LM 675 Improved Howland Current Pump Stability Analysis
LM 675 Improved Howland Current Pump Stability Analysis Tim Green Senior Analog Applications Engineers Precision Analog Linear Applications July 18, 2013 1
Analysis Summary 1) Look at New Circuit and move snubber with values shown directly on output of LM 675. Snubber is for LM 675 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 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*d. I/dt. Do not command faster current than supply Limitations, LM 675 slew rate, and closed loop bandwidth will allow. If the system can demand A step input of this circuit put a slew rate limit on a previous stage. 5) New circuit compensation has Iout/Vin small signal bandwidth at about 541 Hz. 2
Original Circuit 3
Original Circuit Aol, 1/Beta, Loop Gain 4
Original Circuit 1/Beta and Aol 5
Original Circuit Loop Gain 6
Original Circuit Transient 7
Original Circuit Transient 8
New Circuit No Comp & New 1/Beta 9
New Circuit No Comp & New 1/Beta 10
New Circuit Loop Gain, Aol, 1/Beta 11
New Circuit Aol and 1/Beta 12
New Circuit Loop Gain 13
New Circuit Transient 14
New Circuit Transient 15
New Circuit AC Transfer – Iout/Vin 16
TINA Plots 17
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