Design Goals

Design Description

This single–supply, high–side, low–cost current sensing method detects load current between 50mA and 1A and converters it to an output voltage from 0.25V to 5V. High–side sensing allows for the system to identify ground shorts and does not create a ground disturbance on the load.

Design Notes

1. DC common mode rejection ratio (CMRR) performance is dependent on the matching of the gain setting resistors, R₂-R₅.
2. Increasing the shunt resistor increases power dissipation.
3. Verify that the common–mode voltage is within the linear input operating region of the amplifier. The common mode voltage is set by the resistor divider formed by R₂, R₃, and the bus voltage. Depending on the common–mode voltage determined by the resistor divider a rail–to–rail input (RRI) amplifier may not be required for this application.
4. An op amp that does not have a common-mode voltage range that extends to V_cc may be used in low–gain or an attenuating configuration.
5. A capacitor placed in parallel with the feedback resistor limits bandwidth, improve stability, and help reduce noise.
6. Use the op amp in a linear output operating region. Linear output swing is usually specified under the A_OL test conditions.

Design Steps

1. The full transfer function of the circuit is provided below.
   $$\begin{gathered} {\mathrm{V}}_{\mathrm{o}}={\mathrm{I}}_{\mathrm{in}}\times {\mathrm{R}}_1\times \frac{{\mathrm{R}}_5}{{\mathrm{R}}_4} \\ \mathrm{Given} {\mathrm{R}}_2={\mathrm{R}}_4 \mathrm{and} {\mathrm{R}}_3={\mathrm{R}}_5 \end{gathered}$$
2. Calculate the maximum shunt resistance. Set the maximum voltage across the shunt to 100mV.
   ${\mathrm{R}}_1=\frac{{\mathrm{V}}_{\mathrm{iMax}}}{{\mathrm{I}}_{\mathrm{iMax}}}=\frac{100\mathrm{mV}}{1\mathrm{A}}=100\mathrm{m}\mathrm{\Omega }$
3. Calculate the gain to set the maximum output swing range.
   $\mathrm{Gain}=\frac{{\mathrm{V}}_{\mathrm{oMax}}-{\mathrm{V}}_{\mathrm{oMin}}}{{(\mathrm{I}}_{\mathrm{iMax}}-{\mathrm{I}}_{\mathrm{iMin}})\times {\mathrm{R}}_1}=\frac{\text{5}\mathrm{V}-\text{0.25}\mathrm{V}}{(\text{1}\mathrm{A}-\text{0.05}\mathrm{A})\times \text{100}\mathrm{m}\mathrm{\Omega }}=50\frac{\mathrm{V}}{\mathrm{V}}$
4. Calculate the gain setting resistors to set the gain calculated in step 3.
   $$\begin{gathered} \mathrm{Choose} {\mathrm{R}}_2={\mathrm{R}}_4=1.01\mathrm{k}\Omega (\mathrm{Standard} \mathrm{value}) \\ {\mathrm{R}}_3={\mathrm{R}}_5={\mathrm{R}}_2\times \mathrm{Gain}=1.01\mathrm{k}\Omega \times 50\frac{\mathrm{V}}{\mathrm{V}}=50.5\mathrm{k}\Omega (\mathrm{Standard} \mathrm{value}) \end{gathered}$$
5. Calculate the common–mode voltage of the amplifier to establish linear operation.
   ${\mathrm{V}}_{\mathrm{cm}}={\mathrm{V}}_{\mathrm{CC}}\times \frac{{\mathrm{R}}_3}{{\mathrm{R}}_2+{\mathrm{R}}_3}=36\mathrm{V}\times \frac{\text{50.5}\mathrm{k}}{\text{1.01}\mathrm{k}+\text{50.5}\mathrm{k}}=35.294\mathrm{V}$
6. The upper cutoff frequency (f_H) is set by the non–inverting gain (noise gain) of the circuit and the gain bandwidth (GBW) of the op amp.
   ${\mathrm{f}}_{\mathrm{H}}=\frac{\text{GBW }}{\text{Noise Gain}}=\frac{10\mathrm{MHz}}{51\frac{\mathrm{V}}{\mathrm{V}}}=196.1 \mathrm{kHz}$

Design Simulations

DC Simulation Results

AC Simulation Results

References:

Texas Instruments, SBOMAV4 simulation file, software support

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