TINA/Spice/OPA541: Use as a constant current supply

Hello Halo,

The full load current flows through the OPA541 RCL resistor and at high current levels (Amperes) its power dissipation will be Watts. For example, the OPA541 datasheet section 8.1.1 (Eq. 1) provides the RCL equation for the OPA541AP package device:

RCL (Ohms) = (0.813/ |ILIM|) - 0.02

for a current limit of 7 Amps,

RCL (Ohms) = (0.813 / |7|) - 0.02 =  96.1 milliohms

the power dissipation is,

P_D = I² R_CL  = (7²)(96.1∙10^-3) = 4.7 Watts

Therefore, if the load current is continuous at 7 amps the power dissipation is nearly 5 Watts. Certainly, there are 5 Watt power resistors available that can handle this power dissipation, but the key is to identify ones that have a low resistance temperature coefficient, RTC. If the RTC is low, tens of ppm/°C, then the resistance should change only a little as the resistor heats. Nearly all resistors have their RTC specified on the manufacturer's product datasheet. Employing a higher wattage resistor for RCL should help alleviate some of the high heating and should exhibit less resistance change compared to a lower wattage resistor of the same type.

An Improved Howland Current Pump is another approach for a power current source. The OPA541 should work fine in the circuit if it is properly applied. We would need to see your schematic and the load conditions to make sure the OPA541 operating constraints are being observed.

Regards, Thomas

Precision Amplifiers Applications Engineering

Thanks for your reply, Thomas.

Please see attached docx file (or diagram below?) for Howland schematic. Thanks, harry.

OPA541-Howland.docx

Hello Harry,

I evaluated your OPA541 improved Howland Current Pump circuit. All the component values you selected look okay, and with them it will take about  +/- 8.4 V dc input to support a load of +/-7 Amps. You can see my TINA simulation below and the resulting voltages and current levels. Do note that the same issue regarding the RCL power dissipation will be had with this circuit as well. Make sure that your input drive level is sufficient to result in a peak current of 7 Amps.

Note that the OPA541 is rated for a maximum continuous output curren tof 5 Amps, and can produce more current if the duty cycle is reduced. Be sure that you are observing the thermal requirments dictated by the Safe Operating Area curves shown on OPA541 datasheet page 10. You may want to consider the OPA549 can supply +/- 8 Amps.

www.ti.com/.../OPA549

If you are driving the OPA541 with and ac signal and the output load is complex containing both resistance and reactance the load current may be different than what you expect.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

Again, thanks for your help. Two points:

1. I notice your OPA541 in the schematic has a Vcl (sense) pin whereas the TINA spice model doesn't. Have you amended the SPICE model? Can I get a copy of it for my TINA library?

2. Your schematic includes both Rcl and Rs of 100 mOhm and R6 of zero ohms. I would have thought only one of Rcl and Rs was necessary, say Rs, with a connection from VLoad to the sense pin, and set R6 to 100 mOhm. Can you explain why your circuit is different from mine? Does mine not work as it is?

Note: The current of 7 Amps is ramped up from a value of less than 1 Amp and only needs to be held for about 30 seconds before dropping down again.

Regards,

Harry.