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LM675 issue

Jamaal_Charles
Genius 3245 points
Other Parts Discussed in Thread: LM675, LM1875

Hello,

We have a few questions about the LM675.  We are looking to convert 0~10V to 0~15V (gain=x1.5).  We built a non-invert circuit on bread board and found if input voltage on non-invert pin was less than 0.75V, its output is saturated (at 13.77V). The OPAMP is operated at 15Vdc.  This tells me I cannot convert 0~10Vdc to 0~15Vdc. I can only convert 0.78V~10Vdc to 0~13.77Vdc.  Is there a way to resolve this issue?

Also, in the LM675 datasheet, it says, “The LM675 is designed to be stable when operated at a closed-loop gain of 10 or greater”. I am wondering if I pick the right part for my application?

 In LM675 datasheet, there is a following example circuit.  I build a simulation circuit with TI-TINA using OPA541E (because I cannot find LM675 simulation model in TINA).  When I put 0.4V as input, the output current is not 2.5A. Why?

You can use voltage potential across output resistor R5, and divide it by the R5 resistance, which is 4 Ohm, to get the output current Io.

I attach my TINA simulation circuit for your reference.

3527.OPA541E.TSC

Thank you!

  • 0
    TI__Guru 53946 points

    Hello Jamaal,

    The LM675 is not unity gain stable. It needs to be in a gain of 10 or more.

    Obviously, a gain of 1.5 is less than 10, so most likely it is oscillating at the low output level.

    Figure 12 shows how to properly use the LM675 at low gains. Essentially it makes the high frequency AC gain 10, but the low frequency & DC gain is 1.

    If you need DC accuracy...then the simplest thing is to set the gain to 10, and attenuate the input. But this creates drift and error due to the offset.

    Look at the OPA544T - same pinout and package, but is unity gain stable.

    My experience with these high-power devices is they do tend to oscillate easily. That is why you see all the warnings about supply bypassing, layout, output snubbers and inductors....they are not kidding. Use the same precautions that you would designing an audio amplifier - which these essentially are (the LM675 is the same as the LM1875 audio amplifier - but with tighter DC specs).

    As for the TINA circuit, two problems:

    1. You need a split supply. Your TINA circuit only has V+. A positive input creates a negative current, and the output cannot swing negative.

    2. 100 ohms is too small to develop 1A with the provided supply.

    For more info, see AN-1515 on the Improved Howland Current Pump

     http://www.ti.com/lit/an/snoa474a/snoa474a.pdf

    Regards,