Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2000-01-24
2002-07-02
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S170000, C327S374000, C327S078000, C327S077000, C326S086000, C326S087000
Reexamination Certificate
active
06414523
ABSTRACT:
TECHNICAL FIELD
The present invention relates to output drivers for Universal Serial Bus (USB) devices. In particular, the present invention includes a CMOS pull-up circuit for a differential output driver.
BACKGROUND ART
The universal serial bus (USB) is a computer bus architecture used for connection of information processing devices, such as peripheral computer devices, to a personal computer (PC). For a number of reasons, use of a USB in peripheral device interconnection has become desirable. First, a wide range of information processing devices can be interconnected to other information processing devices, such as PCs, via the USB. For example, a PC's keyboard, mouse, printer, scanner, modem, audio devices and video devices, can all be connected via the USB. Also, the USB allows connection of these and other peripheral devices using only a single connector type. Additionally, device attachment is automatically detected by the USB and software automatically configures the device for immediate use, without user intervention.
A USB device is interconnected with, and transfers data to and from, a PC via a USB cable. The USB uses a differential output driver to drive a USB data signal onto the USB cable.
FIG. 1
is a schematic diagram of one example of a differential output driver
10
which can be used to drive the USB cable. Output driver
10
uses both an inverting buffer
12
and a non-inverting buffer
14
. An input data signal is applied to both buffers
12
and
14
, yielding a D− output
16
and a D+ output
18
. Resistors
20
a
and
20
b
are provided in each output line to generate output resistance. In accordance with standard USB specifications, each resistor
20
and
20
b
typically has an impedance of approximately 27 ohms.
An ideal D+ and D− signal generated by output driver
10
is shown in
FIG. 2
which is a voltage versus time output graph showing how D+ and D− signals should appear on outputs
18
and
16
, respectively, of driver
10
. By USB specifications, the high state voltage Voh should be between 2.8 and 3.6 volts. Additionally, to meet USB specifications, an output signal crossover voltage where the output changes digital state must be between 1.3 and 2.0 volts.
Also according to USB specifications, the supply voltage for the USB driver
10
should be from 4.40 to 5.25 volts. Thus, because the high state voltage, Voh, should be between 2.8 and 3.6 volts, the output driver
10
cannot be connected directly to supply voltage. Rather, a separate pull-up circuit is required to maintain a high state voltage of between 2.8 and 3.6 volts.
An example of a previous pull-up circuit
50
which has been used to maintain a high state voltage of between 2.8 and 3.6 volts is shown in FIG.
3
. Pull-up circuit
50
includes a driving transistor
52
, the source of which is connected to an output pad
54
. Output pad
54
is used to drive the USB cable (not shown). A high state voltage on output pad
54
must be between 2.8 and 3.6 volts. The gate of driving transistor
52
is connected to the output of a NAND gate
56
. The drain of driving transistor
52
is connected to the supply voltage
62
(4.40 to 5.25 volts), and the source of driving transistor
52
is connected to output
54
. A first input
56
a
of NAND gate
56
serves as the input to pull-up circuit
50
. When first input
56
a
is high, as explained below, pull-up circuit
50
causes output pad
54
to go high. A second input
56
b
of NAND gate
56
is driven by the output of a comparator
58
. A first input
58
a
of comparator
58
is connected to output pad
54
and a second input
58
b
to comparator
58
is driven by a voltage divider
60
.
When input
56
a
to NAND gate
56
is high, the output of NAND gate
56
can go low. This allows driving transistor
52
to be turned on (because the gate of driving transistor
52
is inverted) to pull-up output pad
54
to a digital high state (that is, to a voltage of from 2.8 to 3.6 volts). When input
56
a
is high, the remainder of pull-up circuit holds output pad
54
in a digital high state. Specifically, voltage divider
60
, including resistors
60
a
and
60
b,
is connected between the power supply voltage of from 4.40 to 5.25 volts and ground. Divider
60
divides this voltage down to the specified high state voltage of between 2.8 and 3.6 volts (Voh). Comparator
58
compares this voltage to the voltage on output pad
54
. If the voltage on output pad
54
is higher than Voh, then the output of comparator
58
is high. If the voltage on output pad
54
is lower than Voh, then the output of comparator
58
is low. Thus, when the voltage on output pad
54
is higher than Voh, the output of NAND gate
56
is high (because input
56
b
is inverted) and driving transistor
52
is turned off (because the gate of driving transistor
52
is inverted). When driving transistor
52
is off, the voltage at output pad
54
will drop below Voh, and the output of comparator
58
goes low to turn on driving transistor
52
. This brings the voltage at output pad
54
back up above Voh.
If a driving transistor
52
is a PMOS device, an approximate resultant voltage versus current characteristic
82
which is generated on output pad
54
is shown in
FIG. 4
which is a voltage versus current graph
80
of the output of pull-up circuit
50
. When V reaches Voh, the output of comparator
58
will go low, shutting off driving transistor
52
. This causes the voltage at output pad
54
to drop below Voh again, turning on driving transistor
52
. The resultant voltage at output pad
54
, when input
56
a
is switched high, is shown in
FIG. 5
, which is a time versus voltage graph of the voltage at output pad
54
. As shown, once the voltage reaches Voh, it is not held constant. Rather the voltage at output pad
54
oscillates about Voh with the largest excursions from Voh occurring just after input
56
a
goes high.
FIG. 6
shows the signal illustrated in
FIG. 5
superimposed on a portion of the ideal differential signal to be generated by output driver
10
shown in FIG.
2
. As shown, particularly just after a state transition occurs, the oscillations of the voltage at output pad
54
can cause the rising differential signal to drop back below a state change voltage. Specifically, under USB specifications, a signal should change from a digital low state to a digital high state between 1.3 volts and 2.0 volts. Oscillating across this voltage could trigger a “false” state change from a high state (1) to a low state (0), as shown in FIG.
6
. This could undesirably cause an error in data transmission on the USB bus.
Accordingly, improvement is needed in USB pull-up circuits. Specifically, the pull-up circuit should be able to drive the USB bus at the appropriate high state voltage without causing excessive oscillations or ringing which might generate data errors.
SUMMARY OF THE INVENTION
The present invention includes a method and apparatus for driving a USB which substantially eliminates ringing or oscillation around a target voltage. An electronic output driver in accordance with a present invention includes an output for providing an electrical signal, a first pull up circuit, a second pull up circuit, and a detection circuit in electrical communication with the first pull up circuit and the second pull up circuit. The first pull up circuit is in at least intermittent electrical communication with the output to provide a first voltage range to the output. The second pull-up circuit is also in at least intermittent electrical communication with the output and drives the output up to a predetermined voltage. The detection circuit detects the voltage of the output and selects the first pull up circuit or the second pull up circuit to drive the output. Preferably, the detection circuit selects either the first pull-up circuit or the second pull up circuit depending upon the voltage at the output.
Because the second pull up circuit drives the output up to a predetermined voltage, there is substantially no ringing or oscillati
Cunningham Terry D.
Matsushita Electrical Industrial Co. Ltd.
Morrison & Foerster / LLP
Nguyen Long
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