Design Goals

Design Description

This single-supply, low-side, current sensing solution accurately detects load current between 0A to 1A and converts it to a voltage between 0V to 4.9V. The input current range and output voltage range can be scaled as necessary and larger supplies can be used to accommodate larger swings. A negative charge pump (such as the LM7705) is used as the negative supply in this design to maintain linearity for output signals near 0V.

Design Notes

1. Use precision resistors to minimize gain error.
2. For light load accuracy, the negative supply should extend slightly below ground.
3. A capacitor placed in parallel with the feedback resistor will limit bandwidth and help reduce noise.

Design Steps

1. Determine the transfer function.
   $V_o=I_i\times R_1\times \left(1+\frac{R_3}{R_2}\right)$
2. Define the full-scale shunt voltage and shunt resistance.
   $V_{\mathrm{iMax}}=100\mathrm{mV}\text{ at } I_{\mathrm{iMax}}=1A$
   $R_1=\frac{V_{\mathrm{iMax}}}{I_{\mathrm{iMax}}}=\frac{100\mathrm{mV}}{1 A}=100\mathrm{m\Omega }$
3. Select gain resistors to set the output range.
   $V_{\mathrm{iMax}}=100\mathrm{mV} \text{and }{V}_{\mathrm{oMax}}=4.9V$
   $\mathrm{Gain}=\frac{V_{\mathrm{oMax}}}{V_{\mathrm{iMax}}}=\frac{4.9V}{100\mathrm{mV}}=49\frac{V}{V}$
   $\mathrm{Gain}=1+\frac{R_3}{R_2}=49\frac{V}{V}$
4. Select a standard value for R₂ and R₃.
   $R_2=7.5\mathrm{k\Omega }\text{ (0.05\% Standard Value)}$
   $R_3=48\times R_2=360\mathrm{k\Omega }\text{ (0.05\% Standard Value)}$

Design Simulations

DC Simulation Results

AC Simulation Results

Design References

Texas Instruments, Simulation for Unidirectional Current Sense with Output Swing to GND, circuit SPICE simulation file

Texas Instruments, 0-1A, Single-Supply, Low-Side, Current Sensing Solution, reference design

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