Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2000-02-08
2001-05-01
Zweizig, Jeffrey (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
Reexamination Certificate
active
06225857
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of linear voltage regulators, and particularly to pass transistor driver circuits for low-dropout voltage regulators.
2. Description of the Related Art
Low-dropout (LDO) voltage regulators, i.e., regulators which must operate with a small difference between their input and regulated output voltages, can be difficult to frequency compensate. U.S. Pat. No. 5,631,598 to Miranda et. al (assigned to the present assignee), described an implementation of an LDO voltage regulator which facilitated the use of a desirable frequency compensation scheme. A simplified schematic of the LDO described therein is shown in
FIG. 1. A
supply voltage V
in
is connected to the emitter
10
of a pass transistor
12
, typically a pnp bipolar transistor, and an output voltage V
out
is taken at the transistor's collector
14
. The output voltage is regulated by controlling pass transistor
12
via its base terminal
16
. Regulation is accomplished with a feedback loop: a signal
17
representative of the output voltage is fed back to the non-inverting input
18
of an error amplifier
20
, usually via a voltage divider
22
. A reference voltage Vref is connected to the inverting input
24
of the amplifier. The amplifier's output
25
is connected to the input
26
of an inverting amplifier
28
, whose output is connected to the base of a drive transistor
30
which provides the drive current for pass transistor
12
. Inverting amplifier
28
and drive transistor
30
provide two inversions, and in combination form a non-inverting driver circuit
32
. With a non-inverting driver circuit employed in this way, the regulator can be frequency compensated by connecting a compensation network
34
between V
out
and the output
25
of error amplifier
20
.
However, using a non-inverting driver of this sort can present a problem, particularly when a MOS pass device is used. The large MOSFET needed to handle large output currents will have a large gate capacitance. At the same time, the pass device is driven from a high impedance node of the driver circuit—in this case, the collector of drive transistor
30
. The high impedance driver combined with the large gate capacitance result in a low frequency pole in the loop transfer function, which can prove troublesome when trying to frequency stabilize the LDO in the face of highly variable load resistance and reactance.
SUMMARY OF THE INVENTION
A non-inverting driver circuit for an LDO is presented which overcomes the problem described above. The novel non-inverting driver enables the use of the frequency compensation scheme referred to above, and also presents a low output impedance, such that the pole resulting from the driver output impedance/gate capacitance combination is moved to a higher frequency than would otherwise be possible, thereby simplifying the frequency compensation task.
The new non-inverting driver circuit employs a level-shifting inverter stage followed by a normalizing inverter stage. The level-shifting inverter converts the output common-referenced error amplifier output (i.e., the amplifier output is referenced to the same common point as the load to which the LDO is connected) to a current. The current is delivered to the normalizing inverter, which produces a drive signal to the LDO's pass device that is referred to the LDO input voltage, enabling the drive signal to remain substantially invariant with respect to the input voltage. The normalizing inverter stage is also preferably configured to have a low output impedance, so that when driving the high gate capacitance of a MOS pass device, the resulting pole is moved to a higher frequency than would be possible with a driver circuit having a high output impedance. With the pole moved higher, frequency compensating the regulator is simplified. Though most beneficial when used with a MOS pass device, the non-inverting driver can also be advantageously employed with a bipolar pass transistor.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
REFERENCES:
patent: 5373226 (1994-12-01), Kimura
patent: 5631598 (1997-05-01), Miranda et al.
patent: 5886570 (1999-03-01), Brokaw
patent: 5929617 (1999-07-01), Brokaw
Analog Devices Inc.
Koppel & Jacobs
Zweizig Jeffrey
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