Low noise output buffer

Details Low noise output buffer Low noise output buffer

1999-08-10

2001-07-24

Tokar, Michael (Department: 2819)

Electronic digital logic circuitry

Signal sensitivity or transmission integrity

Output switching noise reduction

C326S026000, C326S087000

Reexamination Certificate

active

06265892

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to an output buffer. More particularly, this invention relates a low noise output buffer which can simultaneously reduce switching noise and output signal ringing and maintain a DC current supply, or even with the function of reducing ground bounces.
2. Description of the Related Art
In a high operating speed digital circuit, the simultaneous switching noise (SSN) is the main source of noise. The output pad driver is the major contributor to simultaneous switching noise because of large transient currents flowing through the bounding wires, leadframe, and pin parasitic inductance.
FIG. 1
shows a schematic diagram of a parasitic inductance induced after chip-package. A driver
70
having a voltage source connected to pins via pads and bonding wires induces a pin parasitic inductance
10
and a pad/bonding wire parasitic inductance
20
. Similarly, at ground terminal Gnd and Load terminal C
L
, pin parasitic inductance
50
,
30
and pad/bonding wire parasitic inductance
60
,
40
are induced.
The simultaneous switching noise has the following outward effects:
(1) The maximum bouncing voltage of simultaneous switching noise between voltage source and the ground terminal (V
DD
/Gnd (SSN)) increases when the number of simultaneous switching output is increased. As the increase of the bouncing voltage, the time for output voltage to achieve a steady state is delayed. Consequently, the speed of the digital circuit is affected. Referring to both
FIG. 2
a
and
FIG. 2
b
, to the statistic analysis data, the delay time is increased when the number of switching outputs is increased. In addition, the SSN is increased as the increase of the number of the switching output. This indicates that the delay time increases when the slew rate of current increases, so as the SSN. It is known that to obtain a high speed performance, a MOS transistor of an output buffer is designed with a larger channel width to improve the capability of driving current. However, an enhanced driving current capacity induces a larger SSN and a longer delay time. It is very likely to cause a deteriorated performance or wrong function.
(2)
FIG. 3
illustrates a schematic diagram of interference caused by SSN. Assuming that an active driver
90
and a quiet driver
100
have a common of V
DD
/Gnd, because the V
DD
/Gnd line are disturbed by SSN of the active driver
90
, the quiet driver is disturbed through the V
DD
/Gnd line. When a high level H is supplied to the quiet driver
100
from an internal circuit
80
, the output thereof is fixed at a low level L. Meanwhile, a signal of transferring from L to H is transmitted to n active drivers
90
, so that one signal of transferring from H to L is generated by each of the n active drivers
90
. n discharging currents thus flow to the internal ground terminal
95
at the same time. Since a parasitic inductance exists between the internal ground terminal
90
and an external Gnd, a spike noise is generated between internal ground terminal
95
and the external Gnd by n discharging currents (n×i
discharge
). Thus, the quiet driver
100
with an output at L is still disturbed by the spike noise through the internal ground terminal. It is possible that a receiver
110
may receive the spike noise as the signal to perform a wrong function.
FIG. 4
shows a conventional output buffer made in order to reduce SSN. The output buffer
160
includes a quiet driver
120
coupled to a quiet V
DD
/quiet GND (quiet V
DD
/GND) voltage source and a noise driver
130
coupled to a noise V
DD

oise GND (noisy V
DD
/GND) voltage source. The quiet driver
120
further includes a quiet pull-up transistor
122
, a quiet pull-down transistor
124
, and a predriver composed of a first NOR gate
141
and a second NOR gate
142
. The noise driver
130
includes a noise pull-up transistor
132
, a noise pull-down transistor
134
, and a predriver composed of a third NOR gate
143
and a fourth NOR gate
144
. A first feedback NOT gate
150
and a second feedback NOT gate
152
are used to feed back a signal of an output terminal.
Under a steady state, when an input terminal
112
is H, an output terminal
114
is H, the first NOR gate
141
has L and L inputs and an H output, the quiet pull-up transistor
122
is turned on to provide H to the output terminals
114
. The second NOR gate
142
has H and H inputs and an L output, the quiet pull-down transistor
124
is turned off. The third NOR gate
143
has an L input, an H input and an L output. The noise pull-up transistor
132
is turned off. The fourth NOR gate
144
has an L input, an H input and an L input. The noise pull-down transistor
132
is turned off. Meanwhile, only the quiet pull-up transistor
122
of the quiet driver
120
provides H to the output terminal.
When the input terminal enters is switched from H to L, two steps are included:
(1) Before the H state of the output terminal changes, since the input terminal
112
has been converted into L, the first NOR gate
141
has H and L inputs and L output, and the quiet pull-up transistor
122
is turned off. The inputs of the second NOR gate
142
are H and L and the output thereof is L, the quiet pull-down transistor
124
is turned off. The inputs of the third NOR gate
143
are H and H and the output thereof is L, the noise pull-up transistor
132
is turned off. The fourth NOR gate
144
has L and L inputs and H output, the noise pull-down transistor
134
is turned on to provide L to the output terminal
114
. That is, a forepart of state transferring is to turn on the noise pull-down transistor
132
by the noise driver
130
, so as to provide L to the output terminal
114
. Meanwhile, SSN is generated in the noise GND voltage source.
(2) When the output terminal
114
is switched to L by the noise pull-down transistor
134
and fed back to the first and the second feedback NOT gates
150
and
152
, the first NOR gate
141
has H and H inputs and L output, the quiet pull-up transistor
122
is thus turned off. The second NOR gate
142
has L and L inputs and an H output, so that the quiet pull-down transistor
124
is turned on to provide L to the output terminal
114
. The inputs of the third NOR gate
143
are H and L and the output thereof is L, the noise pull-up transistor
132
is turned off. For the fourth NOR gate
144
, the inputs are H and L and the output is H, the noise pull-down transistor
132
is turned off. That is, when the state is switched to a steady state, the quiet pull-down transistor
124
of the quiet driver
120
is turned on to provide L to the output terminal
114
. Thus, SSN at the quiet GND voltage source is greatly reduced without affecting the internal circuit.
Similarly, when the input terminal
112
is switched from L to H before the output terminal
114
is converted to H, the quiet driver
120
is turned off. The noise driver
130
is turned on to bear with a large SSN at the noise V
DD

oise GND voltage source. When the output terminal
114
is switched to H, the noise driver
130
is turned off, the quiet driver
120
is turned on, a smaller SSN at the quiet V
DD
/quiet GND voltage source is resulted.
The conventional output buffer has the following drawbacks:
(1) The output buffer uses two independent voltage sources for operation. The forepart of state transferring for the output terminal uses one voltage source, while the latter part of the state transferring uses another voltage source.
(2) When the output terminal
114
is switched from H to L, or from L to H, with regard to the first feedback NOT gate
150
, a trigger level to turn off the noise driver
130
is the same as that to turn on the quiet driver
120
. As a consequence, the speed of outputting signal is reduced.
(3) When the noise driver
130
is off and the quiet driver
120
are on, the slew rate to turn on the quiet pull-up or pull down transistor of the quiet driver
120
can not be too slow. However, with a very fast slew rate, SSN is increased. For the buffer
160
, the SS

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