50mA-20A, Single-Supply, Low-Side or High

Ed Mullins
TI Precision Designs: Reference Design
50 mA-20 A, Single-Supply, Low-Side or High-Side,
Current Sensing Solution
TI Precision Designs
Circuit Description
TI Precision Designs are analog solutions created by
TI’s analog experts. Reference Designs offer the
theory, component selection, and simulation of useful
circuits. Circuit modifications that help to meet
alternate design goals are also discussed.
This single-supply, low-side or high-side, current
sensing solution can accurately detect load currents
from 50 mA to 20 A. The linear range of the output is
from 0 V to 5 V. A unique yet simple gain switching
network is implemented in order to accurately
measure the load current across this wide dynamic
range.
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High-Side
Current
Sensing
Low-Side
Current
Sensing
INA300
ALERT
RSHUNT
Bus
Voltage
INA225
OUTPUT
Load
Bus
Voltage
INA300
ALERT
RSHUNT
Load
INA225
OUTPUT
An IMPORTANT NOTICE at the end of this TI reference design addresses authorized use, intellectual property matters and
other important disclaimers and information.
TINA-TI is a trademark of Texas Instruments
WEBENCH is a registered trademark of Texas Instruments
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50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution 1
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1
Design Summary
The design requirements are as follows:
•
Bus Voltage: 0 to 36V
•
Load Current: 50mA to 20A
•
Operate from a single 5V power supply
•
Maximum Shunt Voltage: ≤200mV
Two unique current sensing devices are selected to implement this design. The INA225 is a precision
current shunt monitor with pin selectable gain. The INA300 is a current sensing comparator which can be
used to switch the gain of the INA225 to increase the dynamic range of the design. The design goals and
simulated performance are summarized in Table 1. Figure 1 depicts the simulated transfer function of the
design.
Table 1: Comparison of Design Goals and Simulated Performance
Goal
Simulated
Minimum Detectable Current
50mA
25mA
Maximum Detectable Current
20A
20A
Figure 1: Simulated Transfer Function
2 50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution
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2
2.1
Theory of Operation
Low-Side or High-Side Current Sensing
Accurately measuring current is required in systems when any of the following system attributes must be
considered by a designer:
•
Circuit Protection
•
Fault Detection
•
Power Efficiency and Control
•
Product Safety
The most commonly encountered method to measure current is to place a small resistor in series with the
load and measuring the voltage drop developed across the resistor. Commonly such a resistor is referred
to as a “shunt resistor” or RSHUNT. Figure 2 illustrates this concept.
Low-side
sensing
High-side
sensing
RSHUNT
Load Current
Bus Voltage
RSHUNT
Bus Voltage
+
VSHUNT
-
+
VSHUNT
Load Current
Figure 2: Current Sensing using a Shunt Resistor
There are two circuits illustrated in Figure (2). The circuit on the left hand side illustrates a circuit wherein
RSHUNT is connected between the load and ground. The type of connection is referred to as “low-side
sensing”. The circuit on the right hand side illustrates a circuit wherein RSHUNT is connected between the
bus voltage and the load. This type of connection is referred to as “high-side sensing”. In either circuit
configuration the current can be measured accurately, but there are important distinctions that a system
designer must consider when deciding which configuration will work best in a given application. Table (2)
shows a comparison between each configuration in terms of their unique features.
Table 2: Comparison of Features
Can be used to measure current
accurately
Low-side Sensing
High-side sensing
Yes
Yes
Can detect load shorts
No
Yes
Requires high common mode
rejection amplifier
Yes/No
Yes
Poor grounding can result in
measurement error
Yes
No
Low cost
Yes
Yes
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50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution 3
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2.1.1
Fault Detection
The ability to detect a short from the load to ground is an important differentiation between low-side and
high-side sensing. When using low-side sensing should a short occur between the system load and
ground the current that flows from the bus voltage source to ground (via the short) and will not flow through
the shunt resistor and therefore the fault will not be detected. This short circuit could be low-level or
moderate leakage or it could a hard failure representing some catastrophic event. In a high-side sensing
configuration, should a similar short or failure occur, the current passes through the shunt resistor and is
easily detectable. Figure (3) illustrates this concept. For circuits where system protection or user
protection is required high-side sensing is recommended.
High-side
sensing
Low-side
sensing
System
Load
Bus Voltage
short
+
VSHUNT
-
RSHUNT
RSHUNT
Bus Voltage
+
VSHUNT
-
System
Load
short
Figure 3: Fault Sensing using High-side Current Sensing
2.1.2
Common Mode Rejection Ratio
Common mode rejection ratio describes an amplifiers ability to reject signals that are common to both
inputs. Common mode rejection is therefore an important amplifier parameter when attempting to
measure small differential voltages in the presence of large common mode voltages. The voltage
developed across the shunt resistor provides the differential input to an amplifier and it is this voltage that
is intended to be accurately measured. When configured in a low-side current sensing solution the
common mode voltage applied to the amplifier is essentially zero and in many cases can be ignored. This
allows the designer to choose an amplifier that may not have very high common mode rejection ratio, and
thus in some ways allows for a simpler design with fewer error sources to consider. When striving for
maximum performance, an analysis which includes the effects of the amplifier common mode rejection
(even in low-side configurations) is recommended. When configured in a high-side current sensing
solution the common mode voltage is equal to the bus voltage. This requires that the bus voltage be
rejected by the amplifier and therefore always requires an amplifier with high common mode rejection.
4 50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution
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2.1.3
Low-side Sensing and Poor Grounding
Mentioned in Table (2), some low-side sensing configurations are susceptible to poor PCB layout, wiring
issues and grounding issues. Figure (4) illustrates a simple low-side current sense circuit using only an
operational amplifier to monitor the voltage developed across a shunt resistor. This circuit has a singleended input and is susceptible to PCB trace and wiring resistance, ground bounce and noisy grounds.
Notice that any resistance in series with the current shunt will create a voltage drop due to the load current
flowing. Commonly this additional resistance in series with the shunt resistor is due to PCB trace
resistance and or wiring resistance. This additional voltage drop across the undesired PCB trace or wiring
resistance will add to the input signal developed only across the shunt resistor and introduce an error.
This type of error is most significant at high currents and will vary widely over temperature due to the
temperature coefficient of the resistance of the copper PCB trace, copper wiring or poor grounding.
Load Current
Bus Voltage
+
RSHUNT
Parasitic resistance due to
PCB trace, wiring or poor
grounding
Output
+
Error voltage
-
Figure 4: Errors Associated with Single-ended, Low-side Sensing
The solution to these problems is quite simple. Rather than using the single-ended type of amplifier circuit
shown in Figure (4), use instead a current shunt monitor with differential inputs and rely upon the common
mode rejection ratio to eliminate the errors associated with the PCB trace or wiring resistance and ground
noise. Figure (5) illustrates this concept.
TIDU447- September 2014-[Keywords]-[Category]
50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution 5
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Load Current
Bus Voltage
+
RSHUNT
Parasitic resistance due to
PCB trace, wiring or poor
grounding
Output
+
Error voltage is now rejected by the common mode
rejection of the differential amplifier
-
Figure 5: Errors are Eliminated by the Current Shunt Monitor in a Low-side Sensing Solution
2.2
Choosing the Shunt Resistor
The motivation to use a current shunt monitor for sensing the voltage drop across a shunt resistor is based
upon the desire for accuracy and low system cost. The typical current shunt monitor will have low input
offset voltage, low input offset voltage drift with temperature and high common mode rejection ratio.
These error sources are most significant at low load currents (low input voltages). At high currents the
gain error of the amplifier and shunt resistor tolerance also contributes to errors in the measurement.
Minimizing these errors allows the use of increasingly smaller shunt resistors, thus reducing power loss in
the shunt resistor as well as the size and cost of the shunt resistor. The desired current range for this
application is given as 50 mA to 20A. This is a very wide range and consideration to the minimum current
value and maximum current value is required.
2.2.1
Error Analysis at the Minimum Current Value
Consider a current shunt monitor that has an initial input offset voltage maximum of ±150µV and a
minimum common mode rejection of 95dB. For either low-side or high-side sensing we must analyze the
effect of these two error sources upon the ability to monitor small levels of shunt current. Before a proper
worst case error analysis can be performed the details and conditions surrounding the device
specifications of offset and common mode rejection must be taken into account. Such conditions are listed
in the current shunt monitor device specification. Of particular interest to note are the conditions under
which input offset voltage and common mode rejection are given in the data sheet. For this example it will
be given that the conditions under which the current shunt monitor is specified are:
VS = 5 V
Bus Voltage = 12 V
These two conditions are important as any deviation from them in the actual current sensing solution must
be considered and properly calculated. For the following examples it is assumed that in the application the
VS is 5 V. This will minimize errors due to power supply voltage effects from the power supply rejection of
the amplifier as the amplifier used in this design has its offset voltage specified at a VS equal to 5V.
6 50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution
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2.2.1.1
Low-side and High-side Sensing Minimum Current Error Analysis
In a low-side sensing configuration the common mode voltage at the current shunt monitor input is zero.
But since the current shunt monitor is specified with a Bus Voltage of 12 V, this difference between device
specification conditions and circuit application conditions must be considered. In this case there will be an
effective -12 V common mode voltage “seen” by the current shunt monitor. To understand the impact from
this -12 V common mode voltage a calculation of the input referred error must be made. Equation (1)
illustrates this calculation.
Input error dueto common mode effects =
ApplicationCMV − Specified CMV
CMRR
(1)
Where:
ApplicationCMV is the common mode voltage in the application…for low-side sensing this is zero, for high-side
sensing this is equal to the Bus Voltage
SpecifiedCMV is the common mode voltage at which the devices offset volatge is specified (refer to the device data
sheet)
CMRR is the minimum common mode rejection ratio given in the current shunt monitor device specification
Example:
Input error dueto common mode=
effects
0V − 12V
−12V
= = 213µV
95dB
56, 234
(2)
This error due to the common mode effect is an input referred voltage error, much like the initial device input
offset voltage. Combining the two error terms (common mode error and initial input offset) is required to
determine the total error from the current shunt monitor. While it may be tempting to linearly add the two error
terms together to determine a worst case value, in reality these two errors are uncorrelated to one another and
therefore can be added as the square root of the sum of the squares. Equation (3) illustrates this combination of
error terms.
Total input referred=
error
Vosi 2 + CME 2
(3)
Where:
Vosi is the maximum initial input referred offset given in the current shunt monitor specification
CME is the input referred error due to common mode effects as determined previously
Example:
Total input referred error =
150µV 2 + 213µV 2 = 261µV
(4)
This total input referred error is the uncertainty of the input voltage under zero input conditions. With the desire to
monitor load currents as small as 50 mA a minimum value of shunt resistor can be determined by Equation (5):
Total input referred error
Minimum value of shunt resistor =
Minimumload
current
(5)
261µV
Minimum value of shunt resistor
=
= 5.2mΩ
50mA
(6)
Example:
TIDU447- September 2014-[Keywords]-[Category]
50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution 7
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What the example illustrates is that if a 5.2mΩ shunt resistor is selected an uncertainty of up to ±50mA will be
present even when the load current is zero. Another way to think about this is to recognize that for the values
used in the example, having an uncertainty of ±50mA will result in a 100% total measurement error for an ideal
input current of 50 mA. Increasing the shunt resistor value will reduce the amount of uncertainty, and therefore
result in reduced errors. Increasing the shunt resistor too much will result in overvoltage at the amplifier output as
well and lead to increased power dissipation when the load current is at a maximum. Therefore the shunt resistor
must lie within a range. Analysis for the case of high-side sensing is performed the same as for low-side sensing
referenced above.
2.2.1.2
Low-side and high-side Sensing Maximum Current Analysis
The maximum load current to be sensed should correspond to the maximum output voltage from the
current shunt monitor. A current shunt monitor amplifier is configured in either a fixed gain or selectable
gain. Choosing a device with a gain = 25V/V and a 5 V maximum output will require a maximum input
voltage developed across the shunt resistor at the full load current of 200 mV. Determining the required
shunt resistor value is performed by dividing the maximum shunt voltage required by the maximum load
current.
Example:
MaximumOutputVoltage
Maximum value of shunt resistor
=
Gain
=
Maximum Load Current
5V
25V /=
V 10mΩ
20 A
(7)
Equation (7) defines the maximum value for the shunt resistor, beyond which saturation of the output
voltage would occur. A range now has been established for the shunt resistor value bounded on the low
end by the desired minimum detectable current with 100% error and on the high end by the maximum
output voltage swing. Please note that the maximum output voltage swing is a function of power supply
voltage and the amplifier output swing specification in the device datasheet.
5.2mΩ ≤ RSHUNT ≤ 10mΩ
(8)
The power dissipation from the load current flowing through the shunt resistor is determined as shown in
Equation (9):
Maximum power dissipated
by shunt resistor Maximum Load Current 2 ×Shunt Resistor
=
(9)
Example:
Maximum power dissipated by shunt resistor
= 20 A2 10
× m
=
Ω 4W
(10)
It may be tempting to reduce the shunt resistor value from the maximum of 10 mΩ in an effort to reduce
power dissipated, and this certainly can be performed, but it will come at the expense of the low current
accuracy.
8 50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution
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Example:
Reducing the shunt resistor value from 10 mΩ to 1 mΩ will reduce the power dissipation by the same ratio,
to 0.4 W. This will certainly allow for a smaller shunt resistor; however the accuracy on the low end of the
load current range will be degraded. The following example illustrates the amount of error in the current
measurement as a function of reducing the shunt resistor.
Example:
Total input referred error 261µV
Minimum current tobe detected
=
= = 261mA
Shut 1
Resistor
mΩ
(11)
Notice that this is also a factor of 10 times larger than the previously calculated value. There is always a
tradeoff to be made in terms of low current accuracy and power dissipation at full scale. Selecting a
current shunt monitor with minimal errors will allow for the most effective tradeoff, and therefore the most
optimum choice of shunt resistor size and cost. All of the above analysis and examples are applicable to
both low-side sensing and high-side sensing configurations.
3
Component Selection
The most critical passive component with regards to performance and accuracy is the shunt resistor. Factors
such as initial tolerance, temperature coefficient, power rating, size and cost are all important parameters and
must be taken under consideration when choosing the shunt resistor. In this design a CSSH2728 (Stackpole
Electronics, Inc.), 10mΩ, 4W, 0.5% resistor is assumed. This resistor offers high initial accuracy, adequate power
handling capability, a small surface mount footprint, and low temperature drift (15ppm/C). Lower performance
options within this same resistor family are also available at the time of this publication which allows an additional
flexibility between performance and price.
3.1
Amplifier
Two amplifiers are used in this design.
The INA225 is a high-side or low-side current shunt monitor with pin selectable gain.
The INA300 is a high-side or low-side current shunt comparator with open drain output. The open drain
output is used to control the gain selection pins on the INA225.
Because both amplifiers can be used over a common mode voltage range of 0 V to 36 V a high-side or
low-side configuration can be utilized with similar performance. Basic circuit operation is described for the
low-side configuration shown in Figure (6).
TIDU447- September 2014-[Keywords]-[Category]
50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution 9
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5V RPULL-UP
INA300
+
ALERT
Load
LIMIT
-
Bus
Voltage
ALERT
RLIMIT
RSHUNT
5V
INA225
+
GS0
GS1
OUT
-
OUTPUT
REF
Figure 6: Basic Operation of the Low-side Sensing Solution
10 50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution
TIDU447- September 2014-[Keywords]-[Category]
Copyright © 2013, Texas Instruments Incorporated
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Both inputs from the INA225 and INA300 are used to sense directly across the shunt resistor. The basic
operation is described as the INA225 is configured in its maximum gain (200V/V) for small values of load current
and when the current becomes large enough the INA225 gain is switched to its lowest available setting (25V/V).
This gain switching is achieved by connecting the alert output (open drain) of the INA300 to both GS0 and GS1
(gain selection pins) on the INA225. At very small currents the INA300 output is high and thus drives the INA225
gain select pins high, resulting in a gain of 200V/V for the INA225. When the load current increases beyond a
threshold level, the alert output of the INA300 pulls the gain select pins of the INA225 low, resulting in a gain of
25V/V, thereby extending the range of current detection.
Setting the threshold at which the INA300 will trip is determined by the voltage at the LIMIT pin of the INA300.
This voltage can either be applied directly from a DAC output or it can be created by placing the appropriate sized
resistor from the LIMIT pin to ground. Placing a resistor between the LIMIT pin and ground creates a voltage
(VLIMIT) given by Equation (12):
V=
20 µA × RLIMIT
LIMIT
(12)
There is a one to one relationship between VLIMIT and the threshold voltage at which the INA300 will trip, as such:
VTHRESHOLD = VLIMIT
(13)
The value at which the INA300 trips (and switches the gain of the INA225) should occur before the output of the
INA225 reaches its maximum limit. When powered from a 5V power supply, the maximum value of the output
voltage of the INA225 is given in the data sheet as 4.8V. Using a 5 V power supply the desired switch point
would occur before the INA225 output reaches 4.8 V. Considering the condition where the input current is small
and INA225 gain is therefore large (200V/V) requires the input threshold voltage to occur at a maximum value of
4.8V divided by the gain of the INA225. This results in a maximum threshold voltage of 24mV. Choosing a limit
resistor value of 1.13kΩ results in the limit voltage of 22.6mV. Using a 10mΩ shunt resistor results in a threshold
load current value of 2.26A. What this results in is when the load current is less than 2.26A the INA225 is
configured in a gain of 200 V/V and when the load current exceeds 2.26 A the INA225 is configured in a gain of
25 V/V.
4
Simulation
Simulation models for both the INA225 and INA300 are available at www.ti.com. These models can be
used to simulate the operation of the described circuit.
4.1
Transfer Function
Figure (7) illustrates the transfer functions for both the Alert output from the INA300 and the analog output
from the INA225 as a function of load current. The INA300 trip point has been set at 2.26 A as previously
described.
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50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution 11
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Figure 7: Transfer Function
4.2
The Impact of the Hysteresis setting on the INA300
The INA300 is a current shunt comparator that has pin selectable hysteresis. Hysteresis is required to
prevent multiple or false comparator changes of state due to the presence of noise in the system. As an
example of how the amount of hysteresis relates to the transfer function please refer to Figure (8). Notice
that as the load current starts to increase from some very small value (below the trip point of 2.26 A) the
output voltage increases with a gain of 200 V/V until the threshold voltage is reached. This corresponds to
an input referred trip point of 22.6 mV as previously described. The INA225 gain is then reduced to 25 V/V
and the output continues to increase with increasing load current. As the load current begins to decrease
it will eventually decrease to the value of the trip point, however due to the amount of hysteresis set by the
INA300 the trip point occurs at a slightly lower load current level. In the simulation case used to create
Figure (8), the INA300 was configured to have 4 mV of hysteresis. This was achieved by connecting the
Hysteresis pin on the INA300 to ground.
Since the hysteresis refers to the amount of change of threshold voltage at the INA300 input, this
hysteresis value can be converted to the amount of hysteresis in terms of load current by dividing by the
value of the shunt resistor value.
LOAD CURRENTHYSTERESIS =
12 50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution
HysteresisINA300
RSHUNT
(14)
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Figure 8: Hysteresis
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50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution 13
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4.3
The Complete Circuit
5V
RPULL-UP
C1
INA300
Load
Leave open for 10µS delay
ENABLE
+
-
DELAY
ALERT
ALERT
LIMIT
LATCH
HYS
Bus
Voltage
RLIMIT
5V
RSHUNT
C2
+
GS0
GS1
OUT
INA225
-
OUTPUT
REF
Figure 9: Complete Circuit for Low-side Sensing
14 50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution
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5V
RPULL-UP
C1
INA300
Leave open for 10µS delay
ENABLE
+
-
DELAY
ALERT
ALERT
LIMIT
LATCH
HYS
Bus
Voltage
RLIMIT
5V
RSHUNT
C2
Load
+
GS0
GS1
OUT
INA225
-
OUTPUT
REF
Figure 10: Complete Circuit for High-side Sensing
5
Measuring “Zero” Load Current
Other considerations which can impact performance not discussed in the above analysis are the finite
output voltage swing to ground limitations of the INA225. For example when there is zero load current the
output of the INA225 cannot swing all the way to ground if it is configured in a single supply configuration
shown in either Figure (9) or Figure (10). Please refer to the limitations described in the INA225
specification. As an example, if the load current is zero and all other error sources are also zero, the
output may only swing as low as 50 mV due to the output swing to ground limitation. When in a gain of
200 V/V it would appear as if there was in input voltage given by Equation (15):
Output 50
swing
mV
= = 250 µV
Gain
200V / V
(15)
This 250µV of input error can be related to an error in load current given by Equation (16):
Load current=
error
TIDU447- September 2014-[Keywords]-[Category]
input 250
error
µV
= = 25mA
RSHUNT
10mΩ
(16)
50mA-20A, Single-Supply, Low-Side or High-Side Current Sensing Solution 15
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If measuring zero load current is desired it will be required to add a low output impedance voltage input to
the INA225 REF pin. Please refer to the INA225 data sheet for additional information. Other sources
sources of error can arise from specific PCB layout issues that alter the value of the shunt resistance or
issues that can arise from self-heating of the shunt resistor if exposed to prolonged periods of excessive
power dissipation.
6
Maximum Output Voltage Limitations and Power Supply Tolerance
The output voltage of the INA225 is specified to swing to within 200 mV of the power supply voltage. This
implies that with a power supply voltage of 5 V, the maximum output that the INA225 can achieve is 4.8V.
Assuming a 20 A load current and INA225 gain of 25 V/V the maximum value for the shunt resistor must
be reduced to 9.6 mΩ. Further impacting the maximum value for the INA225 output is the tolerance for the
power supply voltage. For example if the power supply voltage is specified as 5 V with a 5% tolerance, the
minimum supply voltage is then 4.75V…this in turn will limit the maximum output voltage of the INA225 to
4.55 V. Reducing the shunt resistor to a maximum value of 9.1 mΩ would be required to ensure a full
scale input of 20 A can be accommodated at the INA225 output. The alternative is to leave the shunt
resistor at the 10 mΩ value and have a maximum measureable load current, under the assumption of a 5%
power supply tolerance, of 18.2A
7
Verification
This design was constructed and verified using the INA225EVM combined with the INA300EVM.
Figure 11: INA225 and INA300 Evaluation Modules
8
About the Author
Ed Mullins has more than 20 years of experience with Texas Instruments with a background in Analog IC
Design and Applications Engineering.
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