STDA041 October   2026 OPA690

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Design Summary
  5. 2Theory of Operation
    1. 2.1 Circuit Overview
    2. 2.2 Closed-loop Operation
    3. 2.3 FET Operation Region
  6. 3Design Procedure
    1. 3.1 Choosing FET
      1. 3.1.1 Ciss
      2. 3.1.2 Compliance Voltage Range
      3. 3.1.3 Safe Operating Area (SOA)
    2. 3.2 Rsense Selection
      1. 3.2.1 Effect of ESL
    3. 3.3 Opamp Selection
      1. 3.3.1 Bandwidth
      2. 3.3.2 Slew Rate
      3. 3.3.3 Opamp Output Current
  7. 4Design Based Component Selection
    1. 4.1 Design for Trise = 100ns
      1. 4.1.1 FET Selection
      2. 4.1.2 Rsense Selection
      3. 4.1.3 Opamp Selection
        1. 4.1.3.1 Bandwidth
        2. 4.1.3.2 Slew Rate
        3. 4.1.3.3 Output Current
    2. 4.2 Architecture Selection (Quick Reference)
  8. 5Circuit Architecture Selection
    1. 5.1 Single Feedback Method
    2. 5.2 Dual Feedback Method
    3. 5.3 Enhanced Dual Feedback Method
  9. 6Results
  10. 7Related Documentation

5.3 Enhanced Dual Feedback Method

RFX is an additional resistor added in series with CF in the dual-feedback path as shown in Figure 5-3. The main purpose of RFX is to increase the high-frequency noise gain. This configuration allows the system to remain stable with a smaller CF, while also providing faster feedback response and quicker settling for input and load transients. The trade-off is increased output noise at high frequencies.

RFX also provides an optional enhancement to the dual-feedback circuit. Setting RFX to 0Ω reverts the circuit to the standard Rdamp dual-feedback topology, which provides good stability. RF cannot be zero in the Rdamp dual-feedback topology because RFX serves as the fundamental DC feedback resistor that senses the output voltage at low frequencies. Without RFX the circuit loses the DC feedback path completely, and the opamp output becomes uncontrolled. Therefore, RF serves as a performance optimization rather than a necessity.

OPA690 VCCS with Enhanced Dual Feedback Figure 5-3 VCCS with Enhanced Dual Feedback