Oscillators – Ring oscillators
Reexamination Certificate
2000-08-22
2001-10-09
Riley, Shawn (Department: 2838)
Oscillators
Ring oscillators
C323S280000
Reexamination Certificate
active
06300839
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to regulating a positive voltage on an integrated circuit, and specifically to regulating this positive voltage by slowing down an oscillator.
RELATED ART
Charge pumps are well known in the art of integrated circuits. In standard applications, a charge pump provides a voltage more positive than the most positive external power supply voltage.
FIG. 1A
illustrates a known prior art charge pump system
100
including an oscillator
120
, a latch circuit
130
, and a pump
140
. Oscillator
120
has a plurality of inverter stages
120
(
1
)-
120
(
12
) connected in series. The output terminal of the last inverter stage
120
(
12
) is coupled to an input terminal of latch circuit
130
.
To disable ring oscillator
120
, an input signal
135
is pulled high. In the disable mode, the last output signal of a NAND gate
132
is latched by cross-coupled NAND gates
132
and
134
. To enable ring oscillator
120
, input signal
135
is pulled low. In the enable mode, latch circuit
130
acts as an inverter, thereby acting as the final inverter stage of oscillator
120
. Specifically, when input signal
135
is low, NAND gate
134
will output a logic one regardless of the signal at its other input terminal. Therefore, NAND gate
132
functions as an inverter. It logically follows that, inverter
131
, NAND gate
132
, and inverter
134
also act as an inverter. The output signal of latch circuit
130
is provided as a feedback signal to node
110
, which is coupled to the first inverter stage
120
(
1
) of oscillator
120
. In this manner, an oscillation is generated by charge pump system
100
. The output signal of latch circuit
130
is also provided to a pump
140
.
FIG. 1B
illustrates one embodiment of pump
140
. The ring oscillator (such as oscillator
120
in
FIG. 1A
) provides a square wave signal
141
that oscillates between approximately supply voltage Vcc and ground. An inverter
142
sharpens the edges of signal
141
. A transistor
144
has its source coupled to capacitor
143
and its gate and drain coupled to Vcc. Therefore, transistor
144
acts as a weak pull-up device.
When signal
141
is approximately Vcc, the output signal of inverter
142
is low. Therefore, capacitor
143
does not discharge and transistor
144
provides a weak pull-up voltage on node
147
of Vcc minus the threshold voltage of transistor
144
. When signal
141
is low, the output signal of inverter
142
is high, thereby coupling node
147
to a value higher than Vcc. The discharge of capacitor
143
, via conducting transistor
145
, provides an output voltage on node
146
of Vcc plus an additional boost, &Dgr;V, provided by capacitor
143
.
FIG. 1B
illustrates a typical waveform of pump
140
at node
146
that varies between approximately Vcc and Vcc+&Dgr;V.
Note that transistors
144
and
145
function like diodes, as known by those skilled in the art, and are not described in detail herein. However, detailed information regarding charge pump system
100
is provided in U.S. Pat. No. 5,519,360, which is incorporated by reference herein.
Input signal
135
(
FIG. 1A
) is provided by a regulator circuit (not shown) which monitors the voltage on node
146
. If the voltage is below a predetermined amount, when the circuit provides a low input signal
135
, thereby enabling oscillator
120
to charge pump
140
. If the voltage is above a predetermined amount, then the circuit provides a high input signal
135
, thereby disabling oscillator
120
and thus turning off pump
140
. Therefore, particularly in cases where a high programming voltage is required on-chip, the voltage on node
146
varies considerably over time. Thus, this method may cause an undesirable voltage ripple.
To resolve this problem, in one embodiment, the regulator circuit that outputs signal
135
is eliminated and oscillator
120
is modified to include an additional inverter stage. In this embodiment, pump
140
is left on and the excessive current generated by pump
140
is sunk into ground via a large resistor. Unfortunately, this method results in excessive and unnecessary current being dumped into ground during the high-voltage cycle.
In another embodiment shown in
FIG. 2
, a charge pump system
200
provides staggered voltages on a common line to minimize the voltage ripple of a pumped voltage Vout. In charge pump system
200
, if internal voltage Vin is low compared to the desired internal voltage, then regulation circuit
201
increases the oscillation of ring oscillator stages
202
, thereby increasing voltage Vout (and also, logically, Vin). If voltage Vin is high compared to the desired internal voltage, then regulation circuit
201
decreases the oscillation of ring oscillator stages
202
, thereby decreasing voltages Vout and Vin (which is a function of voltage Vout).
To trigger this function, regulation circuit
201
includes a voltage divider
206
that receives pumped voltage Vout and generates internal voltage Vin. Differential amplifier
205
receives this internal voltage Vin as well as a reference voltage Vref, which is selected to be above the desired output voltage of voltage divider
206
. Thus, if the output voltage of voltage divider
206
is, for example, 5 volts, then reference voltage Vref could be 5.5 volts. If voltage Vin is greater than voltage Vref, then differential amplifier
205
outputs a logic zero until voltage Vout decreases appropriately. If voltage Vin is less than voltage Vref, then differential amplifier
205
outputs a logic one until voltage Vout increases appropriately.
In charge pump system
200
, ring oscillator stages
202
provide output clock signals to charge pumps
204
(via clock drivers
203
) in a staggered series. It logically follows that charge pumps
204
, all being identical, provide staggered output signals. Because charge pumps
204
provide their staggered output voltages to a common line, voltage Vout remains relatively constant. Detailed information regarding charge pump system
200
is provided in U.S. Pat. No. 5,553,030, which is incorporated by reference herein.
However, charge pump system
200
fails to address the problem of a significant overshoot of voltage Vout. Specifically, if a significant overshoot in Vout occurs, then considerable time is needed to reduce voltage Vout to a desired level. Therefore, a need arises for a charge pump system that quickly compensates for an overshoot in the desired pumped voltage.
SUMMARY OF THE INVENTION
The charge pump system of the present invention includes a ring oscillator for generating a clock signal having a frequency, a charge pump for receiving the clock signal and generating a pumped voltage, and a regulation circuit for receiving the pumped voltage, generating an internal voltage based on the pumped voltage, and modifying the frequency based on the internal voltage. The regulation circuit includes a plurality of differential amplifiers, each differential amplifier receiving the internal voltage.
In accordance with the present invention, the frequency of the oscillator is based on the output signals from the differential amplifiers. Specifically, each differential amplifier, in addition to receiving the internal voltage, also receives a different reference voltage. In this manner, the higher the internal voltage, the larger the number of differential amplifiers that output a predetermined logic signal. In one embodiment, this predetermined logic signal is a logic one signal.
In the present invention, this predetermined logic signal modifies, i.e. reduces, an original frequency of the oscillator. The oscillator of the present invention includes a plurality of stages, each stage including an inverting element and a modifiable load coupled to an output node of the stage. The modifiable load comprises a plurality of load control circuits, each load control circuit receiving an output signal of a differential amplifier.
In one embodiment, the load control circuit includes a pass gate coupled between the node of the stage and a capacitive element. Th
Bazargan Hassan K.
Shokouhi Farshid
Bever Hoffman & Harms
Harms Jeanette S.
Riley Shawn
Xilinx , Inc.
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