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. Low-side sensing should not be used 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.
   1. Since the input current range is symmetric, the reference should be set 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

See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.

See circuit SPICE simulation file SBOC500.

See TIPD175, www.ti.com/tipd175.

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