SBOA326 May 2021 OPA197 , OPA2991

**Design Goals**

Input | Output | Supply | |||||
---|---|---|---|---|---|---|---|

V_{iMin} | V_{iMax} | I_{iMax} | I_{oMin} | I_{oMax} | P_{R5Max} | V_{cc} | V_{ee} |

0V | 10V | 200µA | 0A | 5A | 0.25W | 36V | 0V |

**Design Description**

This high-side voltage-to-current (V-I) converter delivers a well-regulated current to a load, R_{4}. The circuit accepts an input voltage from 0V to 10V and converts it to an output current from 0A to 5A. The current is regulated by feeding the voltage across a low-side, current-sense resistor back to the op amp. The output Darlington pair allows for higher current gain than when using a single, discrete transistor.

**Design Notes**

- A resistor divider, formed by R
_{1}and R_{2}, is implemented at the input to limit the full-scale voltage at the non-inverting terminal of the amplifier and the output sense resistor (R_{5}). - The high current gain of the Darlington pair reduces the demand on the output current of the op amp.
- Smaller values of R
_{4}and R_{5}lead to an increased load compliance voltage and a reduction in power dissipated in the full-scale, output state. - Feedback components R
_{3}and C_{1}provide frequency compensation to ensure the stability of the circuit during transients. They also help reduce noise. R_{3}provides a DC feedback path directly at the current setting resistor, R_{5}, and C_{1}provides a high-frequency feedback path that bypasses the NPN pair. - The input bias current will flow through R
_{3}, which will cause a DC error. Therefore, ensure that this error is minimal compared to the offset voltage of the op amp. - Select an op amp whose linear output voltage swing includes at least 2 ✕ V
_{be}+V_{sense}. The output voltage of the op amp will be greater than the voltage at the sense resistor by approximaly double the base-to-emitter voltage, V_{be}. - Use the op amp in its linear operating region, specified under the A
_{OL}test conditions of the data sheet. - If needed, an isolation resistor may be placed between the high-frequency feedback path and the base of T1 for stability.

**Design Steps**

The transfer function of this circuit is provided in the following steps:

${I}_{o}={V}_{i}\times \frac{{R}_{2}}{{R}_{5}\times \left({R}_{1}+{R}_{2}\right)}$

- Using the specifications for the maximum output power dissipation and the maximum output current, determine the maximum value of V
_{sense}.${V}_{R5\mathrm{Max}}={V}_{\mathrm{senseMax}}=\frac{{P}_{R5\mathrm{Max}}}{{I}_{\mathrm{oMax}}}=\frac{0.25W}{5A}=50\mathrm{mV}$ - Calculate the sense resistance, R
_{5}.${R}_{5}=\frac{{V}_{\mathrm{senseMax}}}{{I}_{\mathrm{oMax}}}=\frac{\mathrm{50mV}}{\mathrm{5A}}=\mathrm{10m\Omega}$ - Select values for R
_{1}and R_{2}based on the maximum allowable input current, I_{iMax}, and the desired V_{senseMax}voltage.${R}_{1}=\frac{{V}_{\mathrm{senseMax}}}{{I}_{\mathrm{iMax}}}=\frac{\mathrm{50mV}}{\mathrm{200\mu A}}=\mathrm{250\Omega}\approx \mathrm{249\Omega}\mathrm{(Standard\; Value)}$${V}_{\mathrm{senseMax}}={V}_{\mathrm{iMax}}\times \left(\frac{{R}_{2}}{{R}_{1}+{R}_{2}}\right)$${R}_{2}=\mathrm{49.6k\Omega}\approx \mathrm{49.9k\Omega \; (Standard\; Value)}$ - See the Design References section [2] for the design procedure on how to properly
size the compensation components, R
_{3}and C_{1}.

**Design Simulations**

**DC Simulation Results**

**Loop Stability Simulation Results**

Loop gain phase is 53 degrees.

**Compliance Voltage Simulation Results**

**Design References**

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

**Design Featured Op Amp**

OPA2991 | |
---|---|

V_{ss} | 2.7V to 40V |

V_{inCM} | Rail-to-rail |

V_{out} | Rail-to-rail |

V_{os} | 125µV |

I_{q} | 560µA |

I_{b} | 10pA |

UGBW | 4.5MHz |

SR | 21V/µs |

#Channels | 1, 2, 4 |

www.ti.com/product/opa2991 |

**Design Alternate Op Amp**

OPA197 | |
---|---|

V_{ss} | 4.5V to 36V |

V_{inCM} | Rail-to-rail |

V_{out} | Rail-to-rail |

V_{os} | 25µV |

I_{q} | 1mA |

I_{b} | 5pA |

UGBW | 10MHz |

SR | 20V/µs |

#Channels | 1, 2, 4 |

www.ti.com/product/opa197 |