Voltage level shifter

Details Voltage level shifter Voltage level shifter

2002-04-19

2003-09-02

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Amplitude control

C327S427000, C326S068000, C326S081000

Reexamination Certificate

active

06614283

ABSTRACT:

FIELD
The subject matter herein relates to a voltage level shifter for electronic circuits, such as for translating electrical signals from within an integrated circuit (IC) to outside the IC, where the IC has an internal operating voltage at a different voltage level than an external transfer voltage.
BACKGROUND
Integrated circuits (IC's) of today typically operate internally at voltages that are lower than those used in IC's of just a few years ago. For example, a few years ago, an “internal operating voltage” of about 5.0 volts was common for many IC's. More recently, an internal operating voltage of about 3.3 volts has become common. Today, internal operating voltages of about 1.8 volts or less have come, or are coming, into use for IC's.
When two IC's having different internal operating voltages are to be used together, a voltage level shifter is used to shift one of the IC's output signals from the IC's internal operating voltage to that of the other IC. Since newer IC's are typically designed to be compatible with older, “legacy,” IC's, the voltage level shifter is commonly incorporated into the newer IC's.
A newer IC may have a lower internal operating voltage at which most of the functions of the IC operate and a higher “external transfer voltage” at which output signals are transferred to other IC's. The voltage level shifter, thus, transitions the output signals from the lower internal operating voltage to the higher external transfer voltage. Additionally, input signals are typically shifted by the IC from the higher external transfer voltage to the lower internal operating voltage.
An exemplary prior art voltage level shifter
100
, as shown in
FIG. 1
, receives an input signal V
IN
102
, typically a digital signal operating at a given clock frequency and voltage level, and produces therefrom an output signal V
OUT
104
at the same frequency, but a higher voltage level. The input signal V
IN
102
is supplied by internal core IC circuitry (not shown), performing the normal functions of the IC (not shown). The output signal V
OUT
104
is supplied to output pins (not shown), which connect to other circuitry or IC's (not shown), possibly through a printed circuit board (not shown). Additionally, a logic one value for the digital input signal V
IN
102
has about the same voltage level as a core voltage V
CORE
106
, i.e. the internal operating voltage, used to power of the internal core IC circuitry. A logic one value for the digital output signal V
OUT
104
has about the same voltage level as an I/O (input/output) voltage V
IO
108
, i.e. the external transfer voltage, used to power the I/O functions of the IC.
The voltage level shifter
100
includes thick oxide N-MOSFET transistors
110
and
112
, thick oxide P-MOSFET transistors
114
and
116
and a thin oxide inverter
118
. The sources of the transistors
110
and
112
are connected to ground
120
. The drains of the transistors
110
and
112
are connected to the drains of the transistors
114
and
116
, respectively. The sources of the transistors
114
and
116
are connected to the I/O voltage
108
. The drain of the transistor
110
is also connected to the gate of the transistor
116
. The drain of the transistor
112
is connected to the gate of the transistor
114
and also supplies the output signal
104
. The inverter
118
is connected to the core voltage
106
and to the ground
120
.
The input signal
102
is supplied to the gate of the transistor
110
and to the input of the inverter
118
. The inverter
118
, powered by the core voltage
106
, inverts the input signal
102
. The inverted input signal
102
is supplied to the gate of the transistor
112
.
Therefore, when the input signal
102
is at a logic zero (i.e. approximately zero volts), the logic zero at the gate of the transistor
110
causes the transistor
110
to turn “off,” and the inverted input signal
102
(i.e. logic one) at the gate of the transistor
112
causes the transistor
112
to turn “on.” Since the transistor
112
is “on,” the drain of the transistor
112
(and, thus, the output signal
104
and the gate of the transistor
114
) is “pulled down” to approximately ground, or zero volts or logic zero. The logic zero on the gate of the transistor
114
, thus, turns “on” the transistor
114
, so the drain of the transistor
114
(and the gate of the transistor
116
) is “pulled up” to the voltage level of the I/O voltage
108
, i.e. a logic one. The logic one on the gate of the transistor
116
, thus, turns “off” the transistor
116
, so as not to interfere with the logic zero on the drain of the transistor
112
and the output signal
104
.
On the other hand, when the input signal
102
is at a logic one (i.e. the internal operating voltage), the logic one at the gate of the transistor
110
causes the transistor
110
to turn “on,” and the inverted input signal
102
(i.e. logic zero) at the gate of the transistor
112
causes the transistor
112
to turn “off.” Since the transistor
110
is “on,” the drain of the transistor
110
(and, thus, the gate of the transistor
116
) is “pulled down” to approximately ground, or zero volts or logic zero. The logic zero on the gate of the transistor
116
, thus, turns “on” the transistor
116
, so the drain of the transistor
116
(and, thus, the output signal
104
and the gate of the transistor
114
) is “pulled up” to the voltage level of the I/O voltage,
108
(i.e. a logic one at the external transfer voltage). The logic one on the gate of the transistor
114
, thus, turns “off” the transistor
114
, so as not to interfere with the logic zero on the drain of the transistor
110
and the gate of the transistor
116
.
With the lower internal operating voltages and higher clock frequencies coming into use with many IC's, the thick oxide N-MOSFET transistors
110
and
112
cannot perform adequately. The internal operating voltages, for example, are becoming so low that they are approaching the “threshold voltage” of the transistors
110
and
112
. The threshold voltage of a transistor is the minimum voltage that can be applied to the gate of the transistor to activate the transistor. Therefore, if the internal operating voltage (i.e. the logic one voltage of the input signal
102
) becomes as low as the threshold voltage of the transistors
110
and
112
, then the transistors
110
and
112
cannot be activated and the voltage level shifter
100
will not operate. Additionally, if the logic on& voltage level of the input signal
102
is relatively larger than the threshold voltage of the transistors
110
and
112
, then the transistors
110
and
112
can be turned on relatively fast. However, if the logic one voltage level of the input signal
102
is relatively close to the threshold voltage of the transistors
110
and
112
, then the transistors
110
and
112
will switch from “off” to “on” relatively slowly. In this case, the transistors
110
and
112
cannot be activated quickly enough for the desired clock frequency of the IC, and the voltage level shifter
100
will not operate.
It is with respect to these and other background considerations that the subject matter herein has evolved.
SUMMARY
The subject matter described herein involves an integrated circuit (IC) having a voltage level shifter capable of operating with the lower internal operating voltages and higher clock frequencies used by current and upcoming IC's. The transistors (i.e. “switching transistors”) within the voltage level shifter that are activated, or “switched on and off,” by the internal operating voltage of the IC incorporate a thinner oxide than in the prior art. Therefore, the threshold voltage of the switching transistors is lower than the thicker-oxide transistors in the prior art, so the internal operating voltage required to turn “on,” or “activate,” the switching transistors is lower than the voltage required to turn “on” the transistors in the prior art. Additionally, the frequency at which the switching tra

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