Design Goals

Design Description

This circuit controls the slew rate of an analog gain stage. This circuit is intended for symmetrical slew rate applications. The desired slew rate must be slower than that of the op amp chosen to implement the slew rate limiter.

Design Notes

1. The gain stage op-amp and slew rate limiting op amp should both be checked for stability.
2. Verify that the current demands for charging or discharging C₁ plus any load current out of U₂ will not limit the voltage swing of U₂.

Design Steps

1. Set slew rate and choose a standard value for the feedback capacitor, C₁.
   ${\mathrm{C}}_1=470\mathrm{nF}$
   $\mathrm{SR}=20\frac{\mathrm{V}}{\mathrm{s}}$
2. Choose the value of R₂ to set the capacitor current necessary for the desired slew rate.
   $\mathrm{SR}=\frac{{\mathrm{I}}_{{\mathrm{C}}_1}}{{\mathrm{C}}_1}$
   $20\frac{\mathrm{V}}{\mathrm{s}}=\frac{{\mathrm{I}}_{{\mathrm{C}}_1}}{470\mathrm{nF}}\text{ where }{\mathrm{I}}_{{\mathrm{C}}_1}=9.4\mathrm{\mu A}$
   $\text{ Gain stage op amp }{\mathrm{V}}_{\mathrm{sat}}=\pm 14.995\text{ (typical)}$
   ${\mathrm{I}}_{{\mathrm{C}}_1}=\frac{{\mathrm{V}}_{\mathrm{sat}}}{{\mathrm{R}}_2}$
   $9.4\mathrm{\mu A}=\frac{14.995\mathrm{V}}{{\mathrm{R}}_2}\text{, so }{\mathrm{R}}_2=1.595\mathrm{M\Omega }\approx 1.6\mathrm{M\Omega }\text{ (Standard Value)}$
3. Compensate feedback network for stability. R₁ adds a pole to the 1/β network. This pole should be placed so that the 1/β curve levels off a decade before it intersects the open loop gain curve (200 Hz, for this example).
   ${\mathrm{f}}_{\mathrm{p}}=\frac{1}{2\mathrm{\pi }\times {\mathrm{R}}_1\times {\mathrm{C}}_1}=200\mathrm{Hz}$
   $200\mathrm{Hz}=\frac{1}{2\mathrm{\pi }\times {\mathrm{R}}_1\times 470\mathrm{nF}}\text{, so }{\mathrm{R}}_1=1.693\mathrm{k\Omega }\approx 1.69\mathrm{k\Omega }\text{ (Standard Value)}$

Design Simulations

Transient Simulation Results

AC Simulation Results

Design References

Texas Instruments, Simulation for Slew Rate Limiter, circuit SPICE simulation file

Texas Instruments, Single Op-Amp Slew Rate Limiter, reference design

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