Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1999-06-15
2001-01-30
Lee, Benny (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C327S359000
Reexamination Certificate
active
06181203
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to transconductance amplifiers. More particularly, the present invention relates to a nonlinear transconductance amplifier suitable to provide a nonlinear response to a differential signal. The nonlinear transconductance amplifier of the present invention is suitable for use as a nonlinear element in the feedback loop of a switching-type closed loop system, such as a DC—DC converter, for improving the response time to load transients, or a phase lock loop for improving the frequency capture rate. It may also be employed in a linear regulator.
2. The Prior Art
Traditional linear and more conventional switched mode power supply topologies are well known to those of ordinary skill in the art. In a typical linear power supply or voltage regulator circuit, a linear control element, such as a pass transistor, in series with an unregulated DC is used, with feedback, to maintain a constant output voltage. The output voltage is always lower in voltage than the unregulated input voltage, and some power is dissipated in the control element. Though the linear power supply has a fast response time, it is not very efficient in comparison to a switched mode converter when the input to output voltage ratio is large.
Typically, in a switching converter, a transistor operated as a saturated switch periodically applies the full unregulated voltage across an inductor for short intervals. The current in the inductor builds up during each pulse, storing ½ LI
2
of energy in its magnetic field. The stored energy is transferred to a filter capacitor at the output that also smooths the output by carrying the output load between the charging pulses. In order to accommodate rapid and transient load changes, and to filter the switch frequency from the output, the output capacitor preferably has a large value with a very low equivalent series resistance (ESR). With feedback, the output of the converter is compared with the input to control the switching frequency or pulse width of the signal applied to the transistor operated as a switch. Since the control element is either off or saturated, the power dissipation in the regulator is minimized. Accordingly, switching regulators are very efficient, even when there is a large voltage drop from the input to the output.
In
FIG. 1
, a known DC—DC converter
10
, referred to by those of ordinary skill in the art as a step-down or “buck” topology, is illustrated. In converter
10
, a switch
12
is controlled by the output a comparator
14
having an inverting input connected to an oscillatory signal and a non-inverting input connected to a feedback signal to be described to form a pulse width modulator (PWM). A first terminal of switch
12
is connected to Vin, and a second terminal of switch
12
is connected a first terminal of inductor
16
and the anode of diode
18
. The second terminal of inductor
16
is connected to a first plate of load capacitor
20
, and also through voltage divider pair
22
-
1
and
22
-
2
to the inverting input of error amplifier
24
. Vout is formed at the second terminal of inductor
16
. A second plate of load capacitor
20
is connected to the cathode of diode
18
to complete a loop for current circulation, and also to a ground reference potential. The non-inverting input of error amplifier
24
is connected to a reference potential
26
. The output of error amplifier
24
is connected to a first end of a resistor
28
. A second end of resistor
28
is connected to a first plate of capacitor
30
and to the non-inverting input of comparator
14
to complete a feedback loop from Vout. A second plate of capacitor
30
is connected to ground.
In the converter
10
, a high input voltage at Vin is converted to a lower input voltage at Vout. When the switch
12
is closed by the output of comparator
14
, the voltage Vout-Vin is applied across the inductor
16
, causing a linearly increasing current to flow through the inductor
16
. When the switch
12
is opened, inductor current continues to flow in the same direction with the diode
18
conducting to complete the circuit. Since the voltage across the inductor
16
is now the difference between Vout and the nominal diode
18
voltage, the inductor current will decrease linearly. The load capacitor
20
operates to minimize current and voltage ripple at the output of the converter
10
. It will be appreciated that as the size of the capacitor
20
increases, the amount of ripple decreases, however, the response time of the converter
10
to changes in the load also increases.
The feedback loop including the error amplifier
24
forms a control circuit to ensure that Vout remains at a selected value with a high degree of precision. In the feedback loop, Vout through the voltage divider
22
-
1
and
22
-
2
is compared to the reference voltage
26
. The difference between the reference voltage
26
and Vout determines the width of the pulse driving the switch
12
from comparator
14
in a manner well understood by those of ordinary skill in the art.
The feedback control of the converter
10
has a finite response time to changing load conditions. The value of the output capacitor
20
is typically chosen to provide continuity in Vout while the feedback loop is responding to the changing load condition. The response times are typically slow in comparison to linear regulators, due to the size of the output capacitor
20
. Further, the output capacitor is typically expensive due to its size, and also quite bulky.
The error amplifier
24
is typically a conventional operational amplifier which employs feedback filtering to provide either phase lag or phase advance responses. The feedback filtering often includes an arrangement of several capacitors. To aid stability and simplify compensation requirements, the error amplifier
24
is often low gain. To improve the response time of the feedback loop, it is known to implement the error amplifier
24
as a linear transconductance amplifier with a capacitive load, though for an increasing number of applications, the response time is inadequate.
For example, in conventional computer systems, the load conditions of a microprocessor may change very small amounts in a very short time due to very high microprocessor speeds or may change significantly in a very short time due to a state change in the microprocessor, such as power up. Further, various input and output devices of the computer system, such as hard drives or CD ROMS may change the load conditions of the microprocessor very rapidly.
Accordingly, it is an object of the present invention to provide a DC—DC converter with a fast response time to load changes.
It is yet another object of the present invention to reduce the size of the output capacitor in a step down DC—DC converter topology.
It is further object of the present invention to reduce the size of a step down DC—DC converter by reducing the size of the output capacitor.
It is yet a further object of the present invention to provide a nonlinear transconductance amplifier with a nonlinear response to a differential input
It another object of the present invention to simplify the phase compensation in an error amplifier of a DC—DC converter.
BRIEF DESCRIPTION OF THE INVENTION
According to the present invention, a nonlinear transconductance provides a nonlinear output in response to a differential signal input. In a switch-type closed loop system, the nonlinear transconductance amplifier provides a very fast response time. The nonlinear transconductance amplifier is a linear transconductance amplifier that has been modified to include a nonlinear output stage of current mirrors having resistive elements connected to the emitters of the diode connected transistors in the current mirrors. The nonlinearity of nonlinear transconductance amplifier is exponential such that when the differential signal at the inputs to the nonlinear transconductance amplifier is less than a predetermined value, the nonlinear transconductance amplifier provides a
D'Alessandro & Ritchie
Lee Benny
Nguyen Khanh Van
Semtech Corporation
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