Design Goals

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 configurable analog signal chain capabilities, visit MSP430 MCUs Smart Analog Combo Training. To get started with your design, download the Low-side Bidirectional Current Sensing Design Files.

This single-supply low-side, bidirectional current sensing method accurately detects load currents from –1A to 1A. The linear range of the output is from 100mV to 3.2V. Low-side current sensing keeps the common-mode voltage near ground, and is thus most useful in applications with large bus voltages. This design leverages two of the four integrated op-amp blocks (SACs) in the MSP430FR2355 MCU. One SAC_L3 peripheral is configured as a general purpose op-amp to amplify the voltage across the shunt resistor, while the other is configured as a buffer to provide the bias voltage (Vref). The latter SAC_L3 block can also be configured in DAC buffer mode to provide Vref, replacing the external voltage divider circuit. The output of the circuit can be internally or externally connected to other integrated peripherals in the MSP430FR2355 MCU. For example, the analog-to-digital converter (ADC) window comparator can sample this output periodically (with no CPU intervention) and trigger an interrupt when the signal crosses a threshold.

Design Notes

• To minimize errors, set R₃ = R₁ and R₄ = R₂.
• Use precision resistors for higher accuracy.
• Set output range based on linear output swing (see A_ol specification).
• Do not use low-side sensing in applications where the system load cannot withstand small ground disturbances or in applications that need to detect load shorts.
• In the schematic above, the first SAC_L3 peripheral in the MSP430FR2355 MCU (U1) is configured in general purpose mode. The second SAC_L3 peripheral (U2) is also configured in general purpose mode, but with an external voltage divider.
• Use the DAC buffer configuration for U2 (as seen in the code examples in the Low-side Bidirectional Current Sensing Design Files) to provide Vref instead of using the external voltage divider circuit.
• This option can also be implemented using the MSP430FR2311 device by using the internal transimpedance amplifier for U1, and the SAC_L1 op-amp for U2.
• The Low-side Bidirectional Current Sensing Design Files include code examples showing how to properly initialize the SAC peripherals.

Design Steps

1. Determine the transfer equation given R₄ = R₂ and R₁ = R₃.
   ${\mathrm{V}}_{\mathrm{o}}=\left({\mathrm{I}}_{\mathrm{i}}\times {\mathrm{R}}_{\mathrm{shunt}}\times \frac{{\mathrm{R}}_4}{{\mathrm{R}}_3}\right)+{\mathrm{V}}_{\mathrm{ref}}$
   ${\mathrm{V}}_{\mathrm{ref}}={\mathrm{V}}_{\mathrm{cc}}\times \left(\frac{{\mathrm{R}}_6}{{\mathrm{R}}_5+{\mathrm{R}}_6}\right)$
2. Determine the maximum shunt resistance.
   ${\mathrm{R}}_{\mathrm{shunt}}=\frac{{\mathrm{V}}_{\mathrm{shunt}}}{{\mathrm{I}}_{\mathrm{imax}}}=\frac{100\mathrm{mV}}{1 \mathrm{A}}=100\mathrm{m\Omega }$
3. Set reference voltage. Because the input current range is symmetric, set the reference to mid supply. Therefore, make R₅ and R₆ equal.
   ${\mathrm{R}}_5={\mathrm{R}}_6=10\mathrm{k\Omega }$
4. Set the difference amplifier gain based on the op amp output swing. The op amp output is able to swing from 100mV to 3.2V, given a 3.3-V supply.
   $\mathrm{Gain}=\frac{{\mathrm{V}}_{\mathrm{oMax}}-{\mathrm{V}}_{\mathrm{oMin}}}{{\mathrm{R}}_{\mathrm{shunt}}\times \left({\mathrm{I}}_{\mathrm{iMax}}-{\mathrm{I}}_{\mathrm{iMin}}\right)}=\frac{3.2\mathrm{V}-100\mathrm{mV}}{100\mathrm{m\Omega }\times \left(1 \mathrm{A}-\left(-1 \mathrm{A}\right)\right)}=15.5\frac{\mathrm{V}}{\mathrm{V}}$
   $\mathrm{Gain}=\frac{{\mathrm{R}}_4}{{\mathrm{R}}_3}=15.5\frac{\mathrm{V}}{\mathrm{V}}$

Design Simulations

DC Simulation Results

Closed Loop AC Simulation Results

Transient Simulation Results

Target Applications

• Motor Drives
• Servo Drive Functional Safety Module
• Battery Charger
• Battery Pack: Cordless Power Tool
• HEV/EV battery-management system (BMS)

Design References

1. Texas Instruments, MSP430 Low-side Bidirectional Current Sensing Circuit, code examples and SPICE simulation file
2. Texas Instruments, 16MHz integrated analog microcontroller with 3.75-KB FRAM, Op Amp, TIA, comparator with DAC, 10-bit ADC, product page
3. Texas Instruments, MSP430 MCUs Smart Analog Combo Training, video

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