• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 48 subscribers
  • Views 707 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

LM675: Supply voltage and output swing clarifications

LM675: Output short circuit protection issues

Nikhil Kulkarni87
Prodigy 40 points

Part Number: LM675
Other Parts Discussed in Thread: TPS55340, OPA548

Hi Experts,

I have built and tested a circuit with LM675 to generate voltage pulses by using an adder circuit.  The pedestal level is 24V and above that pulses go about 6V. The circuit is working successfully. The nominal load is 0.5A.

Now, I wanted to add output short circuit protection to the LM675 incase the output PULS_OUT+ gets shorted to ground and current tries to go above 0.5A. Hence, I added a circuit as shown page 1 of attached circuit. Page 2 shows the 24V and from that 34V output generated using TPS55340.

When I tested the circuit by shorting the output to PULS_OUT+ to ground, the load current becomes zero and the 34V drops to around 8V. However, even after removing the short circuit, the 34V remains around 8V. If I switch OFF the circuit and switch ON, it works as normal. My intention is that in ON condition, if a short circuit occurs, the output path should become open and upon removal or short circuit, the circuit should return to its normal working.

Any suggestion what would be missing in this peice of circuit and how to overcome that?LM375 Short Circuit Protection.pdf

  • 0
    Guru 140200 points
    Hi Nikhil,

    I would strongly recommend to use a modern power OPAmp with a built-in adjustable current limit. Here you will find suited parts:

    www.ti.com/.../products.html

    Kai
  • 0
    TI__Guru* 93105 points

    Hello Nikhil,

    If the Q1, Q2 external current limit circuit removed does the LM675 output short-circuit resume the expected output voltage once the short is removed? The Q1, Q2 looks much like a latch circuit (SCR) having positive feedback. Once a latch is triggered, the power must be removed for them to turn off. the 8 V may be the result of the external circuit latching on.

    By the way, the LM675 is only internally compensated for closed loop gains of 10 V/V, and greater. Your circuit has a closed loop gain of +3 V/V.

    Kai mentions employing a more contemporary power op amps that has adjustable current limit provisions. One example is the OPA548. You wouldn't need to add external current limiting circuitry, and it is unity gain stable. Here's where you can find the datasheet for the OPA548. You can compare its built-in protection functions to that of the LM675:

    http://www.ti.com/lit/ds/symlink/opa548.pdf

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points
    Thanks Kai & Thomas for your suggestions.

    Dear Thomas, if output is short circuit without Q1, Q1 circuit, my PCB resets, since due to 34V output short circuit, 24V power supply (SMPS) also draws more current and it goes into shut down.
    I have gone through the datasheet of OPA548. Looks like it may solve my problem. I have ordered samples of OPA548. Let me check the circuit with OPA548. And thanks for bringing the gain issue to my notice.
    I appreciate your expert guidance in this matter.

    Regards
    Nikhil
  • 0 in reply to Nikhil Kulkarni87
    Guru 140200 points
    Hi Nikhil,

    I had suspected that the SMPS goes into shutdown and wanted to mention this in my reply. But when I saw the circuit I thought that it would profit from a better power OPAmp which can provide a much more accurate current limit, anyway.

    Kai
  • 0 in reply to Nikhil Kulkarni87
    TI__Guru* 93105 points

    Hi Nikhil,

    I think the OPA548 will be easier to apply in your application. The adjustable current limit is easy to use, and there is also the built-in thermal shutdown as added protection.

    If you think you have enough information to go on for now, please close this e2e inquiry. If you find you need assistance with the OPA548, then a new e2e inquiry can be opened.

    Regards, Thomas

    Precision Amplifiers Applications Engineering