Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2000-10-30
2001-12-25
Riley, Shawn (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S281000, C323S224000
Reexamination Certificate
active
06333623
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to low dropout (“LDO”) voltage regulators. More particularly, the present invention relates to improvements in LDO voltage regulators that use a “follower” connected pass element to address the problems of output over-voltage conditions and instability at low output currents.
2. State of the Art
The function of a voltage regulator is to take a varying input voltage supply and generate a stable output voltage. The efficiency of modern power supply systems, particularly battery powered supply systems, is directly related to the amount of power dissipated in the voltage regulator. Minimizing the power consumption is a key parameter in regulator design. The primary method for reducing power consumption is to reduce the voltage drop across the linear regulator. The lowest voltage drop the regulator can tolerate before loss of regulation occurs is called the “dropout voltage” and a low dropout voltage is very desirable. For battery powered systems, power is limited and efficiency is of key importance. Thus, the design of an efficient system that utilizes linear regulation necessarily includes a low dropout (“LDO”) voltage regulator.
As shown in
FIG. 1
, a linear voltage regulator
2
conventionally includes an amplifier
4
which compares the output of a voltage reference
6
to a sample of an output voltage supplied by feedback elements
8
. The output of the amplifier
4
is coupled to a control terminal
10
of a pass element
12
which serves to “pass” current from the unregulated input terminal
14
of the voltage regulator
2
, to the regulated output terminal
16
of the voltage regulator
2
. The feedback control loop
18
formed by the amplifier
4
, pass element
12
and feedback elements
8
acts to force the control terminal
10
of the pass element
12
to a dynamic value that maintains a regulated voltage at the output terminal
16
of the voltage regulator
2
.
The pass element
12
may be used in a common source/emitter configuration or a common drain/collector follower configuration. A voltage follower configuration has the advantages of not requiring a large output capacitance, having a better response time for transient signals, and providing greater immunity to output capacitor characteristics. Greater immunity to output capacitor characteristics is a significant advantage in low power LDO voltage regulators.
In either configuration, however, the pass element
12
functions as a “unipolar” element in conventional designs. A “unipolar” element, as used herein, is one which sources current to the load, but does not sink current from the load. In other words, a unipolar element can supply needed electrical charge to a load, but cannot remove excess electrical charge from the load. A load conventionally includes at least one large output capacitor
20
. A linear voltage regulator
2
configured with a unipolar output stage, however, experiences two common problems: an output over-voltage or “hiccup,” and instability at output current levels below a required minimum output current value.
First, when the load current required at the output terminal
16
of the voltage regulator
2
rapidly changes from a large value (e.g. near a maximum rated output) to a relatively small value (e.g. near zero), more current than is necessary may be supplied to the output terminal
16
until the feedback loop
18
regains control due a finite response time associated with the feedback control loop
18
. The excess charge is stored on the output capacitor
20
and results in an output voltage higher than the desired regulation voltage. The increased voltage at the output terminal
16
causes the feedback control loop
18
to attempt to reduce the output voltage by reducing or stopping the current passing through the pass element
12
. Even with the pass element
12
turned off, however, the output voltage remains high for a time because the feedback control loop
18
cannot remove the excess charge from the output capacitor
20
. As a result, the feedback control loop locks-up, and the output terminal
16
remains in an over-voltage condition until the excess charge drains off of the output capacitor
20
. This transient overvoltage is sometimes called a “hiccup.”
In applications where the load current is small, this discharge process may take a relatively long time. Although the voltage regulator
2
includes a discharge path through the feedback elements
8
, the amount of discharge through the feedback elements
8
is typically insignificant because the feedback elements
8
conventionally comprise large valued resistive elements. While the feedback control loop
18
is locked up and, therefore, unable to regulate, the voltage on the output capacitor
20
may be in a range that is harmful to the load circuitry and, therefore, have serious consequences.
The “hiccup” condition may also be further exacerbated when the excess charge is discharged from the output capacitor
20
and the voltage regulator
2
again begins to pass current through the pass element
12
. As the feedback control loop
18
begins to respond to the need for more charge on the output, the pass element
12
is turned back “ON” to allow current to pass. This rapid change in current may result in another “hiccup” from the pass element
12
again passing too much current before the feedback control loop
18
has time to respond. With each subsequent “hiccup,” the feedback control loop
18
locks up and takes time to recover during which it cannot properly regulate the output voltage. Each subsequent “hiccup” decreases in magnitude until the feedback control loop
18
no longer locks up. In other words, the feedback control loop
18
oscillates between locking-up and being in control of the pass element
12
for a time following an initial “hiccup.”
Second, the stability problem occurs under low or no-load conditions where the only current passing through the pass element
12
is due to the current passing to ground through the feedback elements
8
. As stated previously, because the feedback elements
8
conventionally include large valued resistive elements, this current is very small compared to a current for a load at the output terminal
16
, and is typically below the minimum output current requirements of the pass element
12
. This small current in the relatively large pass element
12
causes low transconductance (g
m
) due to low current density therein, decreases loop gain and increases output impedance, potentially causing an unstable condition. An unstable condition results from the voltage regulator failing to regulate the output voltage which may cause the output voltage to oscillate undesirably until the specified minimum output current again flows through the pass element
12
. This problem is more pronounced with pass elements
12
implemented as “followers” configured as a common drain or a common collector amplifier.
Early linear voltage regulators used a pass clement
12
which was an NPN transistor in an emitter follower configuration. These early voltage regulators did not require LDO characteristics, and conventionally did not have load currents which rapidly transitioned between high and low values during periods where tight output voltage regulation was required. Thus, the above described “hiccup” and minimum current problems were not significant. However, as dropout became more important (i.e. with battery powered systems), LDO voltage regulators
22
were introduced which used the pass element
24
in the common emitter and, later, common source configurations (FIG.
2
).
FIG. 2
illustrates a conventional LDO voltage regulator
2
implementation of the circuit shown in FIG.
1
. For the LDO voltage regulator
22
, the reference voltage
26
(which may be provided by a bandgap reference or any other voltage reference generator known in the art) is applied to the inverting terminal
28
of the error amplifier
30
. The error amplifier
30
compares the voltage reference
26
at the inverting termi
Heisley David A.
Larson Tony R.
Brady W. James
Riley Shawn
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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