Patent Application
20150015222
Low dropout voltage regulators (LDOs) use a pass transistor that conducts current from a source to a load. The amount of current and, thus, the voltage of the output, is controlled by the gate voltage of the pass transistor. The output voltage of the voltage regulator is fed back to an error amplifier that compares the output voltage to a reference voltage. The difference between the two voltages is used to generate the gate voltage of the pass transistor. Therefore, if the output voltage is too low, the error amplifier generates a gate voltage that causes the pass transistor to conduct more current, which increases the output voltage. Likewise, if the output voltage is too high, the error amplifier generates a voltage that causes the pass transistor to pass less current, which lowers the output voltage.
Low dropout voltage regulators are used as voltage supplies in some radio circuits. The voltage supplies in the radios typically require low voltage supply noise at frequencies above 10-20 kHz. Higher frequency noise may interfere with the performance of the radios. In order to conserve power, the circuits in the radios draw very little current, which increases noise. In order to overcome noise, conventional voltage regulators used in radio circuits may have a large capacitance connected to the output of the LDO. However, many radio circuits use a plurality of different power domains, which requires the use of many large capacitors. These capacitors are typically external to the radio circuits and add costs and size to the radios.
Another method of reducing noise is by running large currents through the error amplifier. For example, currents in the range of one half to two milliamps may be used. The noise is inversely proportional to the square of the current in the input stage of the error amplifier, so a higher current results in lower noise. However, the high current has many drawbacks, especially when the radio is a battery-powered device. The most notable drawback is that the higher current reduces the battery life of the battery-powered radio.
Voltage regulators are disclosed herein. An embodiment of a voltage regulator includes a MOS-type pass transistor, wherein a channel of the pass transistor is connectable to a voltage source and wherein a second channel of the pass transistor is connected to the output of the voltage regulator. The voltage regulator also includes an error amplifier having a reference input and an output, the output being connected to the gate of the pass transistor, and the reference input being connected to a reference voltage source.
Low dropout voltage regulators and methods of regulating voltage are disclosed herein. The low dropout voltage regulators are sometimes referred to simply as LDOs or regulators. The LDOs disclosed herein use a very slow error amplifier so the closed-loop LDO bandwidth is below a predetermined frequency. For example, the closed-loop LDO bandwidth may be below 10-20 kHz, which can be achieved by using very small biasing current (10-20 nA) in the input stage as described below. The output noise at frequencies outside of the closed-loop bandwidth is defined by the noise of the pass transistor which is inversely proportional to the square root of the load current. Therefore, a greater load current results in lower noise at the frequencies of interest.
In conventional applications, a low closed-loop bandwidth in the LDO causes long settling times of the output voltage when the load is switched. The LDOs disclosed herein eliminate the loading problem by using class AB input stages in the error amplifier. At steady state, when the load is constant, the error is small and current flow through the error amplifier is small. However during load transients the current flow may rise, which increases the closed-loop bandwidth and ensures very fast settling of the output voltage.
Having briefly described the LDOs, different embodiments will now be described in greater detail. Reference is made to
The regulator 100 includes a pass transistor QPASS, which may be a field effect transistor (FET), connected or coupled between the input 102 and the output 104. The pass transistor QPASS may operate in open collector or open drain mode, which enables it to operate in the saturation mode or close to the saturation mode. The drain and source may be referred to generically as channels. In the saturation mode, the voltage drop across the pass transistor QPASS between the input 102 and the output 104 is very small, which enables the regulator 100 to operate efficiently. In some embodiments, the pass transistor QPASS is a bipolar junction transistor. In other embodiments, the pass transistor QPASS may be an NMOS-type device or a PMOS-type device.
A voltage divider 108 provides feedback of the output voltage VO. In the embodiment of
The voltage reference VR is the replica of or is proportional to the replica of the required output voltage VO at the output 104. As the load on the output 104 changes, the output voltage VO may not be equal to the reference voltage VR. The regulator 100 resolves this problem so that a predetermined and regulated output voltage Vo is output at the output 104. More specifically, the regulator 100 functions to make the output voltage Vo equal to the required output voltage, which is equal to or proportional to the reference voltage VR. The output of the error amplifier 110 is connected to or coupled to the gate or base of the pass transistor QPASS. The voltage output by the error amplifier 110 regulates the current flow through the pass transistor QPASS, which is used to maintain the output voltage VO.
A capacitor CO may be connected to the output 104. The capacitor CO attenuates noise and/or ripple on the output 104. In some embodiments, the regulator 100 is able to attenuate noise and ripple at the output by way of the voltage regulation, so the capacitor CO is not required. A resistance RL is representative of the load connected to the regulator 100. As items are connected to and disconnected from the regulator 100, the value of the load RL changes accordingly. As described above, the regulator 100 is fast enough to maintain a constant output voltage VO as the load RL on the output 104 changes.
Having described the components of the regulator 100, its operation will now be described. The input voltage VIN is present at the input 102, which is connected to the pass transistor QPASS. The pass transistor QPASS enables current to flow to the output 104 based on the gate or base voltage, which is the voltage output by the error amplifier 110 or is a voltage that is proportional to the voltage output by the error amplifier 110. The output voltage VO is measured by way of the voltage divider 108 and input to the error amplifier 110. Accordingly, the output voltage VO or a voltage proportional to the output voltage VO is compared to the reference voltage VR. If the output voltage VO is too low, the error amplifier 110 causes the pass transistor QPASS to output more current, which increases the output voltage VO. Likewise, if the output voltage VO is too high, the error amplifier 110 causes the pass transistor QPASS to reduce the current flow, which reduces the output voltage VO.
An embodiment of a voltage regulator 200 is shown in
The output of the error amplifier 210 is the drain of the transistor Q2, which is connected to the gate of the pass transistor QPASS. As with the regulator 100 of
The regulator 200 operates in a manner that is similar to the regulator 100 of
Another embodiment of an error amplifier 300 is shown in
The second side 304 is the same or substantially the same as the first side 304. The output voltage VO is connected to the gate of the transistor M6. A voltage source I2 is connected between the drain of the transistor M8 and ground. The two sides 302 and 304 are connected at the sources of the transistors M2 and M5. The drains of the transistors M5 and M6 are connected to the drain of the transistor Q4 and the gate of the pass transistor QPASS.
The transistors M5-M8 form a negative feedback loop that controls the minimum current flow through the transistor M5. When the load increases, which can be when the resistance RL decreases, the output voltage VO drops. The voltage drop causes the source voltage and the gate voltage of the transistor M8 to drop accordingly. With the gate voltage of the transistor M8 connected to the gate of the transistor M7, the gate to source voltage on the transistor M7 increases, which increases the current through the transistor M7. It follows that the tail current through the transistors M2 and M5 increases. It can be seen that the sum of the gate to source voltages of the transistors M5 and M7 is equal to the sum of the gate to source voltages of the transistors M6 and M8. The correlation can also be shown by the current where the product of the currents through the transistors M1 and M5 is equal to the current IO2/4, where IO is the current through the current source I2. The same operation applies to the first side 302 of the error amplifier 300. The increased current in the error amplifier 300 increases the closed-loop bandwidth so that the error amplifier 300 can quickly correct the output voltage VO. During steady state operation, the current is low, so the bandwidth is low, which attenuates unwanted noise in the output voltage VO.
Another alternative embodiment of an error amplifier 400 is shown in
In order to optimize the performance of the error amplifier 400, the transistors M9, M10, and M14 may be matched. In addition, the resistor R1 may have a value that is equal to the value of the resistor R2. The resistors R1 and R2 may have a value of half the resistance value of the resistor RO. In such embodiments, the current passing through the transistors M11 and M16 is equal to half the value of the current flowing through the current source I3. When the output voltage VO decreases, the current through the transistor M16 decreases due to a drop in the gate to source voltage. This causes the current through the transistor M14 to increase. The result is that the current passing through the transistor M13 increases in order to maintain it at a value of half the current of the current source I4. The error amplifier 400 has a higher gain than the error amplifier 300 and better regulation of the minimum current. However, it does require the inclusion of resistors and matched transistors.
The regulators described above have been described with various transistors, such as N-type and P-type transistors. Those skilled in the art may switch the transistors to achieve the same results. In addition, other components may be added to the regulators described herein. The other components may include various regulators and biasing circuits as are well-known in the art.
While illustrative and presently preferred embodiments of the invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.
