Output buffer crossing point compensation

Details Output buffer crossing point compensation Output buffer crossing point compensation

2001-06-14

2002-10-22

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Current driver

C327S538000, C327S543000, C327S112000

Reexamination Certificate

active

06469548

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and/or architecture for output buffers generally and, more particularly, to buffer crossing point compensation for supply voltage variations.
BACKGROUND OF THE INVENTION
FIG. 1
shows a block diagram of a conventional output buffer
10
in a differential configuration. The output buffer
10
has an output current source
12
connected to a pair of output transistors
14
A-B. Each of the output transistors
14
A-B presents a signal (i.e., OUTA and OUTB). Each of the output transistors
14
A-B has a gate driven by a pre-drive inverter
16
A-B. The left pre-drive inverter
16
A receives a signal (i.e., INA) and presents a signal (i.e., GA) to the gate of the left output transistor
14
A. The right pre-drive inverter
16
B receives a signal (i.e., IND) and present a signal (i.e., GB) to the gate of the right output transistor
14
B. The signal INA and the signal INB are commonly complementary signals. Under ideal conditions, the signal GA and the signal GB will also be complementary signals. As a result, the signal OUTA and the signal OUTB will be complementary signals.
Referring to
FIG. 2
, a transfer characteristic of the pre-drive inverters
16
A-B is shown. The pre-drive inverters
16
A-B are designed to operate at a specific supply voltage (i.e., VCC) with respect to a ground voltage (i.e., GND). A curve
20
represents the transfer characteristics of the pre-drive inverters
16
A-B for a supply voltage VCC (i.e., 3.0 volts). A curve
22
a represents the transfer characteristics of the pre-drive inverters
16
A-B for a higher supply voltage VCC (i.e., 3.3 volts). A curve
23
represents the transfer characteristics for the pre-drive inverters
16
A-B for an even higher supply voltage VCC (i.e., 3.6 volts). The curves
20
,
22
, and
24
are symmetrical about a line
26
where the signal IN equals the signal G. The curves
20
,
22
, and
24
cross-over the line
26
at different values for the signal IN as indicated by lines
28
,
30
, and
32
respectively. As the supply voltage VCC increases from 3.0 volts to 3.6 volts, a threshold V
M
of the pre-drive inverters
16
A-B moves up from 1.47 volts to 1.8 volts.
Referring back to
FIG. 1
, the crossing point of the pre-drive inverters
16
A-B moves up with increasing supply voltage VCC due to increases in the threshold V
M
. Defining a second crossing point as that point where signal OUTA equals signal OUTB, then the second crossing point is also dependent upon the crossing point of the pre-drive inverters
16
A-B. As the supply voltage VCC changes, so too will the second crossing point.
Referring to
FIG. 3
, transient characteristics of the pre-drive inverters
16
A-B are shown. A pair of curves
34
and
36
represent step responses of the pre-drive inverters
16
A-B at the supply voltage VCC of 3.0 volts. A pair of curves
38
and
40
represent step responses of the pre-drive inverters
16
A-B at the supply voltage VCC of 3.3 volts. A pair of curves
42
and
44
represent step responses of the pre-drive inverters
16
A-B at the supply voltage VCC of 3.6 volts. Defining a third crossing point as a time where the step-up response crosses the step-down response, then the curves
34
-
44
show that the third crossing point will also vary with changes in the supply voltage VCC.
SUMMARY OF THE INVENTION
The present invention concerns a circuit comprising a current source, a first amplifier, and a second amplifier. The circuit may be used to provide for crossing point compensation of a CMOS driver as a function of a supply voltage. The current source may be configured to present a reference current. The first amplifier may be configured to (i) receive the reference current as a load, (ii) receive a first voltage, and (iii) present a second voltage responsive to the first voltage. The second amplifier may be configured to (i) receive the second voltage and (ii) change a current at a node responsive to the second voltage.
The objects, features and advantages of the present invention include providing output buffer crossing point compensation that may (i) reduce and/or eliminate dependency of the crossing point upon the supply voltage and/or (ii) require a small part count to implement.


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