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 method 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.
Design Notes
- Use the op amp
linear output operating range, which is usually specified
under the test conditions.
- The common–mode
voltage is equal to the input voltage.
- Tolerance of the
shunt resistor and feedback resistors determines the gain error of the circuit.
- Avoid placing
capacitive loads directly on the output of the amplifier to
minimize stability issues.
- 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. See Single-Supply, Low-Side, Unidirectional
Current-Sensing Solution With Output Swing to GND
Circuit analog engineer's circuit for
more information.
- Using high–value
resistors can degrade the phase margin of the circuit and
introduce additional noise in the circuit.
- The small–signal
bandwidth of this circuit depends on the gain of the circuit
and gain bandwidth product (GBP) of the amplifier.
- Filtering can be
accomplished by adding a capacitor in parallel with R3 . Adding a capacitor in
parallel with R3 improves stability of the circuit if high–value resistors are
used.
- For more
information on op amp linear operating region, stability,
capacitive load drive, driving ADCs, and bandwidth please
see the Design References section.
Design Steps
The transfer function for
this circuit is given below.
- Define the full–scale shunt voltage and calculate the
maximum shunt resistance.
- Calculate the gain required for maximum linear
output voltage.
- 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)
- Calculate minimum
input current before hitting output swing–to–rail limit.
IiMin represents the minimum accurately
detectable input current.
- Calculate Full
scale range error and relative error. Vos is the
typical offset voltage found in data sheet.
- To maintain
sufficient phase margin, verify that the zero created by the gain setting resistors and
input capacitance of the device is greater than the bandwidth of the circuit
Design Simulations
DC Simulation
Results
AC Simulation
Results
References
Texas Instruments,
Simulation for Single-Supply
Low-Side Unidirectional Current-Sense
Circuit, SBOC523 SPICE simulation file
Texas Instruments, 0-1A, Single-Supply, Low-Side,
Current Sensing Solution, TIPD129
reference design
Texas Instruments, Current sensing reference design
for 10µA to 10mA, low side, single
supply, TIPD104 reference design
Texas Instruments, Single-Supply, Low-Side,
Unidirectional Current-Sensing Solution With Output
Swing to GND Circuit, analog engineer's
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 |
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 |
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 |
LPV821 |