Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
2001-12-04
2003-01-21
Ton, My-Trang Nu (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S185000
Reexamination Certificate
active
06509764
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pre-driver circuits and particularly to turning off the upper output device in such circuits.
2. Description of Background Art
In many tri-state pre-driver circuits, back-gate current blocking diodes in the pull-up path connected to the gate of an upper output transistor may prevent the transistor from turning completely OFF, thereby increasing both the dynamic operating current (I
ccd
) and the power dissipation capacitance (C
pd
) of the device.
FIG. 1
is a schematic for a conventional pre-driver circuit
10
with In and Tri-state signal inputs and an input/output (I/O) pin. The circuit has blocking diodes
11
connected in the V
dd
line to the back-gate of the circuit's control transistors to prevent I
off
current from flowing into V
dd
when a V
dd
+v
t
voltage is applied to the input/output (I/O) pin during the tri-state condition. The pre-driver circuit has a p-channel upper output (UOP) transistor
100
and a n-channel lower output (LOP) transistor
101
with the drains being connected together through a ballast resistor
102
, and the sources being connected between V
dd
and Gnd, respectively. The circuit's input/output (I/O) pin is taken off the drain of the UOP transistor
100
. The blocking diodes
11
, shown as a pn diode
111
in parallel with a Schottky diode
110
, are connected between V
dd
and the V
ddref
source line driving the transistor's back-gates.
During normal operation of the circuit, when both the In and Tri-state signals are LOW, p-channel transistor
103
turns ON, coupling the V
dd
signal through parallel blocking diodes
110
and
111
to the gate of the upper output transistor
100
, thereby attempting to turn OFF output transistor
100
. However, particularly in high frequency switching conditions, the voltage drop across these diodes can prevent the gate of the upper output transistor from pulling all the way up to the V
dd
rail and completely turning OFF, thereby allowing an increase in the circuit's dynamic operating current (I
ccd
). This in turn effectively increases the devices power dissipation capacitance (C
pd
).
The I
ccd
current is given as
I
ccd
=(
C
pd
)*(
V
dd
)*
f
in
+(
i
ccd
—
static
),
where
f
in
=the input frequency in Hz.
In many pre-driver circuits, the back-gate blocking diodes are by-passed to increase circuit speed during normal operation of the pre-driver circuit, while reducing I
off
current and satisfying over-voltage tolerant specifications.
FIG. 2
is a schematic for a conventional circuit that by-passes the back-gate current blocking diodes. The circuit consists of a conventional pre-driver circuit
20
and a blocking diode bypass circuitry
21
. The pre-driver circuit
20
has a p-channel UOP transistor
200
and a n-channel LOP transistor
201
with drains connected together through a ballast resistor
202
and an I/O pin connected to the drain of p-channel transistor
200
.
In the blocking diode circuit
21
, transistors
211
and
214
are used to short around parallel pn/Schottky diodes
210
/
219
and provide V
dd
directly to the back-gate source line V
ddref
, while p-channel transistors
217
,
218
, and
212
serve as the critical I
off
and over-voltage tolerant circuit components. Under normal operation, transistors
212
,
217
,
218
are in an OFF condition and inverter
215
supplies a low voltage to the gate of p-channel transistor
211
through n-channel transistor
214
, thus supplying V
dd
to V
ddref
line, bypassing the current blocking diodes
210
/
219
. This provides a stable V
dd
source at the back-gate of UOP transistor
200
and the back-gate and sources of the UOP p-channel pre-driver transistors
203
and
204
, instead of V
dd
minus the voltage drop across blocking diodes
210
/
219
.
In this circuit, I
off
is the maximum leakage current into and out of the input/output transistors
200
,
201
when V
dd
=0V and the I/O pin is forced to a given DC voltage. During the I
off
condition, n-channel transistor
216
is in an OFF condition, while p-channel transistors
217
and
218
are in an ON condition, shorting the I/O pin to S
1
and S
4
nodes, respectively. When the I/O pin is ramped to a given DC voltage, the voltage at node S
1
through transistor
217
will assure that inverter
213
will pull node S
2
to ground, thereby turning OFF n-channel transistor
214
, releasing control of node S
4
to the I/O pin through p-channel transistor
218
. Notice that n-channel transistor
214
will never be in the ON condition during an I
off
condition since the V
dd
source voltage to inverter
213
is 0 volts for this condition. The DC voltage on the I/O, pin through transistor
218
, also assures that p-channel transistor
211
is in the OFF condition. Therefore, the V
ddref
line will track the given DC voltage on the I/O pin since transistors
212
,
218
, and
203
are all in the ON condition, thereby controlling the signal to the gate and back-gate of UOP p-channel transistor
200
. When the I/O node is ramped during an I
off
test, the ramped signal is fed through transistors
212
and
218
to the gate and back-gate of UOP transistor
200
, assuring that the UOP transistor
200
remains in an OFF condition, thereby reducing the I
off
current through UOP transistor
200
.
These type circuits also must protect against over-voltage applied to the I/O pin while in the tri-state mode, satisfying standard over-voltage tolerant specifications. When the voltage on the disabled I/O pin is pulled to a voltage v
t
above V
dd
, p-channel transistor
217
is in an ON condition and connects signal S
1
to the I/O voltage, assuring that n-channel transistor
214
is in an OFF condition, thus giving control of the signal S
4
to the I/O through p-channel transistor
218
. Also, the voltage on the I/O pin will assure that p-channel transistor
211
remains in an OFF condition, thus eliminating the risk of any current sinking into V
dd
through transistor
211
. Also, in the tri-state mode, when an over-voltage (a voltage v
t
above V
dd
) is applied to the disabled I/O pin, p-channel transistor
212
will be in an ON condition and the V
ddref
line will track the I/O pin voltage. Transistors
212
and
218
will supply the voltage on the I/O pin to back-gate of the UOP transistor
200
, thus clamping the back-gate and drain of the UOP transistor. Also, transistors
212
and
218
supplies the voltage on the I/O pin to the back-gate and source of p-channel transistor
204
, in the main pre-driver circuit, further assuring that the gate and drain of UOP transistor
200
does not turn ON and sink current into V
dd
.
The conventional blocking diode by-pass and over-voltage tolerant circuit approach discussed above requires approximately as many components as the pre-driver circuit being improved. What is needed is an effective solution that requires a minimum number of additional components to by-pass the blocking diodes, reduce the I
off
current, and provide over-voltage tolerant protection. The embodiment of the present invention overcomes this need by providing these functions using only three additional components and, as a result is more efficient and more cost competitive to manufacture.
SUMMARY OF THE INVENTION
In its broader aspect, the present invention discloses a pre-driver circuit, which uses three additional components to (1) bypass the back-gate current blocking diodes, (2) lower the I
off
current, and (3) provide over-voltage tolerant protection in the pre-driver circuit. The circuit accomplishes these three characteristics by adding only three additional components, which is a 65% reduction in required circuitry over conventional approaches.
The solution disclosed in the present invention is unique because it uses the pre-driver's tri-state input signal to control the pull-up path of the pre-driver circuit. This circuit by-passes the blocking diode (or diodes) found in many pre-driver circuit's pull-up path, which is connected i
Brady W. James
Nu Ton My-Trang
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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