1. LM675 Improved Howland Current Pump Stability Analysis
Tim Green
Senior Analog Applications Engineers
Precision Analog Linear Applications
July 18, 2013

2. Analysis Summary
1) Look at New Circuit and move snubber with values shown directly on output of LM675.
Snubber is for LM675 output stage stability when driving reactive loads and not overall
loop compensation.
2) Macromodel is for AC use only and small signal transient to check for stability.
There are no large signal parameters modeled. It is basically an Aol curve. Aol curve is based
on information extracted indirectly from data sheet since there is no Aol curve in data sheet.
3) Key to accuracy of stability analysis is based on modeling of the load.
Double check load DC resistance value with a 4-wire ohmmeter check. If an impedance
analyzer is used detailed curves of load Z might help in double check of the load model.
4) These type of V-I circuits can get in trouble if you try to out-run the laws of physics based
on the input signal. Remember V=L*dI/dt. Do not command faster current than supply
Limitations, LM675 slew rate, and closed loop bandwidth will allow. If the system can demand
A step input out of this circuit put a slew rate limit on a previous stage.
5) New circuit compensation has Iout/Vin small signal bandwidth at about 541Hz.

3. Original Circuit

4. Original Circuit Aol, 1/Beta, Loop Gain

5. Original Circuit 1/Beta and Aol

6. Original Circuit Loop Gain

7. Original Circuit Transient

8. Original Circuit Transient

9. New Circuit No Comp & New 1/Beta

10. New Circuit No Comp & New 1/Beta

11. New Circuit Loop Gain, Aol, 1/Beta

12. New Circuit Aol and 1/Beta

13. New Circuit Loop Gain

14. New Circuit Transient

15. New Circuit Transient

16. New Circuit AC Transfer – Iout/Vin

17. TINA Plots