ADjust voltage with DAC?

[echen1024]

So, I have the circuit below:

And I would like to know how to implement a DAC, such as the MSp4725, to adjust the voltage. I know that the LM317 tries to form a 1.25v reference between the V_out and the ADJ pin, but how would I emulate this digitally. Also, as far as I know, I would need an Op=Amp to amplify the signal of the DAC. Circuit diagrams are very much appreciated, as well as any other help

Cheers,
Eric

[dannyf]

Sum the DAC output + PS output to the ADJ pin: increasing the DAC output will lower the output voltage, and vice versa.

[kizzap]

Just use an op-amp at the adj pin of the LM 317, with the op-amp out to the adj pin, the voltage out to the inverting pin, and the wanted voltage reference fed into the non-inverting pin(+). The op-amp will do the job of offsetting the 1.25V (or whatever it will be) to drive the 317 correctly.

-kizzap

[echen1024]

Ok. What type of Op-Amp would I need. A precision, speed, etc...

[Bored@Work]

Ok. What type of Op-Amp would I need. A precision, speed, etc...

The LM317 doesn't care if its adjust pin is driven directly by a DAC or an OpAmp. It cares that whatever it drives can sink (not source) the current established by the resistor between out and adjust.

If the common 120 Ohm are used that would be 10 mA. And sinking 10 mA is a problem. One way to solve this is to reduce the current. The price to pay for this is that the minimum LM317 current needs to be established in another way, an additional load on the output. Another one would be to drive the adjust pin with a transistor, introducing more non-linearity.

[kizzap]

Ok. What type of Op-Amp would I need. A precision, speed, etc...

The LM317 doesn't care if its adjust pin is driven directly by a DAC or an OpAmp. It cares that whatever it drives can sink (not source) the current established by the resistor between out and adjust.

If the common 120 Ohm are used that would be 10 mA. And sinking 10 mA is a problem. One way to solve this is to reduce the current. The price to pay for this is that the minimum LM317 current needs to be established in another way, an additional load on the output. Another one would be to drive the adjust pin with a transistor, introducing more non-linearity.

As Bored@Work said, it doesn't really matter too much as to the speed, as ideally the power supply will not need to react too much to changes (unless of course you have a Constant Current mode tied into the amplifier as well). The more important thing that you will need to consider is that you will need to power the op-amp from a voltage source that will be able to go above the required output voltage, so that the upper extent will need to go to Vout -1.25 at a minimum, you should be able to simply tie this to the input voltage of the LM317, and if you want to be able to drive the voltage down to zero volts, you will need to provide a negative rail. And one final trick you will have to worry about is the voltage differential between the rails, most Op-amps don't have that great of a voltage range. I'm thinking something like one of the TL07x or TL08x series might be ok.

-kizzap