Abstract:
In an LDO regulator, two feedback loops are created. The first feedback loop includes a high power PNP bipolar power transistor connected in series between the input voltage Vin terminal and the output voltage Vout terminal. The first feedback loop includes a first error amplifier that controls a drive transistor to drive the base of the power transistor such that Vout matches a set voltage Vset. This first feedback loop circuitry uses an operating voltage (the upper rail voltage) that is regulated by a second feedback loop and is approximately 300 mV greater than Vout. As a result, the control circuitry will be powered by a low ripple supply to improve output PSRR. Further, the power transistor is connected such that any noise in the input voltage is a common mode voltage across the base-emitter of the transistor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional application Ser. No. 62/012,661, by Amitkumar P. Patel et al., filed Jun. 16, 2014, assigned to the present assignee and incorporated by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to low dropout (LDO) regulators, also known as linear regulators, and, in particular, to a technique for operating the LDO regulator components from the regulated voltage for improving the PSRR of the LDO. 
       BACKGROUND 
       [0003]    Good Power Supply Rejection Ratio (PSRR) is an important performance metric for modern high performance linear regulators. While switched-mode DC-to-DC converters are usually preferred for their higher efficiencies, they have a relatively high output voltage ripple which makes them unsuitable for powering sensitive analog/RF circuitries, such as ADCs, DACs, PLLs, VCOs, etc. 
         [0004]    An LDO regulator does not have any switching ripple so is commonly used for such analog circuits. An LDO regulator may receive as an input voltage a power supply voltage or an output of a switching voltage regulator whose output is slightly greater than the target voltage output from the LDO regulator. 
         [0005]    As the switching frequencies of modem DC-DC converters increase to reduce the component sizes, to reduce the ripple peak-to-peak voltage, and to improve the transient performance, it is becoming increasingly difficult for LDO regulators to suppress the resulting higher frequency ripple since the high frequency may be beyond the bandwidth of the LDO regulator. While increasing the LDO regulator&#39;s bandwidth may improve its high frequency PSRR, it is rather difficult to achieve such a high bandwidth while maintaining loop stability over a wide range of operating conditions and output capacitor types, especially when a large power transistor is used in the LDO regulator. 
         [0006]    Low dropout PNP bipolar transistor regulators offer a number of benefits which are difficult to replicate with NPN, NMOS, or PMOS transistor based linear regulators, such as reverse input protection, reverse current protection, reverse output protection, low dropout in single supply operation, and low minimum input voltage Vin. 
         [0007]    To improve the PSRR in an LDO regulator, various techniques have been used in the prior art. 
         [0008]      FIG. 1  illustrates cascoding the PNP bipolar power transistor  12  with a cascode transistor  14 , biased by a voltage source  16 , to increase the impedance between the input and output terminals of the LDO regulator. This improves PSRR but at the cost of higher dropout voltage and larger die area, as it requires two power transistors. An error amplifier  18  is used in a feedback loop to adjust the conductivity of the power transistor  12  so that the output voltage Vout matches a set voltage Vset, typically set by the user. A load is typically connected between Vout and ground. 
         [0009]      FIG. 2  illustrates another approach using an NPN/NMOS based power transistor  20  to achieve better PSRR because its input impedance is inherently higher, its output impedance is inherently lower, and it has lower DC loop gain (thereby making it easier to achieve a higher bandwidth). But it also requires a higher dropout voltage or another supply rail to drive the NPN transistor&#39;s base current. 
         [0010]      FIG. 3  illustrates stacking two LDO regulators (LDO 1  and LDO 2 ) in series, and independently controlling them, to improve PSRR. But this brute force approach requires an additional capacitor, doubles the circuit area, and increases the dropout voltage. 
         [0011]    What is needed is an LDO regulator design that has very low dropout voltage, has a high bandwidth, has good PSRR, and achieves the improvement with a minimum of additional circuitry. 
       SUMMARY 
       [0012]    In an embodiment of the improved LDO regulator, two feedback loops are created. The first feedback loop includes a high power PNP bipolar power transistor connected in series between the input voltage Vin terminal and the output voltage Vout terminal. The first feedback loop includes a first error amplifier that controls a drive transistor to drive the base of the power transistor such that Vout matches a set voltage Vset. This first feedback loop circuitry uses an operating voltage (the upper rail voltage) that is regulated by a second feedback loop and is approximately 300 mV greater than Vout. Therefore, the upper rail voltage for the first feedback loop tracks the regulated Vout. Both the first error amplifier and a reference current source used to create Vset are powered by the regulated upper rail voltage. As a result, there will be very little noise, stemming from power supply fluctuations, contributed by the control circuitry at the output of the power transistor. 
         [0013]    The second feedback loop includes a second error amplifier that is directly powered by the regulated Vout, so as to keep the second feedback loop low noise. Vout is also applied to a non-inverting input terminal of the second error amplifier. The second feedback loop generates the regulated Vout plus 300 mV for use as the upper rail voltage for the first feedback loop. A first transistor in the second feedback loop supplies a known current through a resistor to subtract 300 mV from the upper rail voltage, and the resulting signal is applied to the inverting input of the second error amplifier. The second feedback loop operates to match the inputs into the second error amplifier. Other voltages can be generated instead of the Vout plus 300 mV, depending on the required parameters of the system. 
         [0014]    Further, the power transistor and the first transistor are each connected such that any noise in the input voltage is a common mode voltage across the base-emitter of the transistors so does not significantly affect the current conducted by the transistors. 
         [0015]    Since the control circuitry for the PNP bipolar power transistor is virtually noiseless, and there is only common mode injection of Vin noise across the base-emitter of the power transistor, the PSRR is improved while a low dropout voltage is maintained by the power transistor being connected across the input voltage and output voltage terminals. 
         [0016]    In one embodiment, the input voltage is provided by an unregulated power supply. In another embodiment, for higher efficiency, the input voltage is provided by a switching voltage regulator, and the LDO regulator removes the ripple as well as changes the level of the switching regulator&#39;s output voltage. This allows the switching regulator to have a much smaller output (smoothing) capacitor. 
         [0017]    Other embodiments are described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  illustrates a first prior art LDO regulator. 
           [0019]      FIG. 2  illustrates a second prior art LDO regulator. 
           [0020]      FIG. 3  illustrates a third prior art LDO regulator. 
           [0021]      FIG. 4  illustrates an LDO regulator in accordance with an embodiment of the invention. 
       
    
    
       [0022]    Elements in the various figures that are the same or equivalent are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0023]      FIG. 4  illustrates an LDO regulator (or linear regulator) in accordance with the present invention. 
         [0024]    In the LDO regulator  20  of  FIG. 4 , an error amplifier  21  (a transconductance amplifier) has its input power terminals coupled to a regulated (noiseless) upper rail voltage, equal to Vout+300 mV, and to a lower rail ground voltage. Thus, the upper rail voltage tracks the regulated Vout. In contrast, the conventional approach is to directly power all circuitry by the noisy input power supply. As a result, there will be very little noise, stemming from power supply fluctuations, contributed by the control circuitry of the LDO regulator. 
         [0025]    The regulated upper rail voltage of Vout+300 mV is generated by the feedback circuit on the right side of  FIG. 4 . The regulated output voltage Vout of the LDO regulator  20  is coupled to the non-inverting input of the error amplifier  22  (a transconductance amplifier). The output of the error amplifier  22  biases a capacitor  24  depending on the relative levels of the error amplifier&#39;s  22  input voltages. The resulting voltage is applied to the base of an NPN bipolar transistor  26 . 
         [0026]    The emitter of the transistor  26  is connected to ground. The collector of the transistor  26  is connected to the emitter of a PNP bipolar transistor  28  via a diode  30  and a resistor  32 . The emitter of the transistor  28  is connected to the input voltage Vin. In the example of  FIG. 4 , because Vin is the output of a switching converter (used for high efficiency), it will inherently contain ripple at the switching frequency and require post-regulation by a high PSRR LDO regulator. The LDO regulator must thus set the level of Vout, while also removing the relatively high frequency ripple. For the highest efficiency, Vin should only be slightly above the target Vout. 
         [0027]    The diode  30  and resistor  32  drop a voltage determined by the current through the transistor  26 . The collector of the transistor  26  is also directly connected to the base of the transistor  28 . The current though the transistor  26  thus determines the base-emitter forward biasing voltage of the transistor  28  and, as a result, the transistor&#39;s  28  collector current. 
         [0028]    Since transistor  28  and diode  30  are powered from Vin, and given that any noise on Vin is a common mode voltage across the base-emitter of transistor  28 , the output voltage tracking rail (the upper rail voltage) is fairly low noise. 
         [0029]    The collector of the PNP bipolar transistor  28  is connected to a 30K ohm resistor  34  in series between the collector and the inverting terminal of the error amplifier  22 . A 10 uA current source  36  is connected between the inverting terminal and ground to drop 300 mV across the resistor  34 . Other values of the resistor  34  and current source  36  can be used, depending on the operating voltage requirements of the error amplifier  21 . Generally, it is desired to make the upper rail voltage slightly higher than the minimum operating voltage of the error amplifier  21 , given the particular input voltages into the error amplifier  21 . 
         [0030]    The feedback loop tries to match the inputs into the amplifier  22 . Therefore, the collector of the transistor  28  will be at a regulated voltage of about Vout+300 mV, and this voltage serves as the upper rail voltage of the error amplifier  21 . 
         [0031]    The Vout+300 mV upper rail voltage also supplies a substantially ripple-free operating voltage for a current source  38  (a reference current source), which draws a known fixed current through a set resistor  40  connected to ground. The value Rset of the set resistor  40  is selected, such as by the user, so that the voltage drop across the resistor  40  is equal to the desired output voltage Vout of the LDO regulator  20 . This set voltage Vset is applied to the non-inverting terminal of the error amplifier  21 . 
         [0032]    The output of the conventional switching voltage regulator  39 , preferably supplying a regulated Vin close to the desired Vout, is directly applied to the emitter of a PNP bipolar power transistor  42  so there can be a very small voltage drop between Vin and Vout. The power transistor  42  may be large to supply a high current to a load  44 , connected between the output terminal  45  and ground. 
         [0033]    The output of the switching regulator  32  has ripple at its switching frequency. The switching frequency is typically between 100 KHz-5 MHz. The ripple may be smoothed somewhat by increasing the switching regulator&#39;s filter components&#39; size (not shown). With a high PSRR LDO regulator, a large filter capacitor is not needed since the LDO regulator  20  will reject any input ripple from propagating to the output. 
         [0034]    The switching regulator  39  may be any type of regulator, such as a buck, boost, buck-boost, flyback, etc. In a typical switching regulator, a switching transistor is coupled to an inductor and, for a buck regulator, the transistor intermittently couples one end of the inductor to a power supply voltage. The inductor thus is alternatingly charged and discharged. A synchronous rectifier or diode conducts the discharging current when the power switch is off. The ramping current through the inductor is smoothed by an output capacitor to create the input voltage Vin for the LDO regulator  20 . A feedback path between the output capacitor and an error amplifier in the switching regulator  39  causes the switching duty cycle to be adjusted so that the output voltage (or a divided output voltage) matches a reference voltage applied to the switching regulator&#39;s error amplifier. The switching frequency may be set by an oscillator that resets (turns on) the switching transistor at the beginning of each clock cycle. 
         [0035]    The collector of the power transistor  42  supplies the regulated Vout. Vout is connected to the inverting input terminal of the error amplifier  21 , and a feedback loop matches Vout to Vset. 
         [0036]    The output of the error amplifier  21  biases a capacitor  46 , and the resulting voltage is coupled to the base of an NPN bipolar transistor  48 , which is the driver for the power transistor  42 . The collector of the transistor  48  is connected to the base of the power transistor  42  for controlling the current through the power transistor  42  for generating the desired Vout. 
         [0037]    The emitter of the transistor  48  is connected to ground either directly or through a resistor. The collector of the transistor  48  is connected to the base of the power transistor  42  and a diode  50  through a resistor  52 . The diode  50  and resistor  52  drop a voltage, determined by the current through the transistor  48 , to cause the power transistor  42  base-emitter to become forward biased to control the power transistor&#39;s  42  collector current. Any ripple in the switching regulator  32  output is a common mode voltage across the base-emitter of the power transistor  42  so does not significantly affect the current through the power transistor  42 . 
         [0038]    Preferably, the output of the switching regulator  39  is close to the desired Vout so there will not be a large voltage difference across the power transistor  42 . In other words, the voltage dropout will be very low to achieve high efficiency. The output voltage of the switching regulator  32  is typically set by the user by an external resistance. 
         [0039]    Since the set voltage Vset at the non-inverting input of the error amplifier  21  is constant and has low ripple due to the external bypass capacitor placed on this node, and the error amplifier  21  is operated using a low ripple operating voltage, the feedback loop substantially adjusts the conductivity of the power transistor  42  to effectively filter out the high frequency ripple in Vin from the switching regulator  32 . 
         [0040]    The various transistors may instead be MOSFETs or other types of transistors. The polarities of the transistors may be reversed if the regulator operates with a negative voltage relative to ground. 
         [0041]    An output capacitor  54  is connected to the output terminal  45  of the LDO regulator  20  to further filter high frequency input ripple and stabilize the LDO regulator  20 . All capacitors used in the LDO regulator  20  and switching regulator  32  may be small since the LDO regulator  20  offers high power supply ripple rejection. Therefore, compared to the prior art, the circuit offers better noise rejection without sacrificing dropout voltage and without requiring large capacitors. 
         [0042]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.