Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
2002-05-08
2003-05-06
Riley, Shawn (Department: 2838)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S543000, C327S538000
Reexamination Certificate
active
06559684
ABSTRACT:
FIELD OF INVENTION
The present invention generally relates to current sensing, and more particularly, to a system and method for current sensing which is substantially consistent over device, temperature, and process variations.
BACKGROUND OF THE INVENTION
Current measurement techniques in electronic devices provide a variety of functions: fault protection, current control, switching, and/or the like. Various types and configurations of current sensors have been developed for current measurement. In such current sensors, it is desirable to restrict variations in device, temperature, process, and aging characteristics of circuit components and parasitic elements. Despite efforts to restrict, variations in such characteristics remain a problem. Further, correcting inaccuracies in sensing the current waveform can require additional printed circuit board (PCB) area and additional components, e.g., discrete resistors, which can increase cost and decrease system efficiency.
With reference to
FIG. 1A
, a switch mode circuit
100
is illustrated for providing a current to a load device. Circuit
100
includes a pair of switches Q
1
and Q
2
comprising a p-channel FET (Field Effect Transistor) device and an n-channel FET device, respectively, although both switches Q
1
and Q
2
could comprise n-channel FET devices as well. The gates of switches Q
1
and Q
2
are connected to an FET drive circuit
102
, while the drains of switches Q
1
and Q
2
are coupled to a load
104
through various elements, including an inductor L
1
and a capacitor C
1
. FET drive circuit
102
is configured with switches Q
1
and Q
2
to alternately couple inductor L
1
to supply voltage VCC and ground. For example, as illustrated with reference to
FIG. 1B
, as switch Q
1
is turned “on”, during a time T
ON
, the voltage at a node V
SW
is high, i.e., connected to supply voltage V
CC
, causing the current I
L1
to ramp upwards within inductor L
1
. In that the voltage V across an inductor is the product of the inductance L times the change in current over time di/dt, the rate of change of current for inductor L
1
can be derived in equations (1) and (2) below:
V
=
L
·
ⅆ
i
ⅆ
t
(
1
)
ⅆ
i
ⅆ
t
=
(
Vin
-
Vout
)
/
L
(
2
)
During a time T
OFF
, switch Q
2
will turn on, the voltage at a node V
SW
will go low, resulting in a pulsed waveform switching between approximately zero volts and VCC, and current I
L1
will ramp downward through inductor L
1
, at a rate set forth in equation (3) below:
ⅆ
i
ⅆ
t
=
Vout
/
L
(
3
)
Currently, it is difficult to accurately sense this inductor current I
L1
without dissipating significant power. Moreover, variations in discrete circuit elements negatively impact accurate current sensing, e.g., variations in temperature, process, and/or the like will have varying effects on the differing discrete circuit elements, which will produce inaccuracy in current sensing. In addition, other current sensing techniques commonly used, such as Rds(ON) sensing techniques of a power device, can have reduced accuracy. For example, FETs can have variations in resistance of about 40% in such Rds(ON) sensing techniques, which decreases the accuracy of current sensing. Other techniques include the placing of an RC network across the inductor, in which the effective series inductance (ESL) of the inductor is used to select the resistance used in the RC network. Unfortunately, the ESL has initial and temperature variations, which lead to inaccuracies in current sensing. Other techniques may utilize a sense resistor for current sensing, but which exhibit power losses, e.g., I
2
R losses.
Many current sensor applications include the use of current mirror circuits configured with operational amplifiers to provide a sensed current based on a load current. For example, current mirror circuits have included a first resistor of known resistance placed in series with a load device, and have measured the voltage drop across the resistor through use of the operational amplifier and a second resistor to calculate the current passing through the load device, e.g., by knowing the drop across the first resistor and forcing that voltage at both the inverting and non-inverting inputs of the amplifier, a sensed current of known proportion to the load current can be provided at the output of the amplifier. In addition to the difficulty and high costs in fabricating resistors within integrated circuit devices, matching a resistor to the power device is difficult to achieve due to variations in process.
With reference to
FIG. 2
, another prior art application including a current sensor
200
is illustrated. Current sensor
200
is configured as a basic current mirror circuit as described above, with the first and second resistors being replaced with p-channel transistors Q
P
and Q
R
configured to operate as variable resistive devices. For example, in that MOSFET devices can operate as linear resistors whose value can be controlled and/or varied by overdrive voltage when operating within the triode region, such as is disclosed by Behzad Razavi, DESIGN OF ANALOG CMOS INTEGRATED CIRCUITS (McGraw-Hill 2001), current sensor
200
can use the variable resistor devices Q
P
and Q
R
with an amplifier
202
to mirror a load current I
LOAD
, and thus provide an output current I
SENSE
representing a known proportion of load current I
LOAD
. In particular, current sensor
200
can sense the voltage at node V
1
, and by forcing the voltage at node V
2
to equal the voltage at node V
1
through amplifier
202
, generate a scaled current through a follower transistor Q
F
, to provide sensed current I
SENSE
.
However, current sense circuit
200
has various operational deficiencies. For example, current mirror circuit
200
generally requires a high speed amplifier for operational amplifier
202
to force the voltage at node V
2
to equal the voltage at node V
1
, which can be difficult to implement in processes optimized for high voltage power devices, i.e., it is difficult to use high voltage devices in a high speed amplifier. Moreover, during switch mode applications, the voltage at node V
1
will equal the voltage at node V
SW
, including having a pulsed waveform between V
CC
and ground. Thus, when the voltage at node V
SW
is low, and thus low at node V
1
, amplifier
202
will attempt to slew sufficient current to pull the voltage at node V
2
to ground, which can amount to a significant amount of current being pulled from current sense circuit
200
.
Accordingly, a system and method for more accurate current sensing over device, temperature, and process variations are desired, particularly for applications sensing high di/dt currents.
SUMMARY OF THE INVENTION
The present invention includes a system and method for current sensing which is substantially consistent over device, temperature, and process variations. In accordance with one aspect of the present invention, a current sensing system and method are configured to provide a scaled down sense current of a known proportion to a load current being sensed. In an exemplary embodiment of the present invention, an exemplary current sensing system includes a first switch coupled to one or more variable resistive elements, wherein the resistive elements are configured to facilitate scaling of the current output from the first switch. A second switch is coupled to the first switch, wherein the size of the second switch is suitably scaled down relative to the size of the first switch. In addition, both switches can be fabricated using substantially the same process. The resistive elements are configured to scale down the voltage across the first switch, with the scaled down voltage being provided to an input of an amplifier. The amplifier is coupled with the resistive elements and the second switch, and is configured to sense the scaled down voltage across the first switch, and force the voltage across the second switch to be equal to the scaled down voltage across the first switch, such that a current of known proportion to the load current can be p
Goodfellow Ryan
Susak David
Primarion, Inc.
Riley Shawn
Snell & Wilmer
LandOfFree
System and method for current sensing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System and method for current sensing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System and method for current sensing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3055628