Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2000-06-16
2002-04-02
Berhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S351000
Reexamination Certificate
active
06366066
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to voltage regulators. More particularly, this invention relates to circuits and methods for reducing the quiescent current in switching voltage regulators.
The purpose of a voltage regulator is to provide a predetermined and substantially constant output voltage to a load from a poorly-specified and fluctuating voltage source. One type of voltage regulator commonly used to accomplish this task is a switching voltage regulator. Switching voltage regulators are typically arranged to have a switching element (e.g., a power transistor) and an inductor coupled between the voltage source and the load. The switching regulator regulates the voltage across the load by turning the switching element ON and OFF so that power is transmitted through the switching element and into the inductor in the form of discrete current pulses. The inductor and an output capacitor then convert these current pulses into a steady load current so that the load voltage is regulated.
To generate a stream of current pulses, switching regulators include control circuitry that commands the switching element ON and OFF. The duty cycle of the switching element (i.e., the amount of time the switching element is ON compared to the period of an ON/OFF cycle), which controls the flow of current into the load, can be varied by a variety of methods. For example, the duty cycle can be varied by fixing the pulse stream frequency and varying the ON or OFF time of each current pulse, or by fixing the ON or OFF time of each current pulse and varying the pulse stream frequency.
Because switching regulators can operate at high levels of efficiency, they are often used in battery operated systems such as notebook computers, cellular telephones, and hand-held instruments. In such systems, when the regulator is supplying close to its rated output current, the efficiency of the overall circuit is usually high. However, this efficiency is generally a function of output current and typically decreases when the switching regulator is providing small amounts of current. This reduction in efficiency is generally attributable to the losses associated with operating the switching regulator. These losses include, among others, quiescent current losses, losses in the control circuit of the switching regulator, switching element losses, switching element driver losses, and inductor/transformer winding and core losses.
The reduction in efficiency of switching regulators at low output currents is of concern to circuit designers. This is because it is common for battery operated devices to experience short periods of high power use (i.e., periods during which relatively large currents must be supplied to a load), followed by extended periods of low power use (i.e., “standby” time during which a very small load current flows, but a regulated output voltage must be maintained). If the standby periods far exceed the usage periods, the quiescent current (i.e., the input current that flows into the switching regulator when the output is unloaded but still in voltage regulation) will determine the effective life of the battery. Accordingly, it is desirable to reduce quiescent current consumption as much as possible to extend battery life.
In the past, numerous techniques have been employed to reduce quiescent current losses in switching regulators during standby periods. For example, a switching regulator such as the LT1070 from Linear Technology Corporation, Milpitas, Calif., uses a control circuit that includes a comparator circuit and an error amplifier for monitoring the regulated output signal. When the output of the error amplifier drops below a threshold voltage, the regulator shuts down some of its internal circuitry to reduce quiescent current levels.
Other switching regulators from Linear Technology Corporation, such as the LT1307, LT1500, and LTC1625 use a mode of operation called “Burst Mode™” to reduce quiescent current. This mode of operation recognizes that the efficiency of a typical switching regulator drops off as the load decreases, because a fixed amount of power is wasted in the switch drive circuitry that is independent of load size. These switching regulators reduce quiescent current by holding the switching transistor(s) OFF, and turns OFF unneeded internal circuits, when the load current drops below a certain value.
A typical prior art current-mode stepdown switching regulator
100
employing burst mode operation is shown in FIG.
1
. Voltage regulator
100
generally comprises an output circuit
110
, a control circuit
130
, and a filter circuit
125
.
The voltage regulator of
FIG. 1
operates as follows. A switch timing circuit
101
(which may be, for example, a one-shot, an oscillator, or any other suitable circuit) within control circuit
130
supplies a control signal SW ON that sets a latch
104
. While latch
104
is set, a switch driver
106
provides a signal to output circuit
110
that causes a switch
108
in output circuit
110
to turn ON and provide current from an input voltage source VIN to an output node
117
. Latch
104
remains set until an output signal from a current comparator
102
causes latch
104
to reset. When reset, latch
104
turns switch
108
OFF so that current is no longer drawn from VIN. Current comparator
102
determines when to reset latch
104
by comparing a current signal (I
L
) from output circuit
110
with a current threshold value (I
TH
) generated by an error amplifier
122
in control circuit
130
(discussed in more detail below).
The primary purpose of output circuit
110
is to provide current pulses as directed by control circuit
130
and to convert those current pulses into a substantially constant output current. Output circuit
110
includes power switch
108
coupled to VIN and a node
109
, a catch diode
112
coupled from node
109
to ground, an inductor
114
coupled from node
109
to output node
117
, a capacitor
116
coupled from output node
117
to ground, and a voltage divider formed by resistors
118
and
120
coupled from node
117
to ground. Although switching element
108
is depicted as a field-effect transistor (FET) in
FIGS. 1 and 2
, any other suitable switching element may be used if desired.
The operation of output circuit
110
can be divided into two periods. The first is when power switch
108
is ON, and the second is when power switch
108
is OFF. During the ON period, current passes from VIN through switch
108
and flows through inductor
114
to output node
117
. During this period diode
112
is reverse-biased. However, after power switch
108
turns OFF, inductor
114
still has current flowing through it. The former current path from VIN through switch
108
is now open-circuited, causing the voltage at node
109
to drop such that catch diode
112
becomes forward-biased and starts to conduct. This maintains a closed current loop through a load (not shown). When power switch
108
turns ON again, the voltage at node
109
rises such that diode
112
becomes reverse-biased and turns OFF.
As shown in
FIG. 1
, error amplifier
122
in control circuit
130
senses the output voltage of regulator
100
via a feedback signal V
FB
produced by resistors
118
and
120
. Error amplifier
122
, which is preferably a transconductance amplifier, compares V
FB
with a reference voltage (V
REF
) that is also connected to amplifier
122
. An output signal, I
TH
, is generated in response to this comparison. The I
TH
signal is filtered by a filter circuit
125
comprised of resistor
124
and capacitor
126
and coupled to an input of current comparator
102
. The value of I
TH
sets the point at which current comparator
102
trips.
An input of a burst comparator
128
in control circuit
130
is also coupled to the output of error amplifier
122
and receives the filtered I
TH
signal. Burst comparator
128
monitors ITS as an indication of load current and compares the filtered I
TH
signal with a voltage potential V
1
that is connected to another input burst comparator
128
. V
1
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