Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-04-24
2004-03-02
Vu, Bao Q. (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S274000, C323S275000
Reexamination Certificate
active
06700361
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a voltage regulator. The output voltage of which depends on the drive to a transistor contained in the voltage regulator.
A voltage regulator of this type is shown in FIG.
5
.
The configuration shown in
FIG. 5
contains a direct voltage regulator and a load impedance Zout connected thereto.
The voltage regulator contains a differential amplifier (a differential transconductance amplifier) OTA
1
, an NMOS transistor MN
1
, a first resistor Rfb, a second resistor Re, a third resistor Rs
1
, a first capacitor Cs
1
, a second capacitor Cs
2
, and a third capacitor Cs
3
.
The voltage regulator generates an output voltage Vout which is picked up at a source terminal of the transistor MN
1
and which is supplied as a supply voltage to the load Zout. A supply voltage supplying the voltage regulator with power is applied to a drain terminal of the transistor MN
1
, and the gate terminal is connected to the output terminal of the transconductance amplifier OTA
1
. The transconductance amplifier OTA
1
has two input terminals, one of which is supplied with an input voltage Vin and the other of which is supplied with a voltage depending on (fed back from) the output voltage Vout. The transconductance amplifier OTA
1
forms the difference between the voltages and outputs the result to the gate terminal of the transistor MN
1
. The voltage fed back is picked up at a node x
2
located between the resistors Rfb and Re. The resistors Rfb and Re are connected in series and are disposed between the source terminal of the transistor MN
1
and ground.
FIG. 6
shows the small-signal equivalent circuit of the configuration shown in FIG.
5
.
The voltage regulator described is a series voltage regulator with a common-drain NMOS transistor as a driver stage. It should be clear, and does not require further explanation, that the voltage regulator shown is capable of generating a constant output voltage Vout that depends only on Vin and the feedback factor (determined by the resistors Rfb and Re). However, this is not guaranteed under all circumstances, especially in the case of complex loads Zout, i.e. in the case of loads with inductive and/or capacitive components. The system may become unstable in this case.
The stability problems would not occur if it could be ensured, by suitable dimensioning of Rfb and Re, that the current Is
1
flowing through the transistor MN
1
does not drop below a certain minimum value even with a large Zout, that is to say a low load current, that is to say the transistor MN
1
has a certain minimum transconductance (a certain minimum output conductance). However, providing a large (shunt) current flowing via the transistor MN
1
and the resistors Rfb and Re is associated with various disadvantages. In particular, such a voltage regulator has a high intrinsic power requirement, and the transistor MN
1
has to be configured to be larger than would be the case with a low shunt current. In addition, the minimum shunt current necessary for ensuring the stability is not available for driving the load Zout.
The dependence of the stability of the voltage regulator on the minimum shunt current is now explained.
In a simplified way, the configuration according to
FIG. 5
can be understood to be a two-pole system. The stability criterion requires that the two poles are apart by a factor of at least n≧10.
The first pole fp
1
is obtained in a simplified manner in accordance with equation 1.1.
f
p1
≅
1
2
*
π
*
C
ml
*
1
/
gm
OTA1
(
1.1
)
It can be seen that the first dominant pole is determined by the transconductance gm of the transconductance amplifier OTA
1
and by the stabilization capacitance Cm
1
. In practice, the first pole is invariant and is determined by the necessary bandwidth of the configuration.
The second pole is determined in a simplified manner by the load capacitance Cout at the output Vout, the load impedance Zout and the output conductance gds of the driving transistor MN
1
. Equation 1.2 reproduces the mathematical relationship for calculating the second pole.
f
p2
≅
1
2
*
π
*
C
out
*
(
1
/
gds
MN1
&LeftDoubleBracketingBar;
Zout
&RightDoubleBracketingBar;
(
Re
+
Rfb
)
)
(
1.2
)
Using the aforementioned simplified dimensioning rule, according to which fp
2
≧10*fp
1
is to apply for a given load, the necessary minimum shunt current and thus the resistance value Rmin (the sum of resistors Re and Rfb) can be calculated.
The second pole fp
2
is directly proportional to the output conductance of the driving transistor. The minimum output conductance of the transistor is directly proportional to the minimum shunt current Iq=Is
1
set and thus ultimately to the minimum phase margin of the configuration.
As has already been explained above, these relationships are disadvantageous.
For this reason, alternatives for influencing the stability of voltage converters that do not have these disadvantages have long been sought.
One possibility for this consists in providing additional elements by which the transfer function of the system or, more precisely, the position of the pole positions and zero positions of the transfer function can be influenced in order to thus guarantee a minimum phase margin for stabilization purposes. In the case of the voltage regulator shown in
FIG. 5
, these possibilities have been used. The additional elements contain the resistor Rs and the capacitors Cs
1
, Cs
2
and Cs
3
. Of the elements, resistor Rs and capacitor Cs
1
are connected in series and disposed between the output terminal of the transconductance amplifier OTA
1
and ground, the capacitor Cs
2
is disposed between the feedback branch and ground, and the capacitor Cs
3
is disposed in parallel with the resistor Rfb.
The elements make it possible to influence the position of the pole and zero positions of the transfer function and thus also the stability characteristic of the system. However, it is difficult and complex and in some cases even impossible to dimension the elements in such a manner that the voltage regulator operates in a stable manner over the entire load range.
There are a large number of publications in which these and other possibilities for stabilizing voltage regulators are described. Reference is made, for example, to:
a) Thomas M. Frederiksen: “A Monolithic High-Power Series Voltage Regulator”, IEEE Journal of Solid-State Circuits, December 1968, page 380 ff.;
b) Gabriel A. Rincon-Mora et al.: “A Low-Voltage, Low Quiescent Current, Low Drop-Out Regulator”, IEEE Journal of Solid-State Circuits, Vol. 33, No. 1, January 1998, pages 36 ff.;
c) Gerrit W. den Besten et al.: “Embedded 5 V-to-3.3 V Voltage Regulator for Supplying Digital ICs in 3.3 V CMOS Technology”, IEEE Journal of Solid-State Circuits, Vol. 33, No. 7, July 1998, page 956 ff; and
d) the other references mentioned therein.
Among the known methods for stabilizing voltage regulators, there is none which is simple to configure and implement and can guarantee reliable stabilization with little intrinsic power requirement under all circumstances.
This applies not only to the series voltage regulator described above but also to so-called low drop output (LDO) regulators which have a common-source PMOS transistor as the driving transistor.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a voltage regulator which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which it can guarantee reliable stabilization under all circumstances with minimum intrinsic power requirement and, in addition, is simple to configure and implement.
With the foregoing and other objects in view there is provided, in accordance with the invention, a voltage regulator. The voltage regulator contains a transistor and an output supplying an output voltage that depends on a drive to the transistor. The output is connected to the transistor. A stabilizing circuit is connected to the transistor for changing a current flo
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Stemer Werner H.
Vu Bao Q.
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