 1</figref>, NMOS 120 is arranged to conduct a current I_A to ground thus reducing the current I_PASS such that the output current I_OUT matches the required load current. Thus I_PASS is preferably at least as high as the highest load current required by the load, and the value of R_SHUNT and V_A are preferably selected to provide I_PASS accordingly. For example, if the highest load current required is 20 mA, a resistance value of 5 ohms could be chosen for R_SHUNT' and V_A could be chosen to be 0.1 V to maintain the pass current at 20 mA. The value of R_SHUNT is preferably chosen to be relatively low, for example less than 10 ohms, to prevent a large voltage drop, as the voltage drop across this resistor combined with the source-drain voltage across PMOS 102 together define the minimum voltage drop achievable by the LDO circuit 100.

0019<figref idref="f0001">Figure 2</figref> illustrates an alternative embodiment of an LDO circuit 200. A large proportion of the circuitry of LDO circuit 200 is the same as the circuitry of LDO circuit 100 of <figref idref="f0001">Figure 1</figref>, and the common parts have been labelled with the same reference numerals and will not be described again in detail. In LDO circuit 200, NMOS 120 is replaced by a current control block 220 comprising a pair of transistors PMOS 220a and NMOS 220b, and a class AB control block 220c. The drain terminals of transistors 220a, 220b are connected together and to the output line 106. The source terminal of PMOS 220a is connected to V_IN on line 104. The source terminal of NMOS 220b is connected to ground. The gate terminals of transistors 220a, 220b are connected to respective output lines of the class AB control block 220c. Class AB control block 220 also comprises an input line connected to node 129 between the drain terminals of transistors 122, 126, and thus receives an input voltage signal from comparator 121.

0020The voltage source 130 of <figref idref="f0001">Figure 1</figref> is replaced in the circuit of <figref idref="f0001">Figure 2</figref> by a voltage source 230 providing a voltage V_B between the gate of transistor 122 and the first terminal of the shunt resistor.

0021Operation of LDO circuit 200 of <figref idref="f0001">Figure 2</figref> is similar to that of LDO circuit 100, except that current control block 220 allows current to be either routed from the output line 106 to ground, or provided to output line 106 from the supply line 104. Thus whereas in the circuit of <figref idref="f0001">Figure 1</figref> current I_A always flows from the output line 106 through NMOS 120 to ground, in the circuit of <figref idref="f0001">Figure 2</figref> current I_A can either flow from output line 106 through NMOS 220b to ground, or from the supply line 104 through PMOS 220a to output line 106, and in particular to the load.

0022Comparators 108, 121 function in the same way as described in relation to <figref idref="f0001">Figure 1</figref>, except that voltage V_B provided by the voltage source 230 is lower than V_A of the LDO circuit 100, and preferably results in a current through the shunt resistor, and therefore also through PMOS 102, that is half way between the highest and lowest load currents required by the load. For example, if the maximum load current required is 50 mA, and the minimum is 10 mA, the pass current is preferably maintained at approximately 30 mA. If R_SHUNT is for example chosen to be 5 ohms, V_B is preferably therefore selected to be 0,15 V. In alternative embodiments however, V_B could also be selected to be at a different value, depending on how the LDO circuit is to be loaded.

0023Class AB control block 220c comprises circuitry for generating the appropriate control signals for driving transistors 220a and 220b based on the voltage at node 129. Type class AB circuits are generally well known, and variations in their design and operation are possible. In the present case, class AB control block 220 is preferably arranged to control both PMOS 220a and NMOS 220b with voltage signals that follow changes in the voltage at node 129, in other words such that when the voltage at node 129 increases, the voltage provided to the gate of PMOS 220a and/or NMOS 220b increases, and when the voltage at node 129 decreases, the voltage at the gate of PMOS 220a and/or NMOS 220b decreases. The particular voltage levels provided to the gate terminals of PMOS 220a and NMOS 220b will depend on the particular characteristics of each device, and the supply voltage V_IN on line 104. In one example, the voltage V_Gb at the gate of NMOS 220b is equal to the voltage V_c at node 129, and the voltage V_Ga at the gate of PMOS 220a is as follows: <maths id="math0001"><math display="block"><msub><mi mathvariant="normal">V</mi><mi>Ga</mi></msub><mo>=</mo><msub><mi mathvariant="normal">V</mi><mi mathvariant="normal">c</mi></msub><mo>+</mo><msub><mi mathvariant="normal">V</mi><mi>IN</mi></msub><mo>-</mo><mn>2</mn><mo>⁢</mo><msub><mi mathvariant="normal">V</mi><mi mathvariant="normal">T</mi></msub><mo>,</mo></math><img file="EP1865397B1_D0001.tif" /></maths> where V_c is the voltage at node 129, and V_T is the absolute value of the threshold voltage of PMOS 220a and NMOS 220b. Preferably both PMOS 220a and NMOS 220b do not conduct at the same time, as this would imply that current is flowing from supply line 104 through NMOS 220a and PMOS 220b straight to ground.

0024LDO circuit 200 is advantageous in that the current through PMOS 102 does not need to be maintained at a high level, but can instead be maintained at a lower level, thus reducing the power consumption of the circuit. The circuit still includes an NMOS transistor for regulating the current, providing a fast response to changes in the output voltage V_OUT. In particular, if the load current is increased from a value of I_A below I_PASS, to a value above I_PASS' the output current I_OUT can be quickly increased to I_PASS by the control of NMOS 220b, which will stop conducting an thus prevent I_A conducting to ground. The increase from I_PASS to the required current level is provided by PMOS 220a, which is controlled at the same time to conduct current from supply line 104. If, on the other hand, the output current is to be rapidly reduced, this can be achieved quickly by control of NMOS 220b, which will quickly increase the current I_A routed to ground.

0025<figref idref="f0002">Figure 3</figref> illustrates an alternative embodiment of an LDO circuit 300. LDO circuit 300 comprises many of the same circuit elements as LDO circuit 100 of <figref idref="f0001">Figure 1</figref>, and the common parts have been labelled with the same reference numerals and will not be described again in detail. As shown in <figref idref="f0002">Figure 3</figref>, PMOS 102 is replaced by PMOS transistors 302a and 302b, each connected in the same way as PMOS 102, with their source terminals connected to supply line 104, and their gate terminals connected to the node between the drain terminals of transistors 116 and 112. PMOS 302a is a larger device than PMOS 302b, and thus conducts more current. In the present example, PMOS 302a is approximately 50 times larger than PMOS 302b, such that I_PASSa through PMOS 302a is approximately 50 times greater than I_PASSb though PMOS 302b. The drain terminal of PMOS 302a is connected directly to the output line 106, whereas the drain terminal of PMOS 302b is connected to the first terminal of the shunt resistor R_SHUNT. The second terminal of R_SHUNT is connected to output line 106. In this way, the current through R_SHUNT is approximately 50 times less than the total pass current I_PASS' which is equal to I_PASSa + I_PASSb. The shunt resistor R_SHUNT of <figref idref="f0002">Figure 3</figref> can thus have a resistance approximately 50 times larger than the shunt resistor R_SHUNT of <figref idref="f0001">Figure 1</figref>, for the same voltage drop across this resistor. Alternatively, R_SHUNT of <figref idref="f0002">Figure 3</figref> could have the same resistance as R_SHUNT of <figref idref="f0001">Figure 1</figref>, and would thus cause a much lower voltage drop. In alternative embodiments, different ratios between the PMOS pass devices 302a, 302b could be chosen.

0026As with LDO circuit 100 of <figref idref="f0001">Figure 1</figref>, NMOS 120 in <figref idref="f0002">Figure 3</figref> is controlled by regulating the voltage drop across R_SHUNT, however an alternative comparator circuit 321 is provided in place of comparator 121. Comparator 321 comprises resistors R3 and R4 with their first terminals connected to the first and second terminals of R_SHUNT respectively. These resistors preferably have relatively high resistance values such that current through these resistors is kept low. The second terminal of R3 is connected to the source terminals of transistors 322, 324. Transistors 322, 324 are P-channel MOS transistors having their gate terminals connected together. The second terminal of R4 is connected to the source terminals of transistors 326, 328. Transistors 326, 328 are P-channel MOS transistors having their gate terminals connected together. The drain terminal of transistor 322 is connected to the drain terminal of an N-channel MOS transistor 330. The gate terminal of transistor 330 is connected to its drain terminal, and its source terminal is connected to ground. The drain terminal of transistor 324 is connected to its gate terminal and to a current source 332. Likewise, the drain terminal of transistor 326 is connected to its gate terminal and to the current source 332. The drain terminal of transistor 328 is connected to the drain terminal of a further NMOS transistor 334, which has its gate terminal connected to the gate terminal of transistor 330, and its source terminal connected to ground. The gate terminal of NMOS 120 is connected to the drain terminals of transistors 334 and 328.

0027In operation, comparator 321 of <figref idref="f0002">Figure 3</figref> operates in a similar fashion to comparator 121 of <figref idref="f0001">Figure 1</figref>, in that a relatively constant voltage is maintained across the shunt resistor R_SHUNT However, comparator 321 comprises resistors R3 and R4 of different values to provide the required voltage different across the shunt resistor, rather than a voltage source 130. For example, in one embodiment R3 is equal to approximately 2500 ohms and R4 is equal to approximately 250 ohms. If, for example, the output current I_OUT increases, the current I_PASS will also increase, causing an increase in the voltage across the shunt resistor R_SHUNT. In consequence, the current through transistors 326 and 328 will decrease, and the current through transistors 322 and 324 will increase. This causes the voltage at the gate of transistor 120 to drop, thus reducing the current I_A. This reduces the increase in current I_PASS, in other words keeping I_PASS constant.

0028An advantage with comparator 321 of <figref idref="f0002">Figure 3</figref> is that no part of this comparator needs to be connected to a supply source that is higher than the voltage V_IN at the supply line 104.

0029Thus LDO circuitry has been described having a pass device controlled to control the voltage at the output of the LDO circuit, and a current regulating device for regulating the current through the pass device such that the current remains relatively constant. By providing a pass device that is used to control the voltage at the output of the device, and a separate current regulating means, an improved response time can be achieved. Preferably the current regulating means comprises a transistor that has a relatively fast response time when compared to the pass device. For example, the current regulating means comprises an n-channel MOS transistor or an NPN bipolar junction transistor.

0030Embodiments of LDO voltage regulators as described herein can for example be implemented in integrated circuit boards and used in a wide range of devices in which a rapid LDO regulating circuit is required.

0031Advantageously according to embodiments of the present invention a PMOS transistor is used as the pass device. A PMOS device can be controlled at its gate terminal with a voltage that is lower than the voltage at its source terminal (connected to the supply voltage), and therefore small voltage drops can be provided by the LDO voltage regulator with no extra circuitry being required to achieve a gate voltage that is higher than the supply voltage.

0032The current regulating device is preferably controlled based on maintaining the voltage drop across a resistor connected between the pass device and the output of the regulator. In certain embodiments, the pass device comprises a plurality of PMOS transistors connected in parallel, one of these PMOS transistors connected directly to the output of said LDO circuit and arranged to receive a comparatively large proportion of the pass current, and the other connected to the resistor. The resistor thus receives a relatively smaller portion of the pass current, and will cause a smaller voltage drop at the output of the LDO circuit.

0033Whilst a number of specific embodiments of LDO circuits have been described, it will be apparent that there are various modifications that could be applied. In particular, in alternative embodiments, the features described above in relation to any of the embodiments could be combined in any combination.

0034Examples have been described in which the pass device and current regulating means comprise MOS transistors, for example MOSFETs. The principles of the present invention apply equally to bipolar junction transistors as they do to MOS transistors, and in particular an NPN bipolar junction transistor has a faster response time than a PNP bipolar junction transistor. In alternative embodiments, one or more PMOS, NMOS or alternative transistors such as NPN or PNP bipolar junction transistors could be used as the pass device 102, 302a, 302b, or the current regulating device 120, 220a, 220b. Furthermore, in the embodiments of <figref idref="f0001">Figures 1, 2</figref> and <figref idref="f0002">3</figref>, some or all of the NMOS transistors could be replaced by NPN bipolar transistors, and some or all of the PMOS transistors could be replaced by PNP bipolar transistors. Whilst not shown in the figures, in some embodiments one or more small capacitors could be provided at the output of the LDO circuit for providing further voltage fluctuation compensation. Alternative comparator circuits could also be used.

0035In some embodiments the voltage sources 130, 230 of <figref idref="f0001">Figures 1 and 2</figref> and the resistance values of resistors R3 and R4 of <figref idref="f0002">Figure 3</figref> are variable such that the pass current I_PASS can be varied during use of the LDO circuit.

0036Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.