The Improved Howland Current Pump By Tim Green

The Improved Howland Current Pump By Tim Green – PA- Linear Applications Engineering Manager

V-I Improved Howland Current Pump Given: Design Goal: PPSL (Preferred Parts Selection List) Parts: Vcc =+5 V OPA 569 Power Op Amp Vin = 0 V to 500 m. V Iout = 0 to 500 m. A Rload = 8 ohms, Grounded Load 1% FSR accuracy in transfer function (Iout/Vin) Design Considerations: Need to Sense current in High Side Need to drive high current -> Power Op Amp? Need RRO Op Amp, Input CM to Ground Design Preliminary Analysis: If there is voltage headroom then: Power Op Amp offers single IC solution Low drop across sense resistor -> maximize headroom & minimize power dissipation

V-I Improved Howland Current Pump Ideal Op Amp RT 0 can be used to increase the effective output impedance of the Current Source BUT care must be taken to always ensure more feedback to the –input of the op amp than to the +input of the op amp as net positive feedback = INSTABILITY !!

V-I Improved Howland Current Pump Set RX=RF and RZ=RI and move all tolerances to worst case for accuracy analysis 1% Resistors (w/RT=0) could yield 9% Accuracy at T=25°C 0. 1% Resistors (w/RT=0) could yield 0. 9% Accuracy at T=25°C Still useful for V-I control in Motors, Valves, TECs, LEDs, etc Other system feedback re-adjusts VM or VP for final results

V-I Improved Howland Current Pump Set RX=RF and RZ=RI Input Voltage is Gained up and Differentially forced across RS

V-I Improved Howland Current Pump: Vin = 500 m. V, Iout = 500 m. A Always Check Loop Gain & Compensate for Stability

V-I Improved Howland Current Pump: Vin = 0 V, Iout = 0 A Always Check Loop Gain & Compensate for Stability

V-I Improved Howland Current Pump : Vin = 0 V, Iout = 0 A (modified) Add Ros to force Vm > 2 m. V (worst case Vos) to drive Voa as low as possible when Vin = 0 V Always Check Loop Gain & Compensate for Stability