Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2003-04-14
2004-06-08
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S111000, C326S082000, C326S083000
Reexamination Certificate
active
06747487
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to voltage compensation techniques in an output driver for an integrated circuit. More particularly, the present invention relates to coupling a capacitive element between a drive transistor's gate and a power supply and phasing-in portions of a voltage-supporting capacitance at slightly different times to smooth the compensating corrections.
2. State of the Art
As the sizes of semiconductor devices have reduced, so have the power supply voltages driving the devices. With smaller power supply voltages, respective signals within the semiconductor devices have also become smaller and more susceptible to variance caused by the influence of resistance, inductance, capacitance and switching within the semiconductor device. Unintended variances on a signal line can lead to an inability to correctly detect a signal and to detecting a signal when one was not intended. In either case, malfunctions and other signal errors may occur as a result of the variances.
As shown in
FIG. 1
, an output
2
of a semiconductor device conventionally includes an output driver
4
between the primary semiconductor circuitry
6
and the outputs
2
. One purpose of the output driver
4
is to provide sufficient power compensation for the output signal to ensure the signal is output with an appropriate signal strength.
FIG. 2
shows a conventional output driver circuit
8
. For a conventional semiconductor die, each output of the die is coupled to each of constant voltage signal lines envg<
0
> through envg<
6
>
10
,
12
,
14
,
16
,
18
,
20
and
22
. Each of the constant voltage signal lines envg<
0
> through envg<
6
>
10
,
12
,
14
,
16
,
18
,
20
and
22
is further coupled to at least one output driver leg circuit
24
,
26
,
28
,
30
,
32
,
34
,
36
,
38
,
40
,
42
and
44
. Using one of the output driver leg circuits
44
coupled to envg<
6
>
22
as an example, each output driver leg circuit conventionally includes a drive transistor
46
and a switching transistor
48
.
The output driver circuit
8
shown in
FIG. 2
is configured as an open-drain output driver. An open-drain configured output driver is one in which the drain of the drive transistor for each output driver leg circuit
24
,
26
,
28
,
30
,
32
,
34
,
36
,
40
,
42
and
44
is not coupled to circuitry within the semiconductor, but is directly coupled to an externally accessible contact, such as a bond pad. In a conventional open-drain configured output driver circuit
8
that has its output impedance controlled, such as that shown in
FIG. 2
, the drive transistor
46
will have its gate
50
set to a controlled voltage (typically 1.3 V to 1.4 V) such that the drive transistor
46
will be in saturation as much as possible while still achieving the required output drive. The switching transistor
48
is placed between the drive transistor
46
and a reference potential
53
such as a ground, to allow for switching the drive transistor
46
between off and on states.
The array of output driver leg circuits
24
,
26
,
28
,
30
,
32
,
34
,
36
,
38
,
40
,
42
and
44
are conventionally configured such that the transistors used for the output driver leg circuits
32
and
40
coupled to constant voltage signal line envg<
5
>
20
are approximately half the physical size of the transistors used for the output driver leg circuits
34
and
44
coupled to constant voltage signal line envg<
6
>
22
. Likewise, the transistors used for the output driver leg circuits
30
and
38
coupled to envg<
4
>
18
are approximately half the physical size of the transistors used for the output driver leg circuits
32
and
40
coupled to constant voltage signal line envg<
5
>
20
. This pattern of using transistors approximately half the physical size of the transistors coupled to the next sequential envg<> signal line continues down to the transistors coupled to envg<
0
>
10
. The physical size of the output driver leg circuit
42
is half the sum of the physical sizes of both output driver leg circuits
28
and
36
. The output drive supplied by an output driver leg circuit is proportional to the physical size of the transistors used for that output driver leg circuit. By including output driver leg circuits, each providing a different output drive amount, a combination of different output driver leg circuits can provide a wide range of available output drive. Additional circuitry well-known to those of ordinary skill in the art determines how much output drive is needed for a particular output signal and controls which output driver leg circuits are switched “ON” and “OFF” to provide an appropriate level of output drive.
Though there are many advantages to selectively switching the various output driver leg circuits
24
,
26
,
28
,
30
,
32
,
34
,
36
,
38
,
40
,
42
and
44
“ON” and “OFF,” the “ON” and “OFF” action causes undesirable shifts in the drive transistor's gate
50
voltage due to potential changes that couple back through the drive transistor's gate
50
. Namely, the drive transistor's gate voltage may drop 100 mV from its desired level, for example, when the drive transistor
46
is switched to an “ON” state which will reduce the output drive from its intended target. One method of compensating for this drop in voltage, as shown in
FIG. 3
, is to couple a capacitor
54
between the drive transistor's gate
58
and a ground potential. With the capacitor in place, when the drive transistor
56
is turned “ON” indirectly by the switching transistor
62
, the voltage on the drive transistor's gate
58
begins to drop toward a ground potential, but the capacitor
54
, also referenced to the ground potential, reduces the voltage drop experienced. The larger the capacitor
54
used, the smaller the voltage dip caused when the drive transistor
56
is turned on. One example of a support circuit having capacitive support of this kind used in an output driver circuit may be found in Rambus Dynamic Random Access Memory (RDRAM) part 288MD-400-800, designed by Rambus, Inc. of Mountain View, Calif.
The repeated “ON”-“OFF” action, with the voltage on the drive transistor's gate
58
working to remain constant over a period of time, results in a square wave signal at the drive transistor's gate
58
. The output driver leg circuits' control circuit then tries to set the DC average of the drive transistor's gate
58
voltage equal to the desired voltage. As an example, using the output driver leg circuit of
FIG. 3
, if constant voltage signal lines envg<
6
>
60
were set at an operating voltage of 1.4 V and the switching transistor
62
were turned on, the voltage on the drive transistor's gate
58
would initially tend to be pulled down by the voltage on the drain of the switching transistor
62
, perhaps to 1.3 V. The capacitor
54
on the drive transistor's gate
58
of the drive transistor
56
would then tend to reduce the voltage drop experienced. If the switching transistor
62
were turned “ON” and never turned “OFF,” the voltage on the gate
58
of the drive transistor
56
would eventually reattain 1.4 V due to the controlling circuit's effect. For a typical output driver compensation circuit, however, the switching transistor
62
is not left “ON,” but is repeatedly switched “ON” and “OFF.” This toggling “ON” and “OFF” results in a square wave signal on constant voltage signal lines envg<
6
>
60
which eventually reaches a DC average of 1.4 V, toggling, for example, between 1.35 V and 1.45 V.
These variances in the voltage level of constant voltage signal lines envg<
6
>
60
result in variances in the output signal power which can result in signal transmission errors. As a result, part specifications are used to identify the maximum allowable variance for reliable operation. For example, the specifications for the RDRAM 288MD-40-800, referenced previously, re
Huber Brian W.
Lisenbe David
Lam Tuan T.
Micro)n Technology, Inc.
TraskBritt
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