SNOAA73 May   2021 LMP2012QML-SP , LMP7704-SP , TPS7A4501-SP


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Design Notes

  1. Use low-tolerance resistors to achieve high current gain matching accuracy.
  2. All resistors and capacitors must be verified space-grade for this design.
  3. In this design, Iret is a local ground, separate from the supply ground, and must be allowed to float.
  4. The current through R3 can be considered a quiescent current. That is, it is a current that should be minimized so as to not use up the current budget. However, as I3 gets smaller, the matching ratio R3 / R4gets larger, so in practice I3 cannot be made arbitrarily small.
  5. For linear operation, the ratio of the current through R3 when Vin = Vin_max and Vin = Vin_min, I3_max / I3_min, must be equal to the ratio of the maximum output current to the minimum output current, Iout_max / Iout-min.
  6. This discrete 2-wire current transmitter solution allows the flexibility to set the output current to be different than the standard 4–20 mA or 4–24 mA. This may be necessary because options for space-grade components are relatively limited. For example, if the total quiescent current for the current transmitter plus surrounding peripherals on the same supply loop cannot meet the requirement that they sum to less than Iout_min (= 4 mA for this design), the output current range may be shifted to 8–24 mA or something similar.
  7. C1 is a bypass compensation capacitor and is required for the stability of the circuit. Recommended capacitor is 100 nF or greater.
  8. R5 and C2 are additional components for compensating the circuit. They are not required for stability, but are added for flexibility if additional compensation is required.
  9. C3 is a required output capacitor for the TPS7A4501-SP. Recommended capacitor is 1 µF to 10 µF for stability.
  10. R6 linearizes the relationship between the op-amp output and the current through the transistor and limits the current through the transistor.
  11. This design can be implemented with a single LMP2012QML-SP or a similar device. See Design Alternate Op Amp for an alternative device.