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.

  • Resolved

Multislope VI limiter protection to use with LM4702C output stage

Expert 1010 points

Replies: 6

Views: 2152

Hello everyone,

I've designed a multislope VI limiter protection to use with LM4702C (using complementary pair BUZ900P/905P). The design is based on a figure of Randy Slone's book High-Power Audio Amplifier Construction Manual . I've tested the circuit using TINA and it performs very well, protecting the output devices against short circuit and other conditions of low impedance loads. As thermal overload protection I'll use two transistor similar to the ones found in the figure attached. My question is:

The priciple of this protection circuit (multislope VI limiter) is to divert current from the source (which I think that's a current source like a feedback current source or some equivalent), if I divert current from the source transistors the device could be damaged or the circuit are current limited?

About the thermal protection circuit it's a simple transistor (BJT) that if the temperature limit imposed by the NTC is overpassed starts diverting current from the 5V source thru the resistor Rmute, when the current on mute pin is lower then 1mA the device mutes the output signal, giving time to the NTC and the output transistors to lower the temperature, but this kind of protection can damage the mute pin, or produce unpleasent sounds at normal working temperatures?

Thank you very much for your atention,

Best Regards,

Daniel Almeida

Lisbon

  • Hi Daniel,

    It looks like you are making good progress; congrats.

    I do not believe, but I will check to confirm, that the sink and source pins are protected with standard current limit circuitry. There is a limit to how much current the transistors internally will sink/source as indicated by a test in the datasheet. I would be careful trying to pull too much current out of these pins for an extended period of time.

    With respect to controlling the mute pin: the IC doesn't care or notice how the current is controlled, but rather that it doesn't exceed specified limits. So, as long as the 2mA maximum isn't exceeded, you should be fine.

    Best Regards,

    JD

  • In reply to John De Celles:

    Hello, and thank you very much for your reply.

    So you are saying that I shouldn't use the multislope VI limiter.

    I'm confuse, the circuit isn't equiped with short circuit protection and current limit? Why?

    Why that current limitation doesn't appear on Absolute Maximum Ratings?

    Without using sink and source pins I don't have any ideia on how to make a good short circuit protection system, I feel disappointed for not make use of this multislope VI limiter, this circuit as a very good behaviour in TINA simulations with LM4702C.

    It's a big drawback, if you can't protect the output stage pulling current from the source and sink pins.

    In Randy Slone's book is said that you can use that circuit with a voltage gain stage without current limiter, with a voltage gain stage consisting of a constant current source and a driver transistor, but maybe the book is wrong or I don't understand the theory, if I cannot use this circuit with this IC.

    I will not make an amplifier with this cost without a short circuit protection.

    If I can't make one with output protection I'll have to use another IC, like 2 LM4780 in bridge configuration or some equivalent.

    The problem is that I want an amplifier like this one, with better noise and distortion performance and with a distinct output stage.

    Sorry for writing this way, but I feel realy disappointed

    Thank you very much for your atention,

    Best Regards,

    Daniel Almeida

  • In reply to Daniel Almeida:

    Hi Daniel,

    I see your post and will respond as soon as possible.

    thanks.

    JD

  • In reply to John De Celles:

    Thank you very much for your reply,

    I realy want to know if that circuit works and if it's compatible with LM4702C's output.

    Best regards,

    Daniel Almeida

  • In reply to Daniel Almeida:

    Hi Daniel,

    You can rest easy, it turns out that the sink and source pins ARE current limit protected.

    Best Regards,

    JD

  • In reply to John De Celles:

    Thank you very much for your fast reply which was very helpful,

    Now I can go on with my project,

    I'll give news about the development of this project,

    Best regards,

    Daniel Almeida

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.