Amplifiers – With semiconductor amplifying device – Including signal feedback means
Reexamination Certificate
2001-10-31
2002-08-06
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including signal feedback means
C330S067000, C330S307000
Reexamination Certificate
active
06429745
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device for use as an amplifier, a regulator, or the like that requires a phase compensation capacitor to be connected to an output terminal arranged thereof.
2. Description of the Prior Art
Among conventional semiconductor devices requiring a phase compensation capacitor to be connected to an output terminal thereof, a power IC (Integrated Circuit) is taken as an example and described hereinafter.
FIGS. 3 and 4
are circuit diagrams showing the outline of a conventional power IC. A power IC
10
″ as shown in
FIG. 3
is a semiconductor device that sends out signals output from a driver circuit
11
to a load Z
1
via an output terminal T
1
. The driver circuit
11
comprises, for example, multiple amplifying stages (not illustrated).
At the same time, a phase compensation capacitor C
1
is connected to the output terminal T
1
externally in parallel with the load Z
1
. This phase compensation capacitor C
1
is connected in order to suppress abnormal oscillations occurring in the driver circuit
11
and serves to form a pole at a lower frequency where a phase lag in output signals occurs so as to maintain a loop gain below 1 at a zero (phase=0) which is found in an input-output phase characteristic. This arrangement eventually serves to compensate the phase by decreasing the gain at a higher frequency side.
In the conventional cases, electrolytic capacitors or tantalum capacitors having a capacitance of approximately 10 &mgr;F are often used as a phase compensation capacitor. However, since these electrolytic capacitors have a polarity, an accidental connection to an opposite pole when mounting the capacitor induces problems that cause the capacitor to emit smoke or break down. Recently, because of these drawbacks, nonpolarized capacitors, particularly ceramic capacitors, have been used preferably as a phase compensation capacitor instead.
However, the aforementioned ceramic capacitors have a lower equivalent series resistance (ESR) component when compared to the electrolytic or tantalum capacitors. Different from other commonly used capacitors having a low ESR component, the ceramic capacitors display a peculiar phase characteristic in which an inherently low ESR component is further decreased at a specific frequency.
As mentioned above, when a low-ESR capacitor having an extremely low ESR component is used as the phase compensation capacitor C
1
as is, the phase characteristic of the driver circuit
11
is influenced, thereby creating the zero in terms of phase characteristic at a higher frequency side. As a result, the loop gain is increased thereat so that the driver circuit
11
becomes unstable, making it prone to oscillate,
The frequency at which the aforementioned zero occurs is given by
f
=
1
2
π
×
Co
×
ESR
In the above equation, it is assumed that capacitance of the phase compensation capacitor C
1
is Co and the equivalent series resistance component thereof is ESR.
As calculated from the equation, the frequency at which the zero occurs moves to a lower frequency side if a larger ESR value is given to the phase compensation capacitor C
1
. Consequently, a wider phase margin becomes available by which a possibility of oscillation is decreased.
Therefore, when a capacitor such as a ceramic capacitor having a low ESR component is used as the phase compensation capacitor C
1
, it is necessary to place an external resistor R
1
in series so as to compensate for the low ESR component of the capacitor.
As mentioned above, if a low-ESR capacitor (a capacitor having a low ESR component) such as a ceramic capacitor having no polarity is used as the phase compensation capacitor C
1
and is connected to the output terminal T
1
, there is no worry over a reverse connection that leads the capacitor to cause problems such as emitting smoke, thereby lowering the occurrence of troubles when mounting components and enabling a smooth flow of the capacitor mounting process.
On the other hand, a low-ESR capacitor for use as the phase compensation capacitor C
1
requires the additional external resistance R
1
which is not necessary if an electrolytic capacitor or the like is used instead. This requirement causes such problems to be solved as an increase in the number of components to be used, a decrease in reliability because of increased connection points, an increase in mounting area, an increase in cost, and the like.
SUMMARY OF THE INVENTION
An object of the present invention is, in light of the problems caused by conventional devices, to provide a semiconductor device that offers advantages such as a smaller mounting area even if a low-ESR capacitor is used as a phase compensation capacitor.
To achieve the above object, according to one aspect of the present invention, a semiconductor device requiring a phase compensation capacitor to be connected externally to the output terminal has an output transistor and an output terminal to which the output transistor is connected, wherein, to permit a low-ESR capacitor having a low equivalent series resistance component to be used as the phase compensation capacitor, the semiconductor device further comprises: a resistor of which one end is connected to a node between the output transistor and the output terminal; and a capacitor connection terminal to which another end of the resistor is connected, so that, when the low-ESR capacitor is connected externally to the capacitor connection terminal, the low equivalent series resistance component thereof is compensated for by the resistor.
According to another aspect of the present invention, a semiconductor device requiring a phase compensation capacitor to be connected externally to the output terminal has an output transistor and an output terminal to which the output transistor is connected, wherein, to permit a low-ESR capacitor having a low equivalent series resistance component to be used as the phase compensation capacitor, the semiconductor device further comprises: a resistor of which one end is connected to ground; and a capacitor connection terminal to which another end of the resistor is connected, so that, when the low-ESR capacitor is connected externally between the output terminal and the capacitor connection terminal, the low equivalent series resistance component thereof is compensated for by the resistor.
REFERENCES:
patent: 4048590 (1977-09-01), Dobberpuhl
patent: 5939944 (1999-08-01), Gibson
patent: 6316983 (2001-11-01), Kitamura
patent: 6344775 (2002-02-01), Morizuka et al.
Choe Henry
Pascal Robert
Rader & Fishman & Grauer, PLLC
Rohm & Co., Ltd.
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