Operational Amplifier Problem

[Matthew]

Hi everyone. I'm having a problem with an Op-Amp (OPA548) that I'm using for a power supply. The + voltage on the output is lower then expected. The – voltage on the output is spot on.

I'm using it at a 1:10 ratio. The input and output follow each other at the 1:10 ratio starting at -24v and all the way up through 0v. When the output reaches 15v it stops increasing even through the input continues on up to V+=2.75v that should give me an output the same or close to the +supply (+24V).

This is what I'm getting. I have checked my +V Supply at the pin on the IC to make sure its getting 24V and it is. What is going on?

Vout Max = 15.1V?
Vout Min = -24V
+Vin  Max = 2.75V
+Vin Min = -2.48V
+Vsupply = 24V
 -Vsupply = -24V

I hope my explained was clear enough. I'm not sure if I just don't know what I'm doing or if I have a bad IC. If anyone could help me out in getting to the bottom of this I would greatly appreciate it. 

Thanks for any help you can give me.

[ejeffrey]

Looking at the data sheet, pin 3 sets the output current limit by a programming resistor to the negative supply rail.  You have it connected it to a positive voltage, that is probably causing it to cut out.

[Zero999]

You also won't be able to get more than +22V or less than -23V out at low currents and at 3A it'll be worse, you won't be able to go above 20.3V or below -20.7V, again read the datasheet.
http://www.ti.com/lit/ds/symlink/opa548.pdf

[Matthew]

Wow…Thank you for the quick response. I appreciate it very much.

With respect to the current limit I have modeled my circuit after the one in Figure 16 (Page14) in the data sheet. In that circuit they list it as 0V – 4.75V on pin 3. I believe this is incorrect for my application because of the -24V supply. I will have to look at that in more detail. If I understand this correctly now -24V on pin 3 would give me 5A limit and -19.25v would give me 0A limit. Let me know if I’m going down the wrong road.
For experimentation purposes I just connected pin 3 directly to –V supply (-24V) and got the same low output voltage results on the output. I may have a hardware problem because I’m getting some erratic results. I might just make a new board with what I know now and see if the problem goes away.

I expected to only get around 20V @ 1.5A. I was trying to keep the option of increasing the supply voltage to +-30V at a later time. All I have right now is a 24v supply for this.
 
Any other thoughts are greatly appreciated. Thanks,

[amspire]

You are definitely on the right track with the -24V to -19.25V on the current control pin.

I would just test to see if the current control is still working correctly.

Put something like a 220K resistor between pin 3 and -24v.

Check that the voltage across the resistor is about 4.5V.

Then put a load on the output and check it limits at around 260mA. If the current control in the chip is not working right, it could also effect the maximum voltage the chip could produce - the chip may be going into current limit mode at 15V output.

[Matthew]

I have a quick question about testing the current limit.
I built the Dave J. dummy load a long time ago. If your familiar with it can that be used to test -voltage current loads?

[amspire]

Yes, as long as you connect the + terminal of the dummy load to GND and the - end to the 1 voltage output.

On a supply that can do both positive and negative voltages, I would put a diode in series with the dummy load so that reverse voltages cannot get to the dummy load. I think Dave used a MOSFET as his output device which would mean under reverse voltage, it will act like a forward based diode in series with 1 ohm. The MOSFET would survive, but the op-amp may not survive a negative voltage from the 1 ohm resistor under reverse voltage conditions. A series diode will protect the dummy load.

Better still, if you have a bridge rectifier lying around, connect the + terminal of the dummy load to the + on the bridge rectifier, the dummy load negative terminal to the - on the rectifier, and then attached the AC pins on the rectifier to the power supply output.

This would mean that the dummy load will work for both positive and negative voltages from the power supply, except for a few volts either side of 0v.