Design Goals
Input |
Output |
Supply |
Full-Scale
Range Error |
IiMax |
ViMax |
VoMin |
VoMax |
Vcc |
Vee |
FSRError |
1 A |
250
mV |
100
mV |
2.25V |
3.3V |
0V |
2.09% |
Design Description
Some MSP430™ microcontrollers (MCUs) contain
configurable integrated signal chain elements such
as op-amps, DACs, and programmable gain stages.
These elements make up a peripheral called the Smart
Analog Combo (SAC). For information on the different
types of SACs and how to leverage their configurable
analog signal chain capabilities, see the MSP430 MCUs Smart Analog Combo
Training video. To get started with
your design, download the Single-Supply, Low-Side,
Unidirectional Current-Sensing Circuit Design
Files.
This
single-supply, low-side, current sensing solution
accurately detects load current up to 1 A and
converts it to a voltage between 100mV and 2.25V.
The circuit uses the MSP430FR2311 op-amp in a
noninverting amplifier configuration. There is room
for further integration by using the programmable
gain stage block within the MSP430FR2355 peripheral
which allows you to integrate the feedback resistor
ladder (R2 and R3) into the MCU. The input current
range and output voltage range can be scaled as
necessary and larger supplies can be used to
accommodate larger swings. The output of the second
stage op-amp can be sampled directly by the onboard
ADC or monitored by the onboard comparator for
further processing inside the MCU.
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.
- The
tolerance of the shunt resistor and feedback
resistors determine the gain error of the
circuit.
- Avoid placing capacitive loads directly on the
output of the amplifier to minimize stability
issues.
- 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 also
improves the stability of the circuit if
high-value resistors are used.
- If
the solution is implemented with the MSP430FR2355
SAC_L3, the op-amp can be configured in
noninverting programmable gain amplifier mode or
general-purpose mode with external R2 and R3
passives to measure the current-sense
circuit.
- If
the solution is implemented using the
MSP430FR2311, the op-amp can be realized by the
SAC_L1 op-amp or by the transimpedance amplifier
(TIA) op-amp to measure the current-sense
circuit.
- The
enhanced reference module in the MSP430FR2355 can
be used to scale the ADC using a VREF of 2.5 V to
more accurately measure the output of the current
sensing AFE.
- The
Single-Supply, Low-Side,
Unidirectional Current-Sensing Circuit Design
Files include code examples showing how
to properly initialize the SAC peripherals.
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.
Let R2 = 715 Ω (0.1% Standard Value)
Choose R3 = 5.69kΩ (0.1% Standard Value)
Note: The feedback resistor ladder (R
2 and
R
3) can be realized using the integrated programmable gain resistor
ladder of the SAC_L3 with a programmed noninverting gain of 9x. This implementation is
showcased in the
MSP430FR2355 code example. If the SAC op-amps are
being used in general purpose mode, external resistors would be used to build the
feedback resistor ladder.
- 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, ensure 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
References
- Texas Instruments, MSP430 Single-Supply, Low-Side,
Unidirectional Current-Sensing Circuit,
code examples and SPICE simulation files
- Texas Instruments, 16MHz integrated analog
microcontroller with 3.75KB FRAM, OpAmp, TIA,
comparator with DAC, 10-bit ADC,
product page
- Texas Instruments, MSP430 MCUs Smart Analog
Combo, video
Design Featured Op Amp
MSP430FRxx Smart Analog
Combo |
|
MSP430FR2311
SAC_L1 |
MSP430FR2355
SAC_L3 |
Vcc |
2.0V to 3.6V |
VCM |
-0.1V to VCC +
0.1V |
Vout |
Rail-to-rail |
Vos |
±5mV |
AOL |
100dB |
Iq |
350µA (high-speed mode) |
120µA (low-power mode) |
Ib |
50pA |
UGBW |
4MHz
(high-speed mode) |
2.8MHz
(high-speed mode) |
1.4MHz
(low-power mode) |
1MHz
(low-power mode) |
SR |
3V/µs (high-speed mode) |
1V/µs (low-power mode) |
Number of
channels |
1 |
4 |
MSP430FR2311 |
MSP430FR2355 |
Design Alternate Op Amp
MSP430FR2311 Transimpedance
Amplifier |
Vcc |
2.0V to
3.6V |
VCM |
-0.1V to
VCC/2V |
Vout |
Rail-to-rail |
Vos |
±5mV |
AOL |
100dB |
Iq |
350µA
(high-speed mode) |
120µA
(low-power mode) |
Ib |
5pA
(TSSOP-16 with OA-dedicated pin input) |
50pA
(TSSOP-20 and VQFN-16) |
UGBW |
5MHz
(high-speed mode) |
1.8MHz
(low-power mode) |
SR |
4V/µs
(high-speed mode) |
1V/µs
(low-power mode) |
Number of
channels |
1 |
MSP430FR2311 |