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0 to 24VDC power supply outputting 22vdc at 0vdc in

Brian Allinger
Prodigy 20 points
Other Parts Discussed in Thread: LM675, OPA564

Hello and Thank you,

I have an LM675 power opamp wired as shown on the first page of the datasheets typical application.

single +24Vdc rail voltage

the feed back resistors have been changed from 20k to 30k and 1k to 15k.

Here is my problem, as I approach 600mv to 0v the output jumps to near full Vcc.

In my application I actually need the output to sink.

Any help would be appreciated,

Brian Allinger

  • 0
    TI__Guru 50016 points

    Hi Brian,

    I assume you have grounded V-.

    The LM675 is not Rail-to-Rail input, or even "ground sensing". The inputs must be at least 2V from the rails (+2 to +22V).

    You are pinching off the input stage and the internal biasing "collapses". If you look at the internal schematic on page 6, Q7 and it's friends need some headroom to operate. When the +IN is less than 600mV to V- (GND in your case), you pinch off that circuit, and the output "collapses".

    Unfortunately, there is no easy fix for your circuit. You can either supply a small negative supply (-2V), or create a "differential" input that move the common mode up into the legal area (with appropriate level-shifting).

    The other problem is that the output will not swing all the way to V-, so you will not get 0V out...maybe 600-800mV at light loads, and up to 2V with heavy loads. So you are being "hit" from both ends...

    If you need 0.00V output, then you will need the negative supply...no easy way around it...

    How much current do you need? Check out the OPA564 which will swing closer and the input range includes ground.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 20 points

    Hi Paul,

    Thanks for your help.

    I have redesigned my circuit to use a +/- 15 Vdc power supply and tested it.

    and it works as needed.

    Thanks Again,

    Brian Allinger