Texas Instruments | Single-supply, low-side, unidirectional current-sensing circuit | Application notes | Texas Instruments Single-supply, low-side, unidirectional current-sensing circuit Application notes

Texas Instruments Single-supply, low-side, unidirectional current-sensing circuit Application notes
Analog Engineer's Circuit: Amplifiers
SBOA285 – December 2018
Single-supply, low-side, unidirectional current-sensing
circuit
Design Goals
Input
Output
Supply
Full–Scale Range Error
IiMax
ViMax
VoMin
VoMax
Vcc
Vee
FSRError
1A
250mV
50mV
4.9V
5V
0V
0.2%
Design Description
This single–supply, low–side, current sensing solution accurately detects load current up to 1A and
converts it to a voltage between 50mV and 4.9V. The input current range and output voltage range can be
scaled as necessary and larger supplies can be used to accommodate larger swings.
R2 715
R3 13.3k
VCC
Ii
Vbus 5
+
R1 250m
U1 TLV9062
+
VCC
Vi
Vo
R4 10k
Design Notes
1. Use the op amp linear output operating range, which is usually specified under the test conditions.
2. The common–mode voltage is equal to the input voltage.
3. Tolerance of the shunt resistor and feedback resistors will determine the gain error of the circuit.
4. Avoid placing capacitive loads directly on the output of the amplifier to minimize stability issues.
5. If trying to detect zero current with output swing to GND, a negative charge pump (such as LM7705)
can be used as the negative supply in this design to maintain linearity for output signals near 0V. [5]
6. Using high–value resistors can degrade the phase margin of the circuit and introduce additional noise
in the circuit.
7. The small–signal bandwidth of this circuit depends on the gain of the circuit and gain bandwidth
product (GBP) of the amplifier.
8. Filtering can be accomplished by adding a capacitor in parallel with R3 . Adding a capacitor in parallel
with R3 will also improve stability of the circuit if high–value resistors are used.
9. For more information on op amp linear operating region, stability, capacitive load drive, driving ADCs,
and bandwidth please see the Design References section.
SBOA285 – December 2018
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Single-supply, low-side, unidirectional current-sensing circuit
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Design Steps
The transfer function for this circuit is given below.
1. Define the full–scale shunt voltage and calculate the maximum shunt resistance.
2. Calculate the gain required for maximum linear output voltage.
3. Select standard values for R2 and R3.
From Analog Engineer’s calculator, use “Find Amplifier Gain” and get resistor values by inputting
gain ratio of 19.6.
R2 = 715 Ω (0.1% Standard Value)
R3 = 13.3 kΩ (0.1% Standard Value)
4. Calculate minimum input current before hitting output swing–to–rail limit. IiMin represents the minimum
accurately detectable input current.
5. Calculate Full scale range error and relative error. Vos is the typical offset voltage found in datasheet.
6. To maintain sufficient phase margin, ensure that the zero created by the gain setting resistors and
input capacitance of the device is greater than the bandwidth of the circuit
2
Single-supply, low-side, unidirectional current-sensing circuit
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SBOA285 – December 2018
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Design Simulations
DC Simulation Results
T
5
4
Ii = 1A
Vo = 4.91V
Ii = 500mA
Vo = 2.46V
Output
3
2
Ii = 10.2m
Vo = 55.4m
1
0
250m
0
500m
Input current (A)
750m
1
AC Simulation Results
30
Vo
Vo
IiIi
-3dB frequency = 500kHz
Gain (dB)
20
10
0
10
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100
1k
10k
Frequency (Hz)
100k
1MEG
10MEG
Single-supply, low-side, unidirectional current-sensing circuit
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References:
1. Analog Engineer's Circuit Cookbooks
2. SPICE Simulation File SBOC523
3. TI Precision Designs TIPD129, TIPD104
4. TI Precision Labs
5. Single-Supply, Low-Side, Unidirectional Current-Sensing Solution with Output Swing to GND Circuit
Design Featured Op Amp
TLV9061
Vss
1.8V to 5.5V
VinCM
Rail–to–rail
Vout
Rail–to–rail
Vos
0.3mV
Iq
538µA
Ib
0.5pA
UGBW
10MHz
SR
6.5V/µs
#Channels
1,2,4
www.ti.com/product/tlv9061
Design Alternate Op Amp
OPA375
Vcc
2.25V to 5.5V
VinCM
(V–) to ((V+)–1.2V)
Vout
Rail–to–rail
Vos
0.15mV
Iq
890µA
Ib
10pA
UGBW
10MHz
SR
4.75V/µs
#Channels
1
www.ti.com/product/OPA375
For battery operated or power conscious designs, outside of the original design goals described earlier,
where lowering total system power is desired.
LPV821
Vcc
1.7V to 3.6V
VinCM
Rail–to–rail
Vout
Rail–to–rail
Vos
1.5µV
Iq
650nA/Ch
Ib
7pA
UGBW
8kHz
SR
3.3V/ms
#Channels
1
www.ti.com/product/LPV821
4
Single-supply, low-side, unidirectional current-sensing circuit
Copyright © 2018, Texas Instruments Incorporated
SBOA285 – December 2018
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