Voltage regulator compensation circuit and method

Details Voltage regulator compensation circuit and method Voltage regulator compensation circuit and method

2000-04-25

2001-05-08

Riley, Shawn (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S224000

Reexamination Certificate

active

06229292

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of voltage regulators, and particularly to methods of improving a voltage regulator's response to a load transient.
2. Description of the Related Art
The purpose of a voltage regulator is to provide a nearly constant output voltage to a load, despite being powered by an unregulated input voltage and having to meet the demands of a varying load current.
In some applications, a regulator is required to maintain a nearly constant output voltage for a step change in load current; i.e., a sudden large increase or decrease in the load current demanded by the load. For example, a microprocessor may have a “power-saving mode” in which unused circuit sections are turned off to reduce current consumption to near zero; when needed, these sections are turned on, requiring the load current to increase to a high value—typically within a few hundred nanoseconds.
When there is a change in load current, some deviation in the regulator's output voltage is practically unavoidable. The magnitude of the deviation is affected by both the capacitane C
e
and the equivalent series resistance (ESR) R
e
of the output capacitor. The output capacitor may comprise one or more capacitors, generally of the same kind, which, when connected into a series, parallel, or series/parallel combination, provide capacitance C
e
and ESR R
e
. A smaller capacitance or a larger ESR increase the deviation. For example, for a switching voltage regulator (which delivers output current via an output inductor and which includes an output capacitor connected in parallel across the load), a change in load current &Dgr;I
load
results in a change in the regulator's output voltage unless 1)the current delivered to the load instantaneously increases by &Dgr;I
load
, or 2)the capacitance of the output capacitor is so large and its ESR is so small that the output voltage deviation would be negligible. The first option is impossible because the current in the output inductor cannot change instantaneously. The time required to accommodate the change in load current can be reduced by reducing the inductance of the output inductor, but that eventually requires increasing the regulator's switching frequency, which is limited by the finite switching speed and the resulting dissipation in the switching transistors. The second option is possible, but requires a very large output capacitor which is likely to occupy too much space on a printed circuit board, cost too much, or both.
For applications requiring the regulator's output voltage to meet a narrow load transient response specification, i.e., a specification which narrowly limits the allowable output voltage deviation for a bidirectional step change in load current, this inevitable deviation may be unacceptably large. As used herein, “&Dgr;V
out
” refers to a regulator's output voltage deviation specification, as well as to peak-to-peak output voltage deviations shown in graphs. The most obvious solution for improving load transient response is to increase the output capacitance and/or reduce the ESR of the output capacitor. However, as noted above, a larger output capacitor (which provides both more capacitance and lower ESR) requires more volume and more PC board area, and thereby more cost.
One approach to improving load transient response is shown in
FIG. 1. A
switching voltage regulator
10
includes a push-pull switch
12
connected between a supply voltage V
in
and ground, typically implemented with two synchronously switched power MOSFETs
14
and
16
. A driver circuit
18
is connected to alternately switch on one or the other of MOSFETs
14
and
16
. A duty ratio modulator circuit
20
controls the driver circuit; circuit
20
includes a voltage comparator
22
that compares a sawtooth clock signal received from a clock circuit
24
and an error voltage received from an error signal generating circuit
26
. Circuit
26
typically includes a high-gain operational amplifier
28
that receives a reference voltage V
ref
at one input and a voltage representative of the output voltage V
out
at a second input, and produces an error voltage that varies with the difference between V
out
and the desired output voltage. The regulator also includes an output inductor L connected to the junction between MOSFETs
14
and
16
, an output capacitor
30
, shown represented as a capacitance C
e
in series with an ESR R
e
, and a resistor R, connected between the output inductor and the output capacitor. A load
32
is connected across the output capacitor.
In operation, MOSFETs
14
and
16
are driven to alternately connect inductor L to V
in
and ground, with a duty ratio determined by duty ratio modulator circuit
20
; the duty ratio varies in accordance with the error voltage produced by error amplifier
28
. The current in inductor L flows into the parallel combination of output capacitor
30
and load
32
. The impedance of capacitor
30
is much smaller at the switching frequency than that of load
32
, so that the capacitor filters out most of the AC components of the inductor current and virtually all of the direct current is delivered to load
32
.
Without series resistor R
s
, the voltage fed back to circuit
26
is equal to V
out
, and the regulator's response to a step change in load current is that of a typical switching regulator; a regulator's output voltage V
out
is shown in
FIG. 2
a
for a step change in load current I
load
shown in
FIG. 2
b
. Because the current in L cannot change instantaneously, a sudden increase in I
load
causes V
out
to deviate downward; the control loop eventually forces V
out
back to a nominal output voltage V
nom
. Similarly, when I
load
later steps down, V
out
deviates upward before returning to V
nom
. The total deviation in output voltage &Dgr;V
out
for a step change in load current is determined by the difference between the two voltage deviations. If the regulator is subject to a narrow load transient response specification, the total deviation may exceed the tolerance allowed.
Connecting resistor R
s
in series with inductor L (at an output terminal
34
) can reduce &Dgr;V
out
; one possible response with R
s
included is shown in
FIG. 3
a
for a step change in load current shown in
FIG. 3
b
. With R
s
in place, the control loop no longer causes V
out
to recover to V
nom
; rather, V
out
recovers to a voltage given by the voltage at terminal
34
minus the product of &Dgr;I
load
and R
s
. That is, the steady-state value of V
out
for a light load will be higher than it is for a heavy load, by &Dgr;I
load
*R
s
. Making R
s
approximately equal to the ESR of the output capacitor can provide a somewhat narrower &Dgr;V
out
than can be achieved without the use of R
s
.
One disadvantage of the circuit of
FIG. 1
is illustrated in
FIGS. 4
a
and
4
b
. In this case, the load current (
FIG. 4
b
) steps back down before V
out
(
FIG. 4
a
) has settled to a steady-state value. With V
out
higher than it was in
FIG. 3
a
at the instant I
load
begins to fall, the peak of the upward V
out
deviation is also higher, making the overall deviation &Dgr;V
out
greater than it would otherwise be. This larger deviation means that to satisfy a particular narrow output voltage deviation specification, regulator
10
must use an output capacitor with larger capacitance or smaller ESR. This can be achieved either by using more individual capacitors of a given type, or by using a different type of capacitor. Either solution (and because the cost of a capacitor is approximately inversely proportional to its ESR) has an associated cost, which may make meeting the voltage deviation. specification prohibitively expensive.
Another disadvantage of the
FIG. 1
circuit is the considerable power dissipation required of series resistor R
s
. For example, assuming an R
s
of 5 m&OHgr; and a maximum load current of 14.6 A, the dissipation in R
s
will be 1.07 W.
An approach to improving a regulator's load transient r

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