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

3.3.1 Bandwidth

The opamp bandwidth is the first and most fundamental criterion in opamp selection because a slow opamp directly limits the rise time of the output current regardless of how well the rest of the circuit is optimized. Treating the system as a first-order approximation, the minimum required bandwidth of the opamp is given by Equation 6.

Equation 6.   B W   =   0.35 T r i s e

where:

  • Trise is the target rise time specification

However, this value represents the minimum value at which the opamp alone is fast enough and leaves no margin for the additional bandwidth limiting introduced by the rest of the signal path.

In particular, the low-pass filter formed by Rdamp and Cgate at the FET input already consumes a portion of the available bandwidth, meaning if the opamp is chosen at exactly the minimum BW, the cascaded system response falls short of the rise time target. To make sure the opamp does not become the bottleneck and that sufficient bandwidth remains after accounting for the Rdamp – Cgate pole, select an opamp with a bandwidth of 3× to 4× the calculated minimum, providing enough headroom for the overall closed-loop system to meet the rise time specification comfortably.