Design Goals

Design Description

This single-supply low-side, bidirectional current sensing solution can accurately detect load currents from –1A to 1A. The linear range of the output is from 110mV to 3.19V. Low-side current sensing keeps the common-mode voltage near ground, and is thus most useful in applications with large bus voltages.

Design Notes

1. To minimize errors, set R₃ = R₁ and R₄ = R₂.
2. Use precision resistors for higher accuracy.
3. Set output range based on linear output swing (see A_ol specification).
4. 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.

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. Since 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 can 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

Design References

Texas Instruments, Simulation for Low-Side Bidirectional Current-Sensing Circuit, circuit SPICE simulation file

Texas Instruments, ±1A Single-Supply Low-Side Current Sensing Solution, reference design

Design Featured Op Amp

Design Alternate Op Amp

For battery-operated or power-conscious designs, outside of the original design goals described earlier, where lowering total system power is desired.