Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2003-03-04
2004-08-10
Riley, Shawn (Department: 2838)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S540000
Reexamination Certificate
active
06774709
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to integrated digital circuits, and more particularly, to a voltage regulator for a charge pump generating a high voltage (HV) starting from the supply voltage of the integrated circuit.
BACKGROUND OF THE INVENTION
In EEPROM memory devices it is necessary to keep the high voltage, generated by the charge pump for programming and erasing operations (12-15 V), as constant as possible independently of the required driving capability, which is on the order of tens of &mgr;A. It is important that a certain and stable output voltage V
out
be produced to ensure that it will not surpass the limit imposed by the dielectric strength of oxides of transistor structures, as well as of capacitors, to prevent the risk of permanently damaging them.
The basic diagram of
FIG. 1
illustrates a commonly implemented regulator. A desired V
reg
(regulation voltage) is generated inside the integrated device, based on a known and very stable voltage reference (BandGap circuit), and is input to a comparator that compares it with the output voltage V
out
of the charge pump.
When V
out
>V
reg
, the regulator sends a logic signal V
ON
to a control circuit that stops the application of clock pulses to the phase generator. As a consequence, the charge pump starts to discharge, and as soon as V
out
<V
reg
, the regulator re-enables the driving action of the clock on the phase generator.
With this feedback, a dynamic equilibrium at the steady state condition V
out
=V
reg
is eventually reached. In a loadless condition, the charge pump is practically turned off, with the evident advantage (especially for minimizing power consumption) of not wasting power. When a load absorbs current, the regulator enables clock pulses to drive the charge pump at a frequency f
0
sufficient for precisely compensating the drop of V
out
because of the delivering of an electrical charge.
The current-voltage characteristic of an N stage pump of capacitance C, supplied at the supply voltage V
dd
and regulated with a frequency f
0
≦f
max
is shown in FIG.
2
. The charge pump generates a constant output voltage V
out
=V
reg
for any load current, up to the current I
0,max
compatible with that voltage.
When the load absorbs a certain current I
0
, the regulator adjusts the frequency of the clock pulses that reach the phase generator in the neighborhood of a frequency f
0
smaller than f
max
, making the pump work at the operating point (V
out
=V
reg
, I
out
=I
0
). In practice, it is as if the slope of the load line had been modified to determine the appropriate output resistance R
out
.
The ideal characteristic of
FIG. 2
satisfies the following equation:
I
out
=
fC
N
(
(
N
+
1
)
V
dd
-
V
out
)
FIG. 3
shows a commonly used regulator circuit. The circuit uses a resistive voltage divider by which a certain fraction V
R
of the output voltage V
out
is tapped and compared by a comparator with the reference voltage V
BG
usually produced by a bandgap circuit. The output V
ON
of the comparator is a logic signal that enables or disables the passage of clock pulses from the oscillator to the phase generator. Moreover, a properly dimensioned capacitive network is connected in parallel to the voltage divider for reasons that will be explained in the following paragraph.
The regulated voltage (in terms of mean value) is equal to:
V
out
=
V
BG
(
1
+
R
UP
R
DOWN
)
The precision of the regulator is tied essentially to the speed with which the node V
R
responds to voltage variations of V
out
for restoring the condition V
R
=V
BG
. If the resistors were ideal, the response would be instantaneous. In reality, the variation of V
out
propagates to the input of the comparator only after the voltage on the parasitic (distributed) capacitances associated with the integrated resistors have changed, and this slows down the response of the regulation loop.
In order to compensate this effect, a capacitor C
UP
is introduced between the node V
out
and the node V
R
, the size of which is chosen based upon a simulated step response, as depicted in FIG.
4
. With only the resistive line, the voltage V
R
increases quite slowly following a sub-compensated characteristic (curve a). Taking into consideration possible uncontrollable variations of parameters, instead of compensating exactly, it is often preferable to overcompensate, in order to benefit from an enhanced reactivity of the control loop.
To avoid an undershoot (curve b) that would imply an overshoot of the regulated voltage (because the comparator would consider the interval in which V
R
<V
BG
as an absence of regulation, thus letting clock pulses reach the phase generator and thus increasing the HVP level), the value of C
UP
is increased until the undershoot becomes null (curve c). Finally, for reducing the over-elongation peak that could cause spurious switching of the comparator, a capacitor C
DOWN
, about four times greater than C
UP
, is used (curve d).
This common regulation system is subject to dynamic problems. The more frequently the regulator must intervene (that is, the higher is the current required to be delivered by the charge pump), less readily the voltage V
R
follows the variations of the output voltage V
out
. In other words, the response of the regulation loop becomes slower as the load current increases, and any inaccuracy of regulation in terms of an offset (difference) V
R
−V
BG
present at the input of the comparator implies an error on V
out
that is equal to the offset itself amplified by the ratio R
UP
/R
DOWN
, and it may even be on the order of hundreds of mVs.
With modern technologies, this problem becomes even more severe. The reduced value of the bandgap voltage (~840 mV versus ~1.38 V for older technologies), coupled to a high value of R
UP
(for reducing current consumption through the voltage divider), renders even more difficult accurate regulation of the voltage V
out
, and indeed errors of up to 300-400 mV are not infrequent.
SUMMARY OF THE INVENTION
In view of the foregoing background, the purpose of the present invention is to overcome the above described difficulties.
The objective has been fully achieved by adopting an innovative technique that includes comparing the reference bandgap voltage not only with the instantaneous value of V
R
, as it is normally done, but also with a filtered replica thereof obtained by the use of an RC low-pass filter. In this way, even the average value of V
R
, hereinafter called V
Rfilter
, is accounted for compensating an eventual offset that may be present and that, as we have seen in the known circuits, causes a magnified error on the regulated voltage (normally the magnifying ratio R
UP
/R
DOWN
is about 12).
An objective is thus to provide a regulator for a HV charge pump, capable of self adapting itself to the varying level of the load current, compensating the variation of the regulating effect that normally occurs between extreme operating conditions (that is, near a null load and the maximum load).
According to the present invention, a regulator of the output voltage of a charge pump operates through the whole range of variation of the current absorbed by the load of the charge pump The regulator comprises a voltage divider for the output voltage of the charge pump for tapping a certain fraction of the output voltage, and a comparator for comparing the tapped voltage fraction with a constant reference voltage applied to respective inputs of the comparator and outputting a resultant digital signal. A logic control circuit is input with a main clock signal and outputs a timing signal, the frequency of which is determined by the state of the digital signal at every pulse of the main clock signal. A circuit generates control phases of the charge pump as a function of the timing signal.
The regulator is characterized in that it further comprises a low-pass filter for the output voltage fraction, the time constant of which is automatically switched at least between two pre-establi
Castaldo Enrico
Conte Antonino
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Jorgenson Lisa K.
Riley Shawn
STMicroelectronics S.r.l.
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