Abstract:
Apparatus and method to provide high line regulation, which can be combined with a low dropout voltage, for linear regulators. The control for the regulators is referenced to ground rather than a relatively noisy power supply terminal so that the control is substantially free of power supply noise. The pass transistor forms the second transistor of a current mirror mirroring the current from the control. Referencing the control to ground and mirroring the control current to the pass transistor makes the output of the regulator substantially independent of the power supply noise. Various embodiments are disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to regulator circuits, and more specifically, to linear regulator circuits. 
     2. Background Information 
     There are many circuit applications that require linear regulators that are minimally degraded by noise on the power supply. Examples of these applications include high quality signal processing, signal conditioning, laptop/notebook computers, cell phones and portable headphone speaker drivers. These applications require analog circuits that exist on the same printed circuit board (PCB) as other high speed/high power circuitry and, often, very noisy switch-mode power supplies. These high speed/high power circuits tend to corrupt the power supply voltage, resulting in poor analog circuit performance. In a typical low dropout regulator design, the power supply noise appears directly at the gate or base of the pass device, since the pass device is a P-channel or PNP device whose gate or base bias is referred to the unregulated supply voltage. Such a prior art circuit using a p-channel pass transistor is shown in FIG.  1 . It may be seen therein that in the first instance, the noise on the power supply Vdd appears as a gate-source voltage noise, requiring the error amplifier to respond for any control of the effect of this noise. Hence the overall line regulation is determined by the gain of the error amplifier, which may be as low as 40 dB in order to maintain system stability. Therefore there is a need to have a regulator that not only has a low dropout voltage, but also good overall line regulation. 
     SUMMARY OF THE INVENTION 
     Apparatus and methods to provide high line regulation, which can be combined with a low dropout voltage, for linear regulators. The control for the regulators is referenced to ground rather than a relatively noisy power supply terminal so that the control is substantially free of power supply noise. The pass transistor forms the second transistor of a current mirror mirroring the current from the control. Referencing the control to ground and mirroring the control current to the pass transistor makes the output of the regulator substantially independent of the power supply noise. Furthermore, the current mirror can incorporate a bias control circuit that substantially eliminates power supply induced error currents from the current mirror itself, thus further improving the line regulation. Various embodiments are disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent from the following detailed description of the present invention in which: 
     FIG. 1 is a diagram illustrating a typical prior art linear regulator. 
     FIG. 2 is a diagram of a first embodiment of the present invention 
     FIG. 3 is a diagram of a further embodiment of the present invention. 
     FIG. 4 is a diagram of a still further embodiment of the present invention. 
     FIG. 5 is a diagram of a still further embodiment of the present invention. 
     FIG. 6 is a diagram of a still further embodiment of the present invention. 
     FIG. 7 is a diagram of a still further embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Now referring to FIG. 2, a diagram of a first embodiment of the present invention low dropout out linear regulator may be seen. As shown therein, instead of using a pass transistor directly controlled by the error amplifier, the error amplifier instead controls, in this case, npn transistor Q 1 . The npn transistor Q 1  in turn controls current to a current mirror formed by the interconnection of transistors P 2  and P 1 , which mirror the collector current of transistor Q 1  from transistor P 2  to transistor P 1  as required to supply current to the load and feedback resistors R 1  and R 2 . Alternatively, the current mirror may be, by way of example, a PNP transistor pair with a common base. In either case, in practice the pass transistor P 1  to which the current is mirrored would be dimensioned so as to give some substantial current gain with respect to the current in transistor P 2  to achieve high efficiency. 
     The net effect of the circuit of FIG. 2 is that the current through the pass device P 1  is primarily dependent upon the base-emitter voltage on transistor Q 1 , which is referenced to ground, not the noisy power supply Vdd. Similarly, the mirroring device P 2  of the current mirror will establish its gate-source voltage or base-emitter voltage as required to conduct the current of transistor Q 1 , with that gate-source voltage or base-emitter voltage being directly coupled to the current mirror device P 1  to which the current is mirrored. Consequently, the gate voltages or the base voltages of the two devices of the current mirror will generally track the noise on Vdd, with the current mirror accurately mirroring the current of transistor Q 1  to the output circuit. 
     Thus in essence the control of transistor Q 1  is referenced to ground rather than Vdd. Very little of the power supply noise on Vdd is passed to the output Vout by the operation of the current mirror, all without depending upon the gain and response of the control loop which includes the error amplifier. While the diode connected transistor of the current mirror will in essence couple the noise on Vdd to the collector of transistor Q 1 , the resulting Early effect variation in the transconductance of transistor Q 1  with noise on Vdd will be quite small in comparison to the effect of the noise on Vdd on the pass device of the prior art circuit of FIG.  1 . 
     Now referring to FIG. 3, a further improvement in line regulation can be gained by removing the effect of the pass device output impedance on the overall line regulation. Referring to FIG. 2, the gate-drain connection of device P 2  implies that, in response to a stimulus on the supply, the drain voltage of device P 2  follows the stimulus, whereas the drain voltage of the pass device P 1  remains fixed at Vout. This imbalance results in a net error current being mirrored to the output due to the finite output impedance of the pass device P 1 . This problem can be substantially eliminated by ensuring that the drain voltage of the mirror device P 2  is biased to a ground-referred potential REF 2 . In this way, the current mirror devices see the same bias conditions, independent of the supply voltage. FIGS. 4,  5 ,  6  and  7  show more detailed embodiments incorporating this aspect of the invention 
     Now referring to FIG. 4, a circuit diagram for one more detailed embodiment of the present invention may be seen. The circuit shown uses a first ground-referenced reference voltage REF and a second ground-referenced reference voltage REF 2 . The reference voltages may be generated by any of the well known methods for generating reference voltages. The reference voltages usually will be reference voltages independent of temperature, such as by way of example, may be generated by using a band gap reference voltage generator. Alternatively, one or both of the reference voltages may be provided from other circuits on the same chip, or from other circuits off chip. By way of example, the reference voltage REF controls the output voltage VOUT, and might be provided by some external source to provide a means of controlling the regulator responsive to the reference voltage input. 
     Assume for the moment that the output voltage VOUT has dropped slightly out of regulation, such as may occur with a sudden slight increase in the load on the regulator. This will cause the voltage on node A to drop, causing an increase in voltage at node B and increased base voltage and collector current in transistor Q 1 . The increased collector current in transistor Q 1  will flow through devices P 2  and P 3 , and the common gate voltage of devices P 1  and P 2  will decrease as required to allow device P 2  to conduct the increased current. This ensures the drain current in device P 1  increases to mirror this current. Hence the output VOUT rises as required. 
     FIG. 5 shows a further embodiment of the invention, whereby a PMOS device P 4 , configured as a source follower and biased with a current source I 1 , has been inserted between the drain of device P 3  and the common gate connection of devices P 1  and P 2 , thus providing a DC voltage level shift from the common gate of devices P 1  and P 2  to the drain of device P 3 . This configuration provides a greater tolerance to variation in the bias conditions of devices P 2  and P 3  with respect to REF 2 , such as might be experienced during normal operation of the circuit under extremes of current drawn by the load. In particular, the power supply input voltage Vdd might typically be required to vary by ±+10% under normal operation, whereas REF 2 , by design, remains fixed. Thus, device P 4  has the effect of maintaining devices P 2  and P 3  in their saturation regions despite such a variation in the power supply input voltage with respect to REF 2 . 
     Now referring to FIG. 6, a further embodiment of the invention can be seen. As can be seen in FIG. 6, transistor P 3  is diode connected, so the current through transistor P 3  will be mirrored to transistor P 2 , and in a greater magnitude to transistor P 1 , the pass transistor of the regulator. The current in transistor P 2  in turn will equal the current in transistor Q 1 , as any difference between the current in transistor Q 1  and the current mirrored through transistor P 2  will be sensed by the equally sized, differential transistor pair Q 2  and Q 3 . This will readjust the portion of the tail current I 1  passing through transistor Q 3  so as to force the current through transistor Q 3  as mirrored through transistor P 2  to equal the current in transistor Q 1 . By way of example, going back to the previous example, if the output voltage VOUT drops slightly, the base voltage of transistor Q 1  will increase, increasing the collector current through the transistor. The increase in current through transistor Q 1  above that momentarily flowing through transistor in P 2  will decrease the voltage on the base of transistor Q 2  relative to the reference voltage REF 2  on the base of transistor Q 3 . This increases the portion of the tail current I 1  that flows through transistor Q 3 , with this current then being mirrored back to transistor P 2 , thus providing the required feedback action to raise the collector voltage of transistor Q 1 . In this way, the voltage on the base of transistor Q 2  will always be substantially equal to the voltage on the base of transistor Q 3 , namely the voltage REF 2 . This has the net effect of clamping the collector voltage of transistor Q 1  substantially at REF 2 , rather than being subject to the noise on the power supply Vdd, further reducing the coupling by the power supply noise through the output VOUT on the regulator. Note that the rejection of the noise on Vdd from the output VOUT, as in FIG. 2, does not depend on the feedback of the voltage on node A to the error amplifier for the regulator. Errors in the circuit of FIG. 6 are best balanced if the voltage REF 2  is nominally equal to the voltage VOUT. 
     Now referring to FIG. 7, a further embodiment of the invention is illustrated, whereby the fixed tail current source I 1  has been replaced by a transistor with its base connected to the base of transistor Q 1  and having double the emitter area of transistor Q 1 . This refinement provides a means whereby the collector currents of transistors Q 2  and Q 3  can substantially track the collector current in Q 1 , and hence can provide improved compliance between the base voltage of transistor Q 2  and the base voltage of transistor Q 3  at the extremes of the load current of the regulator. 
     The foregoing illustrates exemplary embodiments of the present invention. However while certain specific embodiments of the present invention have been disclosed and described in detail herein, it will be obvious to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.