OPA549: Amplifier, Current 1A, inductive load 350uH, frequency 50kHz to 100Hkz

Hello Pablo,

50 to 100 kHz may be a bit high in frequency for the OPA549. The OPEN-LOOP GAIN AND PHASE vs FREQUENCY graph on datasheet Pg. 4 indicates that the OPA549 only has about 40 dB of open-loop gain in that frequency range which will affect dc accuracy. Also, its slewing rate of 9 V/us will much limit the possible output voltage swing in that frequency range as the MAXIMUM OUTPUT VOLTAGE SWING vs FREQUENCY graph on Pg. 6. Therefore, the OPA549 may not be the best choice just from a frequency standpoint. The OPA564 has much wider bandwidth, but it may be thermally limited in the application.

Regarding the heating issue; that is an indicator of high power dissipation. The current may be higher in the circuit than you are expecting. One common cause of excessive power dissipation is oscillation so check the op amp output with a DSO. Also, make sure that the OPA549 thermal requirements outlined in the datasheet have been followed.

Some voltage-to-current (V-to-I) converters such as the Improved Howland Current Pump are especially sensitive to inductive loads. The combination of negative and positive feedback, and an inductive load can lead to a condition where the phase margin degrades to zero and oscillation takes place. Stabilizing that combination can prove difficult.

Regards, Thomas

Precision Amplifiers Applications

Hello Thomas,

Thank you for your answer.

About the OPA564, if I have a current of 0.5A, with my inductive load (350uH at 80kHz is 176Ω), I am gonna need 90V. With the slew rate of the OPA564, there won't be a problem?

Regards,

Pablo

Hi Pablo,

I am not sure what current source circuit you are planning on using, but I did some simulations using the OPA549 in an Improved Howland Current Pump circuit assuming you may be using it.

I found that when the load was purely resistive the output is close to what thoery would predict, but the accuracy transconductance gain accuracy falls off as the OPA549 open-loop gain drops with increased frequency. Also, there is evidence of increased cross-over distortion with the increased frequency. Nothing too suprising about that result.

A concern I have had about driving a purely inductive load is that voltage and current through it should split as theory tells us; the voltage should lead the current by 90 degrees. This phase difference would be altered from 90 degrees by any resistance associated with the inductor. When the load was changed to a purely inductive 350 uH load the simulation results indicated that the normal Howland current source operation was not being attained.

The OPA549 bandwidth limitations are such a big issue at 50 and 100 kHz, some results are shown for 10 kHz in the image below. The output voltage and current are split by about 97 degrees, the output current is too high (>1 Amp), and the output swing voltage is hitting or exceeding the supply rails. Clearly, the OPA549 based Howland Current Pump is not happy driving the 350 uH inductive load.

I expect there may be similar issues with other op amp based current source designs. They too would need to be evaluated for suitability. 

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

About the OPA549, what is the max frequency of work without problem with a resistive load?

The coil load has 4 ohms(resistive part) +350uH(inductive part). If the current sources are a problem. Maybe I need to do a voltage source. Has TI an amplifier to work with a voltage of 150 V max, current 1A max? These values are the max but if there is a choice near to the specifications, at least the amplifier would be work with a current of 0.5 A and a frequency of 80kHz max.

Regards,

Pablo

Hi Pablo,

TI doesn't produce a power op amp that can be applied in a 150 V, 1 Amp application. When the power op amp application requirements exceed what we can meet with our TI power op amps, we often suggest contacting Apex Microtechnology. They produce lines of very high power op amps that cover high voltage and high current. Their High Current Selector Matrix shows the range of current and voltage their products cover. TI is not assciated with Apex Microtechnology in any manner.

Regards, Thomas

Precision Amplifiers Applications Engineering