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Electronics => Beginners => Topic started by: warpigs330 on June 15, 2021, 01:06:50 am

Title: Will my adjustable voltage reference schematic work?
Post by: warpigs330 on June 15, 2021, 01:06:50 am

I am making an adjustable voltage reference for a DAC. It is based on the TL431. I want this to be adjustable from roughly 3v to 3.3v. The accuracy of the limits of adjustability isn't too important, just that it has a range. Essentially I have a dac that outputs 3.3v and and amplifier stage that gets that to a bit more than 5v, depending on the tolerance of the resistors it is around 5.12v. I want to trim my DAC reference so I can adjust for part tolerances and get 0-5v out. Will this circuit work the way I want it to?

Title: Re: Will my adjustable voltage reference schematic work?
Post by: ledtester on June 15, 2021, 04:05:50 am

With regards to C10, have a look at:

"Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet"
https://www.ti.com/lit/an/slva482a/slva482a.pdf?ts=1623729444831 (https://www.ti.com/lit/an/slva482a/slva482a.pdf?ts=1623729444831)

Figure 1 shows the regions of stability when driving a capacitive load.

With V_KA = 3.3V, you'll want to have a larger C_L, like maybe > 5uF.

Title: Re: Will my adjustable voltage reference schematic work?
Post by: Terry Bites on June 16, 2021, 02:07:40 pm

[attachimg=1]No, you wont get the adjustment range you want even with no load.
Why not use a fixed ref and take care of the gain or offset in in software?

EG REF19x from AD. Have look at this as well LT6650.

Try this. https://www.ti.com/tool/TL431CALC (https://www.ti.com/tool/TL431CALC)
I'm not sure the FB is of much use here.

This circuit pumps up your 3.3V to 5v making life easier for the TL431. It can manage a few mA of load. You need to clock it in the Mhz range. Of course all the load current comes from the driver- a small mosfet or even a gate output say.

Title: Re: Will my adjustable voltage reference schematic work?
Post by: fcb on June 16, 2021, 02:23:35 pm

Watch out for stability with the TL431, there is a zone (depending on mfr, part, etc..) around 10nF to 1uF where they oscillate quite badly.

Title: Re: Will my adjustable voltage reference schematic work?
Post by: KT88 on June 17, 2021, 04:18:53 pm

The input voltage needs to be higher than the output voltage. The shunt reference must take some current (within spec). The input resistor must be able to carry both reference- and load current without drooping below the specified output voltage.
In this case it would be best to tie the input to your 5V-rail...

Cheers

Andreas

Title: Re: Will my adjustable voltage reference schematic work?
Post by: Kleinstein on June 17, 2021, 04:46:57 pm

Unless one needs the 3.3 V for something else too, why not run the DAC with 2.5 V and than do a x2 amplification. x2 is nice, as 2 equal resistors is easy and often quite stable. If really needed fine trim could be at the x 2 part.

TL0431 is cheap, but hardly better than what one finds as DAC internal reference, or with a decent 3.3 V voltage regulator.