OPA548: OPA548 voltage output drop

Hi,

I attach the my target schematics. I test with this also but i have the same problem with the output voltage when i set the output to 4.2V, attach the 2.7R/2W resistor on the output and the output voltage drop to 3V and the max current that is draining is 1.38A.

BR,

Vlatko

Hello Vlatko,

I have studied your OPA548 application circuit. The supply voltage appears to be on the low. In one place of your explanation you mention "The input voltage level that is power the OPA548 is 10V" and on the schematic it shows "+U = 8 to 12 V." So I am not sure what supply voltage level you are using, except that it is certainly to the low end of the OPA548 supply range. Too low supply voltage could be a limiting factor with regard to the OPA548's output capabilities.

The ability of the OPA548 to supply an output current level within the specified range relies upon the op amp remaining within its linear input and output operating ranges. The OPA548 datasheet includes Figure 13, a graph of Output Voltage Swing vs Output Current. I include the graph below. Do note that it is for +/-30 V supplies so the actual results with a +8 to +12 V single supply could be different.

It may very well be that if you are attempting to source 3 Amperes from the OPA548 powered by a low voltage supply, the minimum |VSUPPLY - VOUT| requirement indicated by the graph isn't being met. It appears from the graph that for the +/-30 V condition that VSUPPLY must be at least +3.8 V above VOUT to assure the 3 Ampere output current. The datasheet on Pg. 15 states in section 9.2.1.2.1, Power Supply Requirements, "Select the power supply based on the requirement to achieve a ±10-V output with up to a 2.5-A load. The maximum value for output voltage swing at 3-A is approximately within 4 V of either rail, standard 15-V power supplies rated at >2.5 A each will suffice." And as mentioned, the graph for a low voltage, single supply condition could be somewhat different than that with higher voltage, dual supplies.

Does your application the flexibility to increase VSUPPLY a few volts higher? Doing so, you could test to see if the supply voltage is the issue.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

Thanks fro your explanation. In the schematics i state that the input voltage level for powering the OPA548 is 8V to 10V but i did the tests with 10V. I do not have limitation to power the OPA548 with higher input voltage and now i will do the tests with higher input voltage. I will come back with the results soon as i get the results.

BR,

Vlatko

Hi Thomas,

I did testing with 3 different input voltage levels:

1. Input voltage = 15V

   Output voltage (lo load) = 4,2V

   Output voltage (load 2,7Ohm) = 3,00V

   Output current = 1,45A

2. Input voltage = 20V

   Output voltage (lo load) = 4,2V

   Output voltage (load 2,7Ohm) = 3,07V

   Output current = 1,45A

   

3. Input voltage = 25V

   Output voltage (lo load) = 4,2V

   Output voltage (load 2,7Ohm) = 3,07V

   Output current = 1,45A

Result looks like the output voltage still drop to the same value and the current is the same no mater of the input voltage.

BR,

Vlatko

  

Hi Thomas,

After i remove the resistor 9K1 which was part of the devider for seting the current limit and left only 12K resistor connected from pin 3 to ground i had posibility to drain the current up to 2,25A on the real target device. The power supply for the OPA548 was 15V. I need to do more tests with the target device since this battery simulator is part of the test equipment, than i can come back with more results before we close this ticket. This looks promising for me.

BR,
Vlatko

Hello Vlatko,

I suppose I should have initially checked your OPA548 current limit circuitry more closely. But now, after seeing your test results it is apparent that the current limit was set to 1.45 Amps.

If you need any more help with the OPA548 do let us know. If you think you are on the right track now, please go ahead and close this e2e inquiry.

Regards, Thomas
Precision Amplifiers Applications Engineering

Hi Thomas,

After more tests with the real target test device i notice that current solution is on the edge. Looks that the target device while booting in some step require more then 2,25A and my baterry simulator does not provide and probably output voltage drop to the 3V and then device is rebooted. 

I need to change this resistor (12K) for current limiting to be able to get limitation to the 4A. Can you please help me to calculate correct value of the resistor? This resistor is connected form pin 3 on OPA548 to the ground. In paralel i have 100nF capacitor also. 

BR,

Vlatko

Hello Vlatko,

Applying Eq. 1 in OPA548 datasheet section 8.3.1, I calculate an RCL value of 4.06 kilohm for a 4 Ampere current limit. This is backed up by the value listed in datasheet Fig. 41, Adjustable Current Limit table that lists a 4.02 kilohm (standard 1 % resistor).

Do note the OPA548 current limit function does have a temperature coefficient. Device heating can influence the actual current limit value. See datasheet Figure 3. Current Limit vs Temperature, for the typical behavior.

Regards, Thomas

Precision Amplifiers Applications Engineering