Output voltage of unity-gain buffer varying with current?

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Output voltage of unity-gain buffer varying with current?

Postby kimi » April 27th, 2017, 3:50 am

I'm working on a VLC+IR transmitter that runs from batteries. I decided to use an LM317 so that the discharging battery wouldn't really affect the current through the LEDs. (Disclaimer: I've never worked with this IC before in my life.) I use TINA for simulations and I found a spice model of the 317 somewhere on the internet.* Its output voltage dropped when connecting the LEDs, so I connected a unity-gain buffer op-amp between the two.
The transmitter has two "outputs": a white and an IR LED. They can work together or one-by-one. VG1 and VG2 are square waves with a frequency of 1 MHz. When I turn off SW-SPST2 or SW-SPST3, the other is working properly. However, when I try to operate them at the same time, one of them stops behaving normally. I checked the op-amps output voltage (VM3), and it keeps changing between 2 and 5 volts, approximately. I don't understand why, as the op-amp has a really low output impedance, thus the output voltage shouldn't be affected by the load.
So, my question is: First, what is the cause? Simply a simulation mistake? Second, I'm not really convinced that the LM317 model is correct, so, am I correct assuming, that its output voltage drops with a load?
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Re: Output voltage of unity-gain buffer varying with current

Postby k-ww » April 27th, 2017, 3:00 pm

A good rule of thumb is:

If the power supply droops, increase the capacitance - I would add 100 uf caps to the input and output of the LM317.

Also, how close is the bypass cap to the buffer amp - inches = microhenries of inductance in series with the cap.

Look over the spec sheet of the LM317 and don't rely on the spice model, unless it has a roll-off for frequency response included.

There are many 'flavors' of the LM317, and they vary in current output/heat dissipation ability.
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